Pg. 8 #10-12
Homeostasis and
Feedback Loops
Definitions

Homeostasis – state of balance in the body

Stimulus- something in the environment that causes a
change (can be internal)

Response – action of organism as a result of stimulus

Feedback mechanisms/loops – processes by which the
body maintains levels
– Usually uses nervous and hormonal cues to conduct
processes
Steps in a Homeostatic process
When a stimulus
occurs, each of these
steps must occur in
order for a response
to be initiated
 1) receptor must
recognize stimulus

– Must be enough of a
stimulus to surpass
nervous threshold
Sensory Neuron

2)sensory neuron
must transmit
message from the
receptor, to a control
center
– Neuron = nerve cell
– Most receptors are
bundled together on
individual neurons by
region
Control center

3)message is carried
to a control center
(usually in brain),
where the message is
interpreted, and the
correct response is
coordinated.
– In the case of a reflex
action, spinal chord is
C.C.
Motor neuron

4)Carries message
from control center to
effector organ
dictating response
Effector organ

5)Receives message
and conducts actual
response
Afferent vs. Efferent pathway
Afferent pathway
 Includes receptor and
sensory neuron (aka
afferent neuron)

Efferent pathway
 Includes motor
neuron (efferent
neuron) and effector
organ

Positive Feedback loop

Process by which
stimulus causes a
response which will
increase the stimulation

Ie. Oxytocin in childbirth

Rate constantly
increases until action is
complete

Least common
Graph of Positive Feedback

X-axis is time

Y-axis is hormone
secreted
Negative Feedback loops
Receptors detect change
and initiate response
that will counter that
change
 Tries to maintain a
median
 Ie. Blood glucose, temp,
blood O2/CO2 levels
 (draw graph from board
into your notes)

Example: Breathing Pattern


Body is designed to detect blood pH.
Too high = too much CO2





Receptor = chemoreceptors in arteries
Afferent neuron = vagus nerve
C.C. = brain stem
Efferent neuron = phrenic nerve
Effector organ = intercostals and diaphragm

High pH will trigger quicker breathing, lower pH
will slow it down (negative feedback)