Response to Stimuli (complete)

Question 1

Describe a Simple Neural pathway.

Answer 1

Stimulus — Receptor - (sensory neurone) - CNS - (Relay neurone) - CNS - (motor neurone) - Effector — Response

Question 2

Define Stimulus.
How does an organisms response to a stimuli affect their chances of survival?

Answer 2

A detectable change in the internal or external environment of an organism that leads to a response in the organism.
- Responding to a stimulus/changes in their environment can increase an organisms chances of survival.

Question 3

Name and describe the two types of response to stimulus in animals.
Give two examples of the T… response.

Answer 3

• Taxes - a simple response that can maintain a motile organism in a favourable environment, which is determined by the direction of a stimulus. (e.g: Chemotaxis - response to chemicals, Phototaxis - response to light)
• Kinesis - a simple response that can maintain a motile organism in a favourable environment, which has no specific direction, but the harsher the stimulus the faster the reaction/movement.

Question 4

Name the response to stimulus that occurs in plants.
Give the three main examples.

Answer 4

• Tropisms - responses to directional stimuli that can maintain the roots and shoots of flowering plants in a favourable environment.
• Phototropism - Response to light.
• Gravitropism/Geotropism - Response to gravity.
• Hydrotropism - Response to water.

Question 5

What is the name of their hormone (plant growth factor) that affects cell elongation in plants?
How does its movement affect Phototropism and Gravitropism in roots and shoots.

Answer 5

• IAA (auxin) also called Indoleacetic acid.

Phototropism -
IAA produced in the tip of the shoot moves toward the shaded side of the plant, causing a build up. High IAA conc stimulates cell elongation in the shaded side of the plant, causing the plant to bend towards the light (positive phototropic response)

Gravitropism-
IAA produced in top of root will diffuse down the root due to gravity and inhibit cell elongation causing the root to grow downwards

Question 6

Describe phototropism in plants (shoots)

Answer 6

• cells in tip of shoot produce IAA, transported down shoot.
• light causes movement of IAA away from light side to shaded side.
• IAA causes cell elongation of shoot cells, higher concentration of IAA in shaded side.
• cell elongation on shaded side = bending of shoot towards light.

Question 7

Describe gravitropism in flowering plants.

Answer 7

• cells in tip of root produce IAA which is transported along root.
• gravity influences the movement of IAA to the lower side of the root.
• as the IAA inhibits cell elongations of root cells and there is a greater conc of IAA on lower side, the cells on this side elongate less than those on the upper side, therefore root grows downwards

Question 8

Describe the acid growth hypothesis.

Answer 8

• The effect IAA has on increasing the plasticity of plant cell walls.
• Involves the active transport of H+ ions from the cytoplasm into spaces in the cell walls, causing the cell wall to have higher plasticity, allowing the cell to elongate by expansion.

Question 9

Describe Kinesis.

Answer 9

• A non-directional response to a stimulus.
• The speed of movement of an organism is affected by the intensity of the stimulus.

Question 10

Describe Taxis

Answer 10

• A directional response to stimulus.
• The organism moves directly away from or towards the stimulus.

Question 11

Describe motor neurones

Answer 11

• carry nerve impulses away from the CNS to the effectors

Question 12

Describe the involuntary nervous system

Answer 12

• carry nerve impulses to glands, smooth muscle and cardiac muscle and is involuntary

Question 13

Describe Sensory neurones

Answer 13

• carry nerve impulses from receptors to CNS

Question 14

Describe the voluntary nervous system

Answer 14

• carries nerve impulses to body muscles and is under voluntary control

Question 15

Name the three neurones involved in a reflex arc.

Answer 15

• Sensory neurones
• Motor neurones
• Relay (intermediate) neurones

Question 16

Describe the process of a reflex arc.

Answer 16

1- Stimulus
2- Receptor
3- Sensory neurone
4- Co-ordinator
5- Motor neurone
6- Effector
7- Response

Question 17

Why are reflex arcs important?

Answer 17

• They are involuntary so don’t requires depiction making power of the brain, leaving it free to carry out more complex responses.
• They are fast as the neurone pathway is short with very few synapses.
• They protect the body from harm and don’t need to be learned.

Question 18

Describe the structure of the pacinian corpuscle

Answer 18

A neurone ending surrounded by layers of connective tissue with viscous gel between layers, protected by a capsule.

Question 19

Name the two main features of the pacinian corpuscle.

Answer 19

• Specific to a single type of stimulus (mechanical pressure)
• Produces a generator potential by acting as a transducer.

