Coordination: Nervous System & Senses
The CNS and peripheral nervous system; three neurone types; the five-component reflex arc; Extended neurone structure, synaptic transmission, myelin; eye structures and functions; Extended pupil reflex with iris muscles, accommodation mechanism, and rods vs cones.
The Nervous System
CORE EXTENDEDOverview of the Nervous System
The nervous system allows animals to detect stimuli, process information, and produce coordinated responses. It works alongside the hormonal system but is faster and more precise.
| Division | Components | Function |
|---|---|---|
| Central Nervous System (CNS) | Brain + spinal cord | Receives, processes, and coordinates information; initiates responses |
| Peripheral Nervous System (PNS) | All nerves outside the CNS (sensory and motor nerves) | Carries nerve impulses between CNS and rest of body |
Neurones — Types and Functions
| Neurone type | Function | Location |
|---|---|---|
| Sensory neurone | Carries impulses from receptors (sense organs) to the CNS | Peripheral nervous system |
| Motor neurone | Carries impulses from the CNS to effectors (muscles or glands) | Peripheral nervous system |
| Relay neurone (interneurone) |
Connects sensory and motor neurones within the CNS; processes information | Brain and spinal cord (CNS only) |
Structure of a Motor Neurone — Extended
| Structure | Function |
|---|---|
| Cell body | Contains nucleus and most organelles; metabolic centre of the cell |
| Dendrites | Short, branching extensions from the cell body that receive impulses from other neurones |
| Axon | Long fibre that carries the impulse away from the cell body toward the effector |
| Myelin sheath | Fatty insulating layer around the axon (formed by Schwann cells); speeds up impulse transmission by forcing the impulse to ‘jump’ between gaps (nodes of Ranvier) |
| Nodes of Ranvier | Gaps in the myelin sheath; impulse jumps from node to node (saltatory conduction) — increases speed of transmission |
| Synaptic knobs | Terminal endings of the axon that release neurotransmitters at synapses |
Reflex Arc
A reflex action is a rapid, automatic, involuntary response to a stimulus. Reflex actions are not controlled by conscious thought — they bypass the brain’s decision-making centres (cerebral cortex) and are processed in the spinal cord (or brainstem for cranial reflexes). This makes them faster than voluntary responses.
Receptor → Sensory neurone → Relay neurone (in CNS) → Motor neurone → Effector
Example — hot object: Pain receptor in skin detects heat → sensory neurone carries impulse to spinal cord → relay neurone in spinal cord → motor neurone carries impulse to arm muscle → muscle contracts (withdraws hand). The brain receives the signal later but is not required for the initial response.
Speed protects the body from damage. Because reflexes bypass the conscious brain, the response occurs in milliseconds — before the brain has even processed the pain. This minimises tissue damage in dangerous situations.
Withdrawing hand from heat; blinking when an object approaches the eye; pupil constriction in bright light; knee-jerk reflex; salivation in response to food; coughing and sneezing.
Synaptic Transmission — Extended
A synapse is a junction between two neurones (or between a neurone and an effector). There is a tiny gap — the synaptic cleft — that an electrical impulse cannot cross directly. Instead:
1. An impulse arrives at the synaptic knob (pre-synaptic membrane).
2. Vesicles containing neurotransmitter molecules fuse with the pre-synaptic membrane and release the neurotransmitter into the synaptic cleft.
3. Neurotransmitter diffuses across the synaptic cleft.
4. Neurotransmitter binds to specific receptors on the post-synaptic membrane — if enough molecules bind, a new impulse is generated in the next neurone.
5. Neurotransmitter is then broken down by enzymes (or reabsorbed) so the synapse can reset and receive the next impulse.
1. One-way transmission: Neurotransmitter is only released from the pre-synaptic side — impulses cannot travel backwards. This ensures information flows in one direction only.
2. Integration: A relay neurone may receive inputs from many neurones simultaneously. Only if enough neurotransmitter accumulates will a new impulse be generated — allowing the CNS to filter and process signals.
A person touches a hot surface and immediately withdraws their hand before feeling pain. Which pathway correctly describes the reflex arc involved?
