Topic 3- Homeostasis Flashcards
What is homeostasis?
- The maintenance of relatively stable internal conditions despite continuous changes in the outside world
- Maintained by contributions of ALL organ systems
- Dynamic state of equilibrium, always readjusting as needed
- Chemical, thermal & neural factors interact in complex ways, sometimes helping or hindering the body as it works by regulating variables
- Designed to get body organs and workings back into range – e.g. controlling system failure
- Body is in homeostasis when its needs are adequately met, and everything is functioning smoothly
What are variables?
- Variables are factors that can change :
e.g. blood sugars, body temperature, blood volume, waste elimination, etc. - Regulated by physiological mechanisms so that internal conditions remain stable & relatively constant
Homeostatic Control of Variables
Communication is essential for homeostasis – Accomplished mainly by nervous (neural electrical impulses) & endocrine systems (blood-borne hormones)
Steps of Homeostatic Control
- Stimulus : Produces change in variable
- Change detected by receptor : Constantly monitors internal / external environment, is aware of changes as they occur in order to react
- Input : Receptor responds to stimuli by sending info via afferent pathways to the control center
- Control Center : Receives + analyzes input from receptor & determines appropriate response, compares what is happening to the set point (normal range of levels, what SHOULD be happening)
- Output : Info flows out of control center along efferent pathways to the effector
- Effector : Receives output from control center & provides means for response, which either reduces stimulus (negative feedback) OR enhances stimulus (positive feedback) to return variables to homeostasis
What is the most used homeostatic control feedback mechanism?
Negative feedback mechanisms
Negative Feedback Mechanisms
- Response REDUCES or SHUTS OFF original effect / intensity of stimulus
- Variable changes in opposite direction of initial change —> Returning to its “ideal” value
2 options:
1. 1 hormone / neural pathway regulates a process — Negative feedback decreases secretion
2. Process regulated in opposite directions by 2 different hormones / neural pathways
- e.g. Blood glucose - As blood sugars rise, receptors sense this change and the pancreas (control center) secretes insulin into blood. This change prompts body cells (liver, muscle, adipose tissue) to absorb more glucose if needed to be used as fuel, and excess glucose to be stored as glycogen, removing it from the bloodstream. As blood sugars fall, the stimulus for insulin release ends. Instead, pancreas senses this and activates tissues to release stores of glucose
- e.g. Thermoregulation – As body temp rises, receptors in skin & brain detect this change and the info goes through afferent pathways to the control center. Next, the control center sends commands through sweat glands & the dilation of subcutaneous blood vessels to dissipate the heat and bring body temp down to a normal range. As body temp lowers, blood is sent away from the skin to preserve heat, muscles get activated through shivering to generate heat so the temp can be brought back to normal temperatures.
Positive Feedback Mechanisms
- Response ENHANCES or EXAGGERATES the original stimulus, so the output is further stimulated and responses become increasingly greater
- Variable changes in the same direction as initial change —> Deviates further and further from its original value / range
- Linked sequence of events is set off in a “cascading” manner : After initiation, results of each reaction feed into the next as the original stimulus becomes amplified
- There is a GOAL to be attained
- Some may be localized (e.g. platelet plug formation)
Usually controls infrequent events that do not require continuous adjustment, as they are likely to race out of control
- e.g. Blood clotting + Platelet plug formation – Platelets recognize a tear in the blood vessels + stick to it. The platelets will then release chemicals to attract more platelets and keep doing so until a temporary platelet plug is formed and the tear can be repaired as the blood forms a clot.
