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The human body is composed of about 700 named muscles, which make up roughly half of a person's body weight. These muscles are controlled by the nervous system, which sends signals to the muscles to contract or relax. During early muscle development, myofiber type differentiation is autonomous and does not require neural input. However, in some cases, muscles can be innervated by segmentally inappropriate neurons, leading to a mismatch in muscle innervation. This phenomenon has been observed in embryonic chick hindlimb muscles, specifically in the posterior iliotibialis (pITIB) and the iliofibularis (IFIB) muscles. The electrical properties of adult motor neurons are typically well-matched to the contractile properties of the fast or slow muscle fibers they innervate, but the developmental process that leads to this precise matching is not yet fully understood.
| Characteristics | Values |
|---|---|
| Muscle innervation in non-vertebrates | Nerve terminals on muscle fibres form terminal buttons spread along the muscle fibre |
| Muscle innervation in vertebrates | A motor nerve ends in one button localized near the middle of the fibre |
| Excitatory responses to motor nerve action potentials (APs) | Trigger local APs or full-size APs along the muscle fibre |
| Invertebrate muscle innervations | A single myofibre can be innervated by more than one motor neuron |
| Inhibitory neurotransmitter | GABA (γ-aminobutyric acid) |
| Muscle stretch receptors | Located in crayfish |
| Abdominal muscles innervation | Anterior rami of the thoracic and lumbar spinal nerves |
| Chick muscle innervation | Foreign motoneuron pools exhibited considerable selectivity when innervating the pITIB and IFIB muscles |
| Smooth muscle innervation | Sympathetic nervous system, parasympathetic stimulation, and enteric nervous system |
| Smooth muscle contraction | Dependent on calcium influx |
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What You'll Learn
Motor neurons and muscle receptors
Motor neurons are clustered in columnar, spinal nuclei called motor neuron pools or motor nuclei. Each individual muscle fibre in a muscle is innervated by one and only one motor neuron, but a single motor neuron can innervate many muscle fibres. The combination of an individual motor neuron and all the muscle fibres that it innervates is called a motor unit. The number of fibres innervated by a motor unit is called its innervation ratio. If a muscle is required for fine control or delicate movements, its motor units will tend to have small innervation ratios. For example, the muscles in your neck, arm and shoulder that are used to reach for a book on a shelf.
Motor neurons use a rate code to signal 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. The rate code is one of two principles that govern the relationship between motor neuron activity and muscle force.
Different types of muscle fibres are innervated by small and large motor neurons. 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. Intermediate-sized motor neurons innervate fast-twitch, fatigue-resistant fibres, and large motor neurons innervate fast-twitch, fatigable muscle fibres.
The muscle spindle signals muscle length and velocity to the CNS through two types of specialised sensory fibres that innervate the intrafusal fibres. These sensory fibres have stretch receptors that open and close as a function of the length of the intrafusal fibre.
In terms of muscle innervation, there are differences between vertebrates and invertebrates. In vertebrate muscle, a motor nerve ends in one button localized near the middle of the fibre. In invertebrate muscle, a single myofibre can be innervated by more than one motor neuron, and nerve terminals are distributed along the length and around the perimeter of the muscle fibres.
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Muscle innervation in vertebrates vs invertebrates
Muscle innervation refers to the way inures connect to and interact with muscles. In vertebrates, a single nerve ending will connect to a single muscle fibre, with the nerve ending in one button near the middle of the fibre. In vertebrates, the structure of the joints allows many degrees of rotational freedom, and many muscles, each with its own distinctive sites of attachment, are needed to take advantage of this arrangement.
In invertebrates, a single nerve may serve many muscle fibres, and a single muscle fibre can be innervated by more than one nerve. The contraction strength of invertebrate muscle fibres depends upon the number and types of nerves sending impulses to that muscle cell at any given time. In arthropods, inhibitory factors in muscles can prevent contraction, unlike vertebrate muscles. The neurons in arthropods fire in short bursts to produce rapid movements.
In vertebrates, the muscle spindle is located in parallel with the extrafusal fibres, and it will stretch along with the muscle. The muscle spindle signals muscle length and velocity to the CNS through two types of specialised sensory fibres that innervate the intrafusal fibres. These sensory fibres have stretch receptors that open and close as a function of the length of the intrafusal fibre.
Invertebrate muscle structures exhibit less regularity in the arrangement of sarcomeres, the fundamental units of contraction, which can lead to varied patterns of movement. Invertebrates possess different muscle types, including smooth muscle, striated muscle, and obliquely striated muscle, each adapted to the organism's lifestyle and movement requirements. Invertebrate muscles can be categorized into fast and slow fibres, with fast fibres contracting rapidly and slow fibres contracting more slowly. The contraction mechanism is often initiated by nerve impulses, although some invertebrates, like certain insects, have asynchronous muscles that allow for rapid contractions independent of nerve impulses.
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Smooth muscle innervation
Smooth muscle 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. Smooth muscle is non-striated, meaning it has no sarcomeres and therefore no striations (bands or stripes).
Smooth muscle can be divided into two subgroups: single-unit and multi-unit smooth muscle. In single-unit smooth muscle, a single cell in a bundle is innervated by an autonomic nerve fiber (myogenic). An action potential can be propagated through neighbouring muscle cells due to the presence of many gap junctions between the cells. Due to this property, single-unit bundles form a syncytium that contracts in a coordinated fashion, making the whole muscle contract or relax, such as the uterine muscles during childbirth. Single-unit visceral smooth muscle is myogenic; it can contract regularly without input from a motor neuron.
