Download Overview of Anatomy and Physiology

Homeostasis
• Homeo = similar, stasis = condition
• Defined as the ability to maintain a relatively stable
internal environment
• The human body maintains hundreds of
physiologically controlled parameters (variables) close
to a setpoint
– body temperature, blood glucose levels, blood
pressure, body CO2 levels, body pH…
– these parameters are constantly changing due to
external and internal changes
• The body uses control systems to maintain these
parameters at or close to their respective setpoint
Feedback Loops for Reflex Control
• The beginning of a reflex pathway is a disturbance in a
controlled parameter called a stimulus
• The stimulus is detected by a sensor (receptor)
– continuously monitoring the environment
– when a change is detected, it sends out a signal
• The signal travels from the receptor by way of an
afferent pathway to the control (integrating) center
• The control center evaluates the incoming signal,
compares it to the homeostatic setpoint of the
parameter and decides on the appropriate response
• The control center sends out a signal that travels by
way of an efferent pathway to the effector
• The effector is a cell or tissue that carries out the
appropriate response to bring the parameter back to
within normal limits (setpoint)
Feedback Loops for Reflex Control
Feedback Loops
• Most (over 99%) feedback loops are referred to as
negative feedback loops where the response of the
effector opposes or removes the cause of the
parameter’s imbalance
– can restore the normal state of the parameter, but
cannot prevent the initial disturbance out of the
normal range
• The minority (less than 1%) of the feedback loops are
referred to as positive feedback loops where the
response of the effector reinforces the a stimulus
rather than opposing or removing it
– the response destabilizes the parameter triggering
a viscous cycle of ever increasing response and
sending the system temporarily out of control
Cellular Communication
In order to maintain a state of homeostasis, the body’s
100 trillion cells need to communicate in a manner that
is rapid and conveys a tremendous amount of
information and occurs by 2 types of physiological
signals
• Chemicals
– molecules that are secreted from cells into the
extracellular fluid
– bind to protein receptors on/in target cell to elicit a
response in target cell
• Electrical
– changes in the membrane potential of a cell due to
an increase or decrease in ion diffusion across the
cell membrane
Maintenance of Homeostasis
• Local Control (short distance)
– relatively isolated change occurs in the vicinity of a
cell to evoke a localized response through the
secretion of chemicals from the affected cells
– the secreted chemicals diffuse a short distance and
affect neighboring cells
– the response is restricted to the region of cells that
received the secreted chemical
• Reflex Control (long distance)
– response to more widespread or systemic changes
– control of the response to a change occurs outside
the organ that carries out the response
– uses the nervous and or endocrine system through
feedback loops to receive input about a change,
integrate the information and react appropriately
Local Control (short distance)
• Gap junctions
– direct cytoplasmic transport of electrical (ions) and
or chemical signals between adjacent cells
• Contact-dependent signals
– cell surface molecules on the cell membrane of one
cell attach to cell surface molecules on the cell
membrane of an adjacent cell
• Autocrine and Paracrine
– chemical signals that are released into the
extracellular fluid from one cell diffuses a short
distance to regulate itself (autocrine) and or a
neighboring cell (paracrine)
Reflex Control (long distance)
• Chemical signals (hormones or neurohormones)
transported via the circulatory system to the target
cells
• Electrical signals (action potentials) carried along
axons of nerve cells (nervous) which result in the
secretion of neurotransmitters directly onto the target
cells
Neurotransmitter vs Hormonal Control
• The responses to neurotransmitters are:
– very rapid
• action potentials travel at speeds up to 270 mph
• response occurs within 0.005 sec. after secretion
– very short lived (simple reflex)
• neurotransmitters are either rapidly hydrolyzed in
the synaptic cleft or are endocytosed out of the
synaptic cleft back into the neuron
• The responses to hormones are:
– slow
• distribution by blood can take seconds to minutes
• responses at target can take minutes to hours
before it can be measured
– long lasting
• hormones can stay in the blood for minutes to
days continuously causing an effect on the target