Electrical Signalling in Neurons

Question 1

What is the role of neurons ?

Answer 1

It is to integrate and process incoming information and then rapidly transfer information. It does this via generation of electrical signals which are either graded or all-or-nothing

Question 2

Where is the all-or-nothing action potential generated ?

Answer 2

The all-or-nothing action potential is generated at the axon hillock (AH)

Question 3

Graded potentials occur in ?

Answer 3

Specialised regions of synaptic contact with other cells (e.g. dendrites and cell soma) or membrane regions involved in receiving sensory stimuli (e.g. free nerve endings)

Question 4

Different types of neurons have different types of input?

Answer 4

Motor neuron&Interneuron = Synaptic inputs onto dendritic tree
Sensory neuron = Free nerve endings with ion channels that sense heat/touch/chemicals

Question 5

Synaptic inputs can be ?

Answer 5

Excitatory (depolarising) or inhibitory (hyperpolarising). What determines the nature of the synapse is the neurotransmitter used and the type of ligand-gated ion channel it activates

Question 6

Information about strength of stimulus/synaptic input is encoded in ?

Answer 6

The amplitude of the graded local signal

Question 7

A small stimulus causes ?

Answer 7

A small depolarisation of the cell membrane

Question 8

A larger stimulus causes more?

Answer 8

Depolarisation

Question 9

What does a stimulus of longer durations cause ?

Answer 9

A longer lasting depolarisation, but not of any greater strength than the previous stimulus

Question 10

A larger stimulus that depolarises above the threshold may trigger ?

Answer 10

An action potential in a postsynaptic neuron

Question 11

What is Synaptic integration ?

Answer 11

Summation of graded potentials produced by synaptic inputs occurring close together in time and space determine the frequency of firing of an action potential at the axon hillock

Question 12

In the mammalian CNS, excitatory inputs and inhibitory inputs are mediated by ?

Answer 12

Excitatory (EPSP) and Inhibitory (IPSP) post-synaptic potentials. In the mammalian CNS, excitatory inputs are mediated by neurotransmitter glutamate and inhibitory inputs by GABA

Question 13

How can we electrically measure the electrical response of a neuron to a stimulus by ?

Answer 13

Injection of current through an electrode, rather than relying upon a synaptic or sensory input

Question 14

Critical for the generation of the action potential is the following:

Answer 14

• selectivity of ion channels for particular ions
• the high permeation of ions through the pore
• the ion concentration gradients across the plasma membrane

Question 15

What is primary active transport ?

Answer 15

Energy for transport from hydrolysis of adenosine triphosphate (ATP) to ADP

Question 16

What is Secondary active transport ?

Answer 16

Energy for transport from counter movement of another ion down its concentration gradient

Question 17

Ions move both ways through ion channels but ?

Answer 17

Net flux (i.e. the current) is determined by their chemical gradient and by the electrical gradient (the electrochemical gradient), otherwise known as the driving force

Question 18

What are the Eqm potentials Ex (mV) for Sodium and Potassium?

Answer 18

ENa = +67  
EK = -98

Question 19

What is the Equilibrium/Nernst potential?

Answer 19

The potential at which there is no net flux of ions (i.e. influx = efflux, hence current = zero) because the forces on the ion from the chemical gradient and the electrical gradient are equal in magnitude but opposite in direction

Question 20

Many ligand-gated ion channels are non-selective for Na+ and K+ so the reverse potential ?

Answer 20

Erev ~= 0mV

Question 21

What is the difference between the Nernst or equilibrium potential and The reversal potential ?

Answer 21

• The Nernst or equilibrium potential is something you CALCULATE knowing the concentration of ions on either side of the membrane and assuming that the ion channel is completely selective for the ion of interest
• The Reversal potential (Erev ) is something you measure EXPERIMENTALLY and it is the membrane potential at which there is no current flow through the ion channel even though it is open. Ions will be moving in and out, but there is no net flux and hence no current. No ion channel is completely selective and so the Erev for a Na+ channel will be close to ENa but slightly less positive because this channel lets some K+ through

Question 22

What is the driving force acting on the ion ?

Answer 22

The driving force acting on the ion is the difference between the membrane potential (Vm) and the equilibrium potential for the ion (EX); i.e. for K+ channel driving force =( Vm – Ek

Question 23

Membrane permeability to an ion is determined by?

Answer 23

The number of channels that are open as well as the conductance of the channel

Question 24

What is the Permeability or ‘conductance’ of the membrane to an ion (e.g. for K+, gK) is determined by ?

Answer 24

The number of K+ channels that are open (Nk) as well as how permeable these channels are to the ion (γK).

Question 25

Explain the permeability at rest ?

Answer 25

At rest, the membrane is more permeable to K+ than Na+ (because more K+ channels are open) and so resting membrane potential (Vm) is much closer to Ek than ENa. During firing of AP, membrane potential moves closer to ENa (i.e. it becomes positive inside), because for a short time the membrane is more permeable to Na+ than K+. When a channel opens that is non-selective between Na+ and K+, then Vm will go towards 0 mV (Erev)

Question 26

Explain the Gating of Voltage-gated Ion channels?

Answer 26

• All voltage-gated ion channels have at least two gating mechanisms; one gate moves in response to a change in membrane voltage (the activation/deactivation gate) and one moves independently of voltage (the inactivation gate). The inactivation gate can only move to close the pore when the channel is open. Thus inactivation is coupled to activation.
• Ion flux is only permitted in the Open/Activated state

Question 27

Why is the AP an all-or-nothing response?

Answer 27

1. The upstroke of the AP:
   Na+ channels start to open at threshold potential. Although only a small proportion of the channels will open at this membrane potential, the Na + that enters through these open channels will further depolarise the membrane potential and hence more of the channels will become activated, hence allowing more Na+ influx (a feedforward mechanism)
2. The depolarisation phase:
   K+ channels open more slowly but stay open longer. Na+channels rapidly inactivate
3. The after-hyperpolarisation:
   Na+ channels inactive and K+ channels return to deactivated state slowly, so for a period after the AP the Vm is more negative than at rest. This period is the ‘Refractory period’.

Question 28

The stronger the stimulus (in this case the bigger the injected current), the shorter?

Answer 28

The time it takes for the Vm to reach threshold after firing an AP. Hence the frequency of firing of APs increases.