Dominic Stone
Motor neurons are a type of neuron that connects to muscles and plays a vital role in movement. They are found in the brainstem, spinal cord, and cerebral cortex, and they control both voluntary and involuntary muscle movements. There are two main types of motor neurons: upper motor neurons, which originate in the cerebral cortex and travel to the brainstem or spinal cord, and lower motor neurons, which start in the spinal cord and innervate muscles throughout the body. These lower motor neurons, also known as alpha motor neurons, release the neurotransmitter acetylcholine at the neuromuscular junction, causing muscle contraction. The neuromuscular junction is a dynamic system where neurons and muscles communicate, and its function can change under certain conditions, as discovered by researchers at the Karolinska Institutet. Understanding the intricate relationship between motor neurons and muscles is essential for treating spinal cord injuries and neurological diseases, as well as for comprehending the complex process of motor skill learning.
| Characteristics | Values |
|---|---|
| Types of motor neurons | Upper motor neurons and lower motor neurons |
| Location of motor neurons | Motor cortex, brainstem, or spinal cord |
| Function of motor neurons | Control muscle movement and enable both voluntary and involuntary motions |
| Number of motor neurons innervating a single muscle | One |
| Number of muscle fibers innervated by a single motor neuron | Many |
| Neurotransmitter released by motor neurons | Acetylcholine |
| Site of communication between motor neurons and muscle cells | Neuromuscular junction |
| Type of lower motor neurons | Alpha motor neurons, beta motor neurons, and gamma motor neurons |
| Function of alpha motor neurons | Innervate extrafusal muscle fibers and generate forces needed for movement |
| Function of gamma motor neurons | Regulate sensory input by setting the intrafusal muscle fibers to an appropriate length |
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What You'll Learn
- Neurons and muscles communicate at the neuromuscular junction
- The neurotransmitter acetylcholine is released by motor neurons, causing muscle contraction
- Motor neurons receive input from spinal interneurons, sensory neurons, and descending neurons
- The primary motor cortex controls voluntary muscle movements
- Motor neurons innervate muscle fibres within a single muscle
Neurons and muscles communicate at the neuromuscular junction
The neuromuscular junction is where a neuron connects to a muscle fibre. When a neuron reaches its target muscle, it loses its myelin sheath and forms 100-200 branching nerve endings called nerve terminals or terminal boutons. These nerve terminals have areas of membrane thickening called active zones, which contain a family of SNAP proteins and rows of voltage-gated calcium channels.
At the neuromuscular junction, motor neurons release the neurotransmitter acetylcholine (ACh) at a synapse. ACh diffuses across the synaptic cleft and binds to nicotinic acetylcholine receptors (nAChRs) on the cell membrane of the muscle fibre. These receptors are ionotropic, meaning they serve as ligand-gated ion channels. The binding of ACh to the receptor triggers the opening of ACh-gated ion channels, allowing the influx of sodium ions into the muscle. This sodium influx changes the postsynaptic membrane potential, resulting in a decrease called the endplate potential. The endplate potential is strong enough to propagate action potential over the surface of the skeletal muscle membrane, ultimately leading to muscle contraction.
The development of the neuromuscular junction requires signalling from both the motor neuron's terminal and the muscle cell's central region. During this process, muscle cells produce and express acetylcholine receptors in the central regions through a process called prepatterning. Agrin, a heparin proteoglycan, and MuSK kinase help stabilise the accumulation of acetylcholine receptors in the central regions of the muscle cell. Upon activation by agrin, MuSK signals via two proteins, Dok-7 and rapsyn, to induce the clustering of acetylcholine receptors.
There are two types of lower motor neurons: small γ motor neurons and α motor neurons. Each lower motor neuron innervates muscle fibres within a single muscle, and all the motor neurons innervating a single muscle are grouped together in rod-shaped clusters called motor neuron pools. A single motor neuron can innervate many muscle fibres, and this combination of a motor neuron and the muscle fibres it innervates is called a motor unit. The number of muscle fibres innervated by a motor neuron depends on the type of movement required, with fine control movements requiring smaller innervation ratios and coarse movements requiring higher innervation ratios.
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The neurotransmitter acetylcholine is released by motor neurons, causing muscle contraction
Motor neurons are responsible for muscle contractions that generate movement. Each lower motor neuron innervates muscle fibres within a single muscle. The primary motor cortex is responsible for voluntary muscle movements, and portions of it correlate to different areas of the body.
The neurotransmitter acetylcholine is released by motor neurons at a synapse called the neuromuscular junction. Acetylcholine is a neurochemical that has a wide variety of functions in the brain and other organ systems of the body. It acts as a chemical messenger that is released by neurons, allowing them to communicate with one another and with other specialized cells. Acetylcholine is released from the vesicles into the synaptic cleft, where it binds with receptors.
When acetylcholine binds to acetylcholine receptors on the muscle fibre, it triggers an action potential that causes the contraction of the muscle. The action potential moves across the entire cell, creating a wave of depolarization. The depolarization then spreads along the sarcolemma, which is a large surface area for the neurotransmitter to bind to receptors.
The action potential triggers the release of calcium ions, which bind to actin, causing actin filaments to shift position and revealing myosin binding sites. The muscle contracts until the nerve impulse stops and the calcium ions return to their storage sites. The enzyme acetylcholinesterase removes lingering acetylcholine from the synaptic cleft to prevent continuous activation of receptors.
