Logan Steele
Motor neurons are neuronal cells located in the central nervous system (CNS) that control a variety of downstream targets. They are divided into upper and lower motor neurons, which differ in their origins, synapse locations, routes, neurotransmitters, and lesion characteristics. The upper motor neurons originate in the cerebral cortex and travel to the brain stem or spinal cord, while the lower motor neurons start in the spinal cord and innervate muscles and glands throughout the body. These lower motor neurons carry motor information to muscle fibers, making them directly responsible for movement. Each individual muscle fiber in a muscle is innervated by one motor neuron, and the combination of a motor neuron and its muscle fibers is called a motor unit. The number of fibers innervated by a motor unit is called its innervation ratio, which varies depending on the type of movement required.
| Characteristics | Values |
|---|---|
| Motor neurons control muscles by | Sending commands from the brain to the muscles that carry out functions |
| Types of motor neurons | Upper and lower motor neurons |
| Where are upper motor neurons located? | Motor cortex located in the precentral gyrus |
| Where are lower motor neurons located? | Spinal cord |
| What do upper motor neurons do? | Control conscious movement |
| What do lower motor neurons do? | Carry motor information to muscle fibers |
| What are the three categories of lower motor neurons? | Alpha, beta, and gamma |
| What are alpha motor neurons? | Innervate extrafusal muscle fibers and are the primary means of skeletal muscle contraction |
| What are beta motor neurons? | Poorly characterized but innervate both extrafusal and intrafusal fibers |
| What are gamma motor neurons? | Innervate muscle spindles and dictate their sensitivity |
| What are the two principles that govern the relationship between motor neuron activity and muscle force? | Rate code and size principle |
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What You'll Learn
- Motor neurons are divided into upper and lower motor neurons, with different functions and pathways
- Lower motor neurons carry motor information to muscle fibres, controlling movement
- Upper motor neurons are involved in conscious and subconscious movement, such as balance and posture
- Alpha motor neurons innervate extrafusal muscle fibres, causing skeletal muscle contraction
- Gamma motor neurons innervate muscle spindles, dictating their sensitivity and responding to stretch
Motor neurons are divided into upper and lower motor neurons, with different functions and pathways
Motor neurons are divided into upper and lower motor neurons, each with distinct functions and pathways. Both types of neurons work together to enable movement in the human body.
Upper Motor Neurons
Upper motor neurons originate in the cerebral cortex, specifically in the motor strip of the frontal lobe. They travel through the brainstem or spinal cord, sending electrical impulses that translate to movement. These neurons are involved in both conscious and subconscious motor processes. Conscious movements, such as voluntary actions, require coordination between thought and action, so upper motor neurons are often consulted. On the other hand, subconscious movements like balance and posture are controlled by upper motor neurons in extrapyramidal tracts. Upper motor neurons use glutamate as their neurotransmitter and make up the first-order neurons. They do not leave the neuraxis, remaining within the brain, brainstem, and spinal cord.
Lower Motor Neurons
Lower motor neurons, also known as second-order neurons, begin in the spinal cord and innervate muscles and glands throughout the body. They carry motor information to muscle fibers, playing a more direct role in movement than upper motor neurons. These neurons receive signals from upper motor neurons and transmit them to the effector muscles, enabling movement. Lower motor neurons are of three types: somatic motor neurons, special visceral efferent (branchial) motor neurons, and general visceral motor neurons. They use acetylcholine as their neurotransmitter.
The functions and pathways of upper and lower motor neurons are distinct but complementary. While upper motor neurons initiate movement commands, lower motor neurons execute them by innervating the muscles. Understanding this division is crucial for diagnosing and localizing neuronal injuries and lesions, as they present differently in the two types of neurons.
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Lower motor neurons carry motor information to muscle fibres, controlling movement
Motor neurons are cells in the brain and spinal cord that allow us to move, speak, swallow and breathe by sending commands from the brain to the muscles that carry out these functions. Their nerve fibres are the longest in the body, with a single axon capable of stretching from the base of the spinal cord to the toes.
The motor system hierarchy consists of four levels: the spinal cord, the brain stem, the motor cortex, and the association cortex. The spinal cord is the first level of the motor hierarchy and is the site where motor neurons are located. Motor neurons are divided into upper and lower motor neurons, forming a two-neuron circuit. The upper motor neurons originate in the cerebral cortex and travel down to the brain stem or spinal cord, while the lower motor neurons begin in the spinal cord and innervate muscles and glands throughout the body.
The rate code and the size principle govern the relationship between motor neuron activity and muscle force. Motor neurons use a rate code to signal the amount of force to be exerted by a muscle. An increase in the rate of action potentials fired by the motor neuron causes an increase in the amount of force generated by the motor unit.
Alpha motor neurons (also called lower motor neurons) innervate skeletal muscle and cause the muscle contractions that generate movement. Beta motor neurons innervate both extrafusal and intrafusal fibres, while gamma motor neurons innervate muscle spindles and dictate their sensitivity.
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Upper motor neurons are involved in conscious and subconscious movement, such as balance and posture
Upper motor neurons (UMNs) are involved in both conscious and subconscious movements. The conscious movement is voluntary, while the subconscious movement is involuntary. The former is a result of nerve impulses that travel from the motor strip in the frontal lobe of the brain to the spinal cord. The latter, on the other hand, is a result of extrapyramidal tracts or any pathway outside of the pyramid tract.
