Homeostasis and Intercellular Signaling Flashcards
a particular physiologic parameter that is
monitored and maintained within a relatively narrow range at all times
Stability
Examples of physiological parameters
blood glucose levels, body
temperature, blood pressure, ECF osmolarity
When a process proceeds in the forward and backward direction at the same rate – no net change occurs. What is an example of this?
Equilibrium
- a chemical reaction
No energy is expended or regulation occurs
When a process or system exerts energy to maintain a particular state that is optimal for its function. What is an example of this?
Dynamic steady state
For example – the Na+/K+ pump moves sodium out of the cell, preventing swelling (maintaining a constant volume)
this steady state is not monitored with clear feedback loops and sensors (that we know of)*
component of homeostasis: give an example of the regulated variable
blood glucose level
component of homeostasis: what does an internal disturbance have an impact on?
any change in the structure or function of the organisms that results in a change to the magnitude of the regulated variable
component of homeostasis: what does an external disturbance have an impact on?
any change in the conditions of the external environment that results in a change to the internal environment
a parameter that is measured in the body with sensors and is kept within a set of limits
Regulated variable
Limit = between a low and a high range
i.e. extracellular pH – between 7.35 and 7.45
what is the “thing” that homeostasis is devoted to regulating?
Regulated variable
a process that can measure the regulated variable and deliver a signal about that variable
a sensor
- can be constant or when happen the regulated variable falls outside of the “normal” range
This can be a cell, or a biochemical reaction, or a channel, or a
tissue – good example is a baroreceptor
Sometimes signals only happen when the regulated variable falls ________ of the “normal” range
outside
Usually, signalling is __________, reflecting the overall state of the regulated variable
constant
has a controller and an error detector
Control centre
“calculates” the difference between the set-point value of the regulated variable and the actual value of the regulated variable and sends an error signal to the controller
Error detector
sends output signals to effectors that can change the regulated variable depending on data (error signal) from the error
detector
Controller
– the range of values of the regulated variable that the system tries to maintain
Setpoint
Very difficult to “find where the set-point exists” in a biological system
what the controller manipulates to “get the job done”. ________ respond to information from the controller and change the value of the regulated variable
They will change the regulated variable in such a way that it comes closer to the setpoint
effectors usually change values of the non-regulated variable in order to bring the regulated variable closer to the set point
- Nonregulated variable
example of nonregulated variable: if your pH drops, then your respiratory rate increases in order to “get rid of” excess carbon dioxide. What is the regulated variable and what is the non-regulated variable?
Regulated variable = pH
Non-regulated variable = respiratory rat
Are homeostasis and negative feedback
the same?
Note that there is no:
- Control centre
- Setpoint
- Error signal
- Regulated variable
- Therefore, this is not what physiologists consider homeostasis
where are the major baroreceptors? (2)
- carotid arteries
- arch of the aorta
baroreceptors: once the pressure drops, the message is sent to the brainstem via nerves…then what?
- Activation of the sympathetic nervous system > release of epinephrine, norepinephrine
- Epi and NE > Elevation in HR and constriction of arterioles and increased stroke volume
a proprioceptor that senses muscle stretch
Muscle spindle
- As the muscle is stretched: activates the
muscle to contract against the stretch
by stimulating the _____ neuron in the _______ horn
motor neuron in the ventral horn
- inhibits the antagonist muscle
relaxes: Stretch caused by hitting the _______ with a reflex hammer
tendon
Do homeostatic mechanisms only “turn on” when the regulated variable is outside the setpoint?
No, most sensors usually constantly deliver information to the control centre
The controller responds “more intensely” with a larger error signal… but it’s almost always sending input to the effectors at some basal rate
Homeostatic systems tend to _______, rather than be ________
overlap, rather than be isolated
Effectors can be “turned up” or “turned down” to achieve homeostasis, or different effectors can be called upon to control the regulated variable. What is an example of effectors being turned up down? What is an example of different effectors being called upon?
BP regulation – effectors are “turned up” or “turned down”
Glucose regulation – separate metabolic pathways are activated in hyperglycemia vs. hypogycemia
are oscillations in the regulated variable typical?
yes. Regulated variables fluctuate constantly
what can cause larger fluctuations that are unstable and altered set points
disease
what is an example of a disease state causing an altered set point?
hypertension is partially a “set point
error” disease
- the new normal is just high
The more vital the parameter, the ______ number of systems that regulate it
greater
redundancy is common!
The output of a system is fed back in a manner that tends to increase that system’s output. This tends to result in an exponential “increase” in the output until a limited event is reached
positive feedback
what is an example of a positive feedback loop?
