Excitable cells

Question 1

What does electricity allow for

Answer 1

rapid signalling, beating of heart, integration of information, computation, perception, motor action, thought and consciousness

Question 2

What are excitable cells

Answer 2

can be electrically excited to fire action potentials (all-or-none voltage pulses) above some threshold

Question 3

Examples of excitable cells

Answer 3

neurons, cardiac myocytes and skeletal muscle.

Question 4

What are the key roles of excitable cells

Answer 4

medical physiology and pathophysiology

Question 5

How does a potential difference across a membrane arise

Answer 5

1. Passive movement of ions (Permeability of membrane) (Driving voltage (‘force’), down electrochemical gradients (energy used))
2. Active transport of ions (Against concentration and/or electrical gradients) (Requires expenditure of metabolic energy by cell)

Question 6

What is membrane potential

Answer 6

voltage difference across cell membrane (intracellular – extracellular V)

Question 7

What is the resting membrane potential

Answer 7

steady-state membrane potential when there are no electrical inputs to a cell, generated by its own intrinsic electrical properties

Question 8

What is hyperpolarisation

Answer 8

negative change in membrane potential (away from zero)

Question 9

What is depolarisation

Answer 9

positive change in membrane potential (towards zero)

Question 10

How do unequal concentrations arise?

Answer 10

1. Large organic anions produced by cell that cannot cross membrane
2. Active transport mechanism that expend metabolic energy (e.g. the Na+/K+ ATPase pump actively transports Na+ out of the cell and K+ into the cell, powered by ATP hydrolysis)
3. Kidneys and other organs regulate extracellular concentrations homeostatically

Question 11

What does whether a molecule can pass through a cell membrane depend on

Answer 11

molecule size, electrical charge, molecular shape, solubility, etc.

Question 12

Why does membrane permeability differ

Answer 12

Depend on lipids and proteins present and their arrangement; lipid impermeable to ions

Question 13

Cell membranes in their resting state

Answer 13

Fairly readily permeable to K+ and less to Cl- (via ion channels) Poorly permeable to Na+ Impermeable to various large organic anions formed in cells

Question 14

What needs to happen if both chemical and electrical gradients exist at the same time

Answer 14

the concentration gradients needs to be converted to an equivalent electrical gradient

Question 15

What does the Nernst equation tell us

Answer 15

the magnitude of the electrical gradient that would exactly balance a given concentration gradient of a given ion.

Question 16

Nernst equation

Answer 16

Equilibrium potential = - (R x T/ valence of ion x faraday constant) In (concentration inside/conc outside)

Question 17

How can the nernst equation also be written

Answer 17

61 log (concentration inside/concentration outside)

Question 18

Na+ resting potential

Answer 18

Inside - 15 mM
Outside - 150 mM
= +60 mV

Question 19

K+ resting potential

Answer 19

inside - 150 mM
outside - 5mM
= -90 mV

Question 20

Cl- resting potential

Answer 20

Inside - 7mM
outside - 125mM
= -75mV

Question 21

Nernst equation for multiple ions

Answer 21

Vm = FkEk + FcatEcat

(Fk and Fcat are the fractional permeabilities for different ions)

Question 22

How is Fk and Fcat calculated

Answer 22

Gk(Gcat) / Gtotal

Question 23

What properties give rise to the resting membrane potential

Answer 23

1. Unequal distribution of ions across membrane (maintained by Na+/K+ pump)
2. Selective permeability of the cell membrane (PK&raquo_space; PNa)

(K+ in, Na+ out)

Question 24

Ligan gated ion channels

Answer 24

1. Neurotransmitter binds
2. Channel opens
3. Ions flow across membrane

Question 25

G protein-coupled receptors

Answer 25

1. Neurotransmitter binds
2. G protein is activated
3. G-protein subunits or intracellular messengers modulate ion channels
4. Ion channel opens
5. Ions flow across membrane

Question 26

What causes the rising phase of an action potential

Answer 26

Na+ influx

Question 27

What causes the falling phase of an action potential

Answer 27

K+ efflux