What is a Neuron?

Neurons are electrically excitable cells in the body specialized for receiving and transmitting information. They are referred to as the structural units of the nervous system. The important parts of a neuron include the cell body, dendrites, and axons.

What are the Different Types of Neurons?

• Sensory neuron: It is also known as the afferent neuron that sends signals from the receptors present in the sense organs to the brain.
• Motor neuron: It is also known as the efferent neuron that carries information from the brain or spinal cord to muscles and tendons present throughout the body. It supplies the heart, diaphragm, bladder, intestines, and glands. It has very few axons.
• Interneuron: It intervenes between sensory and motor neurons. It carries information between other interneurons in the spinal cord and brain. The interneurons are easy to recognize and have fewer axons.

Explain Afferent Neurons in Detail

• Afferent neurons are also called sensory neurons that carry signals from the sensory organs towards the central nervous system.
• It collects information from the outside world.
• It also transmits messages from the internal organs such as the sense of pain from appendicitis and ulcer.
• It has very long axons. It is also known as a unipolar neuron because its dendrites and axon terminal arise from the same axis.
• The ends of axons contain lots of mitochondria and transmitter chemicals in the secretory vesicles.
• Dendrites are highly branched and some are myelinated.
• The cell body lies in the central nervous system.
• Examples include sensory inputs, such as smell and feeling of pain, which are carried from the point of reception up the spinal cord and into the brain.
• These neurons are important as they are part of the somatic nervous system associated with the voluntary movements in the body.
• For example, when a person looks at the rotten milk in the fridge, he immediately takes out the milk container and discards it. This occurs due to the function of the afferent neurons. These neurons help in sensing the rotten smell of the milk. The message is sent from the spinal cord to the brain where the association neuron responds.

These association neurons tell the efferent neurons about the plan and travel the information down the spinal cord and towards the muscles. As a result, the muscle movement allows to hold the milk container and take it out of the fridge.

• Somatic motor neurons are involved in the contraction of skeletal muscles and are involved in locomotion.
• Special visceral motor neurons are involved in the motion of gills in fishes and the movement of the neck and facial muscles in vertebrates.
• General visceral motor neurons are directly involved in the contractions of the heart, muscles of arteries, and viscera that are not controlled consciously.

What are the Diseases Associated with Motor Neurons?

The diseases of motor neurons are of several forms and the symptoms are widespread from the onset. There is evidence of wasting and weakness in some muscles and twitching of parts of the muscle that appear visible under the skin. People have either lower motor neuron (LMN) or upper motor neuron (UMN) signs at the beginning. The combination of both LMN and UMN symptoms worsens the condition.

1. Amyotrophic lateral sclerosis (ALS): It affects both upper and lower motor neurons. During this condition, control over muscles that help in walking, talking, swallowing, and breathing is lost. The patient also has stiffness and twitches in his muscles. ALS usually occurs between the ages 40-­60 and few may live for only 3 to 5 years while some survive for more than 10 years.
2. Primary lateral sclerosis: It mainly affects upper motor neurons that cause muscle weakness and stiffness in the arms and the legs. As a result, the patient has poor coordination, balance, and walks very slowly.
3. Progressive bulbar palsy: It is a form of ALS that damages motor neurons in the brain stem located at the base of the brain. The motor neurons present in the brain stem help in chewing, swallowing, and speaking. The disease also makes it hard to control emotions and sometimes a person might cry or laugh meaninglessly.
4. Pseudobulbar palsy: It is similar to the progressive type. It affects the motor neurons that control the ability to chew, swallow and talk. People cry or smile without control.
5. Progressive muscular atrophy: It is an inherited or sporadic disorder. It affects the lower motor neurons and weakness starts in the hands and then spreads to other parts of the body. The muscle becomes weak and sometimes becomes a cramp.
6. Spinal muscular atrophy: It is an inherited condition that affects the lower motor neurons. The defective gene responsible for spinal muscular atrophy is SMN1. The protein that needs to be produced is not produced due to a defective gene and is required to protect the motor neurons. The disorder causes weakness in the upper legs, arms, and trunk. It is of different types based on the symptoms that are produced:
   • Type I or Werdnig­-Hoffmann disease: It affects six months of babies. The babies are unable to sit on their own or hold up their heads. They have a weak muscle tone, poor reflexes, and trouble swallowing and breathing.
   • Type 2: It occurs between the 6­-12 months' age group. The infants can sit but they can't walk or stand alone. They also have trouble breathing.
   • Type 3 or Kugelberg­-Welander disease: It occurs between age group 2-­17. It affects the ability of the child to walk, run, stand up, and climb stairs. The affected children have a curved spine or shortened muscles or tendons around their joints.
   • Type 4: It starts after the age of 30. The affected individuals have muscle weakness, twitching, breathing problems, and shaking. Type 4 mainly affects muscles in the arms and the legs.
7. Kennedy’s disease: It is inherited and affects only males, where females are carriers. Males have shaken hands, muscle cramps, weakness in the face, arms, and legs. They also have trouble swallowing and speaking. Men have large breasts and low sperm count.
8. Other diseases associated with motor neurons include post-­polio syndrome, amblyopia, astigmatism, cataract, color blindness, and deafness.

Context and Applications

This topic is significant in the professional exams for both undergraduate and graduate courses, especially for;

• Bachelor of Science in Anatomy and Physiology
• Master of Science in Anatomy and Physiology
• Bachelor of Medicine and Bachelor of Surgery