Motor Neuron: Diagram, Characteristics and Diseases

Motor neurons, also known as efferent neurons, are nerve cells that run throughout the body and allow both voluntary and involuntary movements by innervating effector muscles and glands. The neuronal cell body is located in the brain or spinal cord and the axon extends to or outside the spinal cord.

Well-Labelled Diagram of Motor Neuron

Characteristics

• A motor neuron is a nerve cell that functions to transmit signals from the central area of the nervous system to an effector site such as muscles or glands.
• A motor neuron can be broadly seen as consisting of three parts – cell body, axon and dendrites.
• They are multipolar neurons, meaning one axon with multiple dendrites as a result of which they can innervate and send action potentials to multiple targets at once.
• There are two types of motor neurons – upper motor neuron and lower motor neuron.
• The upper motor neurons synapse with the interneurons in the spinal cord and also with the lower motor neurons occasionally.
• The upper motor neurons are found only in primates and examples include control of the hand and independent control of each finger.
• The lower motor neurons are efferent nerve fibres that are responsible for carrying signals to the effector organs.
• Based on their effector targets, the lower motor neurons are of three types –

1. Somatic motor neurons that target the limbs, abdomen and intercostal muscles.
2. Special visceral motor neurons target facial expression, swallowing and mastication.
3. General visceral motor neurons target the cardiac and smooth muscles.

• A chemical synapse formed between the motor neuron and muscle fibres is known as a neuromuscular joint.
• Motor neurons innervate multiple muscle fibres at once, thus creating many action potentials. This leads to superimposition of the action potentials that create a greater force required for a single muscle twitch.

Refer: What Is a Neuron? – Definition, Structure, Parts and Function

Related Diseases

1. Upper Motor Neuron Syndrome (UMNS)

It occurs due to lesion formation on the upper motor neuron that drastically affects the motor control of skeletal muscles. It leads to general muscle weakness, altered muscle tone, loss of dexterity and a lot of fatigue. Conditions that can lead to UMNS include multiple sclerosis, spinal cord injury and brain injury.

2. Lower Motor Neuron Syndrome (LMNS)

Lesions on the lower motor neurons affect the nerve fibres that are travelling to the effector muscles. It presents with weakness, flaccid paralysis and atrophy. Conditions that lead to LMNS are Guillain–Barré syndrome, botulism, polio and trauma to peripheral nerves.

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Also Read:

• Difference between Sensory and Motor Neuron
• Difference between Upper and Lower Motor Neurons
• An Overview of Neuron, Nerves and Nervous System