Programme 15: Responding to Stimuli

Background Information

The Eye

The eye has to be able to focus light from distant objects as well as nearby objects. It does this by altering the shape of the lens using two sets of ciliary muscles. The radial muscles contract to stretch the lens and make it thinner. Light from distant objects will be focussed on the retina. The circular muscles contract to make the lens fatter. The lens will now focus light from nearby objects on the retina.

Defects in the eye occur when this system does not operate properly. The lens may lose its elasticity, muscles may be too weak, or the eyeball may not be the correct shape. If the eye is long-sighted it cannot focus light from nearby objects. A convex lens is used in spectacles or contact lenses to correct the focus. If the eye is short-sighted, a concave corrective lens is needed to view objects from a distant light source.

The retina is made up of light sensitive cells called rods and cones. These are connected to nerve fibres which run across the inner surface of the retina to the blind spot. At this point they come together to form the optic nerve, which runs through the sclera and out of the eye into the brain.

The rods are sensitive to all frequencies of light and are linked together in groups to one nerve fibre. These cells are very sensitive but cannot detect colour. They allow us to see in dim light, when we cannot detect colour. There are three kinds of cone which detect light of different frequencies allowing us to distinguish between red, blue and green light. These cells are not as sensitive as cones so we cannot see colour in dim light.

The light sensitive cells are most concentrated in an area at the centre of the retina called the fovea. When we concentrate on an object looking for details we move our eyes so that light falls on the fovea. This is why our eyes track across the page when we are reading.

The Nervous System

A reflex is a rapid unconscious response which protects the body from damage. Because reflexes are simple they are used to explain the functioning of the nervous system. All reflexes have the same general pattern:

• a stimulus is received by a receptor or sensory organ, eg a tap on the knee stimulates a stretch receptor in a muscle tendon
• a nerve impulse passes along a sensory nerve to the spinal cord
• a relay nerve cell in the spinal cord passes on the message
• a motor nerve takes the nerve to an effector, often a muscle
• the effector makes an appropriate response, eg the muscle contracts.

The central nervous system (CNS) is made up of the brain and spinal cord. The CNS coordinates the nervous system. This coordination can take place unconsciously in simple reflexes. More commonly the many links between nerve cells, called synapses, allow information to be exchanged across the nervous system. Nerve impulses can pass along the spinal cord to the brain where different areas of the brain can be involved in the response. This can include conscious control and memory.

Plant Tropisms

A few plants, such as the Venus flytrap, are capable of real movements. Most plants do not move in response to stimuli, instead they grow. These growth movements are called tropisms. They are controlled by hormones called auxins which stimulate cells to elongate rapidly. The appropriate response to a stimulus is achieved by the distribution of auxins and the sensitivity of cells to it.

In phototropism plant stems grow towards the light. This is achieved by an auxin concentration gradient across the stem so that more auxin is found on the shaded side of the stem. This side elongates pointing the plant towards the light.

Geotropism is more complex. When seeds germinate shoots and roots respond to the stimulus of gravity in opposite ways. A concentration gradient from the upper to the lower surface of a shoot causes elongation at the lower surface and the shoot grows upwards. A similar concentration gradient in the root causes the upper surface to elongate and the root to grow downwards.

Auxins made by the terminal growth point of a plant inhibit the growth of side shoots. Removing the apex of the plant causes side branching, a useful effect when growing a hedge. Auxins have practical uses as weedkillers, stimulants for ripening, and rooting powders.