Multi-unit smooth muscle, on the other hand, is neurogenic, meaning its contraction must be initiated by an autonomic nervous system neuron. It is found in the trachea, the iris of the eye, and lining the large elastic arteries. Smooth muscle in the gastrointestinal tract is activated by a combination of smooth muscle cells (SMCs), interstitial cells of Cajal (ICCs), and platelet-derived growth factor receptor alpha (PDGFRα) that work together as a functional syncytium.
The innervation of smooth muscle differs from that of vertebrate striated muscle. In vertebrate striated muscle, a motor nerve ends in one button localized near the middle of the fiber. In contrast, invertebrate motor nerves branch out to form multiple nerve terminals distributed along the length and perimeter of the muscle fibers. This topological distribution of nerve terminals results in different excitatory responses to motor nerve action potentials in the muscle fiber.
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Skeletal muscle innervation
The human body has about 700 named skeletal muscles, which make up roughly half of a person's body weight. Each muscle is a discrete organ constructed of skeletal muscle tissue, blood vessels, tendons, and nerves.
The neuronal innervation of a skeletal muscle typically comprises sensory nerve fibres, motor nerve fibres, and the neuromuscular junction. The nerve fibres are composed of myelinated as well as non-myelinated nerve fibres. The cell bodies of the neurons give rise to large axons, which are generally unbranched and travel to the target muscles for innervation. Near the target muscle, the axons divide into multiple smaller branches to innervate multiple muscle fibres. 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. The combination of an individual motor neuron and all the muscle fibres that it innervates is called a motor unit. The number of fibres innervated by a motor unit is called its innervation ratio.
Motor neurons are clustered in columnar, spinal nuclei called motor neuron pools (or motor nuclei). All of the motor neurons in a motor neuron pool innervate a single muscle. Thus, there is a one-to-one relationship between a muscle and a motor neuron pool.
In densely innervated muscles such as those that move the eyeball, each motor unit may contain an average of about seven muscle fibres. In contrast, a motor unit in the leg may contain more than 1000 muscle fibres. The excitatory impulses of the motor units can be recorded as an electromyogram (EMG). The EMG is a recording of the extracellular potential of the muscle. The electrodes are either placed on the skin over the muscle or inserted into the muscle between individual muscle fibres. In a fully relaxed muscle, no change in potential is recorded. However, with an increasing force of contraction, extracellularly recorded action potentials or impulses show up in the EMG.
Voluntary movement of limbs, regulated by the contraction of skeletal muscles, is made possible by the presence of neuromuscular junctions (NMJ). These form the interface between the nervous and musculoskeletal systems in the body.
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Motor neuron pools and muscle innervation
Motor neuron pools, also known as motor nuclei, are clusters of motor neurons found in columnar, spinal nuclei. Each individual muscle fibre is innervated by a single motor neuron, and a single motor neuron can innervate many muscle fibres. The combination of an individual motor neuron and all the muscle fibres it innervates is called a motor unit. The number of fibres innervated by a motor unit is called its innervation ratio.
The innervation ratio varies depending on the type of movement a muscle is required for. Muscles that are needed for fine control or delicate movements, such as the fingers or hands, have smaller innervation ratios. This means that each motor neuron will innervate a smaller number of muscle fibres, allowing for more nuanced movements. On the other hand, muscles used for coarse movements, like thigh muscles, have higher innervation ratios, where a single motor neuron can innervate 1000 or more muscle fibres.
The size of a motor pool is significant because it determines the activity of the muscle it innervates. Muscles responsible for finer movements, like the tongue and hand, have larger motor pools with higher numbers of individual motor neurons. This allows for region-specific innervation, quick movements, and the various nuances required for complex functions such as speech and tool-making.
Motor neurons themselves fall into three main classes: alpha-motor neurons, gamma-motor neurons, and beta-motor neurons. Alpha-motor neurons control extrafusal muscle fibres and innervate skeletal muscles, leading to movement. Gamma-motor neurons innervate intrafusal muscle fibres, controlling the sensitivity of muscle spindles to stretch. Beta-motor neurons are unique in that they can synapse on either type of muscle fibre.
In summary, motor neuron pools and muscle innervation are essential for the precise control and coordination of muscle movements in the human body. The size and composition of motor pools vary depending on the type of movement required, with finer movements requiring larger motor pools and more nuanced innervation.
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Frequently asked questions
Muscle innervation refers to the process by which motor neurons transmit signals to muscles, enabling movement and various functions in the body. Each motor neuron innervates a small number of muscle fibres, allowing for nuanced movements.
During embryonic chick development, researchers caused the posterior iliotibialis (pITIB) and iliofibularis (IFIB) muscles to be innervated by segmentally inappropriate motor neurons. The pITIB contains only fast fibres, while the IFIB has distinct fast and slow fibre regions. The results showed that the foreign motor neurons selectively innervated these muscles, with the slow region of the IFIB innervated by neurons associated with slow muscles, and the fast regions innervated by neurons associated with fast muscles.
Muscle innervation is crucial for the proper functioning of the nervous system. It allows for the precise control of movements, whether they are coarse or delicate. The nervous system sends signals to the muscles, which then contract or relax in response.
There are three main types of muscles in the human body: skeletal, cardiac, and smooth muscle. Skeletal muscles are voluntary muscles that attach to bones and enable movement. Cardiac and smooth muscles are involuntary and controlled by the autonomic nervous system.
In vertebrates, a motor nerve ends in a single button near the middle of the muscle fibre. In contrast, invertebrate motor nerves branch out, resulting in multiple nerve terminals distributed along the length of the muscle fibre. This allows for a wider range of excitatory responses in invertebrates.
