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