Anatomy

Anatomy of the Voluntary Motor System

The cell body with its dendrites is the input end of the cell. It receives and processes (adds and subtracts) the chemical and electrical input from other neurons. It then initiates an electrical signal or action potential. The axon is the conducting portion along which this action potential is transmitted away from the cell body. It is insulated by a fatty substance known as myelin to increase the speed of conduction
The end of the axon with its terminals is the output end of the cell. It secretes a chemical neurotransmitter at a specialized junction known as a synapse. The released neurotransmitter stimulates the cell bodies and dendrites of other motor neurons. Axons also stimulate voluntary muscles at specialized nerve muscle junctions known as the neuromuscular junction.

There are two types of motor neurons distinguished by location in the brain or spinal cord and by type of neurotransmitter—the upper and the lower motor neurons.  The upper motor neurons (UMN) are cell bodies located in the surface layer or cortex of the frontal lobe in a region known as the motor strip. They are organized according to the body region that they control.

Axons descend to the brainstem and spinal cord in the corticospinal tract (CST). They pass through the cerebral hemispheres and brainstem and down the lateral portion of the spinal cord. Axon terminals secrete the chemical neurotransmitter glutamate.
There are lower motor neurons (LMN) cell bodies located in the brainstem. They are collected in groups known as motor nuclei. There are also LMN cell bodies located in the spinal cord which are collected in groups within the anterior horn. Both receive their input from the UMNs (described previously). 

 

Axons of brainstem LMNs exit via the cranial nerves to innervate muscles of the head and neck. Axons of spinal LMNs exit via the spinal nerves and travel in peripheral nerves to innervate muscles of the cervical, thoracic, and lumbosacral segments. Axon terminals secrete the chemical neurotransmitter acetylcholine. 

 

Voluntary control of each skeletal muscle requires an intact complement of UMNs and LMNs. The axons of frontal and temporal cortical neurons travel away from these regions to other brain hemispheric regions involved with the function of attention, emotion, language, social behavior, and personality.