Motor unit number estimation (MUNE) is a technique that uses

electromyography Electromyography (EMG) is a technique for evaluating and recording the electrical activity produced by skeletal muscles. EMG is performed using an instrument called an electromyograph to produce a record called an electromyogram. An electromyo ...

to estimate the number of

motor unit A motor unit is made up of a motor neuron and all of the skeletal muscle fibers innervated by the neuron's axon terminals, including the neuromuscular junctions between the neuron and the fibres. Groups of motor units often work together as a mo ...

s in a muscle.

Principles

consists of one

alpha motor neuron Alpha (α) motor neurons (also called alpha motoneurons), are large, multipolar lower motor neurons of the brainstem and spinal cord. They innervate extrafusal muscle fibers of skeletal muscle and are directly responsible for initiating their ...

and all the

muscle fibre A muscle cell is also known as a myocyte when referring to either a cardiac muscle cell (cardiomyocyte), or a smooth muscle cell as these are both small cells. A skeletal muscle cell is long and threadlike with many nuclei and is called a mus ...

s it innervates. Muscles differ in the number of motor units that they contain, and how many muscle fibres are within each unit (innervation ratio). In a general sense, muscles that require specificity of movement, such as muscles in charge of eye movement, have fewer fibres per unit, while those that are meant for less specific tasks, such as the calf muscles in charge of jumping, have more. MUNE uses a general formula of: Number of motor units =

compound muscle action potential The compound muscle action potential (CMAP) or compound motor action potential is an electromyography Electromyography (EMG) is a technique for evaluating and recording the electrical activity produced by skeletal muscles. EMG is performed usin ...

size divided by the mean surface-detected motor unit action potential size The compound muscle action potential (CMAP) size is found using supramaximal stimulation of the motor nerve to the muscle or muscle group (similar to a

nerve conduction study A nerve conduction study (NCS) is a medical diagnostic test commonly used to evaluate the function, especially the ability of electrical conduction, of the motor and sensory nerves of the human body. These tests may be performed by medical speci ...

). It is recorded using surface electrodes. This is representative of the sum of the surface detected motor unit action potentials from muscles innervated by that

nerve A nerve is an enclosed, cable-like bundle of nerve fibers (called axons) in the peripheral nervous system. A nerve transmits electrical impulses. It is the basic unit of the peripheral nervous system. A nerve provides a common pathway for the ...

. Surface-detected motor unit action potential (SMUAP) size is the contribution of individual motor units. The way of finding the average size of these action potentials depends on the method used, as described below.

Methods

There are at least six techniques that are currently in use to estimate motor unit numbers. These include incremental stimulation, multi-point stimulation method, F-response method, spike-triggered averaging method and the statistical method. Incremental stimulation is the most illustrative of the concept, and so will be discussed here. According to

Henneman's size principle Henneman’s size principle describes relationships between properties of motor neurons and the muscle fibers they innervate and thus control, which together are called motor units. Motor neurons with large cell bodies tend to innervate fast-twitc ...

motor unit recruitment Motor unit recruitment is the activation of additional motor units to accomplish an increase in contractile strength in a muscle. A motor unit consists of one motor neuron and all of the muscle fibers it stimulates. All muscles consist of a n ...

is orderly such that smaller motor neurons are recruited before progressively larger ones. Additionally, the motor unit action potential is an all-or-none phenomenon - once the recruitment threshold (the stimulus intensity at which a motor unit begins to fire) is reached, it fires fully. Electrical stimulation of nerves reverses the recruitment order, due to the lower resistance of the larger motor neuron axons. Incremental stimulation involves gradually increasing the intensity of the stimulus to reach the recruitment threshold of increasing numbers of motor units until the intensity of the CMAP is reached. A 'step' is noted when an increase in stimulus leads to an increase in recorded EMG (i.e. another motor unit's threshold is reached and it is recruited). The CMAP is then divided by the number of steps required to reach the intensity of the CMAP to get a mean SMUAP size. The number of steps does not correlate to the total number of motor units in the muscle. Instead, the CMAP size is then divided by the mean SMUAP size to get an estimation of the number of motor units in the muscle.

Uses

The number of motor units per muscle can change due to aging, disease, or injury. These techniques are used to diagnose disease or monitor the effects of aging, disease and injury over time. In

neuropathies Peripheral neuropathy, often shortened to neuropathy, is a general term describing disease affecting the peripheral nerves, meaning nerves beyond the brain and spinal cord. Damage to peripheral nerves may impair sensation, movement, gland, or or ...

motoneurons A motor neuron (or motoneuron or efferent neuron) is a neuron whose cell body is located in the motor cortex, brainstem or the spinal cord, and whose axon (fiber) projects to the spinal cord or outside of the spinal cord to directly or indirec ...

die off, reducing the number of motor units progressively. In

myopathies In medicine, myopathy is a disease of the muscle in which the muscle fibers do not function properly. This results in muscular weakness. ''Myopathy'' means muscle disease (Greek : myo- ''muscle'' + patheia '' -pathy'' : ''suffering''). This meani ...

the size of the motor units is reduced because of the death of motor fibres, but the number of motor units remains the same until the disease progresses to a very severe state. In collaboration with other

techniques, these conditions can be diagnosed and monitored. In a similar vein, normal

aging Ageing ( BE) or aging ( AE) is the process of becoming older. The term refers mainly to humans, many other animals, and fungi, whereas for example, bacteria, perennial plants and some simple animals are potentially biologically immortal. In ...

also reduces the number of motor units but not to the same degree as disease. The effects of injury depend on the circumstances.

References

{{Reflist Neurology