Muscle tissue is excitable, responding to stimulation and contracting to provide movement, and occurs as three major types: skeletal (voluntary) muscle, smooth muscle, and cardiac muscle in the heart. Muscle tissue is composed of cells that have the special ability to shorten or contract in order to produce movement of the body parts. The tissue is highly cellular and is well supplied with blood vessels. The cells are long and slender so they are sometimes called muscle fibers, and these are usually arranged in bundles or layers that are surrounded by connective tissue. Actin and myosin are contractile proteins in muscle tissue.
Types of muscle tissue
Muscle tissue can be categorized into:
- skeletal muscle tissue;
- smooth muscle tissue;
- cardiac muscle tissue.
Muscle tissue can be classified functionally, voluntary or involuntary and morphologically striated or non-striated. Voluntary refers to whether the muscle is under conscious control, striation refers to the presence of visible banding within myocytes which occurs due to organization of myofibrils to produce a constant direction of tension.
By applying the above classifications it is possible to describe three forms of muscle tissue which perform the wide range of functions described.
The histological unit of skeletal muscle tissue is the muscle fiber, a cylindrical-shaped syncytium, multinucleated, striated, rounded at the ends and under voluntary control. The skeletal striated muscle fiber is delimited by the plasma membrane (sarcolemma); inside it presents the cytoplasm (sarcoplasm), numerous nuclei, organelles, myofibrils (the structures specialized in contraction). The fiber can reach a length of several centimeters. The nuclei (from a few to thousands) are arranged in the periphery of the syncytium, close to the sarcolemma; in some cases (turbid fibers, muscle fibers of Mammals during embryonic development, muscle fibers of many non-Mammalian Vertebrates), they are centrally located.
Skeletal striated muscle tissue, with its associated connective component, forms skeletal, or voluntary, muscles because their contraction occurs under the control of the central nervous system. Each muscle fiber has a specialized surface structure (motor plate) that represents the point of connection with a motor neuron. The connective component consists of the endomysium (which covers individual muscle fibers), the perimysium (which covers bundles of fibers), the epimysium (which covers the entire muscle). Numerous blood vessels are present in the connective tissue. Each skeletal striated muscle fiber has a nerve ending (neuromuscular junction). The term “striated” muscle fiber derives from the fact that, even in optical microscopy preparations, it is possible to observe a typical transverse banding, due to the banding of myofibrils, in which light and dark disks alternate regularly, and to their perfectly parallel arrangement and in register.
Skeletal muscle mainly attaches to the skeletal system via tendons to maintain posture and control movement for example contraction of the biceps muscle, attached to the scapula and radius, will raise the forearm. Some skeletal muscle can attach directly to other muscles or the skin, as seen in the face where numerous muscles control facial expression.
Skeletal muscle is under voluntary control, although this can be subconscious for example when maintaining posture or balance. Morphologically skeletal myocytes are elongated and tubular and appear striated with multiple peripheral nuclei.
Smooth muscle tissue
Smooth muscle fibrocells are tapered-shaped cells. They have a single centrally located nucleus in the widest portion of the cell, and lack striations. They are called involuntary muscles.
No myofibrils are present in the cytoplasm; therefore, no banding is observed. The fibrocells are arranged with the swollen portion of one cell close to the thinner portion of the cell next to it (embryo-like); they can be organized to form bundles, immersed in more or less abundant connective tissue, or be associated to form thick laminae, in which the cells are close to each other.
Smooth muscle tissue is found associated with numerous other organs and tissue systems such as the digestive system or respiratory system. It plays an important role in the regulation of flow in such tissues for example aiding the movement of food through the digestive system via peristalsis.
Smooth muscle is non-striated, although it contains the same myofilaments they are just organized differently, and involuntary. Smooth muscle myocytes are spindle shaped with a single centrally located nucleus.
Cardiac muscle tissue
Cardiac muscle has branching fibers, one nucleus per cell, striations, and intercalated disks. Its contraction is not under voluntary control. It consists of cylindrical contractile cells (cardiomyocytes), usually branched at the ends; between the cells is connective tissue, in which the vessels are located. The cells are connected at their ends (not along the lateral surfaces) by structures called scalar striae or intercalary discs. The nucleus (one, at most two) is centrally located.
In the cytoplasm are myofibrils parallel to each other, separated by a fair amount of sarcoplasm, which is more abundant than in skeletal striated muscle fibers. Myofibrils exhibit characteristic transverse banding; transverse banding in cardiac cells is less obvious than in skeletal striated muscle fibers because myofibrils are not perfectly in register with each other; instead, longitudinal banding is more evident because myofibrils are separated by more sarcoplasm.
Cardiac muscle tissue is found only in the heart where cardiac contractions pump blood throughout the body and maintain blood pressure.
As with skeletal muscle cardiac muscle is striated, however it is not consciously controlled and so is involuntary. Cardiac muscle can be further differentiated from skeletal muscle by the presence of intercalated discs which control the synchronized contraction of cardiac tissues. Cardiac myocytes are shorter than skeletal equivalents and contain only one or two centrally located nuclei.