Detailed Notes on Muscle Microanatomy

Muscle Tissue Overview

  • Muscle cells are specialized for contraction, producing movement for locomotion, propulsion, and pressure regulation.
  • Muscle cells, also referred to as myocytes or myofibers, are typically spindle-shaped and originate from the mesoderm via myoblasts.

Terminology

  • Sarcoplasm: Cytoplasm of muscle cells, containing glycogen and myoglobin.
  • Sarcolemma: Plasma membrane of the muscle cell.
  • Sarcoplasmic Reticulum: Specialized smooth endoplasmic reticulum in muscle cells that regulates calcium flow.

Muscle Tissue Classification

  • Skeletal: Cross-striated muscle.
  • Cardiac.
  • Smooth muscle.
  • Myoepithelial cells.

Muscle Cell Dimensions

  • Skeletal Muscle:
    • Diameter: 10−110μm10 - 110 \mu m
    • Length: 40 mm up to 50 cm
  • Smooth Muscle:
    • Fusiform, spindle-shaped cells
    • Diameter: 6−8μm6 - 8 \mu m
    • Length: 20−40μm20 - 40 \mu m up to 1 mm in uterus
  • Cardiac Muscle:
    • Cells arranged in fibers
    • Length: 100μm100 \mu m
    • Diameter: 15μm15 \mu m

Muscle Fiber Types

  • Smooth muscle: Single cells.
  • Cardiac muscle: Single cells with intercalated disks.
  • Skeletal muscle.

Skeletal Muscles

  • Make up 50% of body weight.
  • The tongue and masseter muscles are some of the strongest muscles of the body.
  • Skeletal myocytes contain multiple, peripherally located nuclei.
  • Striations are seen in myocytes when cut in longitudinal section.

Skeletal Muscle Contraction

  • Contracts fast and voluntarily.
  • Skeletal muscles are voluntary muscles.
  • Most (not all) skeletal muscles insert on bones of the skeleton.
  • Voluntary sphincters in the gastrointestinal tract and urinary systems, muscles in the esophagus and the tongue are made of skeletal muscle.

Skeletal Muscle Development

  • Mesenchymal cells (myoblasts) align and fuse together forming multinucleated tubes called myotubes.
  • Myotubes differentiate forming functional myofilaments, and the nuclei are displaced against the plasma membrane.
  • Some cells do not differentiate and remain as mesenchymal stem cells called satellite cells, that function in muscle repair.

Organization of Skeletal Muscle

  • Muscles are surrounded by a dense irregular connective tissue layer known as the epimysium (fascia).
  • A whole muscle can be made up of many smaller bundles known as fascicles surrounded by perimysium.
  • Each fascicle is made up of many muscle cells (myocytes or myofibers).
  • Myofibers contain cylindrical bundles of myofibrils, which in turn contain many smaller bundles of myofilaments.

Striated Muscle (Cross Section)

  • Each fascicle is surrounded by dense connective tissue, known as the perimysium.
  • Individual myocytes are surrounded by reticular fibers forming the endomysium.

Skeletal Muscle Features

  • Myofibers: Cylindrical multinucleated cells up to 50 cm long and 10−110μm10 -110 \mu m in diameter.
  • Peripheral / hypolemmal nuclei.
  • Myofibrils are the contractile elements; composed of repeating sections of myofilaments, known as thick MYOSIN and thin ACTIN myofilaments, thus creating light and dark banding.

Myofibers

  • Composed of myofibrils.
  • Myofibrils: Cylindrical bundles of contractile proteins called myofilaments. The arrangement of myofilaments cause the striated appearance of skeletal and cardiac muscle.
  • Myofilaments (can be seen in EM only) are contractile and regulatory proteins:
    • Actin, Myosin - contractile proteins
    • Tropomyosin- regulatory proteins

Skeletal Muscle Striations

  • Striations are due to the presence of two types of myofilaments:
    • A band – dark
    • I band – pale
  • Each I band is bisected by a protein disc known as the Z-line.
  • Actin filaments are anchored onto the Z-line.

Sarcomere

  • Contractile or functional unit of myocyte.
  • Includes all elements from Z line to Z line.
  • At full contraction, Z lines (discs) will be drawn closer to each other.
  • A Band - Actin & Myosin
  • I Band - Actin filaments

A Band and I Band

  • The A band is dark and corresponds to the area where thick and thin filaments overlap. It is composed of myosin (+actin).
  • The I band is pale and is composed of only thin actin filaments.

Structural Basis of Muscle Contraction

  • Motion is mediated by muscle cells and is based on the conversion of chemical energy (ATP) into mechanical energy.
  • Myofibrils are composed of repeating assemblies of thick and thin filaments constituting dark A and pale I segments.
  • During muscle contraction the actin filaments slide over the myosin filaments, which results in a shortening of the I band.
  • Actin and myosin are held in position in the myofibril by other proteins (desmin, tropomyosin, and troponin).

Sliding Filament Model

  • Each sarcomere shortens.
  • Myofilament length is constant.
  • I band shortens, almost disappears.
  • Thin (actin) filaments slide past thick (myosin) filaments.
  • Summation of all sarcomere shortening produces contraction of the muscle cell.

Myofibrils and Sarcoplasmic Reticulum

  • Myofibrils made of myosin and thin actin myofilaments.
  • Sarcoplasmic reticulum: Stores and releases Calcium.
  • Terminal cisterna – expanded ends of sarcoplasmic reticulum.
  • Transverse T-tubule - deep tubular finger-like invaginations of sarcolemma (plasma membrane) of skeletal muscle cell create anastomosing network of tubules.

T Tubules

  • A T-tubule is a deep invagination of the sarcolemma only found in skeletal and cardiac muscle cells.
  • These invaginations allow depolarization of the membrane to quickly penetrate to the interior of the cell allowing calcium release from the sarcoplasmic reticulum.

