Topic 6: Nervous impulses and synaptic transmission

Question 1

Label a myelinated neurone

Question 2

Describe how the resting potential is maintained

Answer 2

The resting potential is maintained by a sodium-potassium pump, involving active transport and therefore ATP. The pump moves 2 K+ions in and 3 Na+ ions out.
This creates an electrochemical gradient and results in K+ diffusing out and Na+ diffusing in. Due to the membrane being more permeable to K+, more are moved out resulting in the -70mV.

Question 3

For each stage in the generation of an action potential, describe what is happening

Answer 3

• Resting potential while there is no stimulus.
• A stimulus provides the energy that can cause the sodium voltage- gated channels in the axon membrane to open. This causes Na+ to diffuse in, which increase the positivity inside of the axon.
• This causes more voltage-gated channels to open, so even more Na+ diffuse in.
• When a threshold of +40mV is reached inside the axon, the voltage- gated sodium channels close and instead voltage-gated potassium ion
  channels open.
• This results in potassium ions diffusing out, and the axon becomes negative again and is repolarised.
• Temporarily the axon becomes more negative than the -70mV and is hyperpolarised.
• resting state again

Question 4

What is meant by the all-or-nothing principle?

Answer 4

If the depolarisation does not exceed the -55 mV threshold, then an action potential and the impulse are not produced (Nothing).
Any stimulus that does trigger depolarisation to -55mV will always peak at the same maximum voltage (All).
Bigger stimuli instead increase the frequency of action potentials.

Question 5

What is the refractory period and give three reasons why it is
important

Answer 5

After an action potential has been generated, the membrane enters a refractory period when it can’t be stimulated, because sodium channels are recovering and can’t be opened.

This is important because:

1. It ensures that discrete impulses are produced, meaning that an action
   potential cannot be generated immediately after another one to make sure
   that each is separate from another.
2. It ensures that action potentials travel in one direction. This stops the action
   potential from spreading out in two directions which would prevent a
   response.
3. It limits the number of impulse transmissions. This is important to prevent
   overreaction to a stimulus and therefore overwhelming the senses.

Question 6

State the three factors that affect the speed of conductance and explain how.

Answer 6

1- Myelination and Saltatory conduction

There are gaps between the myelin sheath, called nodes of Ranvier. The action potential jumps from node to node (saltatory conduction), which means the action potential travels along the axon faster as it doesn’t have to generate an action potential along the entire length (just at the nodes of Ranvier).

2- Axon diameter

With a wider diameter, the speed of conductance increases. A wider diameter means that there is less leakage of ions and therefore action potentials travel faster.

3- Temperature

A higher temperature increases the speed of conductance for two reasons:
1. The ions diffuse faster
2. The enzymes involved in respiration work faster. Therefore there is more ATP for active transport in the Na+/K+ pump.

Question 7

describe the actions at a cholinergic synapse

Answer 7

• An action potential arrives at the synaptic knob. Depolarisation of synaptic knob leads to the opening of Ca2+ channels and Ca2+ diffuses into the synaptic knob.
• Vesicles containing acetylcholine neurotransmitters move towards and fuse with the presynaptic membrane.
• Acetylcholine is released into the synaptic cleft.
  Acetylcholine diffuses down a concentration gradient across the synaptic cleft, to the post-synaptic membrane; acetylcholine binds by the complementarity of shape to receptors on the surface of the post-synaptic membrane.
  Na+ ion channels on the post-synaptic membrane open and Na+ diffuse in; if enough acetylcholine binds, and enough Na+ diffuse in to raise the membrane potential above the -55mV threshold, then the post-synaptic neurone becomes depolarised.

-The acetylcholine is degraded by acetylcholinesterase into choline and acetate and released from the receptor; the Na+ channel close and the post-synaptic neuron can re-establish resting potential; the neurotransmitter is transported back into the presynaptic neuron where it is recycled.

Question 8

What is meant by summation?

Answer 8

The rapid build-up of neurotransmitters in the synapse helps generate an action potential

Question 9

What is the difference between spatial and temporal
summation?

Answer 9

• Spatial summation: many different neurones collectively trigger a new action potential by combining the neurotransmitter they release to exceed the threshold value.
• Temporal summation: One neurone releases neurotransmitter repeatedly over a short period of time to add up to enough to exceed the threshold value.

Question 10

What makes the direction of transmission at a synapse unidirectional?

Answer 10

A neurotransmitter is only released from the pre- synaptic neurone, therefore it diffused from here to the post-synaptic neurone. Secondly, there are only receptors for the neurotransmitter on the post-synaptic neurone.

Question 11

Which ion is often released into the post-synaptic membrane at an inhibitory synapse instead of sodium ions?

Answer 11

Chlorine ions

Question 12

give three reasons why the refractory period is
important

Answer 12

This is important because:

1. It ensures that discrete impulses are produced, meaning that an action
   potential cannot be generated immediately after another one to make sure
   that each is separate from another.
2. It ensures that action potentials travel in one direction. This stops the action
   potential from spreading out in two directions which would prevent a
   response.
3. It limits the number of impulse transmissions. This is important to prevent
   overreaction to a stimulus and therefore overwhelming the senses.

Question 13

What is temporal summation?

Answer 13

one neurone releases neurotransmitter repeatedly over a short period of time to add up to enough to exceed the threshold value

Question 14

What is spatial summation?

Answer 14

different neurones collective trigger a new action potential by combining the nt they release to exceed threshold value

Question 15

What is a neuromuscular junction?

Answer 15

this is a synapse that occurs between a motor neurone and a muscle and is very similar to a synaptic junction