Page 57 - Anatomy and Physiology of Farm Animals, 8th Edition

42 / Anatomy and Physiology of Farm Animals

activity of the heart (cardiac arrhythmias), across the membrane. The rapid changes in
permeability are due to the opening and/or
VetBooks.ir and these can threaten life. closing of membrane channels (i.e., channels
In the typical cell in resting conditions,
formed by transmembrane proteins). The
sodium and potassium are the major deter­
minants of the membrane potential. However, channel itself may respond to physical, chem­
in some cell types and in certain conditions, ical, or electrical stimuli, and the response
the membrane permeability to other ions, of the channel is the link between threshold
hence passage of these ions across the mem­ stimuli and action potentials. A stimulus to
brane, may be a significant contributor to the cell may result in the membrane poten­
the membrane potential. For example, if the tial becoming more or less negative. A stim­
membrane permeability to Cl increased, Cl ulus may actually be several stimuli that are
−
−
would diffuse into the cell down the con­ successively experienced in the cell. If the
centration gradient, and the inside of the sum of these stimuli is able to result in the
cell would become more negative. membrane become less negative and there is
enough change in channel status to bring on
Excitable Cells and Action Potentials a rapid reversal of the membrane potential,
then it is called a threshold stimulus because
the cell has reached the threshold potential
Nerve and muscle cells are excitable cells. for an action potential to occur.
In response to the proper stimulus, their cell When an action potential occurs at a
membrane potential can undergo a rapid single site on an excitable cell, the mem­
but short‐lived reversal in electrical poten­ brane potential of adjacent areas along the
tial, so that the inside of the cell membrane membrane of the same cell also changes.
is more positive than the outside. This event The change in potential in the adjacent area
is known as an action potential (Fig. 2‐15). is due to the movement of charge (ions)
The reversal of the resting membrane between the two areas (Fig. 2‐16). If the
potential is described as depolarization of
the membrane, because during this period
the membrane potential is closer to zero. (A) + + + + + + + +
The changes in membrane potential dur­ – – – – – – – –
ing an action potential are due to rapid
changes in membrane permeability to dif­
ferent ions and movement of those ions
(B) – + + + + + + +
+ – – – – – – –
40
30
20 0 (C) + + + + + + + +
Membrane potential (mV) –10 (D) – – + + + + + +
–
10
–
–
–
–
–
–
–
–20
–30
–40
–50
–60 V Rest + + – – – – – –
–70
–80 Figure 2-16. Propagation of action potential.
0.5 1.0 1.5 2.0 (A) Resting membrane potential. (B) Initial thresh­
Time (msec) old stimulus. (C and D) Propagation. Black arrows
indicate direction of action potential with adjacent
Figure 2-15. Nerve action potential. membrane experiencing threshold stimulus.