Session 5: Action Potentials

Question 1

What are ion channels?

Answer 1

Proteins that enable ions to cross cell membranes. Have an aqueous pore through which ions flow via diffusion.

Question 2

What are some ion channel properties?

Answer 2

• Selectivity = for one (or few) ion species e.g., Na+, K+, Ca2+, Cl- cation channels
• Gating = the pore can open or close by a conformational change in the protein
• Rapid ion flow = always down the electrochemical gradient.

Question 3

What makes cell membranes selectively permeable to ions?

Answer 3

The ion selectivity and types of channels that are open.

Question 4

What is the resting membrane potential?

Answer 4

The difference in electrical charge across the membrane at rest (membrane potential under resting conditions).

Question 5

What provides the basis of signalling in the nervous system?

Answer 5

Changes in the membrane potential.

Question 6

What are the main factors influencing resting potential?

Answer 6

• Ionic permeability of membrane
• Ionic concentrations on either side of membrane
• Potassium is the main ion affecting the resting membrane potential so =
  A) K+ concentration gradient
  B) Selective permeability of membrane to K+ ions

Question 7

What is the resting potential of a neuron?

Answer 7

-70mV

Question 8

Where do changes in membrane potential help cells send signals to each other or within themselves?

Answer 8

1) Action potentials in nerve + muscle cells
2) Triggering and control of muscle contraction
3) Control of secretion of hormones + neurotransmitters
4) Transduction of sensory information into electrical activity by receptors
5) Postsynaptic actions of fast synaptic transmitters

Question 9

What is depolarisation?

Answer 9

An increase in the membrane potential so that the cell interior becomes LESS negative (more positive) e.g., a change from -70 mV to -50 mV.

Question 10

What is hyperpolarisation?

Answer 10

A decrease in the membrane potential so that the cell interior becomes MORE negative. E.g., a change from -70 mV to -90mV.

Question 11

What causes hyperpolarisation?

Answer 11

Opening K+ or Cl- channels.

Question 12

What causes depolarisation?

Answer 12

Opening Na+ or Ca2+ channels.

Question 13

What causes changes in membrane potential?

Answer 13

Changes in the activity of ion channels.

Question 14

Name the three types of gated ion channels.

Answer 14

1. Ligand gating = channel opens/closes in response to binding of chemical ligand
2. ** Voltage gating** = channel opens/closes in response to changes in membrane potential
3. Mechanical gating = channel opens/closes in response to membrane deformation.

Question 15

What is the resting potential of cardiac muscle?

Answer 15

-80mV

Question 16

What is the resting potential of smooth muscle?

Answer 16

-50mV

Question 17

What is the resting potential of skeletal muscle?

Answer 17

-90mV

Question 18

How do neurons communicate?

Answer 18

Via electrical signals.

Question 19

What do motor neurones conduct impulses from the CNS to?

Answer 19

The muscle.

Question 20

What are intracellular gaps called where incoming signals in neurones are received?

Answer 20

Synapses.

Question 21

What are extensions of the cell membrane called that receive signals?

Answer 21

Dendrites.

Question 22

Between which cells do synaptic connections occur?

Answer 22

Nerve cell + nerve cell
Nerve cell + muscle cell
Nerve cell + gland cell
Nerve cell + sensory cell.

Question 23

What happens at the synapse?

Answer 23

A chemical transmitter released from the presynaptic cell binds to receptors on the postsynaptic membrane.

Question 24

What is excitatory post-synaptic potential (EPSP)?

Answer 24

The result of an excitatory neurotransmitter binding onto the postsynaptic receptor, causing an electrical charge in the membrane of that cell (depolarisation).

Question 25

What is **inhibitory post-synaptic potential** (IPSP)?

Answer 25

An event that results in the **hyperpolarization** of a neuron and the **prevention of an action potential**.

Question 26

What are examples of excitatory neurotransmitters?

Answer 26

**Acetylcholine**, **Glutamate**.

Question 27

What are examples of inhibitory neurotransmitters?

Answer 27

GABA and glycine.

Question 28

Where are inputs in the neuron summed?

Answer 28

In the perikaryon at the axon hillock.

Question 29

Where are excitatory and inhibitory inputs received?

Answer 29

At the dendrites.

Question 30

What happens if the summed input at the neuron perikaryon is above a threshold of -55mV?

Answer 30

An action potential is started in the axon.

Question 31

What happens if the axon hillock membrane potential exceeds the threshold of -55mV?

Answer 31

Voltage-gated sodium channels open as the cell depolarises.

Question 32

What is the all or nothing principle?

Answer 32

Regardless of strength of impulse, if the critical threshold (-55mV) is not reached, there is no action potential fired.

Question 33

Describe the phases of an action potential.

