1213 Week 14 P

1213 Week 14 P

1. Nerve Conduction Velocity

   • velocity = distance / time
   • conduction velocity = (x-y)/(a-b)
     • x-y = the distance the action potential travels
     • 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).

2. 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