11. Muscle Tissue -vid

Overview of Muscle Tissue

• Proficiency Expectations:

  • Describe the three types of muscle tissue and their characteristics.

  • Describe organization of muscle tissue.

  • Explain structure of muscle fibers.

Types of Muscle Tissue

1. Skeletal Muscle

• Forms large muscle groups and is responsible for voluntary and gross body movements.

• Made up of long cells called fibers (up to 30 cm length), also known as myocytes with multiple nuclei (up to 200).

• Contains striations due to thick (myosin) and thin (actin) filaments.

• Control: Voluntary; requires nervous system input for movement.

• Structure:

  • Composed of muscle fibers, blood vessels, and nerves.

  • Surrounded by fascia (dense connective tissue) for movement freedom and pathway for vessels/nerves.

  • Bundles of fibers known as fascicles, with individual fibers surrounded by endomysium (thin connective tissue).

2. Cardiac Muscle

• Found exclusively in the heart.

• Striations are apparent but less obvious than in skeletal muscle.

• Muscle fibers are branched and typically contain one nucleus.

• Special Features: Intercalated disks facilitate strength and communication via dense desmosomes and gap junctions.

• Control: Involuntary; contractions are regulated by specialized pacemaker cells.

• Classification: Striated involuntary muscle.

3. Smooth Muscle

• Located lining walls of visceral organs (e.g., stomach, intestines), around blood vessels, and in the reproductive system.

• Lacks striations due to different organization of actin and myosin.

• Control: Involuntary; contraction can be initiated by nerves, hormones, or pacemaker cells.

• Muscle fibers are smaller, usually single-nucleated with tapered ends.

• Important for moving contents through the digestive tract and regulating blood flow.

• Classified as non-striated involuntary muscle.

Shared Properties of Muscle Tissue

• Contractility: Proteins draw together, allowing movement without necessarily shortening the muscle.

• Excitability: Muscle cells respond to stimuli and produce electrical signals.

• Extensibility: Ability to stretch up to three times resting length.

• Elasticity: Capacity to return to original shape post-contraction or extension.

Structure of Muscle Fibers

• Each fiber has a unique cell membrane called the sarcolemma; the cytoplasm is referred to as sarcoplasm.

• Rich in mitochondria and glycogen for ATP production during muscle contraction.

• Transverse Tubules: Narrow tubes that are open to the extracellular environment, helping transmit electrical signals.

• Myofibrils: Bundles of myofilaments extending the length of the muscle, responsible for contraction.

• Arrangement yields striations (in skeletal and cardiac muscles).

• Sarcomeres: The functional unit of striated muscle, consisting of organized actin and myosin.

• Sarcoplasmic Reticulum: Stores calcium ions crucial for muscle contraction.

Summary

• Three types of muscle tissue: skeletal, cardiac, and smooth, each with distinct structures, functions, and control mechanisms.

• Understanding muscle structure is essential for studying muscle physiology and function within the muscular system.

Overview of Muscle Tissue

Proficiency Expectations:

• Describe the three types of muscle tissue and their distinct characteristics.

• Describe the organization and anatomical arrangement of muscle tissue.

• Explain the microscopic and macroscopic structure of muscle fibers and their functional significance.

Types of Muscle Tissue

1. Skeletal Muscle

• Function: forms large muscle groups primarily responsible for voluntary and intricate gross body movements, such as walking, running, and lifting objects.

• Histology: made up of long, cylindrical cells called fibers (myocytes) which can reach lengths of up to 30 cm. These fibers are multinucleated, containing up to 200 nuclei per fiber for enhanced protein synthesis and repair.

• Striations: visible striations are due to the organized arrangement of thick (myosin) and thin (actin) filaments within the sarcomere unit.

• Control: voluntary control; muscle contractions are initiated by signals from the nervous system. This entails precise coordination with motor neurons.

• Structure:

  • Composed of muscle fibers bundled together, surrounded by connective tissue layers.

  • Fascia (a dense connective tissue) surrounds the entire muscle, providing support and allowing for smooth movement alongside adjacent structures.

  • Fascicles are bundles of fibers, with each individual fiber encased in endomysium, a thin layer of connective tissue that provides elasticity and support.

2. Cardiac Muscle

• Location: found exclusively within the walls of the heart, playing a critical role in maintaining blood circulation.

• Histological Characteristics: while striations are present, they are less prominent than those in skeletal muscle. The fibers are branched and typically contain a single nucleus, which differs from the multinucleated structure of skeletal muscle.

• Special Features: intercalated disks are unique to cardiac muscle, facilitating strong mechanical connections and electrical communication between neighboring cells via dense desmosomes and gap junctions. This structural adaptation ensures synchronized contraction of the heart.

• Control: involuntary control; the rhythm of contraction is regulated intrinsically by specialized cells (pacemaker cells) that generate electrical impulses.

• Classification: categorized as striated involuntary muscle due to its combination of striated appearance and involuntary control.

3. Smooth Muscle

• Location: lines the walls of visceral organs (e.g., stomach, intestines, bladder), blood vessels, and is involved in various involuntary functions such as peristalsis and regulating blood flow.

• Characteristics: lacks striations due to the less organized arrangement of actin and myosin filaments, which exist as bundles rather than in regular sarcomeres.

• Control: also involuntary; contraction can be induced by various stimuli, including neuronal input, hormonal influences, or localized pacemaker cells.

• Structure: muscle fibers are smaller in diameter, typically single-nucleated, and have tapered ends, which provide a greater range of motion and contraction.

• Functionality: smooth muscle is crucial for the involuntary movement of substances through the digestive system and for controlling the expansion and contraction of blood vessels.

• Classification: recognized as non-striated involuntary muscle due to its structural features and control mechanisms.

Shared Properties of Muscle Tissue

• Contractility: the ability to shorten and generate force; this occurs as proteins inside muscle fibers draw together, enabling movement without necessarily shortening the overall muscle length.

• Excitability: muscle cells have the unique capability to respond to various stimuli (chemical, electrical, or mechanical) and produce action potentials.

• Extensibility: the ability of muscle tissue to stretch without damage, allowing for movement and flexibility during different activities, capable of stretching up to three times its resting length.

• Elasticity: the inherent ability to return to its original shape following contraction or extension, crucial for sustaining muscle function during repeated activities.

Structure of Muscle Fibers

• Each muscle fiber is surrounded by a specialized cell membrane known as the sarcolemma, which encases the cytoplasm, referred to as sarcoplasm.

• Muscle fibers are rich in mitochondria and glycogen deposits, imperative for ATP production during the energy-consuming process of muscle contraction.

• Transverse Tubules (T-tubules): these narrow tubes extend from the sarcolemma into the interior of the muscle fiber, facilitating the rapid transmission of action potentials to ensure synchronous contraction.

• Myofibrils: these structures are composed of bundled myofilaments (actin and myosin) that run the length of the muscle fiber, and are assembled into functional units called sarcomeres which are responsible for the contractile properties of the muscle.

• Sarcoplasmic Reticulum (SR): a specialized form of the endoplasmic reticulum, it serves as a reservoir for calcium ions which are vital for muscle contraction and relaxation.

Summary

The three distinct types of muscle tissue—skeletal, cardiac, and smooth—exhibit unique structures, functions, and control mechanisms. A comprehensive understanding of muscle structure and function is essential for the study of muscle physiology and its applications within the broader context of the muscular system.