Textbook notes-Chapter 6-Communication Integration and Homeostasis

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11 Aug 2010
Chapter 6- Communication, Integration, and Homeostasis
Cell to Cell Communication
x Electrical signals DUHFKDQJHVLQDFHOOVPHPEUDQHSRWHQWLDOZKHUHDVchemical signals are molecules secreted by cells
into the ECF
- Chemical signals are responsible for most communication within the body and the cells that receive electrical or
chemical signals are called target cells, or targets for short
x Our bodies use 4 basic methods of cell-to-cell communication
1. Gap junctions, which allow direct cytoplasmic transfer of electrical and chemical signals b/w adjacent cells
2. Contact-dependent signals, which occur when surface molecules on one cell membrane bind to surface molecules
on another cell membrane
3. Local communication by chemicals that diffuse through the ECF
4. Long-distance communication through a combo of electrical signals carried by nerve cells and chemical signals
transported in the blood
x A given molecule can function as a signal by more than one method
Gap Junctions Create Cytoplasmic Bridges
x The simplest form of cell-to-cell communication is the direct transfer of electrical and chemical signals through gap
junctions, protein channels that create cytoplasmic bridges b/w adjacent cells- Fig 6-1 (pg. 175)
- A gap junction forms from the union of membrane-spanning proteins, called connexions on two adjacent cells
- The united connexins create a protein channel (connexion) that can open and close
x When the channel is open, the connected cells function like a single cell that contains multiple nuclei
x When gap junctions are open, ions and small molecules like cyclic AMP diffuse directly from the cytoplasm of one
cell to the cytoplasm of the next
- As w/ other membrane channels, larger molecules cannot pass through gap junctions
- Gap junctions are the only means by which electrical signals can pass directly from cell to cell
- Movement of molecules through gap junctions can be modulated or shut off completely
Paracrine and Autocrine Signals Carry Out Local Communication
x Local communication is accomplished by paracrine and autocrine signalling
- Paracrine signals are secreted by one cell and diffuse to adjacent cells
- Autocrine signals act on the same cell that secreted them
- In some cases a molecule may act as both an autocrine and a paracrine signal
- Paracrine and autocrine signals reach their target cells by diffusing through the interstitial fluid
- Distance is a limiting factor for diffusion, therefore the effective range of paracrine signals is restricted to
adjacent cells
- A good eg of a paracrine molecule is histamine, a chemical released from damaged cells
Neural Signals, Hormones, and Neurohormones Carry Out Long-Distance Communication
x All cells in the body can release paracrine signals, but most long-distance communication b/w cells is the responsibility
of the nervous and endocrine systems
x The endocrine system communicated by using hormones, chemical signals that are secreted into the blood and
distributed all over the body by the circulation
- Hormones come in contact w/ most cells of the body, but only those cells w/ receptors for the hormone are
target cells- Fig 6-2 (pg. 177)
x The nervous system uses a combo of chemical and electrical signals to communicate over long distances
- An electrical signal travels along a nerve cell (neuron) until it reaches the very end of the cell, where it is
translated into a chemical signal secreted by the neuron (a neurocrine)
- If a neurocrine molecule diffuses from the neuron across a narrow EC space to a target cell and has a rapid
effect, it is called a neurotransmitter
- If a neurocrine acts more slowly as an autocrine or paracrine signal, it is called a neuromodulator
- If a neurocrine released by a neuron diffuses into the blood for distribution, it is called a neurohormone
- The similarities b/w neurohormones and classic hormones secreted by the endocrine system blur the distinction
b/w the nervous and endocrine systems, making them a continuum rather than two distinct systems
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Cytokines May Act as Both Local and Long-Distance Signals
x Cytokines are among the most recently identified communication molecules
- All nucleated cells synthesize and secrete cytokines in response to stimuli
- Cytokines control cell development, cell differentiation, and the immune response
- In development and differentiation, cytokines function as autocrine or paracrine signals
- In stress and in inflammation, some cytokines may act on relatively distant targets and may be transported
through the circulation just as hormones are
- Cytokines are not produced by specialized glands the way hormones are, and they are made on demand
- Most protein or peptide hormones are made in advance and stored in the endocrine cell until needed
Signal Pathways
x Chemical signals in the form of paracrine and autocrine molecules and hormones are released from cells into the EC
reach nearly every cell in the body
x Why do some cells respond to a chemical signal while other cells ignore it?
- The answer lies in the target-cell receptor proteins to which chemical signals bind
- A cell cannot respond to a chemical signal if the cell lacks the appropriate receptor proteins for that signal
x If a target cell has a receptor for a signal molecule, binding of the signal to the receptor protein will initiate a response
and all signal pathways share the following common features:
1. The signal molecule is a ligand that binds to a receptor. The ligand is also know as a first messenger b/c it
brings info to its target cell
2. Ligand-receptor binding activates the receptor
3. The receptor in turn activates one or more intracellular signal molecules
4. The last signal molecule in the pathway initiates synthesis of target proteins or modifies existing target proteins
to create a response
Receptor Proteins Are Located Inside the Cell or on the Cell Membrane
x Chemical signals fall into two broad categories based on their lipid solubility: lipophilic or lipophobic
x Target-cell receptors may be found in the nucleus, in the cytosol, or on the cell membrane as integral proteins
x Where a chemical signal binds to its receptor largely depends on whether the signal molecule can enter the cell- Fig 6-
4 (pg. 178)
x Lipophilic signal molecules can diffuse through the phospholipid bilayer of the cell membrane and bind to cytosolic
receptors or nuclear receptors
- receptor activation often turns on a gene and directs the nucleus to make new mRNA (transcription)
- The mRNA then provides a template for synthesis of new proteins (translation)
- Many of these signal molecules that follow this pattern are hormones
x Lipophobic signal molecules are unable to diffuse through the phospholipid bilayer of the cell membrane
- Instead, they remain in the ECF and bind to receptor proteins on the cell membrane (some lipophilic signal
molecules also bind to cell membrane receptors
- Generally, the response time for pathways linked to membrane receptor proteins is very rapid, and responses
can be seen within milliseconds to mins
- The simplest receptors are chemically gated (ligand-gated) ion channels called receptor-channels and ligand
opens or closes the channel and alters ion flow across the membrane
- The other 3 receptor types are: receptor enzymes, G protein-coupled receptors, and integrin receptors
- For all 3, info from the signal molecule must be passed across the membrane to initiate an intracellular response
- This transmission of info from one side of a membrane to the other using membrane proteins is known as signal
Membrane Proteins Facilitate Signal Transduction
x Signal transduction is the process by which an EC signal molecule activates a membrane receptor that in turn alters
intracellular molecules to create a response
x The EC signal molecule is the 1st messenger, and the intracellular molecules form a 2nd messenger system
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