2.3.1 Structures and functions of the brain

The Brain and the Nervous System

1. The nervous system is divided into the central nervous system and the nerves that run to and from it.
2. The central nervous system, usually shortened to the CNS, is made up of the brain and the spinal cord.
3. The CNS receives impulses from receptors, coordinates a response, and sends impulses out to effectors.
4. The human brain contains billions of interconnected neurones, and different regions of it carry out different jobs.
5. The brain is protected by the bones of the skull, by three membranes called the meninges, and by a cushioning layer of fluid.

The Cerebral Hemispheres

1. The cerebral hemispheres are the two large halves that make up the top and front of the brain, and together they are the largest part of it.
2. Their outer layer is deeply folded, which packs a far larger surface area of nerve cells into the skull.
3. They control conscious thought, memory, language, intelligence and voluntary movement.
4. They also process the information arriving from the sense organs, so seeing, hearing and feeling pain all reach conscious awareness here.
5. Damage to a cerebral hemisphere, for example after a stroke, can cause loss of speech, memory or movement on one side of the body.

The Cerebellum

1. The cerebellum lies at the back and base of the brain, underneath the cerebral hemispheres.
2. It coordinates muscular activity, so that the muscles of a movement contract in the right order and with the right force.
3. It controls balance and posture, using information from the ears and from receptors in the muscles.
4. It makes movement smooth and precise rather than jerky, and it is involved in learning a physical skill.
5. Damage to the cerebellum causes unsteady walking, a loss of balance and clumsy, poorly aimed movements.

The Medulla Oblongata

1. The medulla oblongata sits at the base of the brain, where it joins the spinal cord.
2. It controls unconscious, or involuntary, activities that continue without any thought.
3. It regulates heart rate, breathing rate and blood pressure.
4. It also controls reflexes such as swallowing, coughing, sneezing and vomiting.
5. Because it controls the processes that keep a person alive, damage to the medulla oblongata is usually fatal.

Conscious and Unconscious Control

1. Conscious control means you decide to act, and the cerebral hemispheres send the instruction.
2. Unconscious control happens automatically, and the medulla oblongata handles the processes that must continue while you sleep.
3. Some actions involve both, and breathing is the clearest case because it is normally unconscious but can be taken over deliberately.
4. The cerebellum sits between the two, refining a consciously chosen movement without any conscious effort.

How the Regions Were Identified

1. Much of what is known about the brain came from studying patients with damage to a particular region.
2. Comparing the position of the damage with the ability the patient had lost suggested what that region normally does.
3. Electrical stimulation during brain surgery gave more direct evidence, because stimulating a small area produces a specific response.
4. Scanning techniques now allow the living brain to be studied without surgery, and these are covered in the next section.

2.3.2 Imaging brain function with scanning

Why the Brain Is Difficult to Study

1. The brain is enclosed by the skull, so it cannot be seen or reached without surgery.
2. Brain tissue is extremely delicate, and any attempt to reach it risks permanent damage to healthy regions.
3. The brain contains billions of neurones with enormous numbers of connections, so its structure is very complex.
4. Regions do not work in isolation, so removing or damaging one region to see what it does is neither ethical nor reliable.
5. Studying patients with brain damage gives useful evidence but is slow and opportunistic, because researchers must wait for a suitable case.

Scanning Solves the Access Problem

1. A scan produces an image of the brain from outside the skull, so no surgery is needed.
2. The patient is conscious and unharmed, so the same person can be scanned repeatedly over time.
3. The two techniques named in the specification are the CT scan and the PET scan.
4. They answer different questions, because one shows what the brain looks like and the other shows what it is doing.

CT Scanning

1. A CT scan uses a beam of X-rays taken from many different angles around the head.
2. Dense tissue absorbs more X-rays than soft tissue, so the amount reaching the detector varies with what the beam passed through.
3. A computer combines the readings into a detailed cross-sectional image of the brain.
4. The image shows structure, so a tumour, a bleed, a blood clot or a region killed by a stroke can be located precisely.
5. Brain function is investigated by linking the damaged region to the ability the patient has lost, which shows what that region normally does.

