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