Muscle Tissue: Types, Structure, and Function

Posted on Aug 14, 2024 in Biology

Muscle Tissue

Muscle tissue is composed of contractile cells derived from the mesoderm. These elongated fibers specialize in contraction and are categorized into two main types:

  • Smooth muscle (leiomyofibers)
  • Striated muscle (rhabdomyofibers), which includes skeletal and cardiac muscle.

Smooth Muscle Tissue

Distribution

Smooth muscle cells are found in various locations throughout the body:

  • As scattered cells: scrotum, nipple, duodenal villi
  • Grouped in coats: vessels, digestive tract, respiratory tree
  • In layers within these coats: longitudinal, circular, or oblique orientations
  • Grouped as muscle cells: eye, hair erector

Structure

  • Spindled and elongated shape
  • Homogenous eosinophilic cytoplasm
  • Single, centrally located nucleus with condensed chromatin
  • Sometimes binucleated
  • Cells in close apposition
  • Bounded by a basal lamina containing reticulin fibers
  • Associated with connective tissue carrying vessels and nerves
  • Variable size depending on location (e.g., vessels: 15µm x 4-8µm, digestive tract: 200µm x 4-8µm, uterus: 500µm x 20-22µm)
Key Components:
  • Sarcoplasm: Contains”cone” of sarcoplasm located on either side of the nucleus, housing ribosomes, rough endoplasmic reticulum (RER), smooth endoplasmic reticulum (REL), Golgi apparatus, and mitochondria.
  • Fibrillar Material: Composed of contractile and intermediate filaments arranged parallel to the cell axis.
  • Adhesion Plaques: Areas where actin filaments attach, dispersed throughout the cytoplasm and particularly abundant near the sarcolemma. Composed of α-actinin, vinculin, and talin.
  • Sarcolemma: Contains caveolae, gap junctions for coordinated contraction, neuromuscular synapses, and a glycocalyx.

Innervation, Contraction, and Regulation

  • Innervation: Primarily by the autonomic nervous system via unmyelinated nerve fibers partially covered by Schwann cells. Axons exhibit varicosities and often release norepinephrine as a neurotransmitter.
  • Types of Innervation:
    • Unit: One nerve fiber per muscle cell.
    • Multiple: Contraction order transmitted between cells through gap junctions.
  • Contraction Mechanism: Sliding of myofilaments.
  • Regulation: Nervous and hormonal control via oxytocin, vasopressin, epinephrine, prostaglandins, peptides, and other regulators.

Special Smooth Muscle Cells

  • Myoepithelial cells in exocrine glands
  • Ruyter’s mioepitelioides in the juxtaglomerular apparatus
  • Myofibroblasts in the connective tissue myoid of the seminiferous tubules
  • Branched cells in the middle layer of the aorta

Striated Muscle Tissue

Striated Skeletal Muscle Cells

Definition and General Characteristics

Striated skeletal muscle cells form striated skeletal muscle tissue, a major component of muscles, constituting 30-40% of body weight. This tissue is characterized by:

  • Muscle tissue itself
  • Connective tissue components:
    • Endomysium: Surrounds each muscle cell
    • Perimysium: Surrounds groups of muscle cells
    • Epimysium: Surrounds the entire muscle
  • Blood vessels, nerves, and receptors
Types and General Features
  • Intrafusal: Found within muscle spindles
  • Satellite: Mononuclear cells located beneath the basal lamina surrounding extrafusal fibers
  • Extrafusal:
    • Oblong shape with rounded ends
    • Parallel arrangement (can be pennate)
    • Multinucleated with peripherally located nuclei
    • Cytoplasm filled with myofibrils exhibiting light and dark bands
    • Size: 1mm to several cm long, 10-100µm in diameter
Ultrastructure
  1. Sarcolemma:
    • Plasma Membrane: Excitable membrane with a glycocalyx and a resting membrane potential of -85mV.
    • Basal Lamina: Composed of reticulin fibers and continuous with the endomysium.
    • Transverse Tubular System (T-tubules): Deep invaginations of the sarcolemma rich in sodium ions (Na+). Closely associated with the sarcoplasmic reticulum (SR), with T-tubules flanking SR terminal cisternae. Propagates depolarization waves into the cell, triggering Ca2+ release from the SR terminal cisternae.
    • Neuromuscular Synapse: Primary synaptic cleft where the nerve fiber terminates. The nerve fiber, an axon from an α-motor neuron with its cell body in the spinal cord’s anterior horn, loses its myelin sheath as it approaches the synaptic cleft, becoming enveloped by an expanded Schwann cell.
  2. Sarcoplasm:
    • Nuclei: Multiple, peripherally located nuclei with condensed chromatin.
    • Mitochondria: Abundant and large, distributed along the myofibrils.
    • Golgi Apparatus: Located near the nucleus (juxtanuclear).
    • Energy Stores: Glycogen and lipid droplets.
    • Myoglobin: A pigment similar to hemoglobin, giving muscles their characteristic red color.
    • ATP and Phosphocreatine: Support anaerobic metabolism.
    • Sarcoplasmic Reticulum: A network of interconnected tubules surrounding each myofibril.
    • Myofibrils: Highly organized contractile units composed of thick and thin myofilaments. Arranged parallel to each other and the long axis of the cell. Each myofibril consists of repeating units called sarcomeres, measuring 2.5µm to 1.5µm in length and delineated by Z lines. Cross-sections of myofibrils reveal the geometric arrangement of myofilaments.
Innervation

