하나의 운동단위에 수천개의 근섬유 지배.

[문형철]

올바른 상상력을 위해..

상완이두근에 58만개의 근섬유가 있다

상완이두근에 750개의 motor neuron이 있다

상완이두근에는 774개의 액손이 있다. 

상완이두근에는 58만개의 근내막(결합조직 보자기가 있다) - 인체에는 600개의 근육이 있다. 근내막 결합조직의 숫자를 추정하면 수천만에서 수억개가 있다.

하나의 운동뉴런은 수천개의 근섬유를 지배한다. 눈근육의 경우 10개의 근섬유지배

작은 움직임에서 큰 힘이 올바로 동원된다는 의미...

운동단위크기가 작은 것부터 동원되고 작은 힘, 느린연축(slow twitch), 피로저항 근육(fatigue-resistant muscle fibers)이 먼저 되고, 차례로 큰힘, 빠른 연축, 속근이 동원된다. 

A motor unit is made up of a motor neuron and the skeletal muscle fibers innervated by that motor neuron's axonal terminals.^[1] 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.

- 운동단위는 운동뉴런과 뉴런의 축삭끝에 의해 신경지배되고 있는 골격근섬유가 만듬. 

All muscle fibres in a motor unit are of the same fibre type. When a motor unit is activated, all of its fibres contract. In vertebrates, the force of a muscle contraction is controlled by the number of activated motor units. 

- 운동단위에서 모든 근육들은 같은 섬유타입임. 운동단위가 활성화될때, 모든 근육섬유가 수축함. 척추동물에서 근수축의 힘은 활성화된 운동단위의 숫자에 의해서 조절됨. 

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.

- 각 운동단위내 근육섬유들의 숫자는 다양함. 

- 각 운동단위에서 큰근육은 좀더 많은 근섬유를 가지고, 작은 근육은 적은 근섬유를 가짐. 

- 예를들어 대퇴근육은 각 운동단위당 천여개의 근섬유를 가질 수 있음. 반면에 눈근육은 10개의 근섬유를 가짐. 

- 좀더 많은 운동단위를 가지는 근육은 세밀함보다는 강한 힘을 조절할 수 있음. 

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.

- 척추동물에서 운동단위는 약간 다르게 조직됨. 각 근육은 10개이하의 운동단위를 가지고, 각 근섬유는 흥분과 억제 뉴런을 포함한 multiple 뉴런에 의해서 신경지배됨. 그래서 척추동물에서 근육의 수축힘은 얼마나 많은 운동단위가 활성화되는냐에 의해 조절됨. 

Contents

  [hide] 

• 1Recruitment (vertebrate)
• 2Motor unit types (vertebrate)
• 3See also
• 4References
• 5External links

Recruitment (vertebrate)[edit]

The central nervous system is responsible for the orderly recruitment of motor neurons, beginning with the smallest motor units.^[2] 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.^[3]

- 중추신경은 작은 운동단위부터  규칙적으로 운동단위의 동원을 책임맡음. 

- 헨네만의 크기 원리는 운동단위는 작은 것부터 큰 것으로 동원하는데, 부하의 크기에 기초함. 

- 작은 부하는 작은 힘, 느린 연축, 낮은 힘, 피로저항 근육을 먼저 동원함. 

- 큰 운동단위는 전형적으로 큰힘을 내는 빠른 근섬유를 포함함. 

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 fibres 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.

- 중추신경은 근육을 두가지 방식으로 동원하는데, 공간 동원과 시간동원이 있음. 

Using electromyography (EMG), the neural strategies of muscle activation can be measured.^[4] 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.^[5] 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.

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).^[6] In medical electrodiagnostic testing for a patient with weakness, careful analysis of the MUAP size, shape, and recruitment pattern can help in distinguishing a myopathyfrom a neuropathy.

Motor unit types (vertebrate)[edit]

Motor units are generally categorized based upon the similarities between several factors:

• Physiological^[7]
  • Contraction speed in Isometric contractions
    • Rate of rise of force
    • Time to peak of a twitch contraction (response to a single nerve impulse)

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.

