9 Faris Haddad Dania AlkouzIV. Voltage-gated channels are inaction mainly in depolarization and...
Transcript of 9 Faris Haddad Dania AlkouzIV. Voltage-gated channels are inaction mainly in depolarization and...
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