Muscle Tissue: Types, Structure, and Function

Posted on Aug 14, 2024 in Biology

Muscle Tissue: An Overview

Muscle tissue is composed of specialized cells called muscle fibers, derived from the mesoderm, that excel in contraction. These elongated fibers specialize in various contraction types, broadly categorized as smooth (leiomyofibers) and striated (rhabdomyofibers). Striated muscle further branches into skeletal and cardiac muscle.

Smooth Muscle Tissue

Distribution

Smooth muscle tissue is found throughout the body, often appearing as:

Scattered cells: scrotum, nipple, duodenal villi
Grouped in coats: vessels, digestive tract, respiratory tree
Layers within these coats: longitudinal, circular, or oblique arrangements
Distinct muscle cells: eye, hair erector

Structure

Smooth muscle cells are characterized by:

Spindle-shaped and elongated morphology
Homogenous, eosinophilic cytoplasm
Single, centrally located nucleus with condensed chromatin (sometimes binucleated)
Close apposition to other smooth muscle cells
Bounded by a basal lamina with inserted reticulin fibers
Association with connective tissue containing vessels and nerves
Variable size depending on location (e.g., 15µm x 4-8µm in vessels, 200µm x 4-8µm in the digestive tract, 500µm x 20-22µm in the uterus)

Key Components:

Sarcoplasm: Contains”cone” of sarcoplasm interspersed with cytoplasm spaces. These cones house ribosomes, rough endoplasmic reticulum (RER), smooth endoplasmic reticulum (SER), Golgi apparatus, and mitochondria.
Fibrillar Material: Composed of contractile and intermediate filaments running parallel to the cell’s axis.
Adhesion Plaques: Areas where actin filaments attach, dispersed throughout the cytoplasm and concentrated near the sarcolemma. Composed of α-actinin, vinculin, and talin.
Sarcolemma: Features caveolae (small invaginations), gap junctions for coordinated contraction, neuromuscular synapses, and a glycocalyx (carbohydrate-rich coating).

Innervation, Contraction, and Regulation

Innervation: Primarily controlled by the autonomic nervous system via unmyelinated nerve fibers partially covered by Schwann cells. Axons of these fibers exhibit varicosities and often release norepinephrine as a neurotransmitter.
Types of Innervation:
- Unit: One nerve fiber innervates each muscle cell.
- Multiple: One nerve fiber innervates multiple muscle cells; contraction order is transmitted through gap junctions.
Contraction Mechanism: Sliding filament mechanism.
Regulation: Influenced by both nervous and hormonal signals. Hormones like oxytocin, vasopressin, epinephrine, prostaglandins, and various peptides play regulatory roles.

Specialized Smooth Muscle Cells

Specialized smooth muscle cells include:

Myoepithelial cells in exocrine glands
Ruyter’s mioepitelioides in the juxtaglomerular apparatus
Myofibroblasts in the connective tissue and myoid of the seminiferous tubules
Cells branching from the middle layer of the aorta

Striated Muscle Tissue: Skeletal Muscle

Definition and General Characteristics

Striated skeletal muscle tissue forms the muscles responsible for voluntary movement, constituting 30-40% of body weight. It comprises muscle tissue and connective tissue components:

Endomysium: Surrounds individual muscle fibers.
Perimysium: Encases groups of muscle fibers.
Epimysium: Envelopes the entire muscle.

These connective tissue layers provide structural support and house blood vessels, nerves, and sensory receptors.

Types and General Features

Intrafusal: Found within muscle spindles (sensory organs).
Satellite: Mononucleated cells located beneath the basal lamina surrounding extrafusal fibers; involved in muscle regeneration.
Extrafusal: The primary muscle fibers responsible for contraction. They are oblong with rounded ends, arranged in parallel or pennate patterns, and multinucleated with nuclei at the periphery. Their cytoplasm is densely packed with myofibrils, giving them a striated appearance. These fibers range in size from 1mm to several centimeters long and 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): Invaginations of the sarcolemma that penetrate deep into the muscle fiber, rich in sodium ions (Na+). T-tubules are closely associated with the sarcoplasmic reticulum, forming triads with terminal cisternae.
Motor Neuromuscular Synapse: The junction between a motor neuron and a muscle fiber. The nerve fiber, an axon from an α-motor neuron in the spinal cord, loses its myelin sheath at the synapse and is enveloped by an expanded Schwann cell.

2. Sarcoplasm

Cores: Multiple nuclei with condensed chromatin, located at the periphery of the fiber.
Mitochondria: Abundant and large, distributed along the myofibrils.
Golgi Apparatus: Located near the nuclei.
Energy Sources: Glycogen and lipid droplets.
Myoglobin: An oxygen-binding protein similar to hemoglobin, giving muscle its red color.
ATP and Phosphocreatine: Support anaerobic metabolism.
Sarcoplasmic Reticulum: A network of interconnected tubules surrounding each myofibril, storing and releasing calcium ions (Ca2+) for muscle contraction.
Myofibrils: The contractile units of muscle fibers, composed of thick (myosin) and thin (actin) filaments arranged in a highly organized pattern. Myofibrils are divided into repeating units called sarcomeres, which are the basic functional units of muscle contraction.

Innervation

Skeletal muscle fibers are innervated by:

α-motor neurons: Initiate muscle contraction.
γ-motor neurons: Innervate intrafusal muscle fibers within muscle spindles.
Sensory fibers: Transmit information about muscle length and tension to the central nervous system.

Cardiac Striated Muscle Tissue

General Characteristics and Structure

Cardiac muscle tissue forms the myocardium, the middle layer of the heart wall. It shares similarities with both smooth and skeletal muscle but possesses unique characteristics:

Branched and Interconnected Cells: Cardiac muscle cells are branched and connected end-to-end by intercalated discs, forming a three-dimensional network that allows for synchronized contractions.
Limited Regeneration: Unlike skeletal muscle, cardiac muscle has limited regenerative capacity due to the absence of satellite cells.

Cell Types

Typical Myocardial Cells: These cells make up the bulk of the myocardium. They are rectangular, measure 15-20µm in diameter, and contain one or two centrally located nuclei. Myofibrils are less defined than in skeletal muscle, and numerous mitochondria, glycogen granules, and myoglobin are present. T-tubules form dyads with the sarcoplasmic reticulum at the Z lines.
Cardionector Cells: Specialized cells responsible for the heart’s inherent rhythmicity (automaticity). They form the conduction system of the heart, including the sinoatrial (SA) node, atrioventricular (AV) node, bundle of His, and Purkinje fibers. These cells are elongated, contain fewer myofibrils, and lack T-tubules.
Mioendocrine Cells: Located in the atria, these cells secrete hormones that regulate blood volume, electrolyte balance, and blood pressure.

Cardiac Muscle Contraction

Cardiac muscle contraction is characterized by:

Inherent Rhythmicity: The heart has an intrinsic ability to generate rhythmic contractions, initiated by the SA node (the pacemaker).
Synchronized Contractions: Intercalated discs allow for rapid electrical coupling between cells, ensuring coordinated contractions.
Autonomic Nervous System Modulation: While the heart can beat independently, the autonomic nervous system (ANS) can influence heart rate and contractility.

Muscle Repair

Smooth Muscle: Retains mitotic ability, allowing for regeneration after injury.
Skeletal Muscle: Limited regenerative capacity; satellite cells can differentiate into new muscle fibers.
Cardiac Muscle: Lacks significant regenerative capacity; damaged tissue is typically replaced by scar tissue.