Lesson 12: The Nervous System and the Brain

The nervous system: fast, targeted control

The nervous system detects change and coordinates response in milliseconds. Its cells are neurons: a cell body with nucleus, branching dendrites that receive signals, and a long axon that carries the signal away. Many axons are wrapped in a fatty myelin sheath (made by Schwann cells) with gaps (nodes of Ranvier); the impulse jumps between nodes, reaching up to 120 m/s instead of ~1 m/s unmyelinated. The system divides into the central nervous system (brain and spinal cord) and the peripheral nervous system (sensory neurons carrying information in, motor neurons carrying commands out).

The impulse: electrical and chemical in turn

A resting neuron maintains a charge difference across its membrane (inside negative, about −70 mV) using sodium–potassium pumps. A stimulus opens ion channels; sodium ions rush in, the inside flips positive (depolarisation), and this action potential sweeps along the axon like a row of falling dominoes — each section triggers the next, then resets during a brief refractory period. At the synapse — a ~20 nm gap between neurons — the signal becomes chemical: the impulse triggers release of a neurotransmitter (e.g. acetylcholine, dopamine, serotonin) from vesicles; it diffuses across, binds receptors on the next neuron, and starts a new impulse. Synapses ensure one-way flow, allow integration (many inputs summed), and are where many drugs and toxins act.

Reflexes: the shortcut

A reflex arc bypasses conscious thought: receptor (skin heat sensor) → sensory neuron → relay neuron in the spinal cord → motor neuron → effector (muscle contracts, hand withdraws). The brain is informed, but after the fact. Reflexes protect tissue because they are fast — no decision time. Other reflexes: pupil constriction in bright light, knee-jerk, blinking, and the newborn reflexes checked by paediatricians.

The brain: regions and jobs

• Cerebrum (cerebral cortex) — conscious thought, memory, language, personality, voluntary movement; two hemispheres, folded to pack ~86 billion neurons into the skull; specific areas map to senses and body parts.
• Cerebellum — balance, posture and fine coordination of movement.
• Medulla (brain stem) — automatic control: heart rate, breathing rate, blood pressure, swallowing — the homeostasis control centres of Lesson 10.
• Hypothalamus — temperature and water regulation, hunger, and the link to the endocrine system via the pituitary gland.

Data: reaction times and what slows them

Typical human visual reaction time is ~0.25 s; auditory ~0.17 s (fewer synapses, faster pathway); a spinal reflex ~0.03–0.05 s. A ruler-drop test converts distance to time: catching at 20 cm means t = √(2d/g) = √(0.40/9.8) ≈ 0.20 s. Fatigue, alcohol and distraction lengthen reaction times measurably — at 50 km/h a car travels ~3.5 m in an extra 0.25 s of delay, which is why phone use while driving is so dangerous: the physics of Lesson 12 meets the biology of synapses.

Worked example: a withdrawal reflex, timed

You touch a hot pan. Receptors fire; the sensory impulse travels ~0.8 m at ~80 m/s (≈0.01 s); two synapses add ~1 ms each; the motor impulse returns in ≈0.01 s; the muscle contracts. Total ≈ 0.025 s — your hand is moving before the conscious sensation of pain (which needs the longer path to the cortex) registers. The delay you *feel* between touching and hurting is your brain catching up with your spinal cord.

Biology in the real world: when signalling fails

Neurology is applied neuron biology. In multiple sclerosis the immune system strips myelin; impulses slow and misfire, causing weakness and vision problems. Parkinson's disease is a shortage of dopamine in movement circuits; its main drug, L-DOPA, is a neurotransmitter precursor. Local anaesthetics block sodium channels so action potentials cannot start — no signal, no pain. Even caffeine works at synapses, blocking adenosine receptors that normally signal tiredness.

Common misconceptions

• The impulse is not an electric current flowing through a wire. It is a moving wave of ion movements across the membrane — more like a stadium wave than electricity in a cable.
• We do not use "10% of our brains." Imaging shows all regions active across a day; the myth survives because it flatters us.
• Reflexes do not "skip the brain entirely" in the sense of secrecy. The spinal cord acts first; the brain receives the information immediately after and can override later responses (you can choose to hold the hot dish if it is precious).

