Midterm 1 Flashcards
Physiology definition
& emergent properties
Comes from Greek word PHUSIOLOGIA – knowledge of nature
Today’s definition – physiology is the study of the normal functioning of a living organism and its component parts
o Structure and function relationships
o Chemical and physical interactions (ex. the way nt bind to nt receptors)
Emergent properties – living organisms possess; cannot be predicated to exist based only on knowledge of the systems individual parts
o Organized in a way that gives rise to complex life – the whole is greater than the sum of it’s parts
o Properties that are a result of non linear interaction between component parts
Ex. mona lisa from component parts
What defines if something is alive (4)
- Made of one or more cells
a. Cell – basic unit of life - Regulate its internal environment
a. Single cell – intercellular enviro
b. Multicellular – Humans; enviro of cells and collection of cells that make up body - Respond to stimuli
a. “sensory systems” to detect stimuli – Important for survival - Capable of reproduction
a. Self-replication – one cell is able to replicate
i. Viruses cannot self replicate – need a host cell in order to replicate; not actually alive (according to physiology)
Levels or organization
- Biomolecules – lipids, carbs, proteins, nucleic acid and nucleotides
- Cells – smallest units of life capable of carrying out all life processes
- Tissues – group of cells with similar function
- Organs – 2 or more tissues; structural and functional
- Organ systems
- Organisms
- Population of one species
- Includes atoms – responsible for electrical excitability in neurons
Organ systems
Integumentary system – skin; protective; separates the internal from external environment
Musculoskeletal – provides support and body mvmt; skeletal muscles and bone
4 systems exchange material between internal and external enviro
- Respiratory/pulmonary – gases; lungs and airways
- Digestive/gastrointestinal – nutrient and water uptake & waste elimination; stomach, intestines, liver, pancreas
- Urinary/renal – removes excess water and waste material; kidneys and bladder
- Reproductive – produces eggs/sperm; ovaries and uterus and testes
Circulatory/cardiovascular – distributes materials by pumping blood through vessels; heart, blood vessels, blood
Nervous (brain and spinal cord) and endocrine – coordinate body functions
- A continuum more than 2 separate and distinct systems
Immune – anatomically the lymphatic system (not limited to this); specialized cells throughout the body
- Protects the internal enviro
Why is physiology important & exciting
Important:
Leads to treatment of diseases in humans and other organisms
o Pathophysiology – treating diseases
Helps us understand how organisms cope with environmental stressors – helps understand the environment
o Ex. climate change
Foundation of understanding of the philosophical question “What is life?”
o What does it mean to be alive – physio helps define critical processes
Organization:
Fundamental understanding of how life works
Post-genomic era
- Personalized medicine, “$1000 genome” – to know your genome
- You can submit a sample of your cells and they’ll send you the entire sequence of your genome
- Can be used for personalized medicine
- Molecular basis of evolution
- Allowed understanding about molecule basis of evolution
- Structure of individual proteins and how they can be modified with new drugs
Pathway and drug discovery
o Intelligent design drugs
New tech
o Computing, robotics, nanotech
Themes in physio (5)
Structure and function are closely related – particular shape (anatomy) of a protein/cell/organism and how its related to what it does o Organs o Cells (ex. neurons have an axon) o Molecular interactions (ex. protein has a particular structure because it will bind a ligand that will allow it to be part of the signaling pathway)
Information flow coordinates body function
o Electrical/neural signaling throughout the body
o Hormonal communication
Need for energy
o Processing of ATP – metabolism that is dependent on it
Evolution
o Nothing in Biology Makes Sense Except in the Light of Evolution -Theodosius Dobzhansky
o Different processes happening in a similar way in different species – studying other species give us insight to how humans function
Homeostasis and control systems
o Mastering A&P -> study area ->animations ->bioflix-homeostasis
Homeostasis definition
- who named & first mention
- internal parameters that must be maintained
- does not mean
The ability to maintain a relatively constant internal environment even when the external environment is variable.
o 1800’s Claude Bernard called this “la fixite du milieu interieur”. – the constancy of internal enviro
o Word was coined by Walter Canon, 1929
Internal environmental parameters must be maintained within a certain window of acceptable values – respond to fluctuations in external enviro
o Temperature
o pH
o Salinity (concentration of ions and other solutes)
o Oxygen, carbon dioxide
o Nutrients
Homeostasis does not mean “equilibrium” or never changing
Homeodynamics – we observe a dynamic steady state inside vs outside cells
- Ex. Ions are not equal on ECF and ICF but these states must be maintained in order to maintain homeostasis
Requires energy to maintain (ATP)
- not an “equilibrium” - often functions to maintain states of disequilibrium
Control systems
- definition
- pathway
- vary in 3 ways (& what affects intensity)
monitors and adjusts regulated variables at set point; respond to loss of homeostasis
o Requires compensation from external (ex. temp) or internal (ex. ate something) change
Pathway:
- Input signal
- Integrating center
- Output signal
- Response
Long or short distance
1. Local – restricted to one tissue
• Relatively isolated change occurs in a tissue
• Nearby cells sense changes within their vicinity and respond
2. Long distance reflexive control – can be more complex and have input from multiple sources and output that acts on multiple targets
• Endocrine & NS control & neuroendocrine
Vary in speed and specificity
1. Neural – aimed at specific target, fast acting, shorter lived
• Frequency of electrical signals proportional to signal intensity
2. Endocrine – target specificity determined by only receptors, longer to act, tends to last longer
• Will contact almost every cell in body – cells must have a receptor to effect it
• Amount of hormone released proportional to signal intensity
Feedback loops
- sides
- process
2 sides
- Response loop
- Feedback loop – to monitor if compensation was effective; Modulate the response loop
Process
- Stimulus
- Sense or receptor
- Afferent pathway – brings info to brain/integrating center; ex. sensory NS
- Integrating center – ex. brain
- Efferent pathway – acts on target; ex. motor NS
- Target or effector
- Response
Negative feedback
- effects/loop
- properties
- ex. glucose
homeostatic; stabilize variable/cancelling out
Initial stimulus – response – decreased stimulus – response loop shut off
o Error signals act to maintain ‘cruise control’ limits
Properties
- Keeps system near a setpoint
- Response acts to negate the stimulus
- Response can restore homeostasis, but cannot prevent the initial perturbation
Ex. control of glucose – video from mastering A&P Bioflix
o Secretion of insulin – decreases glucose
o Low glucose levels no longer generate signal for insulin to be released – within ideal set of limit
Positive feedback
- effects/loops
- properties
- homeostatic?
- ex. labour
reinforce a stimulus; brings further from set point; some argue not homeostatic
Initial stimulus – response – increases stimulus – feeds into response – feeds into greater response
o An outside factor is required to shut off
Properties
- Brings a system further from a setpoint
- Response acts to reinforce the stimulus – makes error signal stronger
- Requires an outside factor to shut off.
Non-homeostatic? – however, the process of the baby being born is homeostatic and is critical to survival of mother
- Must look at larger context
- Ultimately it is homeostatic
Ex. Labour
o Baby drops in uterus and stretches the cervix – stretching is detected by sensory system
o Hypothalamus secrets oxytocin – contacting uterine muscles cause uterine muscles to contract
o Pushes baby down pushing on cervix more and cycle repeats
o Baby being born stops pos feedback cycle
Feedforward control
allow body to anticipate change; generate response before variable change to prevent severity
o Argued not a loop
o A small stimulus sets off a chain of events aimed at preventing a perturbation.
Requires a complex “program”, or a “reflex”
1. Ex. Mouth watering in anticipation of food is an often-used example
• Psychologists may disagree because of the influence of learning
2. Ex. bag of salt and vinegar chips causes increased salivation when you smell it
o Effects PSNS – salivary glands
3. Ex. Response to exercise
• Respiration, heart rate increase at the beginning of exercise, before changes in O2/CO2
4. Ex. “Fight or flight” activation of sympathetic NS
Ex. Bear in woods – it’s not touching you, but it activates the response nonetheless to prepare your body to survive
- Stimulus was a small visual input – response was a complicated series of events from activation of SNS
Walter Cannon’s Postulates
describe regulated variables and control systems (July 1929) – 4 postulates
1. Nervous system has a key role in regulation of internal environment (often controlling endocrine system – slight bias; most people recognize NS and endocrine as equally important)
o Regulates parameters
2. Some systems of the body are under tonic control
o ‘volume button’ – you can turn it up or down
o Ex. Applies to blood vessel dilation
3. Some systems of the body are under antagonistic control
o 2 opposing factors that balance each other out
o Ex. heart rate – one branch of autonomic NS causes increases and the other causes decease
4. One chemical signal can have different effects in different tissues (communication lecture, ANS lecture)
o ex. epinephrine – blood vessels contract in some areas and contract in other areas (tonic control)
Biomolecules
- organic molecules
- types of biomolecules
Organic molecule that is commonly associated with life
Organic molecule – contains carbon; not technically naturally occurring or healthy
o Exceptions – co2, co, h2co3
Types
- carbohydrates
- nucleic acids/nucleotides/nucleosides
- lipids
- proteins
Carbohydrates
- general chemical formula
- common examples
- properties
- types
CnH2nOn
Common types
o Glucose – C6H12O6 (hexose)
o Ribose – C5H10O5 (pentose)
Properties
1. Hydrophilic (mostly) – water soluble/loving/polar
2. Very abundant
3. Used for:
a) Energy – almost all euk cells can use glucose for energy and can store some form of glucose (monomer or polymer) for energy
• Although most energy is stored as fat
b) Structure
i) Glycosylated proteins – have carbs added to them
o May need to become activated
o May be important for transcription
o May determine localization (ex. moving out of cell membrane)
ii) Glycolipids – carbs attached to lipids
Types – Monosaccharides form disaccharides form polysaccharides
o Monosaccharides – glucose, fructose,
o Disaccharides
ex. Sucrose – glucose + fructose
ex. Maltose – glucose + glucose
o Polysaccharides – most abundant carb; used for structure and energy
ex. Glycogen – storage molecule; polymerized glucose stored in organs (ex. liver)
Nucleotides
- structure
- types
Structure a) 1 or more phosphate group b) 5 carbon sugar c) Nucleobase – carbon-nitrogen ring Structure determines type o Adenine o Cytosine o Guanine o Thymine o Uracil
Types:
a. Adenosine – a neurotransmitter
b. Adenosine triphosphate (ATP) – basic molecule of energy storage in most organisms, including animals; energy is stored in bonds between phosphate
- & Adenosine monophosphate & adenosine diphosphate
c. Cyclic AMP (cAMP) – important signalling molecule within cells
- Adenylyl cyclase (enzyme) – converts ATP to cAMP
d. Guanosine triphosphate (GTP) – energy source in physiological reactions; important in communication pathways (ex. adenylyl cylase pathway)
- & Guanosine monophosphate & guanosine diphosphate
e. Cyclic GMP (cGMP) – important signalling molecule within cells
- Guanylyl cyclase (enzyme) – convert GTP to cGMP
Lipids
- properties (contain)
- structure
- roles
Properties
- Hydrophobic (generally; or have parts that are hydrophobic)
- Contain mostly C and H; a few O, N, P
- Very diverse
Fatty acids
a) Structure
- long, unbranched hydrocarbon chains with 8-28 carbons
- carboxyl (= acidic) functional group
b) Saturated FA – no double bonds; form a straight chain
c) Unsaturated FA – has double bonds; create a ‘kink’
- More double bonds = less likely to be solid at room temp
- Don’t stack as well – pack together more loosely
- More double bonds = greater curl
Roles
a. Structure of cells – define the cell
- Waterproof – separates ECF and ICF
- Pliable
b. Energy source – can be metabolized and converted to ATP
c. Communication – within and between cells
Types of lipids (6)
Glycerides – derivative of FA; glycerol backbone with 1-3 FA
- Types
1. Mono = 1 FA
2. Di = 2 FA
3. Tri = 3 FA - Primary storage product – major component of fat (high = lots of body fat)
Phospholipids – derivative of glyceride - Structure 1. Glycerol backbone 2. 2 FA 3. Phosphate group 4. R group – commonly an amino acid molecule (ex. serine, choline, ethanolamine) The R group: a. Identifies species of phospholipid b. Variable polar group - Amphipathic – has hydrophobic and hydrophilic components 1. FA – nonpolar 2. Phosphate head – polar - Form 3 structures in water 1. Bilayer – cell membrane 2. Liposomes – spherical with aqueous core (bilayer forms sphere) 3. Micelles – difficult to form; not often found in nature; energetically unfavourable
Sphingolipids – analogous to phospholipid in structure
- Contributes to cell membrane formation
- Structure
1. 1 FA
2. Phosphate group
3. R group
4. Sphingosine – instead of glycerol and additional FA
5. Contains nitrogen – usually at bend (be able to recognize by this feature)
Glycolipids – attached carb; contributes to structure, function, localization
see diagrams!
