The Nervous System
Command & Control⚡ The Body's Electrical Network
The nervous system is the body's rapid communication network — it detects stimuli from the environment and from inside the body, processes that information, and co-ordinates responses. It operates through electrical impulses (action potentials) travelling along neurones at speeds of up to 120 m/s. The nervous system works alongside the endocrine (hormonal) system to maintain homeostasis and produce behaviour.
Structure
Divisions of the Nervous System
Central vs peripheral. Somatic vs autonomic. Sympathetic vs parasympathetic. Know the full hierarchy.
| Division | Components | Function |
|---|---|---|
| Central Nervous System (CNS) | Brain + Spinal cord | Integrates and processes all sensory input; co-ordinates motor output; higher cognitive functions (thought, memory, emotion) |
| Peripheral Nervous System (PNS) | All nerves outside brain and spinal cord; cranial nerves + spinal nerves | Carries signals to and from CNS; sensory input in, motor output out |
| Somatic Nervous System | Sensory neurones + Motor neurones to skeletal muscle | Voluntary movement; conscious sensory perception (pain, touch, vision, hearing) |
| Autonomic Nervous System | Sympathetic + Parasympathetic divisions | Involuntary control of internal organs (heart, lungs, gut, glands) |
| Sympathetic division | Thoracic/lumbar spinal nerves | "Fight or flight" — increases heart rate, dilates pupils, redirects blood to muscles, inhibits digestion |
| Parasympathetic division | Cranial nerves + sacral spinal nerves | "Rest and digest" — decreases heart rate, constricts pupils, stimulates digestion, conserves energy |
The Basic Unit
Neurone Structure & Impulse Transmission
Three types of neurones. Know every part, its function, and how an impulse travels from receptor to effector.
🏗️ Neurone Parts
- Dendrites — short branched extensions; receive impulses from other neurones or receptors; carry impulse TOWARD cell body
- Cell body (soma) — contains nucleus and organelles; metabolic centre
- Axon — long single extension; carries impulse AWAY from cell body to next neurone or effector
- Myelin sheath — fatty insulating layer around axon (made by Schwann cells); speeds up conduction; gaps = nodes of Ranvier (saltatory conduction)
- Synaptic knob — bulb at axon end; releases neurotransmitters across the synapse
🔄 Three Types of Neurones
- Sensory (afferent) neurone — carries impulse FROM receptor TO CNS; long dendrite, short axon
- Interneurone (relay/connector) — carries impulse WITHIN the CNS; connects sensory to motor; processing and integration
- Motor (efferent) neurone — carries impulse FROM CNS TO effector (muscle or gland); short dendrites, long axon
- Pathway: Receptor → Sensory neurone → Interneurone (CNS) → Motor neurone → Effector
📌 The Synapse — Chemical Gap Between Neurones
Neurones do not physically touch — there is a gap of ~20 nm between them called the synaptic cleft. When an impulse reaches the synaptic knob, vesicles release neurotransmitters (e.g. acetylcholine, dopamine, serotonin) into the cleft. These diffuse across and bind to receptor proteins on the post-synaptic membrane, triggering a new impulse. The neurotransmitter is then broken down by enzymes (e.g. acetylcholinesterase breaks down acetylcholine) or reabsorbed, resetting the synapse. Synapses ensure one-way transmission of signals. Many drugs act at synapses — SSRIs (antidepressants) block serotonin reabsorption; cocaine blocks dopamine reabsorption; curare blocks acetylcholine receptors (muscle paralysis).
Rapid Response
The Reflex Arc
A reflex is a rapid, automatic response to a stimulus that bypasses the brain for speed. Know all five components in order.
1
Receptor Detects the stimulus (e.g. pain receptor in skin detects sharp object)
2
Sensory Neurone Carries impulse from receptor toward the spinal cord (CNS)
3
Interneurone (in spinal cord) Receives, processes, and relays impulse; connects sensory to motor. The brain is informed SIMULTANEOUSLY but the response does not wait for brain instruction.
