Anatomy and Physiology of Cells of the NS Flashcards
Soma
-Cell body of neuron
-Contains nucleus and most other organelles
-Common site of synaptic input (afferent)
-Contain receptors for neurotransmitters (subsynaptic membrane)
Dendrites
-Extension of the soma
-Common site of synaptic input (afferent)
-Contain receptors for neurotransmitters (subsynaptic membrane)
Axon hillock
-Anatomical transition from soma to axon
-Physiological transition from graded potential to action potential (AP)
-initial segment - has lowest graded threshold for AP
Axon
-Long thin, efferent process
-Many microtubules and microfilaments to maintain shape
Axon transport
-Up to 400mm per day
Anterograde transport
-Movement of materials (neurotransmitters, proteins, etc) from soma towards synaptic knobs
Kinesin
-Protein motor molecule
Retrograde transport
-Movement of materials (often waste) from synaptic knobs towards soma
Dynein
-Protein motor molecule
What surrounds the axon?
-May be surrounded by myelin with nodes of Ranvier
AP conduction speed _________ with _________ diameter & ________myelin.
increases, increased, increased
Fibers with largest diameter, most myelin
Sensory (afferent) A-alpha
Motor (efferent) Ia, Ib, II
Conduction speed: 70-120m/sec
Fibers in between
Sensory (afferent) A-beta
Motor (efferent) III
Conduction speed: 5-30m/sec
Fibers with smallest diameter, least myelin
Sensory (afferent) C
Motor (efferent) IV
Conduction speed: 0.5-2m/sec
Synaptic knobs (terminal boutons)
-Swelling at end of axons
-Contain structures of synaptic transmission (esp. vesicles of neurotransmitter)
Membrane potential
-Due (primarily) to differences in [Na] & [K] between ICF and EFC
-Also [Cl] & [Ca]
-When no signal is being sent = resting membrane potential
[-70mV] varies among neurons & types of neurons
Changes in membrane potential (=information)
-Depolarization: toward 0mV
-Repolarization: return from depolarization toward -70mV
-Hyperpolarization (overshoot): more negative than -70mV
Graded (local) potentials
-Small, decremental voltage changes in response to some stimulus (receptor, neurotransmitter, etc.)
-Larger stimulus causes larger voltage change (=graded)
-Can be excitatory (depolarizing) or inhibitory (hyperpolarizing)
-Created/located in the dendrites and soma
Action potential (AP)
-Huge, stereotypical, non-decremental, unidirectional change in membrane potential that is propagated along axon
AP is formed if threshold voltage is met (All or none rule)
-Initial segment of axon hillock (site of transition from local potential to AP)
-Threshold varies among neurons, even of the same type (usually around -55mV)
Propagation along axons
Contiguous or Saltatory
Contiguous Conduction
-On unmyelinated fibers
-Slower
-Must be recreated on every portion of the membrane
Saltatory Conduction
-On myelinated fibers
-Much faster
-AP “jumps” from one Node of Ranvier to the next
Synapses
-Functional connection between neurons
-Usually axodendritic or axosomatic (but can be axoaxonic)
-Usually functions by transforming AP signal into chemical signal (neurotransmitter) that crosses the synaptic cleft
Presynaptic neuron
Releases neurotransmitter (sends information)
Postsynaptic neuon
Senses neurotransmitter (receives information)
EPSP - Excitatory Post-Synaptic Potential
-Depolarizes postsynaptic cell
-Moves membrane potential closer to threshold
-Makes it more likely to fire an AP
IPSP - Inhibitory Post-Synaptic Potential
-Hyperpolarizes postsynaptic cell
-Moves membrane further from threshold
-Makes it less likely to fire an AP
Neurotransmitters
Chemical messengers (hundreds)
Excitatory (fast)
[neurotransmitter]
Acetylcholine (ACh), glutamate (primarily CNS; estimated at 90%+ of synapses)
Inhibitory (fast)
[neurotransmitter]
GABA (gamma-aminobutyric acid; mostly brain), glycine (mostly spinal cord)
Second messenger mediated (slow; many are either excitatory or inhibitory, depending on receptor)
-Catecholamines (epinephrine & norepinephrine; dopamine)
-Serotonin
-Neuropeptides (neuromodulators)
-Several neurotransmitters have both fast & slow receptors (ACh, glutamate, GABA)
Drugs and diseases acting at synapse
-Curare
-Botulsim
-Tetanus
-Myasthenia Gravis
Curare
-Toxin that blocks ACh receptors (nicotinic receptors)
-Causes flaccid paralysis
Botulsim
-Toxin prevents exocytosis of ACh (esp @ neuromuscular junction)
-Causes flaccid paralysis
Tetanus
-Toxin is transported by retrograde axonal transport to inhibitory neurons of spinal cord
-Blocks glycine release
-Prevents inhibition of motor neurons; causes spastic paralysis
Myasthenia Gravis
-Autoimmune destruction of ACh receptors; although ACh release is normal
-Muscle contraction decreases (esp. to repeated contractions)
-Causes weakness, double vision, etc.
Glial cells
90% by number; 50% by volume
Astrocytes (CNS)
-Connective tissue of the CNS
-Hold neurons in position
-Blood-brain barrier - chemically protect neurons
Oligodendrocytes (CNS)
-Myelinate axons in CNS
-Larger, myelinate many regions of multiple axons
Schwann cells (PNS)
-Myelinate axons in PNS
-Smaller, myelinate specific region of one axon
-Provide support, myelinate, and have phagocytotic role
Ependymal cells (CNS)
-Line ventricles, central canal of the spinal cord
-Choroid plexus: specialized ependymal cells within the ventricles that produce CSF
-Function: secretory, absorptive and CSF circulatory role
Microglia (CNS)
-Phagocytosis of degenerative debris of CNS
-Immune function
Charcot-Marie Tooth Disease
(family of disorders)
-Uncurable; genetic (most common inherited neurological disorder: 40 in 100,000)
-Peripheral neuropathy – caused by gap junction channelopathy
-Progressive loss of touch sensation, muscle weakness & wasting
-Affects longest axons first
-(i.e., feet first, then legs, then hands, then arm)
-Mechanism/molecule unknown, but causes demyelination of PNS
Guillain-Barre Syndrome
-Autoimmune inflammation (after viral infection) where macrophages attack and destroy myelin in the PNS (fairly rare: 1 in 100,000)
-Can cause rapid (12 hours! to 2 week) symptoms of weakness, loss of sensation
-often affects breathing muscles & patients require ventilation
-Patients often recover significantly, but 20% are unable to walk unaided after 6 months
Multiple Sclerosis
-Autoimmune attack on CNS myelin (approx. 28 in 100,000)
-Symptoms vary widely – depending on specific area of damage
-Chronic & debilitating changes in sensation, movement, mood and/or cognition
