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Signaling:
Compare and contrast signaling by neurons which secret neurotransmitters at
synapses to signaling carried out by endocrine cells, which secretes hormones into
the blood, Discuss the relative advantage of the two mechanisms.
The synaptic process:
The electrical impulse, called action potential, triggers a process that will send vesicles,
i.e. certain containers with neurotransmitter molecules, towards the membrane of the
presynaptic cell. When reaching the membrane, the vesicles will fuse with it and spill
out the transmitter into the synaptic cleft. The transmitter molecules bind to receptor
molecules on the postsynaptic cell membrane. The receptors respond by generating a
new
electrical
impulse
in
the
postsynaptic
cell.
Used neurotransmitters are either taken back by re-uptake pumps on the presynaptic
membrane and "recycled" into new vesicles and transmitters, or cleared by certain
enzymes.
Synaptic transmission consumes energy. A key part is played by the mitochondria, the
energy providers present in all cells. Without the mitochondria, the vesicles would not be
sent off to the membrane, the re-uptake pumps would not function, and the forming of
vesicles would not be possible, just to mention a few.
Several "signaling molecules", such as the neurotransmitters, allow nerve cells to
communicate across synapses, bind to receptor proteins in the membrane and open their
ion channels.
The intensity and strength of the electrical impulse will decide which neurotransmitter
to be released.
Not all neurotransmitters are known, but among the more profoundly mapped out are
acetylcholine, dopamine, noradrenaline and serotonin.
“Here the transmission of signal is carried by Synapses (pre and post synaptic
junction)and finally the target cell is stimulated or transduction of cell signal is
occurred.
Secreted neurotransmitters are act as chemical messengers which interact with
receptor on post synaptic junction.”
“No need of any carrier medium like blood from one place to another.”
Hormone signalling:
The endocrine system is a collection of glands that secrete chemical messages we call
hormones. These signals are passed through the blood to arrive at a target organ, which
has cells possessing the appropriate receptor.
Most of the molecules that enable signalling between the cells or tissues within an
individual animal or plant are known as "hormones." Hormone-initiated signal
transduction takes the following steps:
1.
2.
3.
4.
Biosynthesis of a hormone.
Storage and secretion of the hormone.
Transport of the hormone to the target cell.
Recognition of the hormone by the hormone receptor protein, leading to a
conformational change.
5. Relay and amplification of the signal that leads to defined biochemical reactions
within the target cell. The reactions of the target cells can, in turn, cause a signal
to the hormone-producing cell that leads to the down-regulation of hormone
production.
6. Removal of the hormone.
Hormones and other signaling molecules may exit the sending cell by exocytosis or other
means of membrane transport. The sending cell is typically of a specialized type. Its
recipients may be of one type or several, as in the case of insulin, which triggers diverse
and systemic effects.
Hormone signaling is elaborate and hard to dissect. A cell can have several different
receptors that recognize the same hormone, but activate different signal transduction
pathways; or different hormones and their receptors can invoke the same biochemical
pathway. Different tissue types can answer differently to the same hormone stimulus.
There are two classes of hormone receptors, "membrane-associated receptors" and
intracellular or "cytoplasmic" receptors.
The recognition of the chemical structure of a hormone by the hormone receptor uses the
same (non-covalent) mechanisms, such as hydrogen bonds, electrostatic forces,
hydrophobe and Van der Waals forces.
The important value for the strength of the signal relayed by the receptor is the
concentration of the hormone-receptor complex, which is defined by the affinity of the
hormone for the receptor, the concentration of the hormone and, of course, the
concentration of the receptor. The concentration of the circulating hormone is the key
value for the strength of the signal, since the other two values are constant. For fast
reaction, the hormone-producing cells can store prehormones, and quickly modify and
release them if necessary. Also, the recipient cell can modify the sensitivity of the
receptor, for example by phosphorylation; also, the variation of the number of receptors
can vary the total signal strength in the recipient cell.
“Here, there is no pre synaptic and post synaptic junction (Synaptic cleft) as in the case
of nerve signaling. The contact is involved between the protein (hormone) and receptor
molecule.
Direct ieteraction between the hormone and receptor on target cell”
“Hormone should be carried by blood then only it can reach the target cell.”
Relative advantages:
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Cell signaling
Hormones are used for communication with distant target cells. For example,
cells can secret a chemical and rely on the blood system to deliver the signal to a
distant cell.
Hormones and other excreted signaling factors regulate growth, differentiation
and survival of cells, and ultimately control, the development, metabolism and
behavior of higher organisms.
Neurotransmitters are secreted by neurons to stimulate an adjoining cell. For
example, a neuron might secrete acetylcholine to stimulate the movement of a
muscle cell.
Neurotransmitters bind to specific receptors on the plasma membrane of a
postsynaptic cell, causing a change in its permeability to ions.
Chemical synapses allow a single postsynaptic cell to amplify, modify, and
compute excitatory and inhibitory signals received from multiple presynaptic
neurons. Such integration is common in the central nervous system.
Impulse transmission at chemical synapses occurs with a small time delay but is
nearly instantaneous at electric synapses.
To control secretion whether for increased secretion or decreased secretion.
Cell signal is to control the physiological or secreting action (by positive or
negative feedback) of target cell.