Steroids
- Basic structure – three 6-carbon rings and one 5-carbon ring (17 carbons)
1. Planar/flat molecule – allows protein receptors to recognize and bind
2. Functional R groups – confer different function - Function in: Communication and cell structure
- Examples – same basic structure with ‘décor’
1. Cholesterol – used to make hormones (ex. estrogen, testosterone, cortisol)
Oxylipins – oxygenated metabolites
Eicosanoids – subset of oxylipins
- Structure:
1. Polyunsaturated FA with 20 carbons (fishhook shape)
a. Many are derived from FA arachidonic acid & other unsaturated FA - metabolization of arachidonic is important
- Function
1. Not generally stored – only synthesized as needed
a. Main function is communication between and within cells
i. Inflammation, pain, platelet aggregation
ii. Include prostaglandins and leukotrienes
Protein structure
oligopeptide vs polypeptide
Macromolecules
o Short chain – peptide
o Long chain – protein
Structure of protein:
- Primary structure – sequence of amino acids
a. Linear chain of amino acids – generally
- Oligopeptide – 2-9
- Polypeptide – 10-100 - Secondary – covalent bond angles between amino acids determine secondary structure
a. Secondary structure – created by hyd bonding (cov bonds) between local (adjacent) interactions of amino acids
- A-helix
- B-pleated - Tertiary – 3 dimensional structure of proteins
a. Combine secondary structures
- Formed from chemical interactions between R groups of individual AA (can be cov, ionic, van der waals; depends on AA R group)
b. Pieces can also be removed if not needed on protein
4. Quaternary – interaction of multiple subunits to form active protein; noncovalent interactions Often either fibrous or globular a. Fibrous – not soluble - Ex. collagen – structural protein b. Globular – often soluble - Ex. hemoglobin
Structure of amino acids
- how many
- structure
- properties
- components
Components
a. R group – determines properties (polar/nonpolar/basic/acidic)
b. *20 amino acids encoded by the universal genetic code (debatable – 2 additional may be incorporated)
- 9 of 20 are essential – we need to consume them; our body does not synthesize them
- 11 of 20 are nonessential – we can synthesize them
Varying properties
- Acidic, basic, polar, nonpolar
- Alphabet – analogous to AA and proteins
- Chains can be up to 10,000 AA - Usually a couple hundred or thousand
Structure of amino acid – same basic structure with varying properties Central carbon – 4 bonds are: 1. Carboxyl group (-COOH) 2. Amino group (-NH2) 3. Hydrogen 4. R group – can have varying properties
Functions of proteins
- how many types within mammalian cell
Highly complex
- Determined by the sequence of AA – encoded in genome
- Don’t need to know specific structures of AA – know they have different properties based on structure
Functions of proteins – extremely versatile
o In mammalian cell – between 10,000-15,000 types of protein expressed
o Don’t often do single job (swiss army knife)
Ex. ATPase – uses energy to move; enzyme and transporter
Fibrous vs globular
1. Fibrous – generally insoluble
• Structural (ex. collagen & keratin)
- Globular – generally soluble
Functionally – 7 categories of soluble protein
a. Enzymes – facilitate chemical reactions; without, reactions may not be possible
- Ex. proteases
b. Membrane transporters – sits in membrane and shuttles solute molecule that could otherwise not cross
c. Signal molecules – within and between cells there is communication
- Ex. g-protein – become activated; part of a signalling cascade
d. Receptors – protein that binds something else; does so as part of a communication process
- Ex. hormone receptors – insulin receptors binds insulin and causes reaction
e. Binding proteins – main job is binding something else; sequestering
- Ex. calcium binding protein – binding free calcium inside cells and keep concentration within low
f. Regulatory proteins – many physiological processes occurring; regulatory proteins turns processes on and off
- Ex. DNA binding proteins regulate transcription
g. Immunoglobins – protein binds to antigens
Ligands
- definition
- effects on proteins
- endogenous vs non
- affinity & specificity
- types of effects on proteins (2)
- how types of ligands enact effects (2)
Ligand - molecule that binds to protein site
Protein binding – In order for protein to activate, it must interact/bind to other protein; binding site and activity
- Very specific – shape of binding site is precise
- Ex. insulin receptors bind insulin – insulin is the ligand
Endogenous ligand – occurs naturally in body (ex. hormone, nt)
Nonendogenous – may be a drug/toxin; still a ligand
Affinity
- High = binds strongly
- Low/weak = binds weakly
Types
1. Agonist – ligand that binds to protein site and alters the state of protein resulting in biological response
• Ex. hormone or nt; insulin – all cause response in cell
• Drugs can also be agonist – mimics nt
2. Antagonist – ligand reduces the action of the agonist; binds but causes no biological response; inhibitors/blockers
• Drugs can be antagonist
Agonist and antagonist can both be
1. Competitive – act to block agonist at its normal binding site; binds to it instead
• Acts at the normal binding site
2. Allosteric – block competitive agonist by binding to the protein away from binding site and inactivating the binding site; prevent normal agonist from binding by changing shape of binding site
• Acts at a distant site
Factors that alter protein binding
- isoforms (example)
- activation of proteins (2 methods) & inactive forms
WHAT CAN ATTACH - types of modulators
- physical
- chemical (3)
- saturation
Isoforms – closely related proteins whose function is similar but affinity differs
i. Quaternary structure – subunits acting together; other proteins that are expressed by the body may be similar
ii. Ex. fetal vs adult hemoglobin
Activation
- Cofactors – proteins may need cofactor to function properly; ion or small organic functional group must attach before binding site will activate
- Ex. Mg++ ions binds to slightly alter 3D shape and expose binding site - Protein processing – when proteins are expressed, that may not be the final version
a. Ex. may need to be glycosylated
b. Proteolytic activation – enzymes chop of 1+ portions to activate
- Common in hormones and enzymes
- Inactive forms – ‘pro’ prefix ang/or ‘ogen’ suffix
Types of modulation
i. Changes the ability to bind to a ligand
ii. Changes proteins activity or ability to create response
Physical factors/modulators
- pH, temp – can cause structural changes
- proteins may become denatured – can often not return to normal shape and resume function
Chemical modulation – can be covalent or non; may increase/decrease activity, activate binding site; reversible or irreversible; not necessarily making it ‘functional’
- covalent modulation – can be activating or antagonistic
a. phosphorylation and dephosphorylation – addition or removal of phosphate from protein
- kinases – enzymes, covalently add phosphates
- phosphatases – enzymes, remove phosphate
- phosphorylation may cause activation or inhibition of protein – may allow it to become fully active/activate binding site
Edwin Krebs and Edmond Fisher 1992 Nobel prize – described how important phosphorylation process was in regulating protein activity
a. Described sites proteins are phosphorylated at
- Serine, threonine, and tyrosine AA side chain
- Addition of lipid or carbohydrate
- Presence of agonist or antagonist
a. Antagonist – inhibitors; decrease activity; block binding sites
- Competitive – reversible; degree of inhibition depends on concentrations of ligand vs antagonist and proteins’ affinity for both; Increasing customary ligand can displace competitors
- Irreversible – tightly bound; cannot be displaced
b. Allosteric
Reaction rates can reach a maximum
i. Constant protein concentration – ligand concentration determines response
ii. Saturation – ligands are plentiful but all proteins are active and no binding sites are available
4 functions of cell membrane
- Physical barrier – separates ICF from ECF
- Gateway for exchange – controls mvmts of solutes and regulates concentration in ECF and ICF
a. Semipermeable – allows some to cross, prevents others - Communication – receptors that detect physical and chemical changes and starts cascade response
a. Ex. contracting muscle cells – detect chemical stimuli that cause contraction - Cell structure – some membrane proteins hold cytoskeleton proteins to give cell structure
a. Neurons – have a very specific shape; cell structure is critical to function
b. May also form specialized junctions
- Synapses – specialized
- Tight junctions
Structure of membrane
- mainly
- models that describe
Made of mostly protein and lipid – ration is different for different cell types
o Types or proteins – determines cell’s function
Early model – butter sandwich
a. Layer of fat between 2 bread
- Bread was initially thought to be protein
- Later discovered to be the phosphate head
b. Shortcomings
- Homogenous – not accurate
More recently/accurate – fluid mosaic
a. Proteins are afloat on a sea of lipid
- Shortcomings – not actually afloat
b. Types of proteins
- Peripheral – sitting on surface; afloat
- Integral – anchored within; span the membrane
Types of lipids in cell membrane
- errors in one may cause what
Phospholipids – majority of lipids
o R-groups differ – do not need to be able to distinguish; know they’re different and are distinguished by R-groups
Cholesterol – flat lipids
a. Regulates membrane fluidity – creates more waxy, solidified consistency
- Creates ideal consistency for movements in 37c environment
b. Flat structure – allows it to sit between FA tails
- Slows diffusion of molecules across membrane – fills up space and prevents movement across
Sphingolipids
a. Create lipid rafts – tend to aggregate together; have longer tails than phospholipids
- Some proteins associate only with lipid rafts – leads to areas of specialization on cell membranes
Ex. g-protein coupled receptors
- Have high density of cholesterol relative to phospholipids
b. Ex. sphingomyelin
c. Recent research – errors in lipid raft composition may play role in development of some disease (ex. Alzheimer’s)
Types of proteins within & surrounding cell (3)
- Membrane proteins
