Summary
Highlights
Muscular tissue performs several key functions. It produces body movement by working with bones and joints, stabilizes body position and posture (essential for activities like yoga), and stores and moves substances using structures like sphincters and peristaltic contractions. Additionally, muscles are crucial for thermogenesis, generating heat to maintain body temperature, with 40-50% of body weight coming from muscle.
Muscular tissue exhibits several properties: excitability, meaning it can start an action potential (electrical impulses) in response to triggers like electrical or chemical signals, leading to muscle contraction. Muscles are also contractile, capable of becoming tense and generating tension, and extensible, able to stretch within limits defined by connective tissues. Finally, muscles possess elasticity, allowing them to return to their original shape and size after contraction or extension.
There are three types of muscular tissue: skeletal, cardiac, and smooth (visceral). Skeletal muscles are located near the skeleton, cardiac muscles in the heart, and smooth muscles in the gastrointestinal tract, uterus, blood vessels, and eyes. Skeletal and cardiac muscles are striated, appearing banded under a microscope, while smooth muscle is not. Skeletal muscles are multinucleated and under voluntary control, whereas cardiac and smooth muscles typically have one central nucleus and are involuntary, regulated by the nervous system.
A muscle fiber is synonymous with a muscle cell, characterized by its long, cylindrical shape. Muscle fibers are grouped into bundles called fascicles. Connective tissues surround these structures: endomysium around individual muscle fibers, perimysium around fascicles, and epimysium around the entire muscle. These connective tissues converge to form tendons (cord-like, like the Achilles) or aponeuroses (sheet-like, flat, like the epicranial aponeurosis), which attach muscles to bones. Muscle cells originate from myoblasts, which fuse to form multinucleated muscle fibers. After fusion, these cells lose the ability to divide, meaning muscle growth (hypertrophy) involves increasing cell size by adding proteins and organelles, rather than increasing cell number.
A muscle fiber contains a plasma membrane called sarcolemma, which invaginates to form T-tubules. The cytoplasm, known as sarcoplasm, contains proteins like myoglobin for oxygen binding, and glycogen granules for energy storage. The sarcoplasmic reticulum (SR) is a network of tubules similar to smooth ER, storing large amounts of calcium ions. Terminal cisternae, located on either side of a T-tubule, along with the T-tubule, form a triad. Inside the muscle fiber are myofibrils, which are columns of proteins. The basic functional unit of a myofibril is the sarcomere, extending from one Z-disc to another.
The sarcomere, the basic unit of muscle contraction, is defined by the distance between two Z-discs. Key regions include the M-line in the middle and the H-zone. Darker regions are called A-bands, and lighter regions outside the A-band are I-bands. Sarcomeres are composed of protein filaments: thin filaments (actin) and thick filaments (myosin). Structural proteins like Titin help secure these filaments, connecting Z-discs.
Muscle proteins are categorized into contractile, regulatory, and structural. Contractile proteins are actin (thin filament) and myosin (thick filament). Actin features myosin binding sites. Myosin, with its head and tail, has ATPase activity, hydrolyzing ATP into ADP and phosphate. Regulatory proteins, tropomyosin and troponin, control muscle contraction. Tropomyosin normally blocks myosin binding sites on actin. When calcium is present, it binds to troponin, causing tropomyosin to shift and expose the binding sites, allowing myosin to interact with actin. Structural proteins like Titin provide structural support, connecting Z-discs and keeping filaments in place. Dystrophin is another crucial structural protein; its absence can lead to muscular dystrophy and muscle disintegration.
Muscle movement occurs via the sliding filament mechanism, where thin and thick filaments slide past each other rather than folding. During contraction, the I-bands shorten, while the A-bands remain the same length. This is achieved by myosin heads grabbing onto actin filaments and pulling them towards the center of the sarcomere. The contraction cycle involves four steps: 1) ATP hydrolysis, where ATP binds to the myosin head and is hydrolyzed, energizing the myosin head. 2) Crossbridge formation, where the energized myosin head binds to actin. 3) Power stroke, where ADP is released, and the myosin head rotates, pulling the actin filament. 4) Detachment, where a new ATP molecule binds to the myosin head, causing it to detach from actin. This cycle continues as long as ATP and calcium ions are available, converting chemical energy into mechanical movement.