Chapter 6, Part 2: Homeostasis - Las Positas...
Transcript of Chapter 6, Part 2: Homeostasis - Las Positas...
Chapter 6, Part 2: Homeostasis
and “Homeodynamics”
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?
Fig 6-19
Control of Processes
Developed by
John Gallagher, MS, DVM
Modulation of Signal Pathways
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.)
Receptors exhibit :
Up- vs. Down-regulation
Up
Receptors (e.g., exocytosis)
Affinity for ligand
Down (think: drug tolerance)
Add competitors
Desensitization of receptors
Intracytoplasmic changes
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
Receptors (or Sensors)
Different meanings for “receptor”:
1. Sensory receptor
Peripheral
Central
2. Membrane receptor
3. Endocrine cells act as receptor and effector
Constantly monitor environment
External or Internal
Threshold (= minimum stimulus necessary to initiate response)
Afferent Integration Efferent
Fig 6-23
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 at 2 levels:
1. Cellular response of target cell, e.g.,
opening or closing of a channel
Modification of an enzyme etc...
2. 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
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. Simple (pure) nervous
2. Simple (pure) endocrine
3. Neurohormone
4. Neuroendocrine (different combos)
Fig 6-30