- Cytoskeleton
- Extracellular matrix
Membrane proteins
- types (& what kinds of bonds) (how many AA in MSR)
- 4 major functions of membrane proteins
Integral – associated tightly with cells membrane; critical part of cell membrane
a. Transmembrane
- Approx. 20-25 hydrophobic amino acids span hydrophobic middle of cell membrane per MSR
- Ex. 3 MSR (20 AA each); 2 hydrophilic regions
i) Polytopic – transmembrane with more than one MSR
ii) Bitopic – transmembrane with one MSR
b. Monotopic – permanently associated to one side; lipid anchored proteins
May:
i) have strong hydrophobic sections that allow it to tightly associate with lipid portion of bilayer
- No long enough to thread through membrane
ii) be modified by the addition of a FA – FA will protrude into membrane and hold tight
iii) be electrostatic or ionic interaction between protein and phospholipid
- Mediated by Ca2+ where Ca2+ binds to protein and polar head group – anchors
Peripheral – more weakly associated; associates non covalently with integral proteins or polar heads of phospholipids
o Can be removed without disrupting integrity
4 major function of membrane proteins
- Structural – 3 roles in cell membrane:
- Create cell junctions that hold tissues together (ex. gap junctions, tight junctions)
- Connect the membrane to the cytoskeleton – maintains the shape of cell
- Attach to the extracellular matrix – links cytoskeleton fibres to extracellular protein fibres (ex. collagen) - Enzymes – catalyze chemical reactions on external or internal surface
- Receptors – chemical signalling system
- Binding of ligand triggers response
- Play role is vesicular transport - Transporters – move across membranes
- Channels
- Carriers
Cytoskeleton and Extracellular matrix proteins
- how is cytoskeleton attached
- functions of glycosylation in ECF matrix
Cytoskeleton – not a membrane proteins; flexible skeleton of fibrous proteins – give shape and structure
a. Often interacts directly or indirectly with membrane proteins
o Indirectly – Can be attached to a cytoskeleton associated protein to a peripheral protein
o Direct – directly attached to integral protein
Extracellular matrix – membrane proteins and secreted proteins found on ECF side of membrane; forms a husk
- Highly variable glycosylation
a. Functions
- Helps body identify as being part of you (immune)
- Sticky – allows cells to stick together; not loosely aggregated; allows organs to stick together - Contribute to physical strength of cells
Muscular dystrophy
Disease of cytoskeleton
a. Dystrophin – protein that provides link between cytoskeleton and extracellular matrix
- Can be either missing or non-functional in MD
b. Strongly expressed in muscle cells – loss of protein causes muscle to have no physical strength; results in easily damaged muscles
- Repeated damage causes muscle to eventually waste away
- Scar tissue
Diffusion
- definition
- properties
- simple diffusion
- diffusion across membrane
Diffusion – process of moving solutes from high to low concentration
a. Passive – no external energy (ex. ATP); uses kinetic energy of molecules (heat from vibrations of molecules)
o Down the concentration gradient
o Continues until equilibrium – solutes concentration is the same throughout
Simple diffusion – no membrane
o Diffusion is fast
o Net movement is high to low – some will move low to high due to randomness of movement
Semipermeable membrane – allows select solutes to pass
a. Slows movement of permeable solutes due to presence of membrane
- Passive diffusion – net movement is down concentration gradient
b. Does not allow some solutes to pass (rate = 0)
- Their concentration gradient is maintained
- requires transport mechanisms
Factors that affect the rate of diffusion (5)
Fast over short distance & slow over long
- Time to get A to B is ‘distance squared’ relationship – if distance doubles from 1 to 2; time increases 1 to 4 (2^2)
Increases with temp
- Temp = heat energy -> vibrational mvmt increase
Faster for smaller molecules
Slower across membrane
- Even if the membrane is easily permeable – crossing membrane inherently slows
Permeability of membrane
Semipermeability of cell membrane
Molecules able to cross via diffusion
1. Hydrophobic nonpolar – can pass through membrane easily; move through FA tails
• Ex. o2, co2, lipids, steroids, fat soluble molecules
2. Small uncharged polar molecules – water and urea
- Polar – does not like to interact with FA
- Water and urea are exceptions – able to move across
a. Water
o Able to move across for 2 reasons:
Size - water is small relative to bilayer
Uncharged
o Water mainly gets through membrane via aquaporins – controversial in biology
Cannot diffuse across cell membrane – require transport mechanisms
- Large uncharged polar molecules – glucose, proteins, amino acids
- Charged ions – Na+, K+, phosphate
Factors that affects the rate of diffusion across cell membrane (5)
- ex of 2 diff sized molecules within cell
Speed of diffusion is affected by
a. Size – larger will move more slowly
o Ex. glucose vs glycogen (made up of glucose) – glycogen has more inertia & is harder to move
b. Lipid solubility – polar or nonpolar or VERY nonpolar
o Polar – do not like to move at all
o Nonpolar (ex. cholesterol) – moves across membrane
o Very nonpolar (ex. benzene ring) – moves across very quickly
Concentration gradients
a. Large = faster diffusion (10:1 vs 1000:1)
Surface area
a. Larger = more area to diffuse across = more quickly than smaller surface area
b. Can increase surface area by increasing folding – same volume but one will have more surface area
Temp
a. Heating = greater KE (vibrational mvmt increases)
Composition of membrane
a. Simple lipid bilayer vs membrane with many proteins and extracellular matrix
• Simple – easier
• More impediments – slower
b. Types of phospholipids and sphingolipids
c. Presence of cholesterol between FA tails – slows rate of diffusion
Ficks Law of Diffusion
Rate of diffusion is proportional to: surface area x concentration gradient x membrane permeability
o Surface area
o Concentration gradient
o Membrane permeability
Membrane permeability is proportional to: Lipid solubility/molecular size
• Smaller molecular size = greater permeability (inversely related)
• More lipid soluble = greater permeability (directly related)
Liposomal drug delivery
Some drugs have low bioavailability due to poor solubility – doesn’t reach concentrations required to induce physiological effects
o May also be toxic at useful doses and must be targeted to a specific cell type (ex. only cancer cells; concentration needed is toxic to the rest of the body)
Can take advantage of liposomes formed by phospholipids in hydrophilic environment
a. Can target specific cells by modifying phospholipids by adding protein tag
- When liposome binds to cell of interest bind – will deliver drug to that cell
- Ex. targeting insulin receptors – attach a molecule similar to insulin
b. Can glycosylate to keep them from being destroyed by immune system
Uses
- Oil soluble drugs – can be dissolved and stored in bilayer
- Solid or water soluble drugs that will not dissolve otherwise – can be broken down into nanoparticles and stored in aqueous core
- Not as effective as soluble – still able to be delivered
Osmosis
- % in ECF vs ICF
- men vs women
Osmosis – the diffusion of water
60% of body weight is water
a. ICF – 2/3 of body water volume
b. ECF – 1/3 of body fluid volume
- Interstitial fluid – between circulatory system and the cells has 75% of ECF volume
- Plasma – liquid matric of blood within circulatory system has 25% of ECF volume
Men vs women
a. 70kg man – 60% water; 42L of water
b. Women – have less water & more adipose tissue
• Must be taken into account when administering drugs – women will have higher concentrations in their blood than men
States of homeostasis within the body
- states of disequilibrium
- osmotic equilibrium & osmotic pressure
- Chemical disequilibrium – distribution in and out of cells
- Electrical disequilibrium – ions in and out of cells; creates voltage difference
- Osmotic equilibrium
a. Water diffuses down concentration gradient (high to low; low solute to high solute)
• Pure water has the highest concentration – solutes lower the concentration
• Movement of water can cause pressure
b. Ex. u tube – selectively permeable membrane
• 2M and 1M glucose – water moves 1M to 2M
o Volume of water on 2M side increases – not complete equilibrium due to volume
c. Osmotic pressure – the amount of pressure that must be applied to oppose osmosis
Osmolarity
- units
- types
- important because
Describes the number of particles per litre of solution
Units:
Osmol/L or OsM
Milliosmoles/L = mOsM
Comparing osmolarity
a. Isometric/isosmotic – same number of particles
b. Hyperosmotic – solution has high osmolarity (more particles)
c. Hypoosmotic – solution has low
• Will always be hypotonic
Ex. 3 solutions
a. 1M glucose = 1 OsM
b. 2M glucose = 2 OsM
c. 1M NaCl = 2 OsM (NaCl will dissociate into Na+ and Cl-)
Important because – changes of osmolarity of ECF causes redistribution of solutes in cells
- Can influence tonicity (will determine whether cell will shrink/swell)
Osmolarity vs Molarity vs Osmolality
- converting between osmolarity and molarity
Osmolarity differs from molarity
- Molarity – does not take into account that some molecules dissociate into 2 ions (ex. NaCl)
- Osmolarity – takes into account the number of particles
To convert between osmolarity and molarity:
(mol/L) x particles (osmol/mol) = osmol/L
- Osmolality – osmol/kg (often interchangeable with osmolarity)
- Used clinically – 1kg = 1L; a loss of 1kg is equivalent to 1L of water lost
Normal physiological concentrations of salts
ECF ~290 mOsM (concentration of solutes & other things) K+ 5mM Na+ 145mM Cl- 108mM Ca2+ 1mM
ICF ~290 mOsM K+ 150mM Na+ 15mM Cl- 5mM Ca2+ 0.0001mM
Tonicity
- definition
- types
Tonicity – describes solution and how solution effects cell if placed within and allowed to come to equilibrium
Concentration in normal/healthy cells is ~290 mOsM
Hypertonic - cell shrivels
ECF = 360 mOsM (ex. 180mM NaCl)
Isotonic - cells don’t change
ECF = 290 mOsM (ex. 145 mM NaCl)
Hypotonic - cells swell
ECF = 200 mOsM (ex. 100 mM NaCl)
Osmolarity vs tonicity
- functionally non penetrating?
