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Receptors: turning a stimulus into something a neurone can carry questions
The sequence from stimulus to response, the spinal reflex arc and the three neurones in it, receptors as transducers, the Pacinian corpuscle and its stretch-mediated sodium channels, the generator potential, and rods against cones on sensitivity, acuity, pigment and retinal convergence.
6 original questions · 22 marks · the receptors: turning a stimulus into something a neurone can carry notes · Nervous coordination, receptors, muscles and behaviour
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Describe how a firm pressure applied to a Pacinian corpuscle leads to an action potential in the sensory neurone it surrounds.
Mark scheme
- B1 the pressure deforms the lamellae of connective tissue, which stretches the membrane of the nerve ending inside them
- B1 stretch-mediated sodium channels in that membrane are pulled open
- B1 sodium ions diffuse into the ending down their electrochemical gradient, so the inside becomes less negative
- B1 this depolarisation is the generator potential, and it is graded, so a firmer pressure opens more channels and gives a larger one
- A1 the generator potential is large enough to bring the membrane to the threshold of about −55 mV, so an action potential is triggered in the sensory neurone
Explain why a withdrawal reflex is faster than a conscious response, and explain why the statement that the brain is not involved in a reflex would not be credited.
Mark scheme
- B1 the pathway is short: the impulses enter the spinal cord and leave it again without travelling up to the brain first
- B1 there are only two synapses to cross, and each synapse adds a delay of about half a millisecond
- B1 the pathway is fixed, so no conscious decision has to be made before the effector is stimulated
- A1 impulses do reach the brain along a branch of the pathway, which is how the pain is felt a moment later; the point is that the response does not wait for a decision from it
Compare rod cells with cone cells in the human retina, referring to their pigment, the light each needs, their distribution and the image each produces.
Mark scheme
- B1 rods contain rhodopsin, which is broken down by dim light, whereas cones contain iodopsin, which needs much brighter light to break down
- B1 rods are found in the peripheral retina and are absent from the fovea, whereas cones are concentrated at the fovea and thin out towards the edges
- B1 rods respond to all wavelengths together and give a monochrome image, whereas cones come in three types with different pigments, so comparing them gives colour
- B1 there are about 120 million rods and only about 6 million cones, and several rods share one bipolar neurone whereas each cone has one to itself
Explain how the wiring of the retina makes rods more sensitive than cones in dim light but gives them lower visual acuity.
In a rare condition the stretch-mediated sodium channels in a person's Pacinian corpuscles are locked permanently open. Suggest what this does to the impulses sent along the sensory neurone when no pressure is applied, and suggest why the person would find it hard to judge how firmly they are being pressed.
Name the type of neurone that carries impulses from a receptor into the spinal cord in a reflex arc, and name the type that carries impulses out of the spinal cord to the effector.
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