The divisions of the nervous system

• Central nervous system (CNS) — the brain (the centre of conscious awareness; its outer layer, the cerebral cortex, is what distinguishes higher mental functions) and the spinal cord (relays information between brain and body, and controls reflex actions without brain involvement).
• Peripheral nervous system (PNS) — transmits messages to and from the CNS. It divides into:
• Somatic nervous system — carries sensory information from receptors to the CNS and motor commands from the CNS to muscles. It governs voluntary movement.
• Autonomic nervous system (ANS) — governs involuntary functions (breathing, heart rate, digestion, stress responses). It has two branches: the sympathetic (arousing: increases heart rate, dilates pupils, inhibits digestion) and the parasympathetic (calming: the 'rest and digest' state, returning the body to baseline).

Neurons and synaptic transmission

Three types of neuron: sensory (receptor → CNS; long dendrites, short axons), relay (connect sensory and motor neurons; short dendrites, short axons — around 97% of all neurons are relay neurons, most in the brain and visual system), and motor (CNS → effectors/muscles; short dendrites, long axons).

Structure: dendrites receive signals → the cell body → the axon carries the impulse, insulated by a fatty myelin sheath broken by gaps called nodes of Ranvier, which speed transmission by making the impulse jump across the gaps → terminal buttons.

Synaptic transmission. Within a neuron, the signal is electrical (an action potential, fired when the neuron is activated by a stimulus and becomes positively charged). Between neurons the signal must cross the synaptic cleft, so it becomes chemical:

• The action potential reaches the terminal button and triggers the release of a neurotransmitter from vesicles.
• The neurotransmitter diffuses across the gap and binds to a postsynaptic receptor site — a lock-and-key fit, so each neurotransmitter has its own specific structure.
• The chemical message is converted back into an electrical impulse. Any neurotransmitter left in the cleft is reabsorbed (reuptake) — the process that SSRIs block.

Excitation and inhibition. Adrenaline is typically excitatory — it causes an excitatory postsynaptic potential (EPSP), making the postsynaptic neuron more likely to fire. Serotonin and GABA are typically inhibitory, causing an IPSP and making firing less likely. Summation: the postsynaptic neuron adds up all the excitatory and inhibitory influences it receives; only if the net effect is sufficiently excitatory does it reach the threshold and fire. Transmission is unidirectional.

Excitation or inhibition?

The endocrine system and fight or flight

The endocrine system works alongside the nervous system but is slower and uses hormones carried in the bloodstream, affecting any cell with a receptor for that hormone. The pituitary gland, in the brain, is the master gland: it controls the release of hormones from all the other glands.

The acute stress response ('fight or flight'):

• A stressor is perceived; the hypothalamus activates the sympathomedullary pathway (SAM).
• The sympathetic branch of the ANS sends a signal to the adrenal medulla, which releases adrenaline into the bloodstream.
• Adrenaline produces the physiological changes needed for action: increased heart rate (pumping blood and oxygen to the muscles), increased breathing rate, dilated pupils, inhibited digestion and saliva production, contracted rectum.
• Once the threat has passed, the parasympathetic branch acts as a brake (the 'rest and digest' response), slowing heart rate and restarting digestion, returning the body to homeostasis.

Evaluation: Taylor et al. (2000) argued that fight-or-flight is a beta-biased, androcentric account: women are more likely to show a 'tend and befriend' response, protecting offspring and forming alliances, which may be mediated by oxytocin. The response is also maladaptive in modern life: the stressors we face are chronic and social rather than physical, and the repeated activation of the cardiovascular system contributes to hypertension and heart disease. Gray (1988) also argued that the first response to threat is not fight or flight but freeze — a hyper-vigilant pause allowing appraisal of the best response.

Which pathway?

Localisation of function

Before the 19th century the dominant view was holistic — all parts of the brain were involved in all behaviour. Broca and Wernicke established that specific areas have specific functions.

• Motor area — back of the frontal lobe; controls voluntary movement. Damage causes loss of fine motor control.
• Somatosensory area — front of the parietal lobe, separated from the motor area by the central sulcus; processes sensory information from the skin. The amount of cortex devoted to a body part reflects its sensitivity — the face and hands take up over half of it.
• Visual area — occipital lobe. Each eye sends information from the right visual field to the left hemisphere and vice versa, so damage to the left hemisphere can cause blindness in the right visual field of both eyes.
• Auditory area — temporal lobe. Damage may cause partial hearing loss.
• Broca's area — left frontal lobe: speech production. Broca's patient 'Tan' (Leborgne) could understand speech but could say only that syllable. Broca's aphasia is slow, laborious, non-fluent speech.
• Wernicke's area — left temporal lobe: language comprehension. Wernicke's aphasia produces fluent but meaningless speech, often containing invented words (neologisms).

Evaluation: localisation is supported by neurosurgical evidence — Dougherty et al. (2002) reported on 44 OCD patients who had undergone a cingulotomy (lesioning the cingulate gyrus); at follow-up, about 30% had met the criteria for a successful response and 14% a partial response, suggesting the symptoms are localised. Brain scans also support it (Petersen et al. showed Wernicke's area active during a listening task, Broca's during reading). But Lashley (1950) removed areas of the cortex in rats learning a maze and found no area was more important than any other, concluding that learning is holistic and requires the whole cortex — the theory of equipotentiality. And the brain's plasticity shows that when one area is damaged, others can take over its function — so strict localisation is too simple.

