9

Faris Haddad

Dania Alkouz

Mohammad-Khatatbeh

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Revision of previous ideas

I. The Action potential stages are

mainly controlled by Na+ and

K+ channels

II. These channels can be either

pumps (chemical gated) or

voltage- gated channels

III. Pumps are mainly used in the

maintenance of RMP

IV. Voltage-gated channels are

inaction mainly in

depolarization and

repolarization

V. Cardiac tissues can be grouped

into 2 types, differentiated by

the different type of action

potential they operate with (to be studied later).

VI. Action potential is initiated by reaching a threshold potential using chemical

gates or existing currents in the membrane

Refractory Period

Action potential is initiated by sodium voltage-gate channels which go through 3

states during the cycle:

• Closed and capable of opening: during RMP and polarisation before threshold,

since these channels are electromotive sensitive they do not open before the

potential threshold is reached , but they do have the capacity to open when it is

reached to allow action potential to occur

• Open: once the potential threshold is reached, which is around -50 to -70

millivolts, the voltage-gate channels have their activation gates open in a

conformation change allowing Na+ huge permeability (from 500x to 5000x under

RMP) in the depolarisation stage. Actually the change of permeability is so great

that the membrane's potential gets neutralised and even becomes positive in a

matter of a few 10000ths of a second.

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• Closed and not capable of opening (inactive):once maximum positive potential

is reached, the inactive gate of the channels become inactive (plug themselves) to

prevent any further gaining of charge or experiencing action potential during the

repolarization period, this period is called refractory period. This state of the

channel can be reversed by polarisation beyond the potential threshold and the

hyperpolarisation that succeeds repolarisation contributes to the deinactivation of

the channels; the greater the negative potential the greater the deinactivation rate

•

The refractory period is a stage in action potential when the membrane resists re-

stimulation by not responding to any small stimuli that initiated the process in the

beginning. It is divided into 2 periods :

• The absolute refractory period (ARP): when there is an over-whelming

concentration of inactive Na+ voltage-Gated channels (closed and not capable to

open ) that respond to no stimuli, no matter how strong they are. Meaning action

potential not possible.

note:- Dr khatatbeh said that Na+ voltage-gated channels are open in ARP, as in

the handout. But in the book, inactive (closed and not capable to open).

• The relative refractory period (RRP): when the membrane is recovering and

coming back to RMP, during this period only very strong stimuli can elicit a

response from the membrane. Action potential can theoretically reignite so Na+

voltage-Gate channels are closed and capable to open , but our bodies usually

experience sub-threshold currents.

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A clear, relative time frame for each period wasn't mentioned above because there

is no clear time frame, different textbooks and data references disagree on when

each start and end, so what has been mentioned will do.

This also leads to the conclusion; the magnitude of the action potential is practically

fixed, its frequency isn’t. The greater the magnitude of Na+ concentration the grater

the stronger the stimulus which means action potential may happen during relative

-RP which means the intensity of the a neural signal isn’t expressed by the

membrane potential’s strength, but by frequency.

* The greater the negative potential of a membrane the easier it'll be for it to be

excited because a negative potential causates to a greater concentration of "closed

but capable for opening" channels, and so greater frequency of action potential. This

bit of information is also the logical conclusion to the question “why do different

cells have different RMP?".

Neurons RMP: -90 mV

Smooth muscle cells RMP: -40 mV

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These numbers make sense when you consider the functions of each cell. Neurons

can recover from action potential quickly so they can start the next cycle quickly.

Even the difference between threshold potential and RMP for each of those cells

gets smaller the more negative RMP.

Pacemaker

• Conductive tissues: a tissue with a higher permeability to sodium, that greater

influx means that the threshold is

reached faster, accelerating

depolarization, these tissues use

calcium channels to accelerate

depolarization since Ca ions are

double the charge of Na ions, but the

channels are slow to open thus they

are called slow channels while Na

ion channels are called fast channels. This tissue is exclusive to cardiac muscles,

which means the heart’s action potential is not dependent upon neural impulses.

These tissues also have higher concentration of K channels to help in

repolarisation.

Now what function does all this serve in the heart?

The Na voltage gates being fast channels will depolarise the membranes of the tissues

first, the Ca channels can maintain the new positive charge of the membrane after the

now deactivated Na channels; prolonging the depolarisation stage. This Plateau phase

insures that the contractions happen in the same direction everywhere. This also leads to

more and more K channels becoming active, ready for repolarisation after the

deactivation of the Ca channels. This is all regulated by one part of the Heart called

pacemaker (SA node). This action potential’s refractory period lasts longer so the heart

doesn’t start contracting again while it’s contracting.

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In these cases where both Ca and Na contribute to action potential, these ions can

have effects on each other’s channels. For example; a deficit in Ca will cause the Na

voltage gated channels to become easily excitable, this can cause the respiratory

muscle cells for example to discharge with no stimulus in a phenomena called

muscle tetany which can be lethal sometimes

Neurons

Neurons are the cells responsible for the transmission of commands to organs to

maintain homeostasis, simply put they generate and transmit action potential.

Action potential gets transmitted in pulses; during action potential the membrane is +ve

compared to RMP. In motor neurons for example the axon is so long that different parts

of the membrane have different potentials, which generates a current starting from the

cell body.

The signal gets transmitted from one neuron to the next through 2

types of synapses:

Chemical synapses: where the presynaptic cell transmits

neurotransmitters over the synaptic cleft to the

postsynaptic cell’s receptors which can stimulate the

channels to start another action potential.

Electrical synapses: where the neurons are attached

together with gap junctions that allow the current to flow.

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Now being electrical impulses in a very fluid and conductive environment, the action

potential impulses are insulated from the extracellular environment of the axon by

myelin sheaths (a lipid based white substance), and those little gaps in the sheath are not

a hindrance they help the spreading of the action potential and are called nodes of

Ranvier. Myelin is really effective in helping action potential conduct through

myelinated axons that the impulse seams to leap from one node to the next, so they

called it saltatory conduction (from Latin saltare, “to leap”)

Myelin is secreted by a type of secondary supportive cell that falls into a group called

neuroglia, or "glial cells" that have many vital functions including:

1. Myelin secretion:

Oligodendrocytes and

Schwann cells

2. Phagocytises to destroy

dead neurons or invading

microbes: Microglial cells

3. Assistance in ion

regulation: Astrocytes

4. Anchoring of the neuron

to capillaries