3.6.2 Nervous Coordination

Question 1

Describe the general structure of a motor neurone

Answer 1

Cell body: contains organelles and high proportion of rough endoplasmic reticulum
Dendrons: branch into dendrites which carry impulses towards cell body.
Axon: long, unbranched fibre carries nerve impulses away from cell body.

Question 2

Describe the additional features of a myelinated motor neurone

Answer 2

Schwan 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

Question 3

Name 3 processss Schwann cells are involved in

Answer 3

• electrical insulation
• phagocytosis
• nerve regeneration

Question 4

How does an action potential pass along an unmyelinated neuron?

Answer 4

1) stimulus leads to influx of Na+ ions. First section of membrane depolarises.
2) local electrical currents cause sodium voltage-gated channels further along membrane to open. The section behind begins to repolarises.
3) sequential wave of depolarisation

Question 5

Explain why myelinated axons conduct impulses faster than unmyelinated axons

Answer 5

Saltatory conduction: impulse jumps from one node of ranvier to another. Depolarisation cannot occur where myelin sheath acts as electrical insulator. So impulse does not travel along whole axon length.

Question 6

What is resting potential?

Answer 6

Potential difference (voltage) across neuron membrane when not stimulated (-70mV)

Question 7

How is resting potential established?

Answer 7

1) membrane is more permeable to K+ than Na+
2) sodium-potassium pump actively transports 3Na+ out of cell and 2K+ into cell.

Establishes electrochemical gradient. Inside the cell is more negative than the exterior 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 an electrochemical gradient
2) potential difference across membrane becomes more positive
3) if membrane reaches threshold potential (-50mV), voltage gated Na+ channels open
4) significant influx of Na+ ions reversed potential difference back 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) potential difference across membrane becomes more negative

Question 11

What happens during hyperpolarisation?

Answer 11

1) overshoot when k+ ions diffuse out/ PD 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 sections of membrane:

• ensures undirectional impulse
• ensures discrete impulses
• limits frequency of impulse transmission

Question 13

What is the ‘all or nothing’ principle?

Answer 13

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

Question 14

Name the factors that affect speed of conductance

Answer 14

• myelin sheath
• axon diameter
• temperature

Question 15

How does axon diameter affect the speed of conductance?

Answer 15

Greater=faster

• less resistance to flow of ions (depolarisation and repolarisation)
• 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
- faster rate of respiration (enzyme controlled) = more ATP for active transport to re- establish resting potential
Temperature too high=membrane proteins denature

Question 17

Suggest appropriate units for the maximum frequency of impulse conduction

Answer 17

Hz

Question 18

How can an organism detect the strength of a stimulus?

Answer 18

Larger= raises membrane to threshold potential more quickly after hyperpolarisation = greater frequency of impulses

Question 19

What is the function of synapses ?

Answer 19

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

Question 20

Describe the structure of a synapse

Answer 20

Presynaptic neuron ends in synaptic knob: contains lots of mitochondria, ER and vesicles of neurotransmitter

Synaptic cleft: 20-30nm gap between neurons

Postsynaptic neuron: has complementary receptors to neurotransmitter (ligand-gated Na+ channels)

Question 21

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

Answer 21

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 22

How do neurotransmitters cross the synaptic cleft

Answer 22

Via simple diffusion