To install click the Add extension button. That's it.

The source code for the WIKI 2 extension is being checked by specialists of the Mozilla Foundation, Google, and Apple. You could also do it yourself at any point in time.

How to transfigure the Wikipedia

Would you like Wikipedia to always look as professional and up-to-date? We have created a browser extension. It will enhance any encyclopedic page you visit with the magic of the WIKI 2 technology.

Try it — you can delete it anytime.

Install in 5 seconds
4,5
Kelly Slayton
Congratulations on this excellent venture… what a great idea!
Alexander Grigorievskiy
I use WIKI 2 every day and almost forgot how the original Wikipedia looks like.
Live Statistics
English Articles
Improved in 24 Hours
Added in 24 Hours
What we do. Every page goes through several hundred of perfecting techniques; in live mode. Quite the same Wikipedia. Just better.
Great Wikipedia has got greater.
.
Leo
Newton
Brights
Milds

Motor unit

From Wikipedia, the free encyclopedia

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]

All muscle fibers in a motor unit are of the same fiber type[citation needed]. 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.

YouTube Encyclopedic

  • 1/5
    Views:
    351 478
    46 968
    10 884
    3 410
    49 584
  • Motor unit | Organ Systems | MCAT | Khan Academy
  • The Motor Unit
  • Recruitment of Small and Large Motor Units
  • Motor unit: motor neurons and skeletal muscle fibers (preview) - Human Histology | Kenhub
  • What is a Motor Unit?

Transcription

Voiceover: In this video I'm going to talk about the motor unit. The motor unit is made up of a couple of parts. The first part are what are called lower motor neurons. These are efferent neurons of the peripheral nervous system, meaning that they're carrying information away from the central nervous system. These efferent neurons synapse on and control skeletal muscle. Let me just draw a red outline here for some of the skeletal muscle in the thigh. Skeletal muscle is the main muscle type of our body. It's all over our body, and mostly connected to our skeleton, to move us around. The neurons of the nervous system that tell skeletal muscle when to contract are the lower motor neurons. The term motor unit refers to one lower motor neuron. Let me just draw a soma, and an axon coming out of this lower motor neuron. I'm going to draw this one just having two axon terminals, although they can have lots of axon terminals, but this one we'll just say has two. We'll say that this motor neuron is contacting just two skeletal muscle cells. Let me just draw these two little red tubes to represent two skeletal muscle cells, that are being contacted by this lower motor neuron, so that these skeletal muscle cells- Let me just write that out. These skeletal muscle cells are the other part of the motor unit. So the motor unit is one lower motor neuron, and all the skeletal muscle cells that it contacts and controls. The place where a neuron contacts it's target cell is called a synapse, but this synapse between a lower motor neuron and a skeletal muscle cell has a special name. That special name is the neuromuscular junction. "Neuro" for the neuron, and "muscular" for the muscle cell. So, neuromuscular junction is the synapse between a lower motor neuron and a skeletal muscle cell, and lower motor neurons will usually synapse with multiple skeletal muscle cells so they'll have multiple neuromuscular junctions. All of this is the motor unit. The reason we call it a unit is that usually, when a lower motor neuron fires an action potential, it causes all of the skeletal muscle cells in its unit to contract, so that instead of these cells doing different things at different times, usually they function as a unit. All of them are activated together. The somas of the motor neurons are in the spinal cord like I've drawn here, or they're up in the brain stem, and then their axons will pass out in the cranial nerves, if they pass out through the skull, or the spinal nerves if they pass out through the spine. The axons will continue through little branches of nerves in the peripheral nervous system, until they reach and synapse on all of the skeletal muscle cells in their motor unit. So, lower motor neurons in the cranial nerves, primarily control the skeletal muscles of the head and the neck, and the lower motor neurons of the spinal cord, primarily control all of the skeletal muscle cells in the limbs and the trunk. Small muscles that need rapid precise control, like those that move the eyes, or those that move the fingers- Let me just draw a little muscle here in the hand, to represent muscles that move the fingers- These muscles tend to have small motor units. They're more like what I've drawn here, where a lower motor neuron is synapsing on just a small number of skeletal muscle cells. Large muscles, that do not need rapid precise control, like those muscles in the trunk, and those muscles in the limbs, like these big muscles in the thigh here, usually have large motor units, with each lower motor neuron synapsing on a large number of skeletal muscle cells. Let me just draw a little bigger group of skeletal muscle cells here. I'll put them right next to each other. There could actually be many hundreds of individual skeletal muscle cells in a single muscle unit, in some of these big muscles of the limb or the trunk. Then the lower motor neuron for that motor unit would have lots more axon terminals, that'll form neuromuscular junctions, these connections with all of these skeletal muscle cells, in the large motor unit of a larger muscle. A number of things can happen, with any kind of abnormality of the motor unit. One abnormality that we could see is weakness, or loss of strength of contraction of skeletal muscle. Problems of other parts of the nervous system can also cause weakness, and we'll get into some of that in later videos. Abnormalities of the lower motor neuron specifically, in addition to potentially causing weakness, can cause several other changes, that are called the lower motor neuron signs. I'll just write LMN for lower motor neurons. The lower motor neuron signs can happen in addition to weakness, if there's some abnormality of these lower motor neurons. The first lower motor neuron sign is atrophy of skeletal muscle. Atrophy means decreased bulk of skeletal muscle, so decreased size. Here's a photograph of a person who has a lower motor neuron abnormality causing atrophy. In this person, they had lower motor neurons coming down here through the wrist that were enervating skeletal myocytes in this part of the hand. They had it on both sides, just like we all do, but then they had some kind of abnormality here in the wrist that caused a problem with these nerves passing through here, and injured these lower motor neurons heading towards these muscles in the hands. If you look at these particular muscles in this part of the hand, they have shrunk, they have shriveled up and kind of wasted away. We call that atrophy of those skeletal muscles. The next lower motor neuron sign is called fasciculations. I can't actually draw these, because what these are are twitches, involuntary twitches of skeletal muscle, that can occur after some problem of the lower motor neurons. So like in this person, if we looked at these areas with atrophy of skeletal muscle, we would see little twitches of the muscle that we could see through the skin. The occasional fasciculation is normal, everybody gets a little bit of a muscle twitch here and there, now and then. But with abnormalities of the lower motor neurons, whichever muscles are effected will often have lots of twitching going on for a very long period of time, just in those muscles that are affected. It's not moving around all sorts of different muscles, unless there's problems in lower motor neurons, all over their body. Fasciculations aren't specific to problems with the lower motor neurons, but if we see a lot of them in one spot, than that suggests there could be a problem with those lower motor neurons. The next lower motor neuron sign is called hypotonia which means a decrease in the tone of skeletal muscle. The tone of skeletal muscle refers to how much the muscle is contracted when a person is trying to relax it, because our muscles are always just a little bit contracted, even when we're not trying to contract them. So for example, let's say that this doctor here tells this patient to relax their leg, to go as relaxed as they can, and go relaxed and floppy, like a wet noodle. Then the doctor here started moving the patient's leg for them, they started bending and unbending their knee. The doctor will feel a little bit of resistance, a little bit of tone of the muscles of the leg, even if this person is trying to relax as best they can. But if there's a problem with the lower motor neurons so that that the lower motor neurons aren't telling the skeletal muscles cells to contract as much, then there won't be as much tone. There'll be hypotonia, and the doctor will be able to feel that the leg is kind of floppy, there isn't as much tone when a person is trying to relax it. Another lower motor neuron sign is called hyporeflexia This refers to decreased muscle stretch reflexes. I'm just going to write MSR for muscle stretch reflexes. This is a reflex that happens if you rapidly stretch a skeletal muscle, like if you hit the tendon of the muscle with a little rubber hammer, like this doctor is doing to this patient right here. I'm going to do a different video on the muscle stretch reflexes, so I'll come back to that, and in that video, we'll talk about why the reflexes can decrease with problems of the lower motor neurons, because that one we understand pretty well. These other three, we don't understand why they happen quite as well. We don't know why you get atrophy if there's problems with the lower motor neurons, but for some reason, if the skeletal muscle cells aren't getting periodically stimulated by lower motor neurons, these muscle cells degenerate, they actually shrink, or they're lost we actually can lose skeletal muscle cells, if we lose the lower motor neurons. We also don't understand why fasciculations occur, but apparently with loss of periodic input from lower motor neurons, some skeletal muscle cells will just start contracting on their own, without being told to do so. Hypotonia is probably just because less skeletal muscle cells are being told to contract in general, but we're not totally sure about that either.

