Smooth Muscle Physiology and Structure

Flashcards covering the anatomical structure, location, innervation types, and cellular mechanisms of contraction and relaxation in smooth muscle, including specific biochemical pathways for bronchoconstriction and bronchodilation.

Arrector pili muscle

Smooth muscle in the skin that causes goosebumps.

S T O V E

Mnemonic for smooth muscle locations: Skin (arrector pili), Tracts (reproductive, respiratory, urinary), Organs (hollow, bladder, uterus, stomach), Vessels (blood vessels), and Eyes (iris).

Fusiform

The shape of a smooth muscle cell, resembling a rugby ball or spindle.

Dense Bodies

Structures found throughout the cytoplasm that serve as attachments for thin and intermediate filaments; they are equivalent to Z-lines in skeletal muscle.

Gap Junctions

Connective structures between smooth muscle cells that provide electrical continuity.

Intermediate Filaments

Non-contractile filaments that form a latticework within the smooth muscle cell structure.

Sarcolemma

The outer sheath or plasma membrane of a smooth muscle cell where actin and intermediate filaments anchor at dense patches.

Varicosity

A swelling on autonomic neurons at contact points with smooth muscle cells responsible for vesicular neurotransmitter release.

Multiunit Smooth Muscle

Organization where each cell receives nervous input and cells are electrically isolated, allowing them to act independently (e.g., iris of the eye).

Unitary Smooth Muscle

Organization where few cells receive nervous input and gap junctions allow for coordinated cellular communication (e.g., blood vessels).

Phasic Contraction

Contractions specifically controlled by Autonomic Nervous System input with no spontaneous activity; characterized by fast contraction.

Tonic Contraction

A basal level of spontaneous contraction that can be increased or decreased by Autonomic Nervous System input; characterized by slow contraction.

Pharmacomechanical coupling

A mechanism where smooth muscle contraction occurs without being dependent on electrical activity or action potentials.

Calmodulin

The $Ca^{2+}$ sensor in smooth muscle (equivalent to troponin in skeletal muscle) that contains 4 $Ca^{2+}$ binding sites.

Myosin light chain kinase (MLCK)

An enzyme activated by $Ca^{2+}$-calmodulin that phosphorylates light chains in myosin heads to increase ATPase activity.

Myosin phosphatase

An enzyme that removes phosphate from myosin, leading to decreased muscle tension and relaxation.

Latch phase

A low energy state where dephosphorylated myosin fibers remain bound to actin, maintaining tension without immediate loss of association.

Inositol tri-phosphate (IP3)

A secondary messenger produced by the cleavage of $PIP_2$ that causes the release of $Ca^{2+}$ from the sarcoplasmic reticulum during bronchoconstriction.

Protein Kinase A (PKA)

An enzyme activated by cAMP during bronchodilation that causes the uptake of $Ca^{2+}$ into the sarcoplasmic reticulum, leading to muscle relaxation.

Adenylate Cyclase

An enzyme activated by G\text{\tiny$\textalpha$s} that converts Adenosine Tri-phosphate (ATP) into cyclic Adenosine Monophosphate (cAMP).