Dominic Stone
Muscle innervation is the process by which motor neurons supply nerves to muscle fibres, allowing them to contract and perform movements. The appearance of limbs and the increasingly sophisticated control of digits are reflected in the increasingly specific control over small groups of muscle fibres by the central nervous system. This process can be disrupted by surgery, trauma, or neurological disease, resulting in muscle denervation and atrophy. De-innervation of a muscle, therefore, refers to the disruption of the nerve supply to the muscle, impairing its ability to contract and function properly.
| Characteristics | Values |
|---|---|
| Definition | To de-innervate a muscle means to remove the supply of nerves to it. |
| Process | Motor neurons supply nerves to muscle fibres, allowing them to contract and perform movements. |
| Examples | In frogs, fast muscle fibres are generally innervated at a single site by nerve endings. |
| Importance | Understanding muscle innervation is fundamental in fields like physical therapy and neurology, as it is crucial for diagnosing and treating motor function disorders. |
| Related Concepts | The somatic nervous system (SNS) deals with the voluntary control of skeletal muscle through motor axons. |
| Alteration | Normal tissue and organ innervation can be altered, partially or completely, due to surgery, trauma, or neurological disease. |
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What You'll Learn
Muscle innervation and muscle contraction
Muscle innervation refers to the process by which motor neurons supply nerves to muscle fibres, allowing them to contract and perform movements. Each muscle is connected to a specific motor neuron that sends electrical signals from the central nervous system, helping to execute precise and coordinated actions. The appearance of limbs and the increasing sophistication of digit control are reflected in the specific control over small groups of muscle fibres by the central nervous system. This is most developed in humans, where pyramidal cells of the motor cortex make direct synaptic input to spinal motor neurons.
The contractile intrafusal fibres of the mammalian muscle spindle (cat) receive multiple motor innervations derived from both α- and γ-motor neurons. The former innervates extrafusal fibres, the highly contracting fibres that supply the muscle with its power. The latter innervates intrafusal fibres, which contract only slightly to keep the muscle spindle taut and sensitive to stretch.
In skeletal muscles, excitation–contraction coupling relies on a direct coupling between two key proteins: the sarcoplasmic reticulum (SR) calcium release channel, and the voltage-gated L-type calcium channel. When the central nervous system instructs a muscle to contract, it sends signals to both the alpha and gamma motor neurons. This coordinated process is referred to as alpha-gamma coactivation.
In the case of smooth and cardiac muscles, contractions are myogenic, meaning they are initiated by the smooth or heart muscle cells themselves, rather than being stimulated by an outside event. However, they can be modulated by stimuli from the autonomic nervous system.
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Muscle cramps
Cramps are normal and quite common. They can occur due to muscle fatigue, inadequate stretching, or exercising in hot weather, which causes the body to lose fluids, salt, and minerals (such as potassium, magnesium, and calcium) through sweating. Certain medications, such as pseudoephedrine (a decongestant), diuretics, and statins (used to treat high cholesterol), can also increase the likelihood of muscle cramps. Additionally, people who are pregnant, over the age of 65, or obese are more prone to experiencing muscle cramps.
To alleviate muscle cramps, you can try muscle relaxants, stretching, and massage. For leg cramps at night (nocturnal leg cramps), it is recommended to stretch, massage, and apply heat or ice to the affected area. Staying hydrated and maintaining adequate electrolyte levels by consuming sports drinks, milk, or electrolyte-rich foods like yogurt, bananas, lentils, and spinach can also help prevent and alleviate cramps.
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Types of muscle tissue
Muscle innervation refers to the process by which motor neurons supply nerves to muscle fibres, allowing them to contract and perform movements. The brain, nerves and skeletal muscles work together to cause movement, and this is known as the neuromuscular system.
There are three types of muscle tissue in the human body: skeletal, smooth, and cardiac. Skeletal muscle is a specialised tissue attached to bones that allows movement. Skeletal muscles work with bones, tendons, and ligaments to support body weight and enable movement. They are also known as voluntary muscles as they are under conscious control. Skeletal muscles can be fast-twitch muscles, which contract quickly and use short bursts of energy, or slow-twitch muscles, which move slowly and maintain posture.
