For each physiological description A–E, select the most appropriate option from the following list of axons.
1. Small unmyelinated axons.
2. Large myelinated axons.
3. Small myelinated axons.
4. Large unmyelinated axons.
5. Motor axons.
6. Sensory axons.
7. Autonomic axons.
8. Somatic axons.
- Axons which are concerned with reflex responses affecting cardiac function.
- Axons which transmit motor impulses in the knee jerk reflex.
- Axons which transmit sensory impulses in the knee jerk reflex.
- Axons which transmit impulses around the lower end of the velocity range, around one metre per second.
- Axons which transmit impulses by saltatory conduction at around the upper end of the velocity range, around 50 metres per second.
Medicine
A. Option 7 Autonomic axons. Cardiac function is modified by sympathetic and parasympathetic autonomic motor nerves. These responses are informed by autonomic sensory fibres, e.g. from baroreceptors. Though in general these are small unmyelinated axons, some of the preganglionic motor fibres are myelinated and a little larger.
B. Option 2 Large myelinated axons. The somatic tendon jerks involve very rapid responses, for which large myelinated axons (around 15 microns diameter) are required.
C. Option 2 Large myelinated axons. It is logical to have impulses carried at similar high speeds in both sensory and motor halves of the reflex arc.
D. Option 1 Small unmyelinated axons. Such axons (around 1 micron diameter) carry impulses where speed of response is low, as with slow pain and many autonomic responses.
E. Option 2 Large myelinated axons. The reason for the high speed is that saltatory con-duction jumps the relatively long internodal distance in these axons at the speed of an electric current.
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