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

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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

  • 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


      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




  • Steps of Reflex

    Control (a review)

    Stimulus (internal or


    Sensory receptor

    Afferent path

    Integration center

    Efferent path

    Effector (target



  • Tonic Control

  • Antagonistic Control

  • Receptors (or Sensors)

    Different meanings for “receptor”:

    1. Sensory receptor



    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


    Endocrine system has

    no afferent pathway

    (stimulus comes

    directly into endocrine


  • 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-


    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


  • 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