Cell signaling allows cells to communicate and coordinate their functions. There are several forms of cell signaling, including endocrine signaling where hormones travel through the bloodstream, paracrine signaling between neighboring cells, contact-dependent signaling through cell junctions, and synaptic signaling across nerve cell junctions. In intracellular signaling, receptor activation leads to the production of second messengers that transmit signals within the cell by activating intracellular signaling pathways, ultimately triggering responses like transcription, survival, movement, or metabolic changes.

1. ©Goutam Mallik, Assistant Professor, Pharmacology
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General Principles of Cell Communication
 Cellular systems in the body communicate with each other to coordinate and integrate their
functions.
 This occurs through a variety of processes known collectively as cell signaling, in which a signaling
molecule produced by one cell is detected by another, almost always by means of a specific
receptor protein molecule.
 The recipient cell transduces the signal, which it most often detects at the plasma membrane, into
intracellular chemical messages that change cell behavior.
Forms of intracellular signaling
The signaling by cell are following types:
1. Endocrine signaling (neurocrine or neuroendocrine signaling)
2. Paracrine Signaling
3. Contact-dependent or Juxtacrine Signaling
4. Synaptic Signaling
Endocrine Signaling
 Most endocrine hormones are circulating hormones-they pass from the secretory cells that make
them into interstitial fluid and then into the blood.
 The endocrine cells are located in endocrine glands like thyroid gland, hypothalamus.
 These types of signals usually produce a slower response, but have a longer lasting effect.
 The ligands released in endocrine signaling are called hormones.
 Hormones travel the large distances between endocrine cells and their target cells via bloodstream
which is relatively slow away to move throughout the body.
Paracrine Signaling
 In this form, the products of the cells diffuse in the extracellular fluid to affect neighboring cells that
may be some distance away. •
 Local hormones that act on neighboring cells are called paracrine and those that act on the same
cell that secreted them are called autocrine.
Contact-Dependent or Juxtacrine Signaling
 In this there is the actual contact between the cells. It is particularly important for normal
development and immunity. Some bacteria and other infectious agents use contact signaling to
identify “preferred” target tissues or organs.

2. ©Goutam Mallik, Assistant Professor, Pharmacology
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 Gap junctions in animals and plasmodesmata in plants are connections between the plasma
membranes of neighboring cells, this water filled channels allow small signaling molecules called
intracellular mediators, to diffuse between the two cells.
 The transfer of signaling molecules communicates the current state of the cell that is directly next
to the target cell, this allows a group of cells to co-ordinate their response to a signal that only one
of them may have received.
Synaptic Signaling
 The point at which the nerve impulse passes from one to another is the synapse. There is no
physical contact between these neurons.
 In this form, neurotransmitters are released at synaptic junctions from nerve cells and act across
a narrow synaptic cleft on a postsynaptic cell.
.
Fig: The different modes of cell–cell signaling

3. ©Goutam Mallik, Assistant Professor, Pharmacology
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Intracellular signaling
 A wide variety of small molecules carry signals within cells, conveying the signal from its source
(e.g., activated plasma membrane receptor) to its target (e.g., the nucleus).
 These secondmessengers convey signals as fluctuations in local concentration, according to rates
of synthesis and degradation by specific enzymes (e.g., cyclase involved in cyclic nucleotide
(cAMP, cGMP) synthesis),or, in the caseof calcium,according to the activities of calcium channels
and pumps.
 Other, lipidic, second messengers such as phosphatidylinositol, derive from membranes and may
act within the membrane to generate downstream effects.
Mechanism Of Cell Signaling
Fig: An overview of the major signaling pathways by which extracellular messenger molecules
can elicit intracellular responses.

4. ©Goutam Mallik, Assistant Professor, Pharmacology
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Two different types of signal transduction pathways are depicted, one in which a signaling
pathway is activated by a diffusible second messenger and another in which a signaling pathway
is activated by recruitment of proteins to the plasma membrane. Most signal transduction
pathways involve a combination of these mechanisms. It should also be noted that signaling
pathways are not typically linear tracks as depicted here, but are branched and interconnected to
form a complex web.
Step-1
 Cell signaling is initiated with the release of a messenger molecule by a cell that is engaged in
sending messages to other cells in the body.
 The extracellular environments of cells contain hundreds of different informational molecules,
ranging from small compounds (e.g., steroids and neurotransmitters) to small, soluble protein
hormones (e.g., glucagon and insulin) to huge glycoproteins bound to the surfaces of other cells.
Step-2
 Cells can only respond to a particular extracellular message if they have receptors that specifically
recognize and bind that messenger molecule.
 The molecule that binds to the receptor is called a ligand. Different types of cells possess different
complements of receptors that allow them to respond to different extracellular messengers.
Step-3
 In mostcases,the extracellular messengermolecule binds to a receptor at the outer surface of the
responding cell.
 This interaction induces a conformational change in the receptor that causes the signal to be
relayed across the membrane to the receptor’s cytoplasmic domain.
 Once it has reached the inner surface of the plasma membrane, there are two major routes by
which the signal is transmitted into the cell interior, where it elicits the appropriate response. The
particular route taken depends on the type of receptor that is activated.
Step-4
 One type of receptor transmits a signal from its cytoplasmic domain to a nearby enzyme.
Step-5
 It generates a second messenger. Because it brings about (effects) the cellular response by
generating a second messenger, the enzyme responsible is referred to as an effector. Second
messengers are small substances that typically activate (or inactivate) specific proteins.

5. ©Goutam Mallik, Assistant Professor, Pharmacology
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 Depending on its chemical structure, a second messenger may diffuse through the cytosol or
remain embedded in the lipid bilayer of a membrane.
Step-6
 The signal is transmitted by a second messenger activate intracellular signaling pathway.
Step-7
 Each signaling pathway consists of a series of distinct proteins that operate in sequence.
Step-8
 Target protein activated and do following task.
 Transcription
 Survival
 Protein synthesis
 Movement
 Cell death
 Metabolic change
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Reference
1. KARP’S Cell and Molecular Biology Concepts and Experiments, 8th
Ed. by Janet Iwasa &
Wallace Marshall
2. Gray’s Anatomy, The Anatomical Basis of Clinical Practice 41st
Ed by Elsevier