Session 5: Action Potentials

What are ion channels

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

What are some ion channel properties?

- 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

What are ion channels

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

What are some ion channel properties?

- 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

The ion selectivity and types of channels that are open make cell membranes ___ permeable to ions

selectively permeable

What is the resting membrane potential?

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

Changes in the membrane ___ provide the basis of ___ in the nervous system and in many other types of cells

potential, signalling

What are the main factors influencing resting potential?

- 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

What is the resting potential of a neuron?

-70mV

Give examples of where changes in membrane potential underlie forms of signalling between and within cells

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

What is depolarisation?

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

What is hyperpolarisation?

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

Opening __ or __ channels will cause hyperpolarisation

K+ or Cl- channels will cause hyperpolarisation

Opening __ or __ channels will cause depolarisation

Na+ or Ca2+

Changes in membrane potential are caused by changes in the activity of ___ channels

ion channels

Name the three types of gated ion channels

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

What is the resting potential of cardiac muscle?

-80mV

What is the resting potential of smooth muscle?

-50mV

What is the resting potential of skeletal muscle?

-90mV

How do neurons communicate?

Via electrical signals

Motor neurones are neurones that conduct impulses from the CNS to the ___

muscle

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

synapses, dendrites

Between which cells do synaptic connections occur?

Nerve cell + ___ cell

Nerve cell + ___ cell

Nerve cell + ___ cell

Nerve cell + ___ cell

Nerve cell + nerve cell

Nerve cell + muscle cell

Nerve cell + gland cell

Nerve cell + sensory cell

At the synapse, a chemical transmitter released from the presynaptic cell binds to receptors on the ___ membrane

postsynaptic membrane

Excitatory post-synaptic potential (EPSP)

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

Inhibitory post-synaptic potential (IPSP)

an event that results in the hyperpolarization of a neuron and the prevention of an action potential (resulting change in membrane potential = inhibitory potential synaptic potential, IPSP)

Excitatory neurotransmitters

Acetylcholine, Glutamate, dopamine

Inhibitory neurotransmitters

GABA and glycine

Inputs in the neuron are summed in the ___ at the axon hillock

perikaryon

Excitatory and inhibitory inputs are received at the ___

dendrites

If the summed input at the neuron perikaryon is above a threshold of ___mV = an action potential is started in the axon

-55mV = an action potential is started in the axon

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

Voltage-gated sodium channels open as the cell depolarises

What is the all or nothing principle?

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

Describe the phases of an action potential

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.

What is the refractory period?

a period of inactivity after a neuron has fired (this occurs at stage 5 of the phases of the action potential) - the membrane is unable to conduct another action potential at this point in time.

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

- 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. In this way, chemical message is communicated to downstream cells.

The ___ membrane left behind prevents the return of the action potential, ensuring unidirectional travel of the action potential down the axon.

refractory

According to the ___ equation (which is used to calculate potentials based on gradients of ions of a particular charge) - the K+ concentration gradient will generate a theoretical potential difference across the cells of ___mV. However, the ACTUAL typical neuronal resting potential is ___mV.

Nernst equation, cells of -95mV. However, the ACTUAL typical neuronal resting potential is -70mV.

The bodies of nerve cells (___) receive inputs from upstream stimuli via extensions of their cell membranes called ___.

(perikaryon or soma), dendrites.

The inputs are summed at a region of the perikaryon of the neuron known as the axon ___.

axon hillock.

Describe the three major phases of the action potential

A) depolarisation

B) repolarisation

C) hyperpolarisation

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).

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

The hyperpolarisation stage is associated with the refractory period meaning the cell is unable to conduct another action potential during this time.

The high conductance of neuronal membranes to which ion is primarily response for establishing the resting membrane potential?

K+

Influx of which ion at the terminal bouton (axon terminus) stimulates the fusion of vesicles and release of neurotransmitters into the synapse?

Ca2+

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

A) Calcium ions

B) Voltage-gated channel

C) Synaptic cleft

D) Neurotransmitters

E) Sodium ions

F) Ligand-gated ion channels

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

A = threshold
B = stimulus
C = depolarization
D = failed initiation
E = repolarization/hyperpolarization
F = refractory period

At which part of the nerve cell body (perikaryon) are changes in the membrane potentials arising from multiple inputs summated? (capital)

Axon hillock

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

Spatial summation

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

Refractory period

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

Temporal summation

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

Hyperpolarization

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

Depolarisation

According to the Nernst equation, the K+ concentration gradient will generate a theoretical potential gradient across the cells of -95mV.

The ACTUAL typical neuronal resting potential is -70mV. This is partly due to a ___ selective permeability of membranes to ___ ions, allowing some of these ions to flow back into the cell.

low, Na+

Na+/K+ ATPase pump

3Na+ out of of cell for every

2K+ into cell

using 1ATP

Describe differences between an excitatory synapse and inhibitory synapse