10

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Omar Sami

Muhammad Abid

Muhammad khatatbeh

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Let’s shock the world …

In this lecture we are going to cover topics said in previous lectures and then start with

the nerve cells (neurons) and the synapses and integration of responses.

Rhythmicity of some excitable tissues:

Well, at first what does the word “Rhythm” mean?

Rhythm: repetitive self-induced discharges which occur normally in the heart, smooth

muscles and in many neurons.

Simply, it means that some structures undergo action potentials in different rates, for

example heart muscle undergo 75 action potentials per minute, while others undergo

60, 40 action potentials per minute and so on…

You may wonder why there is a difference in potentials between tissues.

This is due to the difference in properties, as some tissues may have high leakage for

sodium ions while other may have a lower leakage in sodium ion which in turn will

result in different rhythm.

Let’s take a little bit closer look at the cardiac conduction:

Firstly, the conducting system provides the heart its automatic rhythmic beat, the

electrical signals are transported from the SA (sinoatrial) node to the AV

(artrioventricular) node via the conductive tissue of the heart, which allows the action

potential to be spread towards muscle cells.

Secondly, the cardiac muscle potential has a phenomenon called plateau, so what is

plateau and why does it prolong the period of depolarization?

Well, in some instances, the excited membrane does not repolarize immediately after

depolarization; instead the potential remains in a plateau for many milliseconds, and

only after that does the repolarization begins.

** this type of action potential occurs in heart muscle fibers, where the plateau lasts for

0.2 - 0.3 seconds and causes contraction of heart muscle to last for this same long

period…

But why does the plateau occur?

Well, the cause of plateau is a combination of several factors:

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A- In heart muscle, there is two types of channels that participates into

depolarization process, (1) the usual voltage activated sodium channels, called

fast channels, and (2) voltage activated calcium channels, which are slow to open

so they are called slow channels.

So what actually happens due to these channels?

Opening of fast channels causes the spike portion of the action potential, but

don’t forget that calcium channels are slow channels, so calcium ions will prolong

the action potential because of their slow opening which will cause the plateau.

B- The second factor that may be “partly” responsible for the plateau is that the

voltage-gated potassium channels are slower to open than usual, often not

opening much until the end of the plateau. This factor delays the return of the

membrane potential toward its normal negative value of -80 to -90 millivolts.

The plateau ends when the calcium and sodium channels close and permeability

to potassium ions increases.

Now let’s start with the nerve cells (neurons) and the synapses and integration of

responses.

Firstly, let us have a closer look at the component of the neural cell, and then approach

more to the propagation of the action potential along the neural cell.

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** Around the neural cells there is another types of cells, called supportive cells.

Studying supportive cells is a huge field in neurology, which can’t be covered in a sheet.

So, how are action potentials generated at neural cells? And how are they getting

transported?

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Synapses and integration of responses:

After neurotransmitters are synthesized in the cell body, they are transported to the terminals

and stored there, now let’s assume that a stimulus occurred….

When the impulse from the presynaptic membrane reaches the synaptic knob (end bulb), this

will cause activation of voltage dependent Ca++ channels, which in turn will result in Ca++

diffusion into the synaptic knob, the increase in Ca++ concentration inside the axon terminal

will trigger the release of the neurotransmitters into the synaptic cleft by exocytosis…

*** In some cases after the neurotransmitters are attached to their receptors,

hyperpolarization may occur instead of De-Polarization, but what determines either ways?

1- De-Polarization: depolarization will occur if the receptors that the neurotransmitter binds

to at the postsynaptic membrane are linked to Na+ channels. However, this

depolarization is fairly little which can’t reach the threshold, and won’t cause any action

potential.

**The developed postsynaptic potential is called EPSPs (Excitatory Post Synaptic Potentials)

2- Hyperpolarization: hyperpolarization will occur if the receptors of the postsynaptic

membrane that the neurotransmitter binds to are linked to K+ channels.

** The developed postsynaptic potential is called IPSPs (Inhibitory Post Synaptic Potentials)

** The part of the neuron where the action potential is generated is called the axon hillock

Question: let’s assume that the receptors at the postsynaptic membrane are linked to

chloride channels what would happen when the neurotransmitters are attached to their

receptors?