Osmolarity
a. Only describes number of solute molecules in cell
b. Can compare any two solutions
c. Does not always tell is cell shrinks or swells
d. Measures the concentration of penetrating and nonpenetrating solutes
- Penetrating – able to cross membrane; Small polar (ex. water & urea) and nonpolar
- Nonpenetrating – ions and larger polar molecules; don’t diffuse across bilayer
- Functionally nonpenetrating – able to cross membrane/leak out; concentrations are maintained by homeostatic mechanisms
(Ex. Na+, glucose, amino acids)
Tonicity
a. Comparative – whether or not cell changes volume; compares ECF solution to a cell’s ICF solution
b. Always specifically refers effect on cell – whether cell will shrink swell
c. Disregards penetrating solutes – solutes able to cross and don’t influence water movement in/out of cell (affecting hypo/hyper)
- When penetrating and nonpenetrating are present
Ex. initially isosmolar
- P solutes want to establish their own equilibrium – will move into cell
- Hypotonic solution
Rules for tonicity and osmolarity
Normal physiological ICF solutes are functionally non-penetrating (~290 mOsM)
Hypoosmotic solution – always hypotonic
- Hyperosmotic solution – not necessarily hypertonic – solutes in ECF may be penetrating and create hypotonic solution
When determining tonicity with penetrating solutes
a. Disregards concentrations of P solutes – water will ultimately flow to area of higher NP solutes
b. Penetrating solutes – may influence tonicity if establishing
Types of transport proteins
- Channel proteins
- Carrier proteins
Channel proteins
Channel proteins – water filled; open to both sides; more rapid
- Ex. water or small ion channels
Can be open or gated
a. Open/leakage – open virtually all the time
b. Gated – under certain circumstances they open and close (this is their physiological role)
- May respond to – mechanical, chemical, electrical
Topological representation of ion channel protein – Shows how many MSR there are (all channels will have at least one; they will have more than one)
- Require MSRs in order to allow water through
- Will fold and assemble around central water filled pore
Carrier proteins
- properties
- transportation state
- exhibit 3 things
- direction of transport
- number of molecules
- energy requirements
Properties
a. Never forms open channel between 2 sides
b. Can move larger molecules – glucose & amino acids
c. Dependant on conformational changes
- Chemical will bind somewhere within carrier – causes conformational changes
- Transportation state – closed on both sides; conformational changes allow ligand to release and carrier to open to opposite side
- Opens to ECF/ICF
Exhibit
a. Specificity – the ability of transporters to only bind to/move a certain ligand or a group of closely related ligands
b. Competition – substrates will compete with each other for binding sites on carriers; carriers will have varying affinities for substrates
• Competitive inhibitor – blocks the binding site
c. Saturation – rate of transport is dependent on concentration of substrates; must be within physiological limitations
• Fixed number of carriers – when all are bound, rate of transport cannot continue to increase with increased concentrations; transport maximum
• To avoid saturation – cells can increase number of carriers within membrane
Direction of solute transport and how many molecules:
1. Uniport – allows a single molecule (ex. glucose)
2. Cotransporter – can move multiple molecules at a time
a. Symport – 2 molecules move simultaneously in the same direction
• ex. SGLT: Na+ and glucose
b. Antiport – moves more than one molecule in opposite directions
• Ex. ATPase: Na+ moves in, K+ moves out
Energy requirements – creates 3 kinds
- facilitated diffusion
- primary active
- secondary active
Facilitated diffusion
Facilitated diffusion – moving molecule across membrane down concentration gradient via carrier proteins
Passive – does not require ATP or other solutes (secondary)
• This process alone cannot accumulate solutes against a concentration gradient
o Process will halt will chemical equilibrium is established
Ex. GLUT protein
a. ECF has high concentration of glucose – glucose binds to GLUT transporter and move inside cell
b. Cells use enzymes to phosphorylate glucose and polymerize in glycogen
o This lowers the concentration of glucose within the cell
o Able to accumulate glucose molecules in the form of glycogen; not stored as glucose in cytoplasm
o Chemical disequilibrium is maintained – GLUT transporter can continue to move glucose passively
Primary vs Secondary active transport
- examples of each
Primary active a. Energy requiring – uses ATP directly • Establishes gradients • Sometimes called pumps b. Na+ K+ ATPase – the most widely known example • Concentrations inside and outside are maintained by ATPase o ECF K+ 5mM Na+ 145 mM o ICF K+ 150 mM Na+ 15 mM • Moves 3 Na+ out & 2 K+ in • Hydrolysis ATP (ATP -> ADP + Pi) • Undergoes conformational changes – not a channel c. Other types • Ca2+ ATPase • H+ ATPase (proton ATPase) • H+ K+ ATPase
Secondary active
a. Energy requiring – uses concentration gradient of one molecule/ion going down concentration as energy source to transport another against its gradient
• Does not directly utilize ATP as source of energy
b. Ex. SGLT protein – symport cotransporter
• High Na+ inside, low inside – gradient was initially established by the use of ATP
o Na+ binds to carrier – conformational change allows glucose to bind
o Glucose binds – another conformational change
• Protein releases both into cytoplasm
o Glucose moves against its concentration gradient
Epithelial transport
- types of transport used
- effects of tight junctions (regions created)
- absorption vs secretion
Epithelial transport – molecules entering and leaving body or moving between specific compartments must cross layer of epithelial cells connected by adhesive and tight junctions
Epithelial transport utilizes
o Facilitated diffusion
o Primary active transport
o Secondary active transport
Tight junctions – forces solutes to cross through epithelial cells; not between
1. Create 2 regions:
a. Apical membrane/mucosal membrane – faces lumen; microvilli increases surface area
• Certain transporters only found on apical (ex. SGLT)
b. Basolateral membrane/serosal membrane – 3 surfaces of cell facing ECF
• Certain transporters only found on basolateral (ex. GLUT, Na+ K+ ATPase)
2. Regions are polarized – apical and basolateral have different properties
- Polarization allows one way movement of certain molecules across the epithelial membrane
Absorption vs secretion
o Absorption – mvmt of material from lumen to ECF
o Secretion – mvmt of material from ECF to lumen; can also mean to release from cell
Epithelial transport in the digestive tract
- surface area
- transporters used
Folds in villi – each epithelial cell lining the lumen on the villi has microvilli on lumen facing surface
o Increases surface area
Ex. moving glucose from lumen into circulation
o Steps – 3 transporters used to accumulate glucose
1. Na+ K+ ATPase – uses active transport to move Na+ out of cell & maintains low Na+ concentration within cell
• Higher concentration of Na+ outside of cell (in lumen)
2. SGLT (Na+ & glucose) cotransporter – uses energy of Na+ gradient to move glucose from low to high concentration
• Glucose builds up within the cell
3. GLUT transporter – accumulating glucose within the cell will start move via facilitated diffusion through GLUT transporter (passive – moving high to low)
Paracellular (protein)
- pathology
Transcellular (steps and changes in permeability)
Paracellular – mvmt through junctions between adjacent cells
o Tight junctions – act as barriers; minimize unregulated diffusion & results in very little paracellular transport
o Some epithelia can change ‘tightness’ of their functions
Ex. claudins (junctional proteins) can form large holes that allow water, ions, and small uncharged solutes to pass paracellularly
o Pathology – increased paracellular mvmt indicates disease
Transcellular transport – mvmt through epithelial cells themselves; crosses 2 membranes & uses a combination of active and passive transport
o 2 steps in protein mediated transcellular transport
1. Uphill – requires energy
2. Downhill – moves down concentration gradient
o Molecules too large for membrane proteins must cross in vesicles
o Cells can alter permeability by selectively inserting or withdrawing membrane proteins - Transporters removed may be destroyed in lysosomes or stored in vesicles to be reinserted in response to a signal
4 methods of cell to cell communication
Local
1. Gap junctions – allow direct cytoplasmic transfer of electrical and chemical signals between adjacent cells 2. Contact dependant signals – occur when surface molecules on one cell membrane bind to surface molecules on another cells membrane 3. Chemical that diffuse though the ECF to act on cells close by
Long distance communication
4. Uses a combination of chemical and electrical signals carried by nerve cells and chemical signals transported in the blood
Local signalling
- Gap junctions
- structure
- common where - Contact dependant
- structures
- common where - Paracrine and autocrine
Gap junction – channels that connect adjacent cells
- Structure
a. Two adjacent cells each express a connexon (proteins) – will assemble themselves and form gap junction channels
- Each connexon – 6 connexin monomers/subunits (each cell has)
- 12 connexins/2 connexon line up & form gap junction channel
b. Cells are pulled tight together (2-4nm)
c. intercellular 50-100nm channel – fairly wide
d. Water filled pore – will allow small molecules and ion to move across
- Creates cytoplasmic bridge – hydrophilic channel - Common in heart, smooth muscle, and neurons
i. Electrical in heart – movement of ions
Contact dependant – a molecule/ligand in EC matrix of one cell binds to a receptor in the membrane of an adjacent cell
- Structures - Cell 1 and 2
a. Signal/ligand of cell 1 – often laminin or integrin
b. Binds to receptor of cell 2 – causes physiological response - Important in immune and development
a. Development – cells are moving around
- To get point A to B: there are ‘sign posts’/receptors that help guide developing cells to where they need to go
Paracrine and autocrine – a signal chemical molecule is released
a. Paracrine – signalling to cells in the immediate vicinity
b. Autocrine – signalling to itself (auto for self/autonomous)
Long distance communication
- types of systems and cells within systems
- endocrine vs exocrine
Endocrine system - Secrete hormones/chemicals into blood that affect cells in other parts of organism
a. Endocrine cell – secretes chemical signals into ISF & picked up by circulatory system
b. In circulatory – contacts almost every cell in the body; response is dependant on receptors
• Only cells with receptor will have a physiological response
Endocrine vs exocrine
- Endocrine – refers to substance secreted into blood (ex. insulin)
- Exocrine – refers to substances secreted into a duct (ex. digestive enzymes from pancreas into pancreatic duct, then digestive system; this is exocrine because they are leaving the body)
Neurotransmitters – electrical signal travels distance along nerve; causes the release of a chemical; chemical travels across small gap (synapse) onto target
o Not released into blood stream
Neurohormones – neuroendocrine
o Electrical signal travels distance along nerve cell & causes the release of a chemical – the chemical is released into the blood and can act on a distant target
Ex. release of oxytocin – neurohormone produced by neurons & released into blood stream
Process of cell to cell communication
- requires
- process
- ubiquitousness
- amplification
Cell to cell communication (except gap junction) requires:
o Signal/ligand – usually secreted (ex. hormone)
o Receptor
o Signal transduction pathway – intracellular pathways that convert on form of signal to another to make it a meaningful response (not intercellular)
ex. vinyl record
Process: Signal molecules –> binds to receptor proteins –> activates intracellular signal molecules –> alters target protein (ex. phosphorylation) –> creates physiological response
Pathways are ubiquitous (found everywhere) – all cells have some pathways but not all cells have the same pathways
- ex. only some cells respond to insulin (some don’t have insulin pathways)
Amplification – amplifier enzymes allow the presence of one or just a few molecules to have a large physiological effect
Signalling pathways based on receptor location
Intracellular - Ligands are lipophilic/hydrophobic – receptors are in the cytoplasm & nucleus (ex. steroids)
a. Ligand diffuses through membrane and nuclear membrane -> binds & activates receptors -> alters gene expression
o Nucleus – binds to dna sequence
o Cytoplasm – moves into nucleus and binds to dna sequence to change expression
b. Often afters gene expression (changes protein expression)
c. Slow and long lasting – hours to days
Cell membrane receptors (and membrane bound organelles) - Ligands are usually lipophobic/hydrophilic (ex. Insulin and other water soluble peptide hormones)
a. Ligand binds to membrane receptors – does not diffuse through cell membrane
b. Causes intracellular cascade
c. More rapid cellular response – can also be shorter lasting
Types of membrane receptors in intracellular signalling pathways (for hydrophillic ligands)
- Integrin receptors
- Receptor channel
- Receptor enzyme
- G protein coupled receptor
