Muscular Tissue

Muscular tissue is the basic tissue characterized by the ability to contract upon stimulation. Muscular tissues are vascularized tissues chiefly composed of elongated cells that are excitable and contractile, and usually arranged in parallel. In the body, there are three types of muscular tissue: skeletal muscle, smooth muscle, and cardiac muscle.

Other Names

Synonyms

• Muscle tissue

Related Terms

• Muscle
• Muscular system

Types

• Striated muscular tissues
  • Skeletal muscle tissue
  • Cardiac muscle tissue
• Smooth muscle tissue

Description

Muscular tissue is largely composed of muscle cells. Muscle cells are elongated and surrounded by external lamina, which is similar to basal lamina of epithelial tissues. Muscle cells contain a contractile apparatus composed of actin (thin) and myosin (thick) filaments, and associated proteins. In striated muscle cells, the contractile apparatus is organized into myofibrils, which are oriented in the same direction as the long axis of the muscle cell. The regular repeating segments (sacromeres) of myofibrils give skeletal and cardiac muscle cells transverse striations. In smooth muscle cells, the contractile apparatus, actin and myosin filaments form contractile fibers, which do not appear as highly organized as myofibrils.

Skeletal Muscle

Skeletal muscle cells, also known as skeletal muscle fibers, are very long, multinucleated syncytial cells that were formed during development by fusion of myoblast cells. Relative to other muscle cells, skeletal muscle cells are long and wide.

In cross section, skeletal muscle cells are polygonal in shape, and their nuclei are located peripherally, adjacent to the plasma membrane (sarcolemma).

Cardiac Muscle

Cardiac muscle fibers are composed of branching and anastomosing chains of cardiac muscle cells. Cardiac muscle cells within a fiber are joined to their neighbors by intercalated discs, which contain anchoring and gap junctions. The anchoring junctions (adherens junctions and desmosomes) physically connect the cytoskeletons and contractile apparatuses of the neighboring cells. The gap junctions electrically couple the cells.

In cross section, cardiac muscle cells are rounded in shape, have a single central nucleus, and are intermediate in size between skeletal and smooth muscle cells.

Smooth Muscle

Smooth muscle is composed of sheets or bundles of relatively short, spindle-shaped cells, in a staggered array. Smooth muscle cells are not striated, and have a single central nucleus. In some smooth muscle, the cells are interconnected by gap junctions.

In cross section, smooth muscle cells are circular. Diameters of cross-sectional profiles differ; the largest profiles display a central nucleus.

Role of Muscular Tissue in the Body

The special role of muscular tissues is contraction, an ability the body puts to multiple uses.

• Skeletal muscle makes up the muscles of the muscular system. As part of the musculoskeletal system, skeletal muscle is involved in body posture and movement. Skeletal muscle is also found in the extra-ocular muscles, and muscles of the auditory ossicles, tongue, soft palate and fauces, pharynx, larynx, pelvic diaphragm, and perineum.
• Smooth muscle in the walls of hollow visceral organs, ducts, arteries, and veins controls the movement of contents in the lumen. Some bundles of smooth muscle form sphincters. Smooth muscle is also found in arrector pili muscles of the skin, and in intrinsic muscles of the eye.
• Cardiac muscle in the walls of the atria and ventricles of the heart pump blood through the cardiovascular system

How It Works

Muscle tissue contracts following excitation. Excitation of muscle cells causes an increase in calcium ion concentration in the cytosol. Calcium ions bind to proteins that regulate the interaction of actin and myosin filaments, triggering contraction. Muscle tissue types differ in the details of the excitation and initiation of actin-myosin interactions.

Skeletal muscle

Muscles of the skeletal system are generally considered voluntary muscles, because they can be subject to conscious control. Muscle contraction may also be subconscious, such as reflex movements. Muscles are innervated by cranial or spinal nerves.

Skeletal muscle fibers form neuromuscular junctions with motor neurons, whose cell bodies are located in the spinal cord or brainstem. A motor unit consists of a motor neuron and the fibers that it innervates.

Neurotransmission at the neuromuscular junction causes depolarization of the sarcolemma and transverse tubules. Depolarization releases calcium ions from the sacroplasmic reticulum into the cytosol, where it binds troponin C, allowing interaction of actin and myosin filaments.

Cardiac muscle

The contraction of cardiac muscle is involuntary, strong, and rhythmical. Cardiac muscle cells have an intrinsic pacemaker mechanism. Cardiac muscle cells with the highest pacemaker rate determine the rate of contraction of all cardiac muscle fibers to which they are connected. The rate and force of contraction can also be modified by hormones and the autonomic nervous system.

Cardiac muscle cells are excited by depolarization through the fiber spread by gap junctions. Depolarization leads to increased calcium in the cytosol from the extracellular space, as well as sarcoplasmic reticulum. Actin-myosin interactions are triggered by binding of calcium by troponin C, as in skeletal muscle.

Smooth muscle

Smooth muscle contraction is involuntary. Physiologically, smooth muscle is often described as being either multi-unit or unitary. In multi-unit smooth muscle, such as the muscles in the iris, the cells are not interconnected by gap junctions. These cells are individually controlled by the autonomic nervous system.

In unitary smooth muscle, the cells are interconnected by gap junctions. Contraction of unitary smooth muscle, for example, in the walls of the intestines, is often described as slow and rhythmic. The rate and force of contraction are modulated by the autonomic nervous system and hormones.

Excitation of smooth muscle cells, either by autonomic nerve fibers or through gap junctions, causes extracellular calcium ions to enter the cytosol. Calmodulin binds calcium ions and activates myosin light-chain kinase, which phosphorylates a myosin light chain, unmasking myosin's actin-binding site.

History

In Anatomie générale (1801), Bichat's set of 21 tissus included two muscular tissues:

• Tissu musculaire de la vie animale (voluntary muscle)
• Tissu musculaire de la vie organique (smooth and cardiac muscle)[[Category:|Category:]]