PET Scanning

1. A PET scan requires the patient to be injected with a radioactive tracer, usually a form of glucose.
2. The tracer travels in the blood and collects in the most active regions, because active neurones respire faster and take up more glucose.
3. The tracer emits radiation as it decays, and detectors around the head record where it is coming from.
4. A computer builds a colour coded image in which the most active regions stand out clearly.
5. A PET scan therefore shows function in real time, and the patient can be scanned while reading, listening or moving so the active region can be matched to the task.
6. Comparing a patient's scan with a healthy scan can reveal a region of unusually low activity, which is used in diagnosing conditions such as Alzheimer's disease.

Comparing the Two Techniques

1. What they show. A CT scan shows structure, while a PET scan shows activity.
2. How they work. A CT scan uses X-rays passed through the head, while a PET scan detects radiation emitted by a tracer inside the body.
3. Detail. A CT scan gives sharper structural detail, while a PET scan gives poorer resolution but real time information.
4. Speed and cost. A CT scan is quicker and cheaper, while a PET scan needs a tracer made in a nearby facility because it decays rapidly.
5. The two are often used together, so the activity shown by the PET scan can be placed accurately on the structure shown by the CT scan.

Limitations and Risks of Scanning

1. Both techniques expose the patient to ionising radiation, which slightly increases the long term risk of cancer.
2. Repeated scans increase the total dose, so scans are only carried out when they are clinically justified.
3. The equipment is expensive and requires trained staff, so scanners are concentrated in large hospitals.
4. Patients must stay completely still, which is difficult for young children and for people in pain.
5. A scan shows a correlation between a region and a task, and does not by itself prove that the region causes the ability.

2.3.3 Limitations in treating nervous system damage

Nervous Tissue Does Not Repair Itself

1. Most neurones in the central nervous system do not divide once they are mature, so lost neurones are not replaced.
2. A cut axon in the CNS does not regrow, because scar tissue and chemical signals at the injury site block it.
3. Skin and bone heal because the cells there divide by mitosis, and nervous tissue simply cannot do the same.
4. Damage to the CNS is therefore usually permanent, and treatment aims to limit further damage and manage the symptoms.

The Problem of Access

1. The brain is enclosed by the skull and the spinal cord by the vertebral column, so surgery means cutting through bone.
2. Neurones are densely packed and interconnected, so an instrument reaching a deep structure must pass through healthy tissue.
3. Damaging healthy tissue on the way in can cost the patient an ability the operation was never meant to affect.
4. Some structures lie so deep that they are considered inoperable, and the risk of surgery outweighs the benefit.

The Blood Brain Barrier

1. The capillaries supplying the brain have tightly joined walls, forming a blood brain barrier.
2. This barrier protects the brain by preventing many substances in the blood from reaching the neurones.
3. It also stops many drugs from reaching the brain, including some chemotherapy drugs and antibiotics.
4. A drug that works well elsewhere in the body may therefore be useless against a brain tumour or a brain infection.

Treating Brain Tumours

1. A tumour deep inside the brain may be impossible to reach without destroying healthy tissue.
2. A tumour may have no clear boundary, so a surgeon cannot tell exactly where the tumour ends and healthy brain begins.
3. Radiotherapy damages the surrounding healthy neurones as well as the tumour, and those neurones will not be replaced.
4. Chemotherapy is limited by the blood brain barrier, so the drug may not reach the tumour in a high enough concentration.
5. Even a benign brain tumour is dangerous, because the skull cannot expand and a growing mass raises the pressure inside it.

Treating Spinal Injuries

1. The spinal cord carries the neurones connecting the brain to the rest of the body, so damage cuts that connection.
2. The severed axons do not regrow, so impulses can no longer pass between the brain and the body below the injury.
3. The result is a loss of sensation and of voluntary movement below the level of the injury, which is paralysis.
4. The higher up the spine the injury is, the more of the body is affected, and an injury in the neck can affect breathing.
5. Surgery can stabilise the spine and relieve pressure on the cord, but it cannot reconnect the cut neurones.