Hemispheric lateralisation and split-brain research

Lateralisation: some functions are dominated by one hemisphere. Language is left-lateralised (Broca's and Wernicke's areas are both in the left hemisphere for most people). The brain is contralateral: the left hemisphere controls the right side of the body and receives the right visual field, and vice versa.

Sperry (1968) — split-brain research. Eleven patients who had undergone a commissurotomy (severing the corpus callosum to control severe epilepsy) were tested. An image was projected to one visual field for one-tenth of a second — too fast for the eye to move — so the information reached one hemisphere only and could not be shared.

• Describing what you see: an image shown to the right visual field (→ left hemisphere) could be described in speech. The same image shown to the left visual field (→ right hemisphere) could not be described — patients said there was nothing there. Language is left-lateralised.
• Recognition by touch: although they could not name an object presented to the left visual field, patients could select the matching object with their left hand from behind a screen, and could draw it. The right hemisphere knew — it simply could not speak.
• The right hemisphere was superior at drawing and at recognising faces.

Evaluation: Sperry's methodology was highly standardised and specialised, giving high internal validity, and his conclusions were ground-breaking. But there are causal problems: the patients had a history of epileptic seizures and had taken medication for years, which may itself have caused brain changes, and the control group of people with no epilepsy is arguably inappropriate. The generalisability of an 11-person sample of unusual patients is questionable, and modern researchers argue the 'left brain / right brain' popular idea is a gross over-simplification — the two hemispheres are in constant communication in a normal brain, and many functions are distributed across both.

Split-brain logic

Plasticity and functional recovery

Plasticity — the brain's ability to change and adapt as a result of experience and new learning. Synaptic connections used frequently are strengthened; those rarely used are pruned. It was once thought this happened only in childhood, but research shows it continues throughout life.

• Maguire et al. (2000) scanned London taxi drivers and found significantly more grey matter in the posterior hippocampus (associated with spatial and navigational skills) than a matched control group. The longer they had been in the job, the more pronounced the difference — the demands of 'the Knowledge' had physically altered their brains.
• Draganski et al. (2006) imaged medical students' brains three months before and immediately after their final exams, and found learning-induced changes in the posterior hippocampus and parietal cortex.

Functional recovery after trauma. Following injury, unaffected areas take over lost functions — neural reorganisation. It happens quickly at first (spontaneous recovery) and then slows, at which point rehabilitative therapy may be needed. The brain rewires by:

• Axonal sprouting — the growth of new nerve endings which connect with undamaged nerve cells to form new neuronal pathways.
• Denervation supersensitivity — axons that do a similar job become aroused to a higher level to compensate; the side effect can be over-sensitivity to pain.
• Recruitment of homologous areas — the equivalent area on the opposite hemisphere takes over the function (e.g. Broca's area on the right taking over language).

Evaluation: plasticity has huge real-world application in neurorehabilitation. But it can be maladaptive: prolonged drug use produces poorer cognitive functioning in later life, and 60–80% of amputees experience phantom limb syndrome, thought to result from cortical reorganisation in the somatosensory cortex. Age matters — functional recovery is usually faster in the young — though Bezzola et al. (2012) found that 40 hours of golf training produced changes in the neural representations of movement in participants aged 40–60, showing plasticity persists in later life.

Ways of studying the brain

• fMRI — detects changes in blood oxygenation and flow that occur as a result of neural activity (active areas consume more oxygen). Produces a 3D image showing which parts of the brain are involved in a particular mental process. Strengths: non-invasive, no radiation, very high spatial resolution (about 1–2 mm). Weaknesses: expensive; poor temporal resolution — there is a 5-second lag behind the initial neural activity; it measures blood flow, not neuronal activity directly.
• EEG — scalp electrodes record general electrical activity, producing brainwave patterns. Strengths: invaluable in diagnosing epilepsy and in studying sleep stages; extremely high temporal resolution (down to a millisecond). Weakness: the signal is generalised — it cannot pinpoint the exact source of the activity.
• ERPs — the EEG signal is statistically averaged over many presentations of a stimulus, filtering out background 'noise' to leave the brain's response to that specific event. Strengths: much more specific than raw EEG, with excellent temporal resolution. Weakness: background noise is hard to eliminate completely, so studies need standardised conditions.
• Post-mortem examination — the brain of a person with an unusual deficit is examined after death and compared with a neurotypical brain. Broca and Wernicke both relied on it. Strengths: vital in the early foundations of the field; allows examination of deep structures scans cannot reach. Weaknesses: causation is a problem (damage may be due to unrelated decay or trauma rather than the deficit studied), and there are ethical issues with informed consent — HM could not give it, as he had no memory of the discussion.

Choose the method

Which neuron?

Functional recovery

Broca or Wernicke?

Plasticity

Where does it belong?

Tap a card, then the part of biopsychology it belongs to.

Structure and function

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The big ideas to know

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