Neuronal Excitability

Question 1

What are the 3 parts of a neuron?

Answer 1

• Dendrites
• Cell body
• Axon

Question 2

What is the axon?

Answer 2

• Impulses travel along it

- Can be long or short

Question 3

What is in the cell body of the neuron?

Answer 3

All the organelles

Question 4

What are the roles of dendrites?

Answer 4

• Branch off the neuron and receive signals from other neurons via neurotransmitters
• Generate action potentials
• Neurons use action potentials to signal within a cell and communicate with other cells using neurotransmitters
• Convert chemical signals into electrical signals

Question 5

How do many synapses within a dendrite generate an action potential?

Answer 5

A single neurone can have many dendrites with many synapses which when activated provide a combined effect changing the overall charge in the cell and therefore generating an action potential which is an electrical signal that travels down the axon triggering the release of neurotransmitter at the other end of the neuron and further relaying the signal

Question 6

Why do neurones have an electrical charge?

Answer 6

Neurons have an electrical charge because of different concentrations of ions internally and externally to the cell. Distribution of the ions causes the cell to have a net negative charge of around -65 mV relative to the outside environment. This is the neurons resting membrane potential

Question 7

What are the relative ion concentrations inside and outside of the cell?

Answer 7

• Na+, Cl-, Ca2+ ions in higher concentration outside the cell
• K+ and negatively charged ions in higher concentration inside the cell

Question 8

What happens when a neurotransmitter binds to a receptor on the post synaptic membrane?

Answer 8

ligand-gated ion channels open to allow certain ions to flow into and out of the cell

Question 9

What happens when a ligand-gated sodium channel opens?

Answer 9

Na+ can enter the cell increasing the electrical potential within the cell decreasing the polarity of the cell (depolarisation)

Question 10

What happens if there is a net influx of positive charge in the cell?

Answer 10

it is an excitatory post-synaptic potential (depolarisation)

Question 11

What happens if there is a net influx of negative charge in the cell?

Answer 11

it is an inhibatory post-synaptic potential (hyperpolarisation)

Question 12

What happens if there is a single excitatory or inhibitory post synaptic potential?

Answer 12

Only a small change to the resting membrane potential. Threshold not met

Question 13

What happens if there are multiple excitatory or inhibitory post synaptic potentials from multiple sites on the dendrites?

Answer 13

If there are enough excitatory or inhibitory post synaptic potentials from multiple sites on the dendrites then collectively they can push the membrane potential to a specific threshold potential which is typically around -55mV and an action potential is generated

Question 14

What happens with the threshold potential is met?

Answer 14

• Voltage-gated Na+ channels are opened and there is a large influx of positive charge into the cell. The change in voltage and the opening of Na+ channels causes nearby voltage-gated ion channels to open and the action potential travels along the axon (depolarisation)
• The neuron becomes +vely charged relative to the external environment (around +40 mV) at this voltage voltage-gated Na+ channels are inactivated blocking channels so that no more Na+ can enter. The ion channel remains in this state until the cell repolarises and the gates become closed
• Potassium voltage-gated channels open once the Na+ channels become inactivated and K+ leaves the cell lowering the membrane potential
• Sodium potassium pump (active) moves sodium out of the cell and potassium into the cell (repolarisation) the neurone is in its absolute refractory period as the sodium channels have been inactivated and wont respond to any amount of stimuli
• The action of the sodium potassium pump and the open potassium channels results in hyperpolarisation (membrane potential around -75mv) causing the Na+ channels to close
• As potassium channels close the neuron returns to its resting membrane potential

Question 15

What is the absolute refractory period?

Answer 15

• Sodium channels have been inactivated and wont respond to any amount of stimuli.
• The absolute refractory period prevents action potentials being generated too closely and maintains the movement of the action potential in one direction

Question 16

What is the refractory period?

Answer 16

Na+ channels are closed not inactivated so can be reactivated but the potassium channels remain open during hyperpolarisation so depolarisation during the refractory period requires a stronger stimulus

Question 17

What is the threshold potential?

Answer 17

• The level of membrane potential at which graded depolarisations become an action potential is known as the threshold level
• Different neuronal cell types have different threshold levels
• This feature can be used to distinguish between different neuronal cell-types

Question 18

What does the nearst equation allow us to calculate?

Answer 18

Calculate the membrane potential at equilibrium for each ion with known concentrations of ion within extra and intra cellular

Question 19

Propagation of Action Potentials Along Axons for unmyelinated axons

Answer 19

Continuous Conduction

Question 20

Propagation of Action Potentials Along Axons for myelinated axons

Answer 20

Saltatory Conduction