Nerve Conduction Lab Lecture (Week 1--Schweizer and Tripp) Flashcards
Action potential (AP) vs. compound action potential (CAP)
Action potential is in a single axon/cell; can have subthreshold responses with increasing amplitude then suprathreshold responses with AP then as strength increases, AP occurs earlier; different wave form shape depending on size of stimulus
Compound action potential is multiple axons/cells in a nerve bundle; increasing stimulation strength leads to recruiting increased number of axons to fire AP; maximal amplitude of wave form reached once all axons have been recruited; wave is always same shape just larger version of itself
Nerve conduction lab
Stimulated motor nerve and calculated muscle response
Latency is a TIME (ms) that includes synaptic transmission, etc
When we stimulated the nerve, how did we know when all the muscle fibers had been recruited?
When we turned the stimulating voltage up and the wave form did not get bigger, that’s when we knew all the muscle fibers had already been recruited
Where is the median nerve in the wrist?
Between the palmaris longus and the flexor carpi radialis
Compound motor action potential (CMAP)
Electrical discharge of the muscle fibers resulting from stimulation of the motor nerve
Derived from all the muscle fibers from all the individual motor nerve fibers in that muscle (motor units) which fire in a nearly synchronous fashion due to the electrical stimulus
Information we collected
Latency: time from stimulus to response; stimulus to first negative (upward on the screen) deflection
Amplitude: vertical height of evoked response; baseline to peak of wave
Duration: time-duration of negative portion of response
Waveform (shape): biphasic usually
How do we calculate conduction velocity
Calculated between stimulus sites (not from distal site to muscle)
MNCV (m/s) = D/(Latency1 - Latency2)
Wallerian degeneration
Portion of nerve distal to injury degenerates after complete axonal disruption
Segmental demyelination
Loss of myelin from variety of injuries (disease, compression)
Axon is still intact
Axonal degeneration
Dying back of axon from distal portion
Form of axonal neuropathy
Looks like Wallerian degeneration in picture, but is going distal to proximal (as opposed to proximal to distal as in Wallerian degeneration)
Conduction speed of fully myelinated nerve compared to non-myelinated nerve
Fully myelinated motor nerve conducts 50-60 m/s
Completely non-myelinated nerve can conduct as slow as 2 m/s (but usually just something <50 m/s)
NCS abnormal responses and causes
Prolonged latency due to myelin loss
Slow velocity due to myelin loss
Low amplitude due to axon loss
Temporal dispersion due to myelin status variable
Prolonged latency and low amplitude due to myelin loss and axon loss
Conduction block due to segmental myelin loss
Abnormal shape due to wrong nerve, muscle, stimulation site, or variable myelin and/or axon loss
