membrane potential

Question 1

Resting Membrane Potential (RMP)

Answer 1

-electrical gradient between ECF and ICF

Question 2

What is the reason for the existence of this potential ?

Answer 2

Because of the manner of distribution of different ions across cell membranes

Question 3

Selective permeability of the membrane

Answer 3

-permeability is more for K+.500 to 1000 times
greater than that for Na+.
-The membrane is impermeable to intracellular proteins and organic phosphates.
K+ efflux is faster than the influx of Na+ orCl-.

Question 4

Effect of the polarity of the membrane

Answer 4

An electrical gradient is created in the opposite direction

Question 5

Equilibrium potential

Answer 5

The membrane potential that exactly opposes the concentration gradient of the ion is known as the equilibrium potential or Eion
-The equilibrium potential for K+ is about – 90 mV
-The equilibrium potential for Na+ is positive. About + 60 mV
-The equilibrium potential for Cl- is negative.
About - 70 mV

Question 6

Nernst Equation( for one ion)

Answer 6

E = k log [ion]in / [ion]out
* E – equilibrium potential
* k – constant derived from gas constant, valence of the ion and an electrical constant
* [ion]out – concentration outside
* [ion]in - concentration inside

Question 7

Goldman equation ( for many ions)

Answer 7

E = k1(EK) + k2(ENa) + K3(ECl)
E – actual potential across the membrane
EK, ENa, ECl – equilibrium potentials
K1, K2, K3 – constants derived from the conductances of the ions

Question 8

Maintenance of RMP

Answer 8

Generated mainly by K+ efflux
* Maintained by the Na+-K+-ATPase pumps not constant in all cells due to differences in:
✓ Cell membrane permeability characteristics
✓ Number of Na+-K+-ATPase pumps
✓ Activity of Na+-K+-ATPase
* In all cells it is negative
* –50mV and –90mV

Question 9

Depolarisation:

Answer 9

the membrane potential becomes less negative

Question 10

Hyperpolarisation:

Answer 10

the membrane potential becomes more negative than the resting potential

Question 11

Graded potentials

Answer 11

Local changes in membrane potential is confined to the small, specialized region of the total plasma membrane.

Question 12

Action Potential

Answer 12

brief sequence of changes which occur in the resting membrane potential when stimulated by a threshold stimulus.

Depolarisation due to Na+ influx through voltage gated channels
Repolarisation due to K+ efflux through voltage gated channels
Afterpotential during which Na+ K+ pump restores the ion balance

Question 13

Refractory period

Answer 13

A period during which a new action potential cannot be initiated by normal events in a region that has just undergone an action potential.
* 2 components
✓ Absolute refractory period
✓ Relative refractory period

Question 14

Absolute refractory period

Answer 14

When a particular patch of axonal membrane is undergoing an action potential, it cannot initiate another action potential, however strong the stimulus is.
beacuse inactivation gate of voltage gated sodium channels are closed

Question 15

Relative refractory period

Answer 15

A period during which a second action potential can be produced only by a stimulus stronger than usual.
-During this period the Na+ channels are closed but capable of opening and voltage gated potassium channels are still open
-The membrane potential is more negative
-A larger fraction of the Na channels must be opened to reach the threshold
- a stronger stimulus is to be applied.

Question 16

Action potential – All or none law

Answer 16

Irrespective of the magnitude of the stimulus, once the threshold potential is reached, the events involved in the action potential will be the same – provided the conditions remain the same.

Question 17

Conductivity

Answer 17

the propagation of nerve impulse (action potential) in the form of a wave of depolarization through the nerve fibre
-speed of propagation of an action potential is called the conduction velocity

Question 18

Orthodromic conduction

Answer 18

The action potential is propagated in one direction

Question 19

Antidromic conduction:

Answer 19

The conduction of nerve impulse in the opposite direction,

Question 20

Factors affecting the velocity of nerve conduction

Answer 20

• Diameter of the nerve fiber -greater the diameter of a given nerve fiber, the greater its speed of conduction.(lower resistance)
• Myelination of nerve fibers
• Electrolyte concentrations
• Physical factorsE.g.Temp

Question 21

Advantages of saltatory conduction

Answer 21

-gives a higher conduction velocity
-It conserves energy: Since action potential jumps from node to node, ionic changes associated with action potential occur only at the nodes. the need for enhanced activity of the sodium pump to restore normal ionic concentrations is also reduced. Since sodium pump needs energy, the energy requirement is reduced.

Question 22

Multiple sclerosis

Answer 22

• demyelinating disease of the central nervous system.
• Loss of the myelin sheath around nerves causes a decrease in membrane resistance, which means that current “leaks out” across the membrane during conduction of current.
• Unable to generate an action potential when they reach the next node of Ranvier.

Question 23

classification of mammalian nerve fibres

Answer 23

anki

Question 24

Significance

Answer 24

• Saturday night or Sunday morning paralysis: deep sleep with alcoholic intoxication wherein there is loss of conduction in large-diameter motor, touch, and pressure fibers while pain sensation remains relatively intact.
• Local or regional anesthesia is used to block the conduction of action potentials in sensory and motor nerve fibers.
• Epidural, spinal anesthesia: Application of drugs into the vicinity of a central or peripheral nerve which block the conduction of action potentials.
• Cocaine for having local anesthetic properties.

Question 25

ionic basis of action potential

Answer 25

• membrane is in resting membrane potential
• depolarizing stimulus given
• membrane depolarises to threshold.Voltage-gated Na+ and K+ channels begin to open.
  Rapid Na+ entry depolarizes cell.
  Na+ channels close and slower K+ channels open
• K+ moves from cell to ECF
  -K+ channels remain open and additional K+ leaves cell,hyperpolarizing it
• Voltage-gated K+ channels clos, less K+ leaks out of the cell
• cell return to resting ion permeability and resting membrane potential.

Question 26

Compound action potential

Answer 26

Subthreshold stimulus stimulates none of the axons
* Threshold intensity stimulus makes the axons with low threshold to fire.
* Suprathreshold stimulus, the axons with higher thresholds also fire. The stimulus which excites all the axons is called maximal stimulus.
* Supramaximal stimulus does not produce any further increase in the potential.