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Each of our eyes has about 125 million rods and 7 million cones. However, they are not evenly distributed in the retina. Most of the cones are

concentrated in the centre of the retina, at the position called the fovea (see Figure 17). This is where the main part of an image is formed when we look straight at an object. The rods are spread much more widely and they cover most of the back part of the eye. One exception is the area where the optic nerve leaves the eye, the so-called blind spot.

The retina contains about 130 million receptors, each of which can be stimulated by a spot of light falling on it.You can imagine, therefore, that when an image falls on the retina it will have an effect rather like an old

Figure 19 How the rods and cones are connected to other cells in the retina.

retina

The number of rods and cones in the field of view of a light microscope was counted at frequent intervals along a

horizontal line across the retina of a right eye. The graph in Figure 20 shows the results.

a) Which parts of the retina are at points X and Y? Explain your answer.

b) Describe the distribution of cones.

c) Describe the distribution of rods.

Q 6

Figure 20 Graph to show the distribution of rods and cones in the retina of a right eye.

2000

Look carefully at the diagram of the retina in Figure 19. Describe how the way in which the cones connect to the fibres of the optic nerve differs from the way in which the rods connect.

Q 5

Figure 21 How the cones function to allow the brain to distinguish separate spots of light.

impulses

impulses impulses

brain ‘sees’ one spot of light a) 1 cone stimulated

brain ‘sees’ two separate spots of light

b) 2 cones stimulated c) 2 cones stimulated

newspaper photograph, which consists of a mass of tiny dots. Where light falls the receptors will be stimulated, and where there is a dark patch there will be no stimulation. Each receptor that is stimulated can pass an impulse to the brain and thus the brain can interpret the pattern. This is more or less how it works at the fovea where all of the receptors are cones. Each cone in the fovea connects through a single bipolar cell and ganglion cell to one fibre in the optic nerve.

The optic nerve has about 1.2 million fibres. Since there are 130 million receptors there obviously cannot be individual connections to the brain for all the receptors.You can see from Figure 19 that each bipolar cell has synapses that connect with several rods. The bipolar cells in turn have more than one synapse with an optic nerve fibre. Although the simplified

diagram shows only a few synapses, in practice there must be an average of about 100 receptors with synaptic connections to each optic nerve fibre. The cones outside the fovea may also have several connections. This affects the amount of detail that can be perceived in an image on the retina outside the fovea.

How clearly we can see an image depends on two factors: sensitivity and visual acuity.

• Sensitivity is how much light is needed to stimulate the receptors. Rods are stimulated in much dimmer light than cones, so rods are more sensitive.

• Visual acuity is how far apart two spots of light can be seen as being separate. This is affected by the way in which the receptors are connected to the optic nerve fibres.

A ray of light that falls on just one cone in the fovea will show up as a spot of light, as long as it is bright enough to stimulate the cone. This is because the cone is connected to a single fibre in the optic nerve via one bipolar cell (Figure 21a). If another ray of light falls on another cone, as in Figure 21b, the brain will interpret this as two separate spots.

How many spots of light will be seen in Figure 21c? Explain your answer.

Q 7

Vision

Now consider what will be seen when a ray of light falls on a single rod.

Figure 19 on page 147 shows three rods with synapses to each bipolar cell and six rods that connect via the bipolar cells to a single ganglion cell.

Assuming that the stimulation of one rod cell is sufficient to send an impulse to the brain, there is no way in which the brain can interpret which of the six rods had been stimulated. In reality there may be many more than six rods with synapses to a single ganglion cell.

So, what is the advantage of having many rods connecting to a single fibre in the optic nerve? As we shall see in Chapter 7, synapses can act as barriers to the transmission of impulses. Although the rod is very sensitive to light, a single rod is unlikely to produce a sufficiently large generator potential to be able to stimulate the bipolar cell to transmit a nerve impulse.

However, if a group of rods is stimulated by dim light at the same time, the combined generator potentials will reach the threshold required for

transmission across the synapse to the bipolar cell and the fibre in the optic nerve. This means that an impulse can be generated in the bipolar cell and then transmitted to the optic nerve fibre. This process is called summation, because the effect on several cells is added together. Although the image is less sharp, we are able to see in much dimmer light than would be possible with cones alone. The eyes of most nocturnal mammals have only, or mostly, rods. For an animal that is hunting or hunted at night it is more useful to see a slightly fuzzy image than to be unable to see at all.

How would you expect the visual acuity of the rods to compare with that of the cones? Explain your answer.

Q 8

Coordination

The human brain is estimated to have about 100 billion (10¹¹) neurones. If we gave equal portions of a single brain to every person in England, he or she would get about 2000 cells.

Moreover, each neurone has synapses connecting it to as many as 10 000 other neurones. This creates a neural network that makes the average computer seem like a child’s toy.

