Histology of Skeletal Muscles II PDF

Histology of Skeletal Muscles II By: Dr. Moustafa Al Sawy Dr. Shaimaa Mohamed Amer MBBCH, M.SC. M.D HISTOLOGY MBBCH, M.SC. M.D HISTOLOGY Associate Professor of Histology & Cell Bio...

Histology of Skeletal Muscles II By: Dr. Moustafa Al Sawy Dr. Shaimaa Mohamed Amer MBBCH, M.SC. M.D HISTOLOGY MBBCH, M.SC. M.D HISTOLOGY Associate Professor of Histology & Cell Biology Associate Professor of Histology & Cell Biology Certified Medical Educator Learning objectives Knowledge: Learning Objectives At the end of the lecture , each student will be able to: 1.Discuss ultra-structural organization of myofibrils and muscle contraction & relaxation. 2. Explain the mechanism of muscle contraction. 3. Describe the structure of neuromuscular junction.  Major Proteins of the Muscle (Actin) Actin The major protein component of the thin filament is actin , a globular protein.Under physiological conditions, G actin polymerizes to form double helical filament called F actin. Attached to the F actin helix is tropomyosin  Major Proteins of the Muscle (Myosin) Myosin The thick filament is composed of myosin, consists of 6 polypeptide chains : 2 heavy (H) chains. 4 light (L) chains. The 6 polypeptide chains together form → a structure that has a tail and 2 globular heads. The tail consists of the carboxy terminal portions of the 2 heavy chains wound around each other.  Major Proteins of the Muscle (Myosin) Each globular head contains the amino terminal portions of a heavy chain and 2 light chains and connected to the tail by the short neck. The globular head contains ATP-binding site, ATPase,& actin binding site. The globular heads of the myosin molecule correspond to the cross bridges that project outward from the thick filaments.  Major Proteins of the Muscle (Dystrophin) It is a suspensory protein attached to inner surface of plasma membrane of normal muscle cell. Functions: 1. It maintains the structural integrity of plasma membrane and elements within such as ion channels. 2. It links the actin cytoskeleton to extracellular matrix ECM and is needed for assembly of synaptic junction.  Medical Application Research on Duchenne muscular dystrophy revealed that mutations of the dystrophin gene can lead to defective link ages between the cytoskeleton and the extracellular matrix (ECM). Muscle contractions can disrupt these weak linkages, causing the atrophy of muscle fibers typical of this disease.  Muscular Contraction (Sliding filament theory of Huxely): *During contraction muscle fiber becomes shorter & broader.(change in size), with less distinct light & dark bands. *Shortening of the myofibrils is accompanied by sliding of filaments on one another. *Thin filaments slide to penetrate more deeply into the A band. *A band remains constant whereas the I band is short in a contracted muscle, appears narrow and H-zone is short types of filaments thick and thin during contraction due to overlap of both types of filaments thick and thin during contraction. *In stretched muscle is I band prominent & A band remains constant. Skeletal Muscle Fiber Types Motor nerve endings (Myoneural junctions, motor end plate, neuromuscular junction) They are the effectors which initiate contraction of skeletal muscle. They are the axons of anterior horn cells of spinal cord. Shape & Structure: *Each nerve fiber ends into 100-150 branches called axon terminals and each supplies one muscle fiber. *As the nerve fiber comes near the muscle fiber it dilates.  Myoneural junctions *The myelin sheath is lost and the neurolemma continues as a roof over the terminals. The expanded end of naked axon is rich in microvesicles or synaptic vesicles (containing acetylcholine) & mitochondria. *It pushes the sarcolemma, forming a depression (groove) on surface of the muscle fiber. *Axon ends epilemmaly, i.e. it does not penetrate muscle fiber. *The space between the nerve ending and sarcolemma is called synaptic cleft. *The sarcolemma, at this area, is folded (junctional folds). *The underlying sarcoplasm is rich in nuclei and mitochondria and is known as the sole plate.  Myoneural junctions Function: *A nerve impulse causes depolarization of the axon terminals, with the release of acetylcholine into synaptic cleft, causing depolarization of the sarcolemma. *The created action potential travels to the T- tubule system and initiates contraction.  Medical Application Myasthenia gravis is an autoimmune disorder that involves circulating antibodies against proteins of acetylcholine receptors. Antibody binding to the antigenic sites interferes with acetylcholine activation of their receptors, leading to intermittent periods of skeletal muscle weakness. As the body attempts to correct the condition, junctional folds of sarcolemma with affected receptors are internalized, digested by lysosomes, and replaced by newly formed receptors. These receptors, however, are again made unresponsive to acetylcholine by similar antibodies, and the disease follows a progressive course. The extraocular muscles of the eyes are commonly the first affected.  Case Scenario A 5-year-old boy sustains a small tear in his gastrocnemius muscle when he is involved in a bicycle accident. Regeneration of the muscle will occur through which of the following mechanisms? A.Dedifferentiation of muscle cells into myoblasts B.Differentiation of muscle satellite cells C.Fusion of damaged myofibers to form new myotubes D.Hyperplasia of existing muscle fibers E.Differentiation of fibroblasts to form myoblasts Any Questions? References: 1.Basic Histology: Text & Atlas. Editor: Luiz Carlos Junqueira, MD, PhD; Jose Carneiro, MD, PhD. 14th Ed. 2. Wheatear’s functional histology: A text & color atlas.15th Ed. 3. AMBOSS platform. Thank You