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Muscle innervation is the process by which muscles receive nerve signals that trigger contraction and relaxation. This process is controlled by the central nervous system in vertebrates. Invertebrate muscles can be innervated by multiple motor neurons, leading to multiple nerve terminals distributed along the length of the muscle fibres. The spinal cord, spinal nerves, and lumbar vertebral column are all involved in muscle innervation, with the spinal nerves carrying sensory and motor information to and from the spinal cord.
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What You'll Learn
Invertebrate vs vertebrate muscle innervation
Muscle innervation refers to the coordination between the nervous system and muscles. The nervous system plays a crucial role in muscle function, and its contribution varies between vertebrates and invertebrates.
Invertebrate muscle fibres receive innervation from multiple nerve fibres. A single nerve can serve numerous muscle fibres, and any given muscle fibre is served by more than one nerve. The contraction strength of invertebrate muscles depends on the number and types of nerves sending impulses to the muscle. Invertebrate muscles can be categorised into fast and slow fibres, with fast fibres contracting rapidly and slow fibres contracting slowly. In arthropods, muscle tension is controlled by the gradation of contraction within a motor unit. Inhibitory factors can prevent contraction, unlike vertebrate muscles. Invertebrate muscles can also contract rapidly and independently of nerve impulses, as seen in certain insects.
Invertebrate motor nerves branch out and lead to multiple nerve terminals distributed along the length and perimeter of the muscle fibres. This results in excitatory responses that can trigger local or full-size action potentials (APs) along the muscle fibre. Invertebrate muscles can be innervated by more than one motor neuron, and they exhibit varied patterns of movement due to the less regular arrangement of sarcomeres, the fundamental units of contraction.
In contrast, vertebrate muscles have a more centralised nervous system control. In vertebrates, a motor nerve typically ends in one button localized near the middle of the fibre, resulting in a more consistent pattern of movement. Vertebrate muscles also exhibit an "all-or-none" response, meaning that they either contract fully or not at all, depending on the stimulus.
The spinal cord plays a crucial role in vertebrate muscle innervation, with spinal nerves emerging from each vertebral level to innervate the surrounding musculature and skin. The spinal cord consists of grey and white matter, with the grey matter containing cell bodies and the white matter containing myelinated tracts. The lumbar spinal nerves, in particular, are involved in innervating various muscles in the body, such as the abdominal internal obliques, hip flexors, and ankle dorsiflexion muscles.
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Neurotransmitters and muscle excitation
Muscle innervation refers to the neural stimulation of muscle contraction. Neurotransmitters are chemical molecules that carry messages or signals from one nerve cell to another target cell. They are part of the body's communication system.
Neurotransmitters carry messages across the synaptic junction, which is less than 40 nanometers wide. Each type of neurotransmitter binds to a specific receptor on the target cell. After binding, the neurotransmitter triggers a change or action in the target cell, such as an electrical signal in another nerve cell, a muscle contraction, or the release of hormones from a cell in a gland.
There are at least 100 known neurotransmitters, which can be grouped into types based on their chemical nature. Excitatory neurotransmitters cause neurons to fire off a message, continuing to pass the message along to the next cell. Examples of excitatory neurotransmitters include glutamate, epinephrine, norepinephrine, and acetylcholine. Acetylcholine is released by most neurons in the autonomic nervous system, regulating heart rate, blood pressure, and gut motility. It also plays a role in muscle contractions, memory, motivation, sexual desire, sleep, and learning.
In contrast, inhibitory neurotransmitters, such as gamma-aminobutyric acid (GABA), prevent unwanted actions by inhibiting the target cell's response. GABA is the most common inhibitory neurotransmitter in the brain, regulating functions like anxiety, irritability, concentration, sleep, seizures, and depression.
The type of synapse and the response of the target tissue depend on the type of neurotransmitter. Excitatory neurotransmitters cause depolarization of postsynaptic cells, generating an action potential. For example, acetylcholine stimulates muscle contraction. Inhibitory synapses cause hyperpolarization of target cells, leading them away from the action potential threshold and inhibiting their action.
The neurotransmitter released into the synaptic cleft acts for a short duration before being destroyed by enzymes or reabsorbed and recycled. Acetylcholine, norepinephrine, and epinephrine are known for their fast but short-lived excitatory actions, while GABA is the major inhibitory neurotransmitter.
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Spinal nerve roots and their functions
Muscle innervation refers to the interaction between nerves and muscles. The spinal nerves are mixed nerves that interact directly with the spinal cord to modulate motor and sensory information from the body's periphery. Each nerve is formed from nerve fibres, known as fila radicularia, extending from the posterior (dorsal) and anterior (ventral) roots of the spinal cord.
