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- Describe the human nervous system in terms of the central nervous system (brain and spinal cord as areas of coordination) and the peripheral nervous system which together serve to coordinate and regulate body functions.
The human nervous system consists of two main parts:
- The central nervous system – The main components of the CNS are the brain and the spinal cord.
- The peripheral nervous system – this consists of receptor cells (the cells that detect changes in the stimuli and send information down the sensory neurones), sensory neurones (the neurones that carry information from receptors) and the motor neurones (the neurones that carry information to the effectors).
The role of the CNS is to coordinate the messages travelling through the nervous system. When a receptor detects a stimulus, it sends an electrical impulse to the brain or spinal cord, which then sends an electrical impulse to the appropriate effectors.
Examples of receptors include taste receptors (in your taste buds), thermoreceptors (these detect temperature changes and are present in your skin and in the hypothalamus in your brain) and osmoreceptors (these detect changes in the water potential of your blood).
A stimulus is any factor in the environment (light, temperature, etc.) or inside your body (blood sugar, blood water potential, etc.) that changes.
- Describe the structure and function of the eye, including accommodation and pupil reflex.
Each eye is set in a bony socket called the orbit. Only the front of the eye isn’t surrounded by bone. The front of the eye is covered by a thin, transparent, protective membrane called the conjunctiva. It’s always kept moist by the fluid made in the tear glands. This fluid contains lysozymes – an enzyme that can kill bacteria. When your eyelids blink, this fluid is washed over the conjunctiva. Your eyelids, eyelashes and eyebrows also help to stop dirt from falling into your eyes.
The part of your eye inside the skull is protected by a very tough coat called the sclera.
Your eye is attached to your skull by muscles (shown by the branches that extend up and down from the eyeball in the diagram).
The iris is the coloured part of your eye. It contains pigments that absorb light to prevent it from getting through to the retina. The gap through which light enters the eye is called the pupil. The size of the pupil (ie. the aperture of the eye) can be adjusted to control how much light enters the eye. If too much light enters the eye, this can damage the retina.
The suspensory ligaments hold the lens in place and contract and relax to change the shape of the lens so that light can be focused onto the fovea. The aqueous humour contains salts to nourish the lens.
The main body of the eye (called the vitreous chamber) contains the semi-solid fluid called the vitreous humour. This supports the eyeball by helping it retain its shape.
Next is the retina – this is the part that’s actually sensitive to light (it contains the receptor cells). The part of the retina that contains the most receptor cells (and so is the most sensitive to light) is the fovea. Some receptor cells are sensitive to light of different colours, helping to build up a coloured image.
When light falls onto the retina, impulses are sent down the optic nerve. Note that there are no receptor cells in the part where the optic nerve leaves the eye, so this part is called the blind spot.
Behind the retina is a black layer called the choroid. It absorbs the light after it passes through the retina, to prevent it from scattering around the eyeball. It is also rich with blood capillaries, to help nourish the eye.
There are two types of receptor cells in the retina: rod cells and cone cells. Rod cells are sensitive to dim light, but can’t detect colours. Cone cells, on the other hand, are sensitive to colour but are only functional in bright light. There are three types of cone cells – ones that detect red light, blue light and green light (the three primary colours of light).
The fovea is made up almost entirely of cone cells, allowing a sharp, colourful image to be produced. Rod cells are more commonly found further out in the retina, showing us a less detailed image in dim light.
The iris contains two types of muscles – circular muscles and radial muscles. These control the contraction and relaxation of the pupil to change how much light enters the eye.
Radial muscles run outwards from the edge of the pupil and circular muscles circle the pupil.
In bright light, the radial muscles relax and the circular muscles contract, making the pupil smaller.
In low light, the opposite occurs – radial muscles contract and the circular muscles relax, causing the pupil to dilate.
This action of the muscles in the iris is called pupil reflex or iris reflex.
This is an example of a reflex action (because we do not need to make the conscious decision to do it – our body does it itself.)
This is the ability of the eye to change the shape of its lens to alter its focus from distant to near objects, and vice versa.
To focus light on the fovea, light must be refracted by the eye. This is done by the cornea and the lens. The cornea does most of the refracting, and the lens makes finer adjustments to get a clear, sharp, focused image.
