Logan Steele
The neuromuscular system is a network of nerves and muscles that work together to make the body move and manage important functions like breathing. Nerves, which are cells called neurons, carry messages from the brain to the muscles through the spinal cord, telling them to contract and move. This connection between nerves and muscles is called the neuromuscular junction (NMJ). The NMJ is made up of the nerve terminal, the synaptic cleft, and the postsynaptic surface referred to as the endplate. When a person wants to move, neurons send messages through electrical impulses from the brain to the muscles, causing them to contract and relax. For example, when you want to lift your arms, your brain signals the muscles to do so, and motor neurons release chemicals to the muscle fibres, causing them to contract and move.
| Characteristics | Values |
|---|---|
| What is the neuromuscular system? | Connects muscles and nerves, controlling body movements and functions |
| What are nerves? | Cells called neurons |
| What do nerves do? | Carry messages to and from the brain through the spinal cord to muscles in the body |
| What are outgoing messages from the brain to the body? | Motor pathways that activate the muscles of the body |
| What are the neurons in these pathways called? | Motor neurons |
| What are incoming messages to the brain from the body? | Sensory pathways that come from the senses (eyes, nose, etc.) |
| Where do motor neurons sit? | Very close to a muscle fibre |
| Where do motor neurons and muscle fibres meet? | Neuromuscular junction |
| What do motor neurons release? | A chemical that is picked up by the muscle fibre |
| What does the chemical signal from the motor neurons do? | Signals the muscle fibre to contract, making the muscles move |
| What are the three major elements of the neuromuscular junction (NMJ)? | Presynaptic region containing the nerve terminal, the synaptic cleft, and the postsynaptic surface referred to as the endplate |
| What are the critical cellular components in the neuromuscular synapse? | Synapse-associated glial cells also called Perisynaptic Schwann Cells (PSCs) |
| What is the role of PSCs? | Play an active role in NMJ development and in the maintenance and remodeling of adult neuromuscular endplate |
| What are some neuromuscular diseases? | Neuropathies (Charcot-Marie-Tooth disease, motor neuron disease), myopathies (muscular dystrophy), neuromuscular autoimmune conditions (myasthenia gravis, multiple sclerosis) |
| What are the symptoms of neuromuscular diseases? | Tiredness, weakness, muscle pain, muscle wasting, spasms, trouble breathing and swallowing |
| What is the diagnostic test for nerve and muscle disorders? | Electromyography (EMG) |
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What You'll Learn
The somatic nervous system
The SNS consists of two parts: spinal nerves and cranial nerves. Spinal nerves are mixed nerves that carry sensory information into and motor commands out of the spinal cord. There are 31 pairs of spinal nerves, arranged according to the regions of the spinal cord. On the other hand, cranial nerves are 12 pairs of nerves, 11 of which have connections that are part of the SNS. Cranial nerve II, which connects to the eyes, is technically part of the brain and not the SNS.
The nerves in the SNS are classified based on their location, either in the head regions or the spine region. The spinal nerves send sensory information from the periphery to the spinal cord and muscle commands from the spinal cord to the skeletal muscles. The cranial nerves, on the other hand, are required for the five senses and for the movement of the head, neck, and tongue.
The SNS's principal goal is to facilitate the organs and striated muscles of the central nervous system so that we can carry out our daily responsibilities. The primary motor cortex is home to the higher motor neurons that make up the basic motor pathway. These neurons transmit signals to the lower motor neurons in the spinal cord through axons known as the corticospinal tract. These impulses then move to the neuromuscular junction (NMJ) of skeletal muscle, where the release of acetylcholine by upper motor neurons produces a signal that innervates muscles.
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Motor neurons
Alpha motor neurons innervate extrafusal muscle fibres and are the primary means of skeletal muscle contraction. They contribute to muscle tone, the continuous force generated by non-contracting muscles to oppose stretching. Beta motor neurons innervate both extrafusal and intrafusal fibres, while gamma motor neurons innervate muscle spindles and dictate their sensitivity. These neurons primarily respond to the stretch of the muscle spindle.
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Neuromuscular junction
The neuromuscular junction (NMJ) is a highly specialised synapse between a motor neuron nerve terminal and its muscle fibre. It is responsible for converting electrical impulses generated by the motor neuron into electrical activity in the muscle fibres. This process is crucial for health and disease, as it regulates all voluntary movements.