Integrin receptor
Receptor channel
- speed
- other names
- effects
Integrin receptor
- membrane protein & receptor that beings to extracellular matrix proteins (ex. collagen)
- binding ligand stimulates changes in cytoskeleton - cell movement, growth, wound healing
Receptor channel – transmembrane protein; ion channels
a. Often called
- Ligand gated ion channel
- Neurotransmitter gated ion channel
- Ionotropic receptor
Ionotropic – involves actions of ions
b. Ligand is often a neurotransmitter – binding causes opening of channel
- Allows ions to enter and leave cells – synaptic transmission
- Movement of charged molecules to move across membrane – basis of electrical signals
c. Allows ca2+ into cells – important in intracellular signal
- 2 effects
1. Electrical affect
2. In and of itself an intracellular signal – see Ca2+ as a second messenger notes
d. Very fast – a nt binds, it opens and allows movement of ions in only a few milliseconds
Receptor enzymes
- commonality with GPCR
- TKR
- example of TKR
Activate amplifier enzymes (same as GPCR)
i. Signal amplification – a small amount of ligand creates a large biological effect
Tyrosine kinase receptor (TKR/RTK) – transfers phosphate group from ATP to tyrosine residue (AA) of a protein
a. Has one membrane spanning region
- ECF – has surface receptor; becomes activated by presence of ligand
- ICF – tyrosine kinase
Kinase - Phosphorylates protein on tyrosine when proteins bind to binding site
i. will activate/inactivate protein
ii. increase/decrease activity
ex. insulin receptor
1. 2 receptors must dimerize in order for insulin receptor to become activated
2. Receptor enzyme has:
a. an alpha subunit (in ECF portion) – much smaller; contains insulin binding site
- insulin binds -> activates receptor
b. beta subunit (membrane spanning region) – contains tyrosine kinase
- active receptor -> self phosphorylates beta subunit
- TKR on the receptor -> phosphorylates insulin receptor substrate (another signal within cell)
3. Insulin receptor substrate (IRS) – goes onto activate other intracellular processes & proteins
a. Some activated proteins will be kinases – will phosphorylate other proteins
G protein coupled receptor
- how many known
- also called
- general process
- properties and effects of second messengers
- amplification
Hundreds of known GCPRs – known due to genome sequencing
- Many have unknown functions – orphan receptors (don’t know what they do; active area of research)
Also called
i. Metabotropic receptors – as opposed to ionotropic
- Named for the series of biochemical events that occur after they become activated
ii. 7 transmembrane domain receptors (7TRs)
- 7 hydrophobic regions -
- Hydrophilic on each end
iii. Serpentine receptors – looks like a snake
General process
i. Ligand binds to G protein coupled membrane receptor protein/GPCR (7 MSR) – activation g protein
ii. G protein – inner portion of cell membrane (integral protein; lipid anchored protein); binds GDP and GTP
- Inactive configuration (no ligand) – binds GDP
- Active configuration (ligand bound) – exchanges GDP for GTP
iii. G protein binds amplifier enzyme – leads to release/synthesis of second messenger molecules
iv. Second messenger – causes physiological response when released
Second messengers:
- Properties
a. Stimulates a biological response
b. Either
i. Synthesized
ii. Released from storage compartment
c. Usually small and diffusible* (can move throughout cell)
i. Most common – small hydrophilic
ii. Can also be hydrophobic or gas - Effects
a. activate protein kinase – phosphorylates proteins
b. cause release of intracellular ca2+
i. Most cells have Ca2+ stored (ER) – causes release into cytoplasm
ii. Calcium binding proteins – causes physiological response
c. alters ion channels – alters electrical properties & has implications for neurons in NS
Amplification - Occurs in multiple areas
a. Each activated GPCR – can activate 10x g proteins
b. Each amplifier enzyme – can activates 100s-1000s second messenger molecules
- Each second messenger – activate a single protein kinase
c. Each protein kinase – can phosphorylate 100x proteins
Activation of a single receptor – can generate activation of 100,000 protein via phosphorylation (at a minimum)
a. All contribute to a coordinated cellular response
Types of second messengers
Classic second messengers
a. cAMP, cGMP
b. IP3, DAG
Novel second messengers
a. Ca2+
b. Gasses
c. Lipids
d. Endocannabinoids
Calcium functions as second messenger
- Binds to the calcium-binding protein calmodulin
a. Calmodulin – activate other proteins and molecules (ex. kinases) - Binds to motor proteins and allows action of cytoskeleton and motor proteins
a. Important for muscle contraction - Binds to synaptic proteins to trigger exocytosis – causes release of nt into synapse
- Binds to Ca2+ gated ion channels to modulate their gating (opening & closing)
- In fertilized eggs – initiates development
a. Rapid increase in ca2+ is one of the first steps in initiating development of embryo
Gases as second messengers
Soluble gases are recognized as second messengers (more recent; not in all textbooks but important in understanding homeostasis)
NO (nitric oxide)
a. NO has half-life of 2-30 seconds – makes it difficult to study them
- As soon as you cause they production – they go away
b. Synthesized by NO-synthase
- Synthesized by endothelial cells of arteries – diffuses into adjacent arterial smooth muscle
- In smooth muscle – activates guanylyl cyclase (amplifier enzyme)
- Activated guanylyl cyclase – produces cGMP from GTP & leads to relaxation of smooth muscle
c. Pharmalogical reasons (ex. Viagra)
CO (carbon monoxide)
a. Activates guanylyl cyclase
H2S (Hydrogen sulfide)
Adenylyl cyclase
Process – see diagram!
- Ligand binds to G protein coupled receptor – associated with g protein
- Receptor can stimulate several G proteins - 3 subunits of G protein: α, β, γ – become activated and switch GDP for GTP
- Can diffuse along the inside of the membrane – lipid-anchored proteins - Each G protein activates one Adenylyl cyclase – amplifier enzyme
- still within membrane - Adenylyl cyclase – converts several hundred ATP into cAMP.
- cAMP – the second messenger - cAMP can diffuse throughout the cell – activates protein kinase A (PKA)
- PKA diffuses within cell to phosphorylate many other proteins.
- Many types of proteins can be phosphorylated, giving rise to complex & coordinated cellular responses – important to note
Phospholipase C
Abbreviations/terms
a. PL-C = phospholipase C – amplifier enzyme; acts on and degrades specific membrane phospholipids (primarily Phosphatidylinositol 4,5-bisphosphate) into second messengers:
- (1) Diacylglycerol
- (2) Inositol tri-phosphate.
b. DAG = diacylglycerol – a lipid product of cleaved phospholipid in the cell membrane by PLC (enzyme); remains associated with membrane and can activate PKC
c. PK-C = protein kinase C – phosphorylates other proteins
d. IP3 = inositol trisphosphate – second messenger produced by hydrolysis of phospholipids in the cell membrane by PLC
- Small and water soluble – diffuses easily throughout the cell but not across cell membrane
- Binds to IP3 receptor channel in ER – causes channel to open; releases ca2+; Causes increase in free ca2+
- When channel closes – ca2+ ATPase pumps ca2+ back into ER
e. ER = endoplasmic reticulum
- Storage compartment for proteins and calcium; Membrane bound
Process – see diagram!
- Ligand binds to and activates g protein receptor
- Same process as adenylyl cyclase - G protein activates the phospholipase C – amplifier enzyme
- PLC – degrades membrane phospholipids into two second messengers
a. Diacylglycerol – stays associated with lipid (diglycerides are nonpolar and lipophilic) - DAG activates protein kinase C (PKC)
- PKC – diffuses within the cell & phosphorylates other proteins
- Inositol tri-phosphate – small polar molecules; diffuses throughout cytoplasm - IP3 binds to the IP3 receptor on the endoplasmic reticulum
- Activates IP3 receptor channel – allows stores of Ca2+ to be released into cytoplasm - Ca2+ becomes another second messenger
Arachidonic acid
Process – the Arachidonic acid pathway is similar to the PL-C pathway
- GPCR is activated – activates g protein
- G-proteins activate Phospholipase A2 (PLA2) – amplifier enzyme
- PLA2 degrades phospholipids into Arachidonic Acid
- Arachidonic acid – 20 carbons
Arachadonic acid (and its eicosanoid metabolites) – have a dual function
- Are themselves second messengers within a cell
- Diffuse out of the cell and act as a ligand for GPCR cell membrane and adjacent cells
Metabolites
- Leukotrienes – released during allergic responses
- Singulair – leukotriene receptor antagonist - Prostaglandins thromboxane’s – inflammation
- Aspirin – inhibits the enzymes that produce prostaglandins - Histamines ?
Specificity of signalling pathways
- effects of epinephrine in different areas
- cause of different response
- promiscuity of receptors
Physiologists were not able to explain why the hormone epinephrine (adrenaline) caused some blood vessels to constrict and others to dilate for years
Effects of epinephrine in the same conditions
- Skeletal muscle – dilate
- Abdominal viscera blood vessel – constricts
Different responses are due to the presence of receptor isoforms – ligands can activate multiple receptors
o 2 receptor isoforms for epinephrine – can activate both
a. Blood vessels from intestine – will constrict
• Alpha (α)
b. Blood vessels from skeletal muscle – will dilate
• Beta (β2)
Creates a coordinated response by body – dilates some and constrict some to direct blood where it’s needed during fight or flight response
Some receptors are promiscuous – activated by more than one ligand
a. Epinephrine and norepinephrine – both can activate alpha and beta receptor isoforms
b. Have varying affinities
• alpha receptors – prefer norepinephrine
• β2 receptors – prefer epinephrine
Modulation of receptor activity
- number over time
- up and down regulation
Cells contain hundreds to thousands of receptor proteins
Receptors are not constant – can change the population over time depending on homeostatic state
- The numbers of receptors can change over time & type of receptor can change overtime
Upregulation & downregulation of receptors
Decreasing agonist efficacy:
a. Down regulation – decrease in receptor numbers; physically removed through endocytosis
b. Desensitization – binding chemical modulators to receptor protein (ex. phosphorylation)
Changes occur due to:
a. During development – some receptor signaling pathways are important in development but no longer required later
b. Homeostatic challenges/normal day to day functioning
- Ex. eating a lot of Na+ vs properly hydrating
c. Disease states
- Some g protein coupled receptors are up or down regulated – become a target for treatments
Desensitization of receptors
o Phosphorylation of α and β2 receptors can cause them to have a lower affinity for ligands
Ex. Opioid tolerance as a result continuous exposure to an agonist
- Initially – small doses are effective
- receptors will become desensitized due to various mechanisms after repeated exposure – will require increased dosages to achieve the same effect
Mechanism includes:
1. Phosphorylation – causes confirmational change (can increase or reduce affinity)
o Reduce affinity of receptor for ligand (opioid)
2. Decoupling receptors from signaling pathway – G proteins will no longer associate as effectively as GPCR
o Initially – GPCR activates 10 g proteins
o Later – GPCR activates 1 or 2 g proteins
3. Cell processes that functionally result in less receptors in membrane:
a. Cells no longer express as many receptors
- Unstimulated – express 1000
- Now – only express 100
b. Change in expression of receptors
- Cell makes just as many proteins (100% are being expressed) – but are not longer inserted into membrane
- Pathway in membrane has slowed down
Membrane potential
- electrical properties
- conductors vs insulators
- electrochemical gradient & electrochemical equilibrium
- how membrane potential is measured
- changes terms
Membrane potential - occurs due to electrical disequilibrium; voltage difference is established by ATPase transporters
- Not constant – can change due to the movement of ions
Electrical properties
a. Ion gradients are maintained by primary active transport – energy is required to separate charges
b. Conductors – allow charges to move through
- Ex. ion channels
c. Insulators – separate
- Ex. cell membrane
Electrochemical gradient – combination of an electrical gradient and chemical gradient
a. Ions move according to chemical concentration gradient and electrical gradient
- Ex. K+ wants to move out to establish chemical equilibrium, but in towards neg charged ICF – both forces act upon K+ simultaneously
b. Electrochemical equilibrium – the rate at which ions move in/out of the cell down their concentration is exactly equal to the rate at which the ion moves out/in down it’s electrical gradient
Measured
a. Use volt meter
- 1 electrode in cell
- 1 electrode in ECF – ground electrode
b. Chart recorder – measures membrane potential over time and its charges
- Flat – steady state balance of leak vs transport
- Change in electrical energy will be proportional to change in graded potential
- Threshold reached – change in electrical state is not proportional to change
Changes
o Depolarization – more positive
o Repolarization – back to resting membrane potential
o More neg (lower than RMP) – hyperpolarization
Equilibrium potentials
- is it an equilibrium?
