The Nervous System
All living organisms need to respond to changes in the environment. Although this happens in different ways the pattern of events is always the same.
stimulus > detection > co-ordination > response
Detecting the Stimulus- Receptors are specialised cells that detect a stimulus. Their job is to convert the stimulus into electrical signals in nerve cells.
Some receptors can detect several stimuli but they are usually specialised to detect one type of stimulus:
- light = photo receptors in the eye
- sound = vibration receptors in the ears
- touch, pressure, pain and temperature = different receptors in the skin
- taste and smell = chemical receptors in the tongue and nose
- position of the body = receptors in the ears
A sense organ is a group of receptors gathered together with some other structures. The other structures help the receptors to work more efficiently. An example of this is the eye.
Co-ordination- the body recieves information from many different receptors at the same time. Co-ordination involves processing all the information from the receptors so that the body can produce a response that will benefit the whole organism.
In most animals this is done by the central nervous system (CNS).
Response- Effectors are organs in the body that bring about a response to the stimulus. Usually these effectors are muscles and they respond by contracting. They could however be glands and they may respond by releasing an enzyme. Many responses are reflexes.
An example of a sense organ- the eye
The light enters the eye through the pupil. It is focused onto the retina by the cornea and lens. The size of the pupil can be changed by the muscles of the iris when the brightness of the light changes. The aim is to make sure that the same amount of light enters the eye. The job of the lens is to change shape so that the image is always focused on the light-sensitive retina.
The receptors are cells in the retina called roda and cones. They detect light and send messages to the brain along the optic nerve.
The lens must be a different shape when the eye looks at a close object compared with a distant object. This is to make sure that the light is always focused on the back of the retina. The ciliary muscle changes the shape of the lens shown in the diagram. This is called accommodation.
Some people have problems with their eyes:
- Long or short sight- the eyeball or lens is the wrong shape > long or short sight can be corrected by wearing convex or concave lenses respectively; cornea surgery can now also be used.
- Red-green colour blindness- lack of certain cones in the retina > no treatment
- Poor accommodation- lens becomes less elastic in senior citizens > wearing glasses with half convex and half concave lenses.
The eyes are also used to judge distances. Animals that hunt usually have their eyes on the front of their head. Each eye has a slightly different image of the object. This is called binocular vision and it can be used to judge distance. Animals that are hunted usually have eyes on the side of their heads. This gives monocular vision and they cannot judge distances so well, They can, however, see almost all around.
Neurones and synapses
To communicate between receptors and effectors the body uses two main methods. These are:
A neurone is a specialised call that is adapted to pass electrical impulses.
The central nervous system (CNS) contains millions of neurones but outside the CNS, neurones are grouped together into bundles of hundreds or thousands. These bundles are called nerves.
There are different types of neurones. The three main types of neurones are:
- sensory neurones- they carry impulses from the receptors to the CNS
- motor neurones- they carry impulses from the CNS to the effectors
- relay neurones- they pass messages between neurons in the CNS.
Types of Response
The nervous system is made up of the CNS and the peripheral nervous system.
The CNS is the brain and spinal cord. The peripheral nervous system is all the nerves passing information to and from the CNS.
Once the information reaches the CNS from a sensory neurone there is a choice: Either: The message may be passed straight to a motor neurone via a relay neurone. This is very quick and is called a reflex action.
Or: The message can be sent to the higher centres of the brain and the organism might decide to make a response. This is called a voluntary action. All refelxes are:
- do not need conscious thought
- protect the body
Examples of reflexes include the knee jerk, pupil reflex, accommodation, ducking and withdrawing the hand from a hot object.
- 1.Stimulus is detected by sensory cell.
- 2. Impulse passes down sensory neurones.
- 3.Relay neurone passes impulse to motor neurone.
- 4.Motor neurone passes impulse to effector
- 5.Muscle contracts
A voluntary action
Voluntary actions need a conscious decision in order to take place. They therefore always involve the brain.
The cerebral hemisphere is the area of the brain where the decisions are made. Nerve impulses from here are sent down the spinal cord to effectors via motor neurones.
It is vital that the internal environment of the body is kept fairly constant. This is called homeostasis. The different factors that need to be kept constant include:
- water content
- sugar levels
- mineral content
Many of the mechanisms that are used for homeostasis involve hormones. Hormones are chemical messengers that are carried in the blood stream. They are released by glands and pass to their target organ: brain, trachea, kidney,duodenum, scrotum, pituitary, thyroid, adrenal, ovary, and testes.
Hormones take longer to have an effect than the nerves but their responses usually last longer. Many of these control mechanisms work by negative feedback. This means that if the levels change too much, a hormone is released and this brings the change back to the normal level.
Control of blood sugar
It is vital that the sugar or glucose levels of the blood is kept constant. If it gets too low the cells will not have enough to use for respiration. If it is too high then the glucose may start to pass out in the urine. Insulin is the hormone that controls levels of glucose in the blood. When glucose is too high, more insulin is made. The insulin converts to excess glucose into glycogen to be stored in the liver.
Glucose in the blood > insulin > glycogen in the liver
People with diabetes do not produce enough insulin naturally. They need regular insulin injections in order to control the levels of glucose in their blood. They also need to control their diet carefully.
Hormones and Reproduction
Hormones are responsible for controlling many parts of the reproduction process.
- the development of the sex organs
- the production of the sex cells
- controlling pregnancy and birth
Testosterone > male > testes > stimulates the male secondary sexual characteristics.
Oestrogen > female > ovaries > stimulates the female secondary sexual characteristics; repair of the wall of the uterus; controls ovulation.
Progesterone > female > ovaries and placenta > prevents the wall of the uterus breaking down.
Using hormones to control reproduction
It is now possible to produce synthetic versions of these hormones. They can be used to control the fertility of women. This can happen in two main ways.
Some women find it difficult to get pregnant because they do not produce eggs regularly. These women can take a fertility drug. This contains hormones that are similar to FSH. The drugs stimulate the production of eggs and sometimes a number of eggs are released each month.
Sometimes women are treated with fertility drugs and eggs are removed from their body. The eggs can then be fertilised by sperm outside the body. The embryo can then be put back inside the uterus. This process is called in vitro fertilisation or IVF. It can be used on women who have blocked oviducts.
Other women may want to stop themselves becoming pregnant. They take drugs that are called oral contraceptives. These drugs contain hormones that prevent the pituitary gland releasing FSH. This means that the ovary will not produce eggs.