Motor unit

Last updated

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. [1] 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. [2]

Contents

Usually muscle fibers in a motor unit are of the same fiber type. [3] When a motor unit is activated, all of its fibers contract. In vertebrates, the force of a muscle contraction is controlled by the number of activated motor units.

The number of muscle fibers within each unit can vary within a particular muscle and even more from muscle to muscle: the muscles that act on the largest body masses have motor units that contain more muscle fibers, whereas smaller muscles contain fewer muscle fibers in each motor unit. [1] For instance, thigh muscles can have a thousand fibers in each unit, while extraocular muscles might have ten. Muscles which possess more motor units (and thus have greater individual motor neuron innervation) are able to control force output more finely.

Motor units are organized slightly differently in invertebrates: 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 in vertebrates the force of contraction of muscles is regulated by how many motor units are activated, in invertebrates it is controlled by regulating the balance between excitatory and inhibitory signals.

Recruitment (vertebrate)

The central nervous system is responsible for the orderly recruitment of motor neurons, beginning with the smallest motor units. [4] 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. [5]

The central nervous system has two distinct ways of controlling the force produced by a muscle through motor unit recruitment: spatial recruitment and temporal recruitment. Spatial recruitment is the activation of more motor units to produce a greater force. Larger motor units contract along with small motor units until all muscle fibers in a single muscle are activated, thus producing the maximum muscle force. Temporal motor unit recruitment, or rate coding, deals with the frequency of activation of muscle fiber contractions. Consecutive stimulation on the motor unit fibers from the alpha motor neuron causes the muscle to twitch more frequently until the twitches "fuse" temporally. This produces a greater force than singular contractions by decreasing the interval between stimulations to produce a larger force with the same number of motor units.

Using electromyography (EMG), the neural strategies of muscle activation can be measured. [6] Ramp-force threshold refers to an index of motor neuron size in order to test the size principle. This is tested 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) tend to be small motor units, while high-threshold units are recruited when higher forces are needed and involve larger motor neurons. [7] These tend to have shorter contraction times than the smaller units. The number of additional motor units recruited during a given increment of force declines sharply at high levels of voluntary force. This suggests that, even though high threshold units generate more tension, the contribution of recruitment to increase voluntary force declines at higher force levels.

When necessary, the maximal number of motor units in a muscle can be recruited simultaneously, producing the maximum force of contraction for that muscle, but this cannot last for very long because of the energy requirements to sustain the contraction. To prevent complete muscle fatigue, motor units are generally not all simultaneously active, but instead some motor units rest while others are active, which allows for longer muscle contractions. The nervous system uses recruitment as a mechanism to efficiently utilize a skeletal muscle. [8]

To test motor unit stimulation, electrodes are placed extracellularly on the skin and an intramuscular stimulation is applied. 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). [9] 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 are generally categorized based upon the similarities between several factors:

Expressed in
Gene familyDevelopingFast fibers (II)Slow fibers(I)
MHC Embryonic MHCMHC IIaβ/slow MHC
Neonatal MHCMHC IIb
MHC IIx
MLC1 (alkali)Embryonic1f1s
1f3f
MLC2 (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. [14] The relative expression of these myosin types is determined partly by genetics and partly by other biological factors such as activity, innervation and hormones. [15]

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 myosin 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: [16]

MuscleNumber of Motor AxonsNumber of Muscle FibersInnervation RatioReference
Biceps 774580,000750Buchtal, 1961
Brachioradialis 315129,200> 410Feinstein et al. [17]
First dorsal interosseous 11940,500340Feinstein et al. [17]
Medial gastrocnemius 579946,0001,634Feinstein et al. [17]
Tibialis anterior 445292,500657Feinstein et al. [17]

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.

The muscular system is an organ system consisting of skeletal, smooth, and cardiac muscle. It permits movement of the body, maintains posture, and circulates blood throughout the body. The muscular systems in vertebrates are controlled through the nervous system although some muscles can be completely autonomous. Together with the skeletal system in the human, it forms the musculoskeletal system, which is responsible for the movement of the body.

<span class="mw-page-title-main">Smooth muscle</span> Involuntary non-striated muscle

Smoothmuscle is one of the three major types of vertebrate muscle tissue, the others being skeletal and cardiac muscle. It can also be found in invertebrates and is controlled by the autonomic nervous system. It is non-striated, so-called because it has no sarcomeres and therefore no striations. It can be divided into two subgroups, single-unit and multi-unit smooth muscle. Within single-unit muscle, the whole bundle or sheet of smooth muscle cells contracts as a syncytium.

<span class="mw-page-title-main">Skeletal muscle</span> One of three major types of muscle

Skeletal muscle is one of the three types of vertebrate muscle tissue, the other being cardiac muscle and smooth muscle. They are part of the voluntary muscular system and typically are attached by tendons to bones of a skeleton. The skeletal muscle cells are much longer than in the other types of muscle tissue, and are also known as muscle fibers. The tissue of a skeletal muscle is striated – having a striped appearance due to the arrangement of the sarcomeres.

