3.6.2.1 Nerve impulses

Question 1

What are the key structural components of a myelinated motor neurone?

Answer 1

• Myelinated motor neurone structure includes:
• Cell body: contains the nucleus and rough endoplasmic reticulum (RER) for neurotransmitter synthesis
• Dendrites: carry impulses to the cell body
• Axon: transmits impulses away from the cell body
• Myelin sheath: insulating layer made of Schwann cells
• Nodes of Ranvier: gaps in the myelin sheath that facilitate saltatory conduction
• Axon terminals: involved in synaptic transmission

Question 2

How is a resting potential established across the neurone membrane?

Answer 2

• Resting potential establishment involves:
• Sodium-potassium pump actively transporting:
  3 Na+ ions out of the axon
  2 K+ ions into the axon
• Creation of an electrochemical gradient
  Membrane is more permeable to K+ than Na+
• K+ diffuses back out of the axon
  Inside of the axon becomes negatively charged relative to the outside (-70 mV)

Question 3

What causes depolarisation of the neurone membrane?

Answer 3

• Depolarisation process includes:
  
  • A stimulus causes voltage-gated sodium ion channels to open
  • Na+ ions diffuse into the axon down their electrochemical gradient
  • Influx of Na+ makes the inside of the axon less negative
  • Inside reaches the threshold potential (-55 mV)
  • Generation of an action potential (+40 mV)

Question 4

What is the all-or-nothing principle in action potential generation?

Answer 4

• All-or-nothing principle states that:
  
  • An action potential is generated only if the stimulus reaches or exceeds the threshold potential (-55 mV)
  • No action potential occurs below this threshold
  • Action potentials are always the same size, regardless of stimulus strength

Question 5

How does an action potential travel along a non-myelinated axon?

Answer 5

• In a non-myelinated axon, action potential propagation involves:
  
  • Localised currents depolarising the adjacent section of the axon membrane
  • Opening of voltage-gated sodium channels
  • Sequential depolarisation moves the action potential along the axon

Question 6

How does an action potential travel along a myelinated axon?

Answer 6

• In a myelinated axon, action potential propagation occurs through:
  
  • Jumping between nodes of Ranvier (gaps in the myelin sheath)
  • Saltatory conduction mechanism
  • Significant increase in speed of transmission
  • Depolarisation occurs only at the nodes

Question 7

What is the importance of the refractory period in nerve impulse transmission?

Answer 7

• The refractory period ensures discrete impulses by:
  
  • Preventing the overlap of action potentials
• Limiting the frequency of impulse transmission
• Sodium ion channels remain inactivated during this period
• Ensuring unidirectional propagation

Question 8

What factors affect the speed of nerve impulse conduction?

Answer 8

The speed of conduction is influenced by several factors:

• Myelination: Saltatory conduction significantly increases speed as the action potential jumps between nodes of Ranvier.
• Axon diameter: A larger diameter reduces resistance, which in turn increases speed.
• Temperature: Higher temperatures enhance the kinetic energy of ions, speeding up diffusion and conduction.

Question 9

Explain how a resting potential is maintained across the axon membrane in
a neurone. (3)

Answer 9

1. Higher concentration of potassium ions inside and higher
   concentration of sodium ions outside (the neurone)
   OR
   potassium ions diffuse out
   OR
   sodium ions diffuse in;
2. (Membrane) more permeable to potassium ions (leaving
   than sodium ions entering)
   OR
   (Membrane) less permeable to sodium ions (entering
   than potassium ions leaving);
3. Sodium ions (actively) transported out and potassium ions in;

Question 10

Explain why the speed of transmission of impulses is faster along a
myelinated axon than along a non-myelinated axon. (3)

Answer 10

1. Myelination provides (electrical) insulation;
   Reject thermal insulation.
2. (In myelinated) saltatory (conduction)
   OR
   (In myelinated) depolarisation at nodes (of Ranvier);
3. In non-myelinated depolarisation occurs along whole/length (of axon);

Question 11

why would less ATP change the resting potential of the neurone
changed from –70 mV to 0 mV.

Answer 11

1. No/less ATP produced;
2. No/less active transport

Question 12

Describe the sequence of events involved in transmission across a cholinergic
synapse. (5)

Answer 12

1. Depolarisation of presynaptic membrane;
   Accept action potential for depolarisation.
2. Calcium channels open and calcium ions enter (synaptic knob);
   Accept Ca2+.
3. (Calcium ions cause) synaptic vesicles move to/fuse with presynaptic membrane and release acetylcholine/neurotransmitter;
4. Acetylcholine/neurotransmitter diffuses across (synaptic cleft);
5. (Acetylcholine attaches) to receptors on the postsynaptic membrane;
6. Sodium ions enter (postsynaptic neurone) leading to depolarisation;

Question 13

Describe the process by which a neurotransmitter stimulates the production of nerve impulses in postsynaptic neurones. (3)

Answer 13

1. diffuses across (synapse);
2. Attaches to receptors on postsynaptic membrane
3. Stimulates entry of sodium ions and depolarisation/action potential

Question 14

Explain how negatively charged ions entering the postsynaptic neurones inhibits postsynaptic neurons

Answer 14

1. (Inside of postsynaptic) neurone becomes more negative/hyperpolarisation/inhibitory postsynaptic potential;
   Ignore K+
   Accept ‘decrease in charge’
2. More sodium ions required (to reach threshold)
   OR
   Not enough sodium ions enter (to reach threshold);
   Accept Na+ for sodium ions
3. For depolarisation/action potential;
   Context must covey idea that depolarisation / action potential is less likely

Question 15

When a nerve impulse arrives at a synapse, it causes the release of neurotransmitter from vesicles in the presynaptic knob.
Describe how. (3)

Answer 15

1. (Nerve impulse / depolarisation of membrane) causes Ca 2+ channel
   (proteins) to open;
2. Ca 2+ enter by (facilitated) diffusion;
3. Causes (synaptic) vesicles to fuse with (presynaptic)
   membrane;