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A motor unit is made up of a motor neuron and the skeletal muscle fibers innervated by that axon. Groups of motor units often work together to coordinate the contractions of a single muscle; all of the motor units within a muscle are considered a motor pool.
All muscle fibers in a motor unit will be of the same fiber type. When a motor unit is activated, all of its fibers contract. In vertebrates, control of muscle contraction force is accomplished by controlling how many motor units are activated for a given motion.
The number of muscle fibers within each unit can vary within a particular muscle and still more widely from muscle to muscle; the muscles that act on the largest body masses have motor units that contain most of the muscles fibers whereas smaller muscles consist of less muscle fibers in each unit. For instance, thigh muscles can have a thousand fibers in each unit, while extraocular muscles might have ten. Additionally, muscles which possess more motor units are thus able to control force output more finely.
Invertebrates organize their motor units differently; each muscle has few motor units (typically less than 10), and each muscle fiber is innervated by multiple neurons, including excitatory and inhibitory neurons. Thus, while a human muscle controls force by regulating how many motor units are turned on, invertebrates control force by regulating the balance between excitatory and inhibitory signals.
The central nervous system is responsible for the orderly recruitment of motor neurons, beginning with the smallest motor units. Henneman's size principle indicates that motor units are recruited from smallest to largest based on the size of the load. For smaller loads requiring less force, slow twitch, low-force, fatigue-resistant muscle fibers are activated prior to the recruitment of the fast twitch, high-force, less fatigue-resistant muscle fibers. Larger motor units are typically composed of faster muscle fibers that generate higher forces. The central nervous system has two distinct ways of controlling the force produced by a muscle through motor unit recruitment, spatial recruitment and Rate coding. Simply put, spatial recruitment is the activation of more motor units to produce a greater force, larger motor units will contract along with the small motor units until all muscle fibers in a single muscle are activated and the maximum muscle force is produced. Temporal motor unit recruitment, or rate coding, deals with the frequency of activation of muscle fiber contractions. Consecutive stimuli on the motor unit from the α motor neuron causes the fusion of muscle twitches and produces a greater force than a single contraction, by decreasing the interval between the stimuli, the muscle can produce a larger force with the same amount of motor units.
Using Electromyography (EMG), the neural strategies of muscle activation can be measured. Ramp-force Threshold refers to an index of motor neuron size to test the Size Principle by determining the recruitment threshold of a motor unit during isometric contraction in which the force is gradually increased. Motor units recruited at low force, low-threshold units, associate with small motor units, while high threshold units are recruited at higher forces and involve larger motor neurons. Larger motor units, which were recruited at higher threshold forces, tend to have shorter contraction time than the smaller units. The number of additional motor units recruited during a given increment in force declines sharply at high levels of voluntary force, suggesting that even though high threshold units generate more tension, the contribution of recruitment to increase voluntary force declines at higher force levels.
To test motor unit stimulation, electrodes are placed extra-cellularly on the skin. After the motor unit is stimulated, its pulse is then recorded by the electrode and displayed as an action potential, known as a motor unit action potential (MUAP). When multiple MUAP’s are recorded within a short time interval, a motor unit action potential train (MUAPT) is then noted. The time in between these pulses is known as the inter-pulse interval (IPI). In medical electrodiagnostic testing for a patient with weakness, careful analysis of the MUAP size, shape, and recruitment pattern can help in distinguishing a myopathy from a neuropathy.
Motor unit types (vertebrate)
Motor units possess a range of properties and are generally placed into groups based upon the similarities between those properties. Generally, three or four groups are identified, based upon several factors:
- Contraction speed in Isometric contractions
- Rate of rise of force
- Time to peak of a twitch contraction (re
- Contraction speed in Isometric contractions
- FF — Fast fatigable — high force, fast contraction speed but fatigue in a few seconds.
- FR — Fast fatigue resistant — intermediate force, fatigue resistant — fast contraction speed and resistant to fatigue.
- FI — Fast intermediate — intermediate between FF and FR.
- S or SO — Slow (oxidative) — low force, slower contraction speed, highly fatigue resistant.
- These generally designate fibers as:
- I (Slow oxidative, SO) — Low glycolytic and high oxidative presence. Low(er) myosin ATPase, sensitive to alkali.
- IIa (Fast oxidative/glycolytic, FOG) — High glycolytic, oxidative and myosin ATPase presence,sensitive to acid.
- IIb (Fast glycolytic, FG) — High glycolytic and myosin ATPase presence, sensitive to acid. Low oxidative presence.
- IIi — fibers intermediate between IIa and IIb.
- Histochemical and Physiological types correspond as follows:
- S and Type I, FR and type IIa, FF and type IIb, FI and IIi.
- The Immunohistochemical types are as follows, with the type IIa, IIb and slow corresponding to IIa, IIb and slow (type I) histochemical types:
Expressed inGene familyDevelopingFast fibers (II)Slow fibers(I)MHCEmbryonic MHCMHC IIaβ/slow MHCNeonatal MHCMHC IIbMHC IIxMLC1 (alkali)Embryonic1f1s1f3fMLC2 (regulatory)2f2f2s
- There are currently about 15 known different types of MHC genes recognized in muscle, only some of which may be expressed in a single muscle fiber. These genes form one of ~18 classes of myosin genes, identified as class II which should not be confused with the type II myosins identified by immunohistochemistry. The expression of multiple MHC genes in a single muscle fiber is an example of polymorphism. The relative expression of these myosin types is determined partly by genetics and partly by other biological factors such as activity, innervation and hormones.
The typing of motor units has thus gone through many stages and reached a point where it is recognized that muscle fibers contain varying mixtures of several mysosin types that can not easily be classified into specific groups of fibers. The three (or four) classical fiber types represent peaks in the distribution of muscle fiber properties, each determined by the overall biochemistry of the fibers.
Estimates of innervation ratios of motor units in human muscles:
|Muscle||Number of Motor Axons||Number of Muscle Fibers||Innervation Ratio||Reference|
|Brachioradialis||315||129,000||410||Feinstein et al|
|First dorsal interosseous||119||40,500||340||Feinstein et al|
|Medial gastrocnemius||579||1,120,000||1,934||Feinstein et al|
|Tibialis anterior||445||250,200||562||Feinstein et al|
Table reproduced from
- MYH1, MYH2, MYH3, MYH4, MYH6, MYH7, MYH7B, MYH8, MYH9, MYH10, MYH11, MYH13, MYH14, MYH15, MYH16
- motor unit recruitment
- tetanic contraction
- myopathy (muscle pathology/diseases)
- motor unit number estimation
- Motor pool
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