provides a basic picture of excitable cells and the relative concentration of key electrolytes inside versus outside the cell.

1. OVERVIEW OF EXCITABLE TISSUE
DR Sahar Sasi
Physiology – Part 1
“My Notes”

2. This Lecture is dedicated for all medical student
and for whom preparing for basic science exams
.Hope you will find it helpful.

3. What are the Excitable Tissue??
• cells and tissues in which excitation is accompanied by action
potential, distributed along the cellular membrane. This is a
property of the bodies of nerve cells and their processes—nerve
fibers, muscle fibers .
What is excitability?
• An ability of specialized cells to respond to certain stimuli by
producing electrical signals known as action potential at its
membrane .
Give examples of excitable tissues
• Nerve cells, muscle (cardiac/skeletal)
What are 2 basic properties of excitable cell membranes?
• 1. The membranes have an electrical excitability across the
membrane. (In response to depolarization of the membrane
above a certain threshold voltage) and may transmit an impulse
along the membrane
• 2. The membranes contain a variety of ion channels (pores) that
may be opened or closed, allowing specific ions to flow across.
What is Resting Membrane Potential (RMP)?
• Membrane potential difference (trans membrane voltage) that
exists when cell membranes of excitable tissues are not
producing an action potential (at rest)

4. Why does RMP occur?
• Because of build-up of negative charges inside the cells and
an equal build-up of positive charges outside the cells.
• The greater the difference in charges across the membrane,
the larger the membrane potential (voltage)
What is the RMP of nerve cells?
• -90mV (the potential inside the nerve fiber cells is 90 mV
more negative than the potential of the outside)
What is the RMP of skeletal muscle fibers?
• -80 to -90mV
What is the RMP of non-excitable tissue?
• +20mV out, -20mV in
“A cell that exhibits a membrane potential is said to be polarized”
2 main factors contribute to RMP. They are?
• 1. Distribution of ions across the membrane
• 2. Permeability of the membrane to Na⁺ and K⁺

5. What is the distribution of ions in extra and intracellular
fluid?
• Extracellular: rich in Na⁺ and Cl⁻ ions
• Intracellular: mainly K⁺ & organic phosphates + proteins
Explain about the permeability of the membrane to Na⁺ and K⁺
• Plasma membrane permeability to K⁺ is 50-100 times greater than its
permeability to Na⁺
The ion concentrations do not normally change very quickly (with the
exception of calcium). However, the membrane permeabilities can change in
a fraction of a milisecond, as a result of?
• Activation of ligand-gated or voltage-gated ion channels
The change in membrane potential can be large or small, depending on
how many ion channels are activated and what type they are. This type of
changes of the membrane potential are referred to as graded potentials
Explain a bit on graded potentials
• - affect locally
• - Non-propagated potential
• - In contrast to action potentials, AP have a constant amplitude and time
course, and propagated along the neighboring cell membranes.
2 types of voltage-gated ion channels are involved in these 2 phases.
What are they?
• 1. Na⁺ channels
• 2. K⁺ channels

6. Voltage-gated Na⁺ channels have 2 separate gates. What are
they?
• 1. Activation gates: close in resting membrane, open at
threshold
• 2. Inactivation gates: open in resting membrane, also open at
threshold
Following stimulation of excitable cells, a graded potential
causes membrane to depolarize to a critical level that is called
threshold .
Explain depolarization phase
• Rapid opening of Na+voltage-gated channels leads to Na+
inflow. this leads to loss of membrane polarization. (MP =
+30mV). An action potential rises to a constant and maximum
strength each time
Explain threshold depolarization
• Also stimulate slower opening of voltage gated K+ channels.
That its opening coincides with voltage-gated Na+ channels
closing.
What is the effect of K⁺ outflow?
• K+ outflow causes the resting membrane potential to be
restored = repolarization phase (MP = -70mV)

7. While the voltage-gated K⁺ channels are open, a large enough
outflow of K⁺ may lead to ????
After-hyperpolarization
What is hyperpolarization?
• Polarization more negative than the resting level (about -90mV)
• As the voltage-gated K⁺ channels close, what happens?
• RMP returns to the resting level (-70mV)
What is the refractory period?
• The period when excitable cells cannot generate another action potential
There are two types of refractory periods. What are they?
1. Absolute refractory period
2. Relative refractory period
What is the absolute refractory period?
Refers to the time period during which a 2nd action potential cannot be
initiated, even with a very strong stimulus. It coincides with Na⁺ channels
activation and inactivation. Inactivated Na⁺ channels cannot reopen, they must
first return to resting state.
What is relative refractory period?
• Refers to the time period during which a 2nd action potential can be
initiated, but only with a larger than threshold stimulus (supra threshold). It
coincides with the period when the voltage gated K⁺ channels are still open
after inactivated Na⁺ channels have returned to their resting state
Explain propagation (conduction) of action potentials
• As Na⁺ flows in, depolarization increases to threshold depolarization. This
open voltage-gated Na⁺ channels in adjacent patches of cell membrane

