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uitgebreide Samenvatting hoofdstuk 6 silverthorn human physiology
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Fleur sam Ch. 66.1 cell to cell communication
There are only two basic types of physiological signals within cell communication:
Electrical signals: The change in a cell’s membrane potential
Chemical signals: molecules secreted by cells into the ECF
Cells that respond to electrical signals are called target cells (targets). Chemical
signals are responsible for most communication within the body and act as ligands
that bind to proteins to initiate a response. It obeys(volgt) the general rules for protein
interactions, including specificity, affinity, competition and saturation.
There are four basic methods of cell-to-cell communication:
Local communications:
o 1. Gap junctions allow direct cytoplasmic transfer of electrical and
chemical signals between adjacent cells.
o 2. contact dependent signals occur when surface molecules on 1 cell
membrane bind to surface molecules of another cell’s membrane make use
of CAMs as receptors.
o 3. Chemicals that diffuse through the extracellular fluid to act on cells
close by.
Long distance communication:
o 4. uses a combination of chemical and electrical signals carried by nerve
cells and chemical signals transported in the blood (hormones). A given
molecule can function as chemical signal by more than 1 method, bv. A
molecule can act close to cell that released it (local communication) as well as
in distant parts of body (long-distance communication).
1.Simplest form of cell-to-cell communication is direct transfer of electrical and
chemical signals through gap junctions: are protein channels that create cytoplasmic
bridges between adjacent cells.
A gap junction forms from the union of membrane-spanning proteins, called
connexins on 2 adjacent cells. United connexins create a protein channel
(connexon) that can open and close. When channel open connected cells function
like a single cell that contains multiple nuclei (syncytium). When gap junctions open,
ions and small molecules like amino acids, ATP, cyclic AMP diffuse directly from
cytoplasm of 1 cell to cytoplasm of the next. Larger molecules cannot pass gap
junctions.
Gap junctions are only way in which electrical signals can pass directly from cell to
cell.
Movement of molecules and electrical signals through gap junctions can be
modulated or shut off completely.
More than 20 isoforms of connexins that may mix/match to form gap junctions.
This variety of isoforms allows gap junctions selectively to vary from tissue to tissue.
Gap junctions found in almost every cell type: heart muscle, some smooth muscle,
lung, liver, neurons of brain.
2.Contact-dependant signalling= some cell-to-cell communication requires that
surface molecules on 1 cell membrane bind to membrane protein of another cell.
, occurs in immune system and during growth/development bv. When nerve cells send
out long extensions that must grow from central axis to distal ends on developing
limbs. Cell adhesion molecules CAM, play role in cell-to-cell adhesion, but also as
receptors in cell-to-cell signalling. CAMs are linked to cytoskeleton or to intracellular
enzymes. Through these linkages, CAMs transfer signals in both directions across
cell membranes. Contact-dependent signalling is also known as juxtacrine
signaling.
3. local communication uses paracrine and autocrine signals.
Paracrine signal= chemical that acts on cells in immediate vicinity (nabijheid) of cell
that secreted the signal.
Autocrine signal= chemical signal that acts on cell that secreted it itself.
In some cases, molecule may act as both autocrine & paracrine signal.
Paracrine & autocrine signal molecules reach their target cells by diffusing through
interstitial fluid.
Bc distance is limiting factor for diffusion, paracrine signals restricted to adjacent
cells. Bv. Histamine, chemical released from damaged cells, when you scratch
yourself, the red wheal (striem) that arises is due in part to the local release of
histamine from injured tissue. Histamine acts as paracrine signal, diffusing to
capillaries in intermediate area of injury, making them more permeable to WBCs and
antibodies in plasma. Fluid also leaves blood vessels and collects it in interstitial
space, causing swelling around area of injury. Local signals; cytokines are regulatory
peptides, and eicosanoids are lipid-derived paracrine and autocrine signal molecules.
4. long distance communication may be electrical or chemical.
All cells in body can release paracrine signals, but most long-distance
communication between cells takes place through nervous and endocrine systems.
Endocrine system communicates through hormones, are chemical signals secreted
into blood and distributed all over body by circulation.
Hormones come in contact with most cells of body but only those cells with receptors
for the hormone are target cells.
Nervous system uses combi of chemical & electrical signals to communicate over
long distances. Electrical signal travels along nerve cell (neuron) until it reaches end
of cell and translates into chemical signal secreted by the neuron.
Neurocrine molecules= chemicals secreted by the neurons.
When a neurocrine molecules diffuses from neuron across a narrow extracellular
space to a target cell and has a rapid-onset effect it is called a neurotransmitter. If a
neurocrine acts slower than an autocrine/paracrine signal it’s called a
neuromodulator. If a neurocrine molecules diffuses into the blood for body-wide
distribution it is called a neurohormone.
Similarity between neurohormone and classic hormone secreted by endocrine
system bridge the gap between nervous and endocrine system, making them a
functional continuum instead of 2 distinct systems.
Cytokines refers to peptides that modulate immune responses and include variety of
regulatory peptides. Most of these peptides share similar structure of 4 or more a-
helix bundles. Families of cytokines include interferons, interleukins, colony-
stimulating factors, growth factors, tumour necrosis factors and chemokines.
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