OxfordAQA International AS and A-level Biology: Reflexes, taxes and kineses, and sensory receptors (9610)

This guide is aligned to the OxfordAQA International AS and A-level Biology (9610) specification, Version 5.1, and covers sections 3.4.1, 3.4.2.1 and 3.4.2.2 of Unit 4: Control. The specification lists Unit 4 as International A-level only, so it is not part of the International AS. Use it with the revision notes and the practice questions. The course hub and the printable checklist show where this unit sits, and the free diagnostics help you find weak spots first.

What the specification asks for

Specification reference	Skills to show
3.4.1	Describe a simple reflex arc of three neurones. Explain the importance of simple reflexes in avoiding damage to the body. Describe taxes and kineses as simple responses that keep an organism in a favourable environment and so increase its chance of survival.
3.4.2.1	Use the Pacinian corpuscle as an example of a receptor: receptors respond only to specific stimuli, and stimulation leads to a generator potential. Know its basic structure. Explain how deformation of stretch-mediated sodium ion channels leads to a generator potential.
3.4.2.2	Account for the differing sensitivity of the eye to light, its sensitivity to colour and its visual acuity, in terms of the optical pigments in rods and cones and how these cells connect in the optic nerve. The retina is needed only in enough detail for this.

3.4.1 Stimulus and response

Organisms detect a stimulus with a receptor, and respond through an effector. Responses move animals to favourable places or help to keep conditions suitable for metabolism. Either way, the chance of survival goes up.

The reflex arc

A reflex is a quick, automatic reaction to a stimulus, made without any conscious decision. The route taken by the impulses is a reflex arc. The specification asks for the simple version, with only three neurones:

1. Stimulus detected by a receptor.
2. A sensory neurone takes impulses from the receptor into the central nervous system.
3. A relay neurone in the central nervous system passes the impulse across synapses.
4. A motor neurone carries impulses away to the effector.
5. The effector (a muscle or a gland) produces the response.

There are two synapses: sensory to relay, and relay to motor.

Why this protects the body. The response is quick: the route is short and crosses only two synapses. It is automatic, so it needs no learning and no thought. It is the same each time. Pulling a hand from a hot surface or blinking when something nears the eye limits damage before you are even aware of the stimulus.

Worked example 1: reflex time

A finger touches a hot object. The whole pathway (sensory, relay and motor neurones together) is 1.66 m long. Assume impulses travel at 65 m s⁻¹ throughout, and each of the two synapses adds a delay of 0.6 ms. Calculate the time from receptor to effector.

• Conduction time = distance ÷ speed = 1.66 ÷ 65 = 0.02554 s = 25.5 ms.
• Synaptic delay = 2 × 0.6 = 1.2 ms.
• Total = 25.5 + 1.2 = 26.7 ms (3 significant figures).

Taxes and kineses

These are simple responses that keep a simple organism in a favourable environment.

• A taxis is movement whose direction is set by the direction of the stimulus. Heading towards the stimulus is positive; heading away is negative. Example: the unicellular alga Chlamydomonas swims towards a light source (positive phototaxis), and blowfly larvae move away from light (negative phototaxis). Bacteria that swim towards a higher concentration of a nutrient show positive chemotaxis.
• A kinesis is a change in how fast the organism moves, or how often it turns, depending on how intense the stimulus is. The direction of the stimulus does not matter and the movement is random. The organism moves quickly and turns often in an unfavourable region, so it is likely to leave. In a favourable region it slows down and turns less, so it tends to stay.

The test is direction: heading along the gradient is a taxis; speeding up and turning more as conditions worsen is a kinesis.

Worked example 2: identifying a kinesis

A small soil invertebrate was placed in an arena at three light intensities for 30 s each. Data are supplied.

Light	Distance moved (mm)	Turns
Dim	21	6
Medium	54	15
Bright	96	31

Mean speed: dim 21 ÷ 30 = 0.70 mm s⁻¹; medium 54 ÷ 30 = 1.8 mm s⁻¹; bright 96 ÷ 30 = 3.2 mm s⁻¹. Turns per minute: 12, 30 and 62.

Speed and turning both rise as light intensity rises, and nothing in the data shows movement towards or away from a direction. This is a kinesis. Fast turning in bright light makes it likely the animal leaves bright areas. Slow, straighter movement in dim light keeps it where it is. The survival value: it ends up in the dim region, which is the more favourable one for this animal.

Worked example 3: identifying a taxis

A horizontal tube is lit from one end and divided into five equal sections, A (nearest the lamp) to E. After 10 minutes, counts of Chlamydomonas cells are A 58, B 31, C 17, D 9, E 5. Total = 120.

• Percentage in A = 58 ÷ 120 × 100 = 48.3%.
• Percentage in A and B = 89 ÷ 120 × 100 = 74.2%.

Most cells are at the lit end, and the numbers fall steadily with distance from the lamp. The cells moved in a direction set by the direction of the light: positive phototaxis. Moving towards light helps a photosynthetic organism gain light for photosynthesis.

3.4.2.1 The Pacinian corpuscle

Receptors are transducers: they convert the energy of a stimulus into a change in membrane potential. Each type responds only to a specific stimulus. The Pacinian corpuscle responds to mechanical pressure and does not respond to light, because it contains nothing that is altered by light. Rod cells in the retina, in contrast, respond to light and not to pressure.

