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By the end of this section, you will be able to:
  • Describe four types of signaling found in multicellular organisms
  • Compare internal receptors with cell-surface receptors
  • Recognize the relationship between a ligand’s structure and its mechanism of action

There are two kinds of communication in the world of living cells. Communication between cells is called intercellular signaling    , and communication within a cell is called intracellular signaling    . An easy way to remember the distinction is by understanding the Latin origin of the prefixes: inter- means "between" (for example, intersecting lines are those that cross each other) and intra- means "inside" (like intravenous).

Chemical signals are released by signaling cells in the form of small, usually volatile or soluble molecules called ligands. A ligand    is a molecule that binds another specific molecule, in some cases, delivering a signal in the process. Ligands can thus be thought of as signaling molecules. Ligands interact with proteins in target cells , which are cells that are affected by chemical signals; these proteins are also called receptors . Ligands and receptors exist in several varieties; however, a specific ligand will have a specific receptor that typically binds only that ligand.

Forms of signaling

There are four categories of chemical signaling found in multicellular organisms: paracrine signaling, endocrine signaling, autocrine signaling, and direct signaling across gap junctions ( [link] ). The main difference between the different categories of signaling is the distance that the signal travels through the organism to reach the target cell. Not all cells are affected by the same signals.

The illustration shows four forms of chemical signaling. In autocrine signaling, a cell targets itself. In signaling across a gap junction, a cell targets a cell connected via gap junctions. In paracrine signaling, a cell targets a nearby cell. In endocrine signaling, a cell targets a distant cell via the bloodstream
In chemical signaling, a cell may target itself (autocrine signaling), a cell connected by gap junctions, a nearby cell (paracrine signaling), or a distant cell (endocrine signaling). Paracrine signaling acts on nearby cells, endocrine signaling uses the circulatory system to transport ligands, and autocrine signaling acts on the signaling cell. Signaling via gap junctions involves signaling molecules moving directly between adjacent cells.

Paracrine signaling

Signals that act locally between cells that are close together are called paracrine signals . Paracrine signals move by diffusion through the extracellular matrix. These types of signals usually elicit quick responses that last only a short amount of time. In order to keep the response localized, paracrine ligand molecules are normally quickly degraded by enzymes or removed by neighboring cells. Removing the signals will reestablish the concentration gradient for the signal, allowing them to quickly diffuse through the intracellular space if released again.

One example of paracrine signaling is the transfer of signals across synapses between nerve cells. A nerve cell consists of a cell body, several short, branched extensions called dendrites that receive stimuli, and a long extension called an axon, which transmits signals to other nerve cells or muscle cells. The junction between nerve cells where signal transmission occurs is called a synapse. A synaptic signal    is a chemical signal that travels between nerve cells. Signals within the nerve cells are propagated by fast-moving electrical impulses. When these impulses reach the end of the axon, the signal continues on to a dendrite of the next cell by the release of chemical ligands called neurotransmitters by the presynaptic cell (the cell emitting the signal). The neurotransmitters are transported across the very small distances between nerve cells, which are called chemical synapses ( [link] ). The small distance between nerve cells allows the signal to travel quickly; this enables an immediate response, such as, Take your hand off the stove!

Questions & Answers

write a short note on how the kidney carry out osmoregulation in man
Rhoda Reply
Kidney play a very large role in human osoregulation by regulating the amount of water reabsorbed from glomerular filtrate in kidney as tubules, which is controlled by hormones such as antrdiuvetic hormone (ADH) , aldosterone and angiotensin.
kidney is capable of osmoregulation in mammal ,
simple definition for respiratory system
Gift Reply
Respiratory system is a network of organs and tissues that helps you to breathe or help in getting the rids of oxygen and discharge of carbon dioxide in the body.
how is a aerenchyma tissue adapted to its function
fatuma Reply
what does DNA mean
innocent Reply
dioxiribo nucleic acid
what is a zygote
Victor Reply
zygote is developed or fertilized egg cell
what is the difference between cell wall and cell membrane
Ruhiyatu Reply
cell wall is found in plant while cell membrane is found in animal cell
please this is the main answer to that question okay Cell wall gives shape and support to the cell whiles Cell membrane support the movement of substances into and out of the cell. This question is very tricky that's why I asked.
how cell I form
Elijah Reply
what difference between animal cell and plant cell
Lazarus Reply
what is animal call
Animal cell does not have a cell wall but plant cell have a cell wall
plant have cell Wall well animal have not cell wall
what is fertilization?
Muhamed Reply
Fertilization is the fusion of male sex cell and female sex cell to form a zygote.
What kind of nutrients is composed of plants
Annie Reply
what is hormones
Igwe Reply
hormon is the chemical messanger
Genes can make someone dull?
21ecological instrument and their diagrams
Ayomide Reply
cell biology
I am sorry
for wat
no reason
nothing but speculate
Sorry without reason is nothing but speculate
what are the kidney disease
Immaculate Reply
kidney stones
it is a disease that affects the kidney
what are some lung diseases
kidney cancer
what is the other way of treating kidney failure
After having kidney transplant or dialysis
Kidney stone is a hard deposits of mineral and acid salt that stick together in concentrated urine and they are painful when passing through the urinary tract, but usually don't cause permanent damage
what is micro-organism
Jackson Reply
what is the hypothesis
hypothesis is a proposed explanation for a phenomenon
hypothesis is raw materials
what is biology
biology is the study of living things and their interaction with their environment
what is zyogot?
what is fertilization?
zygote is an unfertilized eggs
fertilization refers to the fusion of a sperm and ovum
organs that can't be seen physically
what does mean stigma
Amira Reply
the stigma receives pollen grains during pollination

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Source:  OpenStax, Biology. OpenStax CNX. Feb 29, 2016 Download for free at http://cnx.org/content/col11448/1.10
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