L8: Membrane potentials and resting membrane potentials

Question 1

What are the types of membrane potentials?

Answer 1

• RMP
• GRADED POTENTIAL
• ACTION POTENTIALv

Question 2

What is the graded potential?

Answer 2

• A local change in the resting membrane potential as a result of stimulation by an ineffective stimulus.
• The duration and magnitude of these changes are variable according to the stimulus

Question 3

What are examples of graded potential?

Answer 3

RSPML

a) Receptor potential, at the beginning of the sensory nerves.
b) Synaptic potential, in the synapses inside the CNS.
c) Pacemaker potential, in the heart.
d) Miniature endplate potential, at the neuromuscular junction.
e) Local excitatory state, at the cell bodies of neuron

Question 4

What is action potential?

Answer 4

• It is the electrical changes that occur in resting membrane potential as a result of stimulation by an effective stimulus
• These changes propagate along the nerve fibers and produce responses or actions.

Question 5

What is RMP?

Answer 5

It is the potential difference between the outside and inside of the nerve fiber during rest.

Question 6

Where is RMP present?

Answer 6

RMP is present in nerves, muscle fibers, and all cells of the body with the inside negative relative to outside.

Question 7

What are the values of RMP?

Answer 7

It varies according to the excitable tissue:

Nerves
-70 mV.
Skeletal muscles
-90 mV
Cardiac muscle
-90 mV
Smooth muscles
-60 mV

Question 8

How is RMP measured?

Answer 8

• By two microelectrodes connected with a special voltmeter.

- One of these microelectrodes is inserted inside the nerve fiber.

Question 9

What are the causes of RMP?

Answer 9

RMP is due to unequal distribution of ions on both sides of the cell membrane with excess cations outside and excess anions inside, which is produced as a result of 2 main factors:

(a) Selective permeability of cell membranes.
(b) Na+- K+ pump

Question 10

What are the substances that pass the membrane by selective permeability?

Answer 10

• proteins
• K+
• Na+
• Cl-

Question 11

Selective permeability of proteins

Answer 11

Cell membranes are impermeable to intracellular protein anions (because of their large size).

Question 12

Selective permeability of K+

Answer 12

K+ ions diffuse from inside the cell to outside due to:

1) Concentration gradient: (inside 30-40 times > outside).
2) High permeability of the membrane to K ions (their permeability to K+ is 50-100 times greater than their permeability to Na+).

 this is limited due to:

a) The electrical gradient is directed inwards.
b) Positive charge on the outside of membrane repels K+ ions inwards.
c) Na+- K+ pump actively drives K+ ions inwards.

Question 13

Selective permeability of Na+

Answer 13

Na+ ions tend to diffuse from outside to inside due to:

a) Concentration gradient (outside > inside 10 times)
b) Electric gradient (inside is negative).

But this is limited due to the low permeability of the cell membrane to Na+, Therefore, Na+ ions accumulate outside the membrane, helped by the Na+-K+ pump producing a positive charge.

Question 14

Selective permeability of Cl-

Answer 14

Cl-ions tend to diffuse from outside to inside due to: Concentration gradient (outside > inside 25 times).

However, this is limited because the interior of the cell has a great negative charge and therefore, they are expelled out of the cell.

Question 15

What is the concentration of ions inside and outside of the cell?

Answer 15

o ECF:ICF ratio for Na+ions is 10:1
o ECF: ICF ratio for K+ ions is 1:35
o ECF: ICF ratio for Cl-ions is 25:1

Question 16

What are the three basic types of ion channels?

Answer 16

1) Passive ion channels: the movement of ions through these channels is the cause of RMP.
2) Chemically activated ion channels.
3) Voltage-activated ion channels, open when they detect a certain voltage. They are responsible for the generation of action potential

Question 17

Na+-K+ Pump

Answer 17

 Na+ ions are not allowed to accumulate inside the nerves.

 They are pumped out by an active process (since it occurs against concentration and electrical gradients) which needs energy (provided from the breakdown of ATP by Na+-K ATPase enzyme).

 It helps to maintain the RMP since it pumps 3 Na+ ions outside the cells and 2 K+ ions inside the cell which produces a net movement of positive charges
out of the cell.

Question 18

What does Na- K Pump have?

Answer 18

a) ATPase property i.e. has the ability to split ATP.
b) 3 Na+ binding sites → at ICF side.
c) 2 K+ binding sites → at ECF side.
d) ATP binding sites → at ICF side.

Question 19

What happens during equilibrium regarding the membrane potential?

Answer 19

At equilibrium and as a result of selective permeability of cell membranes and Na+- K+ pump, the outer surface of the nerve fibers will be positively charged (mainly due to Na) while inner surfaces will be negatively charged (mainly due to protein) producing a potential difference of about -70 mV (which is the RMP).

Question 20

What happens if a condition decreases metabolic activity like cooling?

Answer 20

inhibits Na-K pump, So, Na+ ions will accumulate inside the cell and neutralize the -ve charges of protein ions, and K ions that are held on the outer surface escape away, and RMP becomes progressively lost.