Types of Neurons

Definition of Neurons

A neuron is a specialised cell type in the nervous system responsible for receiving or transmitting biochemical signals in the body. Thus, giving them the potential to receive information, process it, and retransmit it to control sensory inputs, motor outputs, and cognitive functions.

Neurons display great diversity in size and shape. Both structural and functional features are used to classify the various neurons in the body. Structurally and functionally, neurons are classified into three types. All these kinds of neurons interact to maintain proper and integrated functioning of the nervous system.

Structure of A Neuron

The different parts that make up a neuron to be a complete structure are stated below:

Dendrites

• Dendrites are the receiving or input portions of a neuron.

• The plasma membranes of dendrites contain numerous receptor sites for binding chemical messengers from other cells.

• Dendrites usually are short, tapering, and highly branched.

• In many neurons the dendrites form a tree-shaped array of processes extending from the cell body. Their cytoplasm contains Nissl bodies, mitochondria, and other organelles.

Cell Body (Soma)

• The cell body, also known as soma, contains a nucleus surrounded by cytoplasm that includes typical cellular organelles such as lysosomes, mitochondria, and a Golgi complex.

• They also contain free ribosomes and clusters of rough endoplasmic reticulum, termed Nissl bodies.

• The ribosomes are the sites of protein synthesis and the newly synthesized proteins produced are used to replace cellular components and to regenerate damaged axons in the PNS.

Axon & Myelin Sheath

• The single axon of a neuron propagates nerve impulses toward another neuron, a muscle fiber, or a gland cell.

• An axon is a long, thin, cylindrical projection that often joins to the cell body at an elevation called the axon hillock.

• An axon contains mitochondria, microtubules, and neurofibrils. Because rough endoplasmic reticulum is not present, protein synthesis does not occur in the axon.

• Axons are surrounded by a multilayered lipid and protein covering, called the myelin sheath.

• The sheath electrically insulates the axon of a neuron and increases the speed of nerve impulse conduction.

Synapse

• The site of communication between two neurons or between a neuron and an effector cell is called a synapse.

• The tips of some axon terminals swell into bulb-shaped structures called synaptic end bulbs

• These synaptic bulbs contain tiny membrane-enclosed sacs called synaptic vesicles that store a chemical called a neurotransmitter.

Classification of Neurons

Both structural and functional features are used to classify the various neurons in the body.

Based On Structure

Structurally, neurons are classified according to the number of processes extending from the cell body

• Multipolar Neurons: It has one axon and many dendrites and is mainly found in the brain and spinal cord. Perform complex activities such as motor control.

• Bipolar Neurons: These neurons have one axon and one dendrite. They primarily participate in sensory functions for example vision and smell.

• Unipolar {Pseudounipolar) Neurons: They have one process that divides into two branches. Generally found in touch and temperature sensory paths.

Based On Function

Functionally, neurons are classified according to the direction in which the nerve impulse is conveyed with respect to the CNS.

• Sensory Neurons: They are afferent neurons that relay sensory information from receptors to the CNS. Electrically, action potentials arise in response to stimulation.

• Motor Neurons: They are efferent neurons that transmit the signal from the CNS to the muscles or glands to elicit a response or act upon something.

• Interneurons: They are interconnecting neurons that remain inside the CNS. Their job is to connect sensory and motor neurons. They largely take part in reflexes and complex behaviours.

Detailed Functions of Neurons

The description of neurons is given below-

Sensory Neurons

• Sensory neurons relay the action potential from the sensory receptors towards the CNS through peripheral nerves, which are the site of processing.

• Function and Examples: The sensory neurons pick up stimuli from the surroundings and carry them in the form of electrical impulses. For example, neurons play a part in providing vision, hearing, and touch.

Motor Neurons

• Motor Neurons transmit signals from the CNS to the effector organs. Motor neurons give rise to physical action.

• Function and Examples: Motor neurons transmit signals from the CNS to muscles and glands. Examples would be motor neurons controlling muscle contraction and the secretions of glands.