The Consequences for Patients

1. Damage is usually permanent, so patients live with the effects for the rest of their lives.
2. Physiotherapy helps patients make the most of the function they still have, and other regions of the brain sometimes take over part of a lost role.
3. Patients may need long term care, adaptations to their home, and support for the mental health effects of losing an ability.
4. Stem cell research offers a possible future treatment, and trials are testing whether transplanted cells can replace damaged neurones.

2.3.4 Neurones and impulse transmission

The Nervous System Carries Electrical Impulses

1. A neurone is a cell specialised to carry an electrical impulse rapidly from one part of the body to another.
2. Bundles of neurones are wrapped in connective tissue to form the nerves that run through the body.
3. The pathway of any nervous response follows the same order: stimulus, receptor, sensory neurone, CNS, motor neurone, effector, response.
4. Nervous responses are fast and short lived, which is what distinguishes them from the slower, longer lasting effects of hormones.

Receptors Detect the Stimulus

1. A receptor detects a stimulus and converts that stimulus into an electrical impulse.
2. Receptors are usually grouped inside a sense organ, so light receptors sit in the retina of the eye and sound receptors sit in the cochlea of the ear.
3. Each type of receptor responds to only one type of stimulus, which is why the eye cannot detect sound.
4. The skin contains separate receptors for touch, pressure, temperature and pain.
5. Receptors inside the body monitor conditions such as blood glucose concentration, blood pressure and the position of the muscles.

Sensory Neurones

1. A sensory neurone carries the impulse from a receptor to the central nervous system.
2. It has a long dendron that carries the impulse towards the cell body, and a shorter axon that carries it onwards into the CNS.
3. The cell body sits partway along the side of the neurone rather than at one end, which is the easiest way to recognise it in a diagram.

Relay Neurones

1. A relay neurone lies entirely within the central nervous system, in the brain or the spinal cord.
2. It carries the impulse from a sensory neurone to a motor neurone.
3. It is short, with a cell body in the middle and many short branches at both ends.
4. Those branches connect to large numbers of other neurones, which is how the CNS links incoming information to a coordinated response.

Motor Neurones

1. A motor neurone carries the impulse from the central nervous system to an effector.
2. Its cell body sits at one end, inside the CNS, with many short dendrites branching from it.
3. A single very long axon then runs all the way to the effector, and in an adult a motor neurone reaching the foot can be about a metre long.
4. The axon ends in branches that spread across the muscle so that many muscle fibres are stimulated at once.

The Axon and the Dendron

1. The axon carries the impulse away from the cell body, and the dendron carries it towards the cell body.
2. Both are long and thin, which allows a single cell to span a large distance without the impulse being passed between cells.
3. Passing an impulse along one continuous fibre is far faster than passing it from cell to cell, because every junction adds a delay.
4. A sensory neurone has both a dendron and an axon, while a motor neurone has dendrites and one long axon.

The Myelin Sheath

1. Many axons are wrapped in a myelin sheath, a fatty layer made by cells that coil around the fibre.
2. The sheath acts as an electrical insulator, so the impulse cannot leak out through the membrane.
3. The sheath is broken by regular gaps, and the impulse jumps from one gap to the next instead of travelling along the whole membrane.
4. This makes the impulse travel much faster, and a myelinated neurone can conduct at over 100 metres per second while an unmyelinated one manages about 1 metre per second.
5. Myelin also stops impulses crossing between neighbouring axons in the same nerve, so signals stay separate.

Effectors Carry Out the Response

1. An effector is a muscle or a gland.
2. A muscle responds by contracting, which produces movement.
3. A gland responds by secreting a substance such as a hormone, sweat or an enzyme.
4. The impulse itself does not do anything useful until it reaches an effector, so the effector is what converts the signal into a response.