Smooth muscle tissue lines some organs, including the digestive tract, uterus, and blood vessels. It is arranged in layered sheets that contract in waves along the length of the structure. Smooth muscle movement is involuntary and occurs without conscious awareness.
Cardiac muscle, or myocardium, makes up the middle layers of the heart. The heart is the only organ that is also a muscle and the only place in the body with cardiac tissue.
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Muscle palsies
Muscle innervation is the process by which motor neurons supply nerves to muscle fibres, allowing them to contract and perform movements. This is fundamental to understanding physical therapy and neurology, as issues with muscle innervation can cause motor function disorders.
Fourth nerve palsy can often go undetected until late adulthood, when patients may complain of double vision, especially when tired. This type of palsy is associated with head trauma and patient discomfort. Patients may adopt a compensatory head tilt to keep their eyes aligned. In congenital cases, surgery is usually planned as soon as the diagnosis is established to prevent the development of facial asymmetry, abnormal neck muscles, and misalignment of the spine.
In addition, exercise may lead to gains in muscle strength and physical function in patients with Charcot-Marie-Tooth disease, the most common hereditary neuropathy. However, there is limited evidence on the optimal exercise modality and intensity.
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Muscle innervation and vocal fold paralysis
Muscle innervation is the process by which motor neurons supply nerves to muscle fibres, enabling them to contract and perform movements. Muscle innervation and muscle de-innervation are, therefore, important concepts in the understanding and treatment of vocal fold paralysis. Vocal fold paralysis is characterised by abnormal movement of the true vocal folds, which can manifest as reduced mobility (paretic) or complete immobility (paralytic). The condition can be unilateral or bilateral, with the unilateral type being more common. The vocal folds are essential for producing speech and singing sounds, and the abnormal movement of these folds results in vocal fold immobility.
The vagus nerve innervates the larynx and its associated muscles. The nerve fibres arise from the nucleus ambiguus in the brainstem medulla, and upper-motor corticobulbar neurons descend from the cerebral cortex to the nucleus ambiguus, stimulating these vagal nerve fibres. The fibres exit the brainstem via the jugular foramen as the 10th cranial nerve. The vagus nerve then descends inferior to the skull base and passes into the neck, branching into the pharyngeal branch, superior laryngeal nerve (SLN), and recurrent laryngeal nerve (RLN). The SLN is responsible for laryngeal sensation and cricothyroid muscle motion, while the RLN descends into the neck and thorax, looping around the subclavian artery and the aortic arch before ascending back into the neck.
Treatment options for unilateral vocal fold paralysis include vocal fold augmentation, laryngeal framework surgery, and laryngeal reinnervation. Laryngeal reinnervation, in particular, has been suggested to provide "tone" to the paralyzed vocal fold, implying that a loss of tone results from denervation without reinnervation. Laryngeal electromyography (LEMG) can be performed to understand the innervation status associated with a chronically paralyzed vocal fold.
Laryngeal framework surgery, such as medialization laryngoplasty or thyroplasty, involves creating a window in the thyroid cartilage and placing an implant to medialize the true vocal fold. Laryngeal reinnervation, on the other hand, utilises functioning nerves in the vicinity of the RLN to restore laryngeal tone and movement.
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Frequently asked questions
De-innervating a muscle means to remove the nerves from it. The process of innervation is when motor neurons supply nerves to muscle fibres, allowing them to contract and perform movements.
In mammals, most twitch muscle fibres are innervated at a single site. An example of multiple innervation is in the extraocular muscles of sheep, which control the position of the eye.
Each muscle is connected to a specific motor neuron that sends electrical signals from the central nervous system, helping to execute precise and coordinated actions.
Congenital heart surgery in infants can disrupt sympathetic innervation, increasing the risk of sudden cardiac arrest later in adult life.
Understanding muscle innervation is fundamental in fields like physical therapy and neurology. It is crucial for diagnosing and treating motor function disorders.
