Lecture 2: Control Systems Flashcards
Homeostasis definition
- Maintenance of a relatively constant internal environment in the presence of an ever- changing external environment
Homeostasis characteristics
- Dynamic (not an unchanging) condition
- Achieved by physiological processes (homeostatic mechanisms)
Homeostatic mechanisms
- Detect changes in the body and act to counter the changes
- Balance concept is inherent
Homeostasis refers to
- The dynamic mechanisms that detect deviations in physiological variables from their “ideal” values
Homeostasis initiates
- Effector responses that restore these variables to the optimal physiological range
Control systems provide
- Communication between cells, tissues, and organs
Types of control systems
- Nerves only
- Endocrines only
- Dual systems
Dual systems
- Provide backup systems
- Greater plasticity of function or control
- Some faster, some slower
Chemically addressed systems
- Most control is mediated via chemical messengers
- Nervous system (via neurotransmitter)
- Endocrines
- Local chemicals
Types of chemically addressed systems
- Local chemicals
- Local hormones
- Neurotransmitters
- True hormones
Local chemicals
- Oxygen, carbon dioxide, other metabolites
- Act on adjacent cells, geared to metabolism
- Probably play no regulatory function
Local hormones
- Gastrin, secretin, other GI hormones, prostaglandins
- Paracrines
- Autocrines
Autocrines
- Hormone or neuronal secretion that may, after being secreted from a cell, act back onto cell of origin
Neurotransmitters
- Released into interstitial fluids (synapse)
- Affect only cells in synaptic contact
True hormones
- Released into circulating body fluids, reaching all cells of the body
- Endocrines
- Neurocrines (neurosecretions)
Characteristics of a true hormone
- Chemical substance
- Secreted directly into the blood by discrete specialized cells
- In response to a specific stimulus (neural or blood-borne)
- Amounts vary with the strength of stimulus
True hormones are found
- Only in minute (physiological) concentrations in blood
When carried to their target cells, true hormones
- Exert specific effects that frequently involve the regulation of preexisting cellular reactions
Six components of control systems
- Sensor
- Afferent pathway
- Error signal generator
- Integrator
- Efferent pathway
- Effector
Sensor
- Monitors environment
Afferent pathways
- Signals to integrator
Integrator
- CPU; center
Efferent pathway
- Signals from integrator
Effector
- Changes environment
Set point
- Ideal value of a control system’s parameter
Error signal
- Difference between set point and actual value
Gain
- Ratio of correction to error
Negative feedback
- Tries to restore homeostasis
- Reduces error signal
Positive feedback
- Disturbs homeostasis
- Increases error signal
- Often employed to generate the rising phase of a cyclic event (e.g. blood clot, action potential, birth)
- Self-limiting normally
Feedforward
- Disturbs homeostasis
- Regulatory mechanism for meeting future needs
- Works with negative feedback controls
Components of electrical communication involve
- Nerves alone
Components of chemical communication involve
- Endocrines alone
Components of electrical and chemical communication combined
- Neuroendocrine
Balance depends on
- Rate of gain vs. loss of some property
Positive balance
- Net gain
- Rate of gain > rate of loss
Negative balance
- Net loss
- Rate of gain < rate of loss
Stable balance
- No net change
- Rate of gain = rate of loss
Balance incorporates the concepts of
- Steady state
- Equilibrium
- Both are states of stable balance
Steady state
- Requires energy to maintain it
Equilibrium
- Does not require energy to maintain it
Nervous system control
- Extrinsic control system
- Relatively fast system
- Controls many of one’s quick/immediate responses
Endocrine system (extrinsic)
- Controls responses that need to continue for long times or occur more slowly
- Works with/alongside the nervous system
- Keeping systems essentially in balance and allowing cells, tissue, organs and the entire body to adjust to changes in the environment
Fever involves
- Up-regulation and down-regulation
- Thermostatic resetting
Effects of a fever on homeostasis
- Set point for body temp is elevated
- Release of pyrogen(s) by microorganisms (such as certain bacteria)
- Pyrogen changes the set point
- Increased heat production by shivering
- Heat conservation by cutaneous vasoconstriction
