The Nervous System
A Stimulus is a change in the environment. Organisms must respond to stimuli to survive.
Receptors are the cells that detet stimuli, there are many types of receptors such as taste receptors on the tongue. Receptors can form part of larger, complex organs such as the retina of the eye that is covered with light receptors.
Effectors are the response to nerve impulses to bring about change. They can also form part of a larger, complex organ. There are 2 types of effectors- Muscle cells (make up the muscle) and hormone secreting cells (found in glands e.g. cells that give out the ADH hormone are found in the pituitary gland).
The nervous system is made up of different parts: the Central Nervous System, Sensory Neurones, Motor Neurons and effectors.
The CNS only consists of the brain and spinal cord in vertebrates. In mamals it is connected to the body by sensory and motor neurones that make up the peripheral nervous system.
Sensory Neurones carry impulses from the receptors to the CNS. Motor Neurones carry impulses from the CNS to the effector.
The CNS is the processing centre- it recieves information from the receptors and coordinates a response.
For example a bird is being stalked by a cat:
1) Out of the corner of its eye it spots a cat moving towards it (this is the stimulus)
2) The receptors in the birds eye are stimulated. Sensory neurones carry the information from the receptors to the CNS.
3) The CNS decides what to do about it.
4) The CNS sends information via motor neurones to the muscles in the birds wings (the effector).
5) The muscles contract and the bird flies away to safety.
Neurones are nerve cells that link up the nervous system.
Electrical impulses travel down the axon. Axons are made from the nerve cells cyptoplasm stretched out into a long fibre and surrounded by a cell membrane. Some are surrounded by a fatty sheath which acts as an electrical insulator, sheilding neighbouring cells and speeding up the electrical impulse.Electrical impulses carry information around the body quickly so that the responses can happen fast, this means they are really short reactions such as if you touch something hot you instantly move your hand away.
Hormones are also used to carry information around the body but these happen much slower and are longer lasting.
A gap between 2 neurones is called a synapse. There is a synapse between every neurone so transmission is done using transmitter chemicals.
1) When an electrical impulse reaches the end of a neurone it triggers the release of transmitter chemicals in the synapse.
2) The transmitter chemicals diffuse across the gap and bind to the receptor molecules on the neurone (only specific transmitter chemicals can bind to the receptor molecules on the neurone)
3) When the chemicals bind to the right receptors they trigger a new electrical impulse in the next neurone.
Many drugs (like ecstasy and prozac) interfere with the transmission of impulses across a synapse. For example prozac blocks the reuptake channel (after the receptors have been binded the leftover serotonin is reabsorbed) so more serotinin binds making the person happier.
Reflexes are involuntary responses. The route taken by the information in a reflex is called a reflex arc.
The neurones in reflex arcs go through the spinal cord or an unconcious part of the brain.
1)When a stimuli is detected by receptors an impulse is sent along a sensory neurone to the CNS.
2)In the CNS the sensory neurone passes on the message to another type of neurone called a relay neurone.
3) The relay neurone passes the impulse to a motor neurone which goes to the effector. This way no information is processed so the reactions are rapid and involuntary.
Reflexes are necessary for survival. Here are some in humans:
- If a bright light is shone in our eyes they may become damaged so the muscles contract making the pupils smaller allowing less light to go into the eye.
- If someone picks up a hot object they drop it
Modifying and learning reflexes
Reflexes can be modified and you can learn new ones. For example if your holding a hot dinner plate the first reflex is to drop it but then you woudnt have any tea so the reflex is modified and you move quickly to place it down.
A stimulus causes a particular reflex response but animals can learn to produce the same response a to a new (secondary) stimulus, this is called conditioning and the new reflex is called a conditioned reflex. Pavlovs dog shows this:
Pavlov studied the behaviour of dogs and saw that they would salivate everytime they smelt food (this is a simple reflex to a primary stimulus). Pavlov rang a bell and give the dog some food he continued to do so for a while. One time he just rang the bell and the dog began to salivate when there wasnt any food. The dog responded to the secondary stimulus (the bell). This is a conditioned reflex. The final response has no direct connection to the secondary stimulus.
Conditioned reflexes can increase chances of survival. For example brightly coloured insects are often poisonous so a bird would learn not to eat them to enable its survival.
Brain Development and Learning
The brain is made of neurones all interconnected- it contains billions of things. This allows you to do things like:
- Modify behaviour as a result of an experience
- Coordinate complicated behaviour e.g. socialising
As you learn things connections form between neurones that were previously unnconnected.
Babies will learn things from experiences and by the age of 3 most of the connections that will ever form have been formed. The number of connections remains constant until the age of 10.
Repeating experiences makes the pathways that the nerve impulses travel down become strengthened. Strengthened pathways are more likely to transmit impluse than others.
After the age of 10 the pathways that arent often used die off. This makes it harder for old people to learn new things like a foreign language although adults can still learn.
Studying the Brain
Scientists use a range of methods to study the brain:
- Studying patients with brain damage
- Electrically stimulating the brain- this is done by pushing a tiny electrode into the brain and giving it an electric shot. By observing the effects of different areas of the brain they can estimate what each part does
- MRI (Magnetic resnance imaging) scans- it produced a detailed picture displaying the active parts of the brain. This can be used by playing music for instance and seeing which parts are active.
Memory is the storage and retreval of information. There are 2 main types of memory :
- Short term memory- last from a few seconds to a few hours. Its for thinking at the moment
- Long term memory- memories stored days, months or years ago
Humans are more likely to remember things with patterns or that are associated with strong stimuli e.g. smell, sound or colours.
How memory works
We arent really sure how memory works. One way of displaying it is the multistore model:
Learning Skills and Behaviour
Learning makes us more adaptable as there are more pathways in the brain.
Some skills have to be learnt by a certain age. For example if a child hasnt learnt to talk by the age of 10 they probably never will.The ability to communicate depends on the child hearing other people speak. Evidence comes form ferel children as they cant talk.
A famous case was the wild boy of Averyon who was raised by wolves and was 12. He showed signs of intelligence but couldnt talk.
A similar child was discovered at the age of 8 who couldnt talk but managed to pick up some words from the doctor said over time and developed a vocabulary of several hundred words.
The Cerebral Cortex is the outer part of the brain, it has a folded structure making the brain look wrinkled. The cerebal cortex plays a big role in things like intelligence, language and consciousness.