Structure

A Pacinian corpuscle is a small, oval structure in the skin and elsewhere. It has:

• the ending of a sensory neurone at its centre;
• around it, many concentric layers (lamellae) of connective tissue, with viscous gel between them;
• stretch-mediated sodium ion channels in the plasma membrane of the neurone ending.

How pressure produces a generator potential

1. At rest the membrane is polarised (a resting potential, with the inside negative). The stretch-mediated channels are too narrow for sodium ions to pass.
2. Pressure deforms the lamellae, and the deformation reaches the neurone ending and stretches its membrane.
3. The stretch-mediated sodium ion channels widen.
4. Sodium ions diffuse into the neurone ending, down their electrochemical gradient.
5. The inside becomes less negative: the membrane is depolarised. This change is the generator potential.
6. A generator potential is graded: a greater pressure opens more channels, so more sodium enters and the depolarisation is larger.
7. If the generator potential reaches the threshold, voltage-gated channels open and an action potential is set up in the sensory neurone. Below the threshold, none is produced.

Worked example 4: data on a Pacinian corpuscle

For this ending, rest is at −70 mV and firing needs the potential to reach −55 mV. Data are supplied.

Pressure (kPa)	Membrane potential (mV)	Depolarisation (mV)
0.5	−68	2
1.0	−62	8
1.5	−56	14
2.0	−51	19
3.0	−45	25

Depolarisation = potential − (−70). The threshold needs a depolarisation of 15 mV. At 1.5 kPa the depolarisation is 14 mV, so it falls just short and no action potential is set up. At 2.0 kPa the potential reaches −51 mV, which is beyond the threshold, so an action potential is triggered. The generator potential grows with pressure, but the action potential is either produced or not.

3.4.2.2 The human retina

Light passes through the layers in front and reaches the rods and cones, the photoreceptors. Light changes the optical pigment in these cells. This sets up a generator potential, which leads to impulses in a bipolar neurone and then in neurones of the optic nerve. The differences below come from the pigments and the connections.

Feature	Rods	Cones
Pigment	One kind: rhodopsin	Three kinds, each absorbing different wavelengths
Sensitivity	High: work in dim light	Low: need brighter light
Colour	None: one pigment	Yes: three types
Connection	Many rods share one bipolar neurone	Often one cone to one bipolar neurone
Acuity	Low	High

Sensitivity

Rhodopsin is broken down by light of low intensity, so rods respond in dim light. Many rods are connected to one bipolar neurone. Each rod produces only a small generator potential in dim light, but the potentials from all the rods add together at the bipolar neurone: this is summation. Together they can reach the threshold, which a single rod could not. Cone pigments need a higher light intensity to change, and a cone usually has its own bipolar neurone, so no summation is possible. Cones therefore give no response in dim light.

Worked example 5: summation

Suppose 144 rods are connected to 6 bipolar neurones, so 144 ÷ 6 = 24 rods per neurone. In dim light each rod gives a generator potential of 0.8 mV and the threshold of the bipolar neurone is 3.0 mV.

• One rod: 0.8 mV, below 3.0 mV, so no impulse.
• Four rods together: 4 × 0.8 = 3.2 mV, which is above 3.0 mV, so an impulse is passed on.

Visual acuity

Acuity is the ability to tell two close points apart. When two points of light fall on two different cones, each with its own bipolar neurone, two separate sets of impulses reach the brain and two points are seen. When they fall on rods that share a bipolar neurone, only one set of impulses is sent, so the points merge. Cones, with their one-to-one connections, give high acuity. Rods, with many-to-one connections, give low acuity. Because acuity needs cones and cones need bright light, you cannot read fine print in dim light.

Colour

Each of the three cone types has a pigment that absorbs a different range of wavelengths. Light of one wavelength stimulates the three types to different degrees, and the brain interprets the relative stimulation as colour. Rods contain only one pigment, so they give no colour information, which is why colours fade in dim light.

Worked example 6: spacing and acuity

In a region of the retina, cones have a centre-to-centre spacing of 2.5 µm. For two points to be seen as separate, there must be at least one unstimulated cone between the two stimulated ones. The two images must then be at least 2 × 2.5 = 5.0 µm apart. Rods that share one bipolar neurone cannot give this resolution.

Common errors

• Saying a kinesis is steered towards or away from the stimulus. What changes is speed or turning.
• Writing that the Pacinian corpuscle “detects touch” without saying it responds to pressure by deforming the membrane.
• Saying sodium ions “pump” in. They diffuse through opened stretch-mediated channels.
• Treating the generator potential as an action potential. It is graded, and an action potential appears only if the threshold is reached.
• Explaining rod sensitivity without summation, or acuity without saying cones have separate connections.

Next, test yourself with the practice questions and see exam preparation.

Official syllabus

OxfordAQA International AS and A-level Biology (9610), Version 5.1, for International AS exams May/June 2017 onwards and International A-level exams May/June 2018 onwards, from OxfordAQA, covering specification content 3.4.1, 3.4.2.1 and 3.4.2.2.