The humours in the eye are both transparent and colourless, allowing light to pass through easily.
Note that the image formed on the retina is inverted (upside down), but the brain interprets it so that we perceive it the right way up.
Light rays coming from a distance will be almost parallel, so they need to be refracted less.
The lens is held in place by a ring of suspensory ligaments. The tension in the suspensory ligaments, and thus the shape of the lens, is altered by the ciliary muscle.
When focusing on distant objects, the ciliary muscle is relaxed, so the pressure in the vitreous humour causes the suspensory ligaments to be pulled tight. This causes the lens to be stretched thin, so the light is refracted less, allowing it to focus on the fovea.
When focusing on nearby objects, the ciliary muscle contracts, allowing the suspensory ligaments to relax, so the lens gets thicker (it bulges), refracting the light more.
- Identify the motor (effector), relay (connector) and sensory neurones from diagrams.
The part of the neurones that contain the nucleus is called the cell body.
A long cytoplasmic branch stretch out from the cell body – these are called axons. Axons are very long – in fact, there’s actually one that starts in your brain and ends in your big toe! The electrical impulse that neurones transmit sweep along axons.
In some sensory and motor neurones, the axons are insulated by ‘Schwann cells’. These cells wrap around the axon to form myelin sheaths, as a form of insulation. The exposed spaces between the myelin sheaths are called nodes of Ranvier. As the space under the myelin sheath is insulated, impulses can skip this part of the axon, and instead jump from node to node, allowing them to travel along the axons much faster.
The thin cytoplasmic processes that extend from the cell bodies of the motor and relay neurones are called dendrites. While motor neurones and relay neurones have multiple dendrites, sensory neurones only have one – this is the long, thin cytoplasmic branch that picks up signals from the receptor and carries them back up through the sensory neurone towards the CNS. Note that dendrites are the cytoplasmic branches that pick up signals and carry them towards the cell body, while axons are the branches that carry them away from the cell body.
At the end of every axon, the neurone branches out, and at the end of each branch are the synaptic knobs (they’re labelled synaptic endings on the relay and sensory neurone diagrams, and the motor neurone diagram doesn’t even mention them. Don’t worry about that, though. We’ll call them synaptic knobs.)
When there are multiple neurones in one ‘pathway’, they don’t actually touch each other – instead, there is a space between them called the synaptic cleft. The neurone’s membrane before the synaptic cleft (the presynaptic membrane), the synaptic cleft, and the neurone membrane after the synaptic cleft (the postsynaptic membrane) make up the synapse.
When an electrical impulse reaches the end of the neurone (ie, when it reaches the presynaptic membrane), it triggers the release of chemical transmitter substances such as acetylcholine into the synaptic cleft. The chemical transmitter substances diffuse across the synaptic cleft and bind to the receptor molecules on the postsynaptic membrane. When the chemicals bind to the right receptors, they trigger a new impulse in the postsynaptic membrane, which is then swept along the postsynaptic neurone.
Also, this isn’t really mentioned in the syllabus either, but I’ve seen related questions in some past papers:
The brain and spinal cord actually have two major parts: grey matter and white matter.
Grey matter contains cell bodies, dendrites, unmyelinated axons (axons without myelin sheaths) and axon terminals. White matter is composed primarily of myelinated axons (axons with myelin sheaths) and contains some blood vessels.
- Describe a simple reflex arc in terms of sensory, relay and motor neurones, and a reflex action as a means of automatically and rapidly integrating and coordinating stimuli with responses.
A reflex action is a means of automatically and rapidly integrating and coordinating stimuli with responses.
The reflex arc:
Stimulus –> Receptor cells (not always present – some sensory neurones can act as receptors themselves) –> Sensory neurones –> Relay neurones (this part is entirely in the brain or spinal cord) –> Motor neurones –> effector –> the reaction
The interneuron in the diagram is the relay neurone.
Note: Technically, in a scientific context, the spelling is neuron not neurone, but Cambridge uses the spelling neurone, and neurone IS the traditional British spelling, so I recommend that you spell it neurone with an e.
Notes submitted by Sarah.
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