4
Motor Neurone Carries impulse from spinal cord to effector muscle
5
Effector Muscle contracts (withdrawal reflex) — you pull your hand away before consciously feeling the pain
⚠️ Exam Watch — Why Reflexes Are Faster Than Voluntary Actions
Reflexes are faster because the impulse does NOT travel all the way to the brain before a response is initiated — it is processed in the spinal cord. The brain is informed in parallel (which is why you feel pain shortly after the withdrawal reflex, not before). This speed is essential for survival: the reflex arc removes your hand from a hot surface in ~50 ms; conscious processing would take ~300 ms.
The Eye
Light Detection👁️ A Living Camera
The human eye is a complex optical instrument that focuses light onto a layer of photoreceptors at the back. It can adapt to light intensities covering a 10-billion-fold range, detect single photons in darkness, and distinguish approximately 10 million different colours. Every structure has a precise function — understanding the eye means knowing how each part contributes to forming a sharp, accurate image on the retina.
Complete Reference
Eye Structures & Functions
Every structure in the eye has a specific, testable function. Learn them all — inside-out and outside-in.
| Structure | Location | Function |
|---|---|---|
| Sclera | Tough outer white coat | Protects the eyeball; maintains shape; attachment point for extrinsic eye muscles |
| Cornea | Transparent front of sclera | Refracts (bends) incoming light — provides ~70% of the eye's total focusing power; no blood vessels (receives O₂ from tears) |
| Choroid | Middle vascular layer | Contains blood vessels — supplies retina with O₂ and nutrients; dark pigment absorbs excess light, preventing internal reflection |
| Iris | Coloured ring in front of lens | Muscular diaphragm — controls pupil size (and therefore amount of light entering). Contains circular muscles (constrict pupil in bright light) and radial muscles (dilate pupil in dim light) |
| Pupil | Central hole in iris | Aperture through which light enters — not a structure itself, but the opening controlled by the iris |
| Lens | Behind iris; held by suspensory ligaments from ciliary body | Refracts light; changes shape (accommodation) to focus near or distant objects precisely on the retina |
| Ciliary body / muscles | Ring of muscle behind iris | Contracts and relaxes to change lens shape during accommodation |
| Suspensory ligaments | Between ciliary body and lens | Hold lens in position; transmit ciliary muscle movement to lens — tense = flat lens (far); slack = fat lens (near) |
| Aqueous humour | Chamber between cornea and lens | Watery fluid; maintains eye shape; refracts light; supplies cornea and lens with nutrients; produced and drained continuously |
| Vitreous humour | Large chamber behind lens | Transparent jelly; maintains shape of eyeball; transmits light to retina |
| Retina | Inner layer at back of eye | Contains photoreceptors (rods and cones); transduces light energy into nerve impulses |
| Fovea (yellow spot / macula) | Central area of retina, directly behind lens | Highest density of cones; point of sharpest vision; used for colour and fine detail |
| Blind spot (optic disc) | Where optic nerve exits retina | No photoreceptors here — cannot detect light; creates a blind spot in visual field |
| Optic nerve | Exits at back of eye | Bundle of ~1 million nerve fibres; carries visual impulses from retina to visual cortex in occipital lobe of brain |
Photoreceptors
Rods vs Cones
Two types of light receptor with completely different jobs. Understanding their differences explains colour vision, night vision, and the fovea.
| Feature | Rods | Cones |
|---|---|---|
| Number | ~120 million per retina | ~6 million per retina |
| Location | Distributed across retina; absent from fovea | Concentrated in fovea; fewer toward periphery |
| Sensitivity | Extremely sensitive — respond to very low light levels; can respond to a single photon | Less sensitive — require brighter light to activate |
| Colour | Cannot distinguish colours — one type only | Three types (S, M, L) sensitive to blue, green, red wavelengths respectively; colour vision by comparing signals |
| Acuity (sharpness) | Low acuity — many rods share one nerve fibre (convergence); can't resolve fine detail | High acuity — each cone has its own nerve fibre; sharp, detailed images |
| Photopigment | Rhodopsin (visual purple) — breaks down in light, regenerates in dark (dark adaptation) | Iodopsin (three types corresponding to R/G/B) |
| Night vision | Primary receptors in dim light | Non-functional in dim light — that is why we lose colour vision at night |
| Peripheral vision | Provide peripheral and motion detection | Very few at periphery |
📌 Why Stars Disappear When You Look Directly at Them
Faint stars are best seen using peripheral (averted) vision because the periphery of the retina is rich in rods. When you look directly at a faint star, its image falls on the fovea — which is packed with cones that need bright light to function, and virtually no rods. So the star disappears. Astronomers call this technique "averted vision." It directly demonstrates the different distribution and sensitivity of rods versus cones.
