Motor Unit

Some muscles are huge. The gluteus maximus has many thousands of muscle fibers essential to walking. Miniscule muscles are designed to produce the slightest movements of the eardrum or the larynx; these often have fewer than 100 muscle fibers. The set of muscle fibers innervated by a single axon, a motor unit, is very small for muscles that require very fine control, such as the extraocular and finger muscles. All of the muscle fibers within a single motor unit are of the same fiber type. Some motor units are even smaller than 10, that is, 1 motor axon innervating only 10 muscle fibers (e.g., in the extraocular muscles). In contrast, other motor units, not requiring fine control, are very large, hundreds to thousands, these innervate very massive postural (back musculature) as well as girdle and extremity muscles (gluteus maximus and gastrocnemius muscles). Because an action potential impulse heading out an axon in a peripheral nerve will enter all the branches of the axon, the motor unit is the unitary muscle contraction from a single axon. When muscles are activated, motor units are recruited in a fixed order. Typically, the weakest motor units causing the smallest muscle twitches are recruited first. If insufficient numbers are recruited for the task, additional motor units are activated, each progressively producing larger amounts of muscle tension. In this way, there is fine control of small muscle contractions and less control as muscle contraction force is increased. All muscle fibers within a single motor unit have similar contraction properties because they have similar subtypes of the contraction protein myosin.

Within a small section of muscle, the fibers of several (up to 10 or more) different motor units are interspersed with each other. (bottom part of Plate 10-3). This figure illustrates the anatomy of three different “motor units” forming the final peripheral components of the descending motor pathway that emanates from a single anterior horn motor neuron cell body and all axons and muscle fibers innervated by that single neuron. The cell bodies of the motor units lay within the brainstem for motor cranial nuclei, serving the somatic cranial muscles, such as the extraocular, facial, and pharyngeal muscles, and within the anterior horn cells of the spinal cord for the motor neurons serving somatic motor function to the noncranial muscles.

The upper left illustration demonstrates that cells bodies of each of three motor units originate within spinal cord anterior horn gray matter. The peripheral axon arising from each anterior horn cell leaves the spinal cord through the ventral nerve root to course as a peripheral nerve component until reaching the muscle. Here the nerve terminals of different motor units are positioned in a relatively confined intramuscular area named the “end-plate zone” or “motor point.” At this site, there are high concentrations of acetylcholine receptors attached within the muscle fibers. It is here, usually toward the middle of a muscle, that muscle fiber action potentials are generated after acetylcholine is released and bound to the receptors.

The first motor units recruited comprise muscle fibers having “slow” fatigue-resistant myosin that cause slow contractions. The last motor units to be recruited activate muscle fibers that have fast contractions thanks to fast myosin but are highly fatigable. It is possible to see positions of all muscle fibers within each of the motor units in one muscle. Such descriptions reveal “connectomes” that are complete maps of all the positions of all the motor axons and their connections within a muscle.

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