Structure and Function - Week 3 Biophysics

Question 1

How do neutrons communicate with each other? (1)

Answer 1

Through miniature electrical impulses

Question 2

Describe Ohm’s law (2)

Answer 2

Describes the relationships between current, voltage, conductance and resistance in a circuit

Check notes week 3 structure and function

Question 3

What kind of membranes do neutrons have? (1)

Answer 3

Semi-permeable

Question 4

Why is there an asymmetrical distribution of ions and charge across the cell membrane (3)

Answer 4

Generated by Ion channels, transporters and ionic pumps

Question 5

What is equilibrium ? (1)

Answer 5

A state of the system where no further changes are possible.

Question 6

What is the Nernst equation? (2)

Answer 6

Check notes week 3

Question 7

Describe the distribution of the major ions in mammalian skeletal muscle (3)

Answer 7

Check notes week 3

Question 8

Contribution of the different ions to the resting membrane potential can be quantified. How? (1)

Answer 8

Goldman-Hodgkin-Katz equation

Question 9

What is the Goldman-Hodgkin-Katz equation? (1)

Answer 9

Check notes week 3

Question 10

What are the type of ion transport proteins (3)

Answer 10

Ion channels
Transporters/exchangers
Ion pumps

Question 11

Describe ion channels (4)

Answer 11

Integral membrane proteins that are selectively permeable to a particular ion species

An important feature of an ion channel is selectivity, e.g. voltage gated sodium channel have permeability to Na+&raquo_space; 10 fold higher than to any other major ion in biological tissue

Conduct small ionic currents along the electrochamical gradient

Can open/close in milliseconds; this underlies rapid changes in membrane potential

Question 12

Describe transporter/exchangers (2)

Answer 12

Integral membrane proteins that transport or exchange ions across plasma membrane usually along the electrochamical gradient by binding/releasing the ion

Do not normally conduct ionic current

Question 13

Describe ion pumps (2)

Answer 13

Integral membrane proteins that transport ions against the electrochemical gradient

This process requires energy and takes up ATP. Examples: Na+, K+ ATPase, PMCA

Question 14

Describe voltage gated ion channel (6)

Answer 14

Voltage-gated ion channels have an ion conducting pore and a gate(s) controlling the pore

The pore has a region called selectivity filter which allows permeable ion to go through much easier than the others ions

The gate is coupled to a voltage sensor, a moveable portion of the protein molecule, that is sensitive to changes in voltage across the membrane. Movement of voltage sensor controls the ion channel gate

When the gate occludes the channel pore, the channel is closed

The gate opens due to a conformational change in the structure of the channel protein brought about by the voltage sensor. When gate opens, ion channel conducts ionic current

Voltage-gated sodium, potassium and calcium channels are important elements of action potential generation

Question 15

Describe structure of voltage gated ion channel (3)

Answer 15

Contain four domains or subunits, each containing 6 transmembrane helices
One subunit of a voltage gated K+ channel (four are needed for functional channel assembly)

Check notes week 3

Question 16

Describe sodium channel gates (4)

Answer 16

The activation gate (m gate) opens rapidly upon depolarisation, and allows Na+ ions to flow

The inactivation gate (h gate) also responds to depolarisation, and plugs the channel pore (preventing ion flow) after a brief delay (up to 5 ms)

The h gate remains in place for a short time. This is called the refractory period, and means that further Na+ influx is not possible

Question 17

Why is the equilibrium potential of cations 0mV? (2)

Answer 17

Some are selective for K and some for Na
Non-selective cationic channels
Some of Na and K inside the cell is approximately equal to the outside
monovalent cations - 0mv - equal concentration on either side of the membrane

Question 18

Describe how activation of K+ channels affects the equilbirum potential of K and the resting membrane potential (3)

Answer 18

Ek: ~ -90 mV
Activation of K+ channels hyperpolarizes the membrane and increases voltage needed to reach the AP firing threshold (inhibition)

Question 19

Describe how activation of Na+ channels affects the equilbirum potential of K and the resting membrane potential (2)

Answer 19

ENa ~ +65 mV
Activation of Na+ channels depolarizes the membrane (excitation)

Question 20

Describe how activation of Ca2+ channels affects the equilbirum potential of K and the resting membrane potential (2)

Answer 20

ECa ~ +110 mV
Activation of Ca2+ channels depolarizes the membrane (excitation)

Question 21

Describe how activation of non-selective cation channels affects the equilbirum potential of K and the resting membrane potential (2)

Answer 21

Ecat ~ 0 mV
Activation of non-selective cation channels (these cannot discriminate between small cations) depolarizes the membrane (excitation)

Question 22

Describe how activation of Cl- channels affects the equilbirum potential of K and the resting membrane potential (2)

Answer 22

ECl varies dramatically but in most CNS neurons is ~ -90 mV
Activation of Cl- channels in these neurons is inhibitory

Question 23

Give the equation for the driving force of ions? (1)

Answer 23

Check notes for week 3

Question 24

What is reversal potential? (1)

Answer 24

The membrane potential at which the direction of current flow reverses

Question 25

What is a voltage-clamp experiment? (2)

Answer 25

In a voltage-clamp experiment the voltage is controlled and the current across the membrane is recorded.

Voltage clamp provides a unique tool to directly test properties of active ion channels

Question 26

What is a current-clamp experiment? (2)

Answer 26

In a current-clamp experiment one controls the current flowing through the cell membrane and record voltage responses

Such experiment can mimic action potentials and synaptic input

Question 27

Describe the role of voltage gated sodium channels in action potential (5)

Answer 27

open upon membrane depolarisation (e.g. excitatory synaptic input), causing an influx of Na+ ions into the cell.

This influx causes further depolarisa-
tion, and thus more Na+ channels to
open.

This influx of sodium drives the
membrane potential (Vm) towards (and
above) zero mV (towards ENa).

The explosive influx of Na+ ions only
occurs if the membrane potential exceeds
the threshold potential (typically above -55 - -50 mV) at which voltage gated Na+ channels start to activate

Na+ channels inactivate rapidly. While Na+ channels are in the inactivated state, no new action potential is possible, even in response to depolarization. This silent phase is called an absolute refractory period

Question 28

Describe the role of voltage-gated K+ (Kv) channels in the action potential (3)

Answer 28

Look at notes week 3

Question 29

Describe calcium activated potassium channels (3)

Answer 29

Look at notes week 3

Question 30

In CNS neurons action potentials are usually generated where? (1)

Answer 30

In CNS neurons action potentials are usually generated in the Axon Initial Segment (AIS)

Question 31

Oligodendrocytes myelinate which axons? (1)

Answer 31

CNS axons

Question 32

Schwann Cells myelinate which axons? (1)

Answer 32

Peripheral