15- Nervous co-ordination & muscles

Question 1

Steps in the establishment of a Resting potential

Answer 1

• The Na+-K+ pumps actively transport: 3 Na+ OUT of the axon, 2K+ INTO axon
• This creates an electrochemical gradient
• The membrane is more permeable to K+ than Na+ so
• K+ move out of the axon by facilitated diffusion
• The axon is more negatively charged than the tissue fluid so it is polarised and resting potential is restored.

Question 2

Why is the membrane initially more permeable to K+ than Na+?

Answer 2

K+ channels are open and Na+ channels are closed

Question 3

steps in an all or nothing response

Answer 3

Stimulus- the membrane is more permeable to Na+ as Na+ channels are open
- Na+ diffuse into neurone down the electrochemical gradient

Depolarisation- potential difference reaches threshold and an AP is formed
-( this is as more voltage-gated Na+ ion channels open so more Na+ diffuse rapidly and no more can enter)

Repolarisation- Na+ channels close while K+ channels open so K+ diffuse out of axon

Hyperpolarisation- K+ channels are slow to close so there’s a slight overshoot. This means too many K+ diffuse out of the axon.

Resting potential restored- by Na+-K+ pump.

Question 4

importance of the refractory period

Answer 4

• produces discrete & discontinuous impulse (AP’s dont overlap)
• limits frequency of impulse transmission at certain intensities
• ensures unidirectional AP

Question 5

Factors affecting conduction speed

Answer 5

1. Myelination
   - depolarisation at nodes of ranvier only> salatory conduction (impulse jumps)- this is quicker than wave of depolarisation.
2. Axon diameter
   - The bigger the diameter, the less leakage of ions so speed INCREASES
3. Temp
   - increased temp increases rate of movement of Na+ & K+ as more kinetic energy
   - increased temp means higher rate of respiration so ATP produced faster & energy released faster so active transport occurs faster
   - If too high proteins could denature

Question 6

myelinated vs non-myelinated axons

Answer 6

In non-myelinated, AP passes as a wave of depolarisation.

In myelinated, depolarisation occurs at nodes of Ranvier only (salatory conduction)- This is much quicker.

Question 7

non-myelinated axon transmission

Answer 7

• AP passes as a wave of depolarisation
• Influx of Na+ in one region increases permeability of adjoining region to Na+ by causing voltage-gated Na+ channels to open so adjoining region depolarises

Question 8

what is the refractory period?

Answer 8

time to restore axon to RP

Question 9

ALL OR NOTHING - 1. Stimulus

Answer 9

• membrane is more permeable to Na+ as Na+ channels open

- Na+ diffuse into neurone down electrochemical gradient

Question 10

ALL OR NOTHING- 2. Depolarisation

Answer 10

• potential difference reaches the threshold & an AP is formed
• This is as more voltage-gated Na+ ions channels open, so more Na+ diffuse rapidly & no more can enter

Question 11

ALL OR NOTHING- 3. repolarisation

Answer 11

Na+ channels close while K+ channels open so K+ diffuse out of axon

Question 12

ALL OR NOTHING- 4. Hyperpolarisation

Answer 12

K+ channels are slow to close so there’s a slight overshoot. This means too many K+ diffuse out of the axon & resting potential restored (by Na+ - K+ pump).

Question 13

Synaptic transmission across neuromuscular junction

Answer 13

• AP arrives, causing Depolarisation, causing Ca2+ channels to open so Ca2+ diffuse into the pre-SN
• causes vesicles containing Acetylcholine to fuse with pre-SM and release acetylcholine, which diffuses across the cleft
• Acetylcholine binds to the receptors on the post-SM
• Na+ enters the post-SN, leading to depolarisation.

Question 14

Difference between cholinergic synapse & neuromuscular junction

Answer 14

• Neuromuscular is neurone to muscle
• Neuromuscular has lots of folds, leading to cleft formation
• Neuromuscular has more receptors
• Neuromuscular always triggers an AP

Question 15

How you know synapses are unidirectional

Answer 15

• NT only made in pre-SN

- They bind to receptors that are only found on post-SM

Question 16

Spatial vs temporal summation

Answer 16

Temporal= ONE pre-SN releases neurotransmitters MANY times, rapidly
This causes sufficient neurotransmitters to reach threshold for an AP

Spatial= MANY pre-SN share the same Post-SN.
These release sufficient neurotransmitters to reach the threshold for an AP.

Question 17

Inhibitory synapse

Answer 17

• Inhibitory synapses cause hyperpolarisation in post- SM where K+ channels open and Cl- channels open
• K+ diffuse out of Post-SN & Cl- diffuse in.
• This prevents an AP being formed as a lot more Na+ would need to enter.