Module 1: Homeostasis and the Basis for Excitable Cells Flashcards
Define homeostasis
The ability of a cell or organism to regulate and maintain its internal environment regardless of the influences of the external environment
Define ‘set point’
The range/point at which a variable physiological state tends to stabilize. Eg., body temperature’s set point is ~27 degrees
What are the two types of homeostatic regulation?
Intrinsic and extrinsic
What is local regulation/ autoregulation
Intrinsic regulation
What is intrinsic regulation?
A homeostatic regulation where the sensor, integrator, and effector are all located within one tissue. The tissue can regulate its own internal environment
Give an example of intrinsic regulation
An exercising skeletal muscle needing more oxygen to produce more ATP. The blood vessels in the muscle dilate to increase amount of oxygen that can be delivered so cells can product more ATP
What is extrinsic regulation?
A homeostatic regulation where the regulatory mechanisms (sensor, integrator, effector) are outside of the tissue/organ
Provide an example of extrinsic regulation
Body temperature. Thermoreceptors detect changes in temperature and pass this information to the thermoregulation centre in the brain. The brain then decides to stimulate skeletal muscles to contract and produce heat
What is negative feedback?
A mechanism used by homeostatic control systems that causes the effector to initiate a response in the opposite direction of what was caused by the initial environmental change, thus restoring the set point
What is positive feedback?
A mechanism that causes an amplification in the initial signal
What’s an example of positive feedback?
Childbirth - amplifies oxytocin secretion
Is positive feedback a homeostatic mechanism (T/F)?
False! Because it’s amplifying a signal, not returning it back to its set point
What are 3 primary functions of the plasma membrane?
- Ensure the cell’s survival
- Maintain homeostasis
- Function cooperatively and in coordination with surrounding cells
Describe the structure and function of phospholipids in the plasma membrane
- Make up the lipid bilayer
- Have a polar head with a negatively charged phosphate group and 2 non-polar fatty acid tails
- It has a hydrophobic head and hydrophilic tail
Describe the structure and function of cholesterol in the plasma membrane
- Found between the phospholipids
- Prevent the fatty acid chains from packing too tightly and becoming ridged
- Keeps membranes fluid
Describe the structure and function of membrane proteins in the plasma membrane
- Inserted into the phospholipid membrane
- Can be associated with the inner side outer side, or pass completely through the plasma membrane
- Function to maintain cells structure, regulate cell function, allow transportation across cell membrane, and facilitate signalling
Describe the structure and function of ion channels
- Specialized membrane proteins
- Span the entire width of the membrane
- Permit entry and exit of ions
Describe the structure and function of carbohydrate chains
- Short chains of carbohydrates can be attached to proteins or the bilayer to form glycoproteins/glycolipids
- Involved in membrane stability
- Act as cell surface receptors
- Participate in transportation across the cell membrane
What is the structure and function of the Extracellular Matrix?
- It’s a network of fibrous proteins embedded in a gel-like mixture of carbohydrates
- Surrounds all the cells in tissues and keeps them in place
What is interstitial fluid? What is it made of?
The gel like fluid in the ECM that allows diffusion of nutrients from the blood and the removal of waste from the cell
- Made of up collagen, elastin, and fibronectin
What is the role of collagen in the ECM?
Forms the cable-like fibres that give the ECM it’s tensile strength
What’s the role of elastin in the ECM?
A rubber like protein that allows tissues to be stretched
What’s the role of fibronectin in the ECM?
Promotes cell adhesion
What are cadhesions (CAMs)?
Transmembrane proteins that are involved in protein-protein interactions. They help cells stick to each other and their surroundings
What are the 4 main families of cadhesions?
- Cadherins
- NCAMs
- Selectins
- Integrins
What are the three types of cell junctions?
Desmosomes/adherens junctions, tight junctions/impermeable junctions, and gap junctions/communicating junctions
What is the primary function of desmosomes?