- A. Receptor → motor neurone → brain → sensory neurone → effector
- B. Effector → relay neurone → sensory neurone → motor neurone → receptor
- C. Receptor → sensory neurone → relay neurone → motor neurone → effector
- D. Receptor → relay neurone → brain → motor neurone → effector
Sense Organs
CORE EXTENDEDReceptors and Sense Organs
A receptor is a cell (or group of cells) that detects a specific stimulus and converts it into a nerve impulse. Sense organs contain many receptors grouped together.
| Sense organ | Stimulus detected | Receptor type |
|---|---|---|
| Eye | Light | Photoreceptors (rods and cones in the retina) |
| Ear | Sound; balance/head position | Hair cells in cochlea (sound); hair cells in semicircular canals (balance) |
| Nose | Chemicals (smell) | Olfactory receptors |
| Tongue | Chemicals (taste) | Taste receptors in taste buds |
| Skin | Touch, pressure, pain, temperature | Various mechanoreceptors and thermoreceptors |
Structure of the Eye
| Structure | Function |
|---|---|
| Cornea | Transparent outer layer; refracts (bends) most of the light entering the eye — provides most of the eye’s focusing power |
| Iris | Coloured ring of muscle around the pupil; controls the size of the pupil (amount of light entering) |
| Pupil | Hole in the centre of the iris; size controlled by the iris muscles |
| Lens | Transparent, flexible structure that fine-tunes focusing by changing shape (accommodation) |
| Ciliary muscles | Ring of muscle that controls the shape of the lens for accommodation |
| Suspensory ligaments | Fibres connecting ciliary muscles to lens; transmit tension changes to alter lens shape |
| Retina | Light-sensitive layer at the back of the eye containing rods (dim light, black/white) and cones (bright light, colour) |
| Fovea (yellow spot) | Region of densest cone concentration; highest visual acuity; where sharp colour vision occurs |
| Blind spot | Where the optic nerve exits the eye; no photoreceptors; cannot detect light |
| Optic nerve | Bundle of nerve fibres carrying impulses from retina to the brain |
| Sclera | White, tough outer coat; protects the eye and maintains its shape |
| Choroid | Pigmented, blood-vessel-rich layer between sclera and retina; absorbs stray light and nourishes the retina |
| Vitreous humour | Jelly-like substance filling the main chamber of the eye; maintains shape |
| Aqueous humour | Watery fluid between cornea and lens; nourishes cornea and lens; maintains pressure |
Pupil Reflex — Extended
| Condition | Iris response | Pupil size | Purpose |
|---|---|---|---|
| Bright light | Circular (constrictor) muscles of iris contract; radial muscles relax | Pupil constricts (smaller) | Reduces light entering the eye — protects the retina from damage |
| Dim light | Radial (dilator) muscles of iris contract; circular muscles relax | Pupil dilates (larger) | Allows more light to enter — maximises vision in low light |
Think of the iris as having two sets of muscles arranged like a target:
Circular (constrictor) muscles run in rings around the pupil — when they contract, the ring tightens, making the pupil smaller.
Radial (dilator) muscles run like spokes of a wheel from pupil edge outward — when they contract, they pull the iris open, making the pupil larger.
Accommodation — Extended
Accommodation is the process by which the lens changes shape to focus on objects at different distances.
| Focusing on | Ciliary muscles | Suspensory ligaments | Lens shape | Focal length |
|---|---|---|---|---|
| Near object | Contract (ring gets smaller) | Slacken (less tension) | Fatter / more curved | Short (converges light strongly) |
| Distant object | Relax (ring gets larger) | Taut (pulled tight) | Thinner / less curved | Long (converges light gently) |
Many students reverse the ciliary muscle/ligament relationship. Remember: ciliary muscles form a ring. When the ring contracts it gets smaller, which reduces the diameter of the space inside — the suspensory ligaments attached to the lens go slack (no longer being pulled outward). With tension removed, the elastic lens springs back to its natural, fatter shape.
Rods and Cones — Extended
| Feature | Rods | Cones |
|---|---|---|
| Light sensitivity | Very sensitive — work in dim light | Less sensitive — need bright light |
| Colour vision? | No — only detect light/dark | Yes — three types for red, green, blue |
| Visual acuity | Low — many rods converge to one nerve fibre | High — each cone has its own nerve fibre |
| Distribution | Mainly peripheral retina | Concentrated at fovea |
| Pigment | Rhodopsin (breaks down in light, regenerated in dark) | Three different photopsins (red, green, blue) |
A person walks from a brightly lit room into a dark room. Which change occurs in the eye?
- A. Circular iris muscles contract, pupil constricts
- B. Radial iris muscles relax, pupil constricts
- C. Radial iris muscles contract, pupil dilates
- D. Ciliary muscles contract, lens becomes thinner
Describe how the lens changes to focus on a near object. Include the roles of the ciliary muscles, suspensory ligaments, and the lens itself. [4 marks]
- The ciliary muscles contract, reducing the diameter of the ciliary body ring [1 mark]
- This causes the suspensory ligaments to slacken (less tension on the lens) [1 mark]
- The elastic lens becomes fatter / more curved / more convex [1 mark]
- The fatter lens has a shorter focal length and refracts light more strongly, converging it onto the retina for a near object [1 mark]
Comprehensive Practice Questions
Mixed questions across Topics 14.1 and 14.2.
