Electrical Excitability

Question 1

Axon AP

Answer 1

Y-axis = -70mV -> +30 mV
Time = 0.5ms (for the peak)

Question 2

Skeletal muscle AP

Answer 2

Y-axis = -90mV -> +40 mV
Time = 0.5 ms (for the peak)

Question 3

SAN AP

Answer 3

Y-axis = -60 mV -> +30 mV
Time = 100 ms (for the peak)

Question 4

Cardiac ventricle AP

Answer 4

Y-axis = -90mV -> +30mV 
Time = 100ms (for the rise and plateau)

Question 5

What happens during the upstroke of an AP?

Answer 5

Depolarisation to threshold

Na+ channels open -> Na+ enters cell -> membrane depolarises -> opens more Na+ channels etc. (+ve feedback)

Question 6

What happens during the downstroke of an AP?

Answer 6

Repolarisation in two parts
Maintained depolarisation causes Na+ channels to inactivate -> Na+ influx stopped -> repolarisation
Depolarisation causes K+ channels to open -> K+ efflux -> repolarisation

Question 7

Absolute refractory period

Answer 7

Nearly all the Na+ channels are inactivated, an AP cannot be fired no matter how large the stimulation

Question 8

Relative refractory period

Answer 8

Na+ channels are recovering, excitability returns to normal as number of inactivated channels decreases, an AP can be fired if a stimulus large enough is applied

Question 9

Local anaesthetics

Answer 9

Bind and block Na+ channels -> stopping AP generation
Block conduction in the following order…
1. Small myelinated axons
2. Non-myelinated axons
3. Large myelinated axons

Question 10

Conduction velocity

Answer 10

Distance
_______

Time

Question 11

How is the AP conducted along an axon?

Answer 11

A change in membrane potential in one part can spread to adjacent areas of the axon
This occurs because of local current spread
Conduction velocity is determined by how far along the axon the local currents spread
When the local current causes depolarisation of part of the axon to threshold the AP is initiated in that location

Question 12

Properties that lead to a high conduction velocity

Answer 12

• High membrane resistance
• Low membrane capacitance (ability to store charge)
• Large axon diameter

Question 13

Myelin sheath

Answer 13

Increases conduction velocity
Large diameter axons are myelinated (motorneurones)
Reduces capacitance and increases membrane resistance
Formed by Schwann cells (peripheral axons) and oligodendrocytes (CNS axons)

Question 14

Saltatory conductance

Answer 14

Internodal region has reduced capacitance
Local axonal current induced by AP at a Node of Ranvier spreads further down the axon to depolarise next Node without firing an AP in the internodal region
Conduction occurs in a saltatory or jumping manner down the nerve -> greatly increasing conduction velocity

Question 15

Fibre diameter and conduction velocity

Answer 15

Myelinated: velocity proportional to diameter (max = 120m/s for mammalian motorneurones
Non-myelinated: velocity proportional to root diameter (max 20m/s)

Question 16

Demyelination

Answer 16

Some axons lose their myelin sheath
Multiple sclerosis - disease of the immune system where myelin is destroyed in certain areas of the CNS -> dramatic effects on previously myelinated axons to conduct APs -> decreased conduction velocity, complete block or only some APs being transmitted