Motor unit recruitment

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General depiction of a motor unit, consisting of a motor neuron innervating a group of muscle fibers. Motor Unit.png
General depiction of a motor unit, consisting of a motor neuron innervating a group of muscle fibers.

Motor unit recruitment is the activation of additional motor units to accomplish an increase in contractile strength in a muscle. [1] A motor unit consists of one motor neuron and all of the muscle fibers it stimulates. 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. [2] [3] 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 (smallest motor neurons to largest motor neurons, and thus slow to fast twitch) as contraction increases. This is known as Henneman's size principle. [4]

Contents

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 potential (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. [5] Burke later demonstrated that there was a graded decrease of both EPSP and inhibitory postsynaptic potential (IPSP) amplitudes from small to large motoneurons. [6] 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. [7] The topic is probably still regarded as controversial.

In their 1982 paper, Burke and colleagues propose that the small cell size and high surface-to-volume ratio of S motor units allows for greater metabolic activity, optimized for the "highest duty cycles" of motoneurons, while other motor unit types may be involved in "lower duty cycles." However, they state that the evidence is not conclusive "to support or deny the intuitively appealing notion that there is a  correlation between metabolic activity, motoneuron size, and motor unit type." [8]

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. [9] [10] This is thought to be due to the interaction of excitatory and inhibitory motoneuronal inputs.

Recruitment of motor unit types

Another topic of controversy resides in the way in which Burke and colleagues categorized motor unit types. They designated three general groups by which motor units could be categorized: S (slow – slow twitch), FR (fast, resistant – fast twitch, fatigue-resistant), and FF (fast, fatigable – fast twitch, fatigable). [11] [8] These designations have served as the basis for motor unit categorization since their conception, but modern research indicates that human motor units are more complex and possibly do not directly fit this model. [12]

However, it is important to note that Burke himself recognized the risk in classifying motor units:

My friend the late Elwood Henneman told me several times in conversation that he thought classifying motor units into distinct categories was probably a bad idea because, unless used with care, classifications tend to distort reality. I agreed, and still do, that taxonomies can lead to overly rigid thinking (and sometimes even lack of thinking) but they are necessary for communication, which requires that things be named; and scientific communication demands that things be named precisely, according to their attributes. [13]

If a correlation were to be drawn between Henneman's size principle [14] and the motor unit categorization of Burke regarding the order of motor unit recruitment, it would resemble the following order: the smallest units, S (slow), would be recruited first, followed by larger FR (fast, resistant) units, and lastly the largest FF (fast, fatigable) units, reserved for high-energy tasks that require additional motor unit recruitment. [15] [16]

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 contraction. 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. [17]

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 each generates (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 another motor unit and the force threshold at which that unit is recruited remains relatively constant. [4]

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 myopathy from a neuropathy.

See also

Related Research Articles

<span class="mw-page-title-main">Motor neuron</span> Nerve cell sending impulse to muscle

A motor 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 indirectly control effector organs, mainly muscles and glands. There are two types of motor neuron – upper motor neurons and lower motor neurons. Axons from upper motor neurons synapse onto interneurons in the spinal cord and occasionally directly onto lower motor neurons. The axons from the lower motor neurons are efferent nerve fibers that carry signals from the spinal cord to the effectors. Types of lower motor neurons are alpha motor neurons, beta motor neurons, and gamma motor neurons.

<span class="mw-page-title-main">Skeletal muscle</span> One of three major skeletal system types that connect to bones

Skeletal muscles are organs of the vertebrate muscular system and typically are attached by tendons to bones of a skeleton. The muscle cells of skeletal muscles are much longer than in the other types of muscle tissue, and are often known as muscle fibers. The muscle tissue of a skeletal muscle is striated – having a striped appearance due to the arrangement of the sarcomeres.

In biology, 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 motor pool to coordinate the contractions of a single muscle. The concept was proposed by Charles Scott Sherrington.

<span class="mw-page-title-main">Muscle spindle</span> Innervated muscle structure involved in reflex actions and proprioception

Muscle spindles are stretch receptors within the body of a skeletal muscle that primarily detect changes in the length of the muscle. They convey length information to the central nervous system via afferent nerve fibers. This information can be processed by the brain as proprioception. The responses of muscle spindles to changes in length also play an important role in regulating the contraction of muscles, for example, by activating motor neurons via the stretch reflex to resist muscle stretch.

Muscle fatigue is the decline in ability of muscles to generate force. It can be a result of vigorous exercise but abnormal fatigue may be caused by barriers to or interference with the different stages of muscle contraction. There are two main causes of muscle fatigue: the limitations of a nerve’s ability to generate a sustained signal ; and the reduced ability of the muscle fiber to contract.

Renshaw cells are inhibitory interneurons found in the gray matter of the spinal cord, and are associated in two ways with an alpha motor neuron.

<span class="mw-page-title-main">Muscle contraction</span> Activation of tension-generating sites in muscle

Muscle contraction is the activation of tension-generating sites within muscle cells. In physiology, muscle contraction does not necessarily mean muscle shortening because muscle tension can be produced without changes in muscle length, such as when holding something heavy in the same position. The termination of muscle contraction is followed by muscle relaxation, which is a return of the muscle fibers to their low tension-generating state.

