Gap Junctions: Link adjacent cells and are formed by plasma membrane proteins, called
connexins, that form structures called connexons.
- Connexons form channels that allow ions and small molecule to pass directly from
one cell to the other.
- Electrically couples the cell
Communication through chemical messengers
- Ligands Molecules that bind to proteins reversibly.
- Happens when one cell releases a chemical into the interstitial fluid (secretion), and
another cell; target cell, responds to the chemical messenger.
- The target cell is the cell at which the message is aimed.
- Target cell responds because of certain proteins, called receptors, which specifically
recognize and bind the messenger.
- The binding of messengers to receptors produces a response in the target cell through
a variety of mechanisms referred to as signal transduction.
1. Paracrines Communicate with neighboring cells. Target cell must be close
enough so that the paracrine secreted can reach it by simple diffusion. Include:
a. Growth factors Proteins that stimulate proliferation and differentiation
b. Clotting factors Proteins that stimulate formation of a blood clot.
c. Cytokines Peptides, released from immune cells, that function in
coordinating the body’s defense against infections.
Autocrines, are a subclass of paracrines, that act on the same cell that secreted them.
(Secretory cell = Target cell)
2. Neurotransmitters Chemicals released into interstitial fluid from nervous
system cells called neurons. released by axon terminal, which is very close to
the target cell.
- Junction between two cells are called synapses.
- Communication by neurotransmitters is called synaptic signaling.
- The cell that releases the neurotransmitter is called the presynaptic neuron.
- And the target cell is called the postsynaptic cell.
3. Hormones Chemicals released from endocrine glands into the interstitial fluid,
where they can diffuse into the blood.
- Travels through the blood to its target cell, which can be distant.
1 - Specific class Neurohormones, released by a special class of neurons called
neurosecretory cells through a mechanism similar to that of neurotransmitter release.
Lipophilic (hydrophobic) molecules are lipid soluble and, therefore, readily cross the
plasma membrane, but they do not dissolve in water.
Hydrophilic (lipophobic) molecules are water-soluble and so not cross the plasma
Hormones transported into the blood, either in dissolved form, or bound to carrier
- To be dissolved, the messenger must be a hydrophilic messenger. (Peptides and
- Steroids are hydrophobic so they bound to carrier proteins.
o Protected from degradation and have a longer life.
- Usually only bind one messenger or a group of messengers.
- The strength is called affinity.
Target’s cell’s response to chemical messenger
1. Messenger’s concentration Response increase as the concentration increase
2. Number of receptors present The more receptors, the more likely
a. Up-regulation Increase in number of receptors occurs when cells are
exposed to low messenger concentrations for a long time. The cell adapts
by producing more receptors.
b. Down-regulation Decrease in the number of receptors occurs when
messenger concentrations are higher than normal for a long time.
3. Affinity of the receptor for the messenger
Agonists: Ligands that bind to receptors and produce a biological response,
Antagonist: Ligands that bind to receptors but do not produce a response.
The ability of small changes in the concentration of a chemical messenger to elicit
marked responses in target cells. (Feature of second messenger system)
2 Long Distance Communication
- The nervous system consists of neurons and supporting cells called glial cells.
o Neurons can communicate in long distances, by transmitting electrical signals, and
transmitting chemical signals through the release of a neurotransmitter from the
The neurotransmitter then diffuses over a short distance.
o Wired system, because of the cell-to-cell interaction by synapses
o Short and quick, in charge of movements
- The endocrine system lacks any direct anatomical link between secretory cell and
o Communicates through hormones, which travel via the bloodstream.
o Coordinating metabolic activities among organ systems.
Action of lipophilic hormones
- Located in the cytosol or nucleus of target cell.
- Easily permeate the plasma membrane
o If receptor is in the nucleus hormone diffuses into the nucleus and binds to it
forming hormone-receptor complex.
o If receptor is in cytosol hormones binds to it there forming hormone-receptor
complex THAT THEN enters the nucleus.
Inside the nucleus complex binds to region of DNA, called the hormone
response element (HRE) which is located at the beginning of a specific gene.
Binding of the complex to the HRE activates or deactivates the gene affects
transcription of mRNA and increases or decreases synthesis of the protein
coded by the gene. (Ex. mRNA is increased and gene is activated)
The mRNA moves into the cytosol.
The mRNA is translated by ribosomes to yield proteins.
cAMP Second Messenger System
1. The first messenger binds to the receptor, activating a Gsprotein
2. The G protein releases alpha subunit binds to and activates the enzyme adenylate
3. Adenylate cyclase catalyzes the conversion of ATP to cAMP
4. cAMP activates protein kinase A, cAMP-dependent protein kinase.
5. The protein kinase catalyzes the transfer of a phosphate group from ATP to a
protein altering the protein’s activity.
6. Causes a response in the cell
3 Chapter 3a(ii)
1. Sensory neurons conduct information toward the brain and spinal chord.
Generally extend from sensory receptors specialized structures in gathering
information about the conditions within and around the body.
2. Motor neurons carry information away from the brain and spinal chord to an
effector – muscle