Peripheral Nerves (Week 1--Schweizer) Flashcards
Peripheral nerve
Contains many axons of diverse function and diameter, carrying both efferent motor and afferent sensory information
Some axons within peripheral nerve are myelinated by Schwann cells and others are not
Epineurium
Ensheaths the peripheral nerve
Loose connective tissue composed of fibroblasts, mast cells, blood vessels and multiple layers of collagen
Gives tensile strength to nerves
Perineurium
Encloses individual fascicles
Composed of concentric sheets of endothelial cells that are connected by tight junctions
Collagen between layers of endothelial cells
Tight sleeve is the blood-nerve barrier
Endoneurium
Ensheaths each individual axon
Composed of collagen fibers, ECM and basal lamina adjacent to Schwann cells that enwrap individual axons
“Layers” of peripheral nerve
Axons make up fascicles (bundles) which are grouped into nerves
Are non-myelinated axons in PNS wrapped with Schwann cells?
No, they are not WRAPPED with Schwann cells, but they are still supported by Schwann cells!
Schwann cells surround non-myelinated axons, but do not wrap around many times
How many Schwann cells per axon?
One Schwann cell can support many non-myelinated axons
Only one single Schwann cell wraps around one single axon for myelinated axons
Note: in CNS, one single oligodendrocyte can wrap many axons with a myelin layer
Node of Ranvier
High concentration of Na+ channels to allow for AP propagation
Also surrounded by voltage gated K+ channels (helps with repolarization by letting K+ out); also astrocytes at node sop up K+
What kind of axon conducts an impulse the fastest?
Large, myelinated
Ex: muscle spindle primary endings, golgi tendon organs, lower motor neurons are the largest
How is axon diameter related to speed of conduction?
Increasing diameter increases speed of conduction by square root of diameter in non-myelinated axons
Increasing diameter increases speed of conduction linearly in myelinated axons
Shorter internodal distances mean faster saltatory conduction, but why don’t we have nodes very close to one another?
Size of Schwann cells (specifically the nucleus) limits lower end (smallest possible) internodal distance for peripheral axons
Which fibers are the fastest?
Large diameter, myelinated fibers carry proprioceptive information about skeletal muscles (A-fibers)
(note: what about muscle spindle, golgi tendon and lower motor neuron…these look biggest/myelinated…)
AP velocity can reach 200mph!
Which fibers are the slowest?
Small diameter non-myelinated fibers carry mainly pain information (C fibers)
Compound action potential (CAP)
Electrical activity generated when ensemble of axons in a nerve fire AP simultaneously
This only occurs in response to external stimulus delivered by a person! Does not occur during normal nerve activity, but has great diagnostic value
Several peaks visible because individual nerves within peripheral nerve fire APs that travel at distinct speeds, depending on myelination and fiber diameter
Phases we are seeing in CAP wave form
1) Axon at rest so negative on inside
2) Axon depolarized so positive on inside at beginning
3) No voltage difference between recording electrodes because whole inside positive
4) Hyperpolarization?
5) Back to resting membrane potential
How do you calculate the nerve conduction velocity (NCV)?
Test two stimulation sites, take the difference in latency, calculate NCV
Sensory nerve action potential (SNAP)
Activity of all sensory fibers stimulated
No synapse between stimulation and recording sites
SNAPs much smaller than CAMPs (microvolts)
Compound motor action potential (CAMP)
Measure of near simultaneous activation of many muscle fibers
Motor nerve stimulated, AP propagates to NMJ, triggers ACh release and postsynaptic AP in muscle is measured
Includes a synapse
In the millivolt range
EMGs are more sensitive so why don’t we just use those?
EMGs require intramuscular electrodes and thus are more invasive
What happens if you get local demyelination of a nerve?
Can get slowing or even block of AP propagation
Why do you get slowing or block after demyelination?
Current flowing through few Na+ channels at original Node of Ranvier not enough to charge large membrane capacitance of non-myelinated segment
Also, charge lost via dispersion through axon where no more myelination?
How can you repair a demyelinated nerve?
1) Remyelinate
2) Insert additional Na+ channels in demyelinated segments
What happens to the neuron after injury to the axon?
Axon distal to injury site undergoes active process of degeneration (Wallerian degeneration)
Activation of degradative enzymes and “balling up” of myelin sheets into “myelin ovoids”
Area gets infiltrated by macrophages
Proximal cell body undergoes “chromolysis” (RNA containing structures such as polyribosomes and rER disappearing) and Nissl bodies disappear; cell body can also undergo apoptosis
If cell body does not die: proximal axonal stump starts sprouting shortly after injury and distal Schwann cells proliferate to form a tube (Bunger’s band) that helps guide sprouting axon in direction of lost axon; if crush injury, basal lamina and perineum help guide growing axon
If axon reaches its target, eventually will be re-myelinated
If reconnection fails, target atrophies over time
Why is a crush injury better than a cut nerve?
Because you’d still have basal lamina and perineurium intact, which can help guide newly growing axon
What happens to the CAP if you have demyelination?
CAP is slowed
Since demyelination not uniform across different axons, individual APs are slowed to a different extent which leads to temporal dispersion –> reduction in amplitude of CAP
When responses arrive at recording electrode together, get big wave; when responses arrive at different times, get phase cancellation
What is one main function of myelin?
1) Reduces membrane capacitance
2) Lowers lateral “leak” of current since there are hardly any channels in internode
CAP in normal vs. demyelinated nerve
CAP in normal nerve: CAP in different parts all arrive at muscle at same time; productive summation, large amplitude, narrow waveform
CAP in demyelinated nerve: some CAP are slowed at demyelinated site; cancellation, small amplitude, temporal dispersion
CMAP in disease
Axonal degeneration: get smaller signal both proximal and distal to the injury
Local demyelination: get low signal proximal to injury and normal signal distal to injury
Conduction block: get low signal proximal to injury and normal signal distal to injury (similar to local demyelination)
