Unit 5 General principles of reception and response in animals
Key unit competence: Correlate the general principles of reception and response in animals.
Key ideas
- Animals respond to changes in the internal and external environment to find food, avoid danger, find mates and keep homeostasis. The pathway is stimulus → receptor → coordinator (CNS) → effector → response. A receptor detects a stimulus and acts as a transducer, converting stimulus energy into the electrical energy of nerve impulses.
- Types of receptors by stimulus: photoreceptors (light: rods and cones), mechanoreceptors (touch, pressure, sound, balance: Pacinian corpuscles, hair cells), chemoreceptors (chemicals: taste buds, olfactory cells, carotid bodies), thermoreceptors (temperature), nociceptors (pain), proprioceptors (position and stretch of muscles: muscle spindles). By location: exteroceptors (external), interoceptors (internal), proprioceptors.
- Functioning of a receptor: a stimulus changes the permeability of the receptor membrane, producing a generator (receptor) potential. If it reaches the threshold, an action potential is fired. A stronger stimulus gives a larger generator potential and a higher frequency of impulses (not bigger impulses). In a Pacinian corpuscle, pressure deforms the layers (lamellae), opening stretch-mediated Na⁺ channels.
- Sensory adaptation is the decreasing response of a receptor to a constant stimulus (e.g. no longer feeling clothes, getting used to a smell). It prevents overloading the brain and lets the animal detect changes. Dark adaptation in the eye is a different, slower process involving regeneration of rhodopsin.
- The eye: sclera (tough protective coat), cornea (transparent, refracts most light), choroid (pigmented, absorbs light, blood vessels), iris (controls pupil size), lens (fine focusing), ciliary body with ciliary muscles and suspensory ligaments, aqueous and vitreous humour (maintain shape), retina (photoreceptors), fovea (most cones, sharpest vision), blind spot (optic nerve exit, no receptors), optic nerve.
- Accommodation: for near objects, ciliary muscles contract, suspensory ligaments slacken, the lens becomes thicker/rounder, refracting more. For distant objects, ciliary muscles relax, ligaments become taut, the lens is pulled thinner. Pupil reflex: in bright light circular muscles of the iris contract and the pupil constricts; in dim light radial muscles contract and the pupil dilates.
- Retina: light passes through ganglion and bipolar cells to the rods and cones. Rods: pigment rhodopsin, very sensitive (work in dim light), black and white vision, many rods share one bipolar cell (retinal convergence: high sensitivity, low visual acuity), mostly in the periphery. Cones: pigment iodopsin, three types (red, green, blue) for colour vision (trichromatic theory), need bright light, each connected to its own bipolar cell (high acuity), concentrated in the fovea.
- Phototransduction in rods: light converts rhodopsin into retinal and opsin (bleaching). Opsin causes Na⁺ channels to close, the rod hyperpolarises and stops releasing inhibitory transmitter, so the bipolar cell depolarises and an impulse is sent along the optic nerve. Rhodopsin is resynthesised using ATP and vitamin A (deficiency causes night blindness). Cones work similarly but need more light to break iodopsin. Images formed on the retina are inverted; the visual cortex interprets them.
- The ear: outer ear (pinna, auditory canal), middle ear (eardrum/tympanic membrane, ossicles — malleus, incus, stapes — which amplify vibrations about 20 times; Eustachian tube equalises pressure; oval window), inner ear (cochlea for hearing; vestibular apparatus — semicircular canals and utricle/saccule — for balance). Hearing: sound waves vibrate the eardrum → ossicles → oval window → waves in perilymph and endolymph → basilar membrane vibrates → hair cells in the organ of Corti bend against the tectorial membrane → impulses in the auditory nerve. High pitch is detected near the base, low pitch near the apex. The round window releases pressure. Balance: three semicircular canals at right angles detect rotational movement (fluid moves the cupula in the ampulla); utricle and saccule detect head position and linear acceleration using otoliths.
- Taste: taste buds on the tongue papillae contain chemoreceptor cells sensitive to sweet, sour, salty, bitter and umami; chemicals dissolve in saliva and bind receptors. Taste works with smell (olfactory cells in the nasal cavity). Skin receptors: Meissner's corpuscles (light touch, near surface), Pacinian corpuscles (deep pressure and vibration), free nerve endings (pain, temperature), thermoreceptors (heat and cold), Ruffini endings (stretch), hair follicle receptors. Sensitive areas such as fingertips and lips have more receptors.
Common misconceptions
- "A stronger stimulus produces bigger action potentials." No: action potentials are all-or-nothing; a stronger stimulus increases their frequency.
- "The lens bends most of the light." No: the cornea does most refraction; the lens adjusts focus.
- "Ciliary muscles relax for near vision." No: they contract for near objects so the lens becomes rounder.
- "Rods see colours in dim light." No: rods give only black-and-white vision; colour vision needs cones.
Worked examples
Example 1. Explain why we see objects more clearly when we look straight at them but see dim stars better out of the corner of the eye.
