The Concept of a Stimulus and Response
A stimulus is a detectable change in the internal or external environment. An organism's ability to respond to stimuli is fundamental to survival - finding food, avoiding predators, reproducing. The response has to be appropriate, coordinated, and timely.
The general pathway is always:
Receptors are specific - they only respond to one type of stimulus (e.g. photoreceptors respond to light, not sound). This specificity is due to the receptor proteins on the cell surface.
Taxes (Taxis)
A taxis is a whole-organism movement directly toward or away from a directional stimulus. It's a simple, innate behaviour.
- Positive taxis=movement towards the stimulus
- Negative taxis=movement away from the stimulus
Named by the stimulus type:
- Phototaxis - response to light (e.g. Euglena moves towards light for photosynthesis - positive phototaxis)
- Chemotaxis - response to a chemical (e.g. bacteria moving toward glucose; white blood cells moving toward pathogens via chemical signals)
> Why it matters: Taxes are important in single-celled organisms and simple animals that lack a complex nervous system. They're entirely stimulus-driven with no learning involved.
Kinesis
A kinesis is a non-directional response - the organism doesn't move towards or away from anything specifically, but changes its speed of movement or rate of turning depending on stimulus intensity.
- In unfavourable conditions→moves faster / turns more frequently→more likely to randomly end up somewhere better
- In favourable conditions→slows down→stays there
Example: a woodlouse in dry conditions moves rapidly and turns frequently (kinesis); in humid conditions it slows and stays put.
> Kinesis vs taxis is a common exam distinction - taxis is directional, kinesis is non-directional but intensity-dependent.
Tropisms
Tropisms are growth responses in plants to directional stimuli. Because plants can't move, they respond by growing differentially.
- Positive tropism=growth towards the stimulus
- Negative tropism=growth away from the stimulus
| Tropism | Stimulus | Example |
|---|---|---|
| Phototropism | Light | Shoots grow toward light (positive) |
| Gravitropism (geotropism) | Gravity | Roots grow downward (positive), shoots grow up (negative) |
| Thigmotropism | Touch/contact | Climbing plant tendrils coil around supports |
| Hydrotropism | Water | Roots grow toward water |
The mechanism behind phototropism and gravitropism involves IAA (auxin) - we'll go into full detail in the plant responses section later.
Reflex Arcs
A reflex is a rapid, automatic, involuntary response to a stimulus. It doesn't involve conscious thought - the signal doesn't go to the cerebral cortex. This makes reflexes faster than voluntary responses, which is key for protection (e.g. withdrawing your hand from something hot).
The Components of a Reflex Arc
Relay neurone → within the spinal cord, connects sensory to motor; passes signal on
Why Reflexes Bypass the Brain
The synapse connections in a reflex arc are within the spinal cord, not the brain. The signal does travel to the brain simultaneously (which is why you feel pain a fraction of a second after you've already pulled your hand away), but the response doesn't wait for the brain - the arc is complete within the spinal cord.
> This is a high-value exam point: reflexes are fast because they involve few synapses and bypass conscious processing in the brain.
The Spinal Cord in Cross-Section
Worth knowing structurally:
- Grey matter (H-shaped inner region) - contains cell bodies of relay and motor neurones
- White matter (outer region) - contains myelinated axons carrying signals up and down the cord
- Sensory neurones enter via the dorsal root
- Motor neurones exit via the ventral root
Exam Tip - Common Mark Scheme Phrasing
- Don't just say "the signal goes to the spinal cord" - say it travels along the sensory neurone to the dorsal root of the spinal cord
- Don't say "it's fast because there's no brain involved" - say "few synapses involved" and "response does not require processing by the brain/cerebral cortex"
- Taxis: always specify directional movement; kinesis: always specify non-directional change in speed/turning rate
The Eye as a Sense Organ
The mammalian eye converts light into nerve impulses (phototransduction). Knowing the structure and how accommodation/pupil reflex work is essential for exam questions.
Structure and Function
- Cornea - refracts light (accounts for ~70% of focusing power). Transparent, avascular (no blood vessels - supplied by aqueous humour).
- Iris - coloured ring; contains circular (constrictor) and radial (dilator) muscles that control pupil size.
- Lens - flexible; fine-tunes focus by changing shape (accommodation).
- Retina - contains rods and cones; converts light to nerve impulses.
- Fovea - highest cone density; maximum visual acuity; where direct gaze falls.
- Blind spot - where optic nerve exits; no photoreceptors; no image formed here.
