2-10 Smooth and Cardiac Muscle

Muscle Types

• Skeletal Muscle

  • Voluntary control, striated

  • Individual fibers can be selectively activated

  • Capable of sustaining contraction, generating a large range of force.

• Smooth Muscle

  • Involuntary control, non-striated

  • Regulated by the autonomic nervous system

  • Functions through myosin/actin cross bridges activated by calcium

  • Exhibits slow, sustained contractions, changes lumen diameter.

• Cardiac Muscle

  • Like skeletal muscle (striated, sarcomere structure)

  • Functions as a unit, heart acts as a pump with contraction and relaxation cycles.

  • Myocytes contract simultaneously due to electrical coupling (gap junctions).

Muscle Contraction Mechanisms

• Skeletal Muscle

  • Activated by somatic motor neurons; action potentials (AP) lead to muscle contraction.

• Smooth Muscle

  • Uses myosin/actin interaction regulated by calcium.

  • Lacks sarcomeres but contains actin and myosin arranged around dense bodies.

  • Can change shape and volume without losing tension due to its unique filament arrangement.

Neuronal Control and Classification

• Neurons: Divided based on direction of impulse conduction.

  • Sensory Neurons: Convey information from sensory receptors to CNS.

  • Motor Neurons: Transmit signals from CNS to target organs (muscles/glands).

  • Somatic Motor Neurons: Control skeletal muscle with AP leading to muscle activation; no AP means muscle is off.

  • Autonomic Motor Neurons: Modulates contraction strength by tuning AP activity.

Smooth Muscle Characteristics

• Structure and Function

  • Located in blood vessels, airways, digestive organs, urinary and reproductive tracts.

  • Lacks distinct striations yet functions via actin and myosin interactions.

  • Primarily contracts in a coordinated fashion (single-unit) through gap junctions.

Sarcomere Functionality

• Length-Tension Relationship

  • Maximum tension occurs when muscle is 100-120% of resting length.

  • Tension decreases outside this optimal range due to fewer interactions between myosin and actin filaments.

Single-unit vs Multi-unit Smooth Muscle

• Single-unit Smooth Muscle

  • Multiple gap junctions ensure synchronized contractions.

  • Exhibits pacemaker activity, moderated by stretch or neural input.

• Multi-unit Smooth Muscle

  • Requires individual nerve innervation for contraction, behaves more independently.

  • Examples include arrector pili muscles and ciliary muscles.

Excitation-Contraction Coupling in Smooth Muscle

• Calcium Source

  • Calcium primarily enters from the extracellular fluid via voltage-gated channels.

  • Calcium binds to calmodulin (not troponin as in skeletal muscle) and activates myosin light-chain kinase (MLCK).

  • MLCK phosphorylates myosin light chains allowing for cross-bridge formation, leading to contraction.

Relaxation Mechanism

• Key Components to Relaxation

  • Calcium removal is essential for muscle relaxation.

  • Decrease in cytosolic calcium occurs via pumping out of the cell or back into the sarcoplasmic reticulum.

  • Myosin light chain phosphatase removes phosphate from myosin light chains, decreasing myosin ATPase activity and muscle tension.

Autonomic Regulation of Bronchiole Air Flow

• Parasympathetic Regulation

  • Ach binding to muscarinic receptors increases calcium influx, causing constriction and reduced airway radius.

• Sympathetic Regulation

  • Epi binding to beta-2 adrenergic receptors decreases smooth muscle contraction, increasing airway radius.

Cardiac Muscle Structure and Function

• Comparison to Skeletal Muscle

  • Structures are striated; smaller cells with a single nucleus.

  • Cells are electrically linked through gap junctions for synchronous contractions.

• Action Potential (AP)

  • AP is longer, facilitating twitch contractions.

  • Pacemaker potentials allow automatic generation of heartbeat without nervous system input.

Calcium-Induced Calcium Release in Cardiac Muscle

• AP from adjacent cells triggers calcium influx through T-tubules.

• Entry of Ca2+ activates ryanodine receptors for calcium release from SR, resulting in muscle contraction.

Comparison of Muscle Types

• Skeletal Muscle: Voluntary, striated, requires nerve stimulation.

• Cardiac Muscle: Involuntary, striated, cells contract as a unit without motor neuron input but modulated by autonomic nerves.

• Smooth Muscle: Involuntary, non-striated, may contract on its own or be modulated by nerve activity.