Victor Forge
Motor neurons are nerve cells that control the muscles of the trunk, limbs, head, and face, affecting speech, swallowing, and breathing. They are the only way the motor system can communicate with muscles, and all movements ultimately depend on the activity of lower motor neurons. These lower motor neurons innervate the body's skeletal muscles and are visible in histological sections of the ventral horns of the spinal cord. Upper motor neurons, on the other hand, originate in the cerebral cortex and travel to the brain stem or spinal cord. The contact point between neurons and muscles is called the neuromuscular junction, and it has been found to be more dynamic than previously thought, with the ability to fine-tune movements under certain conditions and diseases.
| Characteristics | Values |
|---|---|
| Motor neurons | Also known as motoneurons |
| Motor neuron pools | Group of motor neurons innervating a single muscle |
| Motor neuron pool location | Run parallel to the long axis of the cord for one or more spinal cord segments |
| Upper motor neurons | Originate in the motor cortex located in the precentral gyrus |
| Upper motor neuron function | Adaptive control of the hands |
| Lower motor neurons | Start in the spinal cord and innervate muscles and glands throughout the body |
| Lower motor neuron function | Innervate the body's skeletal muscles |
| Alpha motor neurons | Innervate extrafusal fibers, the highly contracting fibers that supply the muscle with its power |
| Gamma motor neurons | Innervate intrafusal fibers, which contract only slightly |
| Beta motor neurons | Innervate intrafusal muscle fibers of muscle spindles, with collaterals to extrafusal fibers |
| Motor neuron communication | Neurons transfer signal substances that can be taken up by the muscle cells to make them contract |
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What You'll Learn
Motor neurons enable voluntary and involuntary motions
Motor neurons are nerve cells that enable both voluntary and involuntary movements. They are made up of intricate, finely tuned circuits found throughout the body. Motor neurons innervate effector muscles and glands, allowing for motion. There are two types of motor neurons: upper motor neurons and lower motor neurons.
Upper motor neurons originate in the motor cortex, located in the precentral gyrus of the brain, and travel to the brain stem or spinal cord. They are responsible for integrating excitatory and inhibitory signals from the cortex and translating them into signals that initiate or inhibit voluntary movement. The cells that make up the primary motor cortex are Betz cells, which are giant pyramidal cells. The axons of these cells descend from the cortex to form the corticospinal tract.
Lower motor neurons, on the other hand, start in the spinal cord and innervate muscles and glands throughout the body. They are responsible for transmitting the signal from the upper motor neurons to the effector muscles to perform a movement. Each lower motor neuron innervates muscle fibres within a single muscle, and all the motor neurons innervating a single muscle are grouped together into rod-shaped clusters. A single motor neuron may synapse with 150 muscle fibres on average, and they are sometimes referred to as ventral horn cells.
There are three broad types of lower motor neurons: somatic motor neurons, special visceral efferent (branchial) motor neurons, and general visceral motor neurons. Somatic motor neurons are found in the brainstem and can be further divided into alpha, beta, and gamma motor neurons. Alpha motor neurons innervate extrafusal muscle fibres and are the primary means of skeletal muscle contraction. Beta motor neurons innervate intrafusal muscle fibres of muscle spindles, and gamma motor neurons innervate specialized muscle fibres that, in combination with their nerve fibres, act as sensory receptors called muscle spindles.
The motor neuron and all of the muscle fibres to which it connects is called a motor unit. Motor units are categorized as slow (S) motor units, fast fatiguing (FF) motor units, and fast fatigue-resistant motor units. Slow motor units stimulate small muscle fibres, providing small amounts of energy but are resistant to fatigue, making them suitable for sustaining muscular contraction. Fast fatiguing motor units stimulate larger muscle groups and provide large amounts of force but fatigue quickly, making them useful for tasks requiring brief bursts of energy. Fast fatigue-resistant motor units fall somewhere in between, providing more force than S motor units but with a slower reaction time.