Synapses

1. Neurones do not physically touch, so there is a tiny gap between the end of one neurone and the start of the next.
2. The junction, including the gap, is called a synapse.
3. An electrical impulse cannot jump the gap, so the signal must be carried across in a different way.
4. The signal is passed across by a chemical, which means transmission is electrical along the neurone and chemical at the synapse.

How an Impulse Crosses a Synapse

1. The electrical impulse arrives at the end of the first neurone.
2. This causes tiny sacs called vesicles to move to the membrane and release a neurotransmitter into the gap.
3. The neurotransmitter diffuses across the gap, which is fast because the gap is extremely small.
4. It binds to receptor molecules on the membrane of the next neurone, and the receptors have a shape complementary to the neurotransmitter.
5. Binding triggers a new electrical impulse in the second neurone, and the impulse continues on its way.
6. The neurotransmitter is then broken down or reabsorbed, so the second neurone is not stimulated continuously.

Why Synapses Are Useful

1. Vesicles are only present on one side of the synapse, so the impulse can travel in one direction only.
2. One neurone can form synapses with many others, which allows a single input to trigger several different responses.
3. Several weak inputs can be added together at a synapse, so a response happens only when the stimulus is strong enough to matter.
4. The cost is a slight delay, because diffusion takes longer than conduction along an axon, which is why a pathway with fewer synapses is faster.

Measuring the Speed of a Nervous Response

2.3.5 Structure and function of a reflex arc

What a Reflex Action Is

1. A reflex action is a rapid, automatic response to a stimulus that happens without conscious thought.
2. It is involuntary, so it happens whether or not you want it to and you cannot stop it by deciding not to.
3. Every person shows the same response to the same stimulus, because the pathway is not learned.
4. Most reflexes are protective, and they prevent injury by acting before the brain has processed what is happening.

The Reflex Arc

1. The reflex arc is the pathway of neurones that produces a reflex action.
2. A stimulus is detected by a receptor, which converts it into an electrical impulse.
3. A sensory neurone carries the impulse from the receptor to the central nervous system, usually the spinal cord.
4. A relay neurone inside the CNS passes the impulse straight across to a motor neurone.
5. A motor neurone carries the impulse out to the effector.
6. The effector, a muscle or a gland, produces the response by contracting or by secreting.
7. The impulse crosses a synapse between each pair of neurones, and a neurotransmitter carries it over each gap.

Why the Brain Is Bypassed

1. The impulse travels through the spinal cord and does not pass through the conscious part of the brain first.
2. Missing out the brain means fewer synapses, and each synapse adds a delay because the neurotransmitter has to diffuse across the gap.
3. A shorter pathway means a shorter reaction time, so the response happens before serious damage is done.
4. No decision is needed, so nothing is gained by involving conscious thought.
5. An impulse is still sent up to the brain, which is why you feel the pain a moment after your hand has already moved.

When you touch a hot object

1. Stimulus. You touch a hot oven tray, and the heat is the stimulus.
2. Receptor. Temperature and pain receptors in the skin of the finger detect it and generate an impulse.
3. Sensory neurone. The impulse travels up the arm to the spinal cord.
4. Relay neurone. In the spinal cord the impulse is passed across to a motor neurone.
5. Motor neurone. The impulse travels back down the arm to the biceps muscle.
6. Effector and response. The biceps contracts and the hand is pulled away from the tray.

Other Reflexes in the Body

1. The pupil reflex. In bright light the iris contracts the pupil, which protects the retina from damage.
2. Blinking. An object approaching the eye triggers the eyelid to close and shield the cornea.
3. The knee jerk. Tapping the tendon below the knee makes the thigh muscle contract, which helps to maintain posture.
4. Coughing and sneezing. Irritants in the airways trigger a sharp expulsion of air that clears them.
5. Some reflexes are coordinated by the brain rather than the spinal cord, and the pupil reflex and blinking are both handled in the brain.