The Nervous System

The brain weighs about 1.4 kg — roughly the weight of a bag of sugar — and contains approximately 86 billion neurons connected by trillions of synapses. It is the most complex structure in the known universe. The main regions of the brain: Cerebrum — the largest part: The wrinkled, dome-shaped outer part you picture when you think of a brain. Divided into left and right hemispheres joined by the corpus callosum. The outer layer is the cerebral cortex — the "thinking" part responsible for: - Voluntary movement - Sensory perception (feeling, seeing, hearing) - Language, reasoning, planning, personality - Memory and learning The cerebrum is divided into four lobes, each with different specialisations: - Frontal lobe — planning, decision-making, personality, voluntary movement (motor cortex), speech production (Broca's area) - Parietal lobe — touch, pressure, pain, temperature, spatial awareness - Temporal lobe — hearing, memory, language comprehension (Wernicke's area) - Occipital lobe — vision exclusively. A blow to the back of the head can cause visual disturbances ("seeing stars") Cerebellum — the "little brain": Sits at the back and bottom of the brain. Coordinates smooth, precise movement and balance. It does not initiate movement — it fine-tunes it. Alcohol specifically impairs the cerebellum → unsteady walking, slurred speech, difficulty with fine motor tasks. Brainstem — the life-support system: The stalk connecting the brain to the spinal cord. Controls vital automatic functions — breathing, heart rate, blood pressure, swallowing, and consciousness. You cannot survive brainstem injury. The brainstem contains the midbrain, pons, and medulla oblongata. Hypothalamus — the master regulator: A tiny but enormously important region the size of an almond, sitting at the base of the brain. Controls hunger, thirst, body temperature, sleep, sexual behaviour, and the hormonal system. It directly controls the pituitary gland — making it the critical link between the nervous system and the hormonal (endocrine) system.

The spinal cord is a thick bundle of nerve fibres running down through the vertebral column. It is about 45 cm long in an adult, protected by the vertebrae, and further cushioned by cerebrospinal fluid (CSF). The spinal cord serves two purposes: 1. Relay highway — carries sensory signals up to the brain and motor signals down from the brain 2. Reflex centre — processes some reflexes WITHOUT involving the brain Spinal cord injury: When the spinal cord is injured, signals cannot pass between the brain and the body below the injury: - Paraplegia — paralysis of the legs (injury in the chest/lower spine region) - Quadriplegia/tetraplegia — paralysis of all four limbs (injury in the neck region — affecting both arms and legs) The higher up the injury, the more of the body is affected. Crucially, the brain itself is completely intact — the person can think, feel, hear, and see perfectly — they simply cannot send signals to or receive signals from the affected body parts. Reflexes — responses that bypass the brain: A reflex is a fast, automatic response to a stimulus that is processed in the spinal cord, without waiting for the brain to get involved. This makes reflexes much faster than voluntary movements. The knee-jerk reflex (patellar reflex) — the classic example: 1. A tap on the patellar tendon (below the kneecap) stretches the quadriceps muscle 2. Stretch receptors in the muscle send a signal up the sensory nerve to the spinal cord 3. In the spinal cord, the signal directly connects to a motor neuron (no brain involvement) 4. The motor neuron sends a signal back to the quadriceps → it contracts → knee kicks forward 5. (Simultaneously, the signal inhibits the hamstring — the antagonist — so it relaxes) Doctors test this reflex to assess whether the neural pathways in that spinal cord region are intact. Absent reflex = nerve damage below; exaggerated reflex = damage ABOVE (upper motor neuron problem).

Within the peripheral nervous system, there is an important division based on what the nerves control: Somatic nervous system (voluntary): Controls skeletal muscles — the muscles you consciously move. When you decide to pick up a pen, it is your somatic nervous system sending the signal. Autonomic nervous system (automatic): Controls involuntary functions — heart rate, breathing rate, digestion, pupil size, sweating, bladder control. You do not consciously control these — they happen automatically. The autonomic system has two opposing divisions that balance each other: Sympathetic nervous system — "fight or flight": Prepares the body for action or danger. When you are stressed, scared, or exercising: - Heart rate increases ↑ - Breathing rate increases ↑ - Blood is diverted from the gut to muscles - Pupils dilate (widen — to see better) - Sweating increases - Digestion slows down - Adrenaline is released from the adrenal glands Think: everything useful for running from a predator is turned up; everything not immediately needed (digestion) is turned down. Parasympathetic nervous system — "rest and digest": Calms the body down after stress. Controls normal resting functions: - Heart rate slows ↓ - Breathing slows ↓ - Digestion is promoted (saliva, stomach acid, gut movement all increase) - Pupils constrict - Bladder contracts (allows urination) Many drugs work by mimicking or blocking these two systems. Beta-blockers block sympathetic receptors → lower heart rate. Atropine blocks parasympathetic receptors → heart rate increases, pupils dilate (used by doctors when examining the eye).

Sensory information reaches the brain through specialised receptors that detect different types of stimuli and convert them into electrical nerve signals — a process called transduction. The five traditional senses + more: Vision: The eye focuses light onto the retina at the back of the eyeball. The retina contains photoreceptors: - Rods — detect light and dark; work in low light; responsible for night vision - Cones — detect colour; require bright light; concentrated in the centre (the fovea) Signals travel via the optic nerve to the occipital lobe (at the back of the brain). Common problems: short-sightedness (myopia) — the eyeball is too long, so the image focuses in front of the retina rather than on it (corrected with concave lenses). Long-sightedness (hyperopia) — the opposite. Hearing: Sound waves enter the ear canal → vibrate the eardrum → the three tiny bones (ossicles: malleus, incus, stapes — also called hammer, anvil, and stirrup) amplify the vibrations → the vibrations reach the cochlea (fluid-filled snail-shaped organ) → hair cells convert vibrations to nerve signals → auditory nerve → temporal lobe. Prolonged exposure to loud noise damages cochlear hair cells permanently — they do not regenerate in mammals. This is why hearing loss from loud music or heavy machinery is irreversible. Touch: Different types of receptors in the skin detect: light touch, pressure, vibration, temperature (heat/cold), and pain. Pain receptors (nociceptors) are especially important — pain is a warning signal that damage is occurring or about to occur. People born without the ability to feel pain (congenital insensitivity to pain) suffer severe injuries throughout life because they have no warning system. Smell and taste: Both are chemoreceptor senses — they detect chemical molecules. Smell receptors in the nasal cavity send signals directly to the limbic system (the emotional and memory part of the brain) — which is why smells trigger vivid memories so powerfully. Taste (sweet, sour, salty, bitter, umami) works alongside smell — this is why food tastes bland when your nose is blocked.