<span class="mw-page-title-main">Tetanic contraction</span> Type of muscle contraction

A tetanic contraction is a sustained muscle contraction evoked when the motor nerve that innervates a skeletal muscle emits action potentials at a very high rate. During this state, a motor unit has been maximally stimulated by its motor neuron and remains that way for some time. This occurs when a muscle's motor unit is stimulated by multiple impulses at a sufficiently high frequency. Each stimulus causes a twitch. If stimuli are delivered slowly enough, the tension in the muscle will relax between successive twitches. If stimuli are delivered at high frequency, the twitches will overlap, resulting in tetanic contraction. A tetanic contraction can be either unfused (incomplete) or fused (complete). An unfused tetanus is when the muscle fibers do not completely relax before the next stimulus because they are being stimulated at a fast rate; however there is a partial relaxation of the muscle fibers between the twitches. Fused tetanus is when there is no relaxation of the muscle fibers between stimuli and it occurs during a high rate of stimulation. A fused tetanic contraction is the strongest single-unit twitch in contraction. When tetanized, the contracting tension in the muscle remains constant in a steady state. This is the maximal possible contraction. During tetanic contractions, muscles can shorten, lengthen or remain constant length.

Weakness is a symptom of many different medical conditions. The causes are many and can be divided into conditions that have true or perceived muscle weakness. True muscle weakness is a primary symptom of a variety of skeletal muscle diseases, including muscular dystrophy and inflammatory myopathy. It occurs in neuromuscular junction disorders, such as myasthenia gravis.

<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.

<span class="mw-page-title-main">Striated muscle tissue</span> Muscle tissue with repeating functional units called sarcomeres

Striated muscle tissue is a muscle tissue that features repeating functional units called sarcomeres. The presence of sarcomeres manifests as a series of bands visible along the muscle fibers, which is responsible for the striated appearance observed in microscopic images of this tissue. There are two types of striated muscle:

Muscle fatigue is when muscles that were initially generating a normal amount of force, then experience a declining ability 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.

<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.

<span class="mw-page-title-main">Gamma motor neuron</span> Lower motor neuron involved in muscle contraction

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.

<span class="mw-page-title-main">Motor unit recruitment</span> Additional activation of motor units to increase contractile strength

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 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 as contraction increases. This is known as Henneman's size principle.

Muscle weakness is a lack of muscle strength. Its causes are many and can be divided into conditions that have either true or perceived muscle weakness. True muscle weakness is a primary symptom of a variety of skeletal muscle diseases, including muscular dystrophy and inflammatory myopathy. It occurs in neuromuscular junction disorders, such as myasthenia gravis. Muscle weakness can also be caused by low levels of potassium and other electrolytes within muscle cells. It can be temporary or long-lasting. The term myasthenia is from my- from Greek μυο meaning "muscle" + -asthenia ἀσθένεια meaning "weakness".

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

Muscle is a soft tissue, one of the four basic types of animal tissue. 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 interact 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.

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.

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.

Even before the very beginning of human space exploration, serious and reasonable concerns were expressed about exposure of humans to the microgravity of space due to the potential systemic effects on terrestrially evolved life forms adapted to Earth gravity. Unloading of skeletal muscle, both on Earth via bed-rest experiments and during spaceflight, result in remodeling of muscle. As a result, decrements occur in skeletal muscle strength, fatigue resistance, motor performance, and connective tissue integrity. In addition, there are cardiopulmonary and vascular changes, including a significant decrease in red blood cell mass, that affect skeletal muscle function. This normal adaptive response to the microgravity environment may become a liability resulting in increased risk of an inability or decreased efficiency in crewmember performance of physically demanding tasks during extravehicular activity (EVA) or upon return to Earth.

