Motor neuron

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The most important motor pathways of the CNS.

Motor neurons, also known as efferent neurons, are nerve cells responsible for carrying central nervous system signals towards muscles to cause voluntary or involuntary movement through the innervation of effector muscles and glands. Their nerve fibers are considered to be the longest in the human body. Motor neurons are the most common structure for neurons.

Motor neurons are divided into either upper or lower motor neurons. Each type of motor neuron utilizes different neurotransmitters to relay their signals.

• Upper motor neurons originate primarily in the cerebral cortex (primary motor cortex) and extend towards the brainstem or spinal cord, where they synapse. Upper motor neurons travel down specific pathways, including the pyramidal, extrapyramidal, rubrospinal, tectospinal and reticulospinal tracts. Glutamate is used as a neurotransmitter.
• Lower motor neurons originate in the brainstem (cranial nerve nuclei) and the spinal cord (anterior horn) and project to innervate muscles and glands throughout the body. Acetylcholine is used as a neurotransmitter.

The structure of a motor neuron is characterized by three components: the soma, the axon, and the dendrites. Motor neurons have a large cell body, or soma, and long projections used in transmitting information away from the soma. These projections are referred to as axons and dendrites. Axons send impulses away from the soma and dendrites carry incoming information. Motor neurons are typically considered multipolar in terms of their structure, which translates to having a single axon and multiple dendrites.

Myelin sheath surrounds the axon of the motor neuron. One of the key functions of myelin is to insulate the axon length ensuring that electrical impulses are transmitted quickly and efficiently to the target structure. Each motor neuron divides into many terminal branches which synapse with muscle fibers at neuromuscular junctions (NMJ), also known as myoneural junctions. Electrical impulses are transmitted from the motor neuron to the muscle fiber via axon terminals (also known as neuropodia or axon endfeet), to generate muscle contractions. Axon terminals function to release neurotransmitters which are received by specific parts of the muscle fiber membrane known as motor end plates, signalling muscle contraction.

A motor neuron and its associated muscle fibers make up a motor unit. Fine muscles (e.g. extraocular muscles) have small motor units and therefore can be controlled more precisely in comparison to larger muscles (e.g. biceps brachii).