Chapter 8 Excitation and Contraction of Smooth Muscle

            

Chapter 7 🔗:

https://mednotesbyrazaullah.blogspot.com/2026/08/chapter-07-neuromuscular-junction.html

Excitation and Contraction of Smooth Muscle

Characteristics of Smooth Muscle

Smooth muscle is composed of much smaller fibers (1 to 5 micrometers in diameter) compared to skeletal muscle, and it lacks the characteristic cross-striations and troponin complex. Instead, contraction relies on actin and myosin filaments that anchor to intra-cellular dense bodies, regulated by calcium ions binding to calmodulin rather than troponin. Guyton classifies smooth muscle into two fundamental types: multi-unit smooth muscle (where discrete fibers contract independently under neural control, such as in the ciliary muscle of the eye) and unitary or visceral smooth muscle (where thousands of fibers are interconnected by gap junctions to contract together as a single unit, as in the gastrointestinal tract and blood vessels). A defining characteristic highlighted by Guyton is its remarkable energy efficiency; through the latch mechanism, smooth muscle can maintain prolonged, tonic contraction while consuming as little as 1/10 to 1/300 of the energy required by skeletal muscle.

• Smooth muscle fibers are much smaller than skeletal muscle.
• Diameter ranges from 1 to 5 micrometers.
• Length ranges from 20 to 500 micrometers.
• Actin and myosin attractive forces cause contraction.
• Physical arrangement differs from skeletal muscle.

Types of Smooth Muscle

Multi-Unit Smooth Muscle

• Composed of discrete, separate muscle fibers.
• Each fiber operates independently.
• Often innervated by a single nerve ending.
• Covered by a thin basement membrane-like substance.
• Each fiber contracts independently of others.
• Controlled mainly by nerve signals.
• Examples include ciliary and iris eye muscles.
• Includes piloerector muscles erecting skin hairs.

Unitary Smooth Muscle

• Also called syncytial or visceral smooth muscle.
• Hundreds to thousands of fibers contract together.
• Fibers are arranged in sheets or bundles.
• Cell membranes adhere to transmit contraction force.
• Connected by gap junctions for ion flow.
• Action potentials travel easily between adjacent fibers.
• Found in walls of most body viscera.
• Present in gut, bile ducts, and ureters.
• Located in the uterus and many blood vessels.

Chemical Basis of Contraction

• Contains both actin and myosin filaments.
• Lacks the normal troponin complex.
• The contraction control mechanism is distinct.
• Actin and myosin interact similarly to skeletal muscle.
• Contraction is activated by calcium ions.
• ATP degrades to ADP for contraction energy.

Physical Basis of Contraction

• Lacks the striated arrangement of skeletal muscle.
• Large numbers of actin attach to dense bodies.
• Some dense bodies attach to cell membranes.
• Intercellular protein bridges bond adjacent dense bodies.
• Contraction force transmits through these dense body bonds.
• Myosin filaments are interspersed among actin filaments.
• Myosin has double the diameter of actin.
• Contains 5 to 10 times more action.
• Dense bodies serve the same role as Z discs.
• Myosin filaments have side polar cross-bridges.
• Pulls acting in opposite directions simultaneously.
• Allows smooth muscle to contract up to 80%.

Smooth Muscle vs. Skeletal Muscle Contraction

• Smooth muscle contraction is prolonged and tonic.
• Contraction can last hours or even days.
• Myosin cross-bridge cycling is much slower.
• Cycling frequency is 1/10 to 1/300 of skeletal.
• Cross-bridges remain attached to actin much longer.
• Cross-bridge heads have far less ATPase activity.
• ATP degradation rate is greatly reduced.
• Energy Requirements and Dynamics of Smooth Muscle Contraction

Low Energy Requirement

• Requires 1/10 to 1/300 energy of skeletal muscle.
• Energy economy results from slow cross-bridge cycling.
• Requires only one ATP molecule per cycle.
• Low energy utilization is crucial for viscera.
• Visceral organs maintain tonic contraction almost indefinitely.

Slowness of Contraction and Relaxation

• Begins contracting 50 to 100 milliseconds after excitation.
• Reaches full contraction about 0.5 second later.
• Force declines over another 1 to 2 seconds.
• Total contraction time lasts 1 to 3 seconds.
• Duration is 30 times longer than skeletal muscle.
• Contraction duration ranges from 0.2 to 30 seconds.
• Slow onset is caused by slow cross-bridge cycling.
• Calcium initiation of contraction is also much slower.

Maximum Force of Contraction

• Maximum force is often greater than skeletal muscle.
• Force equals 4 to 6 {kg/cm}^2 cross-sectional area.
• Skeletal muscle force is 3 to 4 kilograms.
• Great force results from prolonged cross-bridge attachment.

Latch Mechanism

• Maintained contraction requires far less continuing excitation.
• Energy consumed during sustained contraction is minuscule.
• Energy required can be as low as 1/300.
• Latch mechanism maintains prolonged tonic contraction.
• Requires little continued nerve or hormonal signals.

Stress-Relaxation of Smooth Muscle

• Smooth muscle returns to original contraction force.
• Occurs seconds or minutes after length changes.
• Increased fluid volume temporarily spikes organ pressure.
• Pressure returns to original level within 15-60 seconds.
• Decreased volume causes initial pressure drop before recovery.
• Process is called stress-relaxation and reverse stress-relaxation.
• Maintains constant lumen pressure despite volume changes.

Regulation of Contraction by Calcium Ions

Role of Calcium and Calmodulin 

• Increased intracellular calcium initiates smooth muscle contraction.
• Calcium rises via nerves, hormones, stretch, or environment.
• Smooth muscle does not contain troponin.
• Smooth muscle cells contain abundant calmodulin protein.
• Calmodulin activates myosin cross-bridges to initiate contraction.

Sequence of Myosin Activation

• Calcium ions bind with calmodulin.
• Calcium-calmodulin complex activates myosin light chain kinase.
• Myosin light chain kinase phosphorylates regulatory light chain.
• Unphosphorylated myosin heads cannot cycle with actin.
• Phosphorylated myosin heads bind repetitively with actin.
• Cross-bridge cycling proceeds to cause muscle contraction.