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
Anatomical Differences of Sympathetic NS & Parasympathetic NS
-
Sites / Origin of Nerves
- SNS fibers – Craniosacral originate in brain & sacral spinal cord
- PNS fibers – Thoracolumbar originate in thoracic & lumbar regions of spinal cord -
Relative length of pre- & post-ganglionic fibers
- SNS fibers – Short preganglionic & long postganglionic
- PNS fibers – Long preganglionic & short postganglionic -
Locations of ganglia
- SNS – Ganglia close to spinal cord
-PNS – Ganglia in or near their visceral effector organ
Sympathetic & Parasympathetic Interactions
- Most visceral organs have dual innervation / counterbalance one another ; One stimulates action while other inhibits
- Both divisions are partially active, resulting in a basal sympathetic and parasympathetic tone
- SNS & vascular tone: Blood vessels are kept in a state of partial vasoconstriction; blood shunting possible via variations in sympathetic output to specific vessels
- PNS & vascular tone: Tonic slowing effect on heart, regulation of contractile activity of smooth muscle of GI & urinary tracts- can be overridden by SNS
Antagonist Interactions
- Sympathetic division increases heart and respiratory rates and inhibits digestion and elimination
- Parasympathetic division decreases heart and respiratory rates and allows for digestion and discarding of wastes
Cooperative Interactions
e.g.: regulation of external genitalia during intercourse
PNS: dilation of blood vessels in penis/clitoris
SNS: ejaculation, reflex contraction of female’s vagina
Unique Roles of Sympathetic NS
- Regulation of adrenal medulla, sweat glands, arrector pili muscles of skin, kidneys, most blood vessels
Other unique functions :
1. Thermoregulatory responses to heat
2. Renin release from kidneys
- Sympathetic system causes release of renin from kidneys that in turn activates a system that increases blood pressure
3. Metabolic effects – Mobilizing your body to respond to fight or flight mechanism
- Increases metabolic rate of body cells
- Raises blood glucose levels
- Stimulates mobilization of fats
- Increases mental alertness
- Increases speed/strength of muscle contraction
Control of ANS Function
ANS is under control of CNS centers in:
- Brain stem & spinal cord
- Cerebral cortex
- Hypothalamus
The main integration center of ANS activity
Hypothalamus is the BOSS
Brain Stem & Spinal Cord Controls
Significant, direct effects
Processing centers in medulla oblongata coordinate complex reflexes. Contains centers & nuclei involved in :
- Salivation
- Swallowing
- Digestive secretions
- Peristalsis
Motor centres in the medulla :
- e.g. CV → Heart rate, blood vessels, GI, respiratory
- Spinal cord controls defecation and micturition but are subject to conscious override
Hypothalamus
- Anterior regions → parasympathetic
- Posterior areas → sympathetic
Centres that coordinate
- Heart activity and BP
- Body temperature
- Water balance
- Endocrine activity
- + Centres that help mediate emotions & biological drives
Cortical Controls
- Connections of hypothalamus to limbic system allow cortical influence on ANS
- Voluntary cortical control of some visceral activities is possible – Meditation & biofeedback allow some conscious control over visceral activities
– e.g. During meditation, can lower heart & breathing rates, oxygen use, metabolic rate)
Endocrine System
- Coordinates with NS
- Influences metabolic activities via hormones transported in blood
- Target most body cells, widespread effects
– Acts at diffuse locations (target anywhere blood reaches) -
Slower intitation of responses than NS, but responses have longer duration
– Certain lag period (seconds - days)
– Effects can go on continuously
Endocrine system controls and integrates:
- Reproduction
- Growth and development
- Maintenance of electrolyte, water, & nutrient balance of blood
- Regulation of cellular metabolism and energy balance
- Mobilization of body defenses
Endocrinlogy – Study of hormones & endocrine organs
Short-Distance Chemical Signals
Autocrines : Exert effects on same cells that secrete them
Paracrines : Act locally within SAME TISSUE but affect cell types other than those releasing paracrine chemicals
Endocrine Glands
- Ductless glands, produce hormones
- Release hormones into surrounding tissue fluid
- Rich vascular + lymphatic drainage supply
- Hormone producing cells arranged in cords, branching networks – Maximizes contact between them & surrounding capillaries
Glands : Pituitary, thyroid, parathyroid, pineal glands
Organs : Pancreas, gonads (ovaries / testes), placenta → All contain endocrine tissue
– Hypothalamus (neuroendocrine organ) produces & releases hormones
Hormones
- Long distance chemical signals released into the ECF that regulate metabolic function of other cells in the body
- Travel through blood / lymph
- Must bind to specific receptors to influence target cell function → Influence activity of only those tissue cells that have receptors for the hormone
Target cell activation depends on:
- Hormone concentration
- Target cell receptor content
- Affinity of hormone for receptor
Hormone action on target cells work to :
- Alter membrane permeability / potential (thru channels)
- Stimulate synthesis of enzymes within cells
- Activate / deactivate enzyme
- Induce secretory activity
- Stimulation of mitosis
Classification of Hormones
3 Structural Groups :
- Amino acids derivative
- Peptides hormones
- Lipid derivatives (Steroid hormones, Eicosanoids)
Amino Acids Derivative
- Most hormones
- Usu. water soluble → CANNOT pass plasma membrane
Peptides Hormones
- Usu. water soluble → CANNOT pass plasma membrane
Lipid Derivatives
- Lipid soluble → CAN pass plasma membrane