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Motor neurons are made up of intricate circuits
Motor neurons are nerve cells that control the muscles of the trunk and limbs, and affect speech, swallowing and breathing. They are made up of intricate circuits that enable both voluntary and involuntary movements. These circuits are finely tuned and found throughout the body, innervating effector muscles and glands. Two motor neurons come together to form a two-neuron circuit.
There are two types of motor neurons: upper motor neurons and lower motor neurons. They differ in their origins, synapse locations, routes, neurotransmitters, and lesion characteristics. Lower motor neurons start in the spinal cord and innervate muscles and glands throughout the body. Upper motor neurons, on the other hand, originate in the cerebral cortex and travel to the brain stem or spinal cord.
The cell bodies of lower motor neurons are in the ventral horn of the spinal cord, and they are sometimes called ventral horn cells. A single lower motor neuron may synapse with 150 muscle fibres on average. The lower motor neuron is responsible for transmitting the signal from the upper motor neuron to the effector muscle to perform a movement. There are three types of lower motor neurons: somatic, special visceral efferent, and general visceral. Somatic motor neurons are further divided into alpha, beta, and gamma.
Upper motor neurons originate in the motor cortex, located in the precentral gyrus. The cells that make up the primary motor cortex are Betz cells, which are giant pyramidal cells. The axons of these cells descend from the cortex to form the corticospinal tract. The upper motor neuron is responsible for integrating all the excitatory and inhibitory signals from the cortex and translating them into signals that initiate or inhibit voluntary movement.
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Motor neuron pools and their spatial organisation
Motor neurons are nerve cells that control the muscles of the trunk and limbs, enabling both voluntary and involuntary motions. They are made up of intricate, finely tuned circuits found throughout the body. The two types of motor neurons are upper motor neurons and lower motor neurons. Upper motor neurons originate in the motor cortex, specifically in the precentral gyrus, and travel to the brain stem or spinal cord. Lower motor neurons, on the other hand, start in the spinal cord and innervate muscles and glands throughout the body.
Motor neuron pools are groups of motor neurons that innervate a single muscle. These pools are organised in a specific way within the spinal cord, with their spatial organisation providing clues about the functions of the descending upper motor neuron pathways. The size, composition, and anatomical location of each motor pool are tightly controlled by complex developmental pathways. The number of motor neurons within a specific motor pool is variable and is determined by the level of nuanced control that a specific muscle requires. For example, muscles with highly nuanced control, such as the muscles in the human hand, have higher densities of motor units in their motor pools.
The spatial organisation of motor neuron pools is influenced by various factors. One key factor is the expression of specific transcription factors, particularly Hox transcription factors, which play a central role in the spatial orientation of the motor pool within the spine. The size principle, proposed by Elwood Henneman in the 1960s, also influences the spatial organisation of motor neuron pools. This principle states that larger neurons require a greater input current to reach threshold potential, which in turn influences the recruitment of motor units within the pool. Additionally, the differentiation between alpha and gamma motor neurons contributes to the spatial organisation of motor pools, with gamma-motor neurons displaying simpler branching patterns and slower signal propagation velocity compared to alpha-motor neurons.
The spatial organisation of motor neuron pools is not limited to a strict mapping relationship with the muscles. Instead, it involves significant local clustering of neurons that control neighbouring body regions. For example, topographic representations of the forelimb can be found in multiple, potentially overlapping regions, each associated with a specific type of movement. Furthermore, the spatial organisation of motor neuron pools is influenced by sensory input, as evidenced by studies on proprioceptive sensory neurons in mice.
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Motor neuron injuries and localisation of lesions
Motor neurons are nerve cells that control the muscles of the trunk and limbs, enabling functions such as speech, swallowing, and breathing. They are divided into upper and lower motor neurons, which form a two-neuron circuit. Upper motor neurons originate in the motor cortex, located in the precentral gyrus, and travel to the brain stem or spinal cord. Lower motor neurons, on the other hand, begin in the spinal cord and innervate muscles and glands throughout the body.