- Choroid - pigmented layer behind retina; absorbs stray light; vascularised.
Accommodation
Focusing on near object: ciliary muscles contract → suspensory ligaments go slack → lens becomes more convex (bulges) → greater refraction → near object in focus.
Focusing on far object: ciliary muscles relax → suspensory ligaments become taut → lens flattened → less refraction → distant object in focus.
Memory: Near = contract; Far = relax (ciliary muscles).
Pupil Reflex
Bright light: circular muscles contract + radial muscles relax → pupil constricts → less light in.
Dim light: radial muscles contract + circular muscles relax → pupil dilates → more light in.
The reflex is consensual - both pupils constrict even if only one eye is illuminated (signals cross at the optic chiasma).
Rods vs Cones
| Feature | Rods | Cones |
|---|---|---|
| Distribution | Entire retina (not fovea) | Concentrated at fovea |
| Pigment | Rhodopsin (1 type) | 3 types (R, G, B) |
| Sensitivity | High (dim light) | Low (needs bright light) |
| Colour vision | No (monochromatic) | Yes (trichromatic) |
| Convergence | Many → one bipolar cell → low acuity | One → one bipolar cell → high acuity |
| Use | Night vision, peripheral | Daylight, colour, detail |
Why rods have low acuity but high sensitivity: many rods converge onto a single bipolar cell. Small signals from many rods summate → enough to trigger an action potential even in dim light. But because many rods share one bipolar cell, the brain cannot distinguish which rod was stimulated → low acuity.
Why cones have high acuity but low sensitivity: each cone has its own bipolar cell → one photon from one cone gives very precise spatial information → high acuity. But the single signal from one cone may not be enough to trigger an action potential in dim light → needs bright light.
Phototransduction in Rods
Rhodopsin = opsin (protein) + retinal (light-absorbing chromophore from vitamin A).
In the dark: retinal in cis form; rod cell membrane depolarised; continuously releases inhibitory glutamate onto bipolar cells.
In light:
- Light converts retinal from cis→trans form
- Rhodopsin changes shape (photoactivated)→bleaching
- Ion channels close→rod cell membrane hyperpolarises
- Less glutamate released→bipolar cell depolarises→signal to ganglion cell→action potential along optic nerve
Dark adaptation: in bright light, rhodopsin is bleached faster than it is regenerated. Moving from bright to dim environment, it takes time (~20 minutes) for rhodopsin to be fully regenerated → eyes gradually become more sensitive to dim light.
Summary
- Stimulus→Receptor→Coordinator→Effector→Response
- Taxis: directional movement toward (positive) or away from (negative) a stimulus
- Kinesis: non-directional; changes speed/turning rate in response to stimulus intensity
- Tropisms: plant growth responses; positive = toward, negative = away. Mechanism involves IAA (see Plant Responses)
- Reflex arc: stimulus → receptor → sensory neuron → relay neuron (spinal cord) → motor neuron → effector. Fast because few synapses; bypasses cerebral cortex.
- Eye: cornea + lens focus light; accommodation = ciliary muscle contraction/relaxation changes lens shape; pupil reflex = circular/radial muscles control light entry
- Rods: rhodopsin; low acuity, high sensitivity; convergence; dim light
- Cones: 3 types; high acuity, low sensitivity; no convergence; colour/bright light
- For neurons and synapses see Neurons and Synapses; for hormonal control see Hormonal Control; for plant responses see Plant Responses
AQA Exam Tips
- Accommodation - do not confuse ciliary muscles with sphincter/dilator: ciliary muscles are smooth muscle rings; contracting = smaller ring = suspensory ligaments slack. AQA often gives the scenario (near or far) and asks you to describe the sequence - always go: muscles → ligaments → lens shape → refraction.
- Pupil reflex - antagonistic muscles: circular (constrictor) vs radial (dilator). These are antagonistic - one contracts while the other relaxes. State both what contracts AND what relaxes.
- Convergence and acuity: when explaining why rods have lower acuity than cones, state "many rods share one bipolar neurone (convergence)" - this is the precise mechanism, not just "rods are less precise."
- Rhodopsin bleaching: AQA may show data on rhodopsin regeneration in the dark. Link bleaching to cis→trans retinal change and recovery to cis→trans regeneration (slower in the dark).
- Taxis vs kinesis: taxis = directional (toward/away from); kinesis = non-directional (faster/more turning in unfavourable conditions). The key distinguishing word is "directional."