Chapter 6, Part 2: Homeostasis - Las Positas...

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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

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

α- and β-receptors (fig 6-18)

E.g., Specificity:

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

Constantly monitor environment

External or Internal

Threshold (= minimum stimulus necessary to initiate response)

Afferent Integration Efferent

Fig 6-23

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 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

Homeostasis = Dynamic Equilibrium with

Oscillation around Set Point

Fig 6-26

Negative Feedback Example

fig 6-28:

+ Feedback

Loop

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

NS & ES are

linked in a

continuum