Essential Neurophysiology

, Ali T. Ghouse² and Raghav Govindarajan³

(1)

Parkinson’s Clinic of Eastern Toronto and Movement Disorders Centre, Toronto, ON, Canada

(2)

McMaster University Department of Medicine, Hamilton, ON, Canada

(3)

Department of Neurology, University of Missouri, Columbia, MO, USA

Neurophysiology is physiology and neuroscience that focuses on the analysis of nervous system function, and electrophysiology is the study of the electrical properties of biological cells and tissues. Electrophysiology involves measurements of voltage change or electric current on different scales, from single ion channel proteins to large tissues such as muscle. Recordings of electric signals from the nervous system may also be referred to as electrophysiological recordings.

The Motor Unit

Most mature extrafusal skeletal muscle fibers are innervated by a single α motor neuron. Because of the presence of more muscle fibers than motor neurons, single motor axons branch within muscles in order to synapse on a variety of diverse fibers that are commonly dispersed over a relatively wide area within the muscle. This specific layout decreases the probability that damage to a single or a few α motor neurons will considerably modify a muscle’s action.

Since an action potential produced by a motor neuron typically brings all of the muscle fibers in its association to a threshold, a single α motor neuron and its accompanying muscle fibers collectively construct the lowest possible force that can be elicited to generate movement.

Hence, each motor nerve fiber and the group of muscle fibers that it supplies is a functional entity, since each time the nerve fiber discharges an impulse, the muscle fibers of the whole assembly would be collectively excited. Sherrington was the earliest individual to observe this noteworthy correlation between an α motor neuron and the muscle fibers it innervates, consequently coining the term “motor unit.”

Both the motor units and the α motor neurons themselves vary in size, and they also differ in the types of muscle fibers that they innervate. There are three classes of motor units:

1.

Small motor units innervate small muscle fibers that contract slowly and create rather small forces; however, because of their high myoglobin content, large number of mitochondria, and rich blood supply, these tiny “red fibers” are not susceptible to fatigue. These motor units are known as slow (S) motor units; they have a low threshold for activation, are tonically active, and are notably vital for processes that demand continuous muscular contraction, such as sustaining an upright posture. For example, a motor unit within the soleus, a muscle that is crucial for posture, contains tiny slow units and has an average mean innervation ratio of 180 muscle fibers per motor neuron.
2.

Larger α motor neurons innervate larger muscle fibers that produce more force; these fibers, however, possess scarce numbers of mitochondria, resulting in fatigue that occurs rapidly. These large, fast motor units, are also known as fast fatigable (FF) motor units and reach

their threshold only during rapid movements requiring substantial amounts of force. These units are considerably significant for momentary exertions involving large forces, such as running or jumping. An example of FF motor units is their presence in sprinters, whose legs possess greater amounts of powerful but rapidly fatiguing pale fibers compared with the legs of long distance, marathon runners. A great example is also evident in the motor units of the gastrocnemius, a muscle with both small and large units, which has an innervation ratio of 1000–2000 muscle fibers for each motor neuron; this can produce forces necessary for immediate changes in body position.
3.

Fast fatigue-resistant (FR) motor units are the third class of motor units. FR units have characteristics that lie between those of classes 1 and 2 above. They are intermediate in size and are not as rapid as FF units. But they are also significantly more resistant to fatigue than the slow motor units, as hinted at by their name. Furthermore, they produce almost double the amount of force of a slow motor unit.

Other distinct variations in motor units are correlated with the highly specialized activity of specific muscles. For example, for the eyes, which require fast movement along with precision, but minimal strength, extraocular muscle motor units are remarkably tiny in size (innervation ratio of three muscle fibers per motor neuron!), and possess copious amounts of muscle fibers that can contract with maximal velocity.

Synapse

The contact region between an axon and a muscle fiber is called a synapse or neuromuscular junction. The synapse has particular structural and functional features that allow the impulse from the nerve fiber to be transmitted to the muscle fiber through a chemical neurotransmitter link, acetylcholine. Each muscle fiber has one synapse and is innervated by only one motor neuron.

Synaptic Delay

Synaptic delay is the time necessary for the transmission of an impulse across a synapse. This refers to the interval between the time that the end of a presynaptic fiber experiences the arrival of a new impulse and the initiation of the postsynaptic potential. In synapses that have chemical transmission mechanisms, this delay lasts from 0.3 to 0.5 milliseconds to several milliseconds. For most of this period, mediators are released by presynaptic endings that are governed by nerve impulses.

Motor Unit Action Potential (MUAP) (see Figs. 5.1– 5.3)

The MUAP is the compound action potential of a single motor unit whose muscle fibers lie within the recording range of an electrode. For quantitative analysis, the following features are observed: