Dominic Stone
Muscle innervation is a term used to describe the nerves that supply muscles. These nerves are responsible for both motor functions and sensory functions. For example, the median nerve is responsible for the sensory innervation of the palmar side of the thumb, index finger, middle finger, and the lateral side of the ring finger. In addition, the median nerve is also responsible for the motor functions of muscles in the forearm, such as the flexor digitorum superficialis. The arrangement of muscles and their innervation differ across species, with the appearance of limbs and the increasing control of digits leading to more specific control over small groups of muscle fibres by the central nervous system.
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What You'll Learn
Muscle innervation in vertebrates
Muscle innervation refers to the nerves that supply and control muscles. The nervous system and musculoskeletal system interface at neuromuscular junctions, which enable voluntary movement of limbs through the contraction of skeletal muscles. In vertebrates, the central nervous system exerts specific control over small groups of muscle fibres.
In the case of primitive vertebrates of the class Agnatha, the arrangement of muscles and their innervation differ from that of other vertebrates. Within the segmental myotomes, the muscle fibres are arranged into 'muscle units' with large-diameter 'white' fibres placed centrally, surrounded by a layer of smaller 'red' fibres. In many teleosts (bony fish), as well as sharks, there are fast and slow muscle systems, with slow fibres innervated by multiple branches of fine motor axons. In contrast, the fast white fibres show considerable variation in their innervation patterns across species.
In mammals, an important principle of muscle innervation is that each action potential in a motor neuron causes the contraction of every muscle fibre that it innervates. This is also observed in rat limb muscles, where each muscle fibre is innervated by a single motor neuron. In humans, the pyramidal cells of the motor cortex make direct synaptic input to spinal motor neurons.
The innervation of the upper limb, for example, is divided into anterior (ventral) divisions that supply the anterior muscles, such as the biceps brachii and brachialis, which are innervated by C5-T1 spinal nerves. The median nerve also plays a role in innervation, supplying sensory innervation to the dorsal surface of the index, middle, and ring fingers.
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The median nerve and the hand
The median nerve is one of the five branches of the brachial plexus, a complex network of nerves that helps control the shoulders, arms, and hands. The median nerve is responsible for providing sensory and motor functions to the forearm, wrist, and hand. It starts at the armpit and connects to nerve roots in the brachial plexus, which run from the C5 to C8 cervical vertebrae and T1 thoracic vertebra.
The median nerve controls movement and feeling in the forearm, wrist, hand, thumb, and fingers. It stimulates muscles in the forearm, allowing for the bending and straightening of the wrists, thumbs, and first three fingers, as well as the rotation of the forearm and hand to turn the palm downward. It also provides touch, pain, and temperature sensations to the bottom (palm) side of the thumb, index, and middle fingers, as well as part of the ring finger.
The median nerve can be divided into four layers of the anterior compartment of the forearm. The first layer contains the pronator teres, flexor carpi radialis, and palmaris longus muscles. The second layer contains the flexor digitorum superficialis muscle, which is involved in flexing the fingers at the proximal interphalangeal joint. The first and second layers are innervated by the median nerve proper. The third layer includes the lateral half of the flexor digitorum profundus, which innervates the index and ring fingers, flexor pollicis longus, and pronator quadratus. These are innervated by the anterior interosseous nerve, which has no sensory branches.
The median nerve also gives rise to the anterior interosseous nerve, which supplies the deep flexors. The deep layer includes the flexor pollicis longus, pronator quadratus, and the lateral half of the flexor digitorum profundus. The median nerve provides sensory innervation to the hand via two branches: the recurrent branch, which innervates the thenar muscles, and the palmar digital nerves, which innervate the palmar surface and fingertips of the lateral three and a half digits.
A pinched or compressed median nerve can cause carpal tunnel syndrome, resulting in wrist pain and problems grasping and holding items. It can also lead to numbness, tingling, and pain in the distribution of the median nerve. Treatment for median nerve injuries may involve splinting, compression, applying ice, or surgery, depending on the severity of the injury.
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The upper limb and spinal nerves
The upper limb nerves refer to the network of nerves that originate from the lower cervical region of the spinal cord, forming the brachial plexus. This network of nerves supplies the muscles and skin of the arm and hand. The brachial plexus is formed by the anterior branches of the fifth to eighth cervical nerves (C5-C8) and the first thoracic nerve (T1). These nerve roots exit the spinal cord between the vertebrae, with C5 exiting above the fifth vertebra in the neck, C6 above the sixth, C7 above the seventh, and C8 below the seventh vertebra. T1 is the lowest nerve root, exiting below the first vertebra in the thoracic spine.
The brachial plexus nerves control the muscles of the shoulder, arm, forearm, and hand, providing sensation to the whole upper limb. The nerves that emerge from the brachial plexus include the median, ulnar, and radial nerves. The median nerve supplies sensory innervation to the palmar and dorsal surfaces of the thumb, index finger, middle finger, and part of the ring finger. It also innervates muscles in the forearm, such as the pronator teres, flexor carpi radialis, palmaris longus, and flexor digitorum superficialis.
The ulnar nerve is formed from the C8 and T1 nerve roots. It provides sensation and controls the muscles in the hand, specifically allowing the ring and little fingers to extend. The radial nerve is also formed from the brachial plexus and contributes to the sensory and motor functions of the hand and wrist.
Injuries to the upper limb nerves can occur due to fractures, penetrating trauma, external compression, or improper movement. Damage to the brachial plexus is particularly severe as it affects the whole limb. Injuries can range from mild stretch injuries to complete root tears, with varying degrees of recovery potential.
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Motor neurons and muscle fibres
Motor neurons are essential for muscle function, transmitting electrical signals that prompt muscle fibres to contract. These neurons are large cells with big axons, and they are the first to develop during the embryonic stage. In fish, there are three primary motor neurons that innervate each segmental myotome on each side of the body. These primary motor neurons innervate distinct sets of muscle fibres within the myotome, with their axonal arbors remaining within segment boundaries. This clear organisation allows for easy identification of these neurons from one fish to another.
In addition to primary motor neurons, there are also secondary motor neurons. These are smaller in size and have less focused peripheral fields of innervation, often targeting muscle fibres across multiple segments. Individual white muscle fibres in fish receive input from a combination of one primary and several secondary motor neurons. The white muscle fibres are responsible for generating action potentials and twitch contractions.
In the context of the human spinal cord, the anterior (ventral) roots are associated with motor function, while the posterior roots serve sensation. The spinal cord itself is divided into white columns, composed of myelinated axons, and grey horns, which contain the cell bodies of motor neurons. The anterior muscles, such as the biceps brachii and brachialis, are innervated by the C5-T1 spinal nerves.
The median nerve, a branch of the spinal cord, plays a crucial role in innervating muscles in the forearm and hand. The first layer of muscles, including the pronator teres and flexor carpi radialis, is directly innervated by the median nerve. Lesions in these muscles can lead to weakness in pronation and flexion of the forearm. The second layer, containing the flexor digitorum superficialis, is also innervated by the median nerve, and its tendons insert into the middle phalanx of the fingers, enabling finger flexion.
The third layer of muscles, including the flexor digitorum profundus, flexor pollicis longus, and pronator quadratus, is innervated by the anterior interosseous nerve, a branch of the median nerve. This nerve is responsible for the "OK" sign test, where a patient forms an "OK" sign with their thumb and index finger to assess for anterior interosseous nerve palsy.
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Muscle innervation in teleosts
Muscle innervation refers to the areas of muscles, nerves, and vessels surrounded by bone and fascia. In many teleosts (bony fish), there exist fast and slow muscle systems, with the slow fibres present in a superficial layer. The slow fibres are innervated at multiple sites by branches of several fine motor axons.
There is variation in the patterns of innervation of the fast white fibres in different species of teleosts. In catfish and eels, the pattern is similar to that in sharks, with nerve-muscle contact made at the ends of the fast fibres. In most other teleosts, the pattern of innervation of the white fibres is similar to that of the red fibres, with several sites of nerve-muscle contact, originating from more than one motor neuron along the fibres.
In the zebra fish, two distinct populations of motor neurons innervating the fast white muscles have been identified in the spinal cord. Primary motor neurons have large cell bodies and axons and are the first to appear during development. There are three primary motor neurons innervating each segmental myotome on each side of the fish. The primary motor neurons innervate a non-overlapping set of muscle fibres within the myotome. Their axonal arbors do not cross the boundaries between segments.
Teleost muscle first arises in early embryonic life, and its development is driven by molecules present in the egg yolk and modulated by environmental stimuli, including temperature and oxygen. The dramatic increase in myotomal muscle mass between the embryo and adult requires the continuous production of muscle fibres until 40-50% of the maximum body length is reached. The metabolic and contractile phenotypes of muscle fibres also show significant plasticity with respect to environmental conditions, migration, and spawning.
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Frequently asked questions
Innervation refers to the nerves supplying a particular area of the body.
The pronator teres, flexor carpi radialis, palmaris longus, and flexor digitorum superficialis are all muscles innervated by the median nerve.
The median nerve provides sensory innervation to the palmar side of the thumb, index finger, middle finger, and the lateral half of the ring finger. It also supplies sensory innervation to the dorsal surface of the index, middle, and lateral half of the ring finger past the proximal interphalangeal joint.
Primary motor neurons are larger and innervate a non-overlapping set of muscle fibers within the myotome. Secondary motor neurons are smaller and often innervate muscle fibers in more than one segment.
Muscle innervation differs across species of vertebrates. For example, in teleosts like catfish and eel, nerve-muscle contacts are made at the ends of fast fibers, similar to sharks. In other teleosts, the pattern of innervation of white fibers is similar to that of red fibers, with multiple sites of nerve-muscle contact.
