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Chapter 6, Part 2: Homeostasis and “Homeodynamics”

Chapter 6, Part 2: Homeostasis and “Homeodynamics”

Chapter 6, Part 2: Homeostasis and “Homeodynamics”

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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

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