Vision & Visual Defects
Accommodation & Correction🔬 How the Eye Focuses
Accommodation is the process by which the eye changes the shape of its lens to focus objects at different distances onto the retina. It is controlled by the ciliary muscles and suspensory ligaments. When these mechanisms go wrong — due to a lens that is too curved or too flat, or an eyeball that is too long or too short — the image falls in front of or behind the retina, producing blurred vision that can be corrected with lenses.
Focusing Mechanism
Accommodation — Near vs Far
The ciliary muscles, suspensory ligaments, and lens work as a system. When one changes, all change. Know the exact sequence.
🏔️ Looking at a Distant Object
- Ciliary muscles relax
- Ciliary body ring gets larger
- Suspensory ligaments become taut/tense
- Lens is pulled thin/flat
- Lens refracts light less
- Light from distant objects is nearly parallel — less bending needed
- Image forms sharply on retina
📖 Looking at a Near Object
- Ciliary muscles contract
- Ciliary body ring gets smaller
- Suspensory ligaments become slack/loose
- Lens springs back to fat/round shape (elastic recoil)
- Lens refracts light more
- Light from near objects diverges — more bending needed
- Image forms sharply on retina
⚠️ Exam Watch — The Suspensory Ligament Trick
Students commonly confuse the direction of change. Key: when ciliary muscles CONTRACT, the ring gets SMALLER → ligaments go SLACK → lens gets FAT. Think of it like a drawstring bag: pull the string (ciliary muscles contract → ring tightens) → the bag (lens) gets rounder. When muscles RELAX → ring gets bigger → ligaments PULL tight → lens gets FLAT. The ligaments and muscles always move in opposite directions.
Vision Problems
Myopia, Hyperopia & Presbyopia
Three common visual defects — cause, which image formation goes wrong, and the corrective lens. All three are exam staples.
| Defect | Common Name | Cause | Image Formed | Correction |
|---|---|---|---|---|
| Myopia | Short-sightedness (nearsightedness) | Eyeball too long OR lens too curved/powerful | Image of distant objects falls IN FRONT of retina | Concave (diverging) lens — spreads light rays before they enter the eye so they converge further back, on the retina |
| Hyperopia | Long-sightedness (farsightedness) | Eyeball too short OR lens too flat/weak | Image of near objects would focus BEHIND retina | Convex (converging) lens — converges light rays before they enter the eye so they reach focus sooner, on the retina |
| Presbyopia | Age-related long-sight | Lens becomes less elastic with age; ciliary muscles weaken; lens cannot accommodate to near objects | Near objects blurred (same end result as hyperopia but different cause) | Convex (converging) reading glasses; bifocals (top for distance, bottom for near) |
| Astigmatism | Blurred/distorted vision at all distances | Cornea or lens is irregularly curved (not spherical) | Light focused at different points rather than a single sharp image | Cylindrical (toric) lens that compensates for the uneven curvature |
Autonomic Response
Pupil Reflex & Iris Muscles
The iris has two sets of antagonistic muscles. The pupil reflex is a simple but classic exam question on involuntary nervous system control.
☀️ Bright Light → Pupil Constricts
- Circular (sphincter) muscles of iris contract
- Radial (dilator) muscles relax
- Pupil diameter decreases
- Less light enters eye → protects photoreceptors from damage; improves depth of focus
- Controlled by parasympathetic nervous system
🌙 Dim Light → Pupil Dilates
- Radial (dilator) muscles of iris contract
- Circular (sphincter) muscles relax
- Pupil diameter increases
- More light enters eye → maximises sensitivity in low light
- Controlled by sympathetic nervous system
- Also occurs during "fight or flight" (adrenaline effect)
The Ear
Sound & Balance👂 Converting Air Vibrations Into Nerve Signals
The ear performs a remarkable energy conversion: mechanical vibrations in air → mechanical vibrations in fluid → mechanical deformation of hair cells → electrical nerve impulses. This transformation occurs across three regions: the outer ear collects sound, the middle ear amplifies and transmits it, and the inner ear converts it to nerve signals while also providing the sense of balance. Every structure in the ear is precisely tuned for this chain of energy conversion.