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Answer: the effect of this process will be inhibitory on neural activity. This inhibition is

achieved by holding the membrane at its resting potential and preventing

depolarization.(prevents depolarization in general).

*****

There are a lot of terminals ( of one neuron ) that are synapsing with the cell body ( of another

neuron ) , so whenever the neurotransmitters are released at the synaptic cleft it will result in

many de-polarizations and many hyper-polarizations, so if the sum of all of these polarizations

reached the threshold, action potential will be generated at the second neuron.

This process of finding the sum of these polarizations is called Summation.

However, there are two types of summations:

1- Spatial summation :

*So let’s assume that there is three presynaptic neurons synapsing with a fourth neuron,

post synaptic.

So what will happen?

Actually, each neuron will transport its stimulus to the fourth neuron, either it was

depolarization or repolarization, if the sum of these polarizations reach the threshold, an

action potential will be generated in the fourth neuron in the axon hillock.

*** Spatial stands for space.

2- Temporal summation:

Temporal summation occurs when a single presynaptic neuron fires many times in

succession, causing the postsynaptic membrane to reach its threshold and fire.

*** Temporal stands for Time.

Note that the summation isn’t only between excitatory potentials only; it can be between any kind of

potentials. (Excitatory with excitatory, excitatory with inhibitory or inhibitory with inhibitory)

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The sum of everything is taken by axon hillock to result in action potential or nothing.

However, in our nervous system there is more inhibition than excitation.

Remember:

-most chemical-gated channels are found on the cell body of the neuron or on the

dendrites, while most voltage-gated channels are located in the rest of the neuron, that is why

the action potential is generated In the axon hillock where voltage-gated channels concentrate.

-Action potential travels in one direction (unidirectional)

Let’s assume this hypothetical situation, which doesn’t happen in our bodies.

What if we had stimulated a neuron at the middle, what will happen?

Well , the action potential will move in both directions, right and left , although it is moving in

two ways still we are considering it as Unidirectional; because the point is that the potential

always moves from the excited region to the polarized region despite the hypothetical

situations.

What if we had stimulated a neuron at two opposite sides (may occur in some body

movements), what will happen?

Well, in this case the two action potentials will propagate towards the middle, and they will not

by-pass each other, instead both potentials will die at the middle.

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We know that the action potential is generated from the body towards the terminals, but

can an action potential be generated from the terminals towards the body of the axon?

The answer is yes, and this situation occurs in sensory neurons, which carry sensation from

terminals to the body, as they have receptors on their terminals. However, these receptors

may respond to temperature, touch or any other stimulus, by stimulation an action

potential will be generated and transported to the body of the axon, but not to the

same axon body. However, the action potential is generated at one end and travels

within the first axon towards the cell body of the second axon.

Conclusion: Sensory neurons are generating action potentials from terminal

of one neuron, towards the cell body of the second neuron, not the same

cell body.

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There are two types of nerve fibers (neurons), Myelinated & Unmyelinated

nerve fibers so what is the difference and how action potential is conducted in

both of them?

Myelinated Nerve Fiber:

This nerve fiber is surrounded by myelin sheath which results in having a

special way in conducting action potential along the neuron.

In myelinated nerve fiber the action potential is conducted through a

mechanism called Saltatory (jumping) conduction , the action potential will

propagate using Ranvier nodes to skip larger distances, which will result in

depolarization in each node, so it is considered a fast method for propagation

& also it conserves energy for the axon because only the nodes Depolarize.

Unmyelinated Nerve Fiber:

In this nerve fiber no myelin sheath is present so the action potential can be

propagated along the axon easily though more slowly than the saltatory

conduction….

Let’s assume that there is two Unmeylinated nerve fibers with two different

diameters, in which one of them the velocity will be higher?

Answer: the nerve fiber with the larger diameter will result in faster

propagation due to low resistance through the action potentials’ way

So the internal current is faster in bigger diameter than in small one…

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So far all of us know that in order to measure the potential across a plasma membrane,

two electrodes are placed, one inside and the other outside, but is there any other

method…?

A- Monophasic action potential: Is by placing one electrode inside the cell and other electrode

outside the cell.

B- Biphasic action potential: is by placing the two electrodes outside the cell membrane.

** Two waves are obtained in the recording of biphasic action potential, the first always

represents Depolarization and the second is in the reverse direction of the first and always

represents Repolarization.