One of the processes of all living things is sensitivity. This process involves detecting changes in the environment and responding to these changes. In large multi-cellular organisms like humans, a complex system is required to co-ordinate the necessary responses to the hundreds of changes in the surroundings that we detect. The nervous system enables humans to react to their surroundings and to co-ordinate their behaviour. The nervous system consists of receptors, nerve cells, and the central nervous system or CNS for short. Receptors detect changes in the environment, called stimuli. Nerve cells, called neurones, transmit messages to and from the CNS.

Cells called receptors are located in our sense organs, which are the eyes, ears, nose, tongue, and skin. Receptors detect changes in our surroundings. These changes are called stimuli.See if you can match the sense organs to the stimuli they respond to. Complete the diagram by dragging the qualities below to the correct boxes on the diagram.

Each nerve is a bundle of many neurones lying alongside each other. Each neurone sends electrical impulses from one place to another in the body. Some neurones are very long cells, like the ones that reach from the bottom of your back to the tip of your toes. Others are short, like the neurones in your brain. There are three types of neurone: sensory, relay, and motor neurones.

This is a sensory neurone. It transmits impulses from a receptor into the CNS. Sensory neurones can be over a metre long. The long axon is a special adaptation of the basic animal cell, which ensures very fast transmission of an impulse. The myelin sheath acts as an insulator and it also speeds up the transmission of impulses along the axon. Relay neurones are found in the CNS and, as the name implies, pass messages from one neurone to another, just as runners pass a baton in a relay race. A relay neurone is much shorter than a sensory or motor neurone. A motor neurone transmits impulses away from the CNS to a muscle or a gland, which then brings about a response to the original stimulus.

The CNS is composed of the brain and the spinal cord. Inside the CNS, impulses get passed from one neurone to another. For example, an impulse from a sensory neurone in your hand indicating that something is hot is passed within the spinal cord to the motor neurone, which stimulates your arm muscles to contract and move your hand away. At the same time, the neurone makes connections with other neurones via the many dendrites, sending impulses to the brain so that you register you have touched something hot. The brain receives many impulses at once, and co-ordinates them so that you may give a fairly complex response to a stimulus involving several different sets of muscles, or even a combination of different tissues, organs, and systems.

The nervous system allows us to respond to the changes in our surroundings, which are called stimuli. The principles of a response to a stimulus can be summarised in a key sequence. In this sequence, the stimulus is the change we feel, for example, a change in temperature, a sound, or a sight. The receptor is the specific receptor cell sensitive to one type of stimulus, for example receptors in the eye are sensitive to the stimulus light. The co-ordinator is the brain or spinal cord, which processes the information and makes a decision. The effector is a muscle or a gland. Sometimes there is more than one effector involved in a response. The response is the contraction of muscle tissue causing movement or the production of a secretion by the gland. If more than one effector is involved there will be more than one response.

When someone with a fear of spiders sees a spider, the stimulus is the sight of the spider. The receptor is in the eye and the co-ordinator is the brain. The effectors are the leg muscles, the jaw muscles, and the adrenal glands. The responses are running away, opening the mouth to scream, and the production of adrenaline. Sensory neurones carry messages as electrical impulses from the receptor to the co-ordinator. Motor neurones carry messages as electrical impulses from the co-ordinator to the effectors.

Stimuli may bring about a response by two types of pathways. One is a reflex action, over which you have no conscious control. It happens automatically. The other is a voluntary action, one you decide to take. An example of a reflex action is when you withdraw your finger from a hot object. Reflex actions are adaptations of the body to prevent harm to the body. They take place very rapidly, before you have time to make a decision. This is because very few neurones are involved in the pathway. You register what has happened after it has taken place. Place your mouse pointer over the graphics to see more examples of reflex actions. Click on Next when ready to continue.

Reflex responses are extremely fast because only a few neurones are involved. This arrangement of neurones is called a reflex arc. The sensory neurone transmits the nerve impulse into the back of the spinal cord. The nerve impulse is then transmitted through a small relay neurone to the motor neurone. It then travels along the motor neurone to the arm muscle, causing the arm to move. In a reflex response, the part of the CNS which is closest to the site of the receptor will be used to co-ordinate the response. The coordinator in this example is the spinal cord. For the change in pupil size of the eye, the brain is the coordinator.

Voluntary actions involve the conscious part of the brain and are brought about when a decision or choice is made. For example you can choose to get up and walk away from the computer at this moment, just because you want to! The response of moving away will be brought about in a similar way to the reflex responses, where an impulse travels from the co-ordinator, to an effecter, to cause a response. In the case of deciding to stand up and walk somewhere, the co-coordinator is the CNS. The effectors are the leg muscle cells, and the response is to move away. The difference between reflex responses and voluntary actions is that a voluntary response has to be learned and requires thought. The co-ordination is taking place within your brain in response to a thought of your own.

