Anatomy of Skeletal and Smooth Muscle — Comprehensive Notes

Learning Objectives

  • Describe the structure of smooth and skeletal muscle.

  • Identify the differences between smooth and skeletal muscle.

  • Identify the parts of a microfibril.

  • Describe the function of the sarcoplasmic reticulum (SR) and T tubules (transverse tubules).

Muscle Types

  • Skeletal muscle- Found attached to the skeletal system.

    • Striated in appearance.

  • Smooth muscle- Found in organs and parts of vasculature.

    • No striation.

  • Cardiac muscle- Combination of smooth and skeletal features.

Skeletal Muscle Structure

  • Elongated shape- Diameter: 5−100 μm5-100\,\mu\mathrm{m}

    • Length: 10−30 cm10-30\,\mathrm{cm}

    • Attached end-to-end to form longer structures.

  • Myofibril- Bundles of protein filaments containing contractile elements of muscle fibers.

  • Sarcomere- Basic contractile unit of a muscle fiber.

  • Myofilament- Individual filaments of actin or myosin that make up a myofibril.

  • Actin- A protein that is part of the contractile filament; the “thin” filament.

  • Myosin- Fibrous protein that is part of the contractile filament; the “thick” filament.

  • Sarcolemma- Specialized cell membrane surrounding muscle fiber cells.

    • Contains pores that allow glucose, nutrients, and ions to enter

Actin and Myosin Structure

  • Actin filament composition- Actin, tropomyosin, and troponin.

    • Actin and tropomyosin form helical strands.

    • Troponin is located at regular intervals along strands.

  • Troponin subunits and roles- Troponin-C (TnC): binds Ca$^{2+}$.

    • Troponin-T (TnT): binds to tropomyosin.

    • Troponin-I (TnI): binds to actin to inhibit contraction.

  • Active sites on actin- Located on actin strands and usually covered by tropomyosin strands.

  • Myosin II structure- Made of 2 heterotrimers arranged in helical strands.

    • Head has a binding site for ATP.

    • Hundreds of myosin II molecules assemble together.

    • Cross-link in the middle of the filament.

Key Sarcomeric Features

  • Z line (Z disc)- Located at each end of a sarcomere; marks where they meet.

    • Actin projects from the Z line into sarcomeres toward the center.

  • I band- Actin filaments from two adjacent sarcomeres intersect the same Z line.

  • A band- Central region containing myosin filaments, overlapped with actin filaments.

  • M line- Center of the sarcomere (medial line) where thick filaments are anchored.

  • H zone- The region within the A band with only thick (myosin) filaments when relaxed.

Myofibril Organization (Diagrams referenced)

  • Myofibril

  • Thin filament components: actin, tropomyosin, troponin complex (TnC, TnT, TnI)

  • Thick filament component: myosin heads project toward actin during contraction.

SARCOTUBULAR SYSTEM

  • Located within muscle fiber on the outside of myofibrils.

  • Sarcoplasmic Reticulum (SR)- Tubules arranged parallel to myofibrils and encircle them.

  • T tubules (transverse tubules)- Tubules arranged transversely to myofibrils.

  • Function- Provides a means for conduction of an electrical impulse from the surface of the muscle fiber to interior aspects of the fiber.

T Tubules Details

  • Extend transversely from one side of the fiber to the other.

  • Open to the outside of the fiber and contain extracellular fluid (ECF).

  • Regularly spaced throughout the length and circumference of fibers.

  • Located near the junction of actin with myosin; approximately two T tubules close to each sarcomere.

  • Sarcotubules- Individual tubules of the SR located regularly throughout the length of the muscle fiber between T tubules; contain intracellular fluid (ICF).

  • Triad- The point of closeness of a T tubule with the bulbous ends of two adjoining SR tubules.

Sarcoplasmic Reticulum

  • Storage site for Ca$^{2+}$ ions.

  • Important for initiation and termination of muscle contraction.

  • Contains anastomosing channel-like structures that surround each myofibril.

Skeletal Muscle Fiber Types

  • Type I (red/dark/slow-twitch)- Reddish appearance due to large amounts of myoglobin, capillaries, and mitochondria.

    • Supports greater oxidative metabolism.

  • Type II (white/fast-twitch)- React rapidly and with short duration.

    • Consist of large fibers with great contractile strength.

    • Extensive SR but less extensive blood supply and fewer mitochondria.

  • Intermediate fibers exist.

  • Most mammals have a mixture of all fiber types; dominance depends on the muscle’s primary function.

Harnessing (Connective Tissue Framework)

  • Epimysium- Connective tissue sheath that wraps entirely around the outside of the whole muscle.

  • Perimysium- Connective tissue extensions from the epimysium that surround muscle bundles (fascicles).

  • Endomysium- Extensions from the perimysium that surround each muscle fiber.

    • Attached to the sarcolemma (muscle fiber membrane).

  • Force transmission- The pull exerted during contraction is transmitted by the endomysium, perimysium, and epimysium to a tendon or aponeurosis.

  • Aponeurosis- A sheet of fibrous tissue that can take the place of a tendon in flat muscles with a broad attachment.

Neuromuscular Junction

  • Motor neuron- Neuron that interacts with muscle fibers to elicit a response, usually leading to contraction.

  • Motor unit- Consists of a motor neuron and the muscle fibers it innervates.

    • Large motor units are common in limbs/postural muscles; small units are associated with eye movements.

  • Neuromuscular junction (NMJ)- The contact point between the end bulb of a motor neuron and a muscle fiber.

  • Synaptic cleft- The space between the terminal end of the axon and the muscle fiber.

  • Neurotransmitter- Acetylcholine (Ach) stored in vesicles in terminal branches and released into the cleft to stimulate contraction.

