Biomedical Engineering Reference
In-Depth Information
All signals received by cells fi rst interact with specialized proteins in the cells
called receptors, which are very specifi c to the signals they receive. These signals
can be in various forms. The most common are chemical signals, which include all
the hormones and neurotransmitters secreted within the body as well as the sensory
(external) signals of taste and smell. The internal hormonal signals include
steroid
and
peptide
hormones,
neurotransmitters
, and
biogenic amines
, all of which are
released from specialized cells within the various organs. The external signals of
smell, which enter the nasal
compartment
as
gaseous
chemicals, are dissolved in
liquid and then picked up by specialized receptors. Other external
stimuli
are fi rst
received by specialized receptors (for example, light receptors in the eye and touch
receptors in the skin), which then convert the environmental signals into chemical
ones, which are then passed on to the brain in the form of electrical impulses through
peripheral nerve signaling pathways in order to reach CNS (Fig.
7.2
) [
1
,
2
].
Once
a receptor
has received a signal, it must transmit this information effec-
tively into the cell. This is accomplished either by a series of biochemical changes
within the cell or by modifying the membrane potential by the movement of ions
into or out of the cell. Receptors that initiate biochemical changes can do so either
directly via
intrinsic
enzymatic activities within the receptor or by activating
intracellular
messenger molecules) (Fig.
7.3
). Receptors may be broadly classi-
fi ed in four groups that differ in their mode of action and in the molecules that
activate them.
1. The largest family of receptors is the
G-protein-coupled receptors (GPCRs)
,
which depend on
guanosine triphosphate
(GTP) for their function. Many neu-
rotransmitters, hormones, and small molecules bind to and activate
specifi c
G-protein-coupled receptors
[
4
].
2. A second family of membrane-bound receptors is the
receptor tyrosine kinases
(RTKs)
. They function by
phosphorylating
themselves and recruiting down-
stream signaling components [
4
]. Receptors that have inherent tyrosine kinase
activity bind molecules that have a specifi c SH2 domain (src homology domain).
In turn, another accessory protein may be activated such as SOS (son-of-
sevenless). This can activate a monomeric G-protein known as Ras that essen-
tially acts as a signal transduction switch [
4
]. Its activation can lead to