Which neurone carries impulses from the CNS to a muscle?
- A. Sensory neurone
- B. Relay neurone
- C. Motor neurone
- D. Receptor cell
(a) State what is meant by a reflex action. [1 mark]
(b) Name the five components of a reflex arc in order. [2 marks]
(c) State the function of the fovea in the human eye. [2 marks]
(d) Explain why a person cannot see an object that falls on the blind spot of the retina. [2 marks]
- (a) A rapid, automatic, involuntary response to a stimulus that does not require conscious thought [1 mark]
- (b) Receptor → sensory neurone → relay neurone → motor neurone → effector [1 mark for all five in correct order; allow 1 mark for 3–4 correct] [2 marks]
- (c) The fovea contains a high concentration of cone cells [1 mark]; it is the region of highest visual acuity / sharpest colour vision [1 mark]
- (d) The blind spot is where the optic nerve exits the eye [1 mark]; there are no photoreceptors (rods or cones) at this point, so light falling on it cannot generate a nerve impulse [1 mark]
(a) Describe how an impulse crosses a synapse. [4 marks]
(b) Explain how the myelin sheath speeds up nerve impulse transmission. [2 marks]
- Impulse arrives at the synaptic knob (pre-synaptic membrane) [1 mark]
- Vesicles fuse with the pre-synaptic membrane, releasing neurotransmitter into the synaptic cleft [1 mark]
- Neurotransmitter diffuses across the synaptic cleft [1 mark]
- Neurotransmitter binds to specific receptors on the post-synaptic membrane, generating a new impulse in the next neurone [1 mark]
- The myelin sheath is an insulating layer around the axon; the electrical impulse cannot pass through myelin [1 mark]
- The impulse therefore jumps from node to node (nodes of Ranvier — gaps in the myelin sheath), covering greater distances per jump — this saltatory conduction is much faster than continuous conduction [1 mark]
High-Frequency Mistakes — Topics 14.1 & 14.2
- 🔁Saying the brain is part of the reflex arcThe reflex arc bypasses conscious brain centres — it passes through the spinal cord (relay neurone), not the brain. The brain receives the signal afterwards, which is why you feel pain after you have already withdrawn your hand. The brain is not one of the five components of the reflex arc.
- ⚡Reversing sensory and motor neuronesSensory neurones carry impulses FROM receptors TO the CNS (incoming). Motor neurones carry impulses FROM the CNS TO effectors (outgoing). "Sensory = to the Spinal cord; Motor = away from the Main centre" can help.
- 👀Confusing the pupil and the irisThe iris is the coloured ring of muscle. The pupil is the hole in the middle. The iris muscles change size; the pupil changes as a result. In bright light the iris muscles change so the pupil constricts — iris acts, pupil changes.
- 🔴Ext: Reversing circular vs radial muscle actionCircular muscles contract → pupil constricts (bright light). Radial muscles contract → pupil dilates (dim light). A common reversal: students say radial muscles constrict the pupil. Remember: circular = constrict (both start with C); radial = dilate.
- 👁Ext: Reversing ciliary muscle/suspensory ligament for near focusNear object: ciliary muscles CONTRACT → suspensory ligaments SLACKEN → lens FATTER. Many students say "ciliary muscles relax for near objects" — this is wrong. The ring of ciliary muscle contracts (gets smaller) for near focus, releasing tension on the ligaments.
- 🏭Ext: Saying synapses allow impulses to travel in both directionsSynapses are unidirectional — neurotransmitter is only released from the pre-synaptic side. Impulses can only travel from pre-synaptic to post-synaptic neurone. This one-way property is essential for coordinated signalling.
- 🔳Saying rods detect colourRods detect light intensity only (black, white, grey). Cones detect colour — there are three types sensitive to red, green, and blue. Colour blindness results from missing or defective cone types. Rods are essential for night vision; cones for daylight colour vision.
Highest-yield Core items: the three types of neurone with their functions and directions; the five-component reflex arc in order (receptor → sensory → relay → motor → effector); the definition of a reflex action; all eye structures with functions (particularly cornea, iris, lens, retina, fovea, blind spot). For Extended: the synaptic transmission sequence (four steps); myelin sheath and saltatory conduction; pupil reflex (circular vs radial muscles); accommodation mechanism (ciliary muscles → ligaments → lens shape — near and far); and rods vs cones comparison. Accommodation and the synapse are near-certain Paper 4 targets.