Topic 6A: Stimuli and Response

Question 1

Tactic response (taxis)

Answer 1

Directional movement in response to a stimulus. The direction of stimulus affects the response.

Question 2

Kinetic response (kinesis)

Answer 2

Non-directional movement in response to a stimulus. The intensity of the stimulus affects the response.

Question 3

The roles or receptors and effectors

Answer 3

Receptors - detect a stimuli.

Effectors - bring about a response.

Question 4

Structure of a reflex arc

Answer 4

Stimulus
⬇️
Receptors 
⬇️(sensory neurone)
Central Nervous System (relay neurone) 
⬇️(motor neurone)
Effectors 
⬇️
Response

Question 5

What is a tropism

Answer 5

The response of a plant to a directional stimulus through regulation of growth.

• Phototropism:
The growth of a plant in response to light.

• Gravitropism:
The growth of a plant in response to gravity.

Question 6

Auxins

Answer 6

Produced in the tips of shoots, they diffuse downwards to stimulate cells to elongate.

If the tip is removed, the auxins are also removed and so the plant doesn’t grow.

High concentrations of auxins inhibit growth in the roots.

Question 7

IAA in plants

Answer 7

IAA is moved around the plant to control tropisms.

In the shoots:
IAA moves away from light to the opposite side of the plant, causing cells here to elongate, meaning the shoot grows towards the light.

In the roots:
IAA moves to the bottoms of the root due to gravity, where it inhibits growth, causing the root to grow downwards.

Question 8

Pacinian corpuscles

Answer 8

1) Pacinian corpuscle is stimulated.
2) Lamellae deform and press on the nerve ending.
3) Stretch mediated sodium ion channels open.
4) Sodium ions diffuse into the cell creating a generator potential.
5) If the generator potential exceeds the threshold, an action potential is triggered.

Question 9

How do photoreceptors work?

Answer 9

1) Light hits photoreceptors in the eye where it is absorbed by optical pigments.
2) This causes the pigments to bleach causing a chemical change which creates a generator potential.
3) If this exceeds the threshold, a nerve impulse is sent along the bipolar neurone to the optic nerve.

Question 10

Structure of rods and cones

Answer 10

Rods: 
Black and white.
Many rods join one bipolar neurone.
High sensitivity to light.
Low visual acuity.

Cones:
Colour.
One cone joins to one bipolar neurone.
Low sensitivity to light.
High visual acuity.

Question 11

How do you see different colours?

Answer 11

There are 3 types of cones which when stimulated in different proportions allow you to see different colours.

Question 12

Sensitivity

Answer 12

Rods are very sensitive as many rods join to one bipolar neurone meaning many generator potentials combine to reach a threshold and trigger an action potential.

Cones are less sensitive as one cone joins to one bipolar neurone meaning only very bright lights reach the threshold to produce an action potential.

Question 13

Visual acuity

Answer 13

Rods give low visual acuity as many rods join to one bipolar neurone, meaning light from two points close together cannot be told apart.

Cones give high visual acuity as one cone joins to one bipolar neurone, meaning light from two points close together stimulate two separate action potentials.

Question 14

Control of Heart rate

Answer 14

1) SA Node initiates an impulse and sends it to the AV Node via the atrial walls causing them to contract.
2) Once the atria have emptied, the AV Node passes the impulse down the Bundle of His which splits into Purkyne fibres at the apex of the heart.
3) The carry the impulse into both ventricles causing them to contract from the bottom up.

Question 15

Structure of the Nervous system

Answer 15

Central NS
⬇️
Peripheral NS
⬇️ ⬇️
Somatic NS Autonomic NS
⬇️ ⬇️
Parasympathetic NS Sympathetic NS

Central NS: Brain + spinal cord.

Peripheral NS: Neurones connecting the Central NS to the rest of the body.

Somatic NS: Concious activities.

Autonomic NS: Unconcious activities.

Parasympathetic NS: Calms the body down.

Sympathetic NS: Gets the body ready for action.

Question 16

Communication between the heart and brain

Answer 16

1) Baroreceptors detect changes in blood pressure, and chemoreceptors detect changes in carbon dioxide/oxygen/lactic acid concentration in the blood.
2) Impulses from baroreceptors/chemoreceptorsare sent to the medulla along sensory neurones.
3) The medulla then processes this information and sends it to the SA Node, via parasympathetic neurones which slows down the heart rate or Sympathetic neurones which speeds up the heart rate.