Muscle Tissue PDF - University of Bradford

Systems, Physiology, and Anatomy CLS4011-U Muscle tissue Dr Pip Garner [email protected] Learning outcomes To know the properties of all muscle muscle types Be able to identify the different muscle types histologically To know the different characterist...

Systems, Physiology, and Anatomy CLS4011-U Muscle tissue Dr Pip Garner [email protected] Learning outcomes To know the properties of all muscle muscle types Be able to identify the different muscle types histologically To know the different characteristics For skeletal muscle: Know the stages of development Know the hierarchical structure Understand the relationships between the plasma membrane, transverse tubules, and the sarcoplasmic reticulum Understand the mechanism whereby shortening of muscle occurs during contraction Check your understanding-Clinical case studies (self- directed) Properties of muscle Muscle tissue has 4 special properties that enable it to function and contribute to homeostasis: 1) Electrical excitability – Either as autorhythmic electrical signals arising from the muscle tissue itself, or as chemical stimuli, such as neurotransmitters, pH changes or hormones. 2) Contractility 3) Extensibility – ability to stretch 4) Elasticity – return to its original length Types of muscle Muscle cells are specialised contractile cells. Three types of muscle are described based on: Voluntary or involuntary contractions Are they striated? Location Types of muscle 1. Skeletal muscle (striated) - voluntary muscle. Involved in moving and stabilising bones and other structures. Development of skeletal muscle Myoblasts align and fuse together to form myotubes. Myotubes synthesize the proteins to make myofilaments Part of the myoblast population does not fuse and remains mesenchymal Myotubes cells called satellite cells. Satellite cells proliferate myofilaments and produce new muscle fibres following muscle injury. Skeletal Muscle Structure Skeletal muscles are comprised of multiple bundles These bundles are called fascicles which are made up of muscle fibres Muscle fibres are composed of myofibrils Myofibrils contain the contractile elements, the myofilaments known as actin (thin filament) and myosin (thick filament).  Skeletal Muscle Structure Endomysium around single muscle fibre Perimysium around a fascicle (bundle of fibres) Epimysium covers the entire skeletal muscle Epimysium blends into a connective tissue attachment Neurological Impulse in Skeletal Muscle Somatic motor nerves branch from various levels of the spinal cord and control the contraction of skeletal muscle. The major communication site between the nerve and the muscle is the neuromuscular junction. Neuromuscular Junction 1. Action potential travels down motor neuron 2. Causes release of neurotransmitter acetylcholine (1) 3. Acetylcholine binds to receptors on muscle (1) 4. Depolarisation spreads across the muscle cell (2) 5. Depolarisation triggers release of internal calcium stores (3) 6. If sufficient ATP- and Ca2+-is present - muscle contraction (5)  Muscle fibres The entire muscle fibre contracts simultaneously Signal to contract is distributed evenly by T tubules. T tubules are tubes that extend from the sarcolemma into the sarcoplasm of the muscle fibre, and then around myofibrils.  Sarcoplasmic reticulum The sarcoplasmic reticulum (SR) forms a tubular network around each myofibril, either side of each T-tubule. The SR enlarges and fuse forming large chambers, called the terminal cisternae. The T-tubule and a pair of terminal cisternae is called a triad.  Sarcoplasmic reticulum Calcium is stored in the sarcoplasmic reticulum. The action potential is conducted along the T-tubule system, which allows for the release of calcium from the sarcoplasmic reticulum into the sarcoplasm. Muscle contraction 1. Contraction of a skeletal muscle fibre begins with depolarisation of the muscle fiber membrane 2. This activates dihydropyridine (DHP) voltage sensors of the transverse tubules. 3. Activation of the DHP voltage sensor leads to activation of the ryanodine receptor 4. This results in the release of calcium from the terminal cisterna 5. This release of calcium increases the calcium concentration in the sarcoplasm. 6. Contraction of the muscle fiber follows. 