The Nervous System

This section explains the structure and function of the nervous system, the types of neurones, synapses and reflex arcs.

The nervous system is an essential part of the human body, responsible for coordinating and controlling all of its activities. It enables organisms to respond to stimuli and maintain homeostasis. The nervous system works quickly and efficiently by transmitting electrical signals between different parts of the body. This guide will cover the structure and function of the nervous system, including sensory neurones, relay neurones, motor neurones, synapses, and reflex arcs.

Structure and Function of the Nervous System

The nervous system is made up of two main parts:

The Central Nervous System (CNS)
The CNS consists of the brain and spinal cord. It processes information received from the sensory organs, makes decisions, and sends out instructions to the rest of the body. The brain controls higher functions such as thinking, memory, and emotions, while the spinal cord transmits messages to and from the brain and controls simple reflexes.

The Peripheral Nervous System (PNS)
The PNS consists of all the nerves outside the CNS. It connects the CNS to sensory organs, muscles, and glands. The PNS is divided into the somatic nervous system (which controls voluntary movements) and the autonomic nervous system (which regulates involuntary processes like heart rate and digestion).

Types of Neurones

Neurones are specialised cells that transmit electrical impulses in the nervous system. 

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There are three main types of neurones:

Sensory Neurones
Sensory neurones carry information from sensory receptors (such as the skin, eyes, or ears) to the central nervous system (CNS). They detect stimuli such as heat, pressure, light, and sound, and transmit this information as electrical impulses to the brain or spinal cord.

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• Structure: Sensory neurones have a long dendrite that carries impulses from sensory receptors towards the cell body. The axon then transmits the impulse from the cell body towards the CNS.

Relay Neurones
Relay neurones are found in the CNS (brain and spinal cord) and transmit impulses between sensory neurones and motor neurones. They play a crucial role in processing the information and deciding how the body should respond.

• Structure: Relay neurones have a short dendrite that receives impulses from sensory neurones and a short axon that transmits impulses to motor neurones.

Motor Neurones
Motor neurones carry impulses from the CNS to muscles and glands to trigger a response. They are responsible for moving muscles and enabling actions such as walking, talking, or responding to stimuli.

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• Structure: Motor neurones have a long axon that transmits impulses from the CNS to the effector organs, such as muscles or glands.
  1. Electrical impulse travels down the axon of the presynaptic neurone.
  2. Neurotransmitters are released from vesicles into the synapse.
  3. Neurotransmitters bind to receptors on the postsynaptic neurone.
  4. The impulse is transmitted to the next neurone or muscle.

Synapses

A synapse is the gap between two neurones. When an electrical impulse reaches the end of one neurone (the presynaptic neurone), it triggers the release of chemicals called neurotransmitters. These chemicals diffuse across the synapse and bind to receptors on the next neurone (the postsynaptic neurone), transmitting the electrical signal. Synapses ensure that impulses travel in one direction and allow for complex communication between neurones.

Process at the synapse: 

Reflex Arcs

A reflex arc is a simple pathway taken by an impulse to produce a quick, involuntary response. Reflex actions are automatic and do not require conscious thought, allowing the body to respond rapidly to potentially harmful stimuli.

The structure of a reflex arc involves five main components:

Receptor – The sensory organ detects a stimulus (e.g., pain, heat).

Sensory Neurone – The sensory neurone transmits the impulse from the receptor to the CNS.

Relay Neurone – The relay neurone in the spinal cord processes the information and transfers the impulse.

Motor Neurone – The motor neurone carries the impulse from the CNS to the effector (e.g., a muscle).

Effector – The effector (e.g., muscle or gland) carries out the response (e.g., pulling your hand away from something hot).

Example: The Withdrawal Reflex
If you touch something hot, sensory receptors in your skin detect the pain. A sensory neurone sends this information to the spinal cord, where it is passed to a relay neurone. The relay neurone quickly transmits the message to a motor neurone, which causes the muscles in your arm to contract and pull your hand away from the hot object. This happens so quickly that you may not even feel the pain until after your hand has already moved.

Summary

The nervous system is responsible for detecting stimuli, processing information, and coordinating responses. It consists of the CNS, which includes the brain and spinal cord, and the PNS, which connects the CNS to sensory organs, muscles, and glands. Sensory neurones carry information from receptors to the CNS, relay neurones process the information, and motor neurones transmit instructions to muscles or glands. Synapses are junctions between neurones that allow the transmission of electrical signals through neurotransmitters. Finally, reflex arcs are rapid, involuntary pathways that enable quick responses to stimuli, often without conscious thought.