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Our senses deal with the conscious detection of particular changes or conditions of our external environment.
Sense Sense Organ External Stimulus
Vision Eyes Light
Hearing Ears Vibrations
Smell Nose Soluble chemicals in the air
Taste Tongue Soluble chemicals in food and drink
Touch Skin Heat, pain, pressure
A stimulus is any detectable change in our environment that causes us to change our activity i.e. causes us to make a response.
A response is a reaction or change in activity caused by a stimulus.
The response reduces our chances of injury and increases our chances of survival and/or reproduction.
A sense organ is a specialised group of tissues that converts particular stimuli to electrical signals that are carried by sensory nerve cells to the central nervous system. The specialised cells or parts of cells that detect stimuli by generating electrical signals are called receptors – each of our eyes has 126 million receptor cells.
The Nervous System is a group of co-operating organs to store and transmit information quickly to and form all parts controlling the activities of the body. Two parts form nervous system: central and peripheral
The central nervous system (CNS) is composed of the brain and spinal cord.
The peripheral nervous system (PNS) is composed of the sense organs and nerves.
Role of the Central Nervous System
(a) Role of the Brain
(i) Makes us consciously aware of our environment.
(ii) Stores and retrieves information.
(iii) Site of all our sensations, emotions, imagination, reasoning and decision-making.
(iv) Receives information from all the sense organs.
(v) Sends response signals to the different parts of the body.
(b) Role of Spinal Cord
(i) Connects the brain with the skin and the response organs.
(ii) Role in many automatic responses known as reflexes.
Carry signals to the Brain from the sense organs about are called sensory nerves – they carry information about changes in the external environment. Neurons that carry signals away from the Brain to the response organs are called motor nerves because most responses involve movement brought about by stimulating skeletal muscles to contract. Most nerves are a mixture of sensory and motor neuron fibres.
The Eye
Cornea The transparent strongly curved front of the eye – often described as the ‘window of the eye’.
Allows light to enter the eye and doses most of the focussing of light onto the retina.
Iris A coloured muscle sheet across the eye in behind the retina but in front of the lens. The iris controls the amount of light passing to the retina by changing the size of the pupil. The pupil is a hole in the centre of the iris & since light is not reflected out from the back of the eye then this hole appears black.
Lens This is an elastic transparent structure behind the iris. Its function is to bring about the fine accurate focussing of light onto the retina. This role is known by the special name of accommodation.
Retina The light sensitive layer at the rear of the eye. It contains 120 million rods that operate only in dim light giving us black, white and greys with poor detail.
Also present are 6 million cones that operate in bright light giving us colour. A tiny area, called the yellow-spot or fovea, only contains cones but because they are so tightly packed in this area they give us great visual detail.
Optic Nerve only contains the fibres of sensory neurons – about a million. The optic nerve carries electrical signals from the rods and cones to the visual centre of the brain. Stimulation of the brain’s visual centre gives us our visual sensation. The fibres that form the optic nerve lie in front of the retina. All these fibres converge at a single spot and together pass through the retina as the optic nerve. At this spot there aren’t rods or cones and so light falling here will not produce a visual sensation. Therefore this spot is called the ‘blind spot’.
Ciliary Muscle is a ring of muscle in the wall of the eye connected to the lens by ligaments. The action of this muscle is to change the curvature of the lens so that it will always accurately focus light on the retina no matter what the distance the object.
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