Responding to changes in the environment

Organisms have receptors (such as eyes, ears, tongues, auxins in plants etc) to detect external stimuli (such as light, sound-waves and chemicals). This enables them to detect what is going on around them and so helps improve their chances of survival.

The human nervous system is made up of the central nervous system (the brain and the spinal cord that are able to made decisions) and many nerves (made up of many nerve cells called neurons) that carry messages from receptors and to effectors. The effector is the organ that carries out the response. Sometimes this can be a gland, but it is usually a muscle. Motor neurons are nerve cells that carry an impulse from the central nervous system to a muscle (you need to know the structure of a motor neuron). A neuron can be very long, with the longest ones reaching from the tip of your toes up to your spine.

It takes a short time for a message to reach your brain, for your brain to think of a response and then for the message to travel to the effector and for it to react. Sometimes this would result in a response that is a little too slow to allow the body to protect itself. This is why, sometimes, the spine can make a simple response of its own. This is known as a spinal reflex arc. The impulse travels up the sensory nerve to the spine. It is then joined by a connector neuron to the motor neuron that immediately sends an impulse to respond. This cuts down the response time.

The rods and cones are cells in the retina that detect light. Rods detect how bright the light is. They are good for seeing at night, but only give a black-and-white image. Cones give a clearer image in colour. There are 3 types of cone: one blue, one green and one red. From these all the other colours can be formed. The fovea has only cones. This is where light is focused when you stare directly at something. It gives a clear coloured image (except when its dark). Around the fovea there is a mixture of rods and cones, giving good vision both in the light and in darkness. The outer parts of the retina have only rods. This is why you cannot see colours in your peripheral vision (although you are usually not aware of this). The rods and cones convert the light stimulus into an impulse, which passes down the optic nerve to the brain, which forms it into an image. There are no rods or cones at that part of the retina where the optic nerve leaves the eye. This area is called the blind spot.

The amount of light entering the eye depends on the size of the pupil, which is controlled by muscles in the iris. Light passes through the cornea then the liquid aqueous humour, followed by the pupil (the hole surrounded by the iris), then the lens, then through the liquid vitreous humour to land on the retina at the back of the eye. In order to see an image this light needs to be focused. The cornea, humours and lens all help to focus the light. However, the lens can vary how much it bends the light. When it is thin (stretched by the ciliary muscles) it does not bend the light very much, so it is good for focusing light from distant objects. When it is fatter it bends the light more, so it is better at focusing on close objects. This ability of the eye to change its focal length is called accommodation.

Plants also have receptors to detect stimuli in their environment and have effectors to respond. One example is phototropism, where a plant grows towards a light source. A hormone called auxin causes this to happen. Auxin is produced in the tips of shoots and promotes growth in the area just below the tip. When light falls on the shoot the auxin moves to the opposite side, away from the light. It then promotes the growth of the cells on that side only, resulting in the shoot bending towards the light.