The central nervous system is a model for a physiologic electrical system that produces physical force. The number of motor units activated corresponds to the strength of the muscle contraction. This induces voltage differences large enough to be detected by surface electrodes with an electromyogram (EMG) to record the muscle contraction, appearing as a burst of spike signals.
The average nerve conduction velocity of myelinated white matter is 50 m/s. This velocity varies directly with body temperature. Conduction velocity increases with myelin thickness, though it is not complete at birth and increases over the first few years of life.
Patients with carpal tunnel syndrome (entrapment of the median nerve as it enters the wrist) and neurodegenerative diseases are useful subjects for studying the variance of nerve conduction velocity. Sequential activation of motor units while comparing activation response times to standard controls helps elucidate underlying pathology. For example, nerve conduction velocity is measured in the lab by stimulating the tibial nerve and measuring EMG activity of the soleus muscle. The latency of the nerve is measured as the time for the signal to travel from the stimulating to the conducting electrode.
The mixed tibial nerve has motor and sensory neurons allowing sensory latency period, motor latency period, and amplitude of muscle contraction to be measured. These latency periods are very different, as sensory neurons are much larger and have faster conduction velocities compared to motor neurons. The table below summarizes normal and abnormal values for the tibial nerve.
Table 1. Normal and abnormal latency values for the tibial nerve.
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