- reversed potentials; the membrane potential that exactly opposes the steady state electrochemical gradient of an ion (exactly opposes the concentration gradient of an ion)
Not technically equilibrium – ATPase energy is required to maintain the gradient
Equilibrium potential of each ion is independent of the concentrations of other ions
o Ex. the amount of voltage necessary to keep the K+ inside the cell instead of leaking out to establish chemical equilibrium – when pump is turned off???
Calculating at 37°C for a given concentration gradient
Nernst equation:
Eion = 61/z x log ([ion]out/[ion]in)
- E = equilibrium potential
- [ion]out and [ion]in = concentration of ions in ECF and ICF
- z = charge of the ion
- units in mV
Equilibrium potentials for key ions
K+: -90mV
Na+: +60mV
Ca2+: +122mV
Cl-: -81mV
Resting membrane potential
- definition & properties
- 2 facts that establish
- effects of equilibrium potentials
- relative permeability of ions & changes
Resting membrane potential – steady state balance between active transport and leakage of ions
o Most cells – between -20 and -90mV; negative with respect to the outside
o Not an equilibrium – requires energy to maintain gradient
Established by 2 factors
1. Ion gradients across membrane
a. Under normal physiological circumstances – AP do not change concentration gradients by a significant amount
• Small changes in concentration cause large and rapid changes in voltage
• ATPase does not play a large role in AP
• Multiple and rapid AP over long period of time – may effect
2. Relative permeability of those ions
a. Will change if ion permeabilities change
Established by the equilibrium potentials of each ion and each ions relative permeability through the membrane via leakage channels
- If only one ion was present – RMP would be its equilibrium potential
- Cells are permeable to Na+, K+, and Cl- at rest due to leakage channels
Relative permeability for neurons:
- K+: 50
a. K+ largely influences RMP because it has a high permeability
- Small changes in concentrations cause large changes in RMP
b. Has the most influence on RMP – RMP tends towards equilibrium potential for K+
- Except during depolarization of AP – moves towards Na+ EP - Na+: 1
a. Na+ concentrations does not influence RMP as much
b. Increasing Na+ within the cell will move RMP closer to Na+ equilibrium potential due to increased permeability
- Caused by the opening of voltage gated Na+ channels
c. This occurs during depolarization of an AP - Cl-: 10
- Other anions: 0
- Concentration not technically [0] in ECF – but much lower than ICF [65]
- Non penetrating – permeability it zero
Goldman equation
RMP = 61 log ((Pk[K+]out + PNa[Na+]out + PCl[Cl-]in)/(Pk[K+]in + PNa[Na+]in + PCl[Cl-]out))
Cl- is flipped due to neg charge
Ca2+ not included
- virtually impermeable
- equation would be much more complicated
Predicting direction of movement of ions across membrane
- concentration and exceptions
- what to consider
Under normal physiological circumstances when channels open
a. Concentration will often indicate which way ions will move – not always the case!
b. Disease, experimental, and normal physiological concentrations can alter concentrations
Ex. during early development
a. [Cl-] in neurons is not as great as adults
• There is still more outside than inside
• ECl- is only about -50 (compared to -81)
b. if channel opens – Cl- will move from inside to outside (against chemical concentration gradient) in order to make RMP closer to ECl-
Ex. malfunctioning Cl- transporter in epilepsy patients (KCC2)
a. Don’t establish strong ECl-
b. Cl- no longer hyperpolarizes – it depolarizes and causes neurons in brains to be more hyperactive; causing epilepsy
Electrochemical gradient must be considered to determine which way ions will move
a. According to both charge and concentration
• Equilibrium potential
• Membrane potential
b. When an ion channel opens – ion will move to make membrane potential = equilibrium potential
• Ex. Na+ will move in to make MP more positive/closer to +60mV
• Ex. Cl- will move in to make MP more neg/closer to -81mV
• Ex. K+ will move out to make MP more neg/closer to -90mV
• Ex. Ca2+ will move in to make MP more pos/closer to +122mV
Divisions of NS
- Central NS – brain and spinal cord
a. Integrating center – receives info from sensory PNS, sends out response via efferent PNS - Peripheral NS
a. Sensory/afferent – collects info from viscera, skin, sight, receptors, etc., and sends to CNS
b. Efferent
i) Autonomic NS – control viscera functions; regulation of homeostasis
- Sympathetic
- Parasympathetic
ii) Somatic NS – control skeletal muscle; movement of limbs and diaphragm
Development of neural cells
- types of neural cells
Nervous system is a hollow tube containing neuro stem cells
Stem cells -> common progenitors -> becomes neural tissue
Subdivides into 1. Neuronal progenitors • Become neurons – many kinds 2. Glial progenitors • Become glia cells
Types of glial cells CNS: a. Ependymal b. Oligodendrocyte (pic) c. Microglia* d. Astrocytes PNS a. Satellite cells b. Schwann cells (pic)
Structure of neurons
Cell body - soma
o contains nucleus – transcription and translation
Dendrites – project from cell body; receives incoming signals
o Shape of dendrites is variable – often long, thin, and branched
Axon – longer than dendrites; propagate to communicate with neurons down stream; output info
a. Myelin sheath – insulators that electrically insulate axon; separated by Nodes of Ranvier; speed up conduction and ensure AP are always conducted (saltatory conduction)
- Not on all neurons
b. Hillock – cell body meets axon; initial segment/trigger zone
- Not myelinated
- Specialized to start AP – high density Na+ channels
Synaptic terminals – contain vesicles filled with neurotransmitters that are released onto post synaptic cells
o Can have more than one set
o Signalling molecules – neurotransmitters and neurohormones
Synaptic cleft – always a space
o Some neurons do not have and communicate through gap junctions
High density of ion channels
Transport mechanisms to move material from one end to the other – dependent on cytoskeleton
Types of neurons
- by placement in NS
- by structure
Efferent neurons
a. Multipolar neuron – have many dendrite projections from cell body
Sensory neurons
- Bipolar and pseudounipolar – one branch is axon, other branch is dendrites
a. Pseudounipolar – starts off as bipolar in development; changes to have only one branch off cell body; Somatic senses
b. Bipolar – 2 extensions from body; Smell & vision
Interneurons (within CNS)
a. Multipolar – very elaborate dendritic trees
b. Anaxonic – do not have an axon
Types of glia cells
- within CNS
- within PNS
Within CNS – brain and spinal cord
- Oligodendrocytes
- Myelinate axons
- Can myelinate multiple axons of different neurons (~octopuses) - Ependymal cells
a. Form a water tight layer within ventricles
- Ventricles – fluid filled spaces within the brain lined by these cells
b. Make neural stem cells
- There is not mass regeneration of brain cells
- There is some production throughout adulthood of new neurons – these cells are a source of neurons - Microglia* - “immune cells” of CNS
a. ‘Macrophage’ type of immune cell
- Injury or damage – will migrate to area and phagocytize
b. Not a neuro cell in origin* – arise from mesoderm - Astrocytes
a. Blood brain barrier – structure around blood vessels within brain
- Prevents material from directly accessing neurons
- Ex. proteins in circulation are not able to easily access neurons
b. Trophic factors – secreted by astrocytes into ECF; stimulate neurons to promote neuronal survival
c. Maintains homeostasis – take up excess water and K+
d. Stem cells
e. Metabolize glucose to lactate & pass lactate to neurons
- Neurons consume a lot of energy – Na+ K+ ATPase
- Neurons use glucose AND lactate for energy – lactate is used more easily by neurons to produce ATP
Types of Glia in PNS (outside brain & spinal cord)
- Satellite cells
a. Trophic factors – promote growth and survival - Schwann cells
a. Myelinate axons
- Cannot myelinate more than one (~cinnamon bun)
Ion channel classifications
- Type of ion they carry (ex. Na+ channel)
- Where on the cell they’re located
a. Dendritic channels vs axonal channels - Gating mechanisms – most important
a. Voltage gated ion channel – changes in membrane potential past threshold open the channel
b. Receptor Channels – ligand/chemically gated channels; gate when they bind a ligand (neurotransmitter, cGMP…)
c. Phosphorylation gated – causes conformation change that causes opening or closing
d. Stretch gated – opened or closed depending on how cell membrane is deformed
e. Temperature gated
Graded potential characteristics
a. Communicated between neurons through synapses – postsynaptic potential
o Passive – do not regenerate; no active mechanism that keeps them propagating
b. Can be depolarizing or hyperpolarizing (excitatory or inhibitory)
o Depolarizing – Na+ and ca2+ movement
o Hyperpolarizing – Cl- and K+ movement
c. Gradually dissipates/diffuses as it travels through a cell
i) Decrease in amplitude over time as signal dissipates
- Further from site = lower amplitude
ii) Degrades due to
- Electrical resistance of cytoplasm – not a perfect conductor; causes decrease in amplitude
- Cell membrane is leaky to ions – may leak out
- Nothing is regenerating the wave
d. Amplitude is proportional to the size of the stimulus – release of large amounts of nt = larger postsynaptic potential
o Accumulation of small “subthreshold” changes in membrane potential – need multiple stimulations; one will not be enough to initiate AP
e. Caused by the flow of ions through a few ion channels – does not require the involvement of a lot of ion channels in the cell all at one time
f. Summative – graded potentials occur simultaneously will combine
o If depolarizing and hyperpolarizing are occurring at the same time – may can each other out depending on magnitude
g. Can be long-lasting
o Often only last a few milliseconds
o Can sometimes last seconds to minutes
Action potential characteristics
a. Actively propagates across neuronal membrane
i) Regenerative – not passive; begins at the axon hillock
- Mechanism to keep it going
ii) Often called a “spike” – causes a spike in electrical activity on recording not proportional to initial stimulus
b. Always depolarizing/excitatory – never hyperpolarizing/inhibitory
c. All or none – either occurs or does not
o Intensity is determined by frequency NOT “stronger or weaker”
o Requires depolarization of hillock/cell body past threshold by graded potential
d. Amplitude is not proportional to initial stimulus
i) Large amplitude – about 100 mV from RMP to peak
- NOT +100mV (ex. -70mV, peak is +30mV)
e. Refractory period
i) Absolute refractory – AP can absolutely not fire; either all Na+ are open or are inactivated
- Occurs from passing threshold to afterhyperpolarization
ii) Relative refractory – Na+ are not inactivated but cell is hyperpolarized; requires larger stimulus
- Occurs from afterhyperpolarization to RMP
f. Not summative
o You can’t make a ‘200mV’ by adding 2 together
g. Fast – lasts only a few milliseconds
Drugs that effect Na+
Tetrodotoxin (TTX) – blocks voltage gated Na+ channels & prohibits firing of AP to muscles
- Including respiratory muscles – respiratory arrest
- add notes
Lidocaine – blocks voltage gated Na+ channels & prohibits AP
- Used for numbing – reduces neuronal excitation
Action potential process
- Resting membrane potential (-70mV)
a. Na+ K+ ATPase is pumping
b. Each ion has its own equilibrium potential - Graded potential – cell is depolarized on dendrites and cell body via chemically gated ion channels
a. Signal diffuses to hillock – large enough to reach threshold - Threshold – stimulates opening of voltage-gated Na+ and K+ channels
a. Na+ opens quickly – enters cell immediately
i. Absolute refractory period begins – continues until afterhyperpolarization
b. K+ begin to open, but slowly - Depolarization – caused by influx of Na+ (+30mV)
a. Na+ channels inactivate shortly after Na+ channels open
- Will become closed (not inactivated) after the membrane is at least as negative as resting membrane potential
b. Activation vs inactivation gate
- Activation gate – affected by graded and AP; voltage change causes opening
- Inactivation gate – ball & chain; closes soon after channel opens; pore is open but channel has been blocked off
c. K+ channels fully open - Repolarization – causes by K+ leaving cells
a. Hyperpolarizes due to slow closing of K+ channels (surpasses RMP) - Afterhyperpolarization
a. Voltage-gated K+ channels close
b. Na+ channels are reset – recovery from inactivation (takes a few milliseconds)
- Determines cells ability to propagate a new AP
- Relative refractory period begins – occurs until RMP is reached - Cell returns to RMP – normal ion permeability; Na+ channels mostly recovered
a. Na+ K+ ATPase – re-establishes ionic gradient & RMP
- Does not directly influence AP - If neuron is artificially stimulated from both ends – signal will propagate to the middle and stop since both previous sections will be in a refractory period
Local current flow
Local current flow – When a section of axon depolarizes, positive charges move by local current flow into adjacent sections of the cytoplasm. On the extracellular surface, positive charges flow toward the depolarized region (it’s relatively negative)
- Initial at RMP (~ -70 mV) – normal ion concentrations
- AP potential – causes opening of Na+ channels at hillock
a. Axon hillock Na+ channels open (high density) – makes inside more pos with respect to outside (depolarized)
i. Na+ ions are attracted to
- outside – neg area of depolarized regions
- inside – neg area of adjacent area within cell; will flow and change membrane potential in adjacent areas – local current flow - Na+ moving via local current flow causes depolarization of adjacent portion to its own threshold
a. Hillock is not in refractory period – causes unidirectional flow - Na+ enters, causing depolarization.