8. What is relative refractory period?
• Refers to the time period during which a 2nd action potential
can be initiated, but only with a larger than threshold stimulus
(supra threshold). It coincides with the period when the voltage
gated K⁺ channels are still open after inactivated Na⁺ channels
have returned to their resting state
Explain propagation (conduction) of action potentials
• As Na⁺ flows in, depolarization increases to threshold
depolarization. This open voltage-gated Na⁺ channels in adjacent
patches of cell membrane
What is the All-or-None principle?
• A principle of action potential generation. The depolarization
process travels over the entire membrane but if it does not
generate sufficient voltage to stimulate the next area of the
membrane, the spread of depolarization stops.

9. Compare between different excitable tissues in terms of
resting membrane potential
• The initiation and conduction of nerve and muscle action
potentials are similar, but they have different RMP's.
Neuron = -70mV Skeletal and cardiac muscle fibers = -90mV
Compare between different excitable tissues in terms of
velocity of conduction
• Nerve action potentials are 18x faster than muscle
Compare between different excitable tissues in terms of
duration of AP
• Neuron = 0.5 - 2.0 msec
Skeletal muscle fibers = 1.0 - 5.0 msec
Cardiac & smooth muscle fibers = 10 - 300 msec
What helps spread the action potential deep with the skeletal
muscle fiber?
• Transverse tubules. The T tubule action potentials cause
release of calcium ions inside the muscle fiber

10. • Cardiac muscle tissues contract without neural
stimulation. This property is called Automaticity
• This uploaded video will explain the RMP.
Click play

11. Resting Membrane Potential
When a neuron is not sending a signal, it is "at rest." When a neuron is
at rest, the inside of the neuron is negative relative to the outside.
Although the concentrations of the different ions attempt to balance
out on both sides of the membrane, they cannot because the cell
membrane allows only some ions to pass through channels (ion
channels). At rest, potassium ions (K+) can cross through the membrane
easily. Also at rest, chloride ions (Cl-)and sodium ions (Na+) have a more
difficult time crossing. The negatively charged protein molecules (A-)
inside the neuron cannot cross the membrane. In addition to these
selective ion channels, there is a pump that uses energy to move three
sodium ions out of the neuron for every two potassium ions it puts in.
Finally, when all these forces balance out, and the difference in the
voltage between the inside and outside of the neuron is measured, you
have the resting potential. The resting membrane potential of a neuron
is about -70 mV (mV=millivolt) - this means that the inside of the
neuron is 70 mV less than the outside. At rest, there are relatively more
sodium ions outside the neuron and more potassium ions inside that
neuron.

12. Action Potential
The resting potential tells about what happens when a neuron is at
rest. An action potential occurs when a neuron sends information
down an axon, away from the cell body. Neuroscientists use other
words, such as a "spike" or an "impulse" for the action potential.
The action potential is an explosion of electrical activity that is
created by a depolarizing current. This means that some event (a
stimulus) causes the resting potential to move toward 0 mV. When
the depolarization reaches about -55 mV a neuron will fire an action
potential. This is the threshold. If the neuron does not reach this
critical threshold level, then no action potential will fire. Also, when
the threshold level is reached, an action potential of a fixed sized
will always fire...for any given neuron, the size of the action
potential is always the same. There are no big or small action
potentials in one nerve cell - all action potentials are the same size.
Therefore, the neuron either does not reach the threshold or a full
action potential is fired - this is the "ALL OR NONE" principle.

13. Action potentials are caused when different ions cross the neuron
membrane. A stimulus first causes sodium channels to open. Because
there are many more sodium ions on the outside, and the inside of the
neuron is negative relative to the outside, sodium ions rush into the
neuron. Remember, sodium has a positive charge, so the neuron
becomes more positive and becomes depolarized. It takes longer for
potassium channels to open. When they do open, potassium rushes out
of the cell, reversing the depolarization. Also at about this time, sodium
channels start to close. This causes the action potential to go back
toward -70 mV (a repolarization). The action potential actually goes past
-70 mV (a hyperpolarization) because the potassium channels stay open
a bit too long. Gradually, the ion concentrations go back to resting levels
and the cell returns to -70 mV.
And there you have it...the Action Potential

14. Good Luck