3.6.1 - Stimuli and Response

Question 1

stimulus

Answer 1

a detectable change in the internal or external environment of an organism that leads to a response

Question 2

taxis

Answer 2

• directional response to a stimulus
• organism moves its whole body
• towards a favourable stimulus (positive taxis)
• away from an unfavourable stimulus (negative taxis)

Question 3

advantages of taxis

Answer 3

• to avoid competition
• to find a mate
• to increase dispersal
• to avoid predators

Question 4

kinesis

Answer 4

• non-directional response to a stimulus
• organism changes speed at which it moves and rate at which it changes direction
• rate of turning increases upon entering an unfavourable environment
• may slowly decrease as organism moves further into unfavourable environment (moves in long straight lines)

Question 5

tropism

Answer 5

• growth of part of a plant in response to a directional stimulus
• can be positive or negative
• in response to light = phototropism
• in response to gravity = gravitropism
• involve plant growth factors

Question 6

central nervous system (CNS)

Answer 6

brain and spinal cord

Question 7

peripheral nervous system (PNS)

Answer 7

sensory neurones and motor neurones

Question 8

voluntary nervous system

Answer 8

• carries nerve impulses to muscles
• under voluntary (conscious) control

Question 9

autonomic nervous system

Answer 9

• carries nerve impulses to glands, smooth muscle and cardiac muscle
• involuntary (subconscious)

Question 10

reflex

Answer 10

a rapid, involuntary response to a stimulus

Question 11

reflex arc

Answer 11

stimulus → receptor → sensory neurone → coordinator (relay neurone) → motor neurone → effector → response

Question 12

advantages of simple reflexes

Answer 12

• rapid
• protect against damage to body tissues
• do not have to be learnt
• help escape from predators
• enable homeostatic control

Question 13

Pacinian corpuscle

Answer 13

• pressure receptor
• occurs deep in the skin of fingers and toes
• sensory neurone surrounded by layers of connective tissue with viscous gel between
• stretch-mediated sodium channels found in neurone plasma membrane

Question 14

Pacinian corpuscle function

Answer 14

• resting potential
• pressure applied (stimulus)
• sodium channels are stretched and deformed (widen)
• sodium ions can diffuse into neurone
• depolarisation, generator potential produced
• this creates an action potential (nerve impulse)

Question 15

Explain how a high density of cones near the fovea enables us to see in detail.

Answer 15

• high visual acuity
• each cone is connected to a single neurone
• cones send separate sets of impulses to brain

Question 16

The retina of an owl has a high density of rod cells. Explain how this enables an owl to hunt its prey at night.

Answer 16

• high visual sensitivity (retinal convergence)
• several rods connected to a single neurone
• enough neurotransmitter to reach/overcome threshold
• spatial summation to reach/overcome threshold

Question 17

Explain how a resting potential of –70 mV is maintained in a neurone.

Answer 17

• membrane more permeable to potassium ions and less permeable to sodium ions
• sodium ions actively transported out and potassium ions in

Question 18

Explain how applying pressure to a Pacinian corpuscle produces changes in membrane potential.

Answer 18

• pressure causes membrane to become deformed/stretched
• sodium ion channels in membrane open and sodium ions move in
• greater pressure = more channels open/sodium ions enter

Question 19

Explain why membrane potential does not exceed +40 mV when there is a large enough stimulus.

Answer 19

• threshold has been reached
• threshold or above causes maximal response/all or nothing principle

Question 20

Multiple sclerosis is a disease in which parts of the myelin sheaths surrounding neurones are destroyed. Explain how this results in slower responses to stimuli.

Answer 20

• less/no saltatory conduction/action potential/impulse unable to ‘jump’ from node to node
• more depolarisation over length/area of membranes

Question 21

rod cells

Answer 21

• images seen in black and white
• cannot distinguish different wavelengths of light
• many rod cells connected to a single sensory neurone (retinal convergence → summation)
• detect low intensity light
• rhodopsin pigment broken down
• single impulse generated → brain can’t distinguish between separate sources of light → low visual acuity

Question 22

cone cells

Answer 22

• one cone cell connected to one bipolar cell and one sensory neurone
• respond to high light intensity
• iodopsin broken down
• separate impulses generated → brain can distinguish between separate sources of light → high visual acuity
• found at the fovea

Question 23

sympathetic nervous system

Answer 23

• stimulates effectors
• speeds up activity
• fight or flight response

Question 24

parasympathetic nervous system

Answer 24

• inhibits effectors
• slows down activity
• normal resting conditions
• conserves energy
• replenishes reserves

Question 25

control of heart rate

Answer 25

• sinoatrial node (SAN)
• wave of electrical activity spreads out across both atria, they contract
• layer of non-conductive tissue prevents wave crossing to ventricles
• wave enters atrioventricular node (AVN) between the atria
• after short delay, AVN conveys wave of activity along the bundle of His
• bundle of His conducts wave through to base of ventricles
• wave released from Purkyne tissue causing ventricles to contract