Biomedical Engineering Reference
In-Depth Information
in the case of hormones secreted into the blood or plant sap by endocrine cells. The
other method is paracrine transmission, which is the local distribution of the signal in
the extracellular matrix. Signal transmission through neurons is still another type of
long-distance dissipation of information; in this case, however, the signal is delivered
quickly and to specific cells.
There are a few fundamental characteristics seen during cell communications, like
sensitivity of signal transducers. The cell has number of receptors, but they interact
with specific signals only. Secondly, a single signal itself can produce a wide range
of effects in the same cell. Receptors recognize the signal and relay it to the site of
action through a signaling cascade. During this course, the signal undergoes amplifi-
cation through secondary messenger systems and so forth and simultaneously modu-
lation too. Modulation is observed because, at any time, in each cell a number of
different receptors are being activated to produce varying responses. It is important
for a cell to integrate these signals to produce a unified reaction in the best interest of
the cell. The presence of a persistent signal desensitizes the receptor, and the receptor
returns to its normal state slowly.
There are several types of receptors involved in the signaling process. The major
ones include ion-gated channels or ion channel-linked receptors, where a flow of ions
produces an ultimate response; G protein-coupled receptors that activate G protein,
starting a cascade of events; and receptor enzymes that act as enzymes on activation
or activate further enzymes to produce the response and steroid receptors [5-60] .
1.6.1 Ion-Linked Receptors
These are the major signal transducers at the synaptic junctions, neuromuscular junc-
tions, and myocytes. These signal transducers are working comparatively in a more
simple way and act directly to produce the ultimate action. The signals for such recep-
tors are neurotransmitters that bring about a conformational change in the receptor.
This opens/closes the specific ion channels for Na , K , Ca 2 , and Cl - ( Fig. 1. ). The
driving force for the movement of ions is electrochemical gradient. The flow of ions in
and out of the cell causes a redistribution of charges, and hence a change in the mem-
brane potential of the plasma membrane is observed. Nicotinic acetylcholine receptor
is a very classical example of a ligand-gated ion channel. Calcium ions often act as
second messengers in these cases. The calcium ion channels are opened by depolariza-
tion waves in neurons and increase calcium ions concentration intracellular. This rise
initiates the release of neurotransmitter acetylcholine at the synapse, and the released
acetylcholine acts at the next neuron and brings about a similar chain of events to pass
on the signal to the target cell. A neuron responds to several neurotransmitters, and
each neurotransmitter opens up a particular ion channel. The resultant depolarization
or hyperpolarization is the integrated input from the several signals received [61,62] .
1.6.2 G protein-Coupled Receptors
These are the largest family of cell surface receptors, with around 1000 G protein-
coupled receptors having already been identified in humans. These receptors mediate
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