Neuromotor unit (NMU)

What makes up a motor unit?

The motor unit is a structural and functional complex. According to C. Sherrington's concept, it serves as the final common pathway for impulses arriving at the cells of the anterior horns of the spinal cord.

The NMU comprises:

• Motoneuron — a nerve cell whose cell body is located in the anterior horn of the spinal cord.
• Motoneuron processes — the motor nerve fiber (axon) and its terminals.
• Muscle fibers — all muscle fibers innervated by the branches of this axon via neuromuscular junctions.

Fibers of a single NMU do not lie within the muscle as a single isolated bundle. They are distributed mosaically among the fibers of other motor units. Therefore, adjacent muscle fibers often belong to different motor units.

Types of NMUs and movement precision

The number of muscle fibers comprising a single NMU varies depending on the muscle's function. General rule: the fewer muscle fibers innervated by a single nerve fiber, the more precise the movement.

• Small motor units — provide fine and finely tuned movements. Many neurons are involved in the anterior horn, each innervating 5 to 20 fibers. For example, in the extraocular muscles, there are 3–6 fibers per NMU, whereas in the hand muscles, there are 10–25 fibers.
• Large motor units — provide gross movements that do not require precision. Each neuron innervates hundreds of muscle fibers (100–500); in the gastrocnemius muscle, the number of fibers in a single NMU reaches 7,000.

Mechanism of action and contraction smoothness

The function of the NMU is to integrate impulses from higher brain centers (via descending pathways) and from spinal reflex arcs (intra- and intersegmental). The resulting information is transmitted to the muscle fibers.

Smoothness of overall muscle contraction is achieved because individual muscle fibers within a single NMU contract sequentially (non-simultaneously). This occurs due to the different arrival times of nerve excitation to them.

Clinical significance and diagnostics

Damage to the motor unit at any of the following levels: anterior horn, anterior roots, nerve plexus, or peripheral nerve — cuts the muscle off from central voluntary and reflex innervation. This leads to the development of flaccid paralysis, characterized by:

• Plegia — extreme degree of muscle weakness.
• Hypotonia — marked decrease in muscle tone.
• Areflexia — loss of reflexes due to the disruption of the monosynaptic stretch reflex arc.
• Muscle atrophy — upon denervation over several weeks, muscle tissue begins to be replaced by connective tissue; months or years later, the muscle may be completely replaced by connective tissue.

Needle electromyography (EMG) records the motor unit action potential (MUAP). This signal represents a complex waveform formed through the spatiotemporal summation of individual action potentials from muscle fibers of a single NMU located within the recording radius of the electrode. The amplitude of each fiber's potential depends on its distance from the electrode.

Do not confuse: Myon vs. NMU

It is important to distinguish between concepts describing the innervation of a fiber group and the maintenance of an individual cell. A myon (mion) is a structural complex for the vital activity of only one muscle fiber (the fiber itself, the capillary network, and its intrinsic innervation), whereas an NMU includes a single motoneuron and all the fibers subordinate to it.

Class and exam questions

What else to study on this topic

• Neuromuscular junction
• Reflex arc
• Flaccid paralysis
• Electromyography (EMG)

Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.