6.1 Stimuli, both internal and external, are detected and lead to a response

Question 1

give 7 differences between the hormonal system (HS) and the nervous system (HS)

Answer 1

Hormonal vs nervous:
1. communication by hormones vs communication by nerve impulses
2. transmission in blood vs transmission by neurones
3. slow vs rapid transmission
4. widespread vs localised response
5. slow vs rapid response
6. long-lasting vs short-lived response
7. permanent vs temporary effect

Question 2

describe a neurone cell body

Answer 2

• contains all the usual cell organelles
• including a nucleus and large amounts of rough endoplasmic reticulum

Question 3

describe the structure and function of dendrons

Answer 3

extensions of the cell body which divide into smaller branched fibres called dendrites, which carry nerve impulses towards the cell body

Question 4

describe the structure and function of an axon

Answer 4

• single long fibre
• carries nerve impulses away from the cell body

Question 5

describe the structure and function of schwann cells

Answer 5

• cells which surround the axon in layers
• provide electrical insulation
• carry out phagocytosis
• involved in nerve regeneration

Question 6

describe the structure and function of a myelin sheath

Answer 6

• covers the axon
• made up of Schwann cell membranes

Question 7

describe the structure and function of nodes of Ranvier

Answer 7

• constrictions between adjacent Schwann cells where no myelin sheath is present

Question 8

describe the structure and function of sensory neurones

Answer 8

• transmit nerve impulses from a receptor to an intermediate or motor neurone
• often have just one long dendron

Question 9

describe the structure and function of motor neurones

Answer 9

• transmit nerve impulses from an intermediate or relay neurone to an effector
• have a long axon and many short dendrites

Question 10

how do action potentials pass along myelinated neurones?

Answer 10

saltatory conduction
- jumping from node to node

Question 11

what factors affect the speed at which an action potential can travel?

Answer 11

• if it has a myelin sheath: myelin sheaths increase speed of conduction
• diameter of the axon: a greater diameter increases the speed of conduction
• temperature: a higher temperature results in faster conduction, but if temperature is too high, respiratory enzymes and plasma membranes denature, meaning impulses cannot be conducted

Question 12

how can the size of a stimulus be measured?

Answer 12

• by the number of impulses passing in a given time: the larger the stimulus, the more impulses that are generated in a given time
• by the threshold value of the neurone: neurones have different threshold values, so the brain interprets the number and type of neurone passing the impulse and thereby determines its size

Question 13

define refractory period

Answer 13

the time period after an action potential has been generated when the voltage-gated sodium ion channels are closed, so no sodium can diffuse into the axon, and therefore no new action potential can be generated

Question 14

give the three purposes of the refractory period

Answer 14

1. ensures action potentials/impulses are only transmitted in one direction
2. produces separate impulses: new action potentials cannot form immediately after another due to the refractory period, meaning individual action potentials can be separated
3. limits the number of action potentials: as action potentials are separated, the number of action potentials that can pass along an axon in a given time is limited, thus limiting the strength of stimulus that can be detected

Question 15

define spatial summation

Answer 15

multiple presynaptic neurones simultaneously release neurotransmitter for the same postsynaptic neurone, meaning neurotransmitter concentration is combined and can exceed the threshold, resulting in an action potential

Question 16

define temporal summation

Answer 16

a single presynaptic neurone releases neurotransmitter many times over a short period, meaning neurotransmitter concentration can build and exceed the threshold, resulting in an action potential

Question 17

describe the function of inhibitory synapses

Answer 17

• presynaptic neurone releases neurotransmitter
• neurotransmitter binds to chloride ion channels on postsynaptic membrane
• chloride ion channels and potassium ion channels open
• chloride ions move by facilitated diffusion into the postsynaptic neurone
• potassium ions diffuse out of the postsynaptic neurone
• influx of negative chloride ions and removal of positive potassium ions makes postsynaptic neurone negative
• hyperpolarisation of neurone decreases likelihood of a new action potential being generated, as a larger influx of sodium ions will be needed to produce one