Dominic Stone
A muscle unit, or motor unit, is a functional unit consisting of a single motor nerve and all the nerve fibres that it innervates via its axon terminals. In other words, a motor unit is made up of a motor neuron and all of its associated muscle fibres. The number of muscle fibres that a single motor unit innervates is called its innervation ratio. The central nervous system controls the force produced by a muscle through motor unit recruitment, which can be either spatial or temporal. The activation of more motor units produces a greater force.
| Characteristics | Values |
|---|---|
| Definition | A motor unit is a functional unit consisting of a single motor nerve and all the nerve fibres that it innervates via its axon terminals. |
| Number of Motor Neurons | Each muscle fibre is innervated by one, and only one, motor neuron. |
| Innervation Ratio | The number of fibres innervated by a motor unit is called its innervation ratio. |
| Number of Muscle Fibres | A single motor neuron can innervate many muscle fibres. |
| Motor Unit Activation | When a motor unit is activated, all of its fibres contract. |
| Motor Unit Types | There are three types of motor units based on the physiological and biochemical properties of the muscle fibres: Slow fatigue-resistant motor units (type I), Fast fatigable motor units (type IIb), and Fast fatigue-resistant (type IIa). |
| Motor Unit Recruitment | 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. |
| Motor Unit Disorders | Disorders that primarily affect the motor neuron or its axon are classified as neurogenic diseases, while those that mainly affect the muscle fibres are called myopathic diseases. |
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What You'll Learn
- A muscle unit is a functional unit consisting of a single motor nerve and its nerve fibres
- The central nervous system controls the force produced by a muscle through motor unit recruitment
- Motor units are recruited in an orderly fashion, with weak inputs resulting in a small force
- The number of muscle fibres innervated by a motor unit is called its innervation ratio
- There are three types of motor units based on the properties of twitch speed, force produced, and fatigability
A muscle unit is a functional unit consisting of a single motor nerve and its nerve fibres
A muscle unit, also known as a motor unit, is a functional unit composed of a single motor nerve and its nerve fibres. It is the smallest unit of force that can be activated to produce movement. Each muscle unit consists of a motor neuron and the muscle fibres it innervates. This means that a single motor neuron can innervate many muscle fibres, while each muscle fibre is innervated by only one motor neuron. The number of muscle fibres innervated by a motor unit is called its innervation ratio.
Motor units are responsible for generating skeletal muscle contractions, coordinating motor behaviours, and forming the basis of motor control. The central nervous system controls the force produced by a muscle through motor unit recruitment in two distinct ways: spatial recruitment and temporal recruitment. Spatial recruitment involves the activation of more motor units to produce a greater force. In contrast, temporal motor unit recruitment, or rate coding, deals with the frequency of muscle fibre contraction activation.
The size principle, also known as Henneman's size principle, states 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 fibres are activated before fast-twitch, high-force, less fatigue-resistant muscle fibres. The type of muscle fibres innervated by a motor unit depends on the muscle's function. For example, muscles requiring fine control or delicate movements tend to have small innervation ratios, while muscles used for coarse movements have high innervation ratios.
The activation of a motor unit causes all of its muscle fibres to contract simultaneously. The speed and force of the contraction depend on the type of muscle fibres innervated by the motor unit. Slow-twitch fibres generate less force but can maintain these levels for extended periods, while fast-twitch fibres produce greater force but fatigue more quickly.
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The central nervous system controls the force produced by a muscle through motor unit recruitment
A muscle unit, or motor unit, is a combination of an individual motor neuron and all of the muscle fibres it innervates. The number of fibres innervated by a motor unit is called its innervation ratio. In most mammals, each muscle fibre is innervated by one motor neuron, but one motor neuron can innervate many muscle fibres.
The central nervous system (CNS) controls the force produced by a muscle through motor unit recruitment. The motor system is divided into different areas that control different aspects of movement, and these areas are located throughout the nervous system. The nervous system uses recruitment as a mechanism to efficiently utilise a skeletal muscle.
Motor unit recruitment can be spatial or temporal. 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 fibres in a single muscle are activated, thus producing the maximum muscle force. Temporal recruitment, or rate coding, deals with the frequency of activation of muscle fibre contractions. The rate code signals the amount of force to be exerted by a muscle. An increase in the rate of action potentials fired by the motor neuron causes an increase in the amount of force that the motor unit generates.
Motor units are recruited in an orderly fashion, beginning with the smallest motor units. This is known as Henneman's size principle, which 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 fibres are activated prior to the recruitment of fast-twitch, high-force, less fatigue-resistant muscle fibres. As more strength is needed, larger motor units are enlisted to activate larger muscle fibres. This increasing activation of motor units produces an increase in muscle contraction known as recruitment.
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Motor units are recruited in an orderly fashion, with weak inputs resulting in a small force
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 fibres 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 fibre contractions. An increase in the rate of action potentials fired by the motor neuron causes an increase in the amount of force that the motor unit generates.
