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53 Cards in this Set
- Front
- Back
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What are axons?
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Specialized neuritic extensions from cell body for rapid and faithful transmission of information
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What is the longest cell in the body?
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Bipolar dorsal root ganglion (one process to big toe, other to medulla)
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What are the molecular motors for:
a. fast anterograde transport b. Fast retrograde transport c. Slow anterograde transport |
a. kinesin
b. dynein c. neurofilaments/microtubules |
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What is the difference between APs in axon vs. soma?
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All points along axon do not have same membrane potential in axon
In soma, voltage is spatially uniform |
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What is the effect of the long, narrow geometry of the axon on voltage of the axon?
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Higher resistanace to current flow, so amplitude of voltage change decreases exponentially down the axon
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What happens when depolarizations go above threshold in the axon?
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Depolarization of adjacent membrane generates a new AP --> rapid and lossless transmission
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What are the benefits of larger diameter axons?
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Lower resistance to internal current flow --> propagate subthreshold potentials longer and APs faster
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What are the cons of having larger diameter axons? (2)
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1. Fewer axons can fit in a confined space
2. Metabolically inefficient |
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What is myelination?
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Wrapping of multiple layers of membrane around the axon by glial cells
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Gardner's syndrome
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Colon polyps with osteomas and soft tissue tumors
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How long of a segment does each glial cell myelinate along the axon?
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0.5-1 mm
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What is the difference between the Nodal region (Ranvier) and the internodal region (Myelin) in terms of channels?
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Nodal regions has high density channels, internodal segment very few
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What does myelination do to the transmembrane resistance, and why is this beneficial?
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Increases transmembrane resistance, so decreased leakage
current flows preferentially down axon |
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What happens to capacitance in myelination and what are 2 consequences of this?
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Capacitance decreases more than 10 fold
1. Depolarization can occur by moving 10x fewer ions (dQ = CdV) --> metabolically more efficient 2. Rate of change of membrane potential much faster for given current (dV/dt = I/C) |
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What is salutatory conduction?
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Ionic current flow jumps from node to node at Nodes of Ranvier
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Why can demyelination over a short segment lead to conduction block? (2 reasons)
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1. Loss of 'insulating' properties (decreased transmembrane resistance, decreased membrane capacitance)
2. Distribution of ion channels in 'naked' segment poorly matched for needs of optimal conduction |
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How can APs be detected non-invasively?
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Measure potential difference between two points on surface of fiber
Inward depolarization and outward re-polarization generate a longitudinal current parallel to fiber axis |
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Why can't spontaneous activity from nerve fibers be detected at surface of the skin? (2)
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Contribution from an individual fiber is too small
Asynchronous impulses do not summate to reinforce the waveform |
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How can detection of nerve impulses be improved? (2)
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1. External electrical stimulation to synchronize nerve impulses
2. Signal averaging |
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How do large vs. small diameter axons compare in response to external stimuli?
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Large diameter = most easily activated
Small diameter = require the highest stimulus intensity to reach threshold |
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Why do larger diameter axons have a lower activation threshold?
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Nodes of Ranvier are farther apart --> higher local induced stimulus current density
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How does the order of activation of fibers compare with external stimulus vs. voluntary contraction?
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External stimulus: largest activated first, then smaller and smaller
Voluntary contraction: Smallest activated first, then larger and larger |
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What are the implications of having an inverse order of recruitment when external stimulus is given to nerve fibers? (2)
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1. Clinical measures of nerve fiber conduction are biased towards large fiber conduction
2. Development of prosthetic nerve stimulators is hampered by inverse order of motor fiber recruitment |
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What accounts for the distinct phases of the waveform when external stimulus is applied to sensory nerves? (2)
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1. Population distribution of fiber diameters
2. Extent of myelination |
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What are A, B, and C fibers?
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A = fastest/largest diameter fibers, activate first to external stimuli (alpha, beta, gamma, delta)
B = next largest diameter C = unmyelinated |
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What are large muscle afferents referred to as?
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Group I and II fibers
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What are small pain afferents referred to as?
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A delta and C fibers
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How do the largest sensory fibers and the largest motor axons compare in size?