Figure 1 This photograph was taken by a scanning electron microscope and shows a single neurone and some of its branches that connect it to other neurones in the brain.

The brain is the organ responsible for coordinating the great majority of activities in the body. It synchronises most of the automatic activities such as heartbeat and breathing, as well as such complex movements as walking or playing the piano. The brain receives and processes a constant flow of information from sensory receptors. This information may stimulate appropriate responses, or be stored or ignored. The brain is also responsible for what we consider to be the higher human activities, such as thinking, emotions, memory and

consciousness.

Different areas of the brain are responsible for different functions, as you can see from Figure 2, but neuroscientists have discovered that these subdivisions are not sharply defined and that the key to the

Coordination

brain’s functioning lies in its network of connections. It may seem impossible to understand exactly how such a complex organ works, and our progress has been slow. Much of the understanding has come from evidence based on what happens when things go wrong, for example as a result of damage caused by accidents, blood clots and the effects of drugs.

One of the first and most famous examples was an accident that

happened to Phineas Gage in 1848. 26-year-old Phineas was in charge of a gang of men building a railway in the north of the USA. The men would drill a deep hole into a rock face, half fill it with explosive powder and then add a fuse and sand. Phineas had to compress the explosive mixture with a long iron bar before lighting the fuse. One day, when not paying full attention, he started to ram down the mixture with his iron bar before sand had been added. A spark caused the powder to explode with such force that the metre long iron bar shot through his left cheek and the front part of his brain before passing out of the top of his skull and landing some 25 metres away. Amazingly, after a few minutes he was able to get up and talk intelligibly. Somehow the doctor who looked after him managed to avoid the wound becoming seriously infected, and after a couple of months Phineas recovered almost fully. He had lost none of his powers of speech or mobility. However, his personality changed significantly. He was no longer the careful, responsible foreman. He became impulsive, unreliable, selfish, dishonest and prone to swearing.

As a result he was unable to go back to his old job. He lived for another 12 years but never recovered his previous personality.

This case demonstrated that the frontal lobes of the brain are not essential for basic functions, such as muscle activity, vision or speech, but that they are involved in determining a person’s behaviour. It is now known that the frontal lobes are important in organising social behaviour and in planning complex operations.

Figure 2 Side view of a human brain. The approximate positions of some parts and their functions are shown. Many other functions take place in central areas that cannot be seen from the outside, or involve coordination between different areas.

frontal lobe, involved in control of behaviour and emotions motor cortex, controls voluntary muscle activity

speech control area

medulla, regulates automatic activities, such as breathing and heart rate

cerebellum, controls fine muscle movements and balance

visual cortex, processes information from eyes cerebral hemisphere, contains areas involved in memory, reasoning and emotions sensory cortex, processes

information from receptors

Figure 3 Phineas Gage’s skull clearly showing his injury.

Further observations have helped to clarify what other parts of the brain do. One common observation is that someone who is knocked

unconscious, for example in a road accident, has no memory of the events leading up to the accident. This suggests that there is a delay before memory traces are fixed in the brain. It has now been discovered that damage to the hippocampus, near to the base of the brain, prevents the formation of new short-term memories. However, memories of past events, people and places are not affected. It seems that memories are normally first formed in the hippocampus and then later transferred to the cortex of the brain for permanent storage. This may be a way in which the brain ‘tidies up’ its filing system and ‘weeds out’ items that do not need to be retained for a long time. The hippocampus is often found to have degenerated in elderly people who are suffering from Alzheimer’s disease, which may explain why they have difficulty in remembering day-to-day events but often still retain memories from the distant past.

Other evidence from individuals with localised brain damage suggests that memory and other brain functions depend on complex processes of communication within the brain. In one example a brain-damaged man shown a picture of a rhinoceros was able to give details such as its size and that it lives in Africa, but when asked to describe a rhinoceros he could only say that it is an animal. In another brain-damaged man the opposite was found. He was able to give details when asked to describe a rhinoceros but when shown a picture he was unable to recognise or name the animal or give any information about it. Many similar examples indicate that the brain processes information obtained by sight and by hearing in separate parts.

These separate stores then have to be coordinated. Damage to one or other of the stores, or to the system that connects the stores, can destroy the ability to link the two sources of information.

Chemical substances also can interfere with the working of the brain.

Alcohol is well known for its mind-altering capacity, as are many other drugs. Alcohol, or ethanol to be more chemically precise, interferes with the synapses. The effect is to inhibit activity in the brain by preventing neurones communicating chemically with each other. For example it inhibits activity in the cerebellum, which normally coordinates fine motor skills and balance.

Hence quite small amounts of alcohol make you less steady on your feet and less quick to react. Oddly, alcohol is noted for reducing inhibitions and making people more sociable. This is because it also reduces the neuronal activity that normally inhibits social interactions, making it more likely that someone will reveal personal secrets and do embarrassing things.

