Motoneurons

How is motoneuron size related to excitability and resistance?  

The human central nervous system's reactivity can change dramatically with exercise, injury, inactivity, or disease. Changes occur at both the cortical and spinal levels, but in most situations, the motoneuron pool's excitability must be measured to pinpoint the exact location of adaptation. Motoneurons are integrative transducers in that they combine the voltage responses of the soma and dendrites to millions of synaptic currents and transduce this signal into a voltage change at the motoneuron's most excitable segment, the beginning segment of the axon. Axons in larger motoneurons have greater spike amplitudes and faster conduction velocities. If threshold conditions are satisfied, a single action potential or a sequence of action potentials is generated and propagated down the axon to the muscle fibers' neuromuscular connections. As a result, a moto-intrinsic neuron's excitability plays a significant role in determining the likelihood of it being recruited during motor pool activation. This variability is a fundamental determinant of motor unit recruitment order during exercise.

Factors that affect action potential generation 

Factors that affect action potential generation would be a stimulus which is generated by an internal or external action. Since muscles are highly innervated, action potential may propagate through the entire muscle, recruiting most motor neurons. However, if action potential is weak, meaning the stimulus is weak, then smaller motor neurons will be activated, thus producing lesser effect and smaller recruitments of muscle fibers. The action potential is crucial to understanding how the nervous system works. The timing, synchronization, and efficacy of neural transmission are all dependent on its shape, conduction velocity, and propagation commitment. The amount of current flowing into the cell, the resting membrane potential, and the depolarization amplitude required to exceed the spike generation threshold are all factors that determine whether a motoneuron will be stimulated enough to create an action potential. The amount of current necessary to generate an action potential is mostly determined by resistance. The active depolarization propagates towards the soma and down the axon during axonal action potential initiation.

References:

Kress, G. J., & Mennerick, S. (2009). Action potential initiation and propagation: upstream influences on neurotransmission. Neuroscience, 158(1), 211–222. https://doi.org/10.1016/j.neuroscience.2008.03.021

McNeil, C. J., Butler, J. E., Taylor, J. L., & Gandevia, S. C. (2013). Testing the excitability of human motoneurons. Frontiers in human neuroscience, 7, 152. https://doi.org/10.3389/fnhum.2013.00152