Topic 2 - Cellular excitability

Question 1

Electrical signals can be action potentials or graded potentials. What is the difference?

Answer 1

Action potentials - are of fixed size - coded by frequency:

• all or nothing signals that travel along axon.
• can pass either way along axon

Graded potentials - are variable in size - coded by strength of stimulus.

• local signals not over long distances
• pass both ways along neuronal membrane
• can be excitatory or inhibitory
• graded potentials can summate = addinfg esps
• They are caused by (1) opening of neurotransmitter-gated ion channels or (2) opening/closing of potassium channels.

Question 2

Why do neurons have a resting potential?

Answer 2

There is a selectively permeable membrane (due to channels) and an unequal distribution of charged molecules/ions (due to pumps).

Question 3

what is the Nerst equation used for?

Answer 3

used to calculate the equilibrium potential for an ion.

• At rest the neuronal membrane is very permeable to K+ due to open K+ channels.
• At rest the real membrane potential is close to but not at EK.
• Increasing extracellular [K+] causes membrane potential to ‘depolarize’

Question 4

What is the Goldman Equation?

Answer 4

Used to estimate the real Vm - Goldman equation takes other ions into account.

Question 5

Explain the characteristics of an action potential:

1. resting potential
2. Rising phase
3. Overshoot
4. Falling phase
5. Undershoot

Answer 5

• Threshold - sufficient voltage-gated Na+ channels open so that permeability to Na+&raquo_space;K+
• Rising phase - rapid depolarization caused by large force drives Na+ into the neurone
• Overshoot - Vm approaches ENa
• Falling Phase - voltage-gated Na+ channels inactivate, voltage-gated K+ channels open, large force drives K+ out of neurone
• Undershoot - voltage-gated K+ channels (delayed rectifiers) add to resting K+ membrane permeability and reduced Na+ permeability so Vm ~ EK

Question 6

What are 3 factors which affect the velocity of an action potential?

Answer 6

1. permeability
2. diameter - bigger area of axon reduces resistance to current flow.
3. myelination - prevents loss of axon by increased Rm (membrane resistance) and increases space constant (distance from site of depolarisation where it has fallen to 37%).

Question 7

What is the absolute refractory period and relative refractory period?

Answer 7

The ABSOLUTE REFRACTORY PERIOD ~1 ms no matter what you do, the neuron is incapable of generating another action potential…this is often followed by

RELATIVE REFRACTORY PERIOD (a few more ms) during which you can fire another action potential, but you would require a stronger stimulus because the ‘threshold’ is raised.