Your hand jerks off a hot pan before you decide to. What moved it?

Two main divisions

• The central nervous system (CNS) is the brain and the spinal cord.
• The peripheral nervous system (PNS) is all the nerves outside the CNS. It carries messages between the CNS and the rest of the body.

What the CNS does

• The CNS receives information from the senses, processes it and sends instructions to muscles and glands.
• The brain is the control centre. It makes decisions and controls thinking, memory, emotion and voluntary movement.
• The brain also keeps you alive: its lowest part, the brain stem, controls breathing and heartbeat.

The spinal cord

• The spinal cord runs down your back, protected by the spine.
• It carries messages between the brain and the body: up from the senses, down to the muscles.
• It can also control simple reflexes by itself. The message comes in and goes straight back out to the muscle, which is faster than waiting for the brain.
• The brain finds out a moment later.

Brain signals are electrical, until they hit a gap. How do they cross?

Inside a neuron

• A neuron is a nerve cell that carries messages as electrical impulses.
• Dendrites receive messages from other neurons. The cell body contains the nucleus.
• The impulse travels along the axon, which is often wrapped in a fatty myelin sheath that speeds it up.
• At the end of the axon are axon terminals, where the message is passed on to the next neuron.

Crossing the synapse

• The tiny gap between two neurons is the synapse (synaptic gap). The impulse can't jump it.
• When the impulse reaches the axon terminal, vesicles release a chemical, a neurotransmitter, into the gap: this is release.
• The neurotransmitter crosses the gap and binds to receptors on the next neuron, passing the message on.
• Leftover neurotransmitter is taken back into the first neuron to be reused: reuptake.

Excitation and inhibition

• A neurotransmitter's function is to carry the message across the synapse and affect the next neuron.
• Excitatory neurotransmitters (e.g. glutamate) make the next neuron more likely to fire an impulse.
• Inhibitory neurotransmitters (e.g. GABA) make it less likely to fire.
• One neuron receives messages from many synapses. It adds up the excitatory and inhibitory signals and fires only if excitation is strong enough.

Your eyes are at the front. Which part of your brain does the seeing?

The lobes and the cerebellum

• The wrinkled outer layer of the brain is the cerebral cortex. It has two halves (hemispheres), each divided into four lobes.
• Frontal lobe: at the front, behind the forehead.
• Parietal lobe: at the top, behind the frontal lobe.
• Temporal lobe: at the side, near the ears.
• Occipital lobe: at the back.
• The cerebellum sits underneath, at the back, below the occipital lobe.

What each part does

The cerebellum

• The cerebellum ("little brain") sits at the back, underneath the occipital lobe.
• It coordinates movement: balance, posture and smooth, accurate movements.
• It doesn't start movements. It fine-tunes them, so you can walk in a straight line or reach for a cup without missing.
• It helps you learn movement skills, such as riding a bike or typing, until they become automatic.

Split brain: why can a patient name a key in one hand only?

Two hemispheres, crossed wiring

• The brain has a left and a right hemisphere.
• Each hemisphere controls, and feels touch from, the opposite side of the body: your left hand links to your right hemisphere.
• Vision is crossed too: what you see in your left visual field goes to the right hemisphere, and the right visual field goes to the left hemisphere.
• Asymmetrical function: the hemispheres look alike but don't do the same jobs.

Left and right: different strengths

• This is lateralisation of function. Most tasks still use both hemispheres working together.

The corpus callosum

• The corpus callosum is a thick band of nerve fibres joining the two hemispheres.
• It lets them share information, so each knows what the other is doing.
• To treat very severe epilepsy, surgeons sometimes cut the corpus callosum to stop seizures spreading from one hemisphere to the other.
• People who have had this operation are called split-brain patients. Their two hemispheres can no longer communicate directly.

Lateralisation and sex differences

• Some researchers claim male brains are more lateralised: language is processed mainly in the left hemisphere.
• They claim female brains are less lateralised: language uses both hemispheres more.
• This has been used to explain why, on average, women may do slightly better on some verbal tasks and men on some spatial tasks.
• It may also explain why some women recover language better after left-hemisphere damage.

A rod went through his skull. He walked and talked. So what changed?

