Sensory and motor axons are different: implications for neurological disease

Abstract

Axons are required to maintain specific discharge rates and patterns, and as a consequence, the ability to conduct an impulse with minimal expenditure of energy will create different needs for different axonal populations. There may be only one role for an axon, and that is to conduct an impulse securely from one end to the other, but it is to be expected that the biophysical properties of sensory and motor axons may differ in order for them to fulfil this role. Specifically, motor axons that innervate muscle and cause it to contract will behave differently to sensory axons that arise from skin (or muscle) and provide feedback to the central nervous system.Axons are more than telecommunication cables. Were it not for the investment of channels and pumps and the specific organisation of myelin, which in turn determines the localisation of those channels and pumps, the message would decay with distance in accordance with the "cable properties" of the axon. The present Review will consider the basis for the differences in properties of large sensory and motor axons and how these differences influence their behaviour in neurological disease. Using threshold tracking, differences have been established between large myelinated sensory and α motor axons in humans. Major differences are that sensory axons are relatively depolarised at rest such that they have a greater persistent Na + current, and have greater activity of hyperpolarisation-activated cyclic nucleotide-gated (HCN) channels. Sensory axons may thereby be protected from hyperpolarising stresses, and are less likely to develop conduction block. However, the corollary is that sensory axons are more excitable and more likely to become ectopically active.

Autonomic and Somatic Nerve Functions in Type 2 Diabetes Mellitus Patients: Electrophysiological Aspects

“…The multivariate analysis confirmed that the reduction in the sensory response in the sural nerve (longest sensory nerve) is positively correlated with a reduction in HRV in Valsalva and HRV DB, which suggests a concomitant involvement of sensory-somatic and parasympathetic nerve fibres in T2DM. The differences between biophysical properties (excitability) and in ion channel functions between motor and sensory axons can subject the sensory fibres to greater metabolic disturbances; therefore, in early DN stages, the sensory axons from the peripheral nerves are initially affected [32].…”

Does Essential Tremor Alter the Axonal Excitability Properties of Lower Motor Neurons?

“…Automated nerve excitability testing (ANET) using a threshold tracking protocol is a novel technique used in neurophysiology to assess the axonal excitability within peripheral nerves in vivo . 1–3 The results of ANET reflect the biophysics of the relevant voltage-gated ion channels and resultant membrane potentials of peripheral nerves. 4–6 This technique has contributed to improving the understanding of the pathophysiology of peripheral neuropathies and motor neuron disease.…”