Topic 15 - Response to Stimuli

Question 1

Define effectors

Answer 1

A range of different cells, tissues, organs, or systems that carry out a response.

Question 2

Define stimuli

Answer 2

Detectable change in the internal or external environment.

Question 3

Define receptors

Answer 3

Cells or organs that detect specific stimuli.

Question 4

What is the basic sequence of events, starting with a stimulus?

Answer 4

Stimulus -> receptor -> coordinator -> effector -> response

Question 5

Define coordinator

Answer 5

Intermediary between a receptor and effector, which formulates a suitable response to a stimulus.

Question 6

Define taxes

Answer 6

• Simple directional response.
• Direction determined by direction of stimulus.
• Move towards stimulus = positive taxis.
• Move away from stimulus = negative taxis.
• E.g. chemotaxis, thermotaxis

Question 7

Define kineses

Answer 7

• Random non-directional movement.
• Organisms do not move towards or away from stimulus.
• More unpleasant the stimulus, the more rapidly the organism moves.

Question 8

What is a tropism?

Answer 8

Plant growth in response to a directional stimulus and can either be towards (positive) or away (negative) from stimulus.

Question 9

Indoleacetic acid (IAA/auxin) in the shoot tip - phototropism

Answer 9

1) Cells in shoot tip produce IAA, diffuses down shoot.
2) Light causes movement of IAA from light side to shaded side.
3) Conc. greater in darker side.
4) Attaches to receptors on membrane of cells.
5) Releases hydrogen ions, lowering the pH.
6) Breaks bonds between microfibrils in the cellulose cell wall, making them more easily stretched so they elongate.
7) Shaded side grows faster = shoot bends towards the light.

Question 10

Indoleacetic acid (IAA) in the root tip - geotropism

Answer 10

1) Cells in root tip produce IAA, diffuses down root.
2) Gravity causes movement of IAA to lower region in root tip.
3) Conc. greater in lower side of root tip than top.
4) Attaches to receptors on membrane of cells.
5) Lower side grows slower.
6) Top side grows faster = root bends downwards towards centre of Earth.

Question 11

What are sensory neurones?

Answer 11

Carries nerve impulses from receptors towards central nervous system.

Question 12

What is the voluntary nervous system?

Answer 12

Carries nerve impulses to body muscles and is under conscious control.

Question 13

What are motor neurones?

Answer 13

Carries nerve impulses away from central nervous system towards effectors.

Question 14

What is the central nervous system?

Answer 14

Made up of the brain and spinal cord.

Question 15

What is the autonomic nervous system?

Answer 15

Carries nerve impulses to glands, smooth muscle, and cardiac muscle.

Question 16

What is the peripheral nervous system?

Answer 16

Made up from pairs of nerves that originate from either the brain or spinal cord.

Question 17

Why is the reflex arc important?

Answer 17

• Reduce/avoid damage to tissue.
• Present from birth, not learned.
• Very rapid, allows brain to execute more complex responses.
• Helps with posture/balance.
• Process by association area - helps us to learn.

Question 18

What is the Pacinian corpuscle?

Answer 18

• Transducers, converting pressure energy into electrochemical energy.
• Produce a generator potential.
• The greater the pressure, the greater the frequency of nerve impulses along neurone.
• Consist of a single neurone ending surrounded by a bulb of lamellae and a capsule made up of connective tissue.
• Contains stretch-mediated channels.

Question 19

What is a generator potential?

Answer 19

• When stimulated, receptors change one form of energy into another (transducers).
• Energy change of stimulus converted into an electrical impulse known as a generator potential.
• If generator potential is large enough, it produces an action potential.

Question 20

What are rods in the eye?

Answer 20

• Sensitive to very low intensity of light, enable us to distinguish light from dark in very dim light.
• Respond to just one photon.
• Do not permit us to distinguish different colours.

Question 21

What are cones in the eye?

Answer 21

• Sensitive to different wavelengths of light, can distinguish pigments.
• Human eyes have three types of cones: one responds to wavelengths of red, one to green, and one to blue.
• When combinations of three types of cone are stimulated, we perceive the range of other colours in the visible spectrum.

Question 22

What causes vision using the fovea to be in colour?

Answer 22

Different types of cone cells produce different colours.

Question 23

What causes vision using the fovea to have high visual acuity?

Answer 23

Impulses along separate neurone from each receptor cell.

Question 24

The sympathetic and parasympathetic nervous system (ANS)

Answer 24

• Antagonistic.
• Sympathetic nervous system: stimulates effectors and speeds up any activity to stressful stimuli. Helps prepare for the activity.
• Parasympathetic nervous system: inhibits effectors and slows down any activity. Helps conserve energy and replenish body’s reserves.

Question 25

The initiation and coordination of the heart beat

Answer 25

1) Electrical waves are generated and sent out from sinoatrial node (SAN). Initiates heartbeat, determines heart rate.
2) SAN sends out wave of electrical activity across walls of atrium.
3) Contraction of walls of atrium.
4) Impulses from atria detected by the atrioventricular node (AVN). There is a short delay to allow atria to finish contracting.
5) Impulses sent down bundle of His to Purkinje fibres.
6) Electrical impulses spread from apex up ventricular walls.
7) Causes simultaneous contraction of the ventricular walls.

Question 26

What are pressure receptors?

Answer 26

• In wall of carotid arteries and aorta.
• Sensitive to blood pressure.
• High pressure = lower heart rate via PNS and SAN.
• Decreased frequency of impulses.

Question 27

What are chemoreceptors?

Answer 27

• In wall of carotid arteries.
• Detect changes in pH.
• Low pH = increased heart rate via SNS and SAN.
• Increased frequency of impulses.