Biopsychology: How the Body and Brain Shape Behaviour

The nervous system and its divisions

The nervous system is the body's fast message network. It has two main parts.

• Central nervous system (CNS): the brain and the spinal cord. The brain makes decisions. The spinal cord carries messages to and from the brain and runs simple reflexes.
• Peripheral nervous system (PNS): all the other nerves. It has two branches.

Somatic and autonomic

• Somatic: controls muscles you move on purpose and carries senses (touch, sight) to the CNS.
• Autonomic: runs organs by itself, with no thinking (heart, gut, breathing rate). It has two halves that work in opposite ways:
  • Sympathetic: speeds the body up for action.
  • Parasympathetic: calms the body down ("rest and digest").

Neurons and synaptic transmission

A neuron is a nerve cell. There are three types:

• Sensory neurons carry messages from the senses to the CNS.
• Relay neurons link neurons inside the CNS.
• Motor neurons carry messages from the CNS to muscles and glands.

Parts of a neuron: a cell body with the nucleus, dendrites that receive signals, an axon that carries the signal, covered by a fatty myelin sheath that speeds it up, and terminal buttons at the end.

Crossing the synapse

1. An electrical signal (action potential) reaches the end of the axon.
2. Tiny sacs called vesicles release a chemical, a neurotransmitter, into the gap (the synaptic cleft).
3. The chemical crosses the gap and fits receptors on the next neuron.
4. It is then broken down or taken back up (reuptake).

Transmission goes one way only, because only the sending side has vesicles and only the receiving side has receptors.

Excitation and inhibition

Excitatory neurotransmitters (such as adrenaline) make the next neuron more likely to fire. Inhibitory ones (such as serotonin or GABA) make it less likely. The next neuron adds up all inputs (summation) and fires only if the total is excitatory enough.

The endocrine system and fight or flight

The endocrine system is a set of glands that release hormones into the blood. Hormones are slower than nerve signals but their effects last longer.

• Pituitary gland: the "master gland", controlled by the hypothalamus, it tells other glands what to do.
• Adrenal glands: release adrenaline (and cortisol in long stress).
• Thyroid: thyroxine controls how fast the body uses energy. Pineal gland: melatonin for sleep.

Fight or flight, step by step

1. The brain (amygdala) spots a threat and alerts the hypothalamus.
2. The hypothalamus switches on the sympathetic branch.
3. The adrenal medulla releases adrenaline into the blood.
4. Heart rate and breathing rise, pupils widen, digestion slows, sugar is released for energy.
5. When the threat passes, the parasympathetic branch calms the body.

Some researchers add a third reaction, "tend and befriend" (protecting others and seeking support), seen more often in females. People can also freeze.

Localisation, lateralisation and split-brain research

Localisation means certain jobs are handled by certain areas of the brain.

• Motor area (frontal lobe): plans and starts movement.
• Somatosensory area (parietal lobe): feels touch, heat, pain.
• Visual area (occipital lobe, at the back) and auditory area (temporal lobe).
• Broca's area (left frontal): producing speech. Damage gives slow, effortful speech.
• Wernicke's area (left temporal): understanding language. Damage gives fluent but meaningless speech.

Not all is localised: some researchers argue that complex jobs like memory spread across the brain (a holistic view).

Lateralisation

The brain has two halves (hemispheres). Lateralisation means some jobs are done mainly by one half. Language is usually left; face recognition and space are more right. The left half controls the right side of the body, and the other way round.

Split-brain research

Some patients with severe epilepsy had the corpus callosum (the bridge between the halves) cut. Researchers flashed a picture to one eye-field only. A word shown to the right half could not be spoken (speech is in the left), but the left hand could pick out the matching object by touch. This supports lateralisation. Limits: very few patients, and their brains were not typical before surgery.

Plasticity and functional recovery

Plasticity means the brain changes its structure with experience and learning. New connections grow; unused ones are pruned. Studies of taxi drivers who learned the streets of a big city found a larger memory area (hippocampus). Long-term meditation and learning to juggle also change grey matter.

Functional recovery after damage

After a stroke or injury, the brain can move a lost job to healthy areas:

• Axon sprouting: new nerve endings grow to link undamaged cells.
• Recruitment of similar areas: a matching area on the other side takes over.
• Reformation of blood vessels to feed the area.

Recovery is usually better in younger people, with more education ("cognitive reserve") and with therapy and practice.

Ways of studying the brain

Two key ideas: spatial resolution = how exactly we know WHERE activity is; temporal resolution = how exactly we know WHEN it happens.

• fMRI: measures blood oxygen changes in active areas. Good spatial (1-2 mm), poor temporal (1-4 s delay). Safe, no radiation; costly; shows link not cause.
• EEG: scalp electrodes record overall electrical waves. Excellent temporal (milliseconds), poor spatial. Cheap; useful for sleep and epilepsy.
• ERP: many EEG readings to the same stimulus are averaged so the brain's response to that one event appears. Good temporal; helps study attention and perception; background noise is a problem.
• Post-mortem: studying the brain after death and linking it to behaviour in life (e.g. Broca's patient). Very detailed structure; but no live activity, and damage may have other causes.

Biological rhythms, pacemakers and zeitgebers

• Circadian rhythm: about 24 hours. Example: the sleep-wake cycle, body temperature (lowest early morning, highest late afternoon).
• Ultradian: more than one cycle in 24 hours. Example: sleep stages repeat about every 90 minutes.
• Infradian: longer than 24 hours. Example: the monthly menstrual cycle; seasonal mood changes (winter low mood).

Pacemakers and zeitgebers

Endogenous pacemakers are inner clocks. The main one is the SCN (suprachiasmatic nucleus) in the hypothalamus. It gets light information from the eyes and tells the pineal gland when to make melatonin (the sleep hormone, high in the dark).

Exogenous zeitgebers (German for "time-givers") are outside cues that reset the clock. Light is the strongest; others are meal times and social habits. Cave studies, where people lived without daylight, found their cycle drifted to about 25 hours, showing the inner clock needs light to stay at 24.

Try it at home

For three days, note the time you feel most alert and most sleepy. Then look at when the room gets dark. Do the sleepy times follow the dark?

Key formulas and definitions

• CNS = brain + spinal cord; PNS = somatic + autonomic.
• Autonomic: sympathetic = arousal (fight or flight); parasympathetic = rest and digest.
• Synapse: action potential → vesicles release neurotransmitter → receptors → excitatory or inhibitory → summation.
• Fight or flight: amygdala → hypothalamus → sympathetic → adrenal medulla → adrenaline.
• Broca's area = speech production; Wernicke's area = language understanding.
• fMRI: good WHERE; EEG/ERP: good WHEN; post-mortem: structure after death.
• Circadian ≈ 24 h; ultradian < 24 h; infradian > 24 h.
• SCN = main endogenous pacemaker; light = main exogenous zeitgeber.

Worked examples

Common mistakes

• Saying the sympathetic system is part of the somatic system. It belongs to the autonomic system.
• Thinking synaptic transmission can go both ways. It goes one way only: vesicles on one side, receptors on the other.
• Mixing up Broca's (making speech) and Wernicke's (understanding speech).
• Calling EEG good at showing exactly where activity is. EEG is good for WHEN (temporal), fMRI for WHERE (spatial).