To anchor two adjacent cells together that are otherwise not in direct contact
Describe the structure of desmosomes
- Composed of dense intracellular thickenings called plaques, connected together by glycoprotein filaments containing cadherins to attach neighbouring plaques together
Give an example of a cell that contains desmosomes
A skin cell
What is the primary function of a tight junction?
To create a tight seal between cells that prevents movement of molecules from cell to cell
Describe the structure of a tight junction
- Long strings of junctional proteins in the plasma membrane in neighbouring cells align and adhere to each other
What is a ‘kiss site’?
- The area of opposing junctional proteins from neighbour cells meet in a tight junction
Give an example of a tissue where tight junctions are commonly found
Epithelial cells/epithelial linings and tissues
What is the primary function of gap junctions?
To allow for direct communication between two cells. Allows for electrical activity and secondary messengers to be communicated and relayed
What is the structure of a gap junction?
6 connexion proteins create a connexion, when a connexion from each cell join together, a tunnel is created which connects their intracellular spaces, allowing for direct communication. These can be opened/closed
Give an example of where gap junctions are commonly found
In cardiac and smooth muscle
What are common characteristics of lipophilic substances? Provide an example of a lipophilic substance
Uncharged, non-polar molecules
Ex., O2, CO2, fatty acids
What are common characteristics of lipophobic substances. Provide an example of one.
Charged/polar molecules.
Ex., Ca2+, Na2+, glucose
What are two main factors that determine membrane permeability of a substance. Describe them
Size: the smaller the molecule, the more readily it can cross without any assistance. The larger the molecule, the more likely it will require a transport molecule to take it across the membrane
Permeability: lipophilic molecules can pass through the membrane more readily whereas lipophobic molecules cannot
Describe diffusion
Molecules moving down their concentration gradient. Process will continue until there is no longer a concentration gradient present
Describe osmosis
Diffusion of water through aquaphorins in the membrane
What is osmotic pressure vs hydrostatic pressure?
Osmotic pressure is the force that moves water down its concentration gradient. The greater the gradient, the greater the osmotic pressure
Hydrostatic pressure is the force created by a given volume of water. The greater the volume, the greater the hydrostatic pressure
What is facilitated diffusion?
Diffusion down a concentration gradient with the help of a carrier protein. Does not require energy
Describe the steps in facilitated diffusion
1) Carrier protein takes a conformation in which the solute binding site is exposed to the region of higher solute concentration
2) Solute molecules bind to the carrier protein
3) Carrier protein changes conformation so the binding site is exposed to a region of lower solute concentration
4) Transported solute is released and the carrier protein returns to original conformation
What is active transport
Transport of a substance up its concentration gradient using a carrier protein. Does require energy
What is vesicular transport
Exocytosis and endocytosis
What are the three types of endocytosis
Pinocytosis, receptor-mediated endocytosis, and phagocytosis
Describe pinocytosis`
Endocytosis where the cell membrane engulfs and internalizes a small droplet of extracellular fluid. Coat proteins cause the formation of an endocytic pouobi that is pinched off to form an internalized vesicle filled with extracellular fluid
- This is a non selective transport, and the vesicle will contain whatever substances were close to the membrane at the time it was formed
Describe receptor-mediated endocytosis
- Similar to pinocytosis
- Trigger to create a vesicle is dependent upon the binding of a substance to a specific receptor on the cell
Describe phagocytosis
- Involved the internalization of large multi-molecular particles (i.e., bacteria) and only occurs in a few cell types (e.g., white blood cells)
- The pseudopods surround the prey and close in. Once enclosed in the endocytic vesicle and sinks into the cytoplasm
What is the purpose of exocytosis?
To allow for large, polar molecules (ex., protein hormones) to pass through/into the membrane
Describe the steps of exocytosis
1) Recognition markers in the membrane of the Golgi sac capture cargo from the Golgi lumen. The membrane that will wrap the vesicle is coated with coatomer, causing the membrane to curve and bud out
2) The membrane closes beneath the bud, pinching off the vesicle
3) The vesicle loses its coating and exposes the v-SNARE docking markers on the vesicle surface
4) The vSNAREs bind to the tSNARE docking marker acceptors of the targeted plasma membrane. The vesicles then empty their contents to the cell’s exterior
What is Ohm’s Law? What do each of the variables represent?