References

  1. 1 2 Buchtal, F; H. Schmalbruch (1 January 1980). "Motor Unit of Mammalian Muscle". Physiological Reviews. 60 (1): 90–142. doi:10.1152/physrev.1980.60.1.90. PMID   6766557.
  2. ↑ Kandel, Eric (2013). Principles of Neural Science, 5th ed. McGraw-Hill, New York. p. 768. ISBN   978-0-07-139011-8.
  3. ↑ Purves, Dale; Augustine, George J.; Fitzpatrick, David; Katz, Lawrence C.; LaMantia, Anthony-Samuel; McNamara, James O.; Williams, S. Mark (2001), "The Motor Unit", Neuroscience. 2nd edition, Sinauer Associates, retrieved 2024-11-10
  4. ↑ Milner-Brown HS, Stein RB, Yemm R (September 1973). "The orderly recruitment of human motor units during voluntary isometric contractions". J. Physiol. 230 (2): 359–70. doi:10.1113/jphysiol.1973.sp010192. PMC   1350367 . PMID   4350770.
  5. ↑ Robinson R (February 2009). "In mammalian muscle, axonal wiring takes surprising paths". PLOS Biol. 7 (2): e1000050. doi: 10.1371/journal.pbio.1000050 . PMC   2637923 . PMID   20076726.
  6. ↑ Farina, Dario; Merletti R; Enoka R.M. (2004). "The extraction of neural strategies from the surface EMG". Journal of Applied Physiology. 96 (4): 1486–1495. doi:10.1152/japplphysiol.01070.2003. PMID   15016793.
  7. ↑ Spiegel KM.; Stratton J.; Burke JR.; Glendinning DS; Enoka RM (November 2012). "The influence of age on the assessment of motor unit activation in a human hand muscle". Experimental Physiology. 81 (5): 805–819. doi: 10.1113/expphysiol.1996.sp003978 . PMID   8889479. S2CID   29034955.
  8. ↑ Creative Commons by small.svg  This article incorporates text available under the CC BY 4.0 license.Betts, J Gordon; Desaix, Peter; Johnson, Eddie; Johnson, Jody E; Korol, Oksana; Kruse, Dean; Poe, Brandon; Wise, James; Womble, Mark D; Young, Kelly A (May 14, 2023). Anatomy & Physiology. Houston: OpenStax CNX. 10.3 Muscle Fiber Contraction and Relaxation. ISBN   978-1-947172-04-3.
  9. ↑ De Luca, Carlo; William J. Forrest (December 1972). "Some Properties of Motor Unit Action Potential Trains Recorded during Constant Force Isometric Contractions in Man". Kybernetik. 12 (3): 160–168. doi:10.1007/bf00289169. PMID   4712973. S2CID   11373497.
  10. 1 2 Burke RE, Levine DN, Tsairis P, Zajac FE (November 1973). "Physiological types and histochemical profiles in motor units of the cat gastrocnemius". J. Physiol. 234 (3): 723–48. doi:10.1113/jphysiol.1973.sp010369. PMC   1350696 . PMID   4148752.
  11. ↑ Collatos TC, Edgerton VR, Smith JL, Botterman BR (November 1977). "Contractile properties and fiber type compositions of flexors and extensors of elbow joint in cat: implications for motor control". J. Neurophysiol. 40 (6): 1292–300. doi:10.1152/jn.1977.40.6.1292. PMID   925731.
  12. ↑ Altshuler D.; Welch K.; Cho B.; Welch D.; Lin A.; Dickinson W.; Dickinson M. (April 2010). "Neuromuscular control of wingbeat kinematics in Annas hummingbirds". The Journal of Experimental Biology. 213 (Pt 14): 2507–2514. doi:10.1242/jeb.043497. PMC   2892424 . PMID   20581280.
  13. ↑ Schiaffino S, Reggiani C (August 1994). "Myosin isoforms in mammalian skeletal muscle". J. Appl. Physiol. 77 (2): 493–501. doi:10.1152/jappl.1994.77.2.493. PMID   8002492.
  14. 1 2 Caiozzo VJ, Baker MJ, Huang K, Chou H, Wu YZ, Baldwin KM (September 2003). "Single-fiber myosin heavy chain polymorphism: how many patterns and what proportions?". Am. J. Physiol. Regul. Integr. Comp. Physiol. 285 (3): R570–80. doi:10.1152/ajpregu.00646.2002. PMID   12738613. S2CID   860317.
  15. ↑ Baldwin KM, Haddad F (January 2001). "Effects of different activity and inactivity paradigms on myosin heavy chain gene expression in striated muscle". J. Appl. Physiol. 90 (1): 345–57. doi:10.1152/jappl.2001.90.1.345. PMID   11133928. S2CID   9677583.
  16. ↑ Karpati, George (2010). Disorders of Voluntary Muscle (PDF). Cambridge University Press. p. 7. ISBN   9780521876292. referenced Feinstein, B; Lindegard, B; Nyman, E; Wohlfart, G (1955). "Morphologic studies of motor units in normal human muscles". Acta Anat (Basel). 23 (2): 127–142. doi:10.1159/000140989. PMID   14349537.
  17. 1 2 3 4 Feinstein, Bertram; Lindegård, Bengt; Nyman, Eberhard; Wohlfart, Gunnar (2008-06-18). "Morphologic Studies of Motor Units in Normal Human Muscles". Acta Anatomica. 23 (2): 127–142. doi:10.1159/000140989. ISSN   0001-5180. PMID   14349537.
This page is based on this Wikipedia article
Text is available under the CC BY-SA 4.0 license; additional terms may apply.
Images, videos and audio are available under their respective licenses.