The Nervous System - A2

Question 1

Explain how resting potential is reached.

Answer 1

At a potential difference of around -70. The sodium/potassium pump causes active transport of 3 Na+ out of the neurone membrane and 2 K+ into the membrane; this keeps the outside of the membrane more positive. (Polarised) The sodium ion channels are closed and only some of the potassium ion channels are open. This means that there is a sodium ion electrochemical gradient.

Question 2

Describe the general structure of a motor neurone.

Answer 2

Cell body - contains organelles and high proportion of RER
Dendrons - branch into dendrites which carry impulses towards cell body
Axon - long unbranched fibre carries nerve impulses away from cell body

Question 3

Describe the additional features of a myelinated motor neurone.

Answer 3

Schwann cells - wrap around axon many times
Myelin sheath - made from myelin rich membranes of Schwann cells
Nodes of Ranvier - very short gaps between neighbouring Schwann cells where there is no myelin sheath
Terminal end branch - connects neurone to effector

Question 4

Name 3 processes Schwann cells are involved in.

Answer 4

• electrical insulation
• phagocytosis
• nerve regeneration

Question 5

How does an action potential pass along an unmyelinated neuron?

Answer 5

• stimulus leads to influx of Na+ ions. First section of membrane depolarised
• local electrical currents cause sodium voltage-gated channels further along the membrane to open
• meanwhile, the section behind begins to re-polarise
• sequential wave of depolarisation occurs.

Question 6

Explain why myelinated axons conduct impulses faster than unmyelinated axons.

Answer 6

Saltatory conduction - impulse jumps from one Node of Ranvier to another
- depolarisation can not occur when myelin sheath acts as an electrical insulator
- so impulse does not travel along whole axon length

Question 7

How is resting potential established?

Answer 7

• membrane is more permeable to K+ than Na+
• sodium potassium pump actively transports 3Na+ out of cell and 2K+ into cell
• establishes an electrochemical gradient: cell contents more negative than extracellular environment

Question 8

Name the stages in generating an action potential.

Answer 8

1. depolarisation
2. repolarisation
3. hyperpolarisation
4. return to resting potential

Question 9

What happens during depolarisation?

Answer 9

1. Stimulus - facilitated diffusion of Na+ ions into cell down electrochemical gradient
2. Potential difference across membrane becomes more positive
3. if membrane reaches threshold potential (-50mv) voltage-gates Na+ channels open
4. Significant influx of Na+ ions reverses p.d to +40mv

Question 10

What happens during repolarisation?

Answer 10

1. voltage-gated Na+ channels close and voltage-gated K+ channels open
2. facilitated diffusion of K+ ions out of cell down their electrochemical gradient
3. p.d across membrane becomes more negative

Question 11

What happens during hyperpolarisation?

Answer 11

1. ‘overshoot’ - when K+ ions diffuse out = p.d becomes more negative than resting potential
2. refractory period - no stimulus is large enough to raise membrane potential to threshold
3. voltage-gated K+ channels close and sodium-potassium pump re-establishes resting potential

Question 12

Explain the importance of the refractory period.

Answer 12

No action potential can be generated in hyperpolarised section of membrane.
- ensures un-directional impulse
- ensures discrete impulses
- limits frequency of impulse transmission

Question 13

What is the ‘all or nothing’ principal?

Answer 13

Any stimulus that causes the membrane to reach threshold potential will generate an action potential.
All action potentials have the same magnitude.

Question 14

Name the factors that could affect the speed of conductance.

Answer 14

Myelin sheath
Axon diameter
Temperature

Question 15

How does axon diameter affect speed of conductance?

Answer 15

greater diameter = faster
- less resistance to flow of ions (depolarisation and re-polarisation)
- less leakage of ions (easier to maintain membrane potential)

Question 16

How does temperature affect speed of conductance?

Answer 16

Higher temp = faster
- faster rate of diffusion (depolarisation and repolarisation)
- faster rate of respiration (enzyme controlled) = more ATP for active transport to re-establish resting potential
- if temperature is too high then proteins denature.

Question 17

How can an organism detect the strength of a stimulus?

Answer 17

larger stimulus raises membrane to threshold potential more quickly after hyperpolarisation = greater frequency of impulse

Question 18

What is the function of synapses?

