Motor Neuron: Diagram, Characteristics and Diseases

Motor neurons, often referred to as efferent neurons, are unique nerve cells that facilitate both voluntary and involuntary movements by stimulating muscles and glands. The neuron's cell body resides in the brain or spinal cord, while the axon extends to or beyond the spinal cord.

A Detailed Diagram of a Motor Neuron

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Key Features

Motor neurons are nerve cells that transmit signals from the central nervous system to a target site such as muscles or glands.
A motor neuron can be divided into three main parts – the cell body, axon, and dendrites.
They are multipolar neurons, with one axon and multiple dendrites, enabling them to send action potentials to numerous targets simultaneously.
Motor neurons are of two types – upper motor neuron and lower motor neuron.
The upper motor neurons interact with the interneurons in the spinal cord and occasionally with the lower motor neurons.
The upper motor neurons are unique to primates and are responsible for controlling hand movements and individual finger movements.
The lower motor neurons are efferent nerve fibers tasked with transmitting signals to the effector organs.
Lower motor neurons can be categorized into three types based on their target effectors –

Somatic motor neurons that control movements of the limbs, abdomen, and intercostal muscles.
Special visceral motor neurons that control facial expressions, swallowing, and chewing.
General visceral motor neurons that control the cardiac and smooth muscles.

The chemical synapse formed between a motor neuron and muscle fibers is termed a neuromuscular junction.
Motor neurons can stimulate multiple muscle fibers at once, creating numerous action potentials. This overlap of action potentials generates the force needed for a single muscle twitch.

Diseases Related to Motor Neurons

UMNS is caused by lesion formation on the upper motor neuron, significantly affecting the motor control of skeletal muscles. It can result in overall muscle weakness, changes in muscle tone, loss of dexterity, and extreme fatigue. Conditions like multiple sclerosis, spinal cord injury, and brain injury can lead to UMNS.

LMNS is caused by lesions on the lower motor neurons, affecting the nerve fibers traveling to the effector muscles. It can cause weakness, flaccid paralysis, and muscle wasting. Conditions that can lead to LMNS include Guillain–Barré syndrome, botulism, polio, and trauma to peripheral nerves.