Sarcoplasmic Reticulum

  • Equivalent to the endoplasmic reticulum but seen in smooth and striated muscle.
  • Stores and releases calcium ions to initiate contractions.

Skeletal Muscle Organization Summary

  • Muscle:
    • Level Seen: Gross
    • Surrounded by: Epimysium
  • Muscle fascicles:
    • Level Seen: Subgross and LM
    • Surrounded by: Perimysium
  • Muscle cell (Myofiber):
    • Level Seen: LM
    • Surrounded by: Endomysium
    • Sarcoplasm is filled with myofibrils
  • Myofibril:
    • Level Seen: LM (cross striations) and EM
    • Myofilaments; “sarcomere” subunits
  • Myofilament
    • Level Seen: EM only
    • Actin and Myosin

Cardiac Muscle

  • Cardiomyocyte = myocardiocyte = cardiac myocyte
  • Single cell with one central nucleus
  • Cross striated
  • Has intercalated discs:
    • gap junctions
    • desmosomes
  • Has sarcoplasmic reticulum
  • Has many mitochondria up to 20% cell volume → requires a lot of O2O_2

Intercalated Discs

  • Attach cardiac muscle cells to each other, providing strength and the ability to function as a SYNCYTIUM.
  • A true syncytium is a multinucleated cell e.g. skeletal muscle cell.

Intercalated Disc Components

  • Transverse element
    • Anchor
    • Desmosome – fascia adherens and/or macula adherens
    • Serve for strong attachment
  • Longitudinal element
    • Communication
    • Gap junction (nexus) with ion channels
    • Propagate electric impulse

Types of Cardiomyocytes

  • Contractile cardiac muscle cells
  • Conductile cardiac muscle cells (“Purkinje fibers”)

Purkinje Fibers

  • Purkinje fibers are modified cardiac muscle cells with 1 or 2 nuclei, myofibrils are sparse and restricted to periphery of cell.
  • Purkinje fibers are arranged in groups, cells are bigger and paler than contractile cardiac muscle cells. These groups are also referred to as Purkinje fibers!

Smooth Muscle Cells

  • Single centrally located nucleus
  • No striations
    • No myofibrils!
    • Actin & myosin myofilaments are present, but are not ordered
  • Cytoplasmic dense bodies represent anchors for myofilaments (like Z bodies)
  • Desmosomes and gap junctions
  • No T– tubules and sarcoplasmic reticulum is poorly developed

Smooth Muscle Cell Characteristics

  • Spindle shaped (fusiform) cells surrounded by a basal lamina and reticular fibers
  • Single central nucleus
  • Parasympathetic and sympathetic innervation
  • Contraction is involuntary

Types of Smooth Muscle

  • Single (unitary) smooth muscle
    • Found in visceral organs
    • Cells behave like syncytium contracting in a network
    • Sparse nerve innervation, but cells communicate via multiple gap junctions
  • Multi-unit smooth muscle
    • Found in the iris of the eye
    • Precise contraction
    • Individual innervation of each myocyte
    • Lack of gap junctions, function individually

Smooth Muscle Functions

  • Peristalsis – wave-like contractions e. g. gastrointestinal tract
  • Vascular dynamics – contraction alters blood flow & important in blood pressure
  • Propulsion – urinary bladder, uterus
  • Secretion (minor role)

Tunica Muscularis

  • Inner circular layer
  • Outer longitudinal layer
  • Contraction is non-voluntary

Smooth Muscle Cell Components

  • Nucleus: Oval (cigar-shaped)
  • Dense bodies: the equivalent of Z discs of skeletal and cardiac muscle.
  • Actin + myosin filaments
  • Intermediate filaments

Smooth Muscle Contraction

  • Oblique arrangement of actin/myosin filaments and intermediate filament arrangement allow the entire cell to shorten by up to 80% (significantly more, and more sustained than striated)
  • Note corkscrew appearance of nucleus.

Contractile Non-Muscle Cells: Myoepithelial Cells

  • Ectodermal origin
  • Contain actin/myosin
  • Similar to smooth muscle
  • Can also be stimulated by hormones (mammary gland)
  • Basket-like shape → known as BASKET CELLS
  • Location: Salivary/ Mammary/Lacrimal Glands

Myoepithelial Cells vs. Myofibroblasts

  • Myoepithelial cells
    • basket-like shape
    • enclose clusters of glandular cells
    • actin and myosin
    • from ectoderm
  • Myofibroblasts
    • wound contraction
    • from mesoderm

Muscle Regeneration

  • Skeletal muscle has limited regeneration capacity.
    • SATELLITE CELLS positioned between basal lamina and sarcolemma of the muscle cell.
    • These cells retain mitotic potential and thus can accomplish some repair.
    • Fibroblasts also form connective tissue (scar) as part of the repair process.
  • Cardiac muscle cells lack the ability to regenerate.
  • Smooth muscle regeneration is limited. In addition to some mitotic activity, new smooth muscle cells may be derived from pericapillary mesenchymal cells.
  • In all muscle types, repair is completed by scar tissue formation and requires intact basal lamina.

Skeletal Muscle Terms to Know

  • Myocyte, myofiber, myofibril
  • Myofilaments (actin and myosin)
  • Mesoderm
  • Satellite cells
  • Myoblast
  • Sarcoplasm
  • Sarcoplasmic reticulum
  • Sarcolemma
  • Intercalated disks (desmosome and gap junction)
  • Cross-striation
  • Epimysium
  • Fascicles
  • Perimysium
  • Hypolemmal
  • Sarcomere (A band, I band, Z line, H zone, M line)
  • Tropomyosin
  • Purkinje fiber
  • Myoepithelial cell (basket cells)