Motor Neuron: Diagram, Characteristics and Diseases

Motor neurons are nerve cells that send signals from the brain to the peripheral nervous system to initiate an action. Motor neurons have two main subtypes, upper motor neurons and lower motor neurons. Upper motor neurons originate in the brain and travel downward to connect with the lower motor neurons.

The structure of a motor neuron consists of a cell body, dendrites, and an axon. Motor neurons allow us to speak, swallow, move, and breathe. In this article, we will study the motor neuron and understand the structure and function of motor neurons with a well-labeled diagram.

What are Motor Neurons?

Motor neurons are also known as efferent neurons, which means they carry information from the central nervous system (CNS) to muscles and other peripheral systems. Motor neurons are named so for their role, which is to stimulate muscle contraction and movement. The cell body of the motor neuron is located in the motor cortex, brainstem, or spinal cord, and the axon (fiber) projects to the spinal cord to directly or indirectly control effector organs, mainly muscles and glands.

Motor neurons have the longest nerve fibers in the body, with a single axon stretching from the base of the spinal cord to the toes. The connection between a motor neuron and the muscle it controls is known as the neuromuscular junction. At this junction, the motor neuron releases neurotransmitters (such as acetylcholine), which bind to receptors on the muscle fibers, leading to muscle contraction.

Diagram of Motor Neuron

A diagram of the motor neuron is as follows:

Structure of Motor Neuron

Motor neurons are multipolar, meaning they have a single axon and multiple dendrites. The structure of the motor neuron is as follows:

• Cell Body (Soma): The cell body of a motor neuron is about 100 microns (0.1 millimeter) in diameter. It contains the nucleus and organelles.
• Dendrites: Branch-like extensions from the cell body that receive signals from other neurons.
• Axon: It is a long projection of about 1 meter (1,000 millimeter), that extends from the cell body and transmits information away from the cell body.
• Myelin Sheath: Formed by Schwann cells in the peripheral nervous system, it forms an insulating layer around the axon.
• Nodes of Ranvier: These are gaps in the myelin sheath along the axon that facilitate rapid conduction of nerve impulses.
• Axon Terminals: Present at the end of the axon, it contain vesicles with neurotransmitters.
• Synapse: It form junction between the motor neuron and the target muscle or gland and act as site of communication through neurotransmitter release.
• Supporting Cells (Glial Cells): It include cells like Schwann cells which provide support and insulation to the motor neuron. They maintain the structural integrity and function of the motor neuron.

Characteristics of Motor Neuron

Motor neuron are a type of nerve fibres that controls the muscle movement. Some of the characteristics of the motor neurons are as follows:

• Motor neurons are nerve cells that transmit signals from the nervous system to effector sites, such as muscles and glands. It causes the muscle to contract and relax.
• Motor neurons have cell bodies in the motor cortex, brainstem, or spinal cord.
• It have main processes, or axons, that extend from the ganglia to contractile muscles.
• Motor neurons have minor processes, or dendrites, that connect with the neuropile.
• It connect with muscles at neuromuscular junctions that releases neurotransmitters. It facilitate communication between the motor neuron and the muscle.
• Motor neurons form complex circuits throughout the body that allow for voluntary and involuntary movements.
• It forms multipolar structure, with one long axon and multiple dendrites extending from the cell body.
• Motor neurons have the longest nerve fibers in the body, with a single axon that can stretch from the base of the spinal cord to the toes.

Functions of Motor Neuron

Function of the motor neuron includes:

• Motor neuron allow the brain and spinal cord to communicate with muscles, organs, and glands all over the body.
• They play a role in movement, including voluntary and involuntary movements.
• Motor neurons carry the signal response from the central nervous system to the effector organs.
• When stimulated, motor neurons release neurotransmitters that bind to the receptors on muscles to trigger a response, which leads to movement.
• Maintains posture and balance through muscle contractions.
• Regulates muscles for speech and facial expressions.
• Controls precise and coordinated movements.
• Regulates respiratory muscle contractions.
• Coordinates muscles for swallowing and digestion.

Motor Neuron Diseases

Motor neuron diseases (MNDs) are a group of rare neurological disorders that destroy motor neurons, the cells that control skeletal muscle activity. Motor neuron diseases are progressive, damage parts of the nervous system and cause muscle weakness, atrophy, and eventual loss of motor function. Some of the common motor neuron diseases are:

Amyotrophic Lateral Sclerosis (ALS)

The most common form of motor neuron diseases and accounts for about 60–70% of cases. In ALS, both upper and lower motor neurons degenerate at the same time. Men are more commonly affected than women. It leads to muscle weakness, paralysis, and difficulties in speaking, swallowing, and breathing.

Primary Lateral Sclerosis (PLS)

It is similar to ALS, but only affects upper motor neurons. It leads to muscle stiffness, weakness, and difficulty with balance and coordination. It progresses more slowly than ALS.

Spinal Muscular Atrophy (SMA)

SMA is a type of genetic disorder caused by the degeneration of lower motor neurons in the spinal cord. and effect the central nervous system and voluntary muscle movement. It gradually leads to muscle weakness and atrophy.

Progressive Bulbar Palsy (PBP)

Progressive bulbar palsy (PBP) is a motor neuron disease that affects the brain stem and the nerves that supply the bulbar muscles. It affects both upper and lower motor neurons. It can lead to difficulties in speaking, swallowing, and respiratory problems.

Hereditary Spastic Paraplegia (HSP)

Hereditary spastic paraplegia (HSP) is a group of rare inherited disorders that cause weakness and stiffness in the leg muscles. It’s also known as familial spastic paraparesis. It affect the upper motor neurons.

Kennedy’s Disease (Spinal and Bulbar Muscular Atrophy)

Kennedy’s disease, also known as spinal and bulbar muscular atrophy (SBMA), is a rare, inherited neuromuscular disorder that affects lower motor neurons. It causes progressive muscle weakness and wasting, particularly in the arms and legs.

Motor Neurone Disease Causes

The exact causes of Motor Neuron Disease (MND), are not fully understood. The disease often occurs sporadically without a clear identifiable cause. Some factor that causes disease are:

• Exposure to environmental toxins, heavy metals, pesticides, and chemicals.
• Infection by viral agents.
• Premature aging of motor neurons.
• Accumulation of abnormal proteins, such as misfolded SOD1 protein.
• Damage to the neuron due to the autoimmune response of the body.
• Loss of  growth factor required to maintain motor neuron survival.
• Disruptions in the normal functioning of mitochondria.

FAQs on Motor Neuron

What are Anterior Horn Cells?

Anterior horn cells are motor neurons that extend from the spinal cord’s anterior gray matter to the skeletal muscle. They are also known as alpha motor neurons.

Why is it Called a Motor Neuron?

Motor neurons are called so as it stimulate muscle contraction and movement and also perform other functions, like stimulating glandular secretion.

What Controls Motor Neurons?

Motor neurons are located in the spinal cord which is the first level of the motor hierarchy. Spinal cord also contains many interneurons and complex neural circuits that perform the processing of motor control.

What are the Main Functions of the Motor Neurons?

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

What are Examples of Motor Neuron?

Some examples of motor neurons includes Upper motor neurons, Medial motor column (MMC) and Somatic motor neurons.

Why do Motor Neurons Stop Working?

Motor neuron disease (MND) is a rare neurodegenerative disease. It causes motor neuron cells in the brain and nerves to gradually stop working and it is not known why this happens.

What is the Function of Myelination in Neurons?

Myelination is the process of glial cells forming a membrane around axons called myelin which insulates the axon and increases the speed of electrical signal conduction.