Synaptogenesis is the formation of synapses between neurons in the nervous system. Although it occurs throughout a healthy person's lifespan, an explosion of synapse formation occurs during early brain development, known as exuberant synaptogenesis. Synaptogenesis is particularly important during an individual's critical period, during which there is a certain degree of synaptic pruning due to competition for neural growth factors by neurons and synapses. Processes that are not used, or inhibited during their critical period will fail to develop normally later on in life.

<span class="mw-page-title-main">Gamma motor neuron</span>

A gamma motor neuron, also called gamma motoneuron, or fusimotor neuron, is a type of lower motor neuron that takes part in the process of muscle contraction, and represents about 30% of (Aγ) fibers going to the muscle. Like alpha motor neurons, their cell bodies are located in the anterior grey column of the spinal cord. They receive input from the reticular formation of the pons in the brainstem. Their axons are smaller than those of the alpha motor neurons, with a diameter of only 5 μm. Unlike the alpha motor neurons, gamma motor neurons do not directly adjust the lengthening or shortening of muscles. However, their role is important in keeping muscle spindles taut, thereby allowing the continued firing of alpha neurons, leading to muscle contraction. These neurons also play a role in adjusting the sensitivity of muscle spindles.

Motor control is the regulation of movement in organisms that possess a nervous system. Motor control includes reflexes as well as directed movement.

<span class="mw-page-title-main">Muscle</span> Basic biological tissue present in animals

Muscle is a soft tissue, one of the animal tissues that makes up the three different types of muscle. Muscle tissue gives skeletal muscles the ability to contract. Muscle is formed during embryonic development, in a process known as myogenesis. Muscle tissue contains special contractile proteins called actin and myosin which contract and relax to cause movement. Among many other muscle proteins present are two regulatory proteins, troponin and tropomyosin.

Myomeres are blocks of skeletal muscle tissue arranged in sequence, commonly found in aquatic chordates. Myomeres are separated from adjacent myomeres by connective fascia (myosepta) and most easily seen in larval fishes or in the olm. Myomere counts are sometimes used for identifying specimens, since their number corresponds to the number of vertebrae in the adults. Location varies, with some species containing these only near the tails, while some have them located near the scapular or pelvic girdles. Depending on the species, myomeres could be arranged in an epaxial or hypaxial manner. Hypaxial refers to ventral muscles and related structures while epaxial refers to more dorsal muscles. The horizontal septum divides these two regions in vertebrates from cyclostomes to gnathostomes. In terrestrial chordates, the myomeres become fused as well as indistinct, due to the disappearance of myosepta.

The Mauthner cells are a pair of big and easily identifiable neurons located in the rhombomere 4 of the hindbrain in fish and amphibians that are responsible for a very fast escape reflex. The cells are also notable for their unusual use of both chemical and electrical synapses.

A motor pool consists of all individual motor neurons that innervate a single muscle. Each individual muscle fiber is innervated by only one motor neuron, but one motor neuron may innervate several muscle fibers. This distinction is physiologically significant because the size of a given motor pool determines the activity of the muscle it innervates: for example, muscles responsible for finer movements are innervated by motor pools consisting of higher numbers of individual motor neurons. Motor pools are also distinguished by the different classes of motor neurons that they contain. The size, composition, and anatomical location of each motor pool is tightly controlled by complex developmental pathways.

Beta motor neurons, also called beta motoneurons, are a kind of lower motor neuron, along with alpha motor neurons and gamma motor neurons. Beta motor neurons innervate intrafusal fibers of muscle spindles with collaterals to extrafusal fibers - a type of slow twitch fiber. Also, axons of alpha, beta, and gamma motor neurons become myelinated. Moreover, these efferent neurons originate from the anterior grey column of the spinal cord and travel to skeletal muscles. However, the larger diameter alpha motor fibers require higher conduction velocity than beta and gamma.

The motor unit consists of a voluntary alpha motoneuron and all of the collective muscle fibers that it controls, known as the effector muscle. The alpha motoneuron communicates with acetylcholine receptors on the motor end plate of the effector muscle. Reception of acetylcholine neurotransmitters on the motor end plate causes contraction of that effector muscle.

Normal aging movement control in humans is about the changes in the muscles, motor neurons, nerves, sensory functions, gait, fatigue, visual and manual responses, in men and women as they get older but who do not have neurological, muscular or neuromuscular disorder. With aging, neuromuscular movements are impaired, though with training or practice, some aspects may be prevented.

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-twitch, high-force, less fatigue-resistant muscle fibers, whereas motor neurons with small cell bodies tend to innervate slow-twitch, low-force, fatigue-resistant muscle fibers. In order to contract a particular muscle, motor neurons with small cell bodies are recruited before motor neurons with large cell bodies. It was proposed by Elwood Henneman.

