Muscular Tissues PDF

# Chapter No. 5: Muscular Tissues ## Functions - Movement - Maintenance of posture - Joint stabilization - Heat generation Three types of muscle tissue in humans can be distinguished on the basis of morphologic and functional characteristics. ## Types of Muscle Tissue Muscle tissue is formed of...

# Chapter No. 5: Muscular Tissues ## Functions - Movement - Maintenance of posture - Joint stabilization - Heat generation Three types of muscle tissue in humans can be distinguished on the basis of morphologic and functional characteristics. ## Types of Muscle Tissue Muscle tissue is formed of elongated muscle cells known as **muscle fibers** that contain contractile proteins (myofilaments). There are three types of muscles: - Smooth - Striated or skeletal - Cardiac All types arise from **mesoderm**. ## Smooth Muscle ### Description and Location - This type is present in the wall of hollow viscera, blood vessels, respiratory passage and large ducts of glands. - Muscles are spindle-shaped cells. One oval central nucleus. The cytoplasm contains thin actin filaments, thick myosin filaments and intermediate filaments. No striations (no sarcomeres), contractions are involuntary (unvoluntary). Smooth muscle is responsible for the contractility of hollow organs, such as blood vessels, the gastrointestinal tract, the bladder, or the uterus. Its structure differs greatly from that of skeletal muscle. - Smooth muscle fibers are much smaller than skeletal muscle fibers. The muscle fibers making up the single-unit muscle are gathered into dense sheets or bands. Though the fibers run roughly parallel, they are densely and irregularly packed together, most often so that the narrower portion of one fiber lies against the wider portion of its neighbor. Contractions are slow, sustained and resistant to fatigue. ### Size - Length varies = 18 to 200 μ - Diameter = 3 to 8 μ ### Shape - Smooth muscle cells form sheets, and in cross section the cells appear rounded or polygonal in shape, some contain nuclei and some not. ### Functions of Smooth Muscle: 1- Regulate the size of the lumen of hollow viscera. 2- Produce peristaltic movements of digestive tract. 3- In the blood vessels it maintains a state of partial contraction called muscle tonus. 4- Synthesize of collagen and elastin. ## Skeletal Muscle ### Description, Functions and Location: - Skeletal muscle is composed of bundles of very long, cylindrical, multinucleated cells that show cross-striations. The fibers aggregate in the form of bundles. Attach to and move skeleton. 40% of body weight. Fibers are multinucleate cells. Cells with obvious striations. Contractions are voluntary. - Their contraction is quick, forceful and usually under voluntary control. It is caused by the interaction of thin actin filaments and thick myosin filaments whose molecular configuration allows them to slide upon one another. Fibers (each is one cell) have striations. - Myofibrils are organelles of the cell and made up of filaments. - Sarcomere is Basic unit of contraction. - Myofibrils are long rows of repeating sarcomeres. - Plasma membrane is called sarcolemma (Sarco=flesh and Lemma=sheath). ### Size - Length = 1 to 40 mm - Diameter = 10 to 100 μ - The whole muscle is surrounded by a dense collagen connective tissue called **epimysium**. Each bundle is surrounded by **perimysium**. Each fiber is surrounded by delicate connective tissue called **endomysium**. ### Note: T.S of muscles: - Several bundles of muscle fibers cut transversely. - C.T. around the muscle is called **epimysium.** - C.T. around the bundles is called **perimysium.** - C.T. around each fiber is called **endomysium.** ### Structure of Skeletal Muscle Fiber: #### LM Picture: - The fibers are cylindrical, the nuclei are oval and located peripherally beneath the sarcolemma (cell membrane). The sarcoplasm (cytoplasm) is acidophilic and is full of **myofibrils**. In transverse section the myofibrils occupy most of the fiber and appear as fine dots. In longitudinal section the cytoplasm show striations of alternating light and dark bands. #### EM Picture: - The sarcoplasm contains sarcoplasmic reticulum (endoplasmic reticulum) (SER), mitochondria, glycogen, lipid droplets and **myofibrils** which occupy most of the fiber. ### Myofibrils: - Myofibrils show cross-striations of alternating light and dark bands. The dark bands are called **A-bands**, and the light bands are called **I-bands**. In the center of each A-bands there is pale area called **H-zone** which is bisected by M-line. Each I-band is bisected by dark line called **Z-line**. ### Sarcomere: - It is the contractile unit of skeletal muscle fibers. Sarcomere is the region of myofibrils between two successive Z-lines. - Myofibrils are formed of smaller units called **myofilaments**. Two types of filaments are present: **thin (actin)** and **thick (myosin)** filaments. ## Cardiac Muscle ### Description, Functions and Location: - Cardiac muscles are striated involuntary muscle and found only in the wall of the heart. - Cells are striated. Cardiac muscle cells are single and branched. - Each fiber has single, large, oval central nucleus. - The fibers branch and anastomose to form network. - Fiber diameter = 9 to 22 μ. - Contractions are involuntary (not voluntary). - Unlike skeletal, cardiac muscle can contract without extrinsic nerve or hormonal stimulation. - Cells join together at **intercalated discs** that appear as dark lines. - 1-2 nuclei in center (uni or bi- nucleated). - **Inherent rhythmicity:** each cell (muscle cells beat separately without any stimulation). ### Conducting System of the Heart - Impulse conduction to different regions of the heart occurs through modified cardiac muscle cells. These fibers are located in the S.A node, A-V node and in the atrioventricular bundle (bundle of His) and are called **Purkinje fibers.** - Purkinje fibers: Are of larger diameter and in cross section they are irregular in outline, pale in staining and have few myofibr.