5.3 Neuron

The neuron is the basic structural and functional unit of the nervous system. It is a highly specialized cell that transmits electrical impulses. Nervous coordination mainly comprises highly specialized cells called neurons.

Definition

Neuron: Neurons are the basic structural and functional unit of the nervous system. Nervous coordination mainly comprises highly specialized cells called neurons.

Main Idea

• Neurons are the basic structural and functional unit of the nervous system.
• Although neurons vary considerably in size and shape, they all have three basic components: a cell body, dendrites, and an axon.
• There are three types of neurons based on function: sensory neurons, motor neurons, and interneurons.
• The structure of each type of neuron is related to its function.

Core Concept

• Neuron = basic structural and functional unit of nervous system
• Three basic components = cell body, dendrites, axon
• Cell body = contains nucleus and organelles
• Dendrites = receive stimuli and conduct impulses to cell body
• Axon = conducts action potentials away from cell body
• Types of neurons = sensory, motor, interneurons

Detailed Explanation

Structure of Neuron

• Cell body
• Dendrites
• Axon

Cell Body

• The cell body is the main part of a neuron.
• It contains a mass of granular cytoplasm and cell membrane.
• The single large nucleus is centrally placed with a prominent nucleolus.
• Golgi apparatus, mitochondria and other organelles are present.
• The cytoplasm is characterised by the presence of Nissl's granules.
• These are groups of ribosomes and rough ER associated with protein synthesis.

Dendrites

• Dendrites are short and thin, often highly branched cytoplasmic extensions.
• They are gradually tapered from their bases to their tips.
• Axons of other neurons form synapses with the dendrites.
• The function of the dendrite is to receive stimuli and conduct impulses to the cell body.

Axon

• An axon is comparatively a long and thick nerve fibre.
• It has a constant diameter and can vary in size from a few mm to more than a metre length.
• It may be branched or un-branched.
• Axons terminate by branching to form small extensions with enlarged ends called presynaptic terminals.
• Functionally, axons conduct action potentials from the neuron cell body to the presynaptic terminals, i.e., conduct signal (information) away from the cell body.

Types of Neuron

All neurons vary somewhat in size, shape and characteristics depending on the function and role of the neuron.

Based upon the function there are three types of neurons:

Sensory Neurons

• The sensory neurons convert signals received from the environment into corresponding stimuli.
• The sensory inputs activate the sensory neurons and carry sensory information to the brain and spinal cord.
• They are pseudo-unipolar in structure.

Motor Neurons

• These are multipolar and are located in the central nervous system extending their axons outside the central nervous system.
• This is the most common type of neuron and transmits information from the brain to the effector (muscles or gland) of the body.

Interneurons

• They are multipolar in structure.
• Their axons connect only to the nearby sensory and motor neurons.
• They help in passing signals between two neurons.

Do You Know? Interneurons are also called relay neurons, association neurons, connector neurons, intermediate neurons or local circuit neurons.

Effectors

• The two examples of effectors are muscles and glands.
• The nervous system responds by sending signals to muscles and glands.
• The signals cause muscles to contract.
• The signals cause glands to produce secretions.
• Muscles and glands are called effectors because they cause an effect in response to directions from the nervous system.

Detailed Structure of a Sensory Neuron, Associative and Motor Neuron

a. Sensory Neurons (Afferent Neurons)

• The cell body or soma is located in the dorsal root ganglion outside the CNS.
• Sensory neurons have often their cell bodies close to the spinal cord.
• Dendrites are specialized extensions receive signals from sensory receptors.
• They are often long in peripheral sensory neurons.
• The axon extends from the sensory receptors to the CNS.
• In some cases they may be myelinated to speed up the signal transmission.
• Most sensory neurons are unipolar, having a single elevated process that branches into two.
• One branch connects to the sensory receptors and the other branch connects to the CNS.
• The signal pathway starts from the sensory receptor, moves along the axon and reaches the spinal cord or brain where further processing occurs.

b. Motor Neurons (Efferent Neurons)

• The cell body is located in the spinal cord (for lower motor neurons) or in the brain's cortex (for upper motor neurons).
• Motor neurons have many dendrites that receive inputs from interneuron and other neurons.
• The axon is long and often myelinated to allow rapid signal transmission from CNS to the muscles or glands.
• Most motor neurons are multipolar.
• Motor neurons transmit signals from CNS to muscles and glands, allowing for movement and glandular response.

