Receptors

Question 1

Pacinian corpuscle

Answer 1

1. Increased pressure deforms stretch-mediated sodium channel;
2. Sodium ion channel proteins open and sodium ions diffuse in;
3. Depolarisation leading to generator potential;

specific to pressure only.

Question 2

Explain how pressure on the Pacinian corpuscle produces the changes in membrane potential recorded by microelectrode A.

Answer 2

• (Pressure) deforms / opens (sodium) channels;
• Sodium ions enter;
• Causing depolarisation;
• Increased pressure opens more channels / greater sodium entry;
• [Size of generator potential determines frequency of action potentials]

Question 3

Describe how the Pacinian corpuscle propagates an action potential.

Answer 3

• (Pressure causes) membrane/lamellae to become deformed/stretched;
• Sodium ion channels in membrane open and sodium ions move in;
• Depolarisation leading to generator potential;
• Greater pressure more channels open/sodium ions enter;

Question 4

TWO types of photo-receptor cell on the retina

Answer 4

RODS
- Evenly distributed throughout the macula
- Sensitive to all wavelengths of light
- High visual sensitivity to low levels of light intensity
- Low visual acuity, this means the image is unclear or poorly resolved.
- Retinal convergence due to several rods sharing a single BIPOLAR neuron.
- Generator potentials combine to reach threshold.

CONES
- Densely packed in the fovea
- Each cone detects a specific wavelength of light (R-G-B). There are three types of iodopsin.
- Each cone cell is sensitive to a specific wavelength.
- Iodopsin is less sensitive than rhodopsin so requires higher Light intensity of photons
- High visual acuity giving a sharp image.
- This is because Each Cone cell synapses / connected with a single BIPOLAR neuron.
- Cones send separate impulses to brain.

Question 5

connecting to bipolar neurons

Answer 5

Question 6

Rods

Answer 6

• LOW VISUAL ACUITY as several rod cells are connected to one bipolar neuron. This means that the light which falls on several rod cells will only generate one impulse to the brain. The brain cannot distinguish between the separate light sources that generated them.
• Rods are highly sensitive to low light intensities because of this arrangement. The stimulation of several rods results in enough NETS released to reach threshold value
• Retinal convergence leads to spatial summation.
• Generator potentials combine to reach threshold = action potential produced
• contain rhodopsin

Question 7

CONES

Answer 7

• Provide HIGH VISUAL ACUITY as each cone synapses with an individual bipolar neuron.
• This means that light falling on two cone cells generates two electrical impulses to the brain and the brain can therefore distinguish between the light sources that generated them.
• However, each cone cell must release enough neurotransmitter to reach the threshold in the bipolar neuron and this is only likely to happen in bright light. This is an example of temporal summation.
  In low light intensities there is not enough neurotransmitter to create an action potential in the bipolar neuron so cones only operate when light intensity is high.
• contain iodopsin

Question 8

Explain how the fovea increases the detail of an image

Answer 8

• High (visual) acuity
• Each / single cone is connected to a single bipolar/neurone
• Each cone sends separate impulse to the brain

Question 9

Explain how the connection of several rods to a single bipolar cell (Retinal convergence) influences visual acuity

Answer 9

• (Spatial) Summation means cannot distinguish between stimuli from different rod cells linked to same bipolar cell;
• Decreases acuity;

Question 10

Explain how the connection of several rods to a single bipolar cell (Retinal convergence) influences sensitivity

Answer 10

• (Summation of) sub-threshold stimuli produces threshold stimulation;
• Increases sensitivity;

Question 11

People with red-green colour blindness are unable to distinguish between red and green, and also between other colours

Explain why.

Answer 11

• Cones that detect green light non-functional;
• Three different types of cone;
• Other colours (‘seen’) due to stimulation of more than one cone

Question 12

Control of heart rate

Answer 12

The heart muscle is myogenic: it can initiate its own contraction

• Sino atrial node sends wave or electrical activity across both atria
• Both atria contract
• Layer of nonconductive tissue prevents wave reaching ventricles
• Wave of electrical activity reaches the atrio ventricular node
• 0.1 second delay allowing atria to empty fully of blood
• Wave of electrical activity sent from the atrioventricular node
• Down the bundle of His to the base of the ventricles
• Up the Purkinje fibres
• Causing the ventricles to contract from the apex of the heart upwards

Question 13

autonomic nervous system controlling heart rate

Answer 13

1. sympathetic
   * stimulates effectors
   * speeds up heart rate
   * fight or flight
   * Neurotransmitter is noradrenaline
2. parasympathetic
   * inhibits effectors
   * controls activity at rest
   * Neurotransmitter is acetylcholine

The two branches are antagonistic- they have opposing effects.

Question 14

Blood pressure increases above normal.

Answer 14

• Detected by Baroreceptors in walls of aorta & carotid arteries.
• More frequent impulses sent to medulla oblongata.
• More frequent impulses from inhibitory center in medulla to SA node via parasympathetic nerve.
• Decreases frequency of impulses from SA node across atria. (Acetylcholine)
• Heart rate decreases

Question 15

Blood pressure decreases below normal.

Answer 15

Detected by Baroreceptors in walls of aorta & carotid arteries.
* Less frequent impulses sent to medulla oblongata.
* More frequent impulses from acceleratory center in medulla to SA node via parasympathetic nerve.
* Increases frequency of impulses from SA node across atria. (Acetylcholine)
* Heart rate increases

Question 16

Chemoreceptors

Answer 16

If CO2 levels rise for example during exercise, then blood pH decreases below normal.
* Detected by chemoreceptors in walls of aorta & carotid arteries.
* More frequent impulses sent to medulla oblongata.
* More frequent impulses from acceleratory center in medulla to SA node via sympathetic nerve
* more frequent impulses from SA node across atria (Noradrenaline)
* Heart rate increases

There is no opposite effect for low CO2 levels of the blood.

Question 17

Exercise causes an increase in heart rate.

Describe the role of receptors and of the nervous system in this process.

Answer 17

1. Chemoreceptors detect rise in CO2
2. Send impulses to cardiac centre/medulla;
3. More impulses to SAN;
4. By parasympathetic

Question 18

The rate of ATP consumption of a de-myelinated neurone is greater than that of a myelinated neurone when conducting impulses at the same frequency.

Explain why.

Answer 18

• Greater entry of sodium ions / greater exit of K+
• in de-myelinated neurone;
• Ref. to active transport / ref. to ion pumps;

Question 19

Explain how nervous control in a human can cause increased cardiac output during exercise.

Answer 19

• Coordination via medulla (of brain) / cardiac centre;
• (Increased) impulses along sympathetic nerve;
• To S.A. node;
• Release of noradrenalin;
• More impulses sent from S.A.N (across atria);
• Increased heart rate / increased stroke volume;

Question 20

Explain why increased cardiac output is an advantage during exercise.

Answer 20

• Higher cardiac output - Increases O2 supply (to muscles);
• Increases glucose supply (to muscles);
• More ATP produced by oxidative phosphorylation / more energy release / more aerobic respiration / actively respiring muscles
• Increases CO2 removal (from muscles) / lactate removal;
• Increases heat removal (from muscles) / for cooling;
• Delays the formation of lactate

Question 21

Describe what is meant by the term threshold

Answer 21

• When threshold has been reached;
• (Threshold or above) causes maximal response / all or nothing principle;