Membrane physiology

Question 1

Why is the cell membrane impermeable to ions?

Answer 1

• seperates intracellular and extracellular comartments which have very different ionic compositions
• ion pumps mantain gradients

Question 2

What is the membrane potential and why is it negative?

Answer 2

The membrane potential is the voltage inside a cell

• The inside of a cell contains slight excess of anions (lack of positive charge)
• in neurones, this may be -65mV
• balance of charges determines value of Em
  *

Question 3

How are ion channels classified and what are the three main types?

Answer 3

Classification is based on

• permeant ion: Na⁺, K⁺, Ca²⁺, Cl^-, cation(Na⁺/K⁺)
• Gating mechanism
  
  • non gated(leak)- set E_m of resting membrane
  • Voltage gated- generate action potentials
  • Ligand gated- generate E_m changes at synapse

Question 4

What affects the ion movements across resting membrane?

Answer 4

• Presence of ion channels
  
  • Non gated K⁺ and Na⁺ channels
  • The membrane has a permeability to each ion
  • The permeability is partly dependent on the number of ion channels
• Factors that influence the flux
  
  • chemical gradient- unequal ion idstribution
    
    • K⁺ concentration high inside- drive to leave cell- efflux
    • Na⁺ concentration high outside- drive to enter cell- influx
  • Electrical force- ions attracted/repelled by E_m- voltage inside cell
  • E_m negative, drive for K⁺ and Na⁺ to move in

Question 5

What happens in this hypothetical scenario

1. membrane is permeable to K⁺only
2. membrane potential is initially 0mV

Answer 5

• Chemical gradient would lead to K⁺ efflux
• This leads to a loss of positive charge inside the cell
• This creates a negative membrane potential which sets up an electrical force
• electrical force leads to K⁺influx
• initially K⁺efflux> K⁺influx
• K⁺ continues to leave the cells and electrical force continues to get stronger
• At a sufficiently negative membrane potential, the chemical and electrical influences are balanced
• no net K⁺ movement

Question 6

What happens in this hypothetical scenario

1. membrane is permeable to Na⁺ only
2. membrane poteintial is initially 0mV

Answer 6

• Chemical gradient would lead to Na⁺ influx
• membrane potenital becomes more positive
• Electrical force leads to Na⁺ efflux
• Initially Na⁺ influx>Na⁺ efflux
• When membrane potential reaches a sufficiently positive value, chemical and electrical influences are balanced
• There is no net Na⁺ movement

Question 7

Why is the chemical gradient constant in a resting membrane ?

Answer 7

The changes in concentrations are extremely small

Question 8

What is the Nernst equation ?

Answer 8

The Nernst equation defines the voltage in which ion is in equilibrium and the chemical and electrical force are in balance

Question 9

What is the Ionic Driving Force?

Answer 9

Net force resulting from chemicl and electrical influences

Driving force >0 whenever membrane potential is different from equilibrium potntial for the ion

Question 10

E_m is -65mV, what is the effect of this on ion movements?

Answer 10

• E_K = -80mV, E_m doesnt equal to E_K, the influences on K⁺movements are unequal
  
  • at -65mV, chemical influence> electrical influence
  • K⁺efflux- trying to bring E_m to -80mV
• ENa= +62mV, E_m doesnt equal to ENa​
  
  • at -65mV, chemical and electrical influences both cause Na⁺influx
  • Na⁺ influx- trying to bring E_m to +62mV
• E_m is at rest, Na⁺ influx =K⁺efflux

Question 11

What is the actual value of membrane potential dependent on?

Answer 11

Relative permeablities

There is a large excess of K⁺ leak ion channels

At rest P_K= 40x P_Na

E_m is therefore a lot closer to E_K

Question 12

How is the ionic gradient maintianed

Answer 12

• Ion pumps
  
  • sodium potasium pump
    
    • enzyme - catalyses ATP breakdown
    • provides energy to exchange internal Na⁺ for external K⁺
    • This is done against concentration gradient
  • Calcium pump- transports Ca²⁺ out of the cell
• operate in the backgound continously
  
  • over long time periods, the constant efflux of K⁺ will lead to significant changes in the concentration, which will lead to depolarisation of the membrane
  • pumps maintian concentrations over long term