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Motor neurons receive input from spinal interneurons, sensory neurons, and descending neurons
Motor neurons are an essential component of the nervous system, transmitting messages to skeletal muscles and controlling muscle movements. There are two types of motor neurons: upper motor neurons and lower motor neurons. Upper motor neurons travel between the brain and spinal cord, while lower motor neurons connect the spinal cord to muscles, glands, and organs throughout the body. Lower motor neurons innervate muscle fibres within a single muscle, and all the lower motor neurons innervating a single muscle are grouped together into rod-shaped clusters.
The upper motor neurons are responsible for integrating excitatory and inhibitory signals from the cortex and translating them into signals that initiate or inhibit voluntary movement. Upper motor neuron pathways can terminate in different regions of the spinal cord, with some controlling postural muscles and others controlling movements of the fingers.
Sensory neurons, on the other hand, are activated by sensory input from the environment, such as touch, heat, or sound. They send signals to the nervous system, which are then processed by interneurons and upper motor neurons.
Descending neurons, such as thalamocortical neurons and callosal projection neurons, regulate upper motor neurons. These descending pathways influence spinal circuitry and locomotor behaviours, contributing to the coordination of movements.
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The primary motor cortex controls voluntary muscle movements
The primary motor cortex is responsible for voluntary muscle movements. It is located in the precentral gyrus of the frontal lobe. The primary motor cortex is somatotopically organised, meaning that different areas of the cortex correspond to different parts of the body. For example, stimulating the anterior paracentral lobule elicits movements of the contralateral leg, and stimulating the dorsomedial area elicits torso movements. The representations of body parts that perform precise movements, such as the hands and face, are larger compared to body parts that perform coarse movements, such as the legs.
The primary motor cortex does not directly control individual muscles. Instead, it controls individual movements or sequences of movements that require the activity of multiple muscle groups. It encodes the parameters that define these movements, including the force and direction of movement. The amount of force required to perform a movement depends on the object being moved; for example, lifting a bowling ball requires more force than lifting a balloon. While a minority of primary motor cortex neurons encode individual muscle force, a larger number encodes the amount of force necessary for a particular movement, regardless of which individual muscles are used.
The upper motor neurons of the primary motor cortex are the pyramidal cells of cortical layer V, also known as Betz cells. These neurons descend to the brainstem and spinal motor centres in the corticobulbar and corticospinal tracts. At the caudal end of the medulla, most axons in the pyramidal tract cross to enter the lateral columns of the spinal cord, forming the lateral corticospinal tract. A smaller number of axons enters the spinal cord without crossing, forming the ventral corticospinal tract.
The axons of upper motor neurons in the primary motor cortex extend from the cortex to the spine, where they connect with lower motor neurons. Each lower motor neuron innervates muscle fibres within a single muscle, and all the lower motor neurons innervating a single muscle are grouped together into rod-shaped clusters. These clusters form a motor neuron pool for that muscle, and there is a one-to-one relationship between a muscle and a motor neuron pool. A single lower motor neuron can innervate many muscle fibres, and the combination of a lower motor neuron and all the muscle fibres it innervates is called a motor unit. The number of muscle fibres innervated by a lower motor neuron depends on the type of movement the muscle is used for. If a muscle is used for fine control or delicate movements, its motor units will have smaller innervation ratios, allowing for more nuanced movements. On the other hand, if a muscle is used for coarse movements, its motor units will have higher innervation ratios.
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Motor neurons innervate muscle fibres within a single muscle
Motor neurons are responsible for muscle contractions that generate movement. Each lower motor neuron innervates muscle fibres within a single muscle. The combination of a motor neuron and all the muscle fibres it innervates is called a motor unit.
The motor neuron pool refers to all the motor neurons that innervate a single muscle. These motor neurons are grouped together into rod-shaped clusters that run parallel to the long axis of the cord for one or more spinal cord segments.
The number of muscle fibres in a motor unit varies according to the muscle. For example, in the large muscles of the lower limb, motor units range in size from approximately 500 to 1000 fibres. In contrast, the small muscles in the hand or the extraocular muscles have motor units ranging in size from approximately 10 to 100 fibres. These muscles are capable of producing very fine movements, such as typing, tying a bow, or making small adjustments with the eyes.
The size principle states that motor units are recruited from smallest to largest based on the size of the load. For smaller loads requiring less force, slow-twitch, low-force, fatigue-resistant muscle fibres are activated before fast-twitch, high-force, less fatigue-resistant muscle fibres. The rate code and the size principle govern the relationship between motor neuron activity and muscle force.
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Frequently asked questions
A motor neuron is a neuron whose cell body is located in the motor cortex, brainstem, or spinal cord. Motor neurons directly or indirectly control muscles and glands.
There are two types of motor neurons: upper motor neurons and lower motor neurons. Lower motor neurons include alpha, beta, and gamma motor neurons.
Motor neurons release the neurotransmitter acetylcholine at a synapse called the neuromuscular junction. When acetylcholine binds to acetylcholine receptors on the muscle fiber, an action potential is triggered, causing the contraction of the muscle.
Motor neurons play a vital role in movement by linking the central nervous system with different muscles in the body. They enable both voluntary and involuntary motions.