The size principle states that, with increasing strength of input onto motor neurons, smaller motor neurons are recruited and fire action potentials before larger motor neurons are recruited. This is because small motor neurons innervate slow-twitch fibres, which generate less force than fast-twitch fibres but are able to maintain these levels of force for long periods. These fibres are used for maintaining posture and making other low-force movements. Fast-twitch, fatigue-resistant fibres are recruited when the input onto motor neurons is large enough to recruit intermediate-sized motor neurons. These fibres generate more force than slow-twitch fibres, but they are not able to maintain the force as long. Finally, fast-twitch, fatigable fibres are recruited when the largest motor neurons are activated. These fibres produce large amounts of force but fatigue very quickly.
The distribution of motor unit size is such that 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.
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The number of muscle fibres innervated by a motor unit is called its innervation ratio
Motor units are the combination of a single motor neuron and all the muscle fibres it innervates. The number of muscle fibres innervated by a motor unit is called its innervation ratio.
Each individual muscle fibre in a muscle is innervated by one, and only one, motor neuron. However, a single motor neuron can innervate many muscle fibres. Motor units are recruited in an orderly fashion, with weak inputs onto motor neurons resulting in a small force exerted by the muscle. With stronger inputs, more motor neurons are recruited, resulting in more force being applied to the muscle.
The innervation ratio varies depending on the function of the muscle. If a muscle is required for fine control or delicate movements, such as the movement of the fingers or hands, its motor units will tend to have small innervation ratios. This means that each motor neuron will innervate a small number of muscle fibres (10-100), allowing for many nuances of movement of the entire muscle. On the other hand, if a muscle is required only for coarse movements, such as a thigh muscle, its motor units will tend to have a high innervation ratio, with each motor neuron innervating 1000 or more muscle fibres.
The innervation ratio also varies between different muscle groups. For example, the soleus, a muscle important for posture that comprises mostly small, slow units, has an average innervation ratio of 180 muscle fibres for each motor neuron. In contrast, the gastrocnemius, a muscle that comprises both small and larger units, has an innervation ratio of 1000–2000 muscle fibres per motor neuron, allowing it to generate forces needed for sudden changes in body position.
The innervation ratio can also differ in athletes with different training regimens. For example, sprinters have a larger proportion of powerful but rapidly fatiguing pale fibres in their legs compared to marathoners. Additionally, extraocular muscle motor units, which are involved in rapid and precise eye movements, have an extremely small innervation ratio of only 3.
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There are three types of motor units based on the properties of twitch speed, force produced, and fatigability
A "motor unit" is defined as a single motor neuron and all of the muscle fibres it innervates. In mammals, each muscle fibre is innervated by only one motor neuron, but one motor neuron innervates many muscle fibres as its axon branches in the muscle. The number of fibres innervated by a motor unit is called its innervation ratio.
Motor units cluster into three basic types based on the three physiological properties of twitch speed, the amount of force produced, and fatigability. Type S motor units are defined by the properties of slow twitch, small amounts of force, and high resistance to fatigue. Type FR motor units are defined by fast twitch, moderate force, and moderate resistance to fatigue. Type FF motor units are defined by fast twitch, high force, and low resistance to fatigue.
Small motor neurons innervate slow-twitch fibres, which generate less force than fast-twitch fibres but are able to maintain these levels of force for long periods. These fibres are used for maintaining posture and making other low-force movements. Intermediate-sized motor neurons innervate fast-twitch, fatigue-resistant fibres, which are recruited when the input onto motor neurons is large enough. Large motor neurons innervate fast-twitch, fatigable muscle fibres.
The central nervous system uses recruitment as a mechanism to efficiently utilise a skeletal muscle. When necessary, the maximal number of motor units in a muscle can be recruited simultaneously, producing the maximum force of contraction for that muscle. However, this cannot last for very long because of the energy requirements to sustain the contraction. To prevent complete muscle fatigue, some motor units rest while others are active, allowing for longer muscle contractions.
Fatigue can be divided into two broad categories: central fatigue and peripheral fatigue. Central fatigue describes the uncomfortable feelings that come from being tired, while peripheral fatigue occurs when the muscle can no longer contract optimally.
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Frequently asked questions
A muscle unit, or motor unit, is a functional unit consisting of a single motor nerve and all the nerve fibres it stimulates.
Each individual muscle fibre in a muscle is stimulated by one, and only one, motor neuron. However, a single motor neuron can stimulate many muscle fibres.
The central nervous system controls the force produced by a muscle through motor unit recruitment. Spatial recruitment is the activation of more motor units to produce greater force. Temporal motor unit recruitment, or rate coding, deals with the frequency of activation of muscle fibre contractions.
The size principle states 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 fibres are activated before fast-twitch, high-force, less fatigue-resistant muscle fibres.
Motor units cluster into three basic types based on the three physiological properties of twitch speed, the amount of force produced, and fatigability: Type S, Type FR, and Type FF.