• Biochemical
  • Histochemical (the oldest form of biochemical fiber typing)^[7][8]
    • Glycolytic enzyme activity (e.g. glycerophosphate dehydrogenase (GPD))
    • Oxidative enzyme activity (e.g. succinate dehydrogenase -SDH )
    • Sensitivity of Myosin ATPase to acid and alkali

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)^[9] — 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)^[10]
    • 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^[10]

• • Gene characterization of myosins^[11]

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.^[11] The relative expression‎ of these myosin types is determined partly by genetics and partly by other biological factors such as activity, innervation and hormones.^[12]

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
Biceps	774	580,000	750	Buchtal, 1961
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^[13]

See also

운동단위 숫자 측정원리

Motor Unit Number Estimation (MUNE) is a technique that uses electromyography to estimate the number of motor units in a muscle.

Principles[edit]

A motor unit consists of one alpha motor neuron and all the muscle fibres it innervates.

Muscles differ in the number of motor units that they contain, and how many muscle fibres are within each unit (innervation ratio). In a general sense, muscles that require specificity of movement, such as muscles in charge of eye movement, have fewer fibres per unit, while those that are meant for less specific tasks, such as the calf muscles in charge of jumping, have more.

MUNE uses a general formula of: Number of motor units = compound muscle action potential size divided by the mean surface-detected motor unit action potential size

The compound muscle action potential (CMAP) size is found using supramaximal stimulation of the motor nerve to the muscle or muscle group (similar to a nerve conduction study). It is recorded using surface electrodes. This is representative of the sum of the surface detected motor unit action potentials from muscles innervated by that nerve.

Surface-detected motor unit action potential (SMUAP) size is the contribution of individual motor units. The way of finding the average size of these action potentials depends on the method used, as described below.

Methods[edit]

There are at least six techniques that are currently in use to estimate motor unit numbers. These include incremental stimulation, multi-point stimulation method, F-response method, spike-triggered averaging method and the statistical method. Incremental stimulation is the most illustrative of the concept, and so will be discussed here.

According to Henneman's size principle, motor unit recruitment is always in the same order from smallest to largest motor unit. Additionally, the motor unit action potential is an all-or-none phenomenon - once the recruitment threshold (the stimulus intensity at which a motor unit begins to fire) is reached, it fires fully. Incremental stimulation starts giving electrical stimulation at a very low stimulus intensity and increases gradually to reach the recruitment threshold of successively larger motor units until the intensity of the CMAP is reached. A 'step' is noted when an increase in stimulus leads to an increase in recorded EMG (i.e. another motor unit's threshold is reached and it is recruited). The CMAP is then divided by the number of steps required to reach the intensity of the CMAP to get a mean SMUAP size. The number of steps does not correlate to the total number of motor units in the muscle. Instead, the CMAP size is then divided by the mean SMUAP size to get an estimation of the number of motor units in the muscle.

Uses[edit]

The number of motor units per muscle can change due to aging, disease, or injury. These techniques are used to diagnose disease or monitor the effects of aging, disease and injury over time. In neuropathies, motoneurons die off, reducing the number of motor units progressively. In myopathies the size of the motor units is reduced because of the death of motor fibres, but the number of motor units remains the same until the disease progresses to a very severe state. In collaboration with other electromyography techniques, these conditions can be diagnosed and monitored. In a similar vein, normal aging also reduces the number of motor units but not to the same degree as disease. The effects of injury depend on the circumstances.

[문형철]

상완이두근에 58만개의 근섬유가 있다
상완이두근에 750개의 motor neuron이 있다
상완이두근에는 774개의 액손이 있다.

상완이두근에는 58만개의 근내막(결합조직 보자기가 있다) - 인체에는 600개의 근육이 있다. 근내막 결합조직의 숫자를 추정하면 수천만에서 수억개가 있다.

하나의 운동뉴런은 수천개의 근섬유를 지배한다. 눈근육의 경우 10개의 근섬유지배

작은 움직임에서 큰 힘이 올바로 동원된다는 의미...
운동단위크기가 작은 것부터 동원되고 작은 힘, 느린연축(slow twitch), 피로저항 근육(fatigue-resistant muscle fibers)이 먼저 되고, 차례로 큰힘, 빠른 연축, 속근이 동원된다.