SF2024_Muscle_Tissue_Updated.pdf

Muscle is a tissue that is specialized for contraction in order to move the body or individual components of the body. Classification based on: appearance of cells (striated or nonstriated), and whether controlled by somatic or autonomic nervous system (voluntary or involuntary). Skeletal (s...

Muscle is a tissue that is specialized for contraction in order to move the body or individual components of the body. Classification based on: appearance of cells (striated or nonstriated), and whether controlled by somatic or autonomic nervous system (voluntary or involuntary). Skeletal (striated-voluntary) Cardiac (striated-involuntary) Smooth (nonstriated-involuntary). Nomenclature: Sarcolemma - Cell membrane. Sarcoplasm - Cytoplasm. Sarcoplasmic reticulum - Smooth endoplasmic reticulum (sequester Ca2+ ions). Sarcomere - Functional unit within a myofiber. Muscle is a major tissue of the body and is controlled by the somatic nervous system. Forms the muscles of the musculoskeletal system and some muscles that do not move bones (diaphragm, extraocular muscles, muscles of facial expression). Motor unit: A lower motor neuron (motor neuron in a cranial nerve nucleus, or a motor neuron in the ventral horn of the spinal cord) and the skeletal muscle fibers it innervates. Each skeletal muscle has many motor units and the ratio of neuron to muscle fibers varies from 1/1, to 1/100. The nerve supply to a skeletal muscle is essential for contraction and muscle tone. *Denervation results in flaccid paralysis, areflexia, atonia, and atrophy of a muscle. What kind of lesion do the above signs indicate? Gross Structure: skeletal muscle cells are also called muscle fibers, they are specialized for rapid, forceful, voluntary contraction. Connective tissue: Endomysium - Delicate connective tissue around individual fibers. Perimysium - Connective tissue that surround a group of fibers called a fascicle. Epimysium - Dense irregular connective tissue that surrounds the entire muscle. *The epimysium, perimysium, and endomysium are continuous with the tendons and/or aponeuroses of the muscle. Blood vessels and nerves use these connective tissue sheaths to reach the muscle. Histologic structure: Each skeletal muscle fiber (myofiber) is a multinucleated cell (a.k.a. a syncytium): Elongated cell (length is from 1mm to 35cm, diameter is from 10-100um). Nuclei are peripherally located. Contain myofibrils composed of actin and myosin filaments (myofilaments). Periodic invaginations of the sarcolemma are called transverse tubules (T- tubules). Along with the normal organelles the sarcoplasm contains sarcoplasmic reticulum (SER). The SER forms a system of membrane bound tubules that extend throughout the cytoplasm and expands into terminal cisterna next to a T- tubule. The two terminal cisternae and T-tubule arrangement is called a triad and is located at the junction of the A and I bands. *Action potential spreads across sarcolemma to T-tubules and into SER (which releases Ca2+ ions ). Myofibril structure: Composed of thick myosin and thin actin myofilaments. Observation with the microscope reveals a banding pattern consisting of: Alternating light bands (I bands) and dark bands (A bands). Fine dark lines called Z bands (disks or lines) in the middle of the I band. The I band is composed of actin filaments and the Z line marks their attachment. The region between two Z disks is a sarcomere which is the contractile (functional) unit in skeletal muscle. The A band contains thick myosin filaments and a portion of the actin filaments on each side. The center of an A band is devoid of actin filaments and thus forms an H band. At the center of the H band is a M line which is the attachment point of the myosin filaments. Muscle contraction: The length of thick and thin filaments does not change. The length of the sarcomere decreases because the filaments slide past each other (the size of the H band and I band decreases). The image in the upper left hand corner is a longitudinal section of skeletal muscle under light microscope, can you identify the A and I bands and the peripherally located nuclei of the muscle cells? The image in the bottom left hand corner is a cross section of skeletal muscle in which you can identify the endomysium and perhaps the perimysium. It is easier here to differentiate the nuclei of muscle cells located peripherally within the cell versus the nuclei of satellite cells which are outside the bounds of the muscle fiber. You can also make out some blood vessels, an artery here and a vein here. In the electron microscope image on the top right corner, you can make out the entire sarcomere, including I bands, A bands, the H band, the M line and Z disks. Fiber type: Red fibers: Small fibers with large amounts of myoglobin (oxygen storage molecule and accounts for red color) and many mitochondria (highly aerobic cells that burn oxygen to produce ATP). They are slower and more sustained in contracting (slow twitch motor units which are more resistant to fatigue). White fibers: Large fibers with less myoglobin, fewer mitochondria and high levels of glycogen (highly anaerobic cells whose ATP comes from metabolism of glycogen into glucose). Exhibit rapid sporadic and intense contraction (fast twitch motor units that fatigue rapidly). Intermediate fibers: Exhibit intermediate characteristics between red and white fibers. Skeletal muscles have a rich blood supply that is usually derived from at least a couple of arteries. Innervation: Motor