So, how is it possible that tiny pulses of electrical activity and the transfer of chemicals across synapses and membranes can result in the complex mass of thoughts, feelings, actions, memories and emotions of which humans are capable? Neuroscientists are still a long way from full understanding, but slowly they are discovering the functions of different regions of the brain and how these interact. In this chapter we shall study the basic processes necessary to understand how the brain works and how the nervous system coordinates our activities. This will enable us to explain how chemicals such as alcohol and other drugs can disrupt the system.

Neurones

Neurones

First we need to look at the structure of the nerve cells, called neurones, which form the conducting tissue in the nervous system (Figures 4 and 5).

Figure 4 The structure of a motor neurone.

dendrite nucleus

axon

cell body

myelin sheath node of Ranvier synaptic

knobs

cytoplasm

Figure 5 Light micrograph of a motor neurone and its effector, the muscle fibres.

In which direction do impulses pass through a motor neurone?

Explain your answer.

Q 1

In Chapter 6 (page 134) you learnt that a simple reflex arc has three different types of neurone.

• A sensory neurone transmits impulses to the spinal cord.

• A motor neurone conveys impulses from the spinal cord to a muscle.

• One or more connector neurones act as links between the sensory and motor neurones.

Figure 4 shows the structure of a motor neurone.You can see that the cell body contains cytoplasm and a nucleus and looks similar in structure to other animal cells. The cytoplasm also contains mitochondria and ribosomes.

In other respects a motor neurone is highly specialised. Its cell body is situated inside the spinal cord. It has large numbers of branching fibres, called dendrites. A motor neurone may have over a hundred of these thin branches, each with one or more synapses from a connector neurone.There is also one very long fibre that extends from the spinal cord to a muscle.This axon can be as much as a metre in length and less than a micrometre in diameter. At the far end of the axon are short branches that terminate in tiny synaptic knobs.

These knobs fit into little pockets on the surface of muscle fibres. Large numbers of axons are bundled together into nerves.The sciatic nerve, for example, originates from the spinal cord in the lower back. Branches from it pass all the way down the leg to muscles in the foot.The sciatic nerve also contains the fibres of sensory neurones conducting impulses in the opposite direction. Damage to the lower back can compress this nerve where it passes between the vertebrae, causing a painful condition called sciatica.

axon branches axon of motor

neurone

motor neurone innervating muscle fibres muscle fibres

You can see from Figure 4 on the previous page that the axon is surrounded by the myelin sheath. This sheath is not strictly part of the neurone. It is made from highly specialised cells, called Schwann cells, that lie alongside the axon (Figure 6). As the axon grows these cells wrap round and round the axon until there may be up to a hundred layers of lipid and protein membranes. This makes a fatty ‘bandage’ around the axon shielding it from surrounding tissue fluid. Between each Schwann cell is a tiny gap where the axon is exposed, called a node of Ranvier. These nodes are the only places that ions can pass between the tissue fluid and the axon through the plasma membrane. The nodes play an important part in speeding up the passage of impulses along the axon, as we shall see later.

Axons contain no organelles other than a few mitochondria. To stimulate the muscles to contract the synaptic knobs secrete substances into the synapses. To make these substances they need proteins and polypeptides as well as other compounds. These have to be transported all the way from the cell body to the end of the axon. If you imagine the cell body of a neurone being magnified to the size of a golf ball, this is like sending supplies for about a mile down a tube roughly the diameter of a ball-point pen.

Impulses

Impulses are waves of electrical activity passing through a neurone. The process is not the same as the conduction of electricity through a wire. It is much slower, although it is still pretty quick. When a neurone is stimulated at a synapse, there is a brief change in the plasma membrane of the fibre.

This allows ions to pass through rapidly. As soon as this happens it causes the next small section of the membrane to change, so ions can pass through here. Meanwhile the first section of fibre changes back as the distribution of ions is restored. This process carries on down the fibre. It is rather like a long line of standing dominoes that are knocked down one after another once the first one is pushed over, except that in this case the domino is immediately set upright again as soon as the next one falls over.

Note that we have referred to a fibre and not an axon in this section. This is because in a neurone the term ‘axon’ should only be used for the long fibre that leads away from the cell body towards a muscle, or other effector. The synapses where a motor neurone is stimulated would be on the dendrites, which are also fibres, but not axons. In fact, impulses pass along all parts of the plasma membrane of a neurone in the same way. This includes the surface of the cell body as well as the dendrites and axon.

So, what causes the movement of ions? Before we explain, it might be a good idea to remind yourself of the ways in which ions can pass through a plasma membrane. In the AS course you learnt about the ways that ions

So, what causes the movement of ions? Before we explain, it might be a good idea to remind yourself of the ways in which ions can pass through a plasma membrane. In the AS course you learnt about the ways that ions

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