Page 58 - Anatomy and Physiology of Farm Animals, 8th Edition

Anatomy and Physiology of the Cell / 43

membrane in the adjacent area has mem­ to, a membrane protein or membrane
receptor. Important characteristics of
brane channels sensitive to changes in volt­
VetBooks.ir age (voltage‐gated channels) and if the the ligand–receptor (LR) relationship are
reversibility, specificity, affinity, and satu­
change in membrane potential in the adja­
cent area reaches a threshold potential (mV), ration. Herein we will describe the charac­
then another action potential in the adjacent teristics of all LR interactions, but will
area is elicited. This can occur again and focus on cell surface, or membrane recep­
again, so that a series of action potentials tors. The ability of a chemical to cross the
moves along the membranous surface of a membrane of the cell will determine if it
cell (Fig. 2‐16). This movement of action binds to a receptor on the cell surface or
potentials is propagation of the action within the cell.
potential. This type of propagation is typical The binding of a ligand to a membrane
of all excitable cells (muscle and nerve cells). receptor is usually reversible, for the inter­
Disrupting the activity of membrane action is due to relatively weak chemical
channels can prevent action potentials attractions (L + R ⇌ LR). The weakness of
and their propagation. For example, this binding is also a reason for some of the
propagation of action potentials in other characteristics of the LR relationship.
peripheral nerves depends on the rapid Membrane receptors can bind to only a
+
opening of voltage‐gated Na channels. limited number of particular ligands or
Local anesthetics, such as lidocaine, chemicals; that is, receptors demonstrate
when applied along a peripheral nerve specificity for certain ligands. For example,
+
are able to block these voltage‐gated Na even though a receptor may be capable of
channels and prevent action potential binding any two ligands, the affinity, or
propagation. The result is a loss of sen- strength of binding, between the receptor
sation (analgesia) and motor function to and the ligand may be stronger for one ligand
the region supplied by the nerve. than the other. Ligands that form a strong
bond with a receptor are described as having
a high affinity for the receptor. The dissocia-
Membrane Receptors tion constant (K ) is a term used in biology
d
and Intracellular Signaling and pharmacology to describe the likelihood
of two objects separating into their individ­
The cell membrane protects the cell from ual subunits (LR ‐ > L + R). Thus, the K is
d
the extracellular environment by restrict­ the inverse of the association constant (K )
a
ing exchange between the intracellular and which would describe the likelihood of the
extracellular fluid spaces. However, cells two objects coming together (L + R ‐ > LR).
must also be able to detect the presence When describing the affinity of a ligand for a
of certain chemical substances in the extra­ receptor, the greater the affinity the smaller
cellular fluid and alter appropriate intra­ the K .
d
cellular functions when these chemicals The number of membrane receptors on
are present. These abilities are essential for any given cell or cell population is finite.
communication between individual nerve Thus, it is possible to provide enough ligand
cells, between nerve and muscle cells, and that all receptor molecules have ligand
between cells of endocrine glands and the bound to them. A receptor is described as
cells affected by the hormones secreted by occupied when it has a ligand bound to it.
endocrine glands. The recognition of the Saturation of receptors occurs when all
specific chemical in the extracellular fluid receptors are occupied with ligand.
and the response to its presence are based These concepts and characteristics of
on the interaction between the chemical ligands and receptors are the basis for a
and a membrane protein. Ligand is the variety of pharmaceutical agents. For
general term applied to any chemical that example, β‐blocker drugs are ligands that
is capable of interacting with, or binding bind to β‐adrenergic receptors but do not