innervation: Derived from lower motor neurons in the brainstem (motor nuclei of cranial nerves), and lower motor neurons in the spinal cord (α-motor neurons or ventral horn cells). Sensory innervation: Derived from pseudounipolar neurons in dorsal root ganglia (and some cranial nerve ganglia). Their peripheral process connects to muscle spindles and Golgi tendon organs. *The sensory and motor fibers to a muscle course in one nerve. Repair- Satellite cells within skeletal muscle proliferate after injury, form myoblasts which fuse to form myotubes that mature into skeletal muscle fibers. Exercise- Increases the size of the muscle fibers (hypertrophy), not the number, by increasing the number of myofilaments and myofibrils within a myofiber or by the fusion of satellite cells with an existing myofiber. *Atrophy of a skeletal muscle is usually due to a pathological condition or disuse. Hyperplasia is an increase in the number of muscle fibers and is usually an abnormal condition. Neuromuscular junction: Motor axons branch at muscle cell surface to form presynaptic buttons (covered by Schwann cells, which have been removed in the image on the right). Buttons are separated from the sarcolemma (green in the image on the right) by synaptic cleft. Myasthenia gravis is an autoimmune disease in which antibodies are produced against acetylcholine receptors. Autoantibodies bind to the receptor, preventing the binding of acetylcholine. This blocks normal nerve-muscle interaction and results in progressive muscle weakness. Curare binds to the acetylcholine receptor and prevents binding of acetylcholine. Curare derivatives are used in surgical procedures in which muscle paralysis is necessary. Botulinum toxin, an exotoxin from the bacterium Clostridium botulinum, prevents the release of acetylcholine at the presynaptic end. Muscle paralysis and dysfunction of the autonomous nervous system occur in cases of food poisoning mediated by botulinum toxin. (Kierszenbaum, 3rd ed, pgs 211-212) Cardiac muscle is located in the heart and at the base of large vessels entering and leaving the heart. Epicardium: Covers the outer surface of the heart and is composed of a mesothelium (visceral layer of serous pericardium) and underlying connective tissue containing the blood vessels and nerves to the heart. Myocardium: Cardiac muscle held together by connective tissue. The inner portion of the myocardium contains specialized cardiac muscle fibers and Purkinje fibers that are part of the impulse conduction system of the heart. Endocardium: Covers the inner surface of the heart and consists of an endothelium and subendothelial connective tissue. Histologic structure of cardiac muscle cells (fibers): Cylindrical cells (10-20um in diameter and 100-150um in length). Single centrally located nucleus. Fibers have the same arrangement of contractile proteins as skeletal muscle and are striated. In contrast to skeletal muscle the T-tubules are larger, the sarcoplasmic reticulum is not as extensive, there are diads located at the Z line, and there are more mitochondria than in skeletal muscle. Intercalated discs: Located between the ends of adjacent cardiac muscle fibers (highlighted in green in the image on the bottom). Function: Provide points of anchorage for the myofibrils. Provide for attachment to neighboring cells. Permit extremely rapid spread of contractile stimulus from one cell to another. Blood Supply - Provided by the coronary arteries and their branches. Innervation - By both of the portions of the autonomic nervous system (sympathetic and parasympathetic). Repair: Destroyed cardiac muscle cells are not replaced by new cardiac muscle cells but rather by fibrous connective tissue. Myocardial infarction is a common cause of cardiac cell death. A single large infarct or repeated small infarcts can be fatal. Spindle-shaped cells ranging from (20um to.5mm in length) Single, central, elongated nucleus which appears staggered in longitudinal section and may be observed in cross section. *Identify three diagnostic features that help you distinguish smooth muscle from cardiac muscle. Commonly observed in sheets or bundles in the walls of the gut, bile duct, ureters, urinary bladder, respiratory tract, uterus, and blood vessels. What do the above locations for smooth muscle tell you about its innervation? Contractile proteins are actin, myosin, and intermediate filaments. The contractile proteins criss-cross the cell and attach to dense bodies in the cell membrane and within the cytoplasm. Tension generated by contraction is transmitted through the dense bodies to other cells allowing a group of smooth muscle cells to function as a unit (like a Z disk in striated muscle). Specialized for continuous contractions of low force. Smooth muscle fibers are non-striated. Can you identify smooth muscle in both longitudinal and cross section? Blood supply Derived from small arteries in the surrounding connective tissue. Innervation Most smooth muscle is innervated by both the parasympathetic and sympathetic portions of the autonomic nervous system; however some smooth muscle is only innervated by one portion; and some smooth muscle contracts in response to hormones (e.g. Intravenous oxytocin stimulates uterine contraction during labor). Repair Some smooth muscle responds to injury by undergoing mitosis; and in other smooth muscle, new cells are derived from undifferentiated cells in blood vessels, endothelial cells, and pericytes. The uterus has regularly replicating populations of smooth muscle cells.