The neuron
The nervous system is built from cells called neurons. Each has three main parts:
- Dendrites: branching fibres that receive signals from other neurons.
- Cell body (soma): holds the nucleus and integrates incoming signals.
- Axon: a long fibre that carries the signal away. Many axons are wrapped in myelin, a fatty insulation that speeds transmission.
When incoming signals are strong enough, the neuron fires an action potential, an electrical pulse that travels down the axon in an all-or-nothing fashion.
The synapse
Neurons do not touch. The tiny gap between them is the synapse. When the pulse reaches the end of the axon, it releases chemical neurotransmitters that cross the gap and bind to receptors on the next neuron. Afterward, the transmitter is cleared away or taken back up (reuptake).
Examples: dopamine is linked to reward and movement; serotonin to mood and sleep; acetylcholine to muscle action and learning; GABA is the main inhibitory transmitter, calming neural activity.
Divisions of the nervous system
- Central nervous system (CNS): brain and spinal cord.
- Peripheral nervous system (PNS): all the nerves linking the CNS to the body. It has a somatic part (voluntary muscles) and an autonomic part (automatic organs).
- The autonomic system splits in two. The sympathetic division prepares the body for action ("fight or flight"): faster heart rate, wider airways. The parasympathetic division calms and restores ("rest and digest").
Brain structures
- Hindbrain: the cerebellum coordinates balance and smooth movement; the brainstem controls breathing and heart rate.
- Limbic system: the amygdala processes fear and emotion; the hippocampus forms new explicit memories; the hypothalamus regulates hunger, thirst and body temperature.
- Cerebral cortex, divided into four lobes:
- Frontal: planning, decisions, personality, voluntary movement.
- Parietal: touch and spatial awareness.
- Temporal: hearing and language comprehension.
- Occipital: vision.
Worked example
Suppose you touch a hot pan and pull away.
- Sensory neurons in your skin carry the pain signal along their axons toward the spinal cord.
- At the synapse, neurotransmitters pass the message to a connecting neuron, then to a motor neuron.
- The motor neuron fires, and your arm muscles contract. This is a reflex, handled by the spinal cord before the brain is aware of it.
- A moment later, the signal reaches the brain. The parietal lobe registers the touch, the amygdala adds fear, and your frontal lobe decides to run the burn under cold water.
The same chain shows how drugs act: many block reuptake, so a neurotransmitter lingers in the synapse and has a stronger effect.
Hemispheres
The brain is split into left and right hemispheres joined by the corpus callosum. Each hemisphere mainly processes the opposite side of the body, and the claim that people are "left-brained" or "right-brained" is a myth.