25: Nervous & Synaptic Transmission

Question 1

Name the different parts of a Motor Neurone:

Answer 1

• dendrites
• cell body
• axon
• nodes of Ranvier
• axon terminals
• Schwann cells

Question 2

Describe what are Schwann cells + nodes of Ranvier:

Answer 2

Schwann cells:
- wrap around the axon, providing insulation
- rich in lipid, making up the myelin sheath
Nodes of Ranvier:
- the gaps between Schwann Cells

Question 3

______ neurons can transmit nerve impulses _____ than _____ neurons

Answer 3

1. myelinated
2. faster
3. non-myelinated

Question 4

What is a nerve impulse?

Answer 4

• temporary reversal of the electrical potential difference across the axon membrane, between action + resting potential

Question 5

Which 3 proteins are embedded in the phospholipid bilayer of the Axon?

Answer 5

• leak channels (open all time)
• gated channels
• sodium-potassium pump

Question 6

What is a Resting Potential?
Describe the Resting Potential maintenance:

Answer 6

1. difference between electrical charge inside and outside the neurone (axon), when the nerve isn’t conducting an impulse
2. • active transport of 3Na+ out of axon, and 2K+ into axon occurs via sodium-potassium pumps
   • this results in more K+ inside the axon than outside, and more Na+ outside than inside
   • the membrane is more permeable to K+ than Na+
   • so more K+ diffuses back out of axon (facilitated diffusion) quicker than Na+ diffuses into axon
   • this leads to -65mV inside compared to outside
   • the membrane has been polarised

Question 7

1. define Action potential
2. name the different process in an Action Potential

Answer 7

1. if the axon is depolarised enough, it can lead to the generation of an action potential
2. resting potential (polarisation), depolarisation, repolarisation, hyperpolarisation, refractory period

Question 8

Describe the Depolarisation:

Answer 8

• stimulus causes gated Na+ channels to open, increasing membrane permeability of Na+
• therefore Na+ rapidly diffuses into the axon, causing depolarisation
• inside membrane is now charged at +40mV
• gated Na+ channels close, meaning axon is now again impermeable to Na+

Question 9

Describe Repolarisation:

Answer 9

• the gated K+ channels open, so permeability for K+ increases
• so more K+ diffuses rapidly out of axon

Question 10

Describe Hyperpolarisation:

Answer 10

• so many K+ moves out of the axon that they cause a more negative potential difference than normal
• axon drops to -90mV

Question 11

Describe Refractory Period:

Answer 11

• short period of time where that area of the axon membrane is recovering from its own depolarisation
• gated Na+ channel close and cannot be reopened
• sodium-potassium pumps are then used to restore concentration gradients back to -65mV

Question 12

What are 3 important factors of the Refractory Period?

Answer 12

Gated Na+ channel close, and cannot be reopened:
- ensures action potential is only in one direction
- produces separate action potentials
- limits number of action potentials

Question 13

Describe how Myelinated Neurones have Saltatory Conduction:

Answer 13

• depolarisation only occurs at Nodes at Ranvier instead of across whole Axon
• impulses can jump from node to node

Question 14

Describe the factors affecting the Speed of Conduction:

Answer 14

• Myelination: action potentials do not need to occur at entire length of axon
• Axon Diameter: wider diameter increases speed, due to the less resistance to flow of ions + less leakage of ions
• Temperature:
  
  1. ions diffuse faster
  2. enzymes used in respiration work faster, so more ATP for active transport

Question 15

Describe the All or Nothing principle:

Answer 15

• if stimulus is above the threshold, then an action potential is generated at constant size + speed
• an increase in stimulus doesn’t change size + speed, but would increase the frequency of the action potential
• if the stimulus is not large enough, then the gated Na+ channels don’t open so no depolarisation (this is the nothing)

Question 16

What is a Synapse?
Why are Synapses needed?
What is a Synaptic Knob?

Answer 16

• the gap between the axon of one neurone and the dendrites of another
• action potentials cannot pass in between neurones, so it is transmitted to a neurotransmitter that diffuses across the gap
• end of axon

Question 17

What is A Cholinergic Synapse?

Answer 17

• contains the neurotransmitter ACh (acetlycholine)
• made up of ethanoic acid and choline

Question 18

Describe the Transmission of a Synapse:

Answer 18

1. action potential arrives at the synaptic knob, allowing Ca ion channels in the membrane to open, allowing Ca ions to diffuse in
2. the influx of Ca ions causes synaptic vesicles to fuse with the presynaptic membrane and release ACh by exocytosis into the synaptic cleft
3. ACh diffuses across synaptic cleft and binds to receptors on Na+ channels in the postsynaptic membrane
4. this causes Na+ channel to open and Na+ rapidly into post-synaptic cell, causing depolarisation, if threshold met then an action potential is generated
5. to prevent any further action potentials in the post-synaptic membrane, an enzyme acetylcholinesterase hydrolyses ACh
6. the products of hydrolysis, diffuse back across the synaptic cleft, to be reabsorbed into the pre-synaptic neurone
7. ## ACh is reformed and repackaged into synaptic vesicles

Question 19

Describe an adaptation of the Synaptic Knob, and explain it:

Answer 19

contains many mitochondria:
- exocytosis of ACh requires ATP as it is an active process
- energy from ATP used to generate the neurotransmitter

Question 20

Describe 3 Features of Synapses:
Explain these features:

Answer 20

1. unidirectionality: only pass impulses in one direction
2. summation: rapid build-up of neurotransmitters in the synapse to help generate an action potential

• spatial summation: number of different presynaptic neurones release enough transmitter at the same time to cause enough depolarisation to exceed threshold
• temporal summation: a single presynaptic neurone releases small amounts of neurotransmitter in a short period (higher frequency), which adds to up to cause enough depolarisation exceeding threshold

3. inhibition:
   
   • inhibitory synapses cause Cl- channels on the post-synaptic membranes of some neurones to be opened by certain neurotransmitters
   • this causes more Cl- to flood into the postsynaptic knob, causing hyperpolarisation -90mV, therefore action potential unlikely

Question 21

Describe how Drugs Stimulate Synapses:

Answer 21

makes it easier for create action potentials in postsynaptic neurones:
- mimicking the neurotransmitter
- causing the release of excess neurotransmitter
- reducing the activity of the enzyme which breaks down the neurotransmitter
- these all cause an increase in the number of impulses sent along the neurone

e.g caffeine, nicotine, amphetamines, cocaine

Question 22

Describe how Drugs Inhibit Synapses:

Answer 22

cause fewer action potentials in the postsynaptic neurone:
- inhibiting the release of neurotransmitter
- blocking the receptors for the neurotransmitter on the postsynaptic membrane
- this causes a reduction in the number of impulses sent along that neurone

e.g alcohol, cannabis, ketamine