Complete Reference
Ear Structures — Outer, Middle & Inner
All three regions, every named structure, and its precise function in the hearing pathway.
| Region | Structure | Function |
|---|---|---|
| Outer Ear | Pinna (auricle) | Collects and funnels sound waves into the ear canal; shape helps with sound localisation and depth perception |
| Ear canal (external auditory meatus) | Channels sound waves toward the tympanic membrane; lined with hairs and ceruminous glands (produce wax for protection) | |
| Tympanic membrane (eardrum) | Thin membrane vibrates in response to sound waves; converts sound (air pressure waves) into mechanical vibrations; boundary between outer and middle ear | |
| Middle Ear | Malleus (hammer) | Ossicle attached to tympanic membrane; receives and transmits vibrations |
| Incus (anvil) | Middle ossicle; transmits vibrations from malleus to stapes | |
| Stapes (stirrup) | Smallest bone in the body; transmits vibrations to oval window of cochlea; amplification system — 3 ossicles together amplify sound ~20× | |
| Eustachian tube | Connects middle ear to throat (pharynx); equalises air pressure on both sides of tympanic membrane; opens when you swallow or yawn — essential for comfortable hearing at altitude | |
| Inner Ear | Oval window | Membrane where stapes transmits vibrations into the fluid-filled cochlea |
| Cochlea | Coiled, fluid-filled tube; contains organ of Corti with hair cells; transduces fluid vibrations into nerve impulses; different regions respond to different sound frequencies | |
| Organ of Corti | Hearing organ within cochlea; hair cells with stereocilia detect fluid movement; bend → ion channels open → electrical signal → auditory nerve | |
| Semicircular canals + Vestibule | Balance organs — NOT for hearing. Three semicircular canals detect rotational (angular) acceleration; vestibule (utricle + saccule) detects linear acceleration and gravity (static equilibrium) |
📌 The Hearing Pathway — Step by Step
Sound waves → pinna collects → ear canal funnels → tympanic membrane vibrates → malleus → incus → stapes → oval window vibrates → cochlear fluid moves → basilar membrane moves → hair cells (organ of Corti) bent → ion channels open → generator potential → auditory nerve impulse → auditory cortex (temporal lobe of brain) → perceived as sound.
Second Function of the Inner Ear
Balance — Semicircular Canals
The inner ear does double duty — hearing AND balance. The two systems are anatomically adjacent but functionally separate.
🔄 Semicircular Canals (Rotational Balance)
- Three canals at right angles to each other — detect rotation in three dimensions (pitch, roll, yaw)
- Each canal contains fluid (endolymph) and a sensory structure called the cupula containing hair cells
- When head rotates, fluid lags behind → cupula bends → hair cells stimulated → nerve impulse to cerebellum (balance centre)
- All three canals together detect any direction of rotation
📐 Vestibule — Utricle & Saccule (Linear Balance)
- Detect gravity and linear (straight-line) acceleration
- Contain otoliths — tiny calcium carbonate crystals (ear stones) sitting on a membrane above hair cells
- When body tilts or accelerates, otoliths shift due to gravity → press on hair cells → nerve impulse
- The feeling of acceleration in a car or lift is detected by the utricle/saccule
- Motion sickness occurs when visual input conflicts with inner ear balance signals
Skin Receptors, Smell & Taste
Chemoreception & Mechanoreception🖐️ The Body's Largest Sense Organ
Skin is the body's largest organ and contains a rich array of sensory receptors that detect pressure, temperature, pain, and vibration. Different receptor types are specialised for different modalities and are found at different depths within the skin. Smell (olfaction) and taste (gustation) are chemoreceptors — they detect chemical molecules dissolved in air or liquid. Together these "minor" senses provide critical information about the immediate environment and internal body state.
