a-b = time it takes for the action potential to travel x-y
latency = neuromuscular delay + utilisation time
factors that determine velocity
distance
time
myelination
diameter of the axon - the bigger the diameter the less resistance so the faster the conduction
synaptic delay - time it takes for the chemical to be released diffuse and elicit the appropriate response in the post synaptic cell
acetylcholine is released at the neuromuscular junction
examples of substances that effect nerve conduction
botulinum
causes relaxation
blocks the release of acetylcholine by preventing vescular fusion
botox
neostigamine
causes contraction
inhibits acetylcholinesterase so acetylcholine builds up
treats myasthenia gravis
reversible
myasthenia gravis
causes relaxation
the body produces antibodies that attack acetylcholine receptors inhibiting their ability to bind acetylcholine
treated with neostigamine
NM delay + utilisation times = total latency response (a > t) – nerve conduction time (a > t).
The Course of the Median Nerve
Directional Terms
horizontal plane - parallel to the ground
transverse plane - perpendicular to the longitudinal axis of the structure being considered
the nerve starts at the brachial plexus and runs superficially in the axilla
it then descends the brachium deep to the biceps brachii and enters the cubital fossa
the nerve enters the proximal aspect of the cubital fossa, medial to the bicipital aponeurosis
it leaves the fossa lateral to the midline and in the distal portion
it then descends laterally to the superficial flexor muscles within the anterior compartment
as it descends the antibrachium it gives off innervating motor branches to all muscles except the deep flexor muscles of digits 4 and 5 and a medial wrist flexor.
to enter the hand the nerve passes through the carpal tunnel and enters the palm
here it divides into many branches and provides innervation to the palmar surface and digits 1,2,3 and the lateral aspect of digit 4
the main motor branch ascends to the thenar eminence to innervate the thumb - the recurrent branch
the median nerve doesn't innervate the brachium muscles
the 3 muscles of the thumb are the abductor pollicis brevis, opponens pollicis and flexor pollicis brevis
Damaging the Median Nerve
if it was damaged at the elbow joint wrist flexion would be seriously impaired - there would be some weak flexion from the deep flexor muscles for digits 4 and 5
if it was damaged at the elbow joint fexion of digits 1,2 and 3 would be absent - flexor muscles of digits 1,2 and 3 are supplied by the median nerve distal to the elbow joint
if the median nerve was damaged at the wrist joint wrist flexion would be normal - wrist flexion is controlled by the median nerve proximal to the wrist
the median nerve was damaged at the wrist joint flexion of digit 1 would be slightly impaired - flexor pollicis brevis doesn't function, felxor pollicis longus originates in the antibrachium so wouldn't be affected