Download Lecture 4: Pathways and Communication

Lecture 4: Pathways and Communication
Silverthorn: Chapter 6
Body systems work together in a coordinated fashion to maintain homeostasis. In order to do this, organ systems must communicate
with each other. Today, we’ll explore several ways that the body facilitates this communication.
Cell to Cell Communication
Cells communicate in the language of chemistry. They don’t form words...they initiate chemical reactions. There are only 2 ways
cells initiate these chemical reactions (and thus communicate with each other). They send either chemical signals...or electrical ones.
The signals pass from cell to cell via 4 primary mechanisms.
1. Gap junctions
A. Cells that are connected via gap junctions are able to directly share their cytoplasm.
B. Both chemical and electrical signals can travel between cells using gap junctions.
Do gap junctions enable local or long-distance communication?
2. Surface molecules on one cell membrane bind to surface molecules on another cell membrane.
A. This requires cells to touch each other.
B. Happens often with immune cells that travel around the body checking on the rest of the cells.
Does communication between surface molecules on cells enable local or long-distance
communication?
3. Paracrine and autocrine signals diffuse through interstitial fluid to act on cells in the neighborhood.
A. Autocrine signals are secreted by a cell, then bind to receptors on the SAME cell.
B. Paracrine signals are secreted by a cell, then bind to receptors on CLOSE-BY cells.
Do paracrine and autocrine signals enable local or long-distance communication?
Are these examples of chemical or electrical signals?
4. Long distance signals travel via neurons or in the blood.
A. Neurons transmit electrical signals long distances. For example, the neuron that tells muscles in your big toe to contract,
receives its instructions from the spinal cord, via spinal nerve L5!
B. Hormones transmit chemical signals long distances via the blood.
i. There are many chemicals we will be exposed to. Hormones, cytokines, eicosanoids...oy.
ii. Distinguishing between them can get fuzzy...don’t stress about that.
Cells respond to chemical signals if they have the right RECEPTOR
No matter how the information is passed from cell to cell, the information can only be “understood” if the receiving cell has the
correct equipment. If a cell has the right receptor, then it can transmit a signal...or interpret the message and MAKE SOMETHING
HAPPEN.
1. A signal molecule (hormone, neurotransmitter, cytokine, etc) approaches a target cell.
2. The molecule binds a receptor, initiating some sort of “shape change”.
A. These receptors are often embedded in the cell membrane.
B. They can also be found in the cytoplasm, or in the nucleus.
3. The receptor activates some sort of response.
A. Ion channels open or close.
B. Enzymes are activated, initiating chemical reactions.
C. Second messenger systems begin, producing a large reaction inside the cell.
i. This is called “signal transduction” and can result in an amplified response.
ii. Signal transduction converts one form of information into a response.
Bio 7: Human Physiology
12
Spring 2014: Riggs
Fine tuning the response
Pathways can be quite complicated, as you’re probably beginning to appreciate. There are several ways that signaling molecules
interact with receptors, leading to a diversity of responses (and ultimately, finer control).
1. One signaling molecule can bind to different receptors!
A. The nature of the receptor determines the result of binding.
i. Epinephrine (adrenalin) initiates vasoconstriction in the intestines by binding α-adrenergic receptors.
ii. Epinephrine initiates vasodilation in skeletal muscles by binding β-adrenergic receptors.
B. Receptors are found where certain responses are needed.
i. In other words, α-adrenergic receptors are found in the intestines, where vasoconstriction is warranted.
ii. β-adrenergic receptors are found in the skeletal muscles, where vasodilation is necessary.
Why would different receptors be needed in skeletal muscle vs. intestines?
2. Sometimes more than one molecule can bind to one receptor
A. This can increase the chances of having a receptor activated
i. In a sympathetic NS response, epinephrine and norepinephine have the same effect on the body..they bind to the same
receptors!
B. If the molecule initiates the same response as the original molecule, it is considered an AGONIST.
C. If the molecule inhibits the response, it is considered an ANTAGONIST.
Drugs can act as agonists or antagonists. Describe an example of each. (Feel free to
make up your example!)
3. Sometimes cells change the number of receptors they express.
A. Up-regulation is when a cell increases the number of receptors. This can result in increased sensitivity to a molecule.
B. Down-regulation is when a cell decreases the number of receptors, resulting in decreased sensitivity to the molecule.
C. Desensitiziation happens when a cell prevents the molecule from initiating a response in the first place.
Which of these would result in “tolerance” to a drug?
4. Terminating the signal
A. Digest the signaling molecule with and enzyme...
B. Reabsorb the signaling molecule into wherever it came from.
C. Endocytose the receptor to remove it from the cell membrane!
Speculate on how a drug might give you that “feel good” feeling.
Putting it all together: Pathways to maintain homeostasis
These pathways often include endocrine and neural components.
1.
2.
3.
4.
5.
6.
7.
Stimulus
Activation of a sensory receptor
Afferent message sent to the integrator
Integrating center compares info to the desired state
Efferent message sent
Target cell/organ
Response
Bio 7: Human Physiology
13
Spring 2014: Riggs
Ext Brain 4: Pathways and communication
Complete all the following tasks on a separate sheet of paper and include it in your External Brain binder. Remember the rules for the
External Brain. All work must be your OWN, although you may use UNLABELED images if you want. Make sure you cite all your
sources!
Study Guide Questions
1. Explain how gap junctions are involved in communication between cells.
2. Distinguish between autocrine signals and paracrine signals.
3. Compare and contrast electrical and chemical signals.
4. Describe how information can be transmitted over a long distance.
5. What is a hormone?
6. What is a receptor?
7. What role does a receptor play in transmitting a signal (or relaying information)?
8. Generally compare and contrast α-adrenergic receptors and β-adrenergic receptors.
9. Compare and contrast agonists and antagonists. Given any scenario, be able to identify a molecule as an agonist or
antagonist.
10.Suggest a mechanism by which a cell might become more sensitive to a particular signal.
11. Suggest a mechanism by which a cell might become less sensitive to a particular signal.
12.What would happen if a signal could not be terminated?
13.Describe several mechanisms by which signals are terminated.
14.Be able to identify all of the following components of a communication pathway.
a. Stimulus
b. Activation of a sensory receptor
c. Afferent message sent to the integrator
d. Integrating center compares info to the desired state
e. Efferent message sent
f. Target cell/organ
g. Response
15.Determine whether a given pathway will maintain homeostasis or disrupt homeostasis.
Bio 7: Human Physiology
14
Spring 2014: Riggs