Double Award - Science for GCSE

Chapter 7

Senses and nerves

1.	Senses and stimuli We depend on our senses and sense organs in order to survive. We have five sense organs: eyes to see ears to hear nose to smell tongue to taste skin to touch and feel Our sense organs have specialised cells called receptors which are sensitive to changes around us. These changes around us are called stimuli.
2.	The nervous system The nervous system carries messages (impulses) from one part of the body to another. It co-ordinates and controls the whole body. There are two parts to the nervous system: the central nervous system (CNS) . the brain and spinal cord; a network of nerves linking the CNS to every part of our bodies. When receptors detect a stimulus, they send a signal to the brain. The signal is a tiny electrical impulse which travels along nerve cells. Nerve cells are called neurones. Each neurone has a cell body with a nucleus somewhere along a long, thin nerve fibre.
3.	Neurones and our response to stimuli Neurones carry tiny electrical impulses from one to another. In this way, they can pass messages (signals) and provide a link between receptors in our sense organs, our central nervous system and our muscles which respond to the signals. When we respond to a stimulus (e.g. a bright light), three things happen: Sensory neurones carry impulses from the sensory organ (e.g. the eye) to the CNS. Relay neurones in the spinal cord and the brain connect sensory neurones to motor neurones. Motor neurones carry impulses from the CNS back to muscles, glands and other organs for their response. Muscles, glands and other organs which respond to these impulses are called effectors.
	There are junctions between neurones called synapses which ensure that impulses (messages) are passed on effectively.
4.	Conscious and reflex actions When the telephone rings, you hear it and then answer it. This response involves impulses (messages) going to your brain. This is called a conscious action. When you see a sudden bright light, your response is much quicker. This is called a reflex action. In reflex actions, impulses travel by the shortest route which may not include the brain. These shortest routes are called reflex arcs.

5.	The eye – an important sense organ We see things with our eyes. They respond to the stimulus of light. The structure of the eye and the parts which help us to respond to light are shown in the diagram below: Our eyes respond to changes in light by changing the size of the iris. When it is bright, the iris extends further over the lens and the pupil is smaller. When it is dark, the iris contracts and the pupil gets bigger to let in more light. Our eyes focus using the curved surface of the cornea and the lens. When we look at something close up, the lens becomes fatter. When we look at something in the distance, the lens becomes thinner. This ability of the lens to change its shape and focus clearly is called accommodation.