Motor Neurone

Original Editor - Lucinda hampton

Top Contributors - Lucinda hampton, Rucha Gadgil and Naomi O'Reilly  

Introduction

Motor neurons are cells in the brain and spinal cord that allow us to move, speak, swallow and breathe by sending commands from the brain to the muscles that carry out these functions.

Motor neurons divided into either upper or lower motor neurones, forming various tightly controlled, complex circuits throughout the body. This controls both voluntary and involuntary movements through the innervation of effector muscles and glands. The upper and lower motor neurons form a two-neuron circuit.

• The upper motor neurons originate in the cerebral cortex and travel down to the brain stem or spinal cord
• The lower motor neurons begin in the spinal cord and go on to innervate muscles and glands throughout the body.

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The upper and lower motor neurons together comprise a two-neuron pathway that is responsible for movement. Upper and lower motor neurons utilize different neurotransmitters to relay their signals. Upper motor neurons use glutamate, while lower motor neurons use acetylcholine.

The lower motor neuron is responsible for transmitting the signal from the upper motor neuron to the effector muscle to perform a movement. There are three broad types of lower motor neurons: somatic motor neurons, special visceral efferent (branchial) motor neurons, and general visceral motor neurons.

Somatic motor neurons are in the brainstem and further divide into three categories: alpha, beta, and gamma.

1. Alpha motor neurons innervate extrafusal muscle fibers and are the primary means of skeletal muscle contraction. The large alpha motor neuron cell body can be either in the brainstem or spinal cord. In the spinal cord, the cell bodies are found in the anterior horn and thus are called anterior horn cells. From the anterior horn cell, a single axon goes on to innervate many muscle fibers within a single muscle. The properties of these muscle fibers are nearly identical, allowing for controlled, synchronous movement of the motor unit upon depolarization of the lower motor neuron.
2. Beta motor neurons are poorly characterized, but it has been established that they innervate both extrafusal and intrafusal fibers.
3. Gamma motor neurons innervate muscle spindles and dictate their sensitivity. These neurons primarily respond to stretch of the muscle spindle. Despite being named a “motor neuron,” these neurons do not directly cause any motor function. It is thought that they get activated along with alpha motor neurons and fine-tune the muscle contraction (alpha-gamma coactivation). A disruption in either alpha or gamma motor neurons will result in a disruption of muscle tone

Branchial motor neurons innervate the muscles of the head and neck that derive from the branchial arches.

• Located in the brainstem.
• The branchial motor neurons and sensory neurons together form the nuclei of cranial nerves V, VII, IX, X, and XI.

Visceral motor neurons contribute to both the sympathetic and parasympathetic functions of the autonomic nervous system.

In the sympathetic nervous system, central motor neurons are present in the spinal cord from T1-L2. They appear in the intermediolateral (IML) nucleus. Motor neurons from this nucleus have three different pathways. The first two pathways are to the prevertebral and paravertebral ganglia, from which peripheral neurons go on to innervate the heart, colon, intestines, kidneys, and lungs. The third possible pathway in this system is to the catecholamine-producing chromaffin cells of the adrenal medulla. By targeting these three pathways, the visceral motor neurons in the sympathetic division contribute to the “fight-or-flight” response.

In the parasympathetic system, the visceral motor neurons help give rise to cranial nerves III, VII, IX, and X. Besides in the brainstem, these visceral motor neurons contribute to the parasympathetic system in the spinal cord at levels S2-S4. Similarly to the sympathetic division, these motor neurons directly innervate ganglia in the heart, pancreas, lungs, and kidneys. Thus, in both divisions of the autonomic system, these lower motor neurons take on a different role than somatic motor neurons in that they do not directly innervate an effector muscle, and instead innervate ganglia

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