Basics of the parturition reflex:
Limiting event – baby is expelled through the birth canal, ending the feedback loop
a system where changes in a regulated variable are anticipated, and the controller “pro-actively” activates an effector
Feed-forward loop
a change is anticipated and you prepare for it before it happens
example of feed-forward loop
Example – visualizing performance prior to an athletic event
- Heart rate, stroke volume, BP, and respiratory rate increase
For example – muscle proprioceptors detect an increase in activity
- Signals to the respiratory centre to increase ventilatory rate before any changes in blood gases occur
Voluntary control of respiratory rate
Cerebral Cortex
Regulates respiratory rate based on emotional state, pain, body
temperature set-points -> tells the brainstem to change ventilation
Hypothalamus
When your muscles and joints move, sends a signal to your brainstem -> your ventilation changes in anticipation of increased MSK oxygen and carbon dioxide exchange needs
Proprioceptors
Increase ventilation when arterial oxygen drops and carbon dioxide increases. These have a strong influence on ventilation
Chemoreceptors
* Peripheral
* Central
what are four ways that cells signal to each other?
contact
paracrine
endocrine
Nervous (synatpic)
Membrane receptors contact the ECM or another cell (ligand) > an intracellular signal in one (or both) cells
contact intercellular signalling
Cell “A” produces a soluble messenger which diffuses to a cell “B” > binding to a membrane receptor on cell “B” > an intracellular signal in cell “B”
Paracrine intercellular signalling
what type of intercellular signalling is important for embryologic development, immune signaling, and for limiting/organizing growth
Contact
Cells in endocrine organs release a chemical messenger into the bloodstream > circulation of the messenger (hormone) > an intracellular response in any cell that has a receptor for that hormone
Endocrine intercellular signalling
A neuron “A” sends an electrical signal along an axon to a synapse with cell “B” > release of a neurotransmitter > binding of neurotransmitter to a receptor on cell “B”
> an intracellular response
Cell “B” could be another neuron, a muscle cell (smooth, skeletal, cardiac), or an endocrine cell
Nervous (synaptic) intercellular signalling
signal cell does:
a target cell:
signal cell sends out the signal while a target cell is the cell receiving the singal
example of contact intercellular signalling
Example – epithelial cells contact the
basement membrane via hemidesmosomes
- Integrins are part of the
hemidesmosome complex – when
they bind the ECM, intracellular
signals are generated - These signals help determine
polarity (which way is “up”)
which types of cell signalling are short-distance signalling
contact and paracrine
what type of intercellular signalling is Immunological/defence, signals of local
damage, Regulation of growth/cell division/tissue repair and Local regulation of blood flow
paracine
Everyday (every second) example of paracine signalling
- Metabolically active tissue releases
metabolites (H+, CO2, K+) that cause
local vascular endothelial cells to relax > - Vasodilation and improved blood flow
what type of cell signalling is long-distance
Endocrine
What is the mechanism for endocrine intercellular signalling
Organ secretes a messenger into the bloodstream>
Messenger is widely distributed throughout the body >
Cells with receptors for the messenger respond
The _____________ controls the endocrine secretions of the pituitary gland. Pituitary secretions act on another target gland or
organ.
hypothalamus
hypothalamus to pituitary to another thing
How does “other” endocrine glands work?
These glands are not under hypothalamic or pituitary control
Usually they directly sense a stimulus (they are the sensor and the control centre) and secrete a hormone in response to that stimulus
what are examples of “other” endocrine glands?
- Pancreas and GI tract
- Parathyroid glands
- Adipose tissue
Which of these glands are under hypothalamic control?
The hypothalamus ultimately affects the functions of the pituitary gland, thyroid gland, adrenal glands, kidneys, musculoskeletal system, and reproductive organs (testes, ovaries)
The hypothalamus sits just under the thalamus and is connected to the pituitary gland via a
vascular stalk
The pituitary gland sits within the
sella turcica
middle cranial fossa within your sphenoid bone
General model of the hypothalamic-pituitary system
Hypothalamic signal > stimulates pituitary cells > pituitary cells release a hormone > hormone acts on another gland (usually endocrine) > “target” gland secretes larger quantities of a hormone > general systemic response
what are the major hormones secreted by the posterior pituitary? Where do these act?
ADH and oxytocin
Both of these act directly on target tissues, not on other glands
ADH (anti-diuretic hormone) controls _______ balance in the body
water
One of the two hormones secreted by the posterior pituitary
Other is oxytocin (hormone that controls the positive feedback loop of childbirth)
ADH secretion is controlled by the osmolarity of the blood – when the blood is ______ concentrated (i.e. ______ water), then ADH is secreted
more concentrated, less water
When ADH is secreted, then more water is “recovered” by the kidneys and kept in the bloodstream
When ADH decreases, more water is lost in the urine
Blood osmolarity is detected by ___________ in the hypothalamus
osmoreceptors