Upper motor neuron lesions occur when there is damage to the nerve cells that help initiate and modulate movement. This damage prevents signals from travelling from the brain and spinal cord to the muscles, resulting in muscle stiffness and weakness. Upper motor neuron syndrome, a group of symptoms associated with these lesions, includes muscle weakness, overactive reflexes, tight muscles, clonus, and the Babinski response/plantar reflex in young children. Upper motor neuron lesions can arise from various injuries or conditions affecting the brain or spinal cord, such as cerebrovascular accidents, traumatic brain injury, anoxic brain injury, malignancy, infections, inflammatory disorders, neurodegenerative disorders, or metabolic disorders.
Lower motor neurons, located in the spinal cord, transmit signals from the upper motor neurons to the effector muscles to execute movements. They are responsible for muscle contraction and can be identified through retrograde labelling techniques. Lower motor neuron lesions, while not explicitly mentioned, likely result in impaired signal transmission to the muscles, leading to movement disorders.
The treatment for motor neuron lesions depends on the underlying disease or condition causing the damage. For upper motor neuron lesions, medications such as muscle relaxants and Botox can help manage symptoms like muscle spasms and stiffness. However, these treatments do not stop the progression of diseases like ALS or PLS. In contrast, lower motor neuron lesions may be associated with conditions such as muscular dystrophy, where progressive deterioration of the muscles occurs, leading to weakness and disability.
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The function of the neuromuscular synapses
The neuromuscular junction (NMJ) is a synapse between the motor nerve terminal and the surface of a muscle fibre. It is a complex structure with a critical function. Each muscle fibre is innervated and controlled by a motor neuron, which has its cell body located within the central nervous system. The axons of these motor neurons enter the muscle and divide into branches called axon terminals. At the end of the axon terminal, closest to the muscle fibre, the tip of the axon terminal enlarges and becomes the synaptic end bulb. Across the synaptic cleft from the synaptic end bulb is the motor end plate (or postsynaptic membrane), a specialised region of the muscle fibre.
The motor end plate has two key features that make it ideal for receiving acetylcholine (ACh) released from the synaptic end bulb. Firstly, it has junctional folds, which are deep invaginations of the sarcolemma that provide a large surface area for the ACh to interact with. Secondly, the sarcolemma of the junctional folds contains 30 to 40 million acetylcholine receptors. These receptors are transmembrane proteins that function as ion channels when activated.
The neuromuscular junction is an all-or-none synapse, meaning that if the endplate potential does not reach a certain threshold, or there are insufficient sodium channels, a muscle contraction will not occur. This is the safety factor of neuromuscular transmission, allowing the NMJ to function under various conditions and stresses. If this margin of safety is reduced, it can lead to muscle weakness.
The development of the NMJ requires signalling from both the motor neuron's terminal and the central region of the muscle cell. During development, acetylcholine receptors (AChRs) are expressed on the surface of nascent skeletal muscle fibres, and the axons of motor neurons are guided to innervate these fibres, leading to a clustering of AChRs underneath the motor nerve terminals. In mice, the neuromuscular junction does not form if they are deficient in either agrin or MuSK kinase, a receptor tyrosine kinase that induces cellular signalling.
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Frequently asked questions
Motor neurons are nerve cells that control the muscles of the trunk and limbs, and affect speech, swallowing and breathing. They are made up of intricate, finely tuned circuits found throughout the body that innervate effector muscles and glands to enable both voluntary and involuntary motions.
Neurons communicate with muscles at the neuromuscular junction, where they transfer signal substances that are taken up by the muscle cells to make them contract. The most important neurotransmitter in this process is the molecule acetylcholine.
Upper motor neurons originate in the motor cortex located in the precentral gyrus, whereas lower motor neurons start in the spinal cord. Upper motor neurons travel to the brain stem or spinal cord, while lower motor neurons innervate muscles and glands throughout the body.
