The Nervous System

This lesson covers the structure and function of the human nervous system, including the roles of the central nervous system, peripheral nervous system, and different types of neurone. This is essential content for Edexcel GCSE Biology (1BI0) Topic 2: Cells and Control.

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Overview of the Nervous System

The nervous system allows organisms to detect changes in their environment and respond to them. It coordinates the body's activities rapidly using electrical impulses.

The nervous system can be divided into two main parts:

1. Central nervous system (CNS) — the brain and spinal cord.
2. Peripheral nervous system (PNS) — all the nerves that connect the CNS to the rest of the body, including sensory neurones and motor neurones.

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The Pathway of a Nervous Response

When the body detects a change in the environment, the following pathway is followed:

Stimulus → Receptor → Sensory neurone → CNS (coordinator) → Motor neurone → Effector → Response

Let us break this down:

Component	Role	Example
Stimulus	A change in the environment that is detected.	A hot pan, a bright light, a loud sound.
Receptor	Specialised cells that detect the stimulus and generate an electrical impulse.	Thermoreceptors in the skin detect temperature changes.
Sensory neurone	Carries the electrical impulse from the receptor to the CNS.	Transmits the signal from the skin to the spinal cord.
CNS (coordinator)	Processes the information and decides on a response. The brain and spinal cord.	The brain interprets the signal and decides to move the hand.
Motor neurone	Carries the electrical impulse from the CNS to the effector.	Transmits the signal from the CNS to the muscles in the arm.
Effector	The organ that carries out the response. Either a muscle or a gland.	Muscles in the arm contract to pull the hand away.
Response	The action carried out by the effector.	The hand moves away from the hot pan.

Exam Tip: You must be able to describe this pathway in the correct order. A common mistake is to leave out the CNS as the coordinator. The sequence is: stimulus → receptor → sensory neurone → CNS → motor neurone → effector → response.

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Receptors

Receptors are specialised cells that detect specific types of stimulus. Different receptors detect different stimuli:

Receptor Type	Stimulus Detected	Location
Photoreceptors	Light	Eyes (retina)
Thermoreceptors	Temperature changes	Skin
Chemoreceptors	Chemicals (taste and smell)	Tongue, nose
Mechanoreceptors	Pressure, vibrations, sound	Skin, ears
Pain receptors (nociceptors)	Tissue damage	Throughout the body

Key Points About Receptors

• Receptors convert the energy of a stimulus into an electrical impulse.
• Each type of receptor is specific to a particular stimulus — photoreceptors cannot detect sound, for example.
• Receptor cells are often grouped together in sense organs (eyes, ears, nose, tongue, skin).

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Effectors

Effectors are the organs that carry out a response. There are two types:

1. Muscles — respond by contracting, which produces movement.
2. Glands — respond by secreting (releasing) chemical substances such as hormones or enzymes.

For example:

• When you touch a hot object, your arm muscles contract to pull your hand away (a rapid response).
• When blood glucose rises after a meal, the pancreas (a gland) secretes insulin to lower blood glucose levels (a slower, hormonal response).

Exam Tip: Always specify what effectors do. Muscles contract and glands secrete hormones/enzymes. Saying that an effector "does a response" is too vague and will not earn marks.

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Types of Neurone

There are three types of neurone in the nervous system:

1. Sensory Neurones

• Carry impulses from receptors to the CNS.
• Have a long axon running from the receptor to the CNS.
• The cell body is located in the middle of the neurone (to one side of the axon).
• Have a dendron (long extension) carrying the impulse towards the cell body, and an axon carrying it away.

2. Relay Neurones (Interneurones)

• Found within the CNS (brain and spinal cord).
• Connect sensory neurones to motor neurones.
• Have short axons and many connections.
• Also involved in processing information and making decisions.

3. Motor Neurones

• Carry impulses from the CNS to effectors (muscles or glands).
• Have a long axon running from the CNS to the effector.
• The cell body is at one end of the neurone (in or near the CNS).
• Have many short dendrites extending from the cell body to receive impulses from relay neurones.

Feature	Sensory Neurone	Relay Neurone	Motor Neurone
Direction of impulse	Receptor → CNS	Within the CNS	CNS → Effector
Axon length	Long	Short	Long
Cell body position	In the middle	In the CNS	At one end (near CNS)
Location	Peripheral nervous system	Central nervous system	Peripheral nervous system

Exam Tip: Be able to identify the three types of neurone from a diagram. The key differences are the position of the cell body and the direction of the impulse. Sensory neurones have the cell body off to the side, while motor neurones have it at the end near the CNS.

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Structure of a Neurone

All neurones share some common structural features:

• Cell body — contains the nucleus and most of the cytoplasm and organelles.
• Dendrites / Dendrons — short (dendrites) or long (dendrons) extensions that carry impulses towards the cell body.
• Axon — a long extension that carries impulses away from the cell body.
• Myelin sheath — a fatty, insulating layer that surrounds the axon. It speeds up the transmission of electrical impulses by forcing the impulse to "jump" between gaps in the sheath (called nodes of Ranvier). This is called saltatory conduction.
• Axon terminal — the branched end of the axon that connects to the next neurone or to an effector.

Why the Myelin Sheath Matters