What you'll learn
- How receptors detect stimuli and convert them into signals.
- How the eye and retina produce visual information for the brain.
- Why rods and cones give different kinds of vision.
- How auxin controls phototropism and gravitropism in plants.
From stimulus to response
Living organisms constantly respond to changes in their surroundings and internal environment. Animals often use fast electrical signalling in the nervous system. Plants do not have nerves, but they still detect stimuli and coordinate responses using chemical signals and growth.
Key response terms
- A stimulus is a detectable change in the internal or external environment.
- A receptor is a cell, tissue or organ that detects a stimulus.
- A coordinator processes information and helps produce an appropriate response.
- An effector is a muscle, gland or growing plant tissue that carries out the response.
- A response is the change in behaviour, physiology or growth caused by the stimulus.
A simple animal pathway is:
stimulus → receptor → sensory neurone → coordinator → motor neurone → effector → response
The important idea is transduction: the conversion of one form of energy into another. For example, light energy entering the eye is converted into changes in electrical signalling in neurones.
Receptor potentials and action potentials
A receptor cell has membrane proteins that respond to a particular type of stimulus. For example, a photoreceptor responds to light, while a mechanoreceptor responds to pressure or stretch.
Receptor potential
A receptor potential is a graded change in membrane potential caused by a stimulus. If the change is large enough to reach threshold, it can lead to action potentials in a sensory neurone or connected neurone.
A graded change means that a stronger stimulus usually produces a larger receptor potential. However, action potentials are all-or-nothing: once threshold is reached, each action potential has the same size.
Stimulus strength is frequency-coded
A stronger stimulus does not make bigger action potentials. It usually causes a higher frequency of action potentials, or activates more receptors.
Interpreting impulse frequency
A pressure receptor has a threshold of −55 mV. Three stimuli produce receptor potentials of −58 mV, −54 mV and −48 mV.
- Compare each value with the threshold. A receptor potential of −58 mV is not enough to reach threshold, but −54 mV and −48 mV are both sufficient.
- Compare the two values above threshold. −48 mV is further beyond threshold than −54 mV, so it represents the stronger stimulus.
- Apply all-or-nothing signalling. The −48 mV stimulus would not produce larger action potentials; it would produce a higher frequency of action potentials.
Bigger stimulus, bigger impulse
Do not say a stronger stimulus produces a larger action potential. Say it produces more frequent action potentials.
Vision: detecting light
The human eye focuses light onto the retina, a light-sensitive layer at the back of the eye. The cornea does most of the refraction, while the lens fine-tunes focus. The iris controls the size of the pupil, regulating how much light enters.
The retina
The retina contains photoreceptors, which are receptor cells that detect light. The two main types are rods and cones.
The retina is arranged in layers. Light passes through layers of neurones before reaching the rods and cones. Signals then pass from photoreceptors to bipolar neurones, then to ganglion neurones. The axons of ganglion neurones form the optic nerve, which carries impulses to the brain.
The blind spot is where the optic nerve leaves the eye. It has no photoreceptors, so light falling there cannot be detected. The fovea is the part of the retina with the highest density of cones, giving the sharpest colour vision.
Photoreceptors are unusual
In many receptors, stimulation causes depolarisation. In rods and cones, light causes hyperpolarisation, meaning the membrane potential becomes more negative. The action potentials sent to the brain are produced by ganglion neurones, not directly by rods and cones.
Rods and cones compared
| Feature | Rods | Cones |
|---|---|---|
| Sensitivity | Very sensitive; work well in dim light | Less sensitive; need brighter light |
| Colour vision | Do not give colour vision | Give colour vision |
| Visual acuity | Low acuity because many rods converge onto fewer neurones | High acuity, especially in the fovea |
| Main role | Night vision and detecting movement | Detailed colour vision |
Visual acuity means the ability to distinguish two close points as separate.
Rods contain the photopigment rhodopsin. When rhodopsin absorbs light, it is bleached, meaning it breaks down into retinal and opsin. This changes neurotransmitter release at synapses with bipolar neurones.
Sensitivity versus sharpness
Rods are excellent for detecting faint light because their signals converge, but this makes the image less precise. Cones give sharper detail because there is much less convergence.
Explaining dim-light vision
You can see shapes in a dark room, but colours and fine detail are poor.
- Use the light level to choose the receptor type. Dim light is often too weak to stimulate cones effectively, but rods are sensitive enough to respond.
- Link rod convergence to sensitivity. Many rods can feed into the same bipolar or ganglion neurone, so weak signals can combine.
- Link rod structure to the limitation. Rods do not provide colour information and their convergence reduces visual acuity, so the image appears grey and less detailed.
Accommodation and the pupil reflex
Accommodation is the process of changing lens shape to focus light from objects at different distances.
For a near object, the ciliary muscles contract, the suspensory ligaments slacken, and the lens becomes thicker and more convex. This increases refraction.
For a distant object, the ciliary muscles relax, the suspensory ligaments are pulled tight, and the lens becomes thinner and less convex. This decreases refraction.
