The Action Potential

Question 1

What is the membrane potential

Answer 1

More sodium and chlorine outside, more potassium inside
Potential is about -70mV inside than outside due to negatively charged proteins and organics
Sodium-potassium pump maintains this

Question 2

Describe action potential transmission of nerve impulse

Answer 2

1) Stimulus lets some sodium into post-synaptic neuron causing a little depolarisation e.g. neurotransmitter
2) If threshold reached (-50 mvV), voltaged gated Na+ channels open, Na+ floods in, cell depolarises
3) At +30mV, Na+ channels close and voltage gated K+ channels open, K+ leaves causing repolarisation
4) K+ channels close slowly; hyperpolarisation
5) Hyperpolarisation causes relative refactory period, as a stronger than usual signal is required e.g. more sodium
6) Depolarisation of a patch of neuron by sodium causes adjacent Na+ channels to open, refractory period ensures one directional flow
7) Sodium-potassium pump restores to resting potential

Sodium channels closing causes absolute refactory; action potential cannot be generated

Question 3

What is a myelin sheath ?

Answer 3

Modified cell membrane structure that wraps around nerve axons in a spiral pattern
-Composed of proteins and fats; high cholesterol
-An electrical insulator: electrical impulses can rapidly and efficiently transmit, without losing strength
-Myelenation increases conduction velocity

Question 4

What is a disease which damages myelin sheaths ?

Answer 4

Multiple Sclerosis is an autoimmune disease that damages the myelin sheath: disrupts the transmission of signals through the affected nerves

Question 5

How does myelination increase conduction velocity ?

Answer 5

Action potential can “leapfrog” down the axon going from one Node of Ranvier to the next: much faster and allows long distance rapid communication.
-Sodium opens sodium channels by depolaristion further down, instead of right next to it
(Na+ enters at node, pos charge pushes na already in axon to next node where na+ channels are opened)

Question 6

How can treating hyponatraemia be dangerous ?

Answer 6

Correction via electrolyte supplements is dependent upon the time course of development
-Rapid correction of >48 hour-developed hyponatremia can cause profound changes in osmolality in the brain; change in neuron cell volumes and a detachment from their myelin sheaths (myelinolysis) - uusally fatal
-Less salt outside cell, water wants to go into cell, cell removes salts etc to stop this, adding salt means water wants to move out of cell, cell has no salts to make water want it

>48 hour-developed = took more than 2 days to become hyponatraemic

Question 7

How do nociceptive sensory neurons transmit pain signals ?

Answer 7

Transient receptor potential channels open in response to thermal changes and stimuli that evoke pain, causing Na+ and Ca2+ influx

Are psudeounipolar (check)

These channels are “cellular pain sensors”: they detect heat, cold, chemicals, and mechanical forces; they open in resposne to things which cause pain

Question 8

How does lidocaine work ?

Answer 8

Lidocaine is a topical anaesthetic that blocks voltage gated Na+ channels on axon of nociceptive sensory neurons
-prevents transmission and propogation