Biol 373 Midterm 1 Study Notes
Unit 1: Introduction and Review
Function is the “why” – Why does the system exist? Why does the event occur?
Process is the “how” – How does a system work? physiological mechanisms
Homeostasis is the maintenance of a relatively stable internal envi...
Function is the “why” – Why does the system exist? Why does the event occur?
Process is the “how” – How does a system work? physiological mechanisms
Homeostasis is the maintenance of a relatively stable internal environment (especially
extracellular fluid) – oscillation around a set point
Walter Cannon – “flight or fight response” – expanded on Bernard’s concept of homeostasis
Study of homeostatic mechanisms = physiology
Failure to compensate for change = disease
Study of failure to compensate = pathophysiology
Local vs. Reflex Control
Control Systems and Homeostasis
Response loop: stimulus, sensor, input signal, integrating centre, output signal, target,
response
Negative feedback stabilizes variable
Feedforward control anticipates change
Positive feedback reinforces stimulus – not homeostatic
,Homeostasis is achieved by nervous and endocrine systems with their combination of electrical
and chemical signals
Electrical signal – changes in membrane potential – restricted to nerve and muscle cells
Chemical signal – excreted into ECF by all cells – responsible for most communication
Cells that respond to signals are ‘target cells’
Long range cell-cell communication
Endocrine: chemical (hormone) released into bloodstream and distributed throughout the
body
Neural: electrical signal travels down neuron; reaches end and is translated to chemical
signal (neurotransmitter) which transmits information to next cell
Neuro endocrine: electrical signal travels down neuron; reaches end and is secreted into
blood
Target cell
Only cells that have receptors for that signal will respond to it – signal molecule that
binds to a particular receptor is its ligand
Receptors are proteins that project outside of the membrane, or are within the cell in
the cytoplasm
Chemical properties of signal molecules (ligands) determine what type of receptor
they will interact with
Water soluble = hydrophilic = lipophobic surface receptor
Water insoluble = hydrophobic = lipophilic intercellular receptor
,Location of Receptors and Types of Membrane Receptors
Modulation of Signal Pathways
One ligand may have several different types of receptors – explains how same signal
can have different effects in different cell types
Cells can change their response to signals by changing receptor number or sensitivity
– increase # can increase gene expression (up regulation); decrease # can internalize
surface receptors (down-regulation); change receptor sensitivity (for example
phosphorylation)
, Receptors exhibit saturation, specificity, competition for their ligands (and molecules
similar to their ligands – e.g., relative affinities of adrenergic receptors with endogenous
ligands or agonists and antagonists competing with endogenous ligands
Many diseases and drugs disrupt signal pathways
Cholera
intestinal infection, Vibrio cholerae, from contaminated food and water
need to ingest ~100 million bacteria, but lower doses can cause infection in people
with reduced gastric acidity, young children and immune supressed individuals
a few bacteria survive acidity of the stomach and can reach the small intestine and
attach to and invade intestinal epithelial cells and produce toxin
This G-protein signal last as long as the alpha and beta-gamma subunits are free which is as long
as the G-alpha-subunit is bound to GTP
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