What neurological damage is

• Neurological damage is damage to the brain or nerves. Dead brain cells are not replaced.
• A stroke happens when the blood supply to part of the brain is blocked or a vessel bursts, so cells there are starved of oxygen and die.
• An injury is physical damage, e.g. a blow to the head.
• Effects depend on where the damage is: one ability can be lost while others are untouched.

The prefrontal cortex and behaviour

• The prefrontal cortex is the very front of the frontal lobe.
• It is involved in planning, decision-making, controlling impulses and controlling emotions and social behaviour.
• Damage can leave movement, speech and memory working, yet the person becomes impulsive, takes risks, makes poor decisions and may seem to have a different personality.

Visual agnosia and prosopagnosia

• Visual agnosia: the person can see but cannot recognise objects by sight. They may describe a key's shape and colour but not know what it is until they touch it.
• Prosopagnosia ("face blindness"): the person cannot recognise familiar faces, sometimes even their own. They recognise people by voice, hair or clothes.
• Both usually follow damage where the occipital and temporal lobes meet; prosopagnosia is linked to the fusiform gyrus.

People once read minds from bumps on the skull. What changed?

From skull bumps to evidence

• Around 1800, phrenology claimed each mental ability had its own brain area, shown by bumps on the skull.
• The idea of areas with different jobs survived; skull-reading did not, as skull thickness varies.
• 1800s: studying people with brain damage, e.g. Gage (1848) and Broca's patient who lost speech (1861).
• 1900s: stimulating the brain during surgery; split-brain studies (1960s).
• 1970s on: brain scanning shows the living brain.

Cognitive neuroscience

• Cognitive neuroscience studies how the structure and function of the brain relate to behaviour and cognition (thinking, memory, perception, language).
• It asks: which brain areas are active when we remember, recognise a face or make a decision?
• Its evidence comes from comparing people with brain damage, measuring what happens when the brain is stimulated, and scanning healthy brains while people do tasks.

How brain research has changed psychology

• More scientific: objective, measurable evidence from damaged, stimulated and scanned brains replaced guesses and people describing their own thoughts.
• New explanations: localisation (Gage), lateralisation (Sperry) and neurotransmitters explain behaviour biologically.
• New treatments: e.g. drugs that act on neurotransmitters for depression.
• New questions: scanning lets psychologists study healthy brains at work, not just damaged ones.

Big ideas

• The nervous system has two divisions: the CNS (brain and spinal cord) and the PNS (nerves linking the CNS to the body).
• The CNS receives, processes and sends out information; the spinal cord carries messages and handles simple reflexes.
• A neuron carries an electrical impulse along its axon; at the synapse, the message is carried across by a chemical neurotransmitter.
• Neurotransmitters are released, bind to receptors and are removed by reuptake; excitatory ones make the next neuron more likely to fire, inhibitory ones less likely.
• The cortex has four lobes in each hemisphere (frontal, parietal, temporal, occipital), with the cerebellum underneath at the back.
• Each structure has main functions: frontal (planning, movement, speech), parietal (touch), temporal (hearing, memory, language), occipital (vision), cerebellum (balance and coordination).
• The two hemispheres control opposite sides of the body and have different strengths (asymmetrical function): the left mainly language and logic, the right spatial skills, faces and emotion. The corpus callosum lets them share information.
• Sperry (1968): split-brain patients could name what reached the left hemisphere but not the right, showing language is lateralised and the separated hemispheres work independently.
• Key question link: lateralisation is one proposed explanation of sex differences, but the evidence is mixed and ignores nurture. Debate (4.3.1): split-brain research is an example of how studying the brain changed psychology (see B6).
• Neurological damage (from a stroke or injury) affects whichever functions the damaged area carries out, leaving others intact.
• Damage to the prefrontal cortex can change behaviour and personality: impulsive actions, poor decisions and less emotional control.
• Visual agnosia: can see but can't recognise objects; prosopagnosia: can't recognise faces. Damasio et al. (1994) located Gage's damage in the ventromedial prefrontal cortex.
• Understanding of the brain moved from phrenology to evidence: brain damage case studies, stimulation, split-brain research and brain scanning.
• Debate: brain research has made psychology more scientific and given it biological explanations and treatments, but it can be reductionist and scans show where, not why.