Lower motor neurone: cell body, axon and final common pathway
A lower motor neurone is the nerve cell whose cell body lies in the central nervous system and whose axon runs out to supply skeletal muscle.
Remember
A lower motor neurone is the last neurone before the muscle. Its cell body lies in the ventral horn (the front part of the spinal grey matter) or in a cranial nerve motor nucleus (a cluster of cell bodies in the brainstem).
Memory aid
The upper motor neurone is the boss in the brain and the lower motor neurone is the worker that reaches the muscle.
Use the picture to keep the two neurones apart: the boss sits in the brain, while the worker is the one that reaches the muscle.
The pathway for voluntary movement is a chain of two neurones. The first, the upper motor neurone, stays within the central nervous system, and many of its cell bodies lie in the primary motor cortex, in the precentral gyrus (the motor strip of the frontal lobe). The second is the lower motor neurone, the one that reaches the muscle.
A lower motor neurone is defined by where its cell body sits and where its axon goes. The cell body lies within the central nervous system, in the ventral horn (the front part of the spinal grey matter) or in a brainstem motor nucleus (a cluster of cell bodies) that supplies a cranial nerve. The axon then exits the central nervous system inside a spinal or cranial nerve and synapses directly with skeletal muscle at the neuromuscular junction (the contact point between nerve and muscle).
Upper motor neurones control lower motor neurones, descending through the cord or brainstem to synapse on them. The lower motor neurone also takes part in spinal reflexes without upper motor neurone involvement, and it forms the outgoing part of spinal tendon reflexes such as the knee jerk. Every command to a skeletal muscle passes through it, which is why it is often called the final common pathway.
Key facts
- Cell body
- lies in the spinal cord's ventral horn or in cranial nerve motor nuclei, inside the central nervous system.
- Axon
- leaves the central nervous system in a spinal or cranial nerve and synapses directly with skeletal muscle.
- Place in the chain
- it is the second neurone of the voluntary motor chain, after the upper motor neurone.
- Upper motor neurone
- stays within the central nervous system, and many of its cell bodies lie in the primary motor cortex.
- Control
- upper motor neurones synapse on lower motor neurones.
- Final common pathway
- the name it is often given, because every command to a skeletal muscle passes through it.
Exam traps
- A cell body in the primary motor cortex is not a lower motor neurone cell body, because cell bodies there belong to upper motor neurones, which remain inside the central nervous system.
- A lower motor neurone cell body does not sit in a ganglion along a peripheral nerve, because it lies inside the central nervous system, in the ventral horn or a cranial nerve nucleus, and its axon is the part that runs out to the muscle.
- A cell body in the dorsal root ganglion (the structure that houses sensory neurone cell bodies) belongs to a sensory neurone, not a lower motor neurone, whose cell body lies in the ventral horn or a brainstem motor nucleus.
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