Lecture 2 (Sept 26)Homeostasis, Steady State, and Biological Control Systems
Homeostasis vs. Steady State
Homeostasis: The term used when the body is at rest and an environmental stimulus occurs, to which the body attempts to adapt. It represents the maintenance of a relatively constant internal environment at rest. For instance, blood pressure, even at rest, fluctuates above and below a mean level, which indicates continuous fine-tuning by biological control systems.
Fluctuations: These fluctuations signify two things:
Biological control systems, designed to keep variables stable, are not perfectly precise.
There is a relatively narrow range within which physiological variables, such as blood pressure (e.g., less than 1 mmHg fluctuation), are allowed to deviate before sensors trigger corrective actions to bring them back towards the set point.
Steady State: This term is used when the body is outside its normal resting levels but physiological variables are relatively constant and unchanging at some point during a sustained activity. It is not homeostasis because the individual is not at rest.
Example: Change in body temperature during one hour of light to moderate intensity aerobic exercise (e.g., treadmill). While the body's core temperature (normal resting value around 37 degrees Celsius) may rise for the first 40 minutes, it then plateaus and remains relatively flat for the remaining 20 minutes of the exercise bout.
Achieving Steady State: This indicates that it takes time (e.g., 40 minutes in the example) for the body's biological control systems for temperature to fully take effect. This involves mechanisms like increased respiratory rate and depth (lungs expelling heat) and sweating.
Environmental Considerations: Reaching a steady state, especially for body temperature, depends on the environment. Exercising in a hot, humid environment would likely prevent an individual from achieving a steady state after 40 minutes, leading to continuous temperature rise.
Duration: Once an individual reaches a steady state, they can theoretically maintain it for a long period until muscle fuel depletion or mental fatigue sets in.
Biological Control Systems
These systems are the basic mechanisms by which homeostasis or steady state can occur.
Locations of Control Systems
Intracellular Control Systems: Take place within cells (e.g., active skeletal muscles).
Examples:
Enzymes responsible for muscle protein synthesis and breakdown (involved in hypertrophy).
Enzymes ensuring a constant supply of adenosine triphosphate (ATP) for muscle contraction. If ATP production cannot meet demand, muscle contraction is compromised.
Storage and breakdown of glucose (as glycogen) in skeletal muscles for readily available energy. Depletion of muscle glycogen is a common cause of fatigue in endurance athletes. Muscles also store fat, but its depletion is less common.
Organ Control Systems: Involve multiple organs working in coordination to maintain physiological variables within a constant range.
Examples:
Pulmonary (ventilatory/lung) and Cardiovascular (heart) Systems: Work together to control blood oxygen (O2)and carbon dioxide (CO2) levels.
During exercise, active muscles produce CO2, which enters the bloodstream. The heart circulates this blood to the lungs to expel the CO2 through increased breathing. This partnership maintains CO_2 within narrow ranges, preventing drops in blood pH that could lead to system failure.
Similarly, the pulmonary system brings O_2 into the body, and the circulatory system delivers it to active muscles.
Components of a Biological Control System
A generic example includes four key components:
Stimulus / Stress / Error Signal: Something that changes or is imposed on the body, pushing a variable outside its normal preferred range.
Sensor (Receptor): Specialized cells in the body whose job is to monitor a specific variable (e.g., blood pressure, pH, temperature, O_2 levels).
Examples: Baroreceptors in the aorta and carotid arteries detect stretch to monitor blood pressure; chemoreceptors detect pH or O_2; thermoreceptors detect temperature.
Control Center (Integrator): Receives information from the sensor, integrates it, and determines the appropriate response. Often specialized neural tissue (e.g., in the central nervous system: brain, brainstem, spinal cord).
Communication from sensor to control center is typically via nerves or hormones.
Effectors: Cells or organs that are activated by the control center to do something to correct the initial stimulus, returning the variable to its normal, relatively constant level.
Example: Regulation of Body Temperature When it Rises (Negative Feedback)
Stimulus: Immersion in a hot environment or engaging in exercise, leading to increased body temperature.
Sensors:
Thermoreceptors in the skin: Detect external temperature rise and warm blood flowing through dilated skin blood vessels.
