Lecture 32:

Question 1

An excitable cell is one that responds to external stimuli through a rapid and reversible alteration in membrane potential.
The external stimuli can be either:

Answer 1

• mechanical
• thermal
• chemical or
• light

Question 2

What is the typical membrane potential of a cell?

Answer 2

-60 to -90 mV (negative on inside of the cell)

Question 3

Excitable cells differ from their non-excitable counterparts by their ability to use:

Answer 3

Changes in this potential as a means of communication

Question 4

How is this resting membrane potential developed?

Answer 4

• have high K+ inside and low Na+ inside, which is a result of the activity of the Na+/K+ pump (an active transport process)
• there is a difference in stoichiometry -> 3 Na+ are pumped out for every 2K+ pumped into the cell
• the rate of leakage of Na+ into the cell is far slower than for K+ out of the cell
• therefore there is a great flow of K+ out than Na+ in, resulting in an internal negative potential
• In addition the internal anions (proteins, ATP, nucleic acids) are impermeable to the cell membrane whereas external Cl- are not.

Question 5

What happens when a nerve cell transmits an impulse?

Answer 5

When a nerve is stimulated by another nerve or by external stimuli there is a resultant local depolarisation. This is the result if a marked increase in the permeability of the membrane to Na+.

• the result is reverse polarisation (the inside us now more positive than the outside)
• at this point the membrane permeability to Na+ returns to normal and permeability to K+ increases
• K+ flows out along a concentration gradient and restores the potential to its resting state. The Na/ K+ pump now restores the ion concentration back to their standing potential
• these changes in membrane potential is called ACTION POTENTIAL
• the local depolarisation causes permeability changes in the adjacent area moving the action potential down the axon- a nerve impulse

Question 6

How is the nerve impulse transferred to the next neuron or to an effector cell?

Answer 6

Transmission across a synapse is achieved by chemical signal molecules called neurotransmitters, such as serotonin, adrenalin, or acetylcholine.
These are the events that take place:
-when action potential reaches end of axon, acetylcholine is released from the synaptic vesicles
-acetylcholine diffuses across the synaptic gap
-acetylcholine binds to specific receptors in the post synaptic membrane.
-this causes the opening of an ion channel triggering a marked increase in Na+ entry and initiates an action potential in the adjacent neurone or effector cell
-acetylcholine dissociates from receptors and is hydrolysed by cholinesterase
-the products, acetate and choline, can diffuse back to the presynaptic axon and regenerate acetylcholine.