- e.g. Expecting mom goes into labour – Process driven by oxytocin (hypothalamic hormone), a homeostatic system drives towards the goal of birthing a baby, making contractions increasingly more frequent and powerful. The intensifying contractions release more and more oxytocin, leading to more contractions until the baby is born and the positive feedback is shut off (stimulus for oxytocin release is ended)
Homeostatic Imbalance
- Disturbance of homeostasis – Involves most diseases causing a homeostatic imbalance
- Contributes to changes associated with aging — Control systems become less efficient with age –> increasing risk for illness
e.g. Cancer – Loss of ability for cell proliferation and differentiation, whole regulatory & immune systems lower with age which contribute to the greater risk of cancer past adulthood - May also occur if negative feedback mechanisms become overwhelmed, destructive positive feedback mechanisms may take over
Nervous System
- Acts via APs & neurotransmitters
- NT act over short distances
- Acts at specific locations determined by axon pathways
Somatic Nervous System
- Innervates skeletal muscles
Autonomic Nervous System
System of motor neurons :
- Innervates smooth muscles, cardiac muscle & glands
- Allows responses usu. without our awareness (subconscious control)
- Also called involuntary nervous system OR general visceral motor system
- Visceral organs → CNS, ANS nerves make necessary adjustments
Functions :
- Shunts blood to more needed areas
- Speeds / slows heart & respiratory rates
- Adjusts blood pressure, body temp
- Increases / decreases gastric secretions
Efferent Pathways
Somatic NS – Motor neuron cell bodies in CNS, axons extend in spinal + cranial nerves → All the way to skeletal muscles to be activated
- Thick, myelinated axon from spinal cord to skeletal muscle (group A fibers); rapid conduction of impulses (no ganglia)
Autonomic NS – 2 neuron chain to reach effectors
- Cell body of preganglionic neuron (1nd motor neuron), resides in brain stem or spinal cord; axon synapses with postganglionic neuron (2nd motor neuron) in ganglion outside CNS
- Cell body of postganglionic neuron is in autonomic ganglion outside CNS; postganglionic axon (its axon) extends to effector organ
Conduction
Slower in ANS than SNS
- In ANS, preganglionic axons are thin + lightly myelinated, postganglionic axons are thinner + nonmyelinated
In ANS, preganglionic axons are _____ & _____ , postganglionic axons are thinner + nonmyelinated
Thin, lightly myelinated
In ANS, postganglionic axons are _____ & _____
Thinner, nonmyelinated
Ganglia
ANS – Ganglia = Motor Ganglia, containing cell bodies of motor neurons
- Sites of synapse + info transmission from pre to postganglionic neurons
SNS – NO ganglia
Neurotransmitter Effects
SNS – All motor neurons release Acetylcholine (ACh) at synapses w/ skeletal muscle fibers
Effect : EXCITATORY
ANS – Postganglionic fibers release 2 NT : Norepinephrine (NE) secreted by most sympathetic fibers & ACh secreted by parasympathetic fibers
Effect : Exc / Inhibitory depending on target organ
Overlap of SNS & ANS Function
- Higher brain centers regulate + coordinate both somatic, autonomic motor activities
- Most spinal nerves (many cranial nerves) contain both somatic, autonomic fibers
- Most homeostatic adaptations involve both skeletal muscles + visceral organs : e.g. Skeletal muscles work hard (SNS) → Need more O2, glucose, so ANS speeds up heart rate + dilates airways
Sympathetic & Parasympathetic Divisions of the ANS
Work AGAINST each other to maintain homeostasis (what one promotes, the other inhibits)
Sympathetic Division – Mobilizes body during activity, “fight or flight” helps person get out of terrible / threatening situations
- Increased heart rate, rapid + deep breathing, cold sweaty skin (blood is being shunt away from skin → towards active skeletal muscles & heart), dilated pupils (helps see further)
- Lung bronchioles dilate → Increased air flow, O2 delivery to body cells
- Liver stimulated to release glucose into blood, accomodates for increased energy needs of body cells
- Temp reduces nonessential activities (e.g. GI tract activity)
- “E” system : Exercise, excitement, emergency, embarrassment
Parasympathetic Division – Promotes maintenance of functions, conserves energy
- ”Resting & digesting” system, active in non-stressful situations
- Keeps body’s energy use low while regulating “housekeeping” activities
- Blood shunt away from skeletal muscles
- Low normal blood pressure & heart rate levels
- Constricted pupils, lenses accommodated for close vision
- “D” system : Digestion, defecation, diuresis
Which ANS division has diffuse effects?
Sympathetic division – longer lasting, body-wide effects
- Widespread release across bloodstream
- NE inactivated more slowly than Ach
- NE & Epinephrine hormones from adrenal medulla have prolonged effects lasting past the halt of sympathetic signalling
Which ANS division has localized effects?
Parasympathetic division – short-lived & highly localized control over effectors
- Ach quickly destroyed by acetylcholinesterase
