Neuroscience · BEGINNER LESSON
Motor neuron
A motor neuron is a nerve cell whose cell body is in the central nervous system and whose axon carries signals out to muscles or glands. In movement circuits, upper motor neurons and lower motor neurons work together, with lower motor neurons directly activating effector tissue.
What it is and what is measured
Motor neurons are neurons, meaning cells that send signals in the nervous system. What is being tracked in this topic is the pathway of movement-related signals: where the cell body starts, where the axon travels, and what tissue it connects to. Upper motor neurons begin in the cerebral cortex and help choose or organize movement, while lower motor neurons leave the brainstem or spinal cord and contact muscles or glands directly. Scientists also compare their targets, neurotransmitters, and lesion effects.
[1] Neuroanatomy, Motor Neuron - StatPearls - NCBI Bookshelf [2] pmc.ncbi.nlm.nih.gov › articles › PMC9378604Singling out motor neurons in the age of single-cell ... [3] 10.2 Eliciting Contractions from Lower Levels - OpenStax
How they work
Motor neurons form a two-neuron circuit for movement. Upper motor neurons use glutamate, a signaling chemical, to influence lower motor neurons. Lower motor neurons use acetylcholine, another signaling chemical, at neuromuscular junctions, which are the synapses between a motor neuron and a muscle fiber. When acetylcholine binds receptors on skeletal muscle, it depolarizes the muscle fiber, which means it changes the electrical state in a way that helps trigger contraction. Motor units, defined as one lower motor neuron and all the muscle fibers it controls, let the nervous system vary force by recruiting smaller units first and larger units later. Some lower motor neurons also help reflex arcs, which are rapid spinal responses that bypass higher brain centers.
[1] Neuroanatomy, Motor Neuron - StatPearls - NCBI Bookshelf [3] 10.2 Eliciting Contractions from Lower Levels - OpenStax
Important limitations and caveats
The term motor neuron does not describe just one simple cell type. Sources distinguish upper and lower motor neurons, and lower motor neurons are also divided into several classes, including somatic, special visceral efferent, and general visceral. Not every motor neuron directly causes skeletal muscle contraction: gamma motor neurons adjust muscle spindle sensitivity rather than directly initiating contraction, and autonomic lower motor neurons project to autonomic ganglia instead of directly to effector muscles. Motor neurons are also very large, fragile, and rare in adult tissue, which makes them difficult to study.
[1] Neuroanatomy, Motor Neuron - StatPearls - NCBI Bookshelf [2] pmc.ncbi.nlm.nih.gov › articles › PMC9378604Singling out motor neurons in the age of single-cell ... [3] 10.2 Eliciting Contractions from Lower Levels - OpenStax
SEE THE IDEA
Motor neuron movement pathway
Starts in the cerebral cortex and sends commands downward.
Starts in the spinal cord or brainstem and carries the output signal outward.
Synapse where acetylcholine is released onto skeletal muscle.
Effector tissue that produces movement or another response.
Upper motor neuron influences lower motor neuron, and the lower motor neuron connects directly to the effector tissue.
A simple two-neuron route links the brain to the body’s output tissue; the lower motor neuron is the direct final link in skeletal movement.
TRY IT YOURSELF
Test your understanding
Choose an answer, then check the explanation.
PUT IT TOGETHER
Match each concept to its meaning
Drag a label onto its description, or select a label and then select a description. This also works with a keyboard.
4 pairs to match
Sources and further reading
This general lesson was generated from public source material. It explains a concept; it does not interpret a private paper or provide medical advice.