Smooth Muscle Structure

  • Cell shape- Spindle-shaped with a central nucleus.

    • Fusiform: tapering at both ends; the taper lies adjacent to the wide portion of neighboring fibers, allowing dense packing.

  • Dense bodies- Anchoring sites for actin myofilaments, similar to Z lines in skeletal muscle.

    • Can be scattered throughout the cytoplasm and attach to intermediate filaments, linking several dense bodies together or to the sarcolemma.

  • Actin to myosin ratio- Approximately 15:115:1 in smooth muscle.

  • T tubules- Absent in smooth muscle.

  • Caveolae- Invaginations in the cell membrane that functionally substitute for T tubules; close proximity to portions of rudimentary SR.

Smooth Muscle Types

  • Multi-unit smooth muscle- Found in the ciliary body and iris of the eye, arrector pili muscle of the skin, and walls of large arteries.

    • Composed of discrete smooth muscle fibers; each fiber is innervated separately and contracts only when stimulated.

    • Contractions do not spread between cells (no intercellular conduction).

  • Single-unit (visceral) smooth muscle- Large regions contract simultaneously.

    • Peristalsis: waves of contraction that help move material (often ingesta) through an organ.

    • Harnessing occurs via cell membranes of fibers within a sheet adherent to each other at multiple points; gap junctions allow ions to flow freely between cells, enabling coordinated contraction.

Practical Mnemonic Note

  • Cartoon recap from the AwkwardYeti: Esophagus uses smooth muscle to move food; gravity is not required for movement in the GI tract due to smooth muscle peristalsis. The dialogue emphasizes interactions like a massage and questions about motion, illustrating how smooth muscle functions mechanically in a humorous context.

Connections to Core Principles and Real-World Relevance

  • Structure–function relationship- The arrangement of actin and myosin in sarcomeres enables striated, highly organized contraction in skeletal muscle.

    • The SR and T tubules coordinate rapid, synchronized Ca$^{2+}$ release and muscle contraction.

  • Electrical–chemical coupling- Neuromuscular junction illustrates how electrical signals (nerve impulses) are transformed into chemical signals (Ach release) and then back into electrical signals leading to contraction.

  • Mechanical integration with connective tissue- Epimysium, perimysium, and endomysium transmit force to tendons, illustrating how microscopic events drive macroscopic movement.

  • Adaptation and specialization- Different fiber types (Type I vs Type II) reflect specialization for endurance vs rapid, powerful movements, with mixed fiber composition in most muscles depending on function.

  • Smooth muscle diversity and coordination- The presence of dense bodies and caveolae enables smooth muscle contraction in organs where slow, sustained, or rhythmic contractions are advantageous.

  • Clinical and physiological relevance- Understanding NMJ, SR/Ca$^{2+}$ handling, and fiber-type composition informs treatments for myopathies, neuromuscular disorders, and considerations in athletic training.

Key Formulas and Numerical References

  • Skeletal muscle diameter and length- d=5−100 μmd = 5-100\,\mu\mathrm{m}

    • L=10−30 cmL = 10-30\,\mathrm{cm}

  • Actin to myosin ratio in smooth muscle- Actin:Myosin=15:1\text{Actin} : \text{Myosin} = 15:1

  • Myosin II organization- "2 heterotrimers" form the myosin II filament assembly.

  • Calcium binding and contraction cues- $\mathrm{Ca}^{2+}$ binding to troponin C is a key trigger for skeletal muscle contraction (via removal of inhibition by tropomyosin).

Quick Reference Highlights

  • Major sarcomeric zones: Z line, I band, A band, M line, H zone.

  • Triad architecture at the t-tubule–SR junction is essential for rapid Ca$^{2+}$ signaling.

  • Types of skeletal muscle fibers influence endurance and power capabilities.

  • Dense bodies and caveolae in smooth muscle support non-synaptic, synchronized contraction.

  • Neuromuscular junction is the critical interface for motor control of skeletal muscle.

Summary of Key Distinctions

  • Skeletal vs Smooth:

    • Skeletal: striated, organized sarcomeres, T-tubules present, rapid, forceful contractions; controlled by NMJ.

    • Smooth: non-striated, dense bodies, no T-tubules (caveolae compensate), slower, sustained contractions; can be multi-unit or single-unit with gap junctions.

  • Connective tissue harnessing differs from organ to organ; force transmission relies on layered connective tissues.

  • Calcium handling is central to contraction initiation in skeletal muscle and is modulated differently in smooth muscle due to the absence of T tubules and the reliance on dense bodies and caveolae.

Questions for Review

  • What structural elements define a sarcomere, and what are their basic roles during contraction?

  • How does troponin regulate actin-myosin interaction in skeletal muscle, and what triggers its change in conformation?

  • Why do smooth muscles lack T tubules, and what structures compensate for Ca$^{2+}$ signaling?

  • Compare and contrast multi-unit and single-unit smooth muscle in terms of innervation and coordination.

  • How do connective tissue layers (epimysium, perimysium, endomysium) contribute to muscle contraction and force transmission?

Appendix: Notable Terminology

  • Sarcomere: unit of contraction within a myofibril.

  • myofibril: bundle of contractile proteins within a muscle fiber.

  • sarcolemma: muscle cell membrane.

  • sarcoplasmic reticulum (SR): Ca$^{2+}$ storage network around myofibrils.

  • T tubules: invaginations of the sarcolemma that propagate action potentials; form triads with SR.

  • dense bodies: anchors for actin in smooth muscle.

  • caveolae: membrane invaginations that substitute for T tubules in smooth muscle.

  • perimysium: surrounds fascicles of muscle fibers.

  • endomysium: surrounds individual muscle fibers.

  • aponeurosis: tendon-like sheet in broad attachments.

  • motor unit: motor neuron plus innervated muscle fibers.