Recruitment (vertebrate)

The central nervous system is responsible for the orderly recruitment of motor neurons, beginning with the smallest motor units.[3] 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.[4]

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.[5] 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.[6] 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.[7]

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).[8] 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:

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)[11] — 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.
    • Immunohistochemical (a more recent form of fiber typing)[12]
      • Myosin Heavy Chain (MHC)
      • Myosin Light Chain — alkali (MLC1)
      • Myosin Light Chain — regulatory (MLC2)
The Immunohistochemical types are as follows, with the type IIa, IIb and slow corresponding to IIa, IIb and slow (type I) histochemical types:
Expressed in
Gene family Developing Fast fibers (II) Slow fibers(I)
MHC Embryonic MHC MHC IIa β/slow MHC
Neonatal MHC MHC IIb
MHC IIx
MLC1 (alkali) Embryonic 1f 1s
1f 3f
MLC2 (regulatory) 2f 2f 2s
Table reproduced from[12]
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.[13] The relative expression of these myosin types is determined partly by genetics and partly by other biological factors such as activity, innervation and hormones.[14]

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:

Muscle Number of Motor Axons Number of Muscle Fibers Innervation Ratio Reference
Biceps 774 580,000 750 Buchtal, 1961
Brachioradialis 315 129,200 > 410 Feinstein et al.[15]
First dorsal interosseous 119 40,500 340 Feinstein et al.[15]
Medial gastrocnemius 579 946,000 1,634 Feinstein et al.[15]
Tibialis anterior 445 292,500 657 Feinstein et al.[15]

Table reproduced from Karpati (2010)[16]

See also

References

  • Altshuler, Douglas; K. Welch; B. Cho; D. Welch; A. Lin; W. Dickinson; M. Dickinson (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.
  1. ^ a b 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. ^ 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.
  4. ^ 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.
  5. ^ 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.
  6. ^ 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.
  7. ^  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.
  8. ^ 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.
  9. ^ a b 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.
  10. ^ 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.
  11. ^ 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.
  12. ^ a b 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.
  13. ^ a b 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.
  14. ^ 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.
  15. ^ a b c d 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.
  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.

External links

This page was last edited on 8 June 2024, at 15:47
Basis of this page is in Wikipedia. Text is available under the CC BY-SA 3.0 Unported License. Non-text media are available under their specified licenses. Wikipedia® is a registered trademark of the Wikimedia Foundation, Inc. WIKI 2 is an independent company and has no affiliation with Wikimedia Foundation.
Contact WIKI 2
Introduction
Terms of Service
Privacy Policy