The pupil reflex protects the retina and helps control light entry. In bright light, circular muscles in the iris contract and radial muscles relax, so the pupil constricts. In dim light, radial muscles contract and circular muscles relax, so the pupil dilates.
Mixing up iris muscles
Circular muscles make the pupil smaller when they contract. Radial muscles make the pupil larger when they contract.
Vision and the brain
The optic nerves carry impulses to visual areas of the brain, especially the visual cortex in the occipital lobe. The brain combines information from both eyes to help judge depth and distance.
Critical period
A critical period is a limited time in early development when nervous connections are especially sensitive to environmental input. Normal visual stimulation during this period is needed for normal development of visual pathways.
This is why conditions such as congenital cataracts need early treatment. If clear visual input is blocked during the critical period, the eye may later be physically repaired, but the brain’s visual connections may not develop fully.
Plant responses: detecting light and gravity
Plants do not have a nervous system, but they still detect stimuli. They use receptor proteins, chemical signals and differential growth.
Tropism
A tropism is a directional growth response to a directional stimulus. A positive tropism is growth towards the stimulus; a negative tropism is growth away from it.
Two important examples are:
- Phototropism: growth response to light.
- Gravitropism: growth response to gravity.
The key plant growth substance here is auxin, especially IAA. Auxin is produced in shoot tips and young leaves, then transported through plant tissues. It affects cell elongation.
Auxin in shoots and roots
In shoots, auxin usually stimulates cell elongation. If light comes from one side, auxin accumulates on the shaded side. Cells on the shaded side elongate more, so the shoot bends towards the light. This is positive phototropism.
In roots, high auxin concentrations inhibit cell elongation. In a horizontal root, auxin accumulates on the lower side. The lower side elongates less, the upper side elongates more, and the root bends downwards. This is positive gravitropism.
Auxin does not always mean more growth
Auxin stimulates elongation in shoots, but high auxin concentrations inhibit elongation in roots. Always identify whether the organ is a shoot or a root before explaining the bend.
Predicting tropism from auxin distribution
A horizontal shoot has more auxin on its lower side.
- Identify the organ. It is a shoot, so a higher auxin concentration stimulates cell elongation.
- Apply the distribution. The lower side has more auxin, so the lower side elongates more than the upper side.
- Predict the curvature. Unequal growth makes the shoot bend upwards, away from gravity, so this is negative gravitropism.
Investigating phototropism
A simple investigation could use seedlings grown in agar or moist paper. You can expose them to light from one side and measure the angle of curvature after a fixed time, such as 48 hours.
Good practical design matters:
- Keep temperature, water availability, seedling age and species the same.
- Keep the light source at a fixed distance unless light intensity is the variable.
- Use replicates and calculate a mean.
- Use a ruler, grid or image analysis to reduce measurement subjectivity.
- Include a control group, such as seedlings in even light or darkness.
A clinostat is a device that slowly rotates plants so gravity acts equally on all sides over time. It can be used as a control when investigating gravitropism.
Controlling light intensity
For a lamp acting approximately like a point source, light intensity follows an inverse-square relationship:
I2I1=(d1d2)2\frac{I_2}{I_1} = \left(\frac{d_1}{d_2}\right)^2I1I2=(d2d1)2where III is light intensity and ddd is distance from the lamp.
Calculating relative light intensity
A lamp gives a light intensity of 800 lx at 0.20 m. Estimate the intensity at 0.40 m.
- Compare the distances. The second distance is twice the first distance, so the intensity should fall to one quarter.
- Substitute into the inverse-square relationship: I2=800 lx×(0.20 m0.40 m)2=200 lxI_2 = 800\ \text{lx} \times \left(\frac{0.20\ \text{m}}{0.40\ \text{m}}\right)^2 = 200\ \text{lx}I2=800 lx×(0.40 m0.20 m)2=200 lx.
- Interpret the result. If you changed lamp distance during a phototropism experiment, you would change light intensity as well as direction, which could confound the results.
Animal and plant responses compared
| Feature | Animal nervous response | Plant tropic response |
|---|---|---|
| Main signal | Electrical impulses and neurotransmitters | Plant growth substances such as auxin |
| Speed | Usually rapid | Usually slower |
| Effector | Muscles or glands | Growing cells and tissues |
| Typical outcome | Movement, secretion or physiological change | Directional growth |
In the exam
- For receptor questions, link stimulus → receptor potential → threshold → action potential frequency → response.
- For retina questions, be clear about the pathway: photoreceptors affect bipolar neurones, and ganglion neurones form the optic nerve.
- For rods and cones, always connect structure to function: convergence gives sensitivity but reduces acuity.
- For auxin questions, identify the organ first, then state whether auxin stimulates or inhibits elongation there.
Check yourself
- Why does a stronger stimulus not produce a larger action potential?
- Why do rods give better dim-light vision but poorer visual acuity than cones?
- How would a horizontal root bend, and how does auxin explain this?