Interneurons

• Function and Examples: Interneurons communicate and process between the sensory and motor neurons, because they connect them within the central nervous system.

• Role in reflex arcs: The large responsibility of the interneuron is to pass the signals between sensory and motor neurons and reflex arc, because of which a very quick response is possible.

Specialised Neurons

The specialised neurons are given below-

• Purkinje Cells: Huge neurons with highly branched and elongated dendritic trees. They are present in the cerebellum. They are involved in motor coordination and balance.

• Pyramidal Cells: Neurons whose cell bodies are pyramid-shaped and give off long axons. They are one of the classes of neurons involved in cognitive functions, such as learning and memory. They reside in the cerebral cortex.

• Mirror neurons: These are neurons that fire when a given action is performed by an individual and also in case the same action is witnessed in another thus involved in empathy and social understanding.

Types of Neurons NEET MCQs (With Answers & Explanations)

Important topics that are frequently asked in NEET exam are:

• Structure of a neuron with diagram

• Function of Nissl bodies

• Types of neurons (structural and functional)

• Reflex arc

Practice Questions for NEET

Q1.Which of the following are the connecting neurons?

1. Relay neurons

2. Interneurons

3. Both 1 and 2

4. None of these

Correct answer: 3) Both 1 and 2

Explanation:

Connecting neurons, or interneurons, are a vital component of the cockroach's nervous system, located in its cranial region. These interneurons are situated in the cerebral ganglion, which serves as the brain, and the subesophageal ganglion. Their primary function is to link sensory neurons, which perceive stimuli, to motor neurons, which govern movement. Relay neurons, or interneurons, are situated in the cephalic region of a cockroach, forming an integral part of its central nervous system (CNS). They are essential for processing sensory data received from antennae, ocular structures, and various other sensory organs, facilitating efficient coordination of the cockroach's responses.

Hence, the correct answer is option 3) Both 1 and 2.

Q2. Which of the neurons have only one axon?

1. Unipolar

2. Bipolar

3. Multipolar

4. All

Correct answer: 4) All

Explanation:

Unipolar, bipolar, and multipolar neurons differ in the number of processes extending from their cell bodies. Unipolar neurons have a single process that branches into both an axon and dendrites, typically found in sensory neurons of the peripheral nervous system. Bipolar neurons possess two processes, one axon, and one dendrite, and are usually located in specialized sensory organs like the retina and olfactory system. Multipolar neurons, the most common type, have one axon and multiple dendrites and are primarily found in the central nervous system, where they function as motor neurons and interneurons for complex signal processing.

Hence the correct answer is option 4) All.

Q3. What is the role of the nodes of Ranvier in saltatory conduction?

1. To slow down the conduction of the action potential

2. To allow the action potential to propagate continuously down the axon

3. To speed up the conduction of the action potential by providing a site for ion channels

4. To prevent the action potential from reaching the axon terminal

Correct answer: 3) To speed up the conduction of the action potential by providing a site for ion channels

Explanation:

In myelinated axons, the nodes of Ranvier are the spaces between adjacent myelin sheaths. Ion channels, especially voltage-gated sodium and potassium channels, are abundant in these nodes. When an action potential reaches a node of Ranvier, the membrane depolarizes, allowing sodium ions to enter the axon and depolarize the membrane, which causes the generation of a new action potential. This process also causes voltage-gated ion channels to open. As a result of the rapid depolarization and repolarization that takes place at the nodes of Ranvier, the action potential can "jump" from node to node and spread along the axon with great speed.

Therefore, the nodes of Ranvier play a crucial role in saltatory conduction, as they enable the action potential to propagate more quickly along myelinated axons than it would in unmyelinated axons.

Hence the correct answer is option 3) To speed up the conduction of the action potential by providing a site for ion channels.

Also Read:

• MCQs on Types of Neurons

• Human Brain

• Connective Tissue

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