The spinal nerve roots emerge from the spinal cord at each vertebral level, forming the sensory, motor, and mixed spinal nerves that innervate the regional musculature and skin. Each spinal cord segment has four roots: an anterior (ventral) and posterior (dorsal) root on both right and left sides. Each of these roots is composed of approximately eight nerve rootlets. The rootlets unite to form an anterior (ventral) or posterior (dorsal) root of a spinal nerve.
The anterior spinal roots carry efferent (away from the spinal cord) motor fibres, while the posterior spinal roots carry afferent (towards the spinal cord) sensory fibres. Mixed spinal nerves are always a combination of sensory and motor nerves. Anterior spinal nerves carry motor fibres that affect motor nerves at the motor endplates of muscles.
The spinal nerves are named according to the vertebra they correspond to. There are 31 bilateral pairs of spinal nerves, including eight cervical spinal nerve pairs, 12 thoracic pairs, five lumbar pairs, five sacral pairs, and a coccygeal nerve pair. The nerves branch directly from the spinal cord and the central nervous system, although the spinal nerves are classified as part of the peripheral nervous system.
The lumbar spinal nerves, for example, innervate the abdominal internal obliques, the iliopsoas, a hip flexor, the adductor longus, and the ankle dorsiflexion muscles.
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Muscle innervation and the spinal cord
Muscle innervation refers to the process by which nerve terminals interact with muscle fibres to produce muscle contractions. The spinal cord, along with the peripheral nerves of the lumbar plexus, the lumbar vertebral column, and the lumbar spinal nerves, plays a central role in this process.
The spinal cord is composed of grey and white matter, with the grey matter containing cell bodies and the white matter containing myelinated tracts. The grey matter is found in the centre of the spinal cord and is shaped like a capital letter H. The spinal cord is surrounded by the pia mater, with the subarachnoid space and arachnoid mater lying just outside, and the dura mater forming the outermost layer.
Bilateral pairs of spinal nerve rootlets emerge from the spinal cord at each vertebral level. These rootlets form the sensory, motor, and mixed spinal nerves that innervate the regional musculature and skin. The anterior (ventral) spinal rootlets exit the spinal cord from the anterior horn and then form the anterior spinal nerves, which carry motor fibres that affect motor nerves at the motor endplates of muscles. The posterior (dorsal) spinal roots enter the spinal column and form posterior root ganglions after separating from the motor fibres of the mixed spinal nerve.
The lumbar spinal nerves are particularly important in muscle innervation. For example, L1 innervates the abdominal internal obliques via the ilioinguinal nerve, while L2-4 innervates the iliopsoas (a hip flexor) and other muscles via the femoral nerve. The specific lumbar spinal nerve involved can vary depending on the muscle being innervated.
Invertebrate muscle innervations exhibit unique characteristics, such as multiple innervations, where a single muscle fibre can produce excitatory and inhibitory responses simultaneously. Additionally, a single myofiber can be innervated by more than one motor neuron in invertebrates.
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Muscle innervation in the abdominal muscles
Muscle innervation refers to the process by which nerve terminals on muscle fibres trigger excitatory responses that lead to muscle contractions. The abdominal muscles are innervated by a network of spinal nerves and peripheral nerves that coordinate the contraction and relaxation of these muscles.
The abdominal wall, which encloses the abdominal cavity and gastrointestinal viscera, is composed of several layers, including skin, superficial fascia, muscles, and associated fascia. The anterolateral abdominal wall, which spans the anterior and lateral sides of the abdomen, is of particular interest when discussing abdominal muscle innervation.
The rectus abdominis, a long paired muscle found on either side of the midline in the abdominal wall, is innervated by the thoracoabdominal nerves (T7-T11) and the subcostal nerve (T12). This muscle plays a role in stabilising the pelvis during walking and depressing the ribs. The rectus abdominis is enclosed by the rectus sheath, which is formed by the aponeuroses of the external oblique and half of the internal oblique muscles.
The transversus abdominis, the deepest of the flat muscles, is innervated by the thoracoabdominal nerves (T7-T11), the subcostal nerve (T12), and branches of the lumbar plexus. This muscle originates from various attachments, including the inguinal ligament, costal cartilages 7-12, and the iliac crest, and it assists in compressing the abdominal contents.
The innervation of the abdominal muscles also involves the lumbar spinal nerves. For example, the L1 nerve innervates the abdominal internal obliques via the ilioinguinal nerve, while the L2 nerve, also known as the genitofemoral nerve, has been reported to innervate the OI muscle. Additionally, the lumbar vertebral column and the peripheral nerves of the lumbar plexus are integral to the innervation of the musculature of the back.
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Frequently asked questions
Muscle innervation is the activation of muscles by nerve impulses.
A motor neuron releases a neurotransmitter called acetylcholine, which binds to the cell membrane of a muscle fibre, causing a muscle contraction.
The lumbar spinal nerves innervate the abdominal internal obliques, the iliopsoas (a hip flexor), and the adductor longus (a hip adductor), among other muscles.
