Chapter 6, Part 2: Homeostasis and Homeodynamics

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Chapter 6, Part 2 Homeostasis and
Homeodynamics
Control of Processes
Fig 6-19

• Cannon's Postulates (concepts) of properties of
  homeostatic control systems
• Nervous regulation of internal environment
• Tonic level of activity
• Antagonistic controls (insulin/glucagon)
• Chemical signals can have different effects on
  different tissues (e.g., a and ß receptors)
• Failure of homeostasis?

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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.)

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Up- vs. Down-regulation

• Up
• ? Receptors (e.g., exocytosis)
• ? Affinity for ligand
• Down (think drug tolerance)
• Add competitors
• Desensitization of receptors
• Intracytoplasmic changes

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a- and ß-receptors (fig 6-18)
E.g., Specificity
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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

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Control Pathways Response and Feedback Loops (p
191)

• Maintain homeostasis
• Local paracrines and autocrines
• Long-distance - reflex control
• Nervous
• Endocrine
• Cytokines

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Steps of ReflexControl (a review)
Stimulus (internal or external) Sensory
receptor Afferent path Integration
center Efferent path Effector (target
cell/tissue) Response
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Tonic Control
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Antagonistic Control
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Receptors (or Sensors)

• Different meanings for receptor
• Sensory receptor
• Peripheral
• Central
• Membrane receptor
• 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
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New definition!
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Afferent Pathway

• From receptor to integrating center.
• Same as the Reflex Pathway
• Endocrine system has no afferent pathway
  (stimulus comes directly into endocrine cell)

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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)

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Efferent Pathway

• From integrating center to effector
• NS ? electrical and chemical signals
• Action Potential
• ACh
• ES ? chemical signals
• hormones

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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 -

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In a feedback loop, effectors that bring about
change receive information from

• a stimulus
• the control center
• receptors
• a response mechanism

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Responses at 2 levels

• Cellular response of target cell, e.g.,
• opening or closing of a channel
• Modification of an enzyme etc...
• Systemic response at organismal level
• vasodilation, vasoconstriction
• Lowering of blood pressure etc....

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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
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Homeostasis Dynamic Equilibrium with
Oscillation around Set Point
Fig 6-26
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Negative and Positive Feedback
NOT homeostatic !!
Homeostatic
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Negative Feedback Example
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Feedback Loop
fig 6-28
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Both insulin and glucagon are peptide hormones
that target liver cells. The response of the
target cells to each of these two hormones is
opposite. This information implies that

• the two hormones bind to different cell surface
  receptors.
• one hormone binds to a receptor on the cell
  membrane and the other to an intracellular
  receptor.
• each of the two hormones uses a different second
  messenger.
• both hormones interact with receptors at the cell
  nucleus.
• A and C

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Positive feedback loops in models assume that
secondary effects ________ the basic trend, while
negative feedback loops assume that secondary
effects tend to  _________ the basic trend.

• counteract reinforce
• reinforce counteract.
• self-limit retard.
• enhance promote.

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The Bodys 2 Control Systems

• Variation in speed, specificity and duration of
  action
• The two systems allow for 4 different types of
  biological reflexes
• Simple (pure) nervous
• Simple (pure) endocrine
• Neurohormone
• Neuroendocrine (different combos)

Fig 6-30
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NS ES are linked in a continuum
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