Motor unit recruitment
Encyclopedia
Motor unit recruitment is the progressive activation of a muscle
by successive recruitment of contractile units (motor unit
s) to accomplish increasing gradations of contractile strength.
A motor unit consists of one motor neuron
and all of the muscle fibers it contracts. All muscles consist of a number of motor units and the fibers belonging to a motor unit are dispersed and intermingle amongst fibers of other units. The muscle fibers belonging to one motor unit can be spread throughout part, or most of the entire muscle, depending on the number of fibers and size of the muscle. When a motor neuron is activated, all of the muscle fibers innervated by the motor neuron are stimulated and contract.
The activation of one motor neuron will result in a weak but distributed muscle contraction. The activation of more motor neurons will result in more muscle fibers being activated, and therefore a stronger muscle contraction. Motor unit recruitment is a measure of how many motor neurons are activated in a particular muscle, and therefore is a measure of how many muscle fibers of that muscle are activated. The higher the recruitment the stronger the muscle contraction will be. Motor units are generally recruited in order of smallest to largest (fewest fibers to most fibers) as contraction increases. This is known as "Henneman's Size Principle".
(EPSPs) would result in a higher voltage change (depolarization) across the neuronal membrane of the smaller motor neurons and therefore larger EPSPs in smaller motoneurons. Burke later demonstrated that there was a graded decrease of both EPSP inhibitory post synaptic potential (IPSP) amplitudes from small to large motoneurons. This seemed to confirm Henneman's idea, but Burke disagreed, pointing out that larger neurons with a larger surface area had space for more synapses. Burke eventually showed (in a very small sample of neurons) that smaller motoneurons have a greater number of synaptic inputs from a single input source. The topic is probably still regarded as controversial.
Under some circumstances, the normal order of motor unit recruitment may be altered, such that small motor units cease to fire and larger ones may be recruited. This is thought to be due to the interaction of excitatory and inhibitory motoneuronal inputs.
. Generally, this allows a 2 to 4-fold change in force. In general, the motor unit firing rate of each individual motor unit increases with increasing muscular effort until a maximum rate is reached. This smooths out the incremental force changes which would otherwise occur as each additional unit was recruited.
from a neuropathy.
Muscle
Muscle is a contractile tissue of animals and is derived from the mesodermal layer of embryonic germ cells. Muscle cells contain contractile filaments that move past each other and change the size of the cell. They are classified as skeletal, cardiac, or smooth muscles. Their function is to...
by successive recruitment of contractile units (motor unit
Motor unit
”A motor unit is a single α-motor neuron and all of the corresponding muscle fibers it innervates; all of these fibers will be of the same type . When a motor unit is activated, all of its fibers contract...
s) to accomplish increasing gradations of contractile strength.
A motor unit consists of one motor neuron
Motor neuron
In vertebrates, the term motor neuron classically applies to neurons located in the central nervous system that project their axons outside the CNS and directly or indirectly control muscles...
and all of the muscle fibers it contracts. All muscles consist of a number of motor units and the fibers belonging to a motor unit are dispersed and intermingle amongst fibers of other units. The muscle fibers belonging to one motor unit can be spread throughout part, or most of the entire muscle, depending on the number of fibers and size of the muscle. When a motor neuron is activated, all of the muscle fibers innervated by the motor neuron are stimulated and contract.
The activation of one motor neuron will result in a weak but distributed muscle contraction. The activation of more motor neurons will result in more muscle fibers being activated, and therefore a stronger muscle contraction. Motor unit recruitment is a measure of how many motor neurons are activated in a particular muscle, and therefore is a measure of how many muscle fibers of that muscle are activated. The higher the recruitment the stronger the muscle contraction will be. Motor units are generally recruited in order of smallest to largest (fewest fibers to most fibers) as contraction increases. This is known as "Henneman's Size Principle".
Neuronal mechanism of recruitment
Henneman proposed that the mechanism underlying the Size Principle was that the smaller motor neurons had a smaller surface area and therefore a higher membrane resistance. He predicted that the current generated by an excitatory postsynaptic potentialExcitatory postsynaptic potential
In neuroscience, an excitatory postsynaptic potential is a temporary depolarization of postsynaptic membrane potential caused by the flow of positively charged ions into the postsynaptic cell as a result of opening of ligand-sensitive channels...
(EPSPs) would result in a higher voltage change (depolarization) across the neuronal membrane of the smaller motor neurons and therefore larger EPSPs in smaller motoneurons. Burke later demonstrated that there was a graded decrease of both EPSP inhibitory post synaptic potential (IPSP) amplitudes from small to large motoneurons. This seemed to confirm Henneman's idea, but Burke disagreed, pointing out that larger neurons with a larger surface area had space for more synapses. Burke eventually showed (in a very small sample of neurons) that smaller motoneurons have a greater number of synaptic inputs from a single input source. The topic is probably still regarded as controversial.
Under some circumstances, the normal order of motor unit recruitment may be altered, such that small motor units cease to fire and larger ones may be recruited. This is thought to be due to the interaction of excitatory and inhibitory motoneuronal inputs.
Rate coding of muscle force
The force produced by a single motor unit is determined in part by the number of muscle fibers in the unit. Another important determinant of force is the frequency with which the muscle fibers are stimulated by their innervating axon. The rate at which the nerve impulses arrive is known as the motor unit firing rate and may vary from frequencies low enough to produce a series of single twitch contractions to frequencies high enough to produce a fused tetanic contractionTetanic contraction
A tetanic contraction occurs when a motor unit has been maximally stimulated by its motor neuron. This occurs when a muscle's motor unit is stimulated at a sufficiently high frequency of multiple impulses. Each stimulus causes a twitch. If stimuli are delivered slow enough, the tension in the...
. Generally, this allows a 2 to 4-fold change in force. In general, the motor unit firing rate of each individual motor unit increases with increasing muscular effort until a maximum rate is reached. This smooths out the incremental force changes which would otherwise occur as each additional unit was recruited.
Proportional control of muscle force
The distribution of motor unit size is such that there is an inverse relationship between the number of motor units and the force they generate (i.e., the number of muscle fibers per motor unit). Thus, there are many small motor units and progressively fewer larger motor units. This means that at low levels of recruitment, the force increment due to recruitment is small, whereas in forceful contractions, the force increment becomes much larger. Thus the ratio between the force increment produced by adding an additional motor unit and the force threshold at which that unit is recruited remains relatively constant.Electrodiagnostic testing
In medical electrodiagnostic testing for a patient with weakness, careful analysis of the "motor unit action potential" (MUAP) size, shape, and recruitment pattern can help in distinguishing a myopathyMyopathy
In medicine, a myopathy is a muscular disease in which the muscle fibers do not function for any one of many reasons, resulting in muscular weakness. "Myopathy" simply means muscle disease...
from a neuropathy.