Answer 33

1) Stimulus = **summed input** at neuron perikaryon at axon hillock is **above threshold of -55mV** 2) Rapid influx of Na+ ions (**depolarisation**) = cause upstroke of AP 3) **Na+ channels inactivate and close** = stopping Na+ influx 4) Depolarisation causes **K+ channels to open** 5) Efflux of K+ (**repolarisation**) leads to **overshoot** and short period of **hyperpolarisation** = membrane is refractory 6) Resting membrane potential is restored. **Na+/K+ ATPase restores resting ion balance**.

Question 34

What is the refractory period?

Answer 34

A period of inactivity after a neuron has fired - the membrane is unable to conduct another action potential at this point in time.

Question 35

How is unidirectional travel ensured in action potential conductance in the axon?

Answer 35

- Na+ influx as AP is generated causes **depolarisation of downstream Na+ channels** to open as well. AP propagates along the axon and leaves a **REFRACTORY PORTION of membrane behind it** = ensures unidirectional travel. - Saltatory conduction. - Once the axon potential reaches nerve terminus: changes in membrane potential triggers influx of calcium ions which stimulates intracellular vesicles to fuse with cell membrane and release neurotransmitter contents into the synapse.

Question 36

What prevents the return of the action potential, ensuring unidirectional travel?

Answer 36

The refractory membrane left behind.

Question 37

According to the Nernst equation, what will the K+ concentration gradient generate?

Answer 37

A theoretical potential difference across the cells of -95mV. However, the ACTUAL typical neuronal resting potential is -70mV.

Question 38

What do the bodies of nerve cells (perikaryon) receive inputs from?

Answer 38

Upstream stimuli via extensions of their cell membranes called dendrites.

Question 39

Where are the inputs summed in the neuron?

Answer 39

At the axon hillock.

Question 40

Describe the three major phases of the action potential.

Answer 40

A) **depolarisation** = if summative inputs exceed -55mV, **voltage-gated sodium channels open**. Sodium flows INTO the cell down a concentration gradient. This increases the membrane potential to around +**30mV**. B) **repolarisation** = the Na+ channels are inactivated rapidly - this STOPS the inward rush of sodium. At the same time, **K+ channels open**. As a result, there is a higher [K+] INSIDE the cell than outside [K+ in > K+ out]. As a result, MORE K+ exits the cell down its concentration gradient when K+ channels open. C) **hyperpolarisation** = an **overshoot** leads to the hyperpolarisation of the membrane. This occurs as the K+ channels are still open and **more positive K+ continue to move OUT the cell**. This causes the cell to become temporarily even more negative (hyperpolarised).

Question 41

Which phase of the action potential is associated with the refractory period?

Answer 41

The hyperpolarisation stage.

Question 42

What ion is primarily responsible for establishing the resting membrane potential?

Answer 42

K+.

Question 43

What ion influx at the terminal bouton stimulates the fusion of vesicles and release of neurotransmitters?

Answer 43

Ca2+.

Question 44

What is **spatial summation**?

Answer 44

Post-synaptic neuron depolarisation **threshold achieved** following stimulation from **multiple pre-synaptic cells**.

Question 45

What is the refractory period?

Answer 45

Time during which an action potential cannot be initiated due to hyperpolarization of the membrane.

Question 46

What is temporal summation?

Answer 46

Post-synaptic neuron depolarisation **threshold** achieved following **high-frequency stimulation from one or more pre-synaptic cells**.

Question 47

What is hyperpolarization?

Answer 47

A decrease in membrane potential (e.g., from -70 to -90mV) following the opening of K+ channels.

Question 48

What is depolarisation?

Answer 48

An increase in membrane potential (e.g., from -70 to -30mV) following the opening of Na+ channels.

Question 49

What is the role of selective permeability of membranes to Na+ ions?

Answer 49

It allows some of these ions to flow back into the cell, affecting the resting potential.

Question 50

What does the Na+/K+ ATPase pump do?

Answer 50

**3Na+ out** of cell for every **2K+ into** cell using **1ATP**.

Question 51

Describe differences between an excitatory synapse and inhibitory synapse.

Answer 51

Excitatory synapses increase the likelihood of an action potential, while inhibitory synapses decrease it.

Question 52

Incoming signals in neurones are received across intracellular gaps called ___ on extensions of the cell membrane called ___.

Answer 52

Incoming signals in neurones are received across intracellular gaps called synapses on extensions of the cell membrane called dendrites

Question 53

Match the labels to the most appropriate description in the following image depicting propagation of an action potential from a presynaptic to a post-synaptic neuron

Answer 53

A) Calcium ions B) Voltage-gated channel C) Synaptic cleft D) Neurotransmitters E) Sodium ions F) Ligand-gated ion channels

Question 54

Match the labels in this diagram of an action potential to the most appropriate description

Answer 54