a. Process repeats as local current flow causes waves of depolarization down the axon
Patterns of AP
- types
- effects by
Types
- Single AP
- Tonic AP
- Fire by themselves without graded potential - Bursting AP
- Fires in bursts, then hyperpolarizes – continuing cycle
Shape of AP effected by
o Isoforms of Na+ and K+ channels change AP firing
o Number of channels
o Where channels are located
Hyperkalemia and hypokalemia
- drugs that cause blockage of K+ channels
Small changes in K+ largely influence RMP (high relative permeability) – influence transmission of AP
Hyperkalemia: increased ECF K+ concentration (ex. with kidney failure)
- Increased excitability – a stimulus that would not normally stimulate the cell enough to cause AP now causes firing of AP that would not normally reach threshold
Hypokalemia: decreased ECF K+ concentration
- Decreased excitability – stimulus that previously fired AP is no longer large enough to fire an AP
Drugs that cause blockage of K+ channels
o Will slow repolarization and remain in refractory period longer – result in longer AP
AP velocity
2 ways
- myelination
- diameter
Myelination:
a. Continuous conduction – unmyelinated
- High Na+ channel density down the length of axon
b. Saltatory conduction – myelinated
Myelination – formed from concentric layers of glial cell membrane (lead edge rolls and wraps around axon)
1. Insulate axon – prevent leakage of ions outside axon
a. Very few ion channels in myelin sheath and on axon beneath sheath – mainly leakage channels
2. Nodes of Ranvier – high concentration of Na+ voltage gated channels; AP “jumps” between
Increases efficiency
- Less leakage of Na+ and K+
- Less ATP used to maintain RMP (steady state balance)
More processing power than larger axons (more efficient)
- Conduct AP faster than similar sized unmyelinated
- Can fit more into a space
Increasing diameter:
- decreased internal resistance (inverse square relationship)
Demyelinating diseases effects
Causes loss of myelin sheaths
o They no longer are insulated
o There is more leakage of ions along axon
AP may be stimulated – when depolarization tries to travel, AP fails due to leakage channels normally covered by myelin
Multiple sclerosis
- cause
- symptoms
- treatments
a. Autoimmune disease – Immune system attacks your own myelin sheaths
- Causes demyelination of CNS – not PNS axons
b. Unknown cause – environment, virus, genetics, cerebral blood flow
- Canada has more MS than any other country
- Zamboni hypotheses – scientific studies largely rejected this, DOI: 10.1186/1472-6939-14-6
c. Multiple patterns of progression – relapsing-remitting; several progressive phases
- Have an episode with loss of speech, balance, vision, pupillary reflex
- Will resolve after a week or so
- Each episode is worse than the last
d. Symptoms – highly variable; loss of balance, loss of speech, loss of vision, abnormal pupil reflexes, numbness, pain
e. Treatments – immunosuppressants, other drugs as indicated by symptoms
Terms for GPCR pathways
Terms
a. GTP = guanosine triphosphate – nucleoside similar to ATP
- Not a common energy source
- Very common in intracellular signaling
- Can be converted to cGMP – second messenger (Same process as ATP -> cAMP)
b. Phosphodiesterase – enzymes; breaks down cGMP and cAMP
c. cAMP = cyclic ATP – second messenger; synthesized by adenylyl cyclase
- activates PKA
- some GPCR activate adenylyl cyclase; some inhibit
d. PLA = phospholipase A – enzyme; breaks phospholipids into releasing arachidonic acid and lysophospholipids
- Arachindonic – eicosanoid; metabolized into prostaglandins and leukotrienes (both cause inflammation)
Synapse
- between what types of cells
- summation & synaptic activity & number of channels
- classifications
Synapse – connection between two neurons or a neuron and another cell that is specialized for the transfer of information
a. Communication between neurons – presynaptic neuron synaptic terminals communicate with postsynaptic cell
- Synaptic activity – causes graded potentials in the postsynaptic cell; comes about via activation of just a few ion channels
b. Summation of IPSP and EPSP at hillock – determines if AP occurs
- Depolarizing – excitatory; brings closer to threshold
- Hyperpolarizing – inhibitory; further form threshold
Classifications
a. Functional
- electrical vs chemical
b. Location on post-synaptic cell
- axodendritic, axosomatic, axoaxonic
Electrical synapse
- required structure
- process
- where is occurs in the body
- bidirectional
- differences from chemical
Gap junctions – connect cytoplasm’s of pre and post synaptic neurons; create ion channels
a. Formed from 2 aligned connexons
- Connexon – hemichannels; made of 6 connexin monomers
- 2 form fully functioning channel
b. Pores
- Large – allows for very fast movement between
- Synaptic delay is 0.2ms
- Non-selective – Ions, second messengers (ex. cAMP), other molecules (ex. ATP)
Process
a. AP arrives in synaptic terminal of presynaptic cell
- Causes almost immediate depolarization of post synaptic cell
b. Electrical information (ex. AP) passes directly between two cells – carried by the movement of ions between cells (ions)
In the body
a. Common in cardiac and smooth muscle
b. Relatively uncommon in neurons
- When it does occur – tends to allow group of neurons to fire AP nearly simultaneously; Assists in secretion of neurohormones
Electrical signal can be bidirectional
o Normal physiological circumstances – AP will travel down axon, go through gap junctions; cause EPSP in posy synaptic cell
o Experimental setting – can force signal to go from postsynaptic to presynaptic
Vs chemical
o Faster – 0.2ms
o Less complex – not converted to chemical signal (cannot become IPSP from EPSP)
Chemical synapses
- exocytosis
- targets
- how many
- signal conversion
Specialized form of exocytosis
a. Release of neurotransmitter from presynaptic cells – influence electrical activity in postsynaptic cell
b. Post-synaptic targets
• Other neurons
• Muscle cells – neuromuscular junctions; skeletal, smooth, cardiac muscle
• Glands
c. Can be depolarizing or hyperpolarizing
Estimated 100-600 trillion synapses in brain – 0.5-1 billion per mm3 in some areas of brain
- Single AP will only depolarize postsynaptic cell slightly
- New AP requires many stimuli on neuron
Electrical signal from presynaptic – converted to a chemical signal to cross synaptic cleft – converted back to an electrical signal in postsynaptic
a. Can convert depolarizing AP to EPSP or IPSP
- More information processing
- Can switch from single AP to bursting to tonic
b. Very efficient – a few molecules of nt can have a large electrical effect
Chemical synapse process
- lots of what??
- size of cleft
AP arrives in synaptic terminal of pre synaptic cell – causes depolarization
a. Contain vesicles (made with phospholipids membrane) – filled with neurotransmitters
• Lots of mitochondria – processes require ATP
b. Voltage gated Ca2+ channels open – located on “face” of synapse
• ICF Ca2+ concentration increases (unlike other ions during AP)
o Initial Ca2+ concentrations within the cell is so small – a small change will create a change in concentration
o Occurs in microdomains – does not have to travel far within cell
• Ca2+ facilitates exocytosis of vesicles – causes fusion with membrane
Neurotransmitters diffuse into cleft
a. Cleft is much larger than electrical – 20-40nM
b. Creates longer synaptic delay than electrical
• 2 ms for fastest receptor channel
• 20 seconds for GPCR (longer post synaptic response)
Synaptic receptors on postsynaptic neurons – bind to neurotransmitters
Effect depends on type of receptor activated:
a. Receptor channel
b. GPCR
o Neuromodulation – longer lasting effects; slower acting response
Neurotransmitter is removed from cleft – needs to be ‘reset’; is either
a. Destroyed in the synaptic cleft by a degradative enzyme
b. Transported back into presynaptic terminal via active transport – recycled and repackaged back into vesicles.
c. Diffuses away from synapse – can occur when synapses are not covered by glial cells
• Common in autonomic NS
d. Taken up into postsynaptic cell by endocytosis
Types of neurotransmitters
Many types; use different definitions to classify; many receptors for a single nt
Classic nt – known to be for a long time; small molecules
a. Acetylcholine
b. Amines
- Norepinephrine, dopamine,
- histamine, serotonin
c. Amino acids
- Glutamate
- Gama-amino-butyric acid (GABA)
Novel nt – widely recognized in the past 25+ years; not as commonly used
a. Peptides – usually act by GPCRs
- Oxytocin, melanocortin
b. Purines – usually act via receptor channel
- ATP – widely used in the autonomic NS
Acetylcholine
- where its used
- synthesis and recycling
- kinds
Cholinergic synapse
- Main nt in neuromuscular junctions – muscular excitation and contraction
- Every pathway of the autonomic NS uses
- Used diffusely throughout the CNS as a neuromodulator
Synthesis & recycling:
- Acetylcholine(ACh) – made from choline and acetyl CoA (coenzyme A)
- Acetylcholinesterase – enzyme rapidly breaks down Ach into acetic acid and choline in synaptic cleft
a. Choline – expensive to make - Choline – transported back into the axon terminal via secondary active transport by cotransport with Na +.
- Recycled choline is used to make more ACh.