Summary Table of Neuron Types

Intermediate Level

Structure of Neuron

Types of Neurons

Effectors

Detailed Structure of Sensory and Motor Neurons

Formula / Mathematical Form

Not applicable to this topic.

Solved Example

Not applicable to this topic (descriptive topic).

Important Points

• 1. Neuron is the basic structural and functional unit of the nervous system.
• 2. Neurons have three basic components: cell body, dendrites, and axon.
• 3. Cell body contains nucleus and organelles.
• 4. Dendrites receive stimuli.
• 5. Axon conducts action potentials away from cell body.
• 6. Nissl's granules are groups of ribosomes and rough ER.
• 7. Presynaptic terminals are enlarged ends of axon branches.
• 8. There are three types of neurons: sensory, motor, and interneurons.
• 9. Sensory neurons are pseudo-unipolar.
• 10. Motor neurons are multipolar.
• 11. Interneurons are multipolar.
• 12. Sensory neurons carry information to CNS.
• 13. Motor neurons carry information from CNS to effectors.
• 14. Interneurons relay signals between neurons.
• 15. Effectors are muscles and glands.

Remember

• - Neuron = basic structural and functional unit of nervous system
• - Three basic components = cell body, dendrites, axon
• - Cell body = contains nucleus and organelles
• - Dendrites = receive stimuli
• - Axon = conducts action potentials away from cell body
• - Nissl's granules = ribosomes and rough ER
• - Presynaptic terminals = release neurotransmitters
• - Types of neurons = sensory, motor, interneurons
• - Sensory neurons = pseudo-unipolar
• - Motor neurons = multipolar
• - **Interneurons** = multipolar
• - Effectors = muscles and glands

Common Mistakes

• 1. Confusing dendrites and axons — Dendrites receive; axons transmit.
• 2. Mixing up the types of neurons — Sensory (to CNS), motor (from CNS), interneurons (between neurons).
• 3. Forgetting the structure of sensory neurons — Pseudo-unipolar.
• 4. Forgetting the structure of motor neurons — Multipolar.
• 5. Forgetting the structure of interneurons — Multipolar.
• 6. Mixing up the locations — Sensory: dorsal root ganglion; motor and interneurons: CNS.
• 7. Confusing effectors and receptors — Effectors = muscles and glands; receptors = sensory cells.
• 8. Forgetting the role of Nissl's granules — Protein synthesis.
• 9. Thinking all neurons are the same — Neurons vary in size, shape, and function.
• 10. Mixing up the functions — Sensory input; motor output; interneurons relay.

Quick Revision

The neuron is the basic structural and functional unit of the nervous system. Nervous coordination mainly comprises highly specialized cells called neurons. Although neurons vary considerably in size and shape, they all have three basic components: a cell body, dendrites, and an axon. The cell body is the main part of a neuron and contains a mass of granular cytoplasm and cell membrane. The single large nucleus is centrally placed with a prominent nucleolus. Golgi apparatus, mitochondria and other organelles are present. The cytoplasm is characterised by the presence of Nissl's granules. Dendrites are short and thin, often highly branched cytoplasmic extensions. The function of the dendrite is to receive stimuli and conduct impulses to the cell body. An axon is comparatively a long and thick nerve fibre. Functionally, axons conduct action potentials from the neuron cell body to the presynaptic terminals. Based upon the function there are three types of neurons: sensory neurons, motor neurons, and interneurons. Sensory neurons are pseudo-unipolar and carry sensory information to the brain and spinal cord. Motor neurons are multipolar and transmit information from the brain to the effector. Interneurons are multipolar and help in passing signals between two neurons.

60-Second Revision

• Neuron = basic structural and functional unit of nervous system
• Three basic components = cell body, dendrites, axon
• Cell body = contains nucleus and organelles
• Dendrites = receive stimuli
• Axon = conducts action potentials away from cell body
• Types of neurons = sensory, motor, interneurons
• Sensory neurons = pseudo-unipolar
• Motor neurons = multipolar
• Interneurons = multipolar
• Effectors = muscles and glands
• Warning: Dendrites receive; axons transmit

NEXT TOPIC

• Chapter:
• 5. Nervous System of Man

• Topic:
• 5.3.1 STRUCTURE OF NEURON

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