B3- Response to Internal Stimuli ✅

what are receptors

• cells that detect stimuli

• they are specific so each type of receptor detects a different type of stimulus

what is a generator potential?

the change in electrical potential in a receptor when it is stimulated

what are Pacinain corpuscles

• type of mechanoreceptor found deep in the skin

• which responds to changes in pressure only; specific to pressure

where are Pacinian corpuscles found

at sensory neurone endings

describe the basic structure of Pacinian corpuscles

• made of many layers of membrane that are separated by gel

• membrane layers are sometimes referred to as connective tissue or lamellae

describe how a generator potential is established in a Pacinian corpuscle

1. Mechanical stimulus (eg. pressure) deforms lamellae and stretch-mediated sodium ion channels

2. so Na⁺ channels in membrane open and Na⁺ diffuse into sensory neurone

3. greater pressure causes more Na⁺ channels to open and more Na⁺ to enter

4. this causes depolarisation, leading to a generator potential

5. if generator potential reaches threshold it triggers an action potential

explain what the Pacinian corpuscle illustrates

1. receptors respond only to a specific stimuli

• Pacinian corpuscle only responds to mechanical pressure

2. Stimulation of a receptor leads to establishment of a generator potential

• when threshold is reached, action potential sent ( all or nothing principle )

1. name the 2 photoreceptors in our eye

2. where are they found

1. rods and cones

2. found in retina but specifically

• rods on peripheral retina, cones in fovea

what are more sensitive to light: rods or cones?

rods

why are rods more sensitive to light than cones?

• several rods are connected to a single neurone

• spatial summation to reach threshold ( as enough neurotransmitter released ) to generate an action potential

what are cones less sensitive to light than rods

• each cone connected to a single neurone

• no spatial summation

do rods or cones have a higher visual acuity / resolution

cones

explain how photoreceptors ( rods and cones ) detect light

1. light causes chemical breakdown ( bleaching ) of optical pigments inside rods and cones

2. the bleaching of these optical pigments results in a generator potential

3. if the general potential is large enough then a nerve impulse is sent along a bipolar neurone to the optic nerve

what is the light sensitive optical pigment in rods called

rhodopsin

what is the light sensitive pigment in cones called

iodopsin

describe how the number and distribution of rods and cones across the retina would differ in a nocturnal mammal from the number in a human

1. more rods and fewer cones

2. rods have high sensitivity

3. rhodopsin bleached at low light intensities

a student stared at a purple square, suggest why he saw a green image afterwards?

1. red and blue cones are stimulated

2. red and blue cones become exhausted

3. afterimage is green due to green cones still working

Prolonged exposure to high-intensity light causes cone cells in the retina to become fatigued or "exhausted".

Suggest three reasons why a cone cell stops generating action potentials in the optic nerve after continuous stimulation.

1. Breakdown / bleaching of iodopsin faster than it can be resynthesised

2. Depletion of neurotransmitter (at the synapse between the cone cell and the bipolar neurone→ so fewer vesicles are released / less depolarisation occurs)

3. Depletion of ATP (needed to resynthesise iodopsin / actively transport ions / resynthesise neurotransmitter).

explain why rods give lower visual acuity than cones

• several rods connected to a single neurone

• so several rods send a single set of impulses to brain

• so cant distinguish between separate sources of light

explain why cones give a higher visual acuity

• each cone is connected to a single neurone ( no retinal convergence )

• cones send separate sets of impulses to brain

• so can distinguish between separate sources of light

explain what type of vision rods allow: colour or monochromatic?

monochromatic

• 1 type of rod / 1 pigment

explain what type of vision cones allow: colour or monochromatic?

colour

• 3 types of cones → red, green and blue sensitive

• with 3 different optical pigments→ absorb different wavelengths

• stimulating different combinations of cones gives range of colour perception

Explain how rod cells enable an organism to see in low light intensities. (3 marks)

• Many-to-one connection: Many rod cells synapse with a single bipolar neurone (retinal convergence).

• Spatial summation: Small amounts of neurotransmitter released by multiple rod cells add together (spatial summation).

• Threshold reached: This is enough to exceed the threshold value and trigger an action potential in the bipolar neurone.

Explain why humans cannot see colour in dim light. (3 marks)

• No convergence: Each cone cell connects to a single bipolar neurone, so there is no spatial summation.

• High light requirement: High light intensity is needed to break down enough iodopsin (pigment).

• Reaching threshold: This ensures enough neurotransmitter is released from a single cone cell to exceed the threshold of the bipolar neurone.

why is there a blind spot on the human retina?

1. point where optic nerve leaves eye

2. no rods and cones present

cardiac muscle is myogenic. what does this mean?

it can contract and relax without receiving electrical impulses from nerves

describe the sequence of events that causes the heart to contract

1. S.A. node initiates heart beat

2. sends out a wave of electrical impulse that spread out from SA node

3. wave spreads across both atria causing them to contract and reaches AVN

4. AVN delays to allow atria to empty/ ventricles to fill before ventricles contract

5. AVN sends a wave of electrical impulse down Bundle of His then the Purkinjie fibres and release it at the apex

6. this causes ventricles to contract from base up

which part of the nervous system control heart rate

autonomic

name the 2 branches of the autonomic nervous system

1. sympathetic

• stimulate effectors

• speeds up

• fight or flight

• neurotransmitter is noradrenaline

2. parasympathetic

• inhibits effectors

• controls activity at rest

• neurotransmitter is acetylcholine

what word can we use to describe the 2 branches of the autonomic nervous system

antagonistic

which part of the brain controls heart rate

medulla oblongata

medulla has a cardioregulatory centre with 2 parts:

1. acceleratory centre

• linked to SA node by sympathetic nervous system

2. inhibitory centre

• linked to SA node by parasympathetic nervous system

medulla receives info from receptors which determines whether it sends impulses down sympathetic or parasympathetic branch, to increase or decrease

name the 2 types of receptors important in the control of heart rate

1. chemoreceptors → respond to changes in pH ( due to CO₂ conc increasing/ decreasing )

2. baroreceptors → respond to changes in blood pressure

where are chemoreceptors and baroreceptors found

aorta, carotid artery

if exercise is increased → CO₂ levels …. → so pH … → so heart rate …

1. increase

2. decreases

3. increases ( to allow aerobic respiration to continue for longer )

explain how heart rate is increased as CO₂ levels increase causing pH decrease eg. during exercise

1. chemoreceptors detect fall in pH and send impulses to the medulla oblongata

2. more impulses are sent to the SA node via sympathetic nerves

3. this increases the frequency of impulses from the SA node across atria, increasing the rate at which atria contract

4. this in turn means AV node fires more frequently, so ventricles contract more frequently and heart rate increases

5. this allows oxygen to be delivered to cells faster so aerobic respiration can continue for longer

explain how heart rate decreases as CO₂ levels in blood decrease causing pH to rise

1. chemoreceptors detect decrease in CO₂ concentration and send impulses to the medulla oblongata

2. medulla sends more impulses to the SA node via parasympathetic nerves

3. decreases frequency of impulses from SA node across atria, decreasing frequency at which atria contract

4. this in turn means AV node fires less frequently, so ventricles contract less frequently and heart rate decreases

explain what happens to heart rate if blood pressure rises above normal

1. baroreceptors detect this and send impulses to the medulla

2. more impulses are sent to the SA node via parasympathetic nerves

3. this decreases the frequency of impulses from SA node across atria, decreasing the rate at which atria contract

4. this in turn means the AV node fires less frequently, so the ventricles contract less frequently and heart rate decreases

explain what happens to heart rate if blood pressure falls below normal

1. baroreceptors detect this and send an impulse to the medulla

2. more impulses are sent via the sympathetic nervous system to the SA node

3. increases the frequency of impulses from the SA node across atria, atria contract more frequently

4. this in turn means AV node fires more frequently, so the ventricles contract more frequently and so heart rate increases

LEARN HOW TO DRAW GRAPH OF DISTRIBUTION OF RODS AND CONES IN THE RETINA

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