The nervous system

The nervous system includes the central and peripheral nervous systems. The sense organs, including the eye, contain receptors that are sensitive to stimuli and respond with reflex actions.

Nervous system

The human nervous system consists of:

• the central nervous system (CNS) - the brain and spinal cord
• the peripheral nervous system - nerve cells that carry information to or from the CNS

Nerve cells

Nerve cells are also called neurones. They are adapted to carry electrical impulses from one place to another:

• they have a long fibre (axon) which is insulated by a fatty sheath
• they have tiny branches (dendrons) which branch further as dendrites at each end

Receptors to effectors

The diagram summarises how information flows from receptors to effectors in the nervous system.

Receptors

Receptors are groups of specialised cells. They can detect a change in the environment (stimulus) and produce electrical impulses in response. Sense organs contain groups of receptors that respond to specific stimuli.

Sense organ Stimulus
Skin Touch, temperature
Tongue Chemicals (in food and drink, for example)
Nose Chemicals (in the air, for example)
Eye Light
Ear Sound

Effectors

Effectors are parts of the body - such as muscles and glands - that produce a response to a detected stimulus. For example:

• a muscle contracting to move an arm
• muscle squeezing saliva from the salivary gland
• a gland releasing a hormone into the blood

This diagram summarises how information flows from receptors to effectors in the nervous system.

Reflex actions

A reflex action is a way for the body to automatically and rapidly respond to a stimulus to minimise any further damage to the body. It follows this general sequence and does not involve the brain:

stimulus → receptor → sensory neurone → relay neurone → motor neurone → effector → response

The nerve pathway followed by a reflex action is called a reflex arc. For example, a simple reflex arc happens if we accidentally touch something hot.

1. Receptor in the skin detects a stimulus (the change in temperature).
2. Sensory neurone sends impulses to relay neurone.
3. Motor neurone sends impulses to effector.
4. Effector produces a response (muscle contracts to move hand away).

Antagonistic pairs

Muscles work in antagonistic pairs. This ensures that when a part of the body is moved, it can move back to its original position. Examples are:

• the biceps and triceps in the arm
• the quadriceps and hamstrings in the leg

The slideshow explains how the lower arm is moved upwards (flexed) when the biceps muscle contracts and the triceps muscle is relaxed. It is moved downwards (extended) when the triceps is contracted and the biceps is relaxed.

The synapse

Where two neurones meet there is a tiny gap called a synapse. Information crosses this gap using neurotransmitters, rather than using electrical impulses. One neurone releases neurotransmitters into the synapse. These diffuse across the gap and make the other neurone transmit an electrical impulse.

The eye

The eye is a sense organ that responds to light.

Structure Function
Cornea Refracts light - bends it as it enters the eye
Iris Controls how much light enters the pupil
Lens Focuses light onto the retina
Retina Contains the light receptors
Optic nerve Carries impulses between the eye and the brain

The retina

Light passes through the eyeball to the retina.

There are two main types of light receptors - rods and cones. Rods are more sensitive to light than cones. There are three different types of cone cells which produce colour vision.

Rods Cones
Number in the retina 120 million 7 million
Where concentrated Outer edges of the retina The fovea
Work best in Dim light Bright light

The pupil reflex

The amount of light entering the eye is controlled by a reflex action. The size of the pupil changes in response to bright or dim light. This is controlled by the muscles of the iris.

Accommodation

The ability of the lens to change its shape to focus near and distant objects is called accommodation.

Position of object Ciliary muscles Suspensory ligaments Muscle tension on lens Lens shape
Near Contract Slackened Low Fat
Distant Relax Stretched High Thin 

Information Taken from: https://www.bbc.co.uk/education/guides/zkdnb9q/revision/1