Answer 18

• electrical impulses cannot travel over junction between neurons
• neurotransmitters send impulses between neurons/from neurons to effectors
• new impulses can be initiated in several different neurons for multiple simultaneous responses

Question 19

Describe the structure of a synapse.

Answer 19

Presynaptic neurone ends in synaptic knob - contains many mitochondria, endoplasmic reticulum and vessels of neurotransmitter.
Synaptic cleft - 20/30nm gap between neurons
Postsynaptic neuron - has complementary receptors to neurotransmitter (ligand-gated Na+ channels)

Question 20

Outline what happens in the presynaptic neuron when an action potential is transmitted from one neuron to another.

Answer 20

1. wave of depolarisation travels down presynaptic neuron, causing voltage-gated Ca2+ channels to open
2. vesicles move towards and fuse with presynaptic membrane
3. exocytosis of neurotransmitter into synaptic cleft

Question 21

How do neurotransmitters cross synaptic clefts?

Answer 21

simple diffusion

Question 22

Outline what happens in the postsynaptic neuron when an action potential is transmitted from one neuron to another.

Answer 22

1. neurotransmitter binds to specific receptor on postsynaptic membrane
2. ligand-gated Na+ channels open
3. if influx of Na+ ions raises membrane to threshold potential, action potential is generated

Question 23

Explain why synaptic transmission is undirectional?

Answer 23

Only presynaptic neuron contains vesicles of neurotransmitter and only postsynaptic membrane has complementary receptors.
So impulse always travels from presynaptic to postsynaptic.

Question 24

Define summation and name the 2 types.

Answer 24

Neurotransmitter from several sub-threshold impulses accumulates to generate action potential.
- temporal summation
- spatial summation
NB no summation at neuromuscular junctions

Question 25

What is the difference between temporal and spatial summation?

Answer 25

temporal - one presynaptic neurone releases neurotransmitter several times in quick succession
spatial - multiple presynaptic neurones release neurotransmitter

Question 26

What are cholinergic synapses?

Answer 26

Use acetylcholine as primary neurotransmitter. Excitatory or inhibitory. Located at…
- motor end plate (muscle contraction)
- preganglionic neurons (excitation)
- parasympathetic postganglionic neurons (inhibition eg of heart breathing rate)

Question 27

What happens to acetylcholine from the synaptic cleft?

Answer 27

1. hydrolysis into acetyl and choline by acetylcholinesterase (ACHe) - breaks ester bonds
2. acetyl and choline diffuse back into presynaptic membrane
3. ATP is used to reform acetylcholine for storage in vesicles

Question 28

Explain the importance of acetylcholinesterase.

Answer 28

• prevents overstimulation of skeletal muscle cells
• enables acetyl and choline to be recycled

Question 29

What happens in an inhibitory synapse?

Answer 29

1. neurotransmitter binds to and open Cl- channels on postsynaptic membrane and triggers K+ channels to open
2. Cl- moves in and K+ moves out via facilitated diffusion
3. p.d becomes more negative - hyperpolarisation

Question 30

Describe the structure of a neuromuscular junction.

Answer 30

Synaptic cleft between a presynaptic neurone and a skeletal muscle cell.

Question 31

How may drugs increase synaptic condition?

Answer 31

• inhibit acetylcholinesterase
• mimic shape of neurotransmitter

Question 32

How may drugs decrease synaptic transmission?

Answer 32

• inhibit release of neurotransmitter
• decrease permeability of postsynaptic membrane to ions
• hyperpolarise postsynaptic membrane

Question 33

Depolarisation mark points.

Answer 33

• ion channel proteins open
• sodium ions in
• changes membrane potential / inside of axon less negative
• more channels open / positive feedback

Question 34

Repolarisation mark points.

Answer 34

• potassium channel proteins open
• potassium ions out
• sodium channel proteins close

Question 35

Resting potential mark points.

Answer 35

• pump / active transport against concentration gradient
• of sodium ions from axon / sodium ions out / potassium ions in
• membrane is more permeable to potassium ions than sodium ions - potassium ions can leak through membrane, sodium can not.

Question 36

How synapses work - mark points.

Answer 36

• action potential arrives / depolarisation occurs
• calcium ions enter synaptic knob
• vesicles fuse with membrane
• acetylcholine diffuses (across synaptic cleft)
• binds to receptors (postsynaptic membrane)
• sodium ions diffuse into post synaptic neurone and depolarise membrane