Two main kinds of receptors for ACh
- Receptor channels – nicotinic receptors
a. Nicotinic – nicotine is agonist; will also bind to channel and cause opening
b. Ionotropic receptors – move ions across membrane
c. Fast EPSP - synaptic delay is 5-45ms (not as fast as electrical)
d. Process - Ach is released from presynaptic neuron – facilitated by Ca2+
- Binding to postsynaptic receptor causes opening of channel – entry of Na+ (and exit of small amount of K+); ionotropic
- Depolarizes postsynaptic cell (EPSP) - GPCR – muscarinic receptor
a. Muscarinic – chemical from specific mushroom that acts as agonist
b. Metabotropic receptors
c. Slow EPSP via muscarinic receptor
- Longer synaptic delay – 20 seconds
- Longer postsynaptic response – can last seconds to hours
Process of GPCR - Ach binds to GPCR
- Triggers intracellular events through second messenger
- Often results in activation of protein kinase
- Causes phosphorylation of K+ leak channels – phosphorylation gated ion channels (& other proteins)
- K+ channels close – depolarizes cell - Not all GCPR nt receptors cause closing of K+ channels – there are many targets; depends on cell and receptor
a. highly variable response - can be hyperpolarizing or depolarizing
- K+ channels may be targeted in different ways
- Na+ leakage channels may be targeted
- Ca2+ channels, etc
GPCR is a Neuromodulator
a. Long slow EPSP
- Doesn’t cause fast depolarizing or immediately trigger AP
- Slowly and subtly change the electrical behaviour of a cell
- Ex. make it slightly easier or harder to bring MP to threshold to fire AP
Norepinephrine
Noradrenergic synapse
Used
o Diffusely throughout the CNS
o Sympathetic branch of the ANS (PNS)
Several subtypes of receptors
a. All are GPCR (alpha and beta receptors) – long and slow response (similar to muscarinic)
- Variable effects – can phosphorylate many kinds of post synaptic channels
b. No ligand gated ion channels
Glutamate
Glutamatergic synapse
- Main excitatory neurotransmitter used throughout the CNS
Two main types of glutamate receptors
- Receptor channels (ionotropic) – 2 types
AMPA receptor
a. Activated by 2-amino-3-(3-hydroxy-5-methyl-isoxazol-4-yl) propanoic acid
b. Depolarizing
- Acts similar to Ach nicotinic
- Binds and causes opening of channels – mostly Na+
NMDA receptor
a. Activated by N-Methyl-D-aspartate
b. Depolarizing
- Binding opens channel – allows both Na+ and Ca2+ to enter
- Ca2+ is a second messenger
c. At RMP – they are blocked by Mg2+ ions
- Depolarizing causes Mg2+ to move away
- Cell must be depolarized in order to function
- Often side by side with others (ex. AMPA)
- If glutamate binds at RMP – still can’t open
d. Behave like voltage gated – they are still ligand gated ions channels
- GPCR (metabotropic glutamate receptors)
a. Several subtypes
Gamma-amino-butyric acid: GABA
GABAergic synapse
- Main inhibitory neurotransmitter used throughout the CNS.
- Inhibitory – makes MP more negative; further from threshold; harder to fire AP
Two kinds of receptors
- Receptor channel – ionotropic GABAa receptor
a. GABA is released from presynaptic – binds to GABAa channel
- GABAa – only permeable to Cl-
- Cl- hyperpolarize membrane
b. Fast IPSP – not as fast as electrical; faster than GPCR
- Synaptic delay approx. 2 milliseconds
- Whole thing lasts about 5 milliseconds - GPCR – metabotropic GABAb receptor
a. Effect is variable, depending on channels phosphorylated
Neurotransmitter vs neuromodulator
Neurotransmitters – generally activate receptor channels
a. Can cause fast EPSP or IPSP
Neuromodulators – generally activate GPCR
a. Slower than receptor channels – longer lasting electrical change
b. GPCR – causes the creation of second messengers
- Causes the release of intracellular Ca2+
- Final result may be phosphorylation of ion channels that cause them to open/close
- Causes a post synaptic potential
Summary of neurotransmitters and receptors
Receptor channels
- AChR:
- Na+ (and a little K+) - depolarizing
- Nicotinic - NMDA:
- Na+, Ca++, (K+)
- depolarizing - AMPA:
- Na+ (and a little K+)
- depolarizing - GABAa
- Cl-
- hyperpolarizing
- Main inhibitory nt in CNS
GPCR – variable effects; phosphorylation can open or close or modulate many types of channels
- AChR
- Muscarinic – causes activation of second messengers
- K+ leak channels close – causes depolarization - Norepinephrine
- Glutamate
- GABA
Synaptic integration
- 4 factors that influence
Whether or not the postsynaptic neuron fires an action potential depends on the grand sum of synaptic activity acting on the cell (many synapses)
Factors that influence integration and modulation:
- AP frequency – of presynaptic cells; not in GPCR
a. Amplitude of GP – proportional to size of stimulus
- Small stimulus – smaller graded potential
- Larger stimulus – GP causes increased frequency of AP
b. Single AP will release constant amounts of neurotransmitter
- Higher frequency – releases more neurotransmitters onto postsynaptic cells due to repeated AP - Divergence and convergence – not 1:1; many networks with varying degrees of convergence and divergence
a. Divergence – branching of axons to act on many postsynaptic neurons (varying amounts)
- Axon collaterals – branches of axon (axon terminals at the end); more effects more postsynaptic cells
b. Convergence – many presynaptic neurons acting on single neurons
- Ex. Purkinji neuron may have up to a million synaptic influences - Temporal and spatial summation
a. Temporal – GP can be added together if they occur close enough in time within single synapse
- Time degrades GP
- AP firing rapidly within ms along single axon can cause summated excitation before previous GP degrades (does not have time)
- Able to bring to threshold
b. Spatial – requires a number of presynaptic neurons
- If all presynaptic neurons fire AP simultaneously, they can summate together at hillock
- Must be nearly simultaneous
- Will diffuse across different distances within cell body to hillock
- Can be a sum of EPSP and IPSP – inhibitory can cause AP to not release
- Sum of EPSP and IPSP determines whether cells reach threshold - Location of synapses on postsynaptic cell
a. Axodendritic – on the dendrites
- Will have varying distances to travel down dendrites to hillock – timing is important
- Spatial summation – further would have to occur slightly earlier to be simultaneously with a closer synapse
b. Axosomatic – on cell body
- Takes less time for GP to travel to hillock – less degradation occurs
- All things equal – more powerful than axodendritic
c. Axoaxonic – 2 types
1. Directly over hillock
a. Heavily influence APs – most powerful; GP doesn’t have to travel as far
- All things equal – this one is the most powerful
2. Directly on synaptic terminal (still axoaxonic)
- Does not influence AP – only modifies the amount of nt released
- Subtle regulation
- Causes
a. Presynaptic facilitation – excitatory
i. Depolarizes synaptic terminal
ii. Will activate some Ca2+ channels
- Causes baseline levels of Ca2+ present when AP arrives – causes increased release of nt
b. Presynaptic inhibition – inhibitory
i. Hyperpolarizes synaptic terminal
ii. AP arriving will not depolarize enough to activate ca2+ channels – will not get as much Ca2+
- Will inhibit neurotransmitter
Synaptic plasticity
- types
- thought to underlie
Synaptic plasticity – the ability of neurons to change synaptic strength; long lasting change in whether or not synapse gets stronger or weaker
Types
o Potentiation – stronger over time
o Depression – weaker over time
Long-term potentiation (LTP) and long term depression (LTD)
- Thought to underlie the process of learning – happens in many types of neuron
- The acquisition of new memories – not storage of memories
LTP in hippocampus
Brenda Milner – neuroscientist from Montreal Neurological Institute; discovered that the hippocampus was critical for storage/acquisition of memories
Patient HM – bike accident in early 1940s
a. Developed epilepsy – over the years it got much worse
• Removed portion of temporal lobe – epileptic seizures went away
b. Hippocampus had been removed – HM could no longer acquire new memories
• Could remember his life before surgery – he couldn’t acquire new ones
c. Difficulty with declarative memories (facts and events) & maps
• Could still acquire new motor skills – different kinds of memories
Steps to induce LTP
- High frequency stimulation of presynaptic neurons that cause release of glutamate
a. AP must occur often - AMPA receptors activated – causes EPSP
a. 2 sources of Ca2+ result
- Releases Mg2+ block on the NMDA – allows influx of Ca2+ from ECF
- Activation of metabotropic glutamate receptors – causes the release of intracellular Ca2+ from stores within cell - Postsynaptic terminal has large increase in intracellular Ca2+
a. Activates Ca2+ second messenger pathways – leads to activation of kinases
b. Effects
- Phosphorylation of AMPA receptors – increases affinity and conductance (opens wider)
- Insertion of new AMPA receptors into membrane from stores within cell
- Generation of retrograde messengers that facilitate the release of vesicles:
Nitric oxide – synthesized by enzymes; diffuses out of post synaptic neuron and causes release of more glutamate from pre synaptic neuron
a. Paracrine signal
LTP
- caused be
- requires
- critical synapse in hippocampus
Caused by period of high frequency AP in presynaptic neuron
- Long lasting potentiation of EPSP – increased depolarization of postsynaptic neuron caused by a single AP
- Larger amplitude of EPSP
Requires
o The 3 types of glutamate receptor play a key role
1. GPCR Metabotropic receptor
2. AMPA (carries mostly Na+) receptor
3. NMDA (carries Na+ and Ca++) – the cell must be depolarized in order to gate NMDA-R open
o Rise in intracellular Ca++ is crucial
o Pre and post synaptic changes
Critical synapse in hippocampus
o Between CA3 and CA1 synapses
Long term maintenance of LTP leads to
- Changes in gene expression
- Creation of new synapses
- Co-ordinated pre and post-synaptic effects
Gullian Barre disease
acute inflammatory demyelinating polyneuropathy; AIDM
Autoimmune disease
i. Demyelination of sensory, motor and autonomic axons (PNS) – CNS not effected
- Slowing and/or loss of AP conduction
Cause
i. Appears after “minor” infection – days after a seemingly minor GI or lung infection
ii. May be also associated with chronic illness such as lupus, HIV
iii. 1976 flu vaccine (1 additional case per 100,000)
Symptoms
i. Initial – tingling, weakness, pain in hands/feet
ii. Progress to – inability to speak (similar to MS), paralysis, respiratory distress
- Paralysis – due to ineffectiveness of efferent neurons
- Respiratory distress – axons in diaphragm are part of PNS
Treatment
i. Plasmapheresis – to remove antibodies from blood
ii. Immunoglobulin G (IGG) – to inactivate circulating antibodies.
Most people survive if diagnosed quickly – recovery may take months to years.
Tetratoxin
Tetrodotoxin (TTX)
First sign – slight numbness of lips and tongue; face will tingle; light headed
- High levels – may start to become paralyzed, respiratory distress and cardiac arrythmia
Death can occur quickly depending on how much you’ve consumed
Effects
a. Moves through circulation & binds to voltage gated Na+ channels – prevents Na+ from entering/depolarization during AP
- Prevents AP in neurons and muscle (and in heart)
- Able to cross BBB – blocks AP within CNS & Causes lightheadedness
Antagonist
- High affinity – hard to unstick
- High specificity – for voltage gated Na+ channels
- High efficacy – the amount required to do from 0 effect to full effect is very low
- High potency – it works at a very low dose; Nm concentrations are enough to kill you
Kills 30-100 people worldwide
o Japan – restaurants able to serve this have licensed chefs
o Other areas – less restrictions; higher mortality rate
