Transmission of nerve impulses

What are nerve impulses?

Electrical messages

What is membrane potential?

It describes the difference in electrical charge across a cell membrane

The difference between the inside and outside of a neuron

What is the voltage at resting potential?

-70 mV

What does -70 mV represent?

The charge inside the neuron when resting

What is voltage?

A comparison of electrical charge between two points

Why is there a voltage difference between a neuron and its enviroment?

There is a difference in ion distribution on either side of the membrane of a neuron

What is the ion distribution between a neuron and its extracellular enviroment?

A higher concentration of sodium Na⁺ exists outside the neuron (extracellular fluid)

A higher concentration of potassium K⁺ and large negatively charged organic molecules exist inside the neuron (axoplasm)

What is the axoplasm?

Intracellular fluid of a neuron

What does the concentration difference of sodium and potassium in a neuron compared to the extracellular fluid cause?

The inside of a neuron to be more negative than the outside

-70 mV means that the inside is 70 mV more negative than the ouside

What are the stages of transmission and action potential?

Resting potential

Upswing phase/depolarization

Downswing phase/repolarization

Overshoot/hyperpolarization

Reset

Refractory/recovery

What creates a nerve impulse?

Changes in the membrane potential of a neuron

What happens when the axon or dendrite is stimulated?

Sodium gates open, which allow some Na⁺ to enter the axoplasm

What does the entry of sodium into the neuron cause?

The inside becoming more positive than the outside by 40 mV

How does the voltage inside a neuron change during depolarization?

The charge changes from -70 mV to 40 mV

What is the all or none response?

Action potentials, once stimulated, are all the same (-70 mV to 40 mV)

It is either there if the threshold of -55 mV is surpassed, or it is not

What is the threshold for the all or none response?

-55 mV

What happens during repolarization?

The sodium gates close when not further stimulated and the increased positive charge within the neuron triggers potassium gates to open and K⁺ to exit the axoplasm

What does K⁺ exiting the axoplasm cause?

The charge to return to -70 mV

When do potassium gates close?

After the resting potential is restored

What is hyperpolarization?

When there is a slight overshoot of the resting potential during repolarization

What is needed after polarization and why?

A reset period is needed to reverse the ions

After polarization, the charge/membrane potential is back to normal, but the ions are reversed from where they are normally located

How does the reset period work?

The sodium/potassium pump returns Na⁺ to the outside and K⁺ to the inside

What does the reset period require?

ATP

It is active transport and pushing sodium and potassium against their concentration gradients

What is the exchange rate during the reset period?

3 Na⁺ are pumped out, 2 K⁺ are pumped in

Why is the exchange rate 3 out 2 in?

To reset the charge across the membrane

What is the propulsion of impulse?

The impulse moves down nerve fibres, because an action potential at one point of the fibre stimulates the sodium gates to open at the very next point

This gives a wave motion/chain reaction moving down the nerve fibre

What is the refractory period?

The time after an action potential is generated, during which another action potential cannot be produced

Why is the refractory period important?

The gates that have just opened and closed cannot be restimulated for a very brief period of time, so the propulsion of impulse moves in one direction only (unidirectional)

Which way does the action potential move?

One direction only, dendrite end to axon end

An action potential can only be first initiated at the dendrite end of a neuron (receptor or synapse), so the impulse can never go in the opposite direction

Can the action potential move from the axon end to the dendrite end?

Not naturally, but it can be made to do so artificially

Explain the characteristics of resting potential

Charge is -70 mV

High concentration of Na⁺ outside

High concentration of K⁺ inside

Explain the characteristics of the upswing of action potential

Depolarization

Na⁺ moves inside (sodium gates open, potassium gates closed)

Charge from -70 mV to 40 mV

Explain the characteristics of the upswing of action potential

Repolarization

K⁺ moves outside (potassium gates open/sodium gates close)

Charge from 40 mV back to -70 mV, after a hyperpolarization

What does the voltage go to during hyperpolarization?

-80 mV

Explain the characteristics of recovery phase

Sodium/potassium pump moves Na⁺ out and K⁺ in

Charge is back to -70 mV

How does the action potential travel in an unmyelinated fibre?

The action potential travels down an axon one small section at a time

Why are action potentials moving down unmyelinated fibres slow?

As soon as an action potential has moved on, the previous section undergoes a refractory period

Are unmyelinated fibres fast or slow?

Slow, 0.5 m/s

Where are unmyelinated fibres often found?

They are typically interneurons of the CNS

How does an action potential move down a myelinated fibre?

The gated ion channels (Na⁺ or K⁺ channels) that produce an action potential are concentrated at the nodes of ranvier

Does an action potential move fast or slow down a myelinated fibre?

Fast, up to 200 m/s

Why do action potentials move fast down myelinated fibres?