Cutaneous Receptors
Skin Receptor Types & Functions
Five main receptor types in skin — each specialised for a different stimulus. Location in skin determines what they detect.
| Receptor | Location in Skin | Stimulus Detected | Notes |
|---|---|---|---|
| Meissner's corpuscles | Dermis — just below epidermis, especially in fingertips, lips | Light touch; texture; low-frequency vibration | Rapidly adapting — stop firing with sustained touch (explains why you stop noticing clothes) |
| Pacinian corpuscles | Deep dermis and subcutaneous tissue | Deep pressure; high-frequency vibration | Large onion-shaped receptor; rapidly adapting; detects vibration through tools held in hand |
| Merkel's discs | Epidermis-dermis boundary, fingertips | Light touch; fine spatial detail; sustained pressure | Slowly adapting — continue firing during sustained touch; critical for reading Braille |
| Ruffini endings | Deep dermis | Skin stretch; sustained pressure; finger position | Slowly adapting; provide proprioceptive (body position) information |
| Free nerve endings | Throughout epidermis and dermis | Pain (nociception); temperature (hot and cold); itch | Simplest receptor — just bare nerve endings; most numerous; essential for injury detection and withdrawal reflexes |
📌 Two-Point Discrimination & Receptor Density
The ability to distinguish two separate touch points (two-point discrimination) varies enormously across the body because receptor density varies. Fingertips: ~2 mm discrimination (very dense receptors — essential for manipulation). Back of hand: ~30 mm. Back: ~60 mm. This is why fingertips are used for tasks requiring fine touch and Braille reading, while the back is insensitive to small separations. Receptor density is directly proportional to the area of somatosensory cortex devoted to that body region (homunculus).
Chemoreception
Smell (Olfaction) & Taste (Gustation)
Both senses detect dissolved chemicals. Smell is vastly more sensitive — humans can detect ~1 trillion odours but only 5 basic tastes.
👃 Olfaction (Smell)
- Olfactory receptor cells in the olfactory epithelium at roof of nasal cavity
- Each receptor cell has cilia bearing specific receptor proteins — different proteins detect different odour molecules
- Humans have ~400 types of olfactory receptor (dogs have ~800)
- Odour molecules dissolve in mucus → bind to receptor → nerve impulse → olfactory bulb → olfactory cortex and limbic system (emotion/memory)
- Direct connection to limbic system explains why smells trigger powerful emotional memories (Proustian memory)
- Can adapt rapidly (olfactory adaptation) — explains why you stop noticing your own house's smell
👅 Gustation (Taste)
- Taste buds in papillae on tongue (and soft palate, epiglottis)
- Each taste bud contains ~50 taste receptor cells with microvilli (taste hairs)
- Five basic tastes: sweet, sour, salty, bitter, umami (savoury/glutamate)
- Taste molecules dissolve in saliva → bind to receptor → nerve impulse → gustatory cortex
- ~80% of perceived "taste" is actually smell — food tastes bland when nose is blocked
- Bitter and sour receptors evolved as poison/acid detectors — explains why children are hypersensitive to bitter tastes
🎯 Senses Assessment
Eight questions on the nervous system, eye, ear, and receptors.
Question 1 of 8
Trace the correct path of a nerve impulse in a spinal reflex arc, in order from stimulus to response.
Question 2 of 8
When you move from a brightly lit room into a dark room, your pupils dilate. Describe exactly what happens in the iris to cause this.
Question 3 of 8
What happens to the ciliary muscles, suspensory ligaments, and lens shape when you focus on a nearby object (e.g. reading a book)?
Question 4 of 8
A patient can see near objects clearly but struggles with distant objects. What visual defect do they have, what causes it, and what type of corrective lens is needed?
Question 5 of 8
What is the difference between rods and cones in the retina? Why do we lose the ability to see colour in dim light?
Question 6 of 8
Describe the pathway by which sound waves are converted into nerve impulses in the ear, naming the key structures in order.
Question 7 of 8
What is the function of the Eustachian tube, and what happens if it becomes blocked?
Question 8 of 8
Explain why you can feel the vibration of a tuning fork held against a table surface even though no sound seems to be transmitted through the air. Which skin receptor is responsible?