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largest sensory fibers > largest motor axons
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Large, myelinated fiber
a. Muscle afferent name b. Cutaneous afferent name c. Diameter (microns) d. Conduction velocity (m/s) |
a. I
b. A alpha c. 12-20 d. 72-120 |
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Med, myelinated fiber
a. Muscle afferent name b. Cutaneous afferent name c. Diameter (microns) d. Conduction velocity (m/s) |
a. II
b. A beta c. 6-12 d. 36-72 |
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Small, myelinated fiber
a. Muscle afferent name b. Cutaneous afferent name c. Diameter (microns) d. Conduction velocity |
a. III
b. A delta c. 1-6 d. 4-36 |
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unmyelinated fiber
a. Muscle afferent name b. Cutaneous afferent name c. Diameter (microns) d. velocity |
a. IV
b. C c. 0.2-1.5 d. 0.4-2.0 |
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How can Sensory Nerve AP (SNAP) be used to test integrity of sensory nerve function? (3 things can be checked)
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1. Latency between stimulus and SNAP --> conduction velocity
2. duration of SNAP 3. Amplitude of SNAP |
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How can Compound Muscle AP (CMAP) be used to measure impulse conduction along motor axons? (measure 3 things)
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From synchronous discharges in multiple muscle fibers, measure for:
1. Latency 2. Duration 3. Amplitude |
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What is an M-wave?
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AP generated from orthdromic (normal direction) activation of motor nerve
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What creates an H-wave?
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Activation of a group I muscle sensory afferent via a reflex arc
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What creates an F-wave?
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Antidromic (opposite direction) activation of the motor nerve that rebounds at the soma and returns as an orthodromic volley
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What is the order of M-, H-, and F-wave activation when stimulus of motor neurons increases?
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1. H-wave (because largest, so lowest threshold)
2. M-wave increases and H-wave decreases (antidromic impulses cancel out H wave) 3. F-wave |
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What fibers are affected by sensory neuropathy?
Symptoms? What is not affected? |
Affects Small C fibers
Causes painful burning and tingling Does not change SNAP |
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What are symptoms of sensory neuropathies involving larger sensory fibers? (3)
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Loss of...
1.Touch perception 2. Vibration detection 3. Joint position sense |
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What are the symptoms of motor nerve neuropathies? (3)
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1. Weakness
2. Atrophy 3. Early loss of tendon reflexes |
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What are the symptoms of autonomic nerve dysfunction (4)?
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1. Impaired vasomotor response with decrease bp on standing
2. Impaired sweating 3. Gastric paresis 4. Erectile dysfunction |
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Where do peripheral neuropathies generally start and which fibers are affected first?
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Start distally
Larger fibers affected first |
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How can we use SNAP and CMAP to differentiate between demyelinating or axonal defects in peripheral neuropathies?
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1. Demyelination --> prolonged latency, increased waveform duration, smaller amplitude
2. Axonal neuropathy has smaller amplitude, but without prolonged latency |
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What is neuropraxis and what causes it?
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Neuropraxis = mild injuries to peripheral nerves caused by contusions to soft tissue or chronic irritation
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In neuropraxis, what are 2 possible problems with nerves?
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Axon integrity maintained, but...
1. possible demyelination and conduction block 2. or transient disruption in axonal transport function |
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What is axonotmesis?
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Crush injury to peripheral nerves where axon itself is damaged, but surrounding basal lamina and support structures intact
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What happens to the axon following axonotmesis? (3)
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1. Distal axon degenerates
2. Proximal axon 'dies back' --> Wallerian degeneration 3. Regrowth of axon --> re-innervation |
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How fast is axonal regrowth?
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about 1 mm/day
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What is neurotmesis?
Does reinnervation occur? |
Cut injury --> axon, endoneurial, perineurial, and epineurial sheaths disrupted
Low reinnervation |
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What are neurotrophins and what are their functions (2)?
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Neurotrophins = growth promoting molecules in target tissues of peripheral nerves
1. Establish connections in neurodevelopment 2. Maintain connections, especially between nerves and non-neuronal targets |
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What 3 factors make it difficult to regenerate injured axons in the CNS?
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1. Glial scar formation (activated astrocytes)
2. ECM protein scar 3. Inhibitory environrment for regeneration away from injury (myelin-associated proteins like Nogo) |
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What is Prototypical demyelinating motor neuropathy (aka AIDP aka Guillian-Barre Syndrome)?
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Autoimmune-mediated demyelination
CMAP shows increased distal latency, temporal dispersion, irregular waveform |