- Steroid hormones – Derivatives of cholesterol, gonadal & adrenocortical hormones
-
Eicosanoids – Made from arachidonic acid, released by nearly all cell membranes, leukotrienes (inflammation, allergic reactions), prostaglandins (multiple targets & effects, e.g. raising bp, blood clotting, pain, inflmmation)
– Highly localized, affect only nearby cells (act as paracrines / autocrines not TRUE hormones) - Thyroid hormone
Chemical structure of hormones determines ______ which in turn determines :
Solubility in water (blood made up of mostly water)
Determines :
- How it’s transported
- How long it lasts before being degraded
- What receptors it can act upon
Mechanism of Hormone Action – Peptide / protein hormones
- Water-soluble hormones (all amino acid–based hormones except T3/T4)
- Act on plasma membrane receptors / CANNOT enter cell
- Usually act via G protein / _second messengers _
Steps :
1. Hormone (first messenger) binds to receptor in plasma membrane
2. Receptor activates a G protein – Binding of receptor causes receptor to change shape, allowing to bind to inactive G protein
- G protein activates as GDP bound to it is displaced by GTP
- G protein “off” when GDP bound, “on” when GTP bound
3. G protein activates or inhibits effector enzyme (adenylate cyclase)
4. Adenylate cyclase then converts ATP to cAMP (second messenger)
5. cAMP activates protein kinases that phosphorylate (add a phosphate) other proteins
6. Phosphorylated proteins are then either activated or inactivated
7. cAMP is rapidly degraded by enzyme phosphodiesterase, stopping cascade
8. Cascades have HUGE amplification effect → # product molecules increases dramatically at each step
Mechanism of Hormone Action – Steroid hormones
- Lipid-soluble steroid hormones & thyroid hormone
- DIRECT GENE ACTIVATION
1. Diffuse into target cells + bind with intracellular receptors
2. Receptor-hormone complex enters nucleus + binds to specific region of DNA
3. Helps initiate DNA transcription to produce messenger RNA (mRNA) which is translated into specific proteins
- Proteins synthesized have various functions (e.g. Metabolic activities, structural purposes, or exported from cell)
- Usually act in the nucleus to influence gene transcription
Stimuli for Hormone Release
- Blood levels of hormones are controlled by negative feedback systems / levels vary only within a narrow range
Hormone release is triggered by:
- Endocrine gland stimuli
- Nervous system modulation
Up / Downregulation
Upregulation : Rising hormone levels cause increase in # hormone receptors –> More cellular activity
Downregulation : Rising hormone levels causes decrease in # hormone receptors –> Less cellular activity
Endocrine Gland Stimuli
Synthesize & release hormones in response of 1 of 3 stimuli :
a) Humoral stimuli: Change in blood level of a nutrient or ion directly stimulates secretion of hormones
- e.g. Parathyroid hormone (PTH) & blood calcium; insulin & blood glucose
b) Neural stimuli : Nerve fibers stimulate hormone release; not as common
- e.g. Sympathetic NS & epinephrine release by adrenal medulla, hypothalamic neurons & oxytocin release
c) Hormonal stimuli : Hormones stimulate other organs to release their hormones
- 3-tiered system involving hypothalamus, pituitary & target endocrine gland - concept of hypothalamic-pituitary axis
Nervous System Modulation
- NS can make adjustments to hormone levels when needed, can modify stimulation or inhibition of endocrine glands
- NS can override normal endocrine controls :
e.g. Under severe stress, hypothalamus & Sympathetic NS override insulin to allow blood glucose levels to increase (preparing body for “fight or flight”)
Blood level of hormone depends on:
- Rate of synthesis
- Rate of degradation / clearance from blood
Half - life of hormones
- Time required for level of hormone in blood level to _decrease by half_
- Persistence of hormone in blood; usually < 1 min to 1 week
- Water-soluble shortest half-life, rapidly removed from blood by kidneys
So, the duration of response is _________ ?
Variable
- Ranges from seconds to hours to day
- Effects may disappear rapidly as blood levels drop, but some may persist for hours at low blood levels
Onset of Hormone Activity
- Variable & depends on _specific target cell response :_
If enzyme activation - rapid (minutes)
If enzyme synthesis - hours - days
Hormone Interaction at Target Cells
- Possible outcomes when a cell receives instructions from two hormones at the same time
Antagonistic effect – 1 or + hormones oppose(s) action of another hormone
- Result depends on balance between two hormones
- e.g. Insulin and glucagon
Synergistic effect – More than 1 hormone produces same effects on target cell
- Causes amplification / additive effect
- e.g. Glucagon & epinephrine both cause liver to release glucose
Permissive effect – 1 hormone cannot exert its effects without another hormone being present effect
- e.g. Reproductive hormones need thyroid hormone to have effect
Hypothalamus - Pituitary Interactions
- Hypothalamus is neural – Produces a number of releasing factors (hormones) which travel to anterior pituitary via hypophyseal portal system (special blood vessels that control release of anterior pituitary hormones)
Pituitary (hypophysis)
- Secretes at least eight major hormones
- Connected to the hypothalamus via stalk, funnel-shaped infundibulum
2 major lobes :
- Posterior – Neurohypophosis, composed of neural tissue, glia-like cells, nerve fibers, axon-terminals / hormone storage area → Releases neurohormones (antidiuretic SON & oxytocin PVN)
- Anterior – Adenohypophosis, composed of glandular tissue, receives hormones from neural hypothalamus via hypophyseal portal system
