Unit 1

the science of the function of living systems

function –> why - why does the system exist -why does the event occur Process –> How -how does a system work - physiological mechanism

• maintenance of a relatively stable internal environment around a set point

Local - cells near site of change initiate response reflex control - cells at a distant site control response

stimulus, sensor, input signal, integrating centre, output signal, target, response

• negative feedback- stabilizes variable • feedforward control- anticipates change • positive feedback- reinforces stimulus – not homeostatic

• changes in membrane potential • restricted to nerve and muscle cells

• secreted into extracellular fluid by all cells • responsible for most of the communication • cells that respond to signals are called target cells

• endocrine -neural • neuro endocrine

chemical (hormone) released into blood stream and distributed throughout body

electrical signal travels down neuron; reaches end and is translated to chemical signal (neurotransmitter) which transmits information to next cell

electrical signal travels down neuron; reaches end and is secreted into blood

• 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 to 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 –> intracellular receptor

• ion channel • enzyme coupled receptor • G protein coupled receptor (GPCR) • integrin receptor

• one ligand may have several different types of receptors • receptors exhibit saturation, specificity, competition for their ligands (and molecules similar to their ligands) • cells can change their response to signals by changing receptor number or sensitivity

• same signal can have different effects in different cell types

• e.g. relative affinities of adrenergic receptors for epinephrine versus norepinephrine – e.g. agonists and antagonists competing with endogenous ligands

• increase –> ↑ gene expression (up-regulation) – decrease –> internalize surface receptors (down-regulation) – change receptor sensitivity –> e.g. phosphorylation 19

• a molecule that is similar to the natural ligand and it activates the receptor

• a molecule that is similar to the natural ligand but it does not activate the receptor (blocker)

• the nervous system has a role in maintaining ‘fitness’ of the internal environment (coordinates responses that regulate blood volume, blood pressure, osmolarity, body temp, …) • some systems are under tonic control • some systems are under antagonistic control • one chemical signal can have different effects in different tissues

• regulates physiological parameters in an “up- down” fashion • such as a neuron increasing or decreasing its firing rate