<span class="mw-page-title-main">Golgi tendon organ</span> Proprioceptive sensory receptor organ that senses changes in muscle tension

The Golgi tendon organ (GTO) is a proprioceptor – a type of sensory receptor that senses changes in muscle tension. It lies at the interface between a muscle and its tendon known as the musculotendinous junction also known as the myotendinous junction. It provides the sensory component of the Golgi tendon reflex.

Elwood Henneman was an American neurophysiologist who studied the properties of vertebrate motor neurons.

References

  1. ↑ Betts, J Gordon; Desaix, Peter; Johnson, Eddie; Johnson, Jody E; Korol, Oksana; Kruse, Dean; Poe, Brandon; Wise, James; Womble, Mark D; Young, Kelly A (June 7, 2023). Anatomy & Physiology. Houston: OpenStax CNX. 10.3 Muscle Fiber Contraction and Relaxation. ISBN   978-1-947172-04-3.
  2. ↑ Ounjian, M., R.R. Roy, E. Eldred, A Garfinkel, J.R. Payne, A. Armstrong, A. Toga, and V.R. Edgerton Physiological and Developmental Implications of Motor Unit Anatomy. J. Neurobiol. 22:547-559, 1991. Motor unit territory.
  3. ↑ Bodine-Fowler, S., Garfinkel, A., Roy, Roland R., and Edgerton, V. Reggie. Spatial distribution of muscle fibers within the territory of a motor unit. Muscle and Nerve 13:1133-1145, 1990. - non-random distribution of fibers.
  4. 1 2 Henneman, E., Somjen, G. & Carpenter, D. O. (1965). Functional significance of cell size in spinal motoneurons. J. Neurophysiol. 28, 560-580.
  5. ↑ Henneman, E., Somjen, G. & Carpenter, D. O. (1965). Excitability and inhibitability of motoneurons of different sizes. J. Neurophysiol. 28, 599-620.
  6. ↑ Burke RE, Rymer WZ & Walsh JR. Relative strength of synaptic input from short-latency pathways to motor units of defined type in cat medial gastrocnemius. J Neurophysiol. 1976 May;39(3):447-58.
  7. ↑ Burke RE, Walmsley B, Hodgson JA. HRP anatomy of group Ia afferent contacts on alpha motoneurones. Brain Res. 1979 Jan 12;160(2):347-52
  8. 1 2 Burke, R.E.; Dum, R.P.; Fleshman, J.W.; Glenn, L.L.; Lev-Tov, A.; O'Donovan, M.J.; Pinter, M.J. (1982). "A HRP study of the relation between cell size and motor unit type in cat ankle extensor motoneurons". The Journal of Comparative Neurology. 209 (1): 17–28. doi:10.1002/cne.902090103. PMID   7119171. S2CID   601035.
  9. ↑ Garnett, R. & Stephens, JA. The reflex responses of single motor units in human first dorsal interosseous muscle following cutaneous afferent stimulation. J. Physiol. Land. 303: 351-364, 1980.
  10. ↑ Kanda, K., Burke, R. E., & Walmsley, B. Differential control of fast and slow twitch motor units in the decerebrate cat. Exp. Brain Res. 29:57-74, 1977.
  11. ↑ Burke, R.E.; Levine, D.N.; Zajac, F.E.; Tsairis, P.; Engel, W.K. (1971). "Mammalian Motor Units: Physiological-Histochemical Correlation in Three Types in Cat Gastrocnemius". Science. 174 (4010): 709–712. Bibcode:1971Sci...174..709B. doi:10.1126/science.174.4010.709. PMID   4107849. S2CID   42638100 – via American Association for the Advancement of Science.
  12. ↑ Heckman, CJ; Enoka, R.M. (2012). Terjung (ed.). "Motor Unit". Comprehensive Physiology. 2 (4): 2629–2682. doi:10.1002/cphy.c100087. PMID   23720261.
  13. ↑ Burke, R.E. (1999). Binder, M.D. (ed.). Chapter 15 Revisiting the notion of 'motor unit types'. Progress in Brain Research. Vol. 123. pp. 167–175. doi:10.1016/S0079-6123(08)62854-X. ISBN   9780444502889.
  14. ↑ Henneman, E. "Skeletal muscle: the servant of the nervous system". Medical Physiology. 1: 674–741.
  15. ↑ Hodson-Tole, E.F.; Wakeling, J.M. (2009). "Motor unit recruitment for dynamic tasks: current understanding and future directions". J Comp Physiol B. 179 (1): 57–66. doi:10.1007/s00360-008-0289-1. PMID   18597095. S2CID   6546752.
  16. ↑ Conwit, R. A.; Stashuk, D.; Tracy, B.; McHugh, M.; Brown, W. F.; Metter, E. J. (1999). "The relationship of motor unit size, firing rate and force". Clinical Neurophysiology. 110 (7): 1270–1275. doi: 10.1016/s1388-2457(99)00054-1 . PMID   10423192.
  17. ↑ Carlo J DeLuca. Control Properties of Motor Units. J. exp. Biol. 115, 125-136 (1985) \ 25.
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