Chapter 6, Part 2: Homeostasis and âHomeodynamicsâ
Chapter 6, Part 2: Homeostasis and âHomeodynamicsâ
Chapter 6, Part 2: Homeostasis and âHomeodynamicsâ
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Developed by<br />
John Gallagher, MS, DVM<br />
<strong>Chapter</strong> 6, <strong>Part</strong> 2: <strong>Homeostasis</strong><br />
<strong>and</strong> “Homeodynamics”<br />
Control of Processes<br />
Fig 6-19<br />
Cannon's Postulates (concepts) of properties of<br />
homeostatic control systems<br />
1. Nervous regulation of internal environment<br />
2. Tonic level of activity<br />
3. Antagonistic controls (insulin/glucagon)<br />
4. Chemical signals can have different effects on<br />
different tissues (e.g., α <strong>and</strong> β receptors)<br />
Failure of homeostasis
Modulation of Signal Pathways<br />
Receptors exhibit :<br />
Saturation, yet<br />
• Receptors can be up- or down-regulated (e.g. drug tolerance)<br />
Change the number of or binding affinity of the receptor<br />
Specificity, yet<br />
• Multiple lig<strong>and</strong>s for one receptor: Agonists (e.g. nicotine) vs.<br />
antagonists (e.g. tamoxifen, finasteride)<br />
• Multiple receptors for one lig<strong>and</strong> (see Fig 6-18)<br />
Competition<br />
• Aberrations in signal transduction causes many diseases (table 6-3)<br />
• Many drugs target signal transduction pathway (SERMs, -blockers etc.)
Up- vs. Down-regulation<br />
Up<br />
• Receptors (e.g., exocytosis)<br />
• Affinity for lig<strong>and</strong><br />
Down (think: drug tolerance)<br />
• Add competitors<br />
• Desensitization of receptors<br />
• Intracytoplasmic changes
E.g., Specificity:<br />
α- <strong>and</strong> β-receptors (fig 6-18)
In Summary:<br />
Receptors Explain Why<br />
Chemicals traveling in bloodstream act<br />
only on specific tissues.<br />
• No receptor, no activity<br />
One chemical can have different effects<br />
in different tissues.<br />
• May have + or - effect
Control Pathways: Response <strong>and</strong><br />
Feedback Loops (p 191)<br />
Maintain homeostasis<br />
• Local – paracrines <strong>and</strong> autocrines<br />
• Long-distance<br />
- reflex control<br />
Nervous<br />
Endocrine<br />
Cytokines
Steps of Reflex<br />
Control (a review)<br />
Stimulus (internal or<br />
external)<br />
Sensory receptor<br />
Afferent path<br />
Integration center<br />
Efferent path<br />
Effector (target<br />
cell/tissue)<br />
Response
Tonic Control
Antagonistic Control
Receptors (or Sensors)<br />
Different meanings for “receptor”:<br />
1. Sensory receptor<br />
Peripheral<br />
Central<br />
2. Membrane receptor<br />
3. Endocrine cells act as receptor <strong>and</strong> effector<br />
Constantly monitor environment<br />
• External or Internal<br />
Fig 6-23<br />
Threshold (= minimum stimulus necessary to initiate response)<br />
Afferent Integration Efferent
New definition!
Afferent Pathway<br />
From receptor to<br />
integrating center.<br />
<br />
Same as the Reflex<br />
Pathway<br />
Endocrine system has<br />
no afferent pathway<br />
(stimulus comes<br />
directly into endocrine<br />
cell)
Integrating Center<br />
Neural reflexes usually in the<br />
CNS; endocrine integration in<br />
the endocrine cell itself<br />
Receives info about change<br />
Interprets multiple inputs <strong>and</strong><br />
compares them with setpoint<br />
Determines appropriate<br />
response (→ alternative name:<br />
control center)
Efferent Pathway<br />
From integrating center to<br />
effector<br />
NS electrical <strong>and</strong><br />
chemical signals<br />
<br />
<br />
Action Potential<br />
ACh<br />
ES chemical signals<br />
<br />
hormones
Effectors<br />
Cells or tissues carrying<br />
out response<br />
Target for NS:<br />
Muscles, gl<strong>and</strong>s <strong>and</strong> some<br />
adipose tissues<br />
Target for ES:<br />
Any cell with proper receptor<br />
May be + or -
Responses at 2 levels:<br />
1. Cellular response of target cell,<br />
e.g.,<br />
• opening or closing of a channel<br />
• Modification of an enzyme etc...<br />
2. Systemic response at organismal<br />
level<br />
• vasodilation, vasoconstriction<br />
• Lowering of blood pressure etc....
Feedback Loops Modulate the<br />
Response Loop<br />
Response loop is only half of reflex! <br />
Response becomes part of stimulus<br />
<strong>and</strong> feeds back into system.<br />
Purpose: keep system near a “Set<br />
Point”<br />
• E. g., Household thermostat<br />
• Circadian rhythms are changes in setpoint<br />
Two types of feedback loops:<br />
• - feedback loops (homeostatic)<br />
Fig 6-25<br />
• + feedback loops (not homeostatic)
<strong>Homeostasis</strong> = Dynamic Equilibrium with<br />
Oscillation around Set Point<br />
Fig 6-26
Negative Feedback Example
+ Feedback<br />
Loop<br />
fig 6-28:
The Body’s 2 Control Systems<br />
Variation in speed, specificity <strong>and</strong><br />
duration of action<br />
The two systems allow for 4 different<br />
types of biological reflexes<br />
1. Simple (pure) nervous<br />
2. Simple (pure) endocrine<br />
3. Neurohormone<br />
4. Neuroendocrine (different combos)<br />
Fig 6-30
NS & ES are<br />
linked in a<br />
continuum