Solution
Looking straight focuses light on the fovea, packed with cones, each with its own bipolar cell, giving high acuity. Dim stars are better seen in the periphery, which has many rods; several rods converge on one bipolar cell, so their small generator potentials summate to reach threshold (high sensitivity).
Example 2. A man enters a dark cinema from bright sunlight. Explain why he cannot see at first but sees better after a few minutes.
Solution
In bright light most rhodopsin is bleached, so rods cannot respond. In the dark rhodopsin is gradually resynthesised (using ATP and vitamin A), and the pupil dilates (radial muscles contract). Sensitivity increases: dark adaptation.
Practice questions
State the functions of five parts of the eye.
Show answer
Cornea: refracts light; iris: controls light entering; lens: fine focusing; retina: contains photoreceptors; choroid: absorbs light, nourishes; ciliary muscles: change lens shape; optic nerve: carries impulses.
Common mistake: Saying the iris focuses light.
Explain accommodation for a distant object.
Show answer
Ciliary muscles relax; suspensory ligaments taut; lens pulled thin; less refraction; image focused on retina.
Common mistake: Saying ciliary muscles contract.
Compare rods and cones.
Show answer
Rods: rhodopsin, dim light, black and white, convergence, low acuity, periphery. Cones: iodopsin, bright light, colour (3 types), own bipolar, high acuity, fovea.
Common mistake: Swapping the pigments.
Explain how light stimulates a rod cell.
Show answer
Light bleaches rhodopsin into retinal and opsin; Na⁺ channels close; rod hyperpolarises; stops releasing inhibitory transmitter; bipolar cell depolarises; impulse in optic nerve.
Common mistake: Saying the rod depolarises in light.
Describe how sound is perceived.
Show answer
Pinna collects sound; eardrum vibrates; ossicles amplify; oval window vibrates; fluid waves in cochlea; basilar membrane vibrates; hair cells in organ of Corti bend against tectorial membrane; impulses along auditory nerve to brain.
Common mistake: Leaving out the ossicles.
Explain how the ear maintains balance.
Show answer
Semicircular canals at right angles detect rotation as endolymph moves the cupula and bends hair cells; utricle and saccule detect head position using otoliths; impulses via vestibular nerve to cerebellum.
Common mistake: Saying the cochlea maintains balance.
Explain the importance of sensory adaptation.
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Avoids overloading the brain with unimportant information; allows attention to new changes; saves energy.
Common mistake: Calling it damage to receptors.
Describe how a Pacinian corpuscle produces impulses.
Show answer
Pressure deforms lamellae; stretch-mediated Na⁺ channels open; Na⁺ enters; generator potential; if threshold reached, action potential in sensory neurone; greater pressure gives higher frequency.
Common mistake: Saying larger impulses for more pressure.
Matching and drag-and-drop
Matching
Match each item in column A with the correct answer in column B. Write the letter next to each number.
Column A Column B 1. Pressure is detected deep in the skin by: a. fovea 2. The region of the retina with the most cones is the: b. sensory adaptation 3. Light touch on the fingertips is detected by: c. Pacinian corpuscles 4. The decreasing response to a constant stimulus is: d. Meissner's corpuscles Show answer
1 → c (Pacinian corpuscles) · 2 → a (fovea) · 3 → d (Meissner's corpuscles) · 4 → b (sensory adaptation)
- Layers of lamellae.
- Sharpest vision.
- Near the skin surface.
- Lets us detect change.
Drag and drop
Choose the correct word or phrase from the box for each gap. One word in the box is not needed.
malleus, incus and stapes · contract · proprioceptors · rods, cones and bipolars- Muscle spindles that detect stretch are .
- The ossicles of the middle ear are the .
- For near vision, the ciliary muscles .
Show answer
1: proprioceptors · 2: malleus, incus and stapes · 3: contract
- Body position.
- They amplify vibrations.
- Ligaments slacken.
Matching
Match each item in column A with the correct answer in column B. Write the letter next to each number.
Column A Column B 1. The pathway of a response begins with a: a. rhodopsin 2. In bright light the pupil constricts because the: b. circular muscles contract 3. The pigment in rods is: c. stimulus 4. Colour vision depends on: d. three types of cones Show answer
1 → c (stimulus) · 2 → b (circular muscles contract) · 3 → a (rhodopsin) · 4 → d (three types of cones)
- Detected by a receptor.
- Protects the retina.
- Bleached by light.
- Trichromatic theory.
Drag and drop
Choose the correct word or phrase from the box for each gap. One word in the box is not needed.
semicircular canals · pigment · photoreceptors · umami- The fifth basic taste, savoury, is .
- The blind spot has no .
- Rotational movement of the head is detected by the .
Show answer
1: umami · 2: photoreceptors · 3: semicircular canals
- Detected by glutamate receptors.
- Optic nerve leaves there.
- Fluid moves the cupula.