Lecture 8 Smooth Muscle: Structure, Function, and Comparison with Skeletal Muscle

Muscle Tissue Overview

Introduction to Muscle Types

Muscle tissue is essential for movement and physiological functions in the human body. The three main types are skeletal muscle, cardiac muscle, and smooth muscle. This guide focuses on the structure, function, and contraction mechanisms of smooth muscle, with comparisons to skeletal muscle.

Smooth Muscle Structure and Location

Distribution in the Body

• Walls of hollow organs: Found in the respiratory, digestive, urinary, reproductive, and circulatory systems (excluding capillaries and the heart).

• Organization: Sheets of tightly packed fibers, typically arranged in two perpendicular layers (longitudinal and circular).

Smooth Muscle Layers

• Longitudinal layer: Fibers run parallel to the organ's long axis, shortening the organ when contracted.

• Circular layer: Fibers encircle the organ, constricting the lumen when contracted.

• Function: Alternating contraction and relaxation of these layers mix and propel substances through the organ's lumen.

Cellular Features of Smooth Muscle

Microscopic Structure

• Shape: Spindle-shaped, thinner, and shorter than skeletal muscle fibers.

• Nucleus: Single, centrally located nucleus per cell.

• Striations: No striations or sarcomeres, but overlapping thick and thin filaments are present.

• Connective tissue: Only endomysium is present; no epimysium or perimysium.

• Control: Involuntary, regulated by the autonomic nervous system (ANS).

Innervation and Control

Smooth Muscle Innervation

• Autonomic nervous system: Axons form varicosities (swellings) that release neurotransmitters (NTs) into wide synaptic clefts called diffuse junctions.

Types of Smooth Muscle

• Unitary (Visceral) Smooth Muscle: Found in hollow organs (except heart), arranged in sheets, innervated by varicosities, electrically coupled by gap junctions for synchronized contraction, and responds to chemical stimuli.

• Multiunit Smooth Muscle: Found in large airways, large arteries, erector pili muscles, and iris of the eye. No gap junctions; fibers contract independently, forming motor units and allowing recruitment. Controlled by ANS and hormones.

Myogenic Activity and Pacemaker Cells

Self-Excitability

• Pacemaker cells: Spontaneously active, do not contract but trigger contraction in neighboring cells via gap junctions.

• Membrane potential: Changes cyclically, leading to two types of spontaneous depolarizations:

  • Pacemaker potentials

  • Slow-wave potentials

Intermediate Filament Network

Structural Support

• Intermediate filaments (IFs): Form a lattice that transmits tension to the sarcolemma.

• Dense bodies: Anchor thin filaments and IFs; connect to adherens junctions and desmosomes.

• Caveolae: Membrane invaginations containing Ca2+ channels for rapid influx of extracellular calcium.

Mechanism of Smooth Muscle Contraction

Filament Arrangement and Contraction

• Thick and thin filaments: Arranged diagonally, connected by dense bodies (contain α-actinin).

• Contraction: Muscle fiber contracts in a corkscrew manner, shortening along its length.

• Thick to thin filament ratio: 1:13 (smooth muscle) vs. 1:2 (skeletal muscle).

• Myosin heads: Oriented in opposite directions on either face of thick filament; thin filaments move in opposite directions.

Regulation of Contraction by Calcium (Ca2+)

Calcium Sources and Role

• No T tubules: Smooth muscle lacks T tubules.

• Sarcoplasmic reticulum (SR): Less elaborate; stores Ca2+ but most comes from extracellular fluid (ECF).

• Caveolae: Contain voltage-gated Ca2+ channels for rapid influx.

• Calcium-induced calcium release (CICR): Ca2+ entry triggers further Ca2+ release from SR.

Regulation of Contraction by Ca2+

• Tropomyosin: Present but does not block myosin binding to actin.

• No troponin: Instead, Ca2+ binds to calmodulin (CaM).

• Myosin light chain kinase (MLCK): Activated by CaM, phosphorylates myosin head, enabling crossbridge formation with actin.

