L13 Review of Neuron Properties

Question 1

Is the plasma membrane permeable to ions?

Answer 1

No, the plasma membrane is impermeable to the passage of ions.

Question 2

What are ion channels?

Answer 2

Large, multi-domain proteins that span the width of the cell membrane, allow the flow of ions across the plasma membrane.
Binding of specific ligands, e.g. neurotransmitters, to ion channels can change the conformation of the channel and induce it to open.

Question 3

Is the ion channel composed of charged or uncharged amino acids?

Answer 3

• The membrane spanning domain contains AAs with no charge

- The domains on the outside of the ion channel (in contact with water) contain charged AAs

Question 4

What type of gradient controls the flow of ions through an open ion channel?

Answer 4

Electrochemical gradient: the difference in charge and chemical concentration across a membrane

Question 5

How is the sodium and potassium concentration difference generated?

Answer 5

• ATPase sodium potassium pump
• Pumps potassium ions into neurons, pumps sodium out of neurons
• Neurons contain twenty time more potassium ions than ECF, ECF contains ten times more sodium than neuronal cytoplasm
• Pump requires hydrolysis of ATP to ADP

Question 6

Define equilbrium potential.

Answer 6

The equilibrium potential of an ion is the electrical potential difference across the cell membrane that exactly balances the concentration gradient for that ion if the membrane is permeable only to that ion.

Question 7

How is equilbrium potential calculated?

Answer 7

Using the Nernst equation

Question 8

What is the equilibrium potential for potassium and sodium at 37 degrees?

Answer 8

Potassium: -90.9mV
Sodium: +60.6mV

Question 9

What generates the resting potential of neurons?

Answer 9

The differential permeability of sodium and potassium voltage-gated ion channel in resting neurons generates the resting potential.

Question 10

Describe the generation of the resting potential in neurons.

Answer 10

• Potassium ions diffuse out of the neuron through voltage gated channels and potassium/chloride co-transporters
• Potassium ions diffuse out of the neuron down their conc gradient until electrical forces lead to an equilibrium of potassium ion movement into and out of the neuron
• The inside of the neuron plasma membrane becomes negatively charged compared to the outside

The potential difference across the membrane is -65mV

Question 11

What is the resting membrane potential of a neuron?

Answer 11

-65mV

Question 12

What role do calcium ions play in regulating neuronal excitability and function?

Answer 12

• Calcium is pumped out of neurons
• Intracellular calcium binding proteins sequester free calcium
• Intracellular calcium stores reduce cytosolic calcium levels further

Therefore, calcium is present at 10,000 fold higher concentrations in the ECF compared to inside neurons.

• The opening of voltage or ligand gated calcium channels creates a calcium influx and depolarisation.

Question 13

What role do chloride ions play in regulating neuronal excitability and function?

Answer 13

• Chloride is co-transported out of neurons alongside potassium ions via KCC2, KCC3,KCC4

Therefore, chloride is present at 11.5 fold higher concentrations in ECF compared to inside neurons.

• The opening of ligand gated chloride channels creates chloride influx and hyperpolarisation of neurons.

Question 14

What is an action potential?

Answer 14

A rapid, transient reversal of the resting potential.

Used to convey information over distance in the nervous system.

Question 15

Describe the rising phase of an AP.

Answer 15

• Inititation of depolarisation: sodium or calcium influx through gated ion channels or direct activation of nocicpetor terminals creates a depolarisation
• If this depolarisation reaches the critical threshold, voltage gated sodium channels will rapidly open, massive influx of sodium into neurons, rapid increase in membrane potential (+30/40mV)

Question 16

Describe the falling phase of an AP.

Answer 16

• At the apex of the overshoot, the voltage gated sodium channels close, and potassium channels open
• Inward sodium current stopped, outward potassium current initiated
• Causes rapid repolarisation
• Falling phase is follow by an undershoot: a period of hyperpolarisation

Hyperpolarisation is followed by gradual restoration of membrane to resting potential.

Question 17

What determines the shape of the action potential?

Answer 17

The kinetics of the ion channel opening.

Question 18

What channel closing creates the hyperpolarised undershoot?

Answer 18

Slow kinetics of volatge gated potassium channel closing creates the hyperpolarised undershoot.

Question 19

How is the AP conducted along non-myelinated axons?

Answer 19

• The massive influx of sodium ions in the rising phase of the AP diffuses laterally causing depolarisation of the plasma membrane immediately in front of the rising AP
• When this depolarisation meets another cluster of voltage gated sodium channels it tirggers a second AP
• Repeats until AP moves along entrie length of axon

Question 20

What is saltatory conduction?

Answer 20

The movement of APs from node to node along the full length of a myelinated axon.

Question 21

Which cells produce the myelin sheath?

Answer 21

• Schwann cells in peripheral NS

- Oligodendrocytes in the CNS

Question 22

What are the gaps between the myelin sheath called?

Answer 22

Nodes of Ranvier

Question 23

Describe saltatory conduction.

Answer 23

• Nodes of Ranvier have high concentrations of sodium and potassium voltage gated ion channels
• APs fired and sodium floods into axon at nodes
• Sodium current flows quickly along the inside of the axon
• Voltage-gated sodium channels at the next node are triggered to open by the depolarising internal sodium current

Question 24

What is nerve fibre conduction velocity proportional to?

Answer 24

Proportional to:

• Nerve fibre diameter
• Thickness of the mylein sheath

Question 25

What are the 4 categories of nerve fibre?

Answer 25

• Aα fibre: largest diameter, heavily myelinated, 80-120m/sec
• Aβ fibre: next largest, moderately myelinated, 35-75m/sec
• A𝛿 fibre: second smallest, lightly myelinated, 5-35m/sec
• C-fibre: smallest, non-myelinated, 0.5-2m/sec

Question 26

What is the effect on AP conduction when axon diameter is reduced?

Answer 26

When axon diameter is narrow, the sodium current cannot travel as quickly, therefore the AP conduction speed is lowered.

Question 27

Describe chemical synapses.

Answer 27

• Axon swells to form bouton
• In the axon shaft there are voltage gated sodium channels
• In the bouton the main depolarising ion channel is coltage gated calcium channels
• Calcium entry causes synaptic vesicle exocytosis and release of neurotransmitter into the synaptic cleft
• Post-synaptic neurotransmitter gated ion channels either depolarise post syanptic neurons (excitatory neurotransmitters) or hyperpolarise them (inhibitory neurotransmitters)

Question 28

What are the 2 types of neurotransmitter receptors?

Answer 28

• Ionotropic

- Metabotropic

Question 29

Describe ionotropic neurotransmitter receptors.

Answer 29

• Ligand gated ion channels
• When neurotransmitter binds, they undergo a conformational change in shape leading to an open ion pore in the centre
• Give rise to rapid, short-lived postsynaptic effects
• E.g. ligand-gated sodium channels, chloride channels, cation channels

Question 30

Describe metabotropic neurotransmitter receptors.

Answer 30

• Ions do not actually pass through the receptor protein
• Activation initiates an intracellular signalling cascade
• Often GPCRs
• When receptor activated, G-protein undergoes conformational change in shape, initiates signalling cascade, can alter gene expression, protein function and modulate ion channel gating
• E.g. adrenergic receptors, metabotropic glutamate receptors