Homeostasis and Integration

• The parameters of homeostasis• Integration through feed back loops• Nervous, endocrine and neuroedocrine

reflexes• Midterm test:

– Monday Oct. 7: 8:30 – 9:20– multiple choice questions – Location: A- P in the Education Gym– Q-Z in Erie Hall (lecture hall)

What does all this have to do with homeostasis?

• Maintain a stable internal environment

• Parameters that are under homeostatic control– Factors affecting cells: osmolarity, pH,

temperature– Materials that cells need: nutrients, water,

sodium, calcium, oxygen, signaling molecules

Properties of homeostasis

• The nervous system co-ordinates many functions

• Many functions have “tone”. These are not merely “on or off”. E.g. blood vessels, hearing

• Some functions occur by antagonistic control (glucagon/insulin)

• The effect of a ligand depends on the receptor

The effect of a ligand depends on the receptor

Antagonistic control of heart rate:-parasympathetic stimulation

Antagonistic control of heart rate:-sympathetic stimulation

Homeostatic mechanisms are local or over long distances

• E.g. of local control, paracines.

• Long distance, reflex controls usually include nervous or endocrine components

A response loop for a reflex pathway

• An input signal• Integration of the signal• Output signal

• Stimulus receptor afferent pathwayintegrating center efferent pathway

effector response

Sensory receptor systems

• Stimulation can be external or internal• External – special senses• Internal – pH, osmolarity, baroreceptors

• All sensory receptors have a threshold stimulus for evoking a response

Afferent Pathway

• From the sensory receptor to the integrating center

Integrating center

• Receives information• Response is initiated• Sometimes two conflicting inputs arrive to

the integrating center

Efferent pathway

• If nervous reflex, the pathway of the nerve• If endocrine, the target cell with

appropriate receptors

The response loop

Name different response loops

• Automatic pet watering system

Negative feedback loops

• preserve homeostasis (osmolarity, oxygen tension)

• Stabilize the physiological variable• Can restore normal state• What if the disturbance is beyond the

normal range? (catastrophic)

Positive feedback loops

• Foster change• Reinforce the stimulus• E.g childbirth, development,

Feed forward

• A stimulus warns a regulatory system• E.g. the odor of food is a feed forward

stimulus. salivation and secretion of digestive enzymes

Some reflexes are cyclic

• Circadian rhythm, on a 24 hour cycle