What Constitutes a Motor Unit?
A motor unit is a structural and functional complex. According to Sir Charles Sherrington's classic concept, it serves as the final common pathway for impulses entering the anterior horn cells of the spinal cord.
A motor unit consists of:
- Motor neuron — a nerve cell whose cell body is located in the anterior (ventral) horn of the spinal cord.
- Motor neuron processes — the motor nerve fiber (axon) and its terminal branches.
- Muscle fibers — all muscle fibers innervated by the terminal branches of this axon via neuromuscular junctions.
Fibers of a single motor unit do not lie within a muscle as an isolated bundle. They are distributed in a mosaic pattern among the fibers of other motor units. Therefore, adjacent muscle fibers often belong to different motor units.
Types of Motor Units and Movement Precision
The number of muscle fibers belonging to a single motor unit varies depending on muscle function. The general rule is: the fewer muscle fibers innervated by a single motor neuron, the more precise the movement.
- Small motor units — provide fine, highly calibrated movements. Many anterior horn neurons are involved, each innervating 5 to 20 fibers. For example, extraocular muscles have 3–6 fibers per motor unit, while intrinsic hand muscles have 10–25 fibers.
- Large motor units — provide coarse movements that do not require precision. Each neuron innervates hundreds of muscle fibers (100–500); in the gastrocnemius muscle, a single motor unit may contain up to 7,000 fibers.
Mechanism of Function and Smoothness of Contraction
The function of the motor unit is to integrate signals from higher brain centers (via descending pathways) and spinal reflex arcs (intrasegmental and intersegmental). This integrated signal is then transmitted to the muscle fibers.
The smoothness of overall muscle contraction is achieved because individual muscle fibers within a single motor unit contract sequentially (asynchronously). This occurs due to slight variations in the arrival time of action potentials at different neuromuscular junctions.
Clinical Significance and Diagnostics
Damage to the motor unit at any level — anterior horn, ventral roots, nerve plexus, or peripheral nerve — severs the muscle from central voluntary and reflex control. This results in flaccid paralysis (lower motor neuron lesion), characterized by:
- Plegia (or paresis) — severe muscle weakness or complete paralysis.
- Hypotonia — marked reduction in muscle tone.
- Areflexia — loss of deep tendon reflexes due to disruption of the monosynaptic stretch reflex arc.
- Muscle atrophy — denervation leads to replacement of muscle tissue by connective tissue within weeks; over months to years, the muscle may be completely replaced by fibrous tissue.
Needle electromyography (EMG) records the motor unit action potential (MUAP). This signal is a complex waveform generated by the spatial and temporal summation of individual action potentials of muscle fibers within the pickup range of the electrode. The amplitude of each fiber's action potential depends on its distance from the electrode.
Not to Be Confused: Myon vs. Motor Unit
It is important to distinguish between terms describing the innervation of a group of fibers versus the support of a single cell. A myon (mion) is a structural complex supporting the vital functions of one individual muscle fiber (the fiber itself, its hemocapillary network, and its own innervation), whereas a motor unit includes one motor neuron and all the muscle fibers it innervates.