Question 20

Describe how a generator potential is formed by a pacinian corpuscle

Answer 20

• When pressure if applied to the pacinian corpuscle, it is deformed and the membrane around its neurone becomes stretched.
• The stretching widens the sodium ions channels in the membrane, allowing sodium ions to diffuse into the neurone.
• The influx of sodium ions changes the potential of the membrane (depolarisation) producing a generator potential.
• This in turn creates am action potential that passes along the neurone.

Question 21

Name the two main receptors found in the retina.

Answer 21

• Rod cells
• Cone cells

Question 22

Describe rod cells.

Answer 22

• High visual sensitivity.
• Much more numerous than cone cells.
• Low visual acuity
• Can’t distinguish different wavelengths of light, therefore only black and white imaging.
• To create a generator potential, the pigment rhodopsin in rod cells must be broken down, which can easily be done in low light levels.
• Less concentrated in the fovea, while spread through the retina.

Question 23

Define low visual acuity, and high visual sensitivity in rod cells.

Answer 23

• Low visual acuity - Many rod cells connect to single bipolar cell means the rod cells sharing the bipolar cell will only generate a single impulse to the brain, so the brain unable to distinguish between separate sources of light
• High visual sensitivity - Only low light intensity needed to stimulate a generator potential as multiple rod cells able to connect to a single bipolar cell (retinal convergence), so easier to exceed threshold.

Question 24

Describe cone cells.

Answer 24

• Low visual sensitivity.
• High visual acuity.
• To create a generator potential the pigment Iodopsin in cone cells must be broken down which require higher light intensity levels.
• Can distinguish different wavelengths of light as three different types of cone cells (red, green and blue cones - trichromatic theory)
• More concentrated in the fovea.

Question 25

Describe High visual acuity and low visual sensitivity in cone cells.

Answer 25

- High visual acuity - each cone cell has its own bipolar cell connection, so if two adjacent cone cells are stimulated, the brain receives two separate impulses, so is able to distinguish between two separate light sources. - Low visual sensitivity - Higher light intensity is required to exceed the threshold to produce a generator potential as only one cone cell connected to each bipolar cell.

Question 26

Describe the two centres in the medulla oblongata.

Answer 26

- Increasing heart rate centre - linked to the SAN by the sympathetic nervous system. - Decreasing heart rate centre - linked to the SAN by the parasympathetic nervous system.

Question 27

Describe chemoreceptors

Answer 27

- found in the wall of the carotid artery and aorta. - sensitive to changes in the pH of the blood that results from changes in CO2 concentration.

Question 28

Describe how chemoreceptors control the pH of the blood. (e.g: lower pH than normal)

Answer 28

- Chemoreceptors detect the decrease in pH, and increase the frequency of nervous impulses to the centre of the medulla oblongata that increases heart rate. - The centre increase the frequency of nervous impulses via the sympathetic nervous system, to the SAN, increasing the heart rate. - The increases heart rate increases blood flow which leads to more CO2 being removed in the lungs, returning CO2 levels to normal and so raising pH.

Question 29

Describe how pressure receptors control blood pressure. (e.g: higher than normal blood pressure)

Answer 29

- The pressure receptors detect an increase in blood pressure, so increase the frequency of nervous impulses to the centre of the medulla oblongata that decreases heart rate. - This centre sends impulses via the parasympathetic nervous system to the SAN of the heart, decreasing heart rate.

Question 30

Describe how the sympathetic nervous system works.

Answer 30

- Stimulates effectors, so speeds up any activity. - Helps us cope with stressful situations by heightening our awareness and preparing us for activities.

Question 31

Describe how the parasympathetic nervous system works.

Answer 31

- Inhibits effectors and so slows down any activity. - Controls activities under normal resting conditions. - Concerned what conserving energy and replenishing the body’s reserves.

Question 32

How does the SAN and AVN ‘beat’ the heart?

Answer 32

- A wave of electrical excitation (wave of depolarisation) spreads out (from the SAN contraction) through the atrial walls across both atria, causing the cardiac muscle to contract simultaneously. - A layer of non-conductive tissue prevents the wave crossing to the ventricles. - The AVN, after a short delay, conveys a wave of electrical excitation between the ventricles along Purkyne Tissue, which makes up the Bundle of His. This conducts the wave through the atrioventricular septum to the base of the ventricles, where the bundle branches into the smaller Purkyne tissues. - The wave of excitation is released from the apex of the heart, causing the ventricles to contract.