human physiology chap 4 1st topic dr amna dr muzammil

1. Introduction to Neural
Communication by (DR Muzammil and DR
Amna saleem )
All body cells display a membrane potential, which is a separation
of positive and negative charges across the membrane
Potential
This is related to the uneven distribution of Na, K, and large
intracellular protein anions between the intracellular fluid (ICF)
and extracellular fluid (ECF), and to the differential permeability
of the plasma membrane to these
NERVE AND MUSCLE TISSUES ARE EXCITABLE
•The constant membrane potential present when a cell is electrically
at rest, that is, not producing electrical signals, is referred
to as the resting membrane potential. Two types of cells, neurons
•(nerve cells) and muscle cells, have developed a specialized use
for membrane potential. They can undergo transient, rapid
•fluctuations in their membrane potentials, which serve as electrical signals
•Nerve and muscle are considered excitable tissues because
•they produce electrical signals when excited. Neurons use these
electrical signals to receive, process, initiate, and transmit messages.
In muscle cells, these electrical signals initiate contraction.
Th us, electrical signals are critical to the function of the
nervous system and all muscles we examine
how neurons undergo changes in potential to accomplish their functions

2. • Membrane potential becomes less negative
• during depolarization and more negative
• during hyperpolarization.
• Before you can understand what electrical signals are and how
• they are created, you must become familiar with several terms
• used to describe changes in potential, which are graphically repressented
• POLARIZATION :Charges are separated across the plasma
membrane, so the membrane has potential. Any time the value
of the membrane potential is other than 0 mV, in either the
positive or negative direction, the membrane is in a state of
polarization. Recall that the magnitude of the potential is directly proportional to the number of
positive and negative charges separated by the membrane and that the sign of the
potential ( or ) always designates whether excess positive or
excess negative charges are present, respectively, on the inside
of the membrane. At resting potential, the membrane is polarized
• at 70 mV in a typical neuron
• 2. Depolarization: The membrane becomes less polarized; the
inside becomes less negative than at resting potential, with the
potential moving closer to 0 mV (for example, a change from
70 to 60 mV); fewer charges are separated than at resting
potential. This term also refers to the inside even becoming
positive as it does during an action potential (a major type of
electrical signal) when the membrane potential reverses itself
• (for example, becoming 30 mV).

3. • 3. Repolarization: The membrane returns to resting
potential after having been depolarized
• 4. Hyperpolarization: The membrane becomes more
polarized;the inside becomes more negative than at
resting potential,with the potential moving even
farther from 0 mV (for instance,a change from 70 to 80
mV).
o One possibly confusing point should be clarified. On the
device used for recording rapid changes in potential,
during a depolarization when the inside becomes less
negative than at resting,this decrease in the magnitude
of the potential is represented as an upward deflection.
By contrast, during a hyperpolarization when the inside
becomes more negative than at resting, this increase in
the magnitude of the potential is represented by a
downward deflection

4. • Electrical signals are produced by changes in ion movement across the
plasma membrane.
• Changes in membrane potential are brought about by changes in ion
movement across the membrane. For example, if the net inward flow of
positively charged ions increases compared to the resting state, the
membrane depolarizes (becomes less negative inside). By contrast, if the
net outward fl ow of positively charged ions increases compared to the
resting state, the membrane hyperpolarizes (becomes more negative inside)
• Changes in ion movement are brought about by changes in membrane
permeability in response to triggering events. Depending on the type of
electrical signal, a triggering event might be (1) a change in the electrical
field in the vicinity of an excitable
membrane; (2) an interaction of a chemical messenger with
a surface receptor on a nerve or muscle cell membrane; (3) a
• stimulus, such as sound waves stimulating specialized neurons
in the ear; or (4) a change of potential caused by inherent cyclical
changes in channel permeability. (You will learn more about the
nature of these various triggering events as our discussion of
electrical signals continues.)

5. Because the water-soluble ions responsible for carrying
charge cannot penetrate the plasma membrane’s lipid bilayer,
these charges can cross the membrane only through channels
specific for them or by carrier-mediated transport. Membrane
channels may be either leak channels or gated channels.
, leak channels, which are open all the time,
permit unregulated leakage of their specifi c ion across the membrane
through the channels. Gated channels, in contrast, have
gates that can be open or closed, permitting ion passage through
the channels when open and preventing ion passage through the
channels when closed. Gate opening and closing results from a
change in the conformation (shape) of the protein that forms the
gated channel. Th ere are four kinds of gated channels, depending
on the factor that causes the change in channel conformation:
(1) voltage-gated channels open or close in response to
changes in membrane potential; (2) chemically gated channels
change conformation in response to binding of a specifi c extracellular
chemical messenger to a surface membrane receptor;
(3) mechanically gated channels respond to stretching or other
mechanical deformation; and (4) thermally gated channels respond
to local changes in temperature (heat or cold).
Triggering events alter membrane permeability and consequently
alter ion fl ow across the membrane by opening or
closing the gates guarding particular ion channels. Th ese ion
movements redistribute charge across the membrane, causing
membrane potential to fluctuate.
There are two basic forms of electrical signals: (1) graded
potentials, which serve as short-distance signals; and (2) action
potentials, which signal over long distances. We next examine
these types of signals in more detail, beginning with graded
potentials, and then explore how neurons use these signals to convey messages.