Between different neurones, there are small gaps called synapses. The synapse ensures that an impulse can only travel one way. The impulse is transmitted across the synapse by a chemical called a neurotransmitter. As an impulse reaches a synapse it causes the secretion of a neurotransmitter, which diffuses across the synapse and attaches to the next neurone. This causes an electrical impulse to be generated in the next neurone. Any excess neurotransmitter is quickly broken down to prevent an ongoing response.

A drug is any chemical substance that changes the chemical processes of the body. Drugs can be used to help prevent or cure disease in the form of medicines. All drugs have harmful side effects and they can be misused, resulting in damage to the body and dependency or addiction. Drug abuse is the term used to describe the misuse of drugs. Even drugs originally designed to be medicines can be misused. Please move the mouse pointer over each graphic to find out what harmful effects some drugs can have. Click on Next when ready to continue.

Many drugs affect the nervous system, particularly the brain, changing mood and behaviour. Drugs may interfere with the transmission of impulses at the synapses. Stimulants increase the speed of transfer of impulses and may even increase the frequency of transfer. Sedatives act by slowing down the transfer of impulses at the synapses, for example, by inhibiting the receptor sites. Alcohol slows down the transfer of impulses and therefore slows down the body’s reactions.

There are many ways in which drugs can harm the body. The liver is particularly at risk of damage because this organ breaks down toxins in the blood. If you take drugs the liver will have more toxins to break down. Other organs that are affected are the kidney, heart, lungs, and brain. Some of the dangers of drugs are involved in the actual method of taking the drug, for example the damage to the nose from sniffing glue or cocaine. One of the biggest concerns is the danger of contracting diseases by use of intravenous drugs. These drugs are injected into the veins using a sharp needle and syringe. This method is preferred by some regular drug users, because the effects of the drugs are more instant and may be stronger because the drug enters the blood system immediately. The danger is that so does any other foreign particle that may be contaminating a shared or non-sterile needle, such as HIV, the virus which causes AIDS, or the virus which causes hepatitis, another fatal disease. Solvents, tobacco, and alcohol are three drugs which are often abused by young people. All are addictive and all are harmful to the body in a number of ways.

The eyes are the sense organs which contain receptors that are sensitive to light. These receptor cells are found in the retina, the light sensitive layer, lining the back of the eyeball. The most sensitive part of the retina is the fovea because there are many receptor cells here. The blind spot is the part of the retina where blood vessels and neurones join the eyeball. There are no receptor cells here, hence its name, the blind spot. The optic nerve carries nerve impulses from the retina to the brain. Around the outside of the eye is a tough layer called the sclera. It is the white part of the eye that you can see.The front part of the sclera is transparent to allow light to pass through it. This is called the cornea. The iris is the coloured part of the eye. It contains muscles that alter the size of the pupil to control the amount of light entering the eye. The pupil is just a hole at the front of the eye to allow light into the eye. Just behind the pupil is the lens. This helps to focus light onto the retina. The lens is held in position by the suspensory ligaments, which are attached to the ciliary muscles. The ciliary muscles change the shape of the lens during focusing.

Light from an object enters the eye through the cornea and pupil. The size of the pupil is controlled by the muscles of the iris. There are two types of muscles in the iris, radial muscles and circular muscles. There are two types of muscles in the iris, radial muscles and circular muscles. In dim light the radial muscles contract to pull the pupil wider so that more light can enter the eye. In bright light the circular muscles of the iris contract to reduce the size of the pupil and prevent too much light from entering the eye, which could damage the retina.

Light enters the eye through the cornea, which is transparent and also slightly curved. It therefore refracts, or bends the light rays, beginning the process of focusing the light on the retina. The light then travels through the transparent lens, which focuses it to form an image on the retina at the back of the eye. The receptor cells then send impulses to the brain along sensory neurones in the optic nerve. The image formed on the retina is actually upside down. When the brain receives the message it turns it the right way up.

To see objects clearly at different distances, the lens has to change shape to refract light rays by differing amounts. When looking at a near object the rays of light coming from it will be diverging, or spreading out. Therefore they will have to be refracted by a larger amount in order to form a clear image. To do this, ciliary muscles relax so the suspensory ligaments slacken and do not pull the lens. The lens therefore has a rounded shape which refracts the light rays a lot, focusing them onto the retina. Light rays coming from a distant object are not diverging, they are almost parallel, so they do not need to be refracted as much in order to focus them onto the retina. The lens is pulled flatter when the ciliary muscles contract and pull the suspensory ligaments tight, which in turn pull on the lens. The process of changing the thickness of the lens is called accommodation and is brought about by contraction and relaxation of the ciliary muscles. The muscles contract or relax as much as is needed to ensure the lens is the correct thickness to focus the image of the object on the retina.