The nervous system

The nervous system carries electrical signals around the body. It has two main parts:

• Central nervous system (CNS) — the brain and spinal cord.
• Peripheral nervous system — nerves connecting the CNS to the rest of the body. It includes the autonomic nervous system (automatic functions) and the somatic nervous system (voluntary movement).

Autonomic nervous system has two branches: the sympathetic (arousal — fight or flight) and parasympathetic (rest and digest).

Parts of the nervous system

Neurons & synaptic transmission

Neurons are nerve cells that carry electrical impulses. Types include sensory, relay and motor neurons.

• An impulse travels electrically along a neuron, from dendrites to the axon terminal.
• At the gap between neurons — the synapse — the signal is passed chemically.
• The neuron releases neurotransmitters that cross the synapse and bind to the next neuron; any left over is taken back in (reuptake).

Key point: the signal is electrical within a neuron but chemical across the synapse.

Across the synapse

The fight-or-flight response

When we sense a threat, the autonomic nervous system triggers fight or flight:

• The sympathetic branch is activated and the adrenal glands release adrenaline.
• Heart rate and breathing speed up, pupils dilate and the body prepares for action.
• Afterwards the parasympathetic branch returns the body to rest ('rest and digest').

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Fight or flight

Hebb's theory of learning

Hebb's theory explains how learning changes the brain at the level of neurons:

• When one neuron repeatedly and persistently helps to fire another, the connection between them strengthens.
• Often summarised as: 'cells that fire together, wire together.'
• Repeated activity leads to neuronal growth and stronger synaptic connections, forming neural pathways — the biological basis of learning.

Penfield's study of the interpretive cortex

Penfield studied conscious epilepsy patients during brain surgery. He gently stimulated areas of the cortex with a mild electrical probe:

• Stimulating the temporal lobe (the 'interpretive cortex') caused patients to re-experience vivid past memories, sounds and feelings.
• This suggested that specific brain areas are linked to storing and interpreting experience.
• Evaluation: highly detailed on conscious patients, but the sample was people with epilepsy, so findings may not generalise to all brains.

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Hebb's theory

Localisation of function

Localisation means different functions are handled by different brain areas:

• Frontal lobe — planning, decision-making and personality.
• Occipital lobe — processing vision.
• Temporal lobe — hearing and memory; parietal lobe — touch and sensation.
• Penfield's work supported the idea that specific areas do specific jobs.

Neuroplasticity

The brain can change in response to experience — this is plasticity:

• Hebb's theory explains it at the neuron level: repeated firing strengthens connections.
• After damage, other areas can sometimes take over lost functions.
• Plasticity is the biological basis of learning new skills.

The big ideas to know

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