Muscle 2 (cardiac & smooth) Flashcards
What are the key characteristics of cardiac muscle?
Specific to the heart
Striated like skeletal muscle
Cells (cardiomyocytes) are shorter and more branched
Connected at intercalated disks
Contains gap junctions for electrical coupling
Action potentials initiate in SA node pacemaker cell
What are the main features of smooth muscle?
Involved in mechanical control of organ systems (digestive, urinary, reproductive)
Controls blood vessel and airway diameter
Non-striated structure
Multiple actin fibers join at dense bodies
Thick filaments intersperse around thin filaments
Can be controlled by hormones, autonomic nervous system, or inflammatory mediators
Exists in two classes: multiunit and unitary
How does calcium increase in skeletal muscle during excitation-contraction coupling?
Depolarization activates L-type Ca²⁺ channels in T-tubule membrane
This has two effects:
Opens L-type Ca²⁺ channels for calcium influx
Creates mechanical tethering between L-type channels and ryanodine receptors
Ca²⁺ release channels in SR open to release calcium into cytoplasm
How does cardiac muscle calcium regulation differ from skeletal muscle?
Has T-tubules but only near one SR branch (called a Dyad)
T-tubules located at Z-line region
No mechanical interaction between voltage-gated Ca²⁺ channels and ryanodine receptors
Uses Calcium Induced Calcium Release (CICR) mechanism
How is calcium removed from the cytoplasm to end muscle contraction?
Two main mechanisms:
1-Across cell membrane via:
Plasma Membrane Calcium ATPase (PMCA)
Sodium/Calcium exchanger (NCX)
2-Back into SR via sarco/endoplasmic reticulum calcium ATPase
How does smooth muscle handle calcium regulation?
Lacks T-tubules and Triad/Dyad structures
Has shallow invaginations called caveolae
Contains peripheral SR (encircles caveolae) and central SR (runs through cell)
Can activate L-type Ca²⁺ channels through voltage changes
Can also use IP₃ pathway through Gq-coupled receptors
What is the role of troponin in skeletal and cardiac muscle contraction?
Present in both skeletal and cardiac muscle
Regulates interaction between actin and myosin
Responds to calcium to enable cross-bridge formation
How does smooth muscle contraction differ from skeletal/cardiac muscle?
No troponin present
Uses calponin and caldesmon for tonic inhibition
Contraction involves calmodulin activation by Ca²⁺
Requires myosin light chain kinase activation
Phosphorylation of MLC removes inhibitory effects
Relaxation requires MLCP to dephosphorylate MLC
What controls smooth muscle activity?
Circulating hormones
Autonomic nervous system input
Inflammatory mediators (e.g., histamine)
Can respond to both action potentials and graded potential changes
What are the key structural differences between cardiac and skeletal muscle?
Cardiac muscle cells are shorter and more branched
Cardiac muscle has intercalated disks for cell connection
Contains gap junctions for electrical coupling
T-tubule arrangement differs (dyads vs. triads)
Both are striated but have different organizational patterns