Because ion exchange occurs only at the nodes, the action potential travels fast

What is the movement of the action potential on a myelinated fibre called?

Saltatory conduction

What is saltatory conduction?

Comes from latin “to hop or leap”

The action potential jumps from node to node

What is multiple sclerosis (MS)

An individual’s immune system’s T cells (autoimmune) attack and break down the myelin sheath, leading to a loss of signal speed

What is multiple sclerosis considered and why?

It is also called a neurodegenerative disease because the impacts on the nervous system worsen over time

How does an action potential’s movement differ from a myelinated vs unmyelinated fibre?

Unmyelinated: Impulse goes point to point (slower)

Myelinated: Impulse goes node to node (faster)

What happens at the end of an axon?

The impulse has to jump since there is a junction between neurons

What is the jump of an action potential between neurons called?

Synapse

Where does the action potential travel during a synapse?

From the axon terminal of one neuron to the dendrites of a second terminal

What structure is at the end of an axon?

Each axon branches off and ends with a swelled tip or terminal knob that lies close to but not touching the dendrite of another neuron (or a tissue of an organ/gland)

This entire region is called a synapse

What is the first neuron and second neuron called in a synapse?

The membrane of the first neuron is called the presynaptic membrane and the membrane of the next neuron/cell is called the postsynapstic membrane

What is the tiny gap separating the two membranes called?

The synaptic cleft

Can an action potential/electrical message cross a synapse?

No, so the transmission of nerve impulses across a synaptic cleft is carried out by chemicals called neurotransmitters

Where are neurotransmitters stored?

In vesicles at the end of an axon

What special structure is at the end of the axon?

Calcium²⁺ ions channels

What happens at the end of an axon?

Calcium binds with contractile proteins that pull the neurotransmitter vesicles to the presynaptic membrane surface

What happens after calcium binds with contractile proteins?

The vesicles join with the cell membrane, forcing the neurotransmitter into the cleft (exocytosis)

What happens after the neurotransmitters are released into the cleft?

The neurotransmitters bind to specific receptor proteins on the postsynaptic membrane (on the dendrite of the next neuron or cell surface of an effector)

What happens when neurotransmitters bind to receptor proteins?

Neurotransmitters and receptor proteins will trigger a specific response

They do not go into the calcium gate, they bind onto the gate to trigger a response

How do neurotransmitters move across the cleft?

By diffusion

What is the job of an excitatory neurotransmitter that stimulates another neuron?

To increase permeability of the sodium ions on the postsynaptic membrane

If enough is received by the next neuron, it will fire and continue the impulse

What do inhibitory neurotransmitters that stimulate other neurons do?

Decrease permeability of the sodium ions on the postsynaptic membrane

What is the all or none law for neurotransmitters?

If enough neurotransmitter is received by the postsynaptic fibre, it will fire 100% (all)

If not enough substance is received, it will not fire (none)

What are the two outcomes that a neurotransmitter can cause?

Depending on the type of neurotransmitter and receptor, the response can be toward excitation (causing an action potential to happen) or toward inhibition (stopping an action potential from happening)

What was the first neurotransmitter discovered and what does it do?

Acetylcholine (Ach) can both excite and inhibit organs of the autonomic system, using different types of receptors

What is integration and why is it important?

The dendrites and cell body of a neuron have synapses with many other neurons, thus a single neuron may receive many excitatory and inhibitory signals

Integration is the summing up of excitatory and inhibitory signals by a neuron

How long does the neurotransmitter stay in the synaptic cleft?

For a very short amount of time

What are the two fates of neurotransmitters after they have been released into the cleft?

Enzymes rapidly breaking them down to clear the synapse so the next impulse can be transmitted

The presynaptic membrane rapidly reabsorbs the neurotransmitter (endocytosis)

What is an example of an enzyme that breaks down a neurotransmitter?

Acetylcholinesterase breaks down acetylcholine

How do drugs that affect the nervous system act?

Enhancing or blocking the release of a neurotransmitter

Mimicking the action of a neurotransmitter

Blocking the receptor protein

Interfering with the removal of a neurotransmitter from a synaptic cleft

How do drugs that enhance or block the release of a neurotransmitter work and what is the result?

Drug blocks excitatory neurotransmitter, result is depression of nerve impulses

Drugs blocks inhibitory neurotransmitter, result is stimulation of nerve impulses

Drug enhances excitatory neurotransmitter, result is stimulation of nerve impulses

Drug enhances inhibitory neurotransmitter, result is depression of nerve impulses

What is an example of a drug that mimics the action of a neurotransmitter?

Heroine

What is an example of a drug that blocks the receptor protein?

Nicotine

What is an example of a drug that interferes with the removal of a neurotransmitter?

Cocaine