1. 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 CO 2 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

2. 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)

3. Feedback Loops for Reflex Control

4. 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

6. 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

7. 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

8. 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)

10. 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

12. 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