V=IR
V= voltage (membrane potential)
I = current (ions moving across the membrane)
R = Resistance (ability of plasma membrane to resist movement across it)
What are voltage gated ion channels?
Ion channels that open and close in response to changes in membrane potential
What are chemically gated ion channels?
Ion channels that open when a specific chemical messenger (a ligand) interacts with it
What are mechanically gated ion channels?
Ion channels that open in response to mechanical deformations (i.e., stretch)
What are thermally gated ion channels
Ion channels that respond to changes in temperature
In Ohm’s Law, what is the relationship between R and I
If R increases, I decreases
If IR decreases, I increases
What is the purpose of the Nernst Equation?
To allow us to calculate the equilibrium potential (E)
What is the Input Zone of the neuron?
The part of the neuron where incoming signals are received. Contains the dendrites and cell body
What is the Trigger Zone of a neuron?
Where action potentials are initiated. Contains the axon hillock
What is the Conducting Zone of the neuron?
The part where action potentials are conducted to their target locations. Contains the axon
What is the Output Zone of the neuron?
The part that releases chemical messengers. Contains the axon terminals
What is the difference between an absolute and relative refractory period?
An absolute refractory period is where under no conditions another action potential can be triggered. A relative refractory period is another action potential can be triggered, but only if the triggering stimulus is strong enough
How does diameter of an axon and conduction velocity relate?
The increased diameter of an axon, causes a decrease in resistance to propagating local currents, therefore fibres can conduct at a faster rate
Describe the mechanism for synaptic transmission
When an electrical action potential reaches the axon terminal, the change in membrane potential opens voltage gated calcium channels, where calcium flows down its concentration gradient into the cell. This triggers exocytosis of synaptic vesicles containing neurotransmitters. the neurotransmitters diffuse across the synaptic cleft and bind to receptors on the postsynaptic neuron. This then opens chemically gated ion channels where modulate the postsynaptic membrane potential
What are EPSPs?
An excitatory synapse that stimulates. Interaction of a neurotransmitter with its receptor opens nonselective cation channels that allow movement of Na and K ions. This leads do a slight depolarization that although won’t result in an action potential, will bring the membrane potential closer to its action potential threshold
What is an IPSP?
An inhibitory synapse that inhibits. Results when the neurotransmitter interacts with its receptor to activate either Cl- or K ion channels. This causes the inside of the cell to become more negative and hyper-polarizes the membrane potential away from the threshold.
What is temporal summation?
The summing of several EPSPs occurring close together in time due to the repetitive firing of a single presynaptic neuron
What is spatial summation?
The summation of EPSPs and IPSPs originating from several different presynaptic inputs having simultaneous effects on the membrane potential. Depending on if the EPSPs or IPSPs dominate, an action potential may or may not be reached.
How does the opening of a K+ channel affect the cell
It hyperpolarizes the cell
How does the opening of an Na+ channel affect the cell?
It depolarizes the cell
What is depolarization?
Occurs when the magnitude of polarization decreases (moves towards 0mV)
What is repolarization?
Occurs after either a depolarization or hyper-polarization, where the polarizing begins to return towards the original RMP
What is hyperpolarization?
Occurs when the magnitude of a polarization increases (becomes more negative than the RMP)
What are graded potentials?
Local changes in membrane potential used for short distance signalling. Their magnitude and duration are dependent on the strength and duration of the triggering stimulus. Generally caused by triggering events that make voltage gated ion channels open. Magnitude of graded potential decreases as it moves away from the site of initial triggering event
What is an action potential?
Caused by a triggering event or stimulus that results in a localized depolarization. Can be conducted throughout entire membrane and does not diminish in strength along the way. Used for long-distance signalling. An all-or-nothing mechanism
