The Nervous System
The Nervous system detects and reacts to stimuli
It is made up of different parts:
- Central Nervous System (CNS)- In vertebrates (animals with backbones) this consists of the brain and spinal cord only. In mammals, the CNS is connected to the body by sensory neurones and motor neurones -- these make up the peripheral nervous system (PNS)
- Sensory Neurones - The neurones that carry impulses from the receptors to the CNS.
- Motor Neurones- The neurones that carry impulses from the CNS to effectors.
- Effectors- All your muscles and glands, which respnd to nervous impulses.
The Central Nervous System (CNS) coordinates the response
The CNS is a processing centre- it recieves information from the receptors and then co-ordinates the response.
Receptors and Effectors
Receptors and Effectors can form part of complex organs
- Receptors are the cells that detect stimuli.
- There are many different types of receptors, such as taste receptors on the tongue and sound receptors in the ears.
- Receptors can form part of larger complex organs e.g. the retina of the eye is covered in light receptor cells.
- Effectors respond to nervous impulses and bring about a change. Effectors can also form part of complex organs.
- There are two type of effector:
Muscle cells -- which make up muscles
Hormone secreting cells - which are found in glands, e.g. cells that secrete the hormone ADH are found in the pituitary gland.
Neurones and Synapses (1)
Information is transmitted around the body by Neurones
When stimulated, neurones transmit information around the body as electrical impulses.
- The electrical impulses pass along the axon of the nerve cells.
- Axons are made from the nerve cell's cytoplasm stretched out into a long fibre and surrounded by a cell membrane.
- Some axons are also surrounded by a fatty sheath that acts as an electical insulator, shielding the neurone from the neighbouring cells and speeding up the electrical impulse.
ELECTRICAL IMPULSES- carry information around the body really quickly so the response happenes fast but they're short lived.
HORMONES are also used to carry information around the body- they're produced in glands and travelled around in the blood. The responses they cause are brought about more slowly and they're longer lasting that the responses caused by nerve impulses.
Neurones and Synapses (2)
The Gap Between Two Neurones is called a synapse
- When an electrical impulse reaches the end of a neurone it triggers the release of transmitter chemicals into the synapse.
- The transmitter chemicals diffuse across the gap and bind to receptor molecules on the membrane of the next neurone.
- Only specific transmitter chemicals can bind to the receptor molecules on the neurone.
- When the chemicals bind to the right receptors they trigger a new electrical impulse in the next neurone.
Some drugs affect transmission across synapses
Many drugs and toxins can interfere with the transmission of impulses across a synapse. For example:
Ecstasy works by blocking sites in the brain's synapses where the transmitter chemical serotonin is removed. Because the serotonin can't be removed, the concentration increases- which affects the person's mood.
Reflexes are involuntary responses
- Reflexes are rapid, automatic responses to certain stimuli
- Reflexes are quick because you don't think about them -- they're involuntary
- The route taken by the information in a reflex (from receptor to effector) is called a reflex arc.
The reflex arc goes through the Central Nervous System
- The neurones in reflex arcs go through the spinal cord or through an unconscious part of the brain.
- when a stimulus (e.g. a [painful bee sting) is detected by receptors, an impulse is sent along a sensory neuron to the CNS.
- In the CNS, the sensory neuron passes on the message to another type of neuron-- a relay neuron.
- The realy neuron passes the impulse to the motor neuron, which then travels to the effector (e.g. a muscle). The muscle then contracts. An impulse always takes the same direct route through the reflex arc. This is why reflexes are involuntary and rapid.
Simple Reflexes improve the chance of survival
Simple animals have no brain- they rely entirely on simple reflex actions. Simple reflexes cause these animals to respond to some stimuli in a way that helps them survive, for example:
- Finding food
- Sheltering from predators
Humans also have simple reflexes that may protect them from damage or increase their chances of survical:
Very bright light can damage the eye- so there's a reflex to drop it. In very bright light, muscles in the eye contract making the pupil smaller, allowing less light in.
Newborn babies have reflexes that are lost as they develop, for example:
- they'll automatically suckle from their mothers
- they'll grasp when their palms are touched
- they'll try to take steps when their feet are put on a flat surface.
Modifying and Learning reflexes
Reflex responses can be modified by the brain
E.g. when you pick up a hot object the reflex response is drop it in order to protect your skin, but the reflex arc is over-ridden by a neuron between the brain and the motor neuron of the reflex arc.
Reflex responses can also be learned
A stimulus causes a particular reflex response, but animals can learn to produce the same response to a new (secondary) stimulus. This is called conditioning - the new reflex is called a conditioned reflex. An example of this is Pavlov's dogs
Conditioned Reflexes can increase chances of survival
Birds develop a conditioned response to brightly coloured insects so they are not harmed. By learning to avoid these insects, the birds are increasing their own chances of survival.
Brain Development and Learning
The brain is complex so it can modify behaviour as a result of experience and co-ordinate complicated behaviour e.g. social behaviour.
The Environment can affect Brain Development and Learning
When experiences are repeated over and over again the pathways that the nerve impulses travel down become strengthened. Strenghtened pathways are more likely to transmit impulses than others.
Learning Skills and Behaviour
Being able to learn means you can adapt to new situations
Complex animals are incredibly adaptable - they're able to cope with whatever the environment throws at them.
They're adaptable because of the variety of potential pathways in the brain.
Simple animals (like worms) have les flexible nervous systems, making their behaviour more predictable and less adaptable.
Some Skills only develop at certain ages
Some nerve pathways need to be strengthened at a particular age.
The ability to communicate by language depends on a child hearing other people speak. It is thought that they must hear this during a critical period. Evidence that demonstrates this comes from studies of feral children.
It is an important part of the brain:
- the outer part of the brain
- it has a folded structure
- plays an important part in things like intelligence, memory, language and consciousness.
Studying the Brain
Scientists use a range of methods to study the Brain:
- Studying patients with brain damage
- Electronically stimulating the brain
- MRI scans.
Memory is the storage and retrival of information
- Short-term memory- uswed for information that you are thinking about at the moment
- Long-term memory
Humans are more likely to remeber things when they see a pattern, or repeated or associated with a strong stimuli, e.g. bright light
Memory models try and explain how memory works
- Information that you've paid attention to is temporarily stored in short-term memory. If it's repeated enough it's transferred to long-term memory and stored there.
- Memories that are never transfereed from the short term memory to the long term memory are forgotten, but but information can be retrieved from the long term memory and remembered.
So far no model has provided a satisfactory explanation of human memory.