Hypothalamus (in the brain): Also acts as a sensor, detecting when warm blood flows through it.
Control Center:
Hypothalamus: Unique in this case, it acts as both a sensor and the primary control center. It integrates information from itself and the skin thermoreceptors, recognizing that body temperature is rising and requires correction.
Effectors: The hypothalamus activates several mechanisms to lower body temperature:
Skin Blood Vessels: Special nerves cause skin blood vessels to dilate (get bigger), bringing warm blood closer to the skin surface to facilitate heat dissipation. (Unique: This is the only instance where sympathetic nerves cause vasodilation; in other organs, sympathetic nerves cause vasoconstriction).
Sweat Glands: The hypothalamus tells sweat glands (via sympathetic nerves) to increase sweat production. Evaporation of sweat cools the skin.
Respiratory Control Center (in the brainstem): The hypothalamus signals this center to increase the rate and depth of breathing, forcing more hot air out of the body.
Types of Feedback Mechanisms
Negative Feedback Systems
Definition: The end result of these systems is to correct the original stimulus or error in the opposite direction, bringing the variable back to its normal level.
Non-Biological Example: A home thermostat/furnace/air conditioning system.
Sensor: Thermostat detects room temperature.
Set Point: Homeowner sets a desired temperature.
Control Center: Within the thermostat, determines if temperature is too high/low.
Effectors: Furnace (to heat) or air conditioner (to cool) kicks in to return room temperature to the set point.
Biological Examples:
Body Temperature Regulation: (as described above) When body temperature rises, biological systems lower it.
Blood Carbon Dioxide Levels: When blood CO2 levels rise, chemoreceptors detect this through changes in pH. The central nervous system initiates an increase in the rate and depth of breathing to expel CO2, thus lowering it back to original levels.
Prevalence: Over 90\% of all biological control systems operate via negative feedback.
Positive Feedback (Feed-Forward) Systems
Definition: The response increases the original signal, leading to an escalating effect, rather than correction.
Non-Exercise Related Examples:
Childbirth: Pressure exerted by the baby's head on stretch receptors during labor leads to the release of oxytocin, which causes stronger uterine contractions. More pressure, more oxytocin, more contractions until the baby is delivered. This is a self-amplifying system.
Mental Stress/Panic Attacks: Stress causes heart rate and blood pressure to rise, and the awareness of these rising vitals can increase stress/panic, further worsening the physiological response.
Exercise-Related Example: Central Command
Definition: Parallel activation of the locomotor and autonomic circuits of the central nervous system. It originates in the brain (central) and coordinates multiple organ systems (command) simultaneously when physical activity begins.
Mechanism: Not a specific brain region, but a process activating multiple systems.
Locomotor System: Nerves controlling skeletal muscles are activated when you initiate movement (e.g., thinking about using hands activates primary motor cortex, sending signals down the spinal cord to muscles).
Autonomic System: Simultaneously, parallel nerves from the same brain regions activate specific nerves controlling the sympathetic and parasympathetic nervous systems.
Effects: This parallel activation leads to immediate increases in:
Skeletal muscle activity.
Heart rate.
Blood pressure.
Ventilation rates (breathing deeper and more rapidly).
Hormone release: Sympathetic nerves stimulate the liver to break down stored carbohydrates and release them into the blood for muscle use. The pancreas also promotes hormone release for digestion.
Why it's Feed-Forward: As the intensity of physical activity increases, a greater amount of skeletal muscle must be activated, leading to a larger central command signal. This results in progressively larger increases in heart rate, blood pressure, and ventilation rate. It is feed-forward because there is no mechanism to correct or dampen these responses; they are beneficial for exercise and are meant to rise with increasing intensity. We want heart rate, blood pressure, and ventilation to rise to meet the demands of exercise, hence no negative feedback corrects them to resting levels.
Failure of Biological Control Systems
When control systems cannot function due to disease or disorder, they fail to maintain physiological variables within normal ranges.
Example: Type 1 Diabetes
Problem: Damage to the beta cells in the pancreas, which are responsible for producing insulin.
Normal Response: After a meal, blood glucose levels rise. Pancreatic beta cells detect this, produce and release insulin. Insulin binds to receptors on body cells, opening