7. Removal of sarcoplasmic calcium terminates skeletal muscle contraction. Muscle contraction  Removal of what results in termination of muscle contraction? Sarcoplasmic calcium Sliding filament theory The accepted theory of how fibres contract is the sliding filament theory. Myosin filaments use ATP to walk along the actin filaments using cross bridges. This pulls the actin filaments closer together, bringing the z lines closer together, shortening the sarcomere. Contraction of muscle Each muscle fibre contains hundreds of myofibrils. Each myofibril is made up of actin and myosin filaments. Actin filaments are anchored to Z lines. The region between two z lines is called a sarcomere. Contraction of muscle The physical lengths of the actin and myosin filaments do not change during contraction. Therefore, the A band, which is composed of myosin filaments, does not change either. The distance between Z disks decreases, but the Z disks themselves do not change. The I band decreases in length as the muscle contracts. Contraction of muscle Only the I band and the H zone decreases in length as the muscle contracts. Contraction of muscle Sliding filament theory What band decrease in length during contraction? Histology of Skeletal muscle Large multinucleated cells. Fused myoblasts surrounded by loose connective tissue containing collagen and elastin fibres, which merge into the tendon at sites of muscle attachment. Types of muscle 2. Cardiac muscle (striated) - involuntary muscle that forms most of the walls of the heart. Cardiac muscle Never fatigues Cryogenic) Found in heart (cardiocytes) Single centrally located nucleus Have a limited ability to divide so repair following injury is limited. Regulated intrinsically by a pacemaker, an impulse conducting system composed of specialised cardiac muscle fibres. Cardiac muscle Striated (consists of a single branched cell). Connected by intercalated discs (thickenings of sarcolemma) Within intercalated discs are desmosomes (hold cells together) and gap junctions. Gap junctions provide connections that enable electrical signals to travel from cell to cell. Types of muscle 3. Smooth muscle (not striated)- involuntary muscle that forms part of the walls of most vessels and hollow organs. Moving substances through them by pulsations or peristaltic contractions. Smooth muscle Named for the absence of striations Fatigues slowly Cardiovascular system (blood vessels) Digestive system Urinary system Smooth muscle Non striated, involuntary muscle No sarcomeres Actin and myosin filaments scattered throughout the cell. Coordination of contraction is limited to the local area (via gap junctions) and usually under the control of autonomic nervous system. Smooth muscle Classified as multi-unit or unitary depending on whether the cells are electrically coupled Unitary smooth muscle has gap junctions between cells which allow for the fast spread of electrical activity Multi-unit have little or no coupling between cells. A combination of unitary and multi-unit is found in vascular smooth muscle Smooth muscle: excitation- contraction In smooth muscle there is no troponin The interaction of actin and myosin is controlled by the binding of calcium to another protein, calmodulin. Calmodulin regulates cross-bridge cycling Types of muscle Skeletal muscle Cardiac muscle Smooth muscle Voluntary movement Involuntary movement Involuntary movement Striated in appearance Striated in appearance Lack striations Multiple nuclei Single nuclei Single nuclei Can become fatigued Do not fatigue Do not fatigue Clinical cases Lambert-Eaton Myasthenic Syndrome (LEMS) LEMS is a rare condition that affects the signals sent from the nerves to the muscles The muscles are unable to contract properly, resulting in muscle weakness Lambert-Eaton Myasthenic Syndrome (LEMS) Antibodies attack the voltage- gated calcium channels on the presynaptic membrane Without proper function of these channels, insufficient amounts of acetylcholine are released into the neuromuscular junction Myasthenia Gravis Myasthenia gravis (MG) is an autoimmune disease that occurs in 1 in 10,000 people. It occurs primarily in women between 20 and 40 years of age. Myasthenia Gravis Eye and facial muscles are often attacked first, producing double vision and drooping eyelid. These clinical features are r often followed by dysphagia, limb weakness and decreased stamina. Can result in fatalities from paralysis of the respiratory muscles. Myasthenia Gravis Antibodies attack the acetylcholine receptors on the postsynaptic muscle fiber membrane. The abnormally clustered Ach receptors are removed from the muscle fibre sarcolemma by endocytosis, reducing the number of receptors in the sarcolemma Damage to the acetylcholine channels results in small endplate potentials that do not reach a threshold value required for generation of an action potential in the muscle fiber. This causes decreased muscle stimulation This decreased stimulation results in fatigue and weakness.  LEMS = Presynaptic disease MG = Postsynaptic disease Recommended reading Guyton and Hall textbook of Medical Physiology