What is the Structure and Function of the Neuron?

A neuron is defined as the functional unit of the body system called the nervous system. It is significant in transmitting signals that include receiving and sending the signals from a portion of the brain. The neurons are considered specialized cells in terms of their structure. 

The Nervous System

The nervous system in human beings is widely classified into two significant sections, namely the peripheral and the central nervous system.

• The nervous system that specifically contains the spinal cord and the brain is termed the central nervous system (CNS). The CNS is completely responsible for analyzing the information from the body parts.
• The nervous system that consists of the neurons and their parts and exists outside the CNS is termed the peripheral nervous system (PNS). The PNS includes motor and sensory neurons. Sensory neurons are significant in bringing signals to the CNS, whereas the motor neurons are vital in carrying signals out of the CNS. 

Neurons

These are the building blocks that constitute the nervous system. The neurons are otherwise termed nerve fibers or nerve cells. These are considered individual cells in terms of their function and their structure. In addition to transmitting information to and from the brain, the neurons particularly relay signals to the glands, muscles, or other nerve cells. The signal can be transmitted in either form, such as an electrical signal or a chemical signal. Varying types of neurons are involved in facilitating the transmission of information. Each neuron has a distinct shape, size, and task. For example, the sensory neurons tend to carry information from the sensory receptors found all over the body to a specific region in the brain.

Classes of Neurons

Based on the different roles played by the neurons, they are classified into three types:

1. Sensory neurons: These neurons receive information from different body parts and transmit that information to the CNS for further processing. For example, when an individual touches a hot pan, the information about the pan’s temperature is transmitted to CNS with the help of the sensory neurons present in his fingertips.
2. Motor neurons: The motor neurons tend to transmit the information from the brain to different regions such as muscles, glands, and organs. If the above example is considered again, the motor neurons innervating the muscles of the fingers would immediately cause the fingers to let go of the hot pan.
3. Interneurons (relay neurons): Interneurons are present only in the CNS and tend to connect one neuron to another. They are capable of receiving and transmitting information from and to other neurons.

Note: Differentiate the three classes of neurons with the help of an example to remember them easily.

Basic Functions of a Neuron

• A neuron is significant in generating action potentials through which signals are transmitted and received. The action potential can trigger both synapse types, namely chemical and electrical synapses.
• It receives information or signals.
• It helps in integrating incoming signals to determine if the information should be passed further or not.
• It also plays a role in communicating signals to the targeted cells like other neurons, glands, or muscles.

These neuronal functions are exhibited in the neuron’s anatomy.

Neuron Structure

The three significant segments of neurons are the cell body, dendrites, and axon. Glial cells and myelin sheath are the other structures involved in improving the neuron’s efficiency. The varied structure of neurons is based on the number of dendrites and axons.

• A nerve cell with a single dendrite and axon is termed bipolar. This type of neuron usually exists only in the eye’s retina.
• A nerve cell that has one axon and multiple dendrites are termed multipolar.  Multipolar neurons are found in the cerebral cortex.
• A nerve with only one axon is termed unipolar and is present in the embryonic stage.

Dendrites

The branch-like or tree-like structures that extend away from the nerve cell body are referred to as dendrites. The dendrites play a significant role in receiving information from another neuron and further transmitting that information towards the cell body. Dendrites are vital for creating the nerve impulse, which is otherwise termed action potential. The small protrusions associated with the dendrites are termed dendritic spines that are important for increasing the surface area to establish every possible connection with other neurons.

Cell bodies

Like every other cell in the human body, a neuron is composed of certain cell organelles, including the endoplasmic reticulum, nucleus, and Golgi apparatus. The significant difference between nerve cells and other cells is the mechanism of replication. Unlike other cells, the nerve cells do not replicate. However, the nerve’s myelin sheath is capable of regeneration.

Axons

The tube-like structure responsible for carrying the electric impulses from the dendrites through the cell body is termed an axon. This particular impulse is then transmitted to the structures present at the opposite end of the dendrites, called axon terminals. The axon terminals transmit the impulse to the adjacent neuron. Axon hillock is a special structure in the cell body that acts as a junction between the nerve cell body and axon.

Synapse

The chemical junction between the axon terminals of a particular neuron and the dendrites of another neuron is termed synapse. The synapse acts as the nerve impulses’ transmission site between two neighboring nerve cells. Two types of synapses are triggered by the action potentials generated by the neurons. These are:

1. Electrical synapses: These types of synapses transmit electrical impulses between two neurons with the help of gap junctions.
2. Chemical synapses: In chemical synapses, the nerve impulse is usually transmitted through neurotransmitters or ion channels.

Myelin Sheath

The layer of fat that surrounds the axon is termed myelin sheath. This particular layer acts as an insulator to the nerve and prevents electric impulse leakage. It further improves the internal conductivity of the neuron. The periodic gaps within the myelin sheath are known as “nodes of Ranvier”; they help recharge the impulse.

Glial Cells

The non-neuronal cells that provide support to the nerve cells by supplying nutrients are termed glial cells. These cells are also responsible for producing myelin. The glial cells are referred to as oligodendrocytes in the CNS and Schwann cells in the PNS.

Context and Applications    

This topic is significant in the professional exams for both undergraduate and graduate courses that specifically deal with the concepts of physiology and anatomy. Understanding the functions and anatomy of the neurons helps in a better understanding of the nervous system. Thus, it helps in understanding and treating various medical conditions related to neurons.

Practice Problems

• Which structure of neuron first receives a nerve impulse?

The dendrites are the first structure in the neuron to receive a nerve impulse.

• Which cells derive myelin?

Myelin sheath is derived from the non-neuronal cells known as glial cells.