The motor system hierarchy consists of four levels: the spinal cord, the brain stem, the motor cortex, and the association cortex. The spinal cord is the first level of the motor hierarchy and is the site where motor neurons are located. The motor neurons that control limb and body movements are located in the anterior horn of the spinal cord, while those that control head and facial movements are located in the motor nuclei of the brainstem. The anterior horn cells, or lower motor neurons, are responsible for transmitting the signal from the upper motor neuron to the effector muscle to perform a movement.
The upper motor neurons in the pyramid tract play a role in controlling conscious movement. These neurons originate in the cerebral cortex and travel down to the brain stem or spinal cord. The nerve impulses travel from the motor strip, a section of the frontal lobe, to the spinal cord. The upper motor neurons in extrapyramidal tracts are involved in subconscious motor processes such as balance and posture. The rubrospinal tract, for example, is heavily involved in involuntary movements to improve and maintain the body's balance. The extrapyramidal system includes projections from the brain stem and higher centres that influence movement, mostly to maintain balance and posture, as well as to maintain muscle tone.
Upper motor neurons are also involved in the control of axial and proximal limb movement and help with posture. The rubrospinal tract, for instance, is related to the movements of the muscles in the neck. The reticulospinal tract has an important role in the control of autonomous actions in the body. The reticular formation, a complicated network of circuits located in the core of the brainstem, has various functions, including motor control. The neurons within the reticular formation influence the local circuit neurons that coordinate axial and proximal limb muscles.
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Alpha motor neurons innervate extrafusal muscle fibres, causing skeletal muscle contraction
Motor neurons are located in the spinal cord, which is the first level of the motor system hierarchy. They are responsible for transmitting signals from the upper motor neuron to the effector muscle, ultimately resulting in movement. The amount of force exerted by the muscle fibres is controlled by the motor neurons.
Somatic motor neurons are found in the brainstem and are divided into alpha, beta, and gamma motor neurons. Alpha motor neurons, also known as lower motor neurons, innervate extrafusal muscle fibres, which are the striated muscle fibres that generate the forces required for movement. These extrafusal fibres are the main fibres within muscles that cause muscle contraction.
Each alpha motor neuron cell body is found either in the brainstem or spinal cord. From the anterior horn cell, a single axon can innervate many muscle fibres within a single muscle. The properties of these muscle fibres are nearly identical, allowing for controlled, synchronous movement of the motor unit upon depolarization of the lower motor neuron.
Alpha motor neurons are responsible for initiating the contraction of skeletal muscles. They supply the acetylcholine that stimulates contraction. When acetylcholine binds to acetylcholine receptors on the muscle fibre, an action potential is triggered, resulting in muscle contraction. If the ends of the muscle are fixed, keeping the muscle at the same length, the contraction results in increased force. If the muscle shortens against no resistance, the contraction results in a constant force.
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Gamma motor neurons innervate muscle spindles, dictating their sensitivity and responding to stretch
Motor neurons are responsible for muscle movement and control. They release the neurotransmitter acetylcholine, which binds to receptors on muscle fibres and triggers contraction. The motor neurons that control limb and body movements are located in the anterior horn of the spinal cord, while those that control head and facial movements are found in the motor nuclei of the brainstem.
Gamma motor neurons, in particular, innervate muscle spindles, which are sensory receptors located within muscles. Muscle spindles provide information to the spinal cord and brain about the body's position in space and the velocity of body limbs. They are mechanoreceptors that respond to stretch and can signal changes in muscle length.
Gamma motor neurons specifically innervate the intrafusal muscle fibres of the muscle spindle. These fibres are surrounded by sensory nerve endings that become active when the muscle lengthens or shortens, providing feedback about the degree of lengthening or shortening. The gamma motor neurons regulate the length of these intrafusal muscle fibres, thereby controlling the sensitivity of the muscle spindles.
Static gamma motor neurons innervate static nuclear bag fibres (bag2 fibres) and nuclear chain fibres, which are part of the intrafusal muscle spindle fibres. Dynamic gamma motor neurons innervate dynamic nuclear bag fibres (bag1 fibres), which are smaller than the static nuclear bag fibres. The firing of gamma motor neurons in sync with alpha motor neurons pulls muscle spindles from the polar ends of the fibres, maintaining the muscle spindle's sensitivity to changes in muscle length.
Overall, gamma motor neurons play a crucial role in dictating the sensitivity of muscle spindles and responding to stretch, contributing to motor control, locomotion, and balance.
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Frequently asked questions
Motor neurons are neuronal cells located in the central nervous system (CNS) that control a variety of downstream targets. They are divided into upper and lower motor neurons, which form a two-neuron circuit.
Motor neurons innervate muscles, meaning they send signals to them that cause them to contract. Each individual muscle fibre in a muscle is innervated by one motor neuron, and a single motor neuron can innervate many muscle fibres. The combination of a motor neuron and all the muscle fibres it innervates is called a motor unit.
Upper motor neurons originate in the motor cortex in the cerebral cortex and travel to the brain stem or spinal cord. Lower motor neurons start in the spinal cord and innervate muscles and glands throughout the body. Upper motor neurons are more involved in controlling conscious movement, while lower motor neurons are more directly responsible for movement.
