Trending Popular articles on BrainFacts. The Neuron. Proteins That Balance Our Moods. Animals in Research Advancing science, improving health. Like Subscribe Follow Follow Subscribe. About BrainFacts. Some pages on this website provide links that require Adobe Reader to view. The upper motor neurons originate in the brain and travel downward to connect with the lower motor neurons.
The latter localize both in the brainstem and the spinal cord and are the mediators between the brainstem and the spinal cord and their peripheral targets, the muscles. The activity of motor neurons is modulated by a network of other neurons, located within the spinal cord and the brain. Motor neurons can be divided into two types of neurons, depending on their function: upper motor neurons and lower motor neurons.
These neurons form a variety of controlled and complex circuits throughout the body, controlling both voluntary and involuntary movements.
The upper motor neurons are located in either the motor cortex of the brain or the brainstem and are the main neurons which initiate voluntary movement throughout the body by connecting the cerebral cortex to the brain stem and spinal cord.
There are a few tracts, or pathways that upper motor neurons can travel, which serve different functions: pyramid, extrapyramidal, rubrospinal, tectospinal, and reticulospinal tracts. The upper motor neurons travel down one of the aforementioned tracts where they will then synapse, or form connections with neurons located within the spinal cord, called lower motor neurons. Lower motor neurons, located in the spinal cord, are directly responsible for communicating signals directly to the skeletal muscles, organs, and glands.
They receive information from the upper motor neurons, either directly or via interneurons, and stimulate their activity, extending their fibers all the way to their appropriate destinations. There are a few subtypes of lower motor neurons: somatic, special visceral efferent, and general visceral motor neurons. Somatic motor neurons originate in the central nervous system, specifically the brainstem and project their axons to skeletal muscles.
Somatic motor neurons can be further divided into alpha, beta, and gamma motor neurons. Special visceral efferent neurons also known as branchial motor neurons are responsible for innervate the muscles of the head and neck. These are located in the brain stem, and together with the sensory neurons form the nuclei of the some of the cranial nerves trigeminal, facial, glossopharyngeal, vagus, and accessory nerves.
General visceral motor neurons contribute to both the sympathetic and parasympathetic functions of the autonomic nervous system ANS. The functions of the ANS are not consciously controlled and rely on the visceral motor neurons to stimulate all peripheral functions, such as innervating the heart, smooth muscle, and glands.
The only area they do not stimulate are the skeletal muscles. Acetylcholine ACh is a neurotransmitte r chemical messenger found in the central nervous system and is used by lower motor neurons. ACh is an excitatory neurotransmitter at the neuromuscular junction in skeletal muscle, causing the muscle to contract. ACh is stored in the vesicles at the ends of the motor neurons which produces the chemical. When a nerve impulse arrives at the terminals of a motor neuron, ACh is released into the neuromuscular junction.
The neuromuscular junction is a specialized chemical synapse between a motor neuron and a muscle fiber which is responsible for converting electrical impulses generated by the neuron into electrical activity in the muscle. Synaptic transmission at this junction begins when an action potential travels down the motor neuron and reaches the presynaptic terminal of that neuron.
This activates voltage-gated calcium channels, allowing calcium ions to enter the neuron. Calcium ions then trigger the release of ACh into the neuromuscular junction. Once released into the neuromuscular junction, ACh then attaches to receptor molecules in the postsynaptic membrane of the muscle fiber.
Glutamate is another neurotransmitter utilized by motor neurons. Glutamate is a neurotransmitter used by the upper motor neurons, in contrast with ACh being used by the lower motor neurons. Glutamate is therefore mostly prevalent within the central nervous system, known as the most abundant neurotransmitter. Although not specific to motor neuron function, it is still an important neurotransmitter to begin the process of muscular movements. Damage to the upper and lower motor neurons can result in different side effects.
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