Nerve action potential

1. Nerve Action Potential
Aditya Johan Romadhon, SST.Ft, M.Fis

2. Introduction
Disruption of this mechanism can have drastic effects resulting in lack of impulse generation and conduction
They also are crucial for communication among neurons through synapses
Neuronal action potentials are vital for propagation of impulses along any nerve fiber even at a distance
This change in membrane potential will open voltage-gated cationic channel (sodium gated channel) resulting in the process of depolarization and generation of the neuronal action potential
A neuronal action potential gets generated when the negative inside potential reaches the threshold (less negative)
This state is the resting membrane potential of about -60mV
Normally, the cell’s interior is negative, compared to its outside
These action potentials are generated and propagated by changes to the cationic gradient (mainly sodium and potassium) across their plasma membranes
Neurons are electrically excitable, reacting to input via the production of electrical impulses, propagated as action potentials throughout the cell and its axon

3. Generation of the action potential
After-
hyperpolarization,
a recovery from a
slight overshoot of
the repolarization
Repolarization, a
return to the
membrane’s resting
potential, primarily
caused by
potassium efflux
Depolarization,
changing the
membrane’s
potential from -60
mV to +40 mV
primarily caused by
sodium influx
There are three
stages in the
generation of the
action potential

4. Chemical-electricity activity
There are two major
potentials to play unique
roles in the production of
action potential that allows
transmission of signals
through the neurons
These are known as the
resting potential and
threshold potential of
neurons
In the axon of a typical
neuron, the resting potential
and threshold potential are
approximately -70 and -55
mV
There are more Na+
accumulated outside the cell
than the K+ inside the cell,
and thus the resting
potential of the cells is
negatively charged
In order to be activated or
inactivated, thus allowing
for the influx or efflux of
specific ions

5. Action potential
Generally, the action potential
starts, when Na+ enters
through a voltage-activated
Na+ channel (Nav), which
creates the depolarizing
nature of the membrane
potential
When threshold potential is
achieved, all the Nav channels
are stimulated to open,
leading to a complete
depolarization till achieving
peak potential (+40 mV) of
the neurons
At this point, the Nav
channels return to their
resting state, and the voltage-
activated K+ channels (Kv)
are activated and opened to
allow the efflux of K+, causing
repolarization of the neurons

6. Refractory periode
Immediately after an action
potential generates, the
neuron cannot immediately
generate another action
potential; this is the
absolute refractory period
At this moment, the sodium
channels are inactivated and
remain closed, whereas the
potassium channels are still
open
This state is followed by the
relative refractory period
when the neuron may only
generate an action potential
with a much higher
threshold.

7. Action potential generates muscle
contraction
Synaptic cleft (area between the nerve terminal and motor endplate)
Postsynaptic part (motor endplate)
Presynaptic part (nerve terminal)
The structure of NMJ can be divided into three main parts
It is the site for the transmission of action potential from nerve to the muscle, it is also a site for many diseases and a site of action for
many pharmacological drugs
A synaptic connection between the terminal end of a motor nerve and a muscle called the neuromuscular junction (NMJ)

8. Presynaptic (nerve terminal)
Nerve Terminal: A myelinated motor
neuron, on reaching the target muscle,
loses its myelin sheath to form a complex
of 100-200 branching nerve endings
These nerve endings are called nerve
terminals or terminal boutons, the nerve
terminal membrane has areas of
membrane thickening called active zones
Active zones have a family of SNAP
proteins (syntaxins and synaptosomal-
associated protein 25) and rows of
voltage-gated calcium (Ca) channels
A nerve terminal also has potassium
channels on its membrane and contains
mitochondria, endoplasmic reticulum,
and synaptic vesicles (SVs)
Each SV stores around 5000-10000
molecules of acetylcholine (ACh)
neurotransmitter at NMJ
The membrane of SVs has synaptobrevin
proteins, these proteins are essential for
fusion and docking of SVs at active zones
On arrival of an action potential at the
nerve terminal, Ca channels open to
cause influx, increased Ca inside the
nerve terminal causes a series of events
leading to docking of SVs at active zones
and exocytosis of the ACh from the
synaptic vesicles into the synaptic cleft

9. Synaptic cleft/ junctional cleft
Synaptic Cleft / Junctional
Cleft: The space between the
nerve terminal and the plasma
membrane of muscle is called
synaptic/junctional cleft
It is the site where presynaptic
neurotransmitters, ACh is
released before it interacts
with nicotinic ACh receptors
on the motor endplate
Synaptic cleft of NMJ contains
acetylcholinesterase enzyme,
responsible for the catabolism
of released ACh so that its
effect on the post-synaptic
receptors is not prolonged

10. Postsynaptic
Motor End Plate forms the
postsynaptic part of NMJ
It is the thickened portion of the
muscle plasma membrane
(sarcolemma) that is folded to
form depressions called
junctional folds
The terminal nerve endings do
not penetrate the motor
endplate but fit into the
junctional folds
Junctional folds have nicotinic
ACh receptors concentrated at
the top, these receptors are ACh
gated ion channels
Binding of ACh to these
receptors opens the channels
allowing the influx of sodium
ions from the extracellular fluid
into the muscle membrane, this
creates endplate potential and
generates and transmits AP to
the muscle membrane

11. THANK
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