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Types of Muscle and Smooth Muscle Mechanisms (Smooth muscle contraction…
Types of Muscle and Smooth Muscle Mechanisms
Cardiac Muscle
Muscles are short but join together; striations; 1 nucleus per cell
Myocardial Cell
Intercalated discs
Cell membranes that separate individual cardiac muscle cells
Located at the ends of individual cells
Maintain cell-to-cell adhesion
Gap junction
Present at intercalated disks
Allows heart to behave as an electrical syncytium (single unit)
Allows flow of ions and small molecules
Allows sharing of common intracellular environment
Formed by aligning half channels called "Connexon" between adjacent cells (found in some smooth muscles, neurons, and epithelial cells)
T tubules
Continuous with cell membrane
Carry action potential
Well developed in VENTRICLES but poorly developed in ATRIUM
Sarcoplasmic Reticulum
Site of storage and release of Ca++
Better developed in ventricles
Less developed than skeletal muscle
Some calcium comes from outside the cell via calcium channels
Heart can be affected by changing slope of leaky current
Some hormones can open calcium channels of the heart
Smooth Muscle
In different organs
Circulatory
Change diameter of vessel
Control distribution of blood
Help regulate blood pressure
Respiratory
Tracheal and bronchial
Alter diameter of respiratory passageways
Change resistance to airflow
Becomes more relaxed when one needs to breathe more
Digestive System
Walls of digestive tract
Move materials along the tract
Walls of gallbladder
Contract to eject bile into digestive tract
Sphincters
Anus: Internal and external
Urinary System
Walls of blood vessels
Alter rate of filtration at the kidneys
Walls of ureters
Transport urine to the urinary bladder
Walls of bladder
Force urine out of the body
Reproductive System
Male
Vas deferens
Helps move sperm along reproductive tract
Causes ejection of glandular secretions of accessory glands
Female
Fallopian tube
Helps move oocytes and maybe sperm
Uterus
Expels fetus
Integumentary System
Piloerector muscle (arrector pili)
Elevates hairs
Regulates body temperature
Nervous System
Ciliary muscles
Changes thickness of lens
Accommodation for near vision
Iris sphincter muscle
Misosis (constricts pupil)
Iris dilator muscle
Mydriasis (dilates pupil)
Types
Multi-Unit
Each fiber operates independently
Often innervated and controlled by separate nerve varicosities
Ciliary muscle, iris muscle, piloerector
Single-unit (syncytial;visceral)
Many cells contract as a unit
Gap junctions allow ion flow and transmission of action potential among cells
Walls of most visceral organs
Innervation
Sometimes connection is loose; can reach cell or just in "neighborhood"
Control is not well regulated
Smooth muscle contraction and initiation
Neuronal Stimulation
Autonomic nerve fibers make diffuse or contact junctions with smooth muscles
Through varicosities, nerve fiber releases neurotransmitters
Receptors can be excitatory or inhibitory
Excitatory
Increases intracellular calcium (more often in single-unit smooth muscles) or without action potential
Inhibitory
Prevents intracellular calcium increase
Norepinephrine
Activates alpha or beta adrenergic receptors
Acetylcholine
Activates muscarinic receptors
With Action Potential (smooth muscle does not always need action potential)
Action potential generated via opening of mainly calcium channels
Calcium channels are slower than sodium channels
Slow waves lead to rhythmic spontaneous action potentials
Plateau = delayed opening of potassium channel, causing prolonged contraction
Without Action Potential
Neurotransmitter (e.g. ACh) activates muscarinic receptor
G protein activated
Activation of phospholipase C (PLC)
Increased production of inositol-tris-phosphate (IP3)
Stimulates calcium release from ER, which leads to calcium influx from extracellular fluid
Via Local Factors or Hormones
Vascular Smooth Muscles
Low oxygen causes relaxation
High CO2 causes relaxation
Low pH causes relaxation
Hormones
Ex: Epinephrine, vasopressin, oxytocin, etc.
Activates respective receptors, activates signaling pathways, eventually affects intracellular calcium concentration
Contractions regulated by calcium
Calcium ions come from outside of the cell or ER
Binds to calmodulin
Activates myosin light chain kinase (MLCK)
MLCK hydrolyses ATP and phosphorylates myosin light chain (MLC)
Myosin activated
Crossbridge cycling occurs (like in skeletal muscle)
When calcium is pumped out, MLC is dephosphorylated by MLC phosphatase
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Increasing calcium
Contractions
Decreasing calcium
Relaxation/difficulty contracting (can be caused by cyclic GMP increase)
Drugs
Antihypertensive
Angiotensin Converting Enzyme (ACE) inhibitors
Angiotensin II Receptor Antagonists (ARB)
Calcium channel blockers
Vasodilators for Hypertensive Emergencies
Nitric Oxide (NO) donors
Relax airway of smooth muscle
Relief of Asthma
Beta-Agonist (salbutamol)
Comparison
Skeletal
Location: Attached to bones
Characteristics: Long & cylindrical, multi nucleated; heavily striated
Control: Voluntary
Action: Movement of joints, contracts and relaxes rapidly, controlled by motor neurons
T tubule: Yes
SR: Well developed
Troponin: Yes
Ca regulation site: Troponin
Ca regulation mechanism: Reposition of troponin-tropmyosin
Gap junction: No
Cardiac
Location: Wall of heart
Characteristics: Branching networks, intercalated disks, single nucleus, lightly striated
Control: Involuntary
Pumps blood, self-excitatory, modulated by autonomic system and hormones
T-tubule: Yes
SR: Moderately developed
Source of Ca: Extracellular fluid and SR
Troponin: Yes
Ca regulation site: Troponin
Ca regulation mechanism: Reposition of tropnin-tropomyosin
Gap junction: Yes
Smooth
Location: Wall of hollow organs with some exceptions
Characteristics: Tapered at teach end, single nucleus, non-striated
Control: Involuntary
Action: Peristalsis, contracts and relaxes slowly, may sustain contraction, controlled by autonomic nervous system, hormones, local factors, etc.
T-tubule: No
SR: Poorly developed
Source of Ca: Extracellular fluid and ER
Troponin: No
Ca regulation site: Myosin
Ca regulation mechanisms: Phosphorylation of myosin
Gap junction: Yes in some
FOR ALL MUSCLES
Mechanism of contraction = sliding filament mechanism, cross-bridges
Calcium needed for activation
ATP is used