Exam Experience

Question 1. The myelin sheath speeds up nerve impulses by —

• A. making the axon shorter
• B. insulating the axon so the impulse jumps between nodes
• C. producing neurotransmitters
• D. increasing the number of dendrites

Question 2. In a reflex arc, the correct order is —

• A. effector → motor neuron → spinal cord → sensory neuron → receptor
• B. receptor → sensory neuron → relay neuron → motor neuron → effector
• C. receptor → motor neuron → brain → sensory neuron → effector
• D. brain → sensory neuron → motor neuron → effector → receptor

Question 3. At a synapse, the signal crosses the gap by —

• A. an electrical spark
• B. direct contact between axons
• C. diffusion of a neurotransmitter
• D. movement of the myelin sheath

Answers. 1: B. 2: B. 3: C.

Constructed response. A student measures reaction time with a ruler-drop test. Catches in cm: 18, 22, 19, 25, 21 (use t = √(2d/g), g = 9.8 m/s²). (a) Calculate the reaction time for the 18 cm catch (1 point). (b) Calculate the mean reaction time for all five catches (2 points). (c) Explain why a spinal reflex such as pulling away from a pin is faster than the ruler-drop response (2 points). Scoring guide. (a) d = 0.18 m; t = √(0.36/9.8) = √0.0367 ≈ 0.19 s (1 pt). (b) Times: 0.19, 0.21, 0.20, 0.23, 0.21 s; mean ≈ 0.21 s (1 pt for conversions, 1 pt for mean). (c) Reflex path is shorter (spinal cord, not brain) and has fewer synapses (1 pt); ruler-drop requires sensory input to reach the cortex and a voluntary motor decision, adding processing time (1 pt).

Going deeper: sense organs — transducers with tuning

Sense organs convert stimulus energy into impulses (transduction), and each is tuned to a slice of the world. The eye: light focused by cornea and lens onto the retina, where rods (dim light, no colour, ~120 million) and cones (colour, detail, ~6 million, concentrated at the fovea) convert photons into signals. The blind spot — where the optic nerve exits — is invisible to you because the brain fills the gap: perception is construction, not photography. The ear: sound waves vibrate the eardrum and three tiny bones (amplifying pressure ~20×) into the fluid-filled cochlea, where hair cells at different positions respond to different frequencies. Both organs feed Lesson 12's neurons; damage their transducers (loud-sound destruction of hair cells, a cloudy lens = cataract) and the wiring downstream never gets the message.

Going deeper: the endocrine partner — hormones in the blood

Alongside the wired nervous system runs the broadcast endocrine system (compared in Lesson 9's table): pituitary (the "master gland", itself commanded by the hypothalamus), thyroid (thyroxine sets metabolic rate), adrenals (adrenaline for seconds-scale emergencies; cortisol for longer stress), pancreas (Lesson 10), ovaries and testes (Lesson 15). Hormones only affect cells carrying their receptors — the broadcast is public, the audience is whoever has the receiver. This explains hormone specificity without wiring, and it is why hormone-mimicking pollutants (endocrine disruptors) can have body-wide effects at tiny doses.

Worked example 2: pupil reflex — a measurable arc

Shine a light in one eye: *both* pupils constrict (the consensual reflex) because the pathway splits in the brain stem. Clinicians test exactly this with a pen torch — a pupil that fails to constrict signals nerve or brain-stem damage, making a 0.2-second reflex a diagnostic instrument. The same arc logic scales up: every reflex test in a neurological exam (knee, ankle, pupil, corneal) is a probe of a specific circuit's integrity.

Case study: the split-brain experiments

In the 1960s, patients with severe epilepsy had the corpus callosum — the bridge between the cerebral hemispheres — cut to stop seizures spreading. Sperry and Gazzaniga's experiments on these patients revealed how specialised the hemispheres are: an image flashed to the left visual field (processed by the right hemisphere) could be *used* (the left hand picks the matching object) but not *named*, because speech sits in the left hemisphere and the bridge was cut. The right hand "didn't know" what the left hand was doing. These studies won a Nobel Prize and rewired our understanding of consciousness: the unified "self" is an achievement of connected hardware — a profound, slightly unsettling conclusion from pure neuroanatomy.

Final check: the autonomic system — accelerator and brake

Motor output divides into voluntary (somatic) control and the autonomic nervous system, which runs the internal world without conscious input. Its two branches oppose each other at most organs: sympathetic prepares for action ("fight or flight": heart rate up, pupils dilate, gut activity down, adrenaline reinforced) and parasympathetic restores ("rest and digest": heart rate down, digestion up). Homeostasis (Lesson 10) is partly the balance between them, adjusted moment to moment — standing up quickly, a near-miss in traffic, and sleep are all autonomic rebalancings. Link this to Lesson 13: the medulla sets heart rate through autonomic nerves plus the hormone adrenaline, which is why both a nerve signal and a blood-borne signal can raise your pulse — the fast one starts the response, the slower one sustains it.