The document discusses excitable tissues, such as muscle and nervous tissues, that can generate and propagate electrical impulses. It explains the concept of membrane potential, detailing resting membrane potential and action potential, which is the rapid rise and fall of electrical potential in excitable cells. Additionally, the document covers the classification of neurons and the role of supporting cells in the central and peripheral nervous systems.

1. PHS 213- Human Physiology I
L.O. Adegbite

2. MEMBRANE POTENTIALS AND EXCITABLE TISSUES

3. Excitable Tissues
• These are specialized tissues with the ability to generate and
propagate electrical impulses, respond to stimuli and transmit signals.
• Examples are:
• 1. Musce Tissues: Skeletal muscle, smooth muscle and cardiac muscle
• 2. Nervous Tissues: Neurons and Neuroglial cells

4. 29-4
Classification of neurons
• The functional classification of neurons is based on the direction in
which they conduct impulses
• Afferent or sensory neurons
• Sensory information (impulse) from environment or inside body
to CNS for interpretation
• Efferent or motor neurons
• Impulses from CNS to PNS to allow for movement or action
• Interneruons
• Interpretive neurons between afferent and efferent nerves in
the CNS

5. PNS = afferent neurons (their activity
“affects” what will happen
next) into the CNS
+
efferent neurons (“effecting” change:
movement, secretion, etc.)
projecting out of the CNS.
CNS PNS
CNS = brain
+
spinal cord;
all parts of
interneurons
are in the CNS.

7. Supporting Cells
• Aid the functions of neurons
• About five times more abundant than neurons
• In the CNS, supporting cells are collectively called neuroglia, or simply
glial cells

8. • There are two types of supporting cells in the PNS:
1. Schwann cells which form myelin sheaths around peripheral axons
2. Satellite cells which support neuron cell bodies within the ganglia of the PNS.
• There are four types of supporting cells in the CNS:
1. Oligodendrocytes which form myelin sheaths around axons of the CNS
2. Microglia which migrate through the CNS and phagocytose foreign and degenerated material
3. Astrocytes which help to regulate the external environment of neurons in the CNS.
4. Ependymal cells, which line the ventricles (cavities) of the brain and the central canal of the
spinal cord.

10. Membrane Potential
• Transmembrane potential/membrane voltage) is the
difference in electric potential between the interior and
the exterior of a biological cell. With respect to the
exterior of the cell, typical values of membrane potential
range from –40 mV to –80 mV.

11. Differences in the concentrations of ions on opposite sides of a cellular
membrane lead to a voltage called the membrane potential.

12. Resting Membrane potential
• Resting membrane potential is:
• the unequal distribution of ions on the both sides of the cell
membrane
• the voltage difference of resting cells
• the membrane potential that would be maintained if there weren’t
any stimuli or conducting impulses across it

13. • determined by the concentrations of ions on both sides of the
membrane
• a negative value, which means that there is an excess of negative
charge inside of the cell, compared to the outside
• much depended on intracellular potassium level as the membrane
permeability to potassium is about 100 times higher than that to
sodium.

16. Resting Membrane potential
• Resting membrane potential varies according to types of cells
For example:
• Skeletal muscle cells: −95 mV
• Smooth muscle cells: −50 mV
• Astrocytes: −80/−90 mV
• Neurons: −70 mV
• Erythrocytes: −12 mV

17. 29-17
• Neuron cell membrane at rest is in a polarized state
• Inside of cell membrane is negative
• Outside of cell membrane is positive due to more Na+
and K+
• As Na+
and K+
move into the cell, the membrane becomes
depolarized
• Inside becomes more positive
• Action potential (nerve impulse) is created
• Repolarization occurs when K+
and later Na+
move to the outside of
the cell membrane
• Return of the cell to polarized (resting) state

18. ACTION POTENTIAL
• Action potential is a short-lasting event in which the
electrical membrane potential of a cell rapidly rises and
falls, following a consistent trajectory.
• Action potentials occur in several types of animal cells,
called excitable cells, which include neurons and muscle
cells

19. BIOPHYSICAL BASIS OF ACTION POTENTIAL
Action potentials occur due to the presence special types of
voltage-gated ion channels in a cell membrane
A voltage-gated ion channel is a cluster of proteins embedded
in the membrane that has three key properties:
• It is capable of assuming more than one conformation.
• At least one of the conformations creates a channel through
the membrane that is permeable to specific types of ions.
• The transition between conformations is influenced by the
membrane potential.

20. A plot of an Action Potential

21. "All-or-none" principle
• The amplitude of an action potential is independent of the
amount of current that produced it. In other words, larger
currents do not create larger action potentials.
• Action potential obeys all-or-none law, since either they
occur fully or they do not occur at all.
• This is in contrast to receptor potentials whose
amplitudes are dependent on the intensity of a stimulus.

22. Action Potential explanation
• As the membrane potential is increased,
sodium ion channels open, allowing the entry
of sodium ions into the cell.
• This is followed by the opening of potassium
ion channels that permit the exit of potassium
ions from the cell.
• The inward flow of sodium ions increases the
concentration of positively charged cations in
the cell and causes depolarization, where the
potential of the cell is higher than the cell's
resting potential.

23. Cont’d
• The sodium channels close at the peak of the action
potential, while potassium continues to leave the cell.
• The efflux of potassium ions decreases the membrane
potential or hyperpolarizes the cell.
• For small voltage increases from rest, the potassium
current exceeds the sodium current and the voltage
returns to its normal resting value, typically −70 mV.

24. Cont’d
• However, if the voltage increases past a
critical threshold, typically 15 mV higher than
the resting value, the sodium current
dominates.
• This results in a runaway condition whereby
the positive feedback from the sodium current
activates even more sodium channels. Thus,
the cell fires, producing an action potential.
• The frequency at which cellular action
potentials are produced is known as its firing
rate.

25. An action potential
is an “all-or-none”
sequence of changes
in membrane potential.
Action potentials result
from an all-or-none
sequence of changes
in ion permeability
due to the operation
of voltage-gated
Na+
and K +
channels.
The rapid opening of
voltage-gated Na+
channels
allows rapid entry of Na+
,
moving membrane potential
closer to the sodium
equilibrium potential (+60 mv)
The slower opening of
voltage-gated K+
channels
allows K+
exit,
moving membrane potential
closer to the potassium
equilibrium potential (-90 mv)

26. The rapid opening of voltage-gated Na+
channels
explains the rapid-depolarization phase at the
beginning of the action potential.
The slower opening of voltage-gated K+
channels
explains the repolarization and after hyperpolarization
phases that complete the action potential.