MCS - The Muscular System

Role of muscle tissue - Contraction and relaxation - Chemical energy changing into mechanical energy What are the three types of muscle tissue? - Skeletal - Cardiac - Smooth Skeletal muscle - A muscle that is attached to the bones, skin or fascia of the skeleton and provides the force that moves the bones. - Striated - the light and dark bands are visible under the microscope. - Voluntary control Cardiac muscle - Muscle of the heart -Striated - light and dark bands are visible under the microscope. - Involuntary control - Autorhythmic - built in pacemaker Smooth muscle - Hair follicles in skin - Found in walls of hollow organs such as; blood vessels and the GI - Non-striated - Involuntary control Functions of muscle tissue - Body movements - Stabilising position - Regulating organ volume: smooth muscle - sphincters - Movement of substances within body: blood, lymph, urine, air, food, and fluids. - Producing heat: involuntary contractions of skeletal muscle = shivering Properties of muscle tissue - Excitability - respond to neurotransmitters - Conductivity - propagate electrical signals over membrane - Contractility - shorten and generate force - Extensibility - can be stretched without 1/6 change - Elasticity - return to original shape after being stretched Skeletal muscle fibre (myofibre) - Long, cylindrical and multinucleated. - Sarcolemma - muscle cell membrane - Sarcoplasm filled with myofibrils and myoglobin (a red-coloured, oxygen-binding protein). Skeletal muscle cell structure Myofibrils are composed of actin (thin filaments), myosin (thick filaments) and support proteins. Myofilaments Myofibrils are composed of thick and thin myofilaments, which help give the muscle its striped appearance. The thick filaments are made of myosin, and the thin filaments are actin, along with two other muscle proteins, tropomyosin and tropinin. Proteins of muscle The proteins of muscle are: - Contractile proteins - Regulatory proteins - Structural proteins Contractile proteins Myosin and Actin Regulatory proteins They turn contractions off and on. -Troponin - Tropomyosin Structural proteins These provide proper alignment, elasticity and extensibility. - Titin - Myomesin - Nebulin - Dystrophin Myosin Myosin consists of an elongated tail region attached to a globular head via a flexible neck structure. Two myosin molecules twist around one 2/6 another to form a dimer. Myosin is a motor protein that functions to move material within the cell or cause a muscle contraction. Actin Actin molecules are comprised of two protein strands that twist to form a simple helix. The helix is surrounded by a protein called tropomyosin, that helps the actin filaments move past each other. Titin - Anchors thick filament to Z disc. - Between Z disc and end of thick filament can stretch to four times the resting strength and recoil. - Recovery of muscle from being stretched Myomesin - Found in the M line - Binds to titin and thick filaments to connect them together at the M line Nebulin An inelastic protein helps align the thin filaments. Dystrophin Links thin filaments to proteins of sarcolemma and transmits tension generated to the tendon. Sliding filament mechanism - Myosin cross bridges pull on thin filaments. - Thin filaments slide inwards. - Z discs come toward each other. - Sarcomeres shorten. - Muscle fibre shortens. - Muscle shortens. - Thick and thin filaments do not change length. Where do muscle contractions begin? Contractions begin at the neuromuscular junction. The physiological process of converting an electrical stimulus to a muscle con3/6 Excitation - contraction coupling The contