Chapter 12 - Nervous System Flashcards
Neuroglial Cells
- AKA “Nerve Glue Cells”
- Small + numerous
- Capable of mitosis
- 6 Types Total (4 in CNS, 2 in PNS)
- Gliomas = Brain Cancer
Neuropeptides (4 Main Examples)
- Opiate Peptides
- Substance P
- Hypothalamic releasing & release inhibiting hormones
- Cholecystokinin-Pancreozymin (AKA “CCK-PZ” or “CCK”)
Microglia
Microgilal Cells
-Phagocytic cells that are found on CNS injury sites
Distribution of Different Ion Species
Na+ = Main extracellular cation Cl- = Main extracellular anion K+ = Main intracellular cation A= Polyvalent anion "A" (Intracellular fixed anions, e.g., carboxyl groups of proteins)
3 Parts of the Neuron
- Dendrite
2 .Axon - Cell Body
Subcellular Locations of Voltage-gated Channels
Always located in places 2/ action potentials:
- Axons of motor neurons, interneurons, bipolar & unipolar sensory neurons
- Sarcolemmas of skeletal muscle cells
Biogenic Amines (3 Main Examples)
- Noradrenaline
- Dopamine
- Serotonin
* Can be either excitatory or inhibitory
Interneurons
Interneurons = 90% of all neurons
- Only found in CNS
- Conduct nerve impulses between different parts of CNS
- AKA “Association Neurons
Absolute Refractory Period
- Period during which nerve/muscle cell cannot generate another AP, even w/ a strong stimulus
- Caused by open/inactive Na+ channels (Not yet returned to resting state)
- Large diameter fibers: short absolute refractory period
- Small diameter fibers: long absolute refractory period
- Refractoriness = one-way transmission of AP; direction of AP propagation always away from membrane areas which have just fired
3 Steps During Action Potentials
- Receptor Binding
- Depolarization
- Repolarization
Differences in Plasma Membrane Permeability to Ions
Resting Membrane is 50-100x more permeable to K+ than to Na+, so RMP will be near K+ equilibrium potential (-90 mV)
-Defined as membrane potential that just balances K+ outflow
*RMP = -70 mV, so slightly leaky to Na+
Temperature
A low temperature rate will lower AP conduction rate. and sometimes even block conduction
Action Potential (AKA “Nerve Impulse”)
Action Potential = Moving transient electrical polarity reversal; moves from one end of neuronal fiber to the other
Synaptic Cleft
Small gap between 2 chemical synapses
Nitric Oxide
- Is both a neurotransmitter & hormone
- “Endothelium-derived relaxation factor” (EDRF)
- Made by MP Synthetase from arginine
- Very brief activity
- Very dangerous free radical
- NO -> Guanyl cyclase activation -> cyclic-GMP
Agonist
- Agents which enhance synaptic transmission (e.g., metoclopramine; releases ACh)
- Agents which mimic the effects of a hormone/neurotransmitter, showing both affinity for receptor & efficacy (e.g., salbutamol; helps bronchodilation)
3 PNS Subdivisions
- Somatic Nervous System (SNS)
- Autonomic Nervous System (ANS)
- Enteric Nervous System (ENS)
Opiate Peptides
- Endorphins, enkephalins & dynorphins are all examples of Opiate Peptides
- Inhibit pain by blocking substance P
- Aid in learning & memory
- Regulate mood, body temperature & sex drive
- Associated w/ schizophrenia & depression
2 Tract Types
- Myelinated = CNS white matter
- Unmyelinated (+ Neuron cell bodies, dendrites & neuroglia) = CNS gray matter
- “H-shaped” in spinal cord
- Outer covering of cerebral & cerebellar hemispheres in brain
Relative Refractory Period
- Period during which nerve/muscle cell can generate another AP, but only w/ a supra-threshold stimulus
- Caused by Na+ channels being in resting state, but K+ channels still being open
2 Features Affecting Nerve Impulses
- Resting Membrane Potential = Electrical charge separation across lipid bilayer
- Presence of ion channels in membrane = Path for ion movement in electrical current, leading to change of membrane potential (Can be gated or ungated)
3 Factors Affecting Graded Potentials
- # of Opened Channels
- Duration
- Channel type - excitatory or inhibitory
- Mechanically-gated = usually excitatory
- Ligand-gated w/ ACh ligand = usually excitatory
- Ligand-gated w/ glycine ligand = usually inhibitory
Oligodendrocytes
Oligodendrocytes
- Form myelin sheaths around CNS fibers, however do NOT form a neurolemmal sheath
- One oligodendrocyte myelinates many fibers
- CNS fibers cannot regenerate
- Demyelination occurs in some disease conditions (e.g. Tay-Sachs disease)
Suprathreshold Graded Potential
Firing of a series of action potentials, only stopping when electrical potential falls bellow -55mV
Hyperkinesis
Excessive dopamine levels; caused by Huntington’s Disease
Astrocytes
Astrocytes
- Can be protoplasmic or fibrous
- Cytoplasmic processes that support neurons + capillaries
- Participate in the “blood-brain barrier” by regulating cerebral capillary permeability
- Homeostasis of K+ in ECF
- Terminate neurotransmitter action by glial uptake
Neurotransmitters
- Chemical messengers that transmit signals across chemical synapses
- Can be either excitatory or inhibitory
2 ANS Subdivisions
- Sympathetic Division = Used for energy expending “fight or flight” activities
- Parasympathetic Division = Used for energy conserving/ restoring “vegetative” activities
Intracellular Ion Movement
Most intracellular anions simply cannot leave the cell
Central Nervous System
CNS = Brain + Spinal Cord; integrates sensory info, motor commands to muscles + glands, processes thoughts, memories + emotions
3 Mechanisms of Neurotransmitter Termination
- Diffusion from cleft
- Enzymatic degradation
- Re-uptake into neurons (uptake 1) and uptake into glial cells (uptake 2) by neurotransmitter transportrers
* SSRIs block re-uptake of serotinin, which leads to serotonin buildup in cleft
* MAO inhibitors block intraneuronal neurotransmitter breakdown
3 Functional Classifications of Neurons
- Motor Neurons
- Sensory Neurons
- Interneurons
Additional Axonal Features
Axon Collaterals = Side branches
Axon Terminals
-Have synaptic end-bulbs + varicosites, containing synaptic vesicles filled w/ neurotransmitters
-Slow Axonal Transport = Movement of axoplasm
-Fast Axonal Transport = Movement of substances (e.g., organelles, debris) along microtubules between cell body + synaptic end-bulbs/ synaptic varicosities
-Kinesins = Microtubule-dependent carrier ATPases
Parallel After-discharge Circuits
-Generate many APs in quick succession
Myelination
- Unmyelinated fibers undergo continuous conduction; the slow, sequential depolarization of each adjacent membrane area
- Myelinated fibers undergo saltatory conduction; ion currents flow through axoplasm & extracellular fluid, basically “leap” from one Node of Ranvier to the other (much faster)
Tracts
Bundles of neuronal fibers within CNS
Ganglia
Clusters of PNS neuron cell nodies
Motor Neurons
Motor Neurons = Always mulit-polar
- Starts from CNS to various types of effectors
- Smooth + cardiac muscle (involuntary)
- Skeletal muscle (voluntary)
- Glands
- “Innervation” of above structures allows muscle contractions/ gland secretions
- AKA “Efferent Neurons”
Mechanically-gated Channels
-Open/close in response to vibration, touch, pressure or sheer (e.g., hearing, balance, or touch)
Satellite Cells
Satellite Cells
-Responsible for supporting PNS nerve ganglia + regulating exchange between interstitial fluid + neurons
Neurotransmitter Co-localization
When neurons may contain more than one neurotransmitter type
GABA
- Inhibitory neurotransmitter in CNS
- Used by 1/3 of brain synapses
- Opens GABA-gated Cl- channels, creating IPSPs
- Ativan has same effect
- Reduces anxiety + relaxes skeletal muscles
Sensory Receptors
- Specialized endings sensitive to stimuli; are either free dendrites/ non-neuronal structures associated w/ dendrites
- Transduce sensations
- Stimulation causes nerve impulse to travel along sensory neuron to CNS
Fiber Diameter
- A Fibers: Up to 20 nanometers in diameter & myelinated, conduction speeds of up to 130 m/sec, found in SNS sensory & motor fibers, rapid input & response
- B Fibers: 2-3 nanometers in diameter & myelinated, conduction speeds of up to 15 m/sec, found in ANS sensory & motor neurons in viscera
- C Fibers: 1 nanometer diameter & unmyelinated, conduction speeds of only 2 m/sec, found in ANS
Resting Membrane Potential (RMP)
RMP = Voltage drop measurable across neuronal fiber membrane when cell isn’t conducting a nerve impulse
- Results from differences in electrical charges located on either side of the membrane (polarization)
- Outside = (+Charge), Inside = (-Charge)
- Average neuronal RMP = -70 mV
- Non-neuronal cells: +5 to -100 mV
2 Types of PNS Neuroglia
- Schwann Cells
2. Satellite Cells
Repolarization
Repolarization, caused by closing of Na+ channels and K+ outflow from opening of voltage-gated K+ channels (Actually causes hyperpolarization to -85 mV)
*Approx. 20,000 K+ ions flow outward
Trigger Zone Threshold
- 55 mV has to be reached in order for an action potential to fire
- If threshold isn’t reached, the potential dies away
Nuclei
Clusters of CNS neuron cell bodies
2 Types of Ion Channels
- Leaky (non-gated) channels
- randomly alternates between open & closed
- explains high K+ permeability of resting neuron - Gated channels
- Open in response to specific stimuli
- 3 different subtypes
4 Drugs that Modify Synaptic Transmission
- L-dopa (Parkinson’s Disease)
- Amphetamines (Depression & obesity)
- Botulinum Toxin (NMJ Blockade)
- Cocaine
CCK-PZ
- Regulates satiety
- Also responsible for gall bladder contraction , bile & pancreatic enzyme secretion
Peripheral Nervous System
PNS = Cranial + Spinal Nerves (w/ all branching nerves); carries info into + out of CNS. Has 3 Subdivisions
7 Neurotransmitter Examples
- Acetylcholine
- Amino Acids
- Biogenic Amines
- Purines
- Nitric Oxide
- Carbon Monoxide
- Neuropeptides
Na+/K+ -ATPase Pump
Restores resting state of nerve cell by pumping 2 K+ ions in for every 3 Na+ ions pumped out
Autonomic Nervous System
Autonomic Nervous System (ANS)
- Sensory neurons from viscera to CNS
- Motor neurons from CNS to smooth + cardiac muscles + glands
- Involuntary
- Has 2 more subdivisions
Neurolemmal Sheaths
- AKA “Sheath of Schwann” (Part of Schwann Cell containing cytoplasm + nuclei)
- May cover unmyelinated fibers
- Also cover myelinated fibers (outside myelin sheath)
- Neurolemma only around PNS fibers
- Nissl body chromatolysis = When the neurolemmas of axon’s many Schwann cells form a regeneration tube that guides/stimulates regrowth of axon (Role of Neurolemmal Sheath during Wallerian Degeneration)
Electrical Synapses
- Current flows between cells through gap junctions (connexons)
- Fast communication
- Able to synchronize activity of a group of cells (e.g., heart, “functional syncytium”)
Acetylcholine
- Found in both the CNS & PNS
- Can be Excitatory or Inhibitory
- Excitatory at NMJ
- Inhibitory at vagal synapses w/ heart
- Action terminated by acetylcholinesterase
Converging Circuits
-Several pre-synaptic neurons synapse w/ a single post-synaptic neuron
2 Types of Synapses
- Electrical Synapses
2. Chemical Synapses
Enteric Nervous System
Enteric Nervous System (ENS)
- Consists of neurons of enteric plexuses in gut wall
- Independent of CNS + ANS
- Communicates w/ CNS via ANS
- Monitors stretch in gut wall + chemical composition of luminal environment
- Stimulates GI smooth muscle contractions + stimulates exocrine gland secretions
- Also involuntary
3 Gated Channel Subtypes
- Voltage-gated
- Ligand-gated
- Mechanically-gated
Reverberating Circuits
-Input recycles, allowing prolonged output
Subcellular Locations of Ligand-gated Channels
- Dendrites & somas of motor neurons & interneurons
- Dendrites of unipolar & bipolar sensory neurons
- Motor end plates of skeletal muscle cells
Simple Series Circuits
-Two neurons connect to each other and one neuron connects to the first two
6 Steps of Neurotransmitter Substances
- AP depolarizes end-bulb membrane
- Opening of voltage-gated Ca+2 channels
- Influx of Ca+2
- Exocytotic release of contents of synaptic vesicles (neurotransmitters)
- Neurotransmitter diffusion across gap
- Neurotransmitter binding to receptors of post-synaptic cell membrane
Schwann Cells
Schwann Cells = “Neurolemmocytes”
- Responsible for myelination of some PNS Fibers (Provides electrical insulation)
- Myelin sheaths formed by many Schwann cells wrapping around each PNS fiber
- Nodes of Ranvier = Gaps between Schwann Cells (Enable rapid nerve impulse conduction, AKA “Saltatory Conduction”)
2 Nervous Tissue Types
- Neurons = Used to conduct nerve impulses
2. Neuroglial Cells = Used to support, protect, + nourish neurons
Hypokinesis
Inadequate dopamine levels; caused by Parkinson’s DIsease
Sensory Neurons
Sensory Neurons = Mostly uni-polar
- 2 long axonic extensions; one segment from periphery w/ dendrites, one segment to CNS w/ terminal synaptic end-bulbs
- Bipolar neurons found in retina, inner ear + olfactory epithelium
- Conduct nerve impulses from peripheral body parts to CNS
- AKA “Afferent Neurons”
All-or-None Principle
- If a stimulus is strong enough to reach the trigger zone threshold, an AP will propagate along the neuronal fiber at constant & maximum strength
- If threshold isn’t reached, no AP occurs
- If stimulus goes beyond threshold, AP will occur, but still at the same strength
Nerves
A bundle of PNS fibers surrounded by connective tissue
4 Types of CNS Neuroglia
- Astrocytes
- Microgilal Cells
- Ependymal Cells
- Oligodendrocytes
2 Possible Outcomes of Neurotransmitter Substances
- Very small partial depolarization of post-synaptic membrane (excitatory post-synaptic potential; EPSP)
- Very small partial hyperpolarization of post-synaptic membrane (inhibitory post-synaptic potential; IPSP)
* Information transfer only happens one way pre-synap. cell -> post-synap. cell
Glycine
- Inhibitory neurotransmitter in CNS
- Used by 1/2 of synapses in spinal cord
- Inhibitory glycinergic neurons (Renshaw cells) hyperpolarize lower motor neurons
Voltage-gated Channels
- Gates open when membrane depolarizes
- Depolarization allows nerve & muscle cells to propagate action potentials along their membranes
- Found in axonal membranes of neurons & in muscle sarcolemma & T-tubules
- Subcellular locations: Motor neurons & interneurons, unipolar & bipolar sensory neurons, skeletal muscle cells
Purines
- ATP, ADP, AMP, adenosine, etc. are all examples of purines
- All are excitatory
- Mediates “purinergic neurotransmission”
- In PNS, some neurons release ATP & noradrenaline or ATP & ACh
Dopamine
- A catecholamine (made from tyrosine by decaroboxylation and terminated by cellular uptake)
- Responsible for pleasure, emotions addiction & schizophrenia
- Modulates skeletal muscle activity of pyramidal pathway
Synapse
Synapse = Functional junction between 2 neurons/ between a neuron & an effector / between 2 effector cells
Neurotransmitter Substances
Transmission via chemicals at synapses (Causes synaptic delay, approx. 0.5 msecs)
Serotonin
- Made from tryptophan by decarboxylation
- Released by neurons of raphe nuclei
- Responsible for sensory perception, appetite, body temperature, mood & sleep
- May be involved in anorexia, depression & sleep disorders
- SSRIs prevent re-uptake
3 Nerve Types
- Sensory = Contain only sensory fibers + carry nerve impulses from peripheral receptors to CNS
- Motor = Contain only motor fibers + carry impulses from CNS to peripheral effectors
- Mixed = Contain both sensory & motor fibers + carry impulses both to & from CNS
5 Types of Neural Circuits
- Simple Series CIrcuits
- Diverging Circuits
- Converging Circuits
- Reverberating Circuits
- Parallel After-discharge Circuits
Ligand-gated Channels
- Open when hormones/neurotransmitters bind to receptors
- Ionotropic receptors: Receptor & channel are the same protein
- Metabotropic receptors: Receptor & channel are located on 2 different proteins
- Other components: A G-protein & a 2nd messenger system
- Subcellular locations: Same as voltage-gated channels
- Opening of one ligand-gated channel = small graded potential
Inhibitory Neurotransmitters
- Hyperpolarize post-synaptic membrane; creates IPSP
- IPSP summation = electrical potential further away from threshold for firing
4 Factors Affecting Speed of AP Conduction
- Myelination
- Fiber Diameter
- Temperature
- Anesthetics & Neurotoxins
2 Ways to Interpret Stimulus Intensity
- AP Frequency = The more frequently an AP is fired, the stronger the stimulus is
- Size of Sensory Field = The bigger the affected area of the stimulus, the stronger the stimulus is
Depolarization
Depolarization (+30 mV), caused by inflow of Na+ from opening of voltage-gated Na+ channels (Only occurs if threshold is reached)
*Approx. 20,000 Na+ ions flow inward
Excitatory Neurotransmitters
- Increase likelihood of AP
- Depolarize post-synaptic membrane; creates EPSP
- Summation of EPSP’s = Increased chance threshold for firing reached
Different AP Durations
Nerves: 1-2 msec
Skeletal Muscle: 1-5 msec
-electrical events mechanical events
Carbon Monoxide
CO functions as an excitatory CNS neurotransmitter
Amino Acids (4 Main Examples)
- Glutamate
- Aspartate
- GABA
- Glycine
Ependymal Cells
Ependymal Cells
- Line brain ventricles + central canal of spinal cord
- Aid CSF circulation + monitor its composition
- Also involved in the “blood-brain barrier”
Dendrite
Dendrite = Short, branched, unmyelinated processes, stimulated by other neurons, conduct electrical impulses to cell body via afferent transmission
Noradrenaline
- A catecholamine (made from tyrosine by decaroboxylation and terminated by cellular uptake)
- Responsible for arousal, mood, & dreaming
- Some neurons release adrenaline
- Both released as hormones
3 Types of Summation
- Spatial = Many pre-synap. neurons release neurotransmitter at the same time
- Temporal = Rapid neurotransmiter release at different times by one pre-synap. neuron
- Mixed = Combo of both
Antagonist
-Agents which block the action of a hormone/neurotransmitter, showing affinity but little/no efficacy (e.g., atropine & its derivatives like ipratropium)
Anesthetics & Neurotoxins
- Some block Na+ channel openings in axons, therefore blocking AP propagation
- Novocaine=Anesthetic
- Tetrodotoxin=Neurotoxin
Glutamate & Aspartate
- Excitatory neurotrasnmitters in CNS
- NMDA-type glutamate receptors linked to Ca+2 channels
- In brain hypoxia, decreased glutamate re-uptake leads to cell death of post-synap. neurons (“excitotoxicity”)
3 Types of Chemical Synapses
- Axodendritic = Axon->Dendrite
- Axosomatic = Axon->Cell Body
- Axoaxonic = Axon->Axon
Electrogenic Mode of Na+/K+ Pump
- For every 3 Na+ pumped out, 2 K+ are pumped in
- Mainly functions to eliminate Na+ that leaks into cell & replace K+ that leaks out of the cell
Strychnine Poisoning
When strychnine blocks glycine receptors on lower motor neuron in spinal cord, leading to skeletal muscles fully contracting, then eventually, death by asphyxiation
4 Factors Affecting RMP
- Distribution of Different Ion Species
- Differences in Plasma Membrane Permeability to Ions
- Intracellular Ion Movement
- Electrogenic Mode of Na+/K+ Pump
Non-systematic Naming of Neurons
Purkinje Cells of Cerebellum -Discovered by a Czech anatomist Jan Purkyne Pyramidal Cells -Controls skeletal muscle activity *Dendrites + axons are processes *Axons AKA "Neuronal Fibers"
2 Major Nervous System Divisions
- The Central Nervous System (CNS)
2. Peripheral Nervous System (PNS)
Hypothalamic Releasing & Release Inhibiting Hormones
- GHRH & GHIH are examples of releasing & release inhibiting hormones
- Function to regulate the anterior pituitary hormones
Receptor Binding
Neurotransmitters bind receptors in neuronal dendrite & cell body membranes; causes ligand-gated channels to open, creating small graded potentials
Net Effect of Neurotransmitters
Balance of EPSPs & IPSPs
Somatic Nervous System
Somatic Nervous System (SNS)
- Sensory neurons from skin, head, body wall + extremities to CNS
- Motor neurons from CNS to skeletal muscles
Chemical Synapses
-Regions where axon terminal’s synaptic end-bulb comes very close to a 2nd cell (called the “neuro-muscular/-glandular/-neural” junction)
3 Structural Classifications of Neurons
- Multipolar Neurons = Several short dendrites + one long axon
- Bipolar Neurons = One dendrite + One axon
- Unipolar Neurons = One process w/ the cell body on the side
Cell Body
Cell Body = Soma; Contains nucleus, organelles + inclusions
- Lipofuscin = colored cytoplasmic pigment inclusions
- Nissl bodies = RER
- Neurofibrils = Intermediate Filaments
- Microtubules = Used for transportation from cell body to axon terminus
Axon
Axon = Long projection, filled w/ axoplasm + surrounded by axolemma, may be myelinated, joins cell body at axon hillock, conducts nerve impulses away from cell body (Initial segment = “trigger zone”)
Diverging Circuits
-One pre-synaptic neuron influences several post-synaptic neurons