• Relaxation: Ca2+ dissociates from CaM, is actively transported into SR and ECF, and myosin is dephosphorylated to inactivate it.

Excitation-Contraction Coupling (Smooth Muscle)

1. Calcium ions (Ca2+) enter the cytosol from ECF via voltage-gated or non-voltage-gated channels, and from SR.

2. Ca2+ binds to and activates calmodulin.

3. Activated calmodulin phosphorylates and activates myosin light chain kinase.

4. Activated kinases phosphorylate myosin, activating myosin ATPase.

5. Activated myosin forms cross bridges with actin, initiating contraction.

Regulation of Smooth Muscle Contraction

Neural Regulation

• Neurotransmitter (NT) binding: Can cause graded or action potentials, increasing intracellular Ca2+.

• Response: Depends on NT and receptor type (e.g., norepinephrine inhibits bronchiole contraction but stimulates blood vessel contraction).

Hormonal and Local Chemical Regulation

• Some smooth muscle is not innervated; can depolarize spontaneously or in response to hormones binding GPCRs.

• Factors such as histamine, high CO2, low pH, and low O2 can enhance or inhibit Ca2+ entry.

Special Reflexes and Responses

Smooth Muscle Stretch Reflex

• Quick or sustained stretch leads to contraction, even without neural input.

• Involves stretch-activated ion channels; important in organs like the stomach, bladder, and small arterioles.

Stress-Relaxation Response (Unitary Smooth Muscle)

• Unitary smooth muscle contracts briefly when stretched, then adapts to new length.

• Allows organs to store contents and accommodate large volume changes.

• Reverse stress-relaxation: Removal of external stress decreases wall tension, followed by contraction to restore tension.

Special Features of Smooth Muscle Contraction

Contraction Speed and Tone

• Contraction and relaxation are slower but can be maintained for prolonged periods (~3 seconds) with minimal energy expenditure.

• Slower ATPases, longer-lasting crossbridges, and slower Ca2+ removal contribute to this property.

• Smooth muscle tone: Moderate, constant contraction without fatigue; ATP is synthesized by aerobic respiration.

Summary of Smooth Muscle Contraction

• Contractions are slow and synchronized; rate and intensity are modulated by neural and chemical stimuli.

• Actin and myosin interact via the sliding filament mechanism.

• Final trigger for contraction is increased intracellular Ca2+.

• ATP energizes the sliding process.

• Contraction ends when Ca2+ is no longer available.

Comparison of Skeletal and Smooth Muscle

Structural and Functional Differences

Characteristic	Skeletal Muscle	Smooth Muscle
Connective tissue components	Epimysium, perimysium, endomysium	Endomysium only
Presence of myofibrils composed of sarcomeres	Yes	No; actin and myosin filaments present but not organized into sarcomeres
Presence and location of T tubules	Yes; two per sarcomere at A-I junctions	No; only caveolae
Elaborate sarcoplasmic reticulum	Yes; large terminal cisterns	Limited (except in cardiac muscle)
Presence of gap junctions	No	Yes in unitary muscle; no in multiunit muscle
Cells exhibit individual neuromuscular junctions	Yes	No in unitary muscle; yes in multiunit muscle
Regulation of contraction	Voluntary via axon terminals of the somatic nervous system	Involuntary; regulated by intrinsic pacemaker cells (in unitary muscle), ANS, hormones, local chemicals, and stretch

Key Equations

• Sliding Filament Mechanism:

• $\text{Force} \propto \text{Number of crossbridges formed}$

• Calcium-Induced Calcium Release (CICR):

• $\text{[Ca^{2+}]_{i}} = \text{[Ca^{2+}]_{ECF}} + \text{[Ca^{2+}]_{SR}}$

Example Application

• Bladder function: Smooth muscle allows the bladder to stretch and store urine, then contract to expel it, demonstrating the stress-relaxation response.

• Blood vessel regulation: Smooth muscle contraction in vessel walls regulates blood pressure and flow.

Additional info: These notes are based on lecture slides and textbook references (Marieb & Hoehn, 11th Ed.), suitable for college-level Anatomy & Physiology students studying muscle tissue structure and function.