Random Pictures in Neurophysiology

This picture is showing the effect of motor neuron size on excitability, see below.

Hypothetically, these two neurons (different sizes) will have the same resting potential and will receive an identical stimulus.

The small motor neurons

• has a smaller surface area so less ion channels available and therefore a higher resistance to change (it also has less cytoplasm which increases resistance further)
• Ohms law(V=IR) states that a higher resistance (same current) will produce a higher potential
  • therefore the smaller motor neuron will reach threshold before the larger one
• The smaller cell has increased resistance, excitability, EPSP amplitudes, post-synaptic delay, duration of after-hyperpolarisation…. et etc primarily because of this feature (less ion channels and less cytoplasm)

A is showing the rate of firing among the different motor units as muscle tension is increased with time. B and C show the difference in the forces produced by each motor unit in response to one action potential. This is also represented by the dot/plot graph.
The body recruits different motor units for different tasks. In general, the slow, non-fatigable units/fibres are recruited first, with fast-non-fatigable (intermediate) fibres next, then fast-fatigable fibres recruited for more strenuous/complex activity.

see also this page about motor units

This shows a neuron being stimulated at its dendrite. This causes a graded potential (not at threshold), which slowly diminishes as it travels through the cell body. However, at the axon hillock, there are a far greater proportion of ion channels than anywhere else. The axon hillock, therefore, has a much lower threshold for AP generation than anywhere else.
This allows cells to generate APs in response to stimuli that would otherwise be useless.

 

 

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