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Download Overview of Anatomy and Physiology
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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