Conduction Nerve Velocity Flashcards
Which nerve axons are activated by an electrical stimulus
Both (provided the stimulus is delivered at an appropriate strength). The stimulus activates sensory axons and the subject feels the stimulus. It also stimulates the motor axons resulting in a twitch of the thumb.
We have assessed the conduction velocity of which nerve axons?
Motor. We are recording the muscle action potentials of the abductor pollicis brevis muscle as a result of stimuli delivered to proximal sites (i.e. closer to the body; at the wrist and at the elbow). Although the stimulus activates both types of nerves axons (sensory and motor) we are only recording those that have travelled down the motor axons to elicit action potentials in the muscle fibres.
The nerve axons activated at lowest electrical stimulus intensities are -[large diameter/small diameter]?
Large diameter. The threshold of a nerve axon to electrical stimulation is inversely proportional to its diameter. So, as the stimulus intensity is increased, the largest diameter axons (those carrying some sensations) are stimulated first and the subject feels, or senses, the stimulus. As the intensity is increased further those axons with smaller diameters (e.g. those carrying motor impulses) are stimulated and a twitch is induced from activation of muscles fibres innervated by these motor fibres. If the stimulus were to be increased even further then smaller unmyelinated axons would be stimulated and the subject would perceive the stimulus as being painful (in addition to sensing the stimulus and the twitch being evoked).
Slowed conduction velocity might indicate [demyelination/hypothermia/increased pressure to the nerve bundle/all of these]
All of these. Myelin sheaths surrounding axons greatly increase their conduction velocity. Diseases in which myelin is lost (e.g. multiple sclerosis) are associated with slowing of nerve conduction. Ambient temperature affects nerve conduction velocity with lower temperatures slowing conduction velocity. Increased pressure on the nerve fibres also slows conduction velocity.
The motor axons activated by the stimulus send action potentials [towards the hand/towards the spinal cord/both directions]
Towards the hand (motor or efferent – away from the spinal cord). The cell body of the alpha motor neuron is located in the ventral (anterior) horn of the spinal cord. Its axon exits the spinal cord and travels in the mixed nerve (along with fibres of other types) down to the muscle.
The sensory axons activated by the stimulus send action potentials [towards the hand/towards the spinal cord/both directions]
Towards the spinal cord (sensory or afferent - towards the spinal cord). Activation of the sensory neurons by the stimulus induces action potentials to be is carried by the sensory nerves to the spinal cord, where they synapse with neurons carrying the impulse up to the brain. NB. For questions 5 & 6, note that axons under normal physiological conditions conduct axon potentials in one direction only. The artificial nature of the electrical stimulus can induce action potentials to travel in both directions from the stimulating electrode (the cathode). This is taken advantage of in certain clinical tests (e.g. measurement of the F-wave)
The median nerve contains motor axons that supply only the abductor pollicis brevis muscle [true/false]
False. The motor axons in the median nerve supply other muscles which move the thumb well as longer muscles which move some of the fingers and the wrist.
Absence of EMG action potentials in response to the stimulus could indicate that the median nerve is blocked [true/false/maybe].
True. However, an absence of EMG action potentials in response to the stimulus could also indicate that transmission of the nerve impulses has not occurred across the neuromuscular junction (i.e. no release of acetylcholine), despite normal conduction of the nerve impulses down the median nerve. Alternatively, the stimulus may not have been of a sufficiently high level to activate the motor axons at all, only the sensory axons (see question 3).
Nerve compression in the forearm can result in slowing of median nerve conduction at the wrist. Could you diagnose such a condition using the stimulation and recording sites in this demonstration [yes/no]?
No. Slowing of impulses at the wrist would not be picked up since the 2 stimulation sites are proximal to the site of slowing (i.e. they are towards the body). The total time recorded for the latency of each response would be longer. However, the difference between these latencies divided by the distance between the 2 stimulation sites would reveal normal conduction velocity. If the stimulation sites were either side of the site of slowing, then this setup could diagnose this condition. The latency of the response to stimulation proximal to the site of slowing would be longer than normal. The latency of the response to stimulation distal to the site of slowing would be normal. The difference between these latencies divided by the distance between the 2 stimulation sites would reveal abnormally slow conduction velocity.
List three factors which can influence conduction velocity of peripheral nerve axons in vivo
Hypothermia/increased pressure to the nerve bundle/loss of myelin (see question 4).
Describe the difference between action potentials in muscle fibres and in peripheral nerves
ction potentials in muscle fibres are of larger amplitude than those in peripheral nerve axons; this makes them easy to record through the skin using surface electrodes. Because of their large amplitude it is easy to record their precise time of occurrence and any changes in amplitude as stimulus intensity is varied.
Where is the adductor pollicis muscle found
thenar eminence of the hand
Where is the anode placed
The positive stimulating electrode (anode) will be sited at a location on the arm proximal to the cathode.
Where is the cathode placed
S1 The medial aspect of the forearm at the wrist
S2 The ulnar groove at the elbow
What does latency time include
Activation time.
Conduction delay from the cathode to the neuromuscular junction.
Delay at the neuromuscular junction.
Conduction delay along the muscle fibres to the EMG recording electrodes.