The NMJ consists of the presynaptic motor nerve terminal and the postsynaptic motor "endplate". At the NMJ, the distal motor axon loses its myelin sheath and expands to form the boutons of the presynaptic nerve terminal. These contain mitochondria and the synaptic vesicles that store the neurotransmitter acetylcholine (ACh). The vesicles are organised within specialised active zones, alongside voltage-gated calcium channels (VGCCs). Voltage-gated potassium channels (VGKCs) are also present on the presynaptic nerve terminal.
The postsynaptic membrane is deeply infolded to create junctional folds, which contain the highest density of acetylcholine receptors (AChRs). At the depths of the folds, there are relatively few AChRs but an abundance of voltage-gated sodium channels (VGSCs). The clustering of the AChRs is critical for efficient neurotransmission. The development of the NMJ is dependent on a number of key proteins, including agrin, rapsyn, muscle-specific kinase (MuSK), low-density lipoprotein receptor-related protein 4 (Lrp4), and docking protein 7 (Dok-7).
The synaptic cleft between the pre- and postsynaptic membranes contains a meshwork of acetylcholinesterase (AChE) at a density of 2,600 enzyme molecules/μm2, held in place by the structural proteins dystrophin and rapsyn. Also present is the receptor tyrosine kinase protein MuSK, which is involved in the development of the NMJ.
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Neuropathies
Nerves and muscles work together in the neuromuscular system to make the body move as desired and manage important functions like breathing. When a person wants to move, their brain sends messages through electrical impulses to their muscles. The neuromuscular junction (NMJ) is the region where muscles and nerves communicate.
There are many different types of neuropathies, and many causes. Symptoms and treatment depend on the cause and which nerves in the body are involved. Neuropathies are usually named according to the number of nerves affected and their location. Mononeuropathy involves damage to a single nerve, polyneuropathy affects multiple peripheral nerves, and autonomic neuropathy occurs when there is damage to the nerves that control the body's automatic functions, such as digestion, blood pressure, and bladder function. Diabetic neuropathy is a common example of autonomic neuropathy, often affecting the hands and feet. Peripheral neuropathy primarily presents with symptoms such as tingling sensations, numbness, pain, diminished muscle strength, and a lack of deep tendon reflexes.
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Myopathies
Nerves and muscles work together in the neuromuscular system to make the body move as desired and manage important functions like breathing. When a person wants to move, neurons in the nerves carry messages through electrical impulses from the brain to the muscles. The neuromuscular junction (NMJ) is the region where the muscle and nerve communicate.
There are several types of myopathies, including congenital myopathies, mitochondrial myopathies, muscular dystrophies, metabolic myopathies, autoimmune/inflammatory myopathies, toxic myopathies, endocrine myopathies, and infectious myopathies. Congenital myopathies are often characterised by developmental delays in learning motor skills like crawling or walking and usually affect all skeletal muscles. Mitochondrial myopathies are caused by mutations in the mitochondria, which are the energy-producing parts of the body's cells. In addition to muscle weakness, they may also cause problems with the heart, brain, or gastrointestinal tract. Muscular dystrophies are characterised by progressive weakness in skeletal muscles due to the degeneration of muscular tissue caused by structural support protein abnormalities. Metabolic myopathies are characterised by episodic muscle weakness that is sometimes random but often caused by exercise or muscle exertion. They are caused by defects in the genes that code for certain enzymes. Autoimmune/inflammatory myopathies cause issues with muscular function due to an autoimmune condition. Toxic myopathies are caused by toxins like alcohol and certain medications. Endocrine myopathies are caused by issues with the endocrine system, which controls hormone production, and can be caused by thyroid or adrenal diseases. Infectious myopathies are caused by viral, bacterial, parasitic, or fungal infections.
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Frequently asked questions
The neuromuscular system connects nerves and muscles, which work together to control body movements and functions. Nerves called motor neurons carry messages from the brain to the muscles, causing them to contract and move.
Neuromuscular diseases can cause tiredness, muscle weakness, cramps, pain, and in severe cases, trouble breathing and swallowing. There is currently no cure for many neuromuscular disorders, but treatments are used to improve quality of life.
If you suspect a muscle or nerve injury, it is important to seek medical advice as soon as possible. Doctors may use electromyography (EMG) imaging to diagnose nerve and muscle disorders. EMG measures the electrical activity of muscles at rest and during movement.
