Transcript Action potential

‫بسم اللة الرحمن الرحيم‬
Muscle and neuron as excitable
tissue
• In general sense all living cell are excitable because
they respond to external stimuli.
• These cells have some properties which necessary for
excitation:
1. They can maintain different concentration between
positively and negatively charge
2. Their permeability change when they are stimulated
3. When stimulated only specific types of ions can pass
in certain direction
Nerve and muscle cells are called excitable tissue
because they respond to chemical, mechanical and
electrical stimuli.
The membrane serves as both an insulator and a
diffusion barrier to the movement of ions.
Anion and cation
• An anion (−) is an ion with more electrons
than protons, giving it a net negative charge
(since electrons are negatively charged and
protons are positively charged).
• A cation (+) is an ion with fewer electrons
than protons, giving it a positive charge.
The concentration of major cations and anions
ECF
ICF
Na+
140
10
K+
4
145
Ca+
5
1
Mg+
2
40
Total
156
196
Cl-
110
3
HCO3-
31
10
Protein-
4
45
HPO4-
6
138
Total
156
196
Cations (mmolL)
Anions (mmolL)
Resting potential
• When the membrane potential of a cell can go
for a long period of time without changing
significantly, it is referred to as a resting
potential or resting voltage. This term is used
for the membrane potential of non-excitable
cells, but also for the membrane potential of
excitable cells in the absence of excitation.
• In case of resting membrane potential there is
negatively voltage of about - 70 to -90 mV inside
the cell with respect to outside because of the
following:
1. The resting cell membrane is 10 - 100 times
more permeable to K than to Na.
2. The non - diffusible anions (protein, sulphat and
phosphate ions)can not leave the cell.
3. Avery small amount of Na diffuses into the cell
down its concentration gradient.
The membrane potential has two basic functions:
• First, it allows a cell to function as a battery,
providing power to operate a variety of voltage
in the membrane.
• Second, in electrically excitable cells such as
neurons and muscle cells, it is used for
transmitting signals between different parts of
a cell. Signals are generated by opening or
closing of ion channels at one point in the
membrane, producing a local change in the
membrane potential.
Origin of resting membrane potential
A number of forces act on cell membranes. This
force are responsible for:
1. Maintenance of resting membrane potential
2. Development of action potential.
3. Bringing the cell back to its resting state after
the action potential.
These forces are:
• Diffusion
• Electrical gradient
• Active transport
Ion transporter/pump
Is a trans-membrane protein that moves ions
across a plasma membrane against their
concentration gradient.
Ion channels allow ions to move across the
membrane down those concentration gradients, a
process known as active transport or facilitated
diffusion.
Na K pumping
The most important of active transport
( facilitated diffusion) system is Na K pumping
, the ion transporter Na+/K+-ATPase pumps
which transport sodium cations from the inside
to the outside, and potassium cations from the
outside to the inside of the cell. There is an
enzyme called Na-K adenosine triphosphatase
(ATPase) present on the cell membrane, which
activated by Na and k to hydrolyse ATP and
release the energy.
Voltage-dependent calcium channels (VDCC)
• Are a group of voltage-gated ion channels
found in excitable cells ( muscle, neurons, etc.)
with a permeability to the ion Ca2+.These
channels are slightly permeable to sodium
ions, so they are also called Ca2+-Na+
channels, but their permeability to calcium is
about 1000-fold greater than to sodium under
normal physiological conditions.
• At physiologic or resting membrane potential,
VDCCs are normally closed.
• VDCC are activated ( opened) at depolarized
membrane potentials and this is the source of
the "voltage-dependent". Activation of
particular VDCCs allows Ca2+ entry into the
cell, which, depending on the cell type, results
in muscular contraction, excitation of neurons,
up-regulation of gene expression, or release of
hormones or neurotransmitters.
Action potential
In physiology, an action potential is a short-lasting
event in which the electrical membrane potential of a
cell rapidly rises and falls. The action potential is a
sudden reversal of membrane polarity produced by a
stimulus. Action potential occur in living organism to
produce physiological effects such as:
• Transmission of impulses
• Release of neurosecretions or chemical transmtters in
synapses.
• Contraction of muscle.
• Activation or inhibition of glandular secretion
Development of action potential
When a cell membrane is stimulated by a physical or
chemical stimulus, the cell membrane permeability to
Na is increased. Sodium channels open and the sodium
ions rush through the channels to the inside of the cell.
This is called depolarization. The membrane potential
actually becomes reversed and reaches +35 mV.
At the end of depolarization, Na permeability stops and
K permeability increased abruptly and K ions leaves the
the cell down their concentration gradient causing the
inside membrane return to its original potential. This
called repolarization.
The duration of DP and RP in muscle and nerve about
1-5 ms (1/1000 s)
Threshold stimulus
Is a stimulus which is just strong enough to
move the resting membrane potential from – 70
mV to – 55 mV that leads to production of action
potential.
Synaptic transmission
A synapse is the junction between tow neurons
where the electrical activity of one neuron is
transmitted to the other. Most synapses occur
between the axon terminals of one neuron and
the cell body (dendrites). The presynaptic
endings enlarge slightly to make the synaptic
Knob.
The synaptic knob contains vesicles which
contain a transmitter substance. When AP arrives
from the axon it cause the calcium channels to
open and increasing the membrane permeability
to Ca.
Calcium attracts the vesicles to the membrane
and once they are in contact they rupture and
neurotransmitter is released into synaptic cleft
and combine with specific receptors for that
transmitter on the postsynabtic membrane. This
changes the permeability of postsynaptic
membrane to specific ions and results in
postsynaptic potential.
The postsynaptic membrane usually contains no
transmitter; this why nerve conduction occur
only in one direction.
Neurotransmitter
Synaptic vesicles
Reuptake pump
Axon terminal
Voltage gated
Ca++ channel
Receptors
Post-synabtic density
Synaptic cleft
dendrite