Muscle Tissue (General Overview)
Muscle tissue is one of the four basic tissue types in the body, alongside epithelia, connective, and nervous tissues. Its primary function is contractility, enabling movement within organ systems, of blood, and of the body as a whole. Muscle cells, also known as myocytes, are generally elongated and originate from mesodermal cells during embryonic development.
There are three distinct types of muscle tissue based on their structure and function:
- Skeletal (striated) muscle: Characterised by very long, multinucleated cells with cross-striations, allowing quick, forceful, voluntary contractions.
- Cardiac muscle: Also has cross-striations but consists of often branched cells connected by intercalated discs, exhibiting vigorous, rhythmic, involuntary contractions.
- Smooth (visceral) muscle: Composed of fusiform (spindle-shaped) cells that lack striations and undergo slow, involuntary contractions.
Muscle cell cytoplasm is often called sarcoplasm, its smooth endoplasmic reticulum (ER) is the sarcoplasmic reticulum, and the cell membrane is the sarcolemma.
Smooth Muscle Tissue
Smooth muscle is specialised for slow, steady, involuntary contraction. It lacks the distinct striations seen in skeletal and cardiac muscle.
- Location:
- Found in the walls of visceral (hollow) organs, such as the gastrointestinal tract, urinary tract, and birth canal.
- Present in the walls of all blood vessels larger than capillaries.
- Also in the uterus (myometrium) and bladder.
- Cellular Characteristics:
- Cells are elongated, tapering (fusiform), and typically small.
- Each cell has a single, long, centrally located nucleus.
- They are enclosed by a thin basal lamina and a fine network of reticular fibers (endomysium).
- Smooth muscle cells are connected by numerous gap junctions, allowing electrical communication and coordinated contraction.
- They contain actin (thin) and myosin (thick) filaments.
- Unlike striated muscle, these filaments are not organised into myofibrils or sarcomeres, hence the lack of striations.
- Dense bodies, which contain α-actinin, are scattered throughout the sarcoplasm and attached to the cell membrane; these serve as attachment points for thin and intermediate filaments, functionally similar to Z discs in striated muscle.
- An elaborate array of intermediate filaments (primarily desmin) also inserts into these dense bodies.
- The sarcoplasmic reticulum is rudimentary, and there is no transverse tubule (T-tubule) system.
- Caveolae, small membrane invaginations, are frequent on the cell surface and may play a role in organising proteins for calcium release.
- Troponin is absent in smooth muscle; instead, proteins like myosin light-chain kinase (MLCK) and calmodulin regulate the sliding filaments and contraction.
Contraction:
- Controlled by involuntary mechanisms, including autonomic nerves, hormones, and local physiological conditions (e.g., stretch).