Skeletal muscle cells receive innervation from:

  • α-motor neurons
  • γ-motor neurons
  • Sensory fibers innervating muscle spindles (proprioceptive receptors located in connective tissue septa, sensing muscle tone and stretch)
    • Muscle spindles contain modified striated muscle cells of two types: nuclear bag fibers and nuclear chain fibers, with sparse and poorly defined myofibrils.
    • Each intrafusal muscle cell synapses with a γ-motor neuron and receives sensory innervation from fibers that wrap around the muscle cell with varicose (beaded) dilatations.

Cardiac Striated Muscle Tissue

Cardiac Striated Muscle Cells

General Characteristics and Structure

The myocardium, the heart’s muscular layer, comprises groups of cardiac striated muscle cells separated by connective tissue. Notably, cardiac muscle lacks satellite cells and therefore cannot regenerate.

  1. Typical Myocardial Cells:
    • Bifurcated and interconnected, forming a three-dimensional syncytium
    • Rectangular shape, approximately 150µm long and 20µm in diameter
    • Contain one or two centrally located nuclei
    • Less defined myofibrils compared to skeletal muscle
    • Abundant mitochondria, glycogen, and myoglobin between myofibrils
    • T-tubules form dyads with the sarcoplasmic reticulum (SR) at the Z lines
    • Connected by intercalated discs
  2. Cardionector Cells:
    • Modified myocardial cells forming the heart’s conduction system
    • Responsible for the heart’s automaticity (intrinsic rhythm)
    • Located in the sinoatrial (SA) node, atrioventricular (AV) node, bundle of His, and Purkinje fibers
    • Elongated cells with sparse fibrillar material and a lack of T-tubules and well-developed SR
  3. Myoendocrine Cells:
    • Specialized myocytes found in the atria
    • Secrete hormones involved in regulating blood volume, composition, and extracellular fluid electrolyte balance
Peculiarities of Cardiac Muscle Contraction
  • Inherent Rhythmicity: Cardiac muscle possesses an intrinsic rhythm, allowing it to contract spontaneously.
  • Conduction System: A specialized system of cells ensures coordinated contraction. The SA node acts as the heart’s pacemaker, initiating depolarization waves that spread through gap junctions to the atria and ventricles, determining the heart rate. The autonomic nervous system can modulate this intrinsic rhythm.
  • Internodal Tracts: Conduct the depolarization wave from the SA node to the AV node.
  • Bundle of His and Purkinje Fibers: Transmit the depolarization wave from the AV node to the ventricular myocardium, ensuring a coordinated contraction that begins at the apex and spreads towards the base, effectively ejecting blood into the aorta.

Repair of Muscle Tissue

  • Smooth Muscle: Retains mitotic ability, allowing for regeneration. Loss of muscle tissue is compensated by mitosis of existing cells and differentiation of pericytes.
  • Skeletal Muscle: Lacks mitotic capacity but can regenerate from satellite cells.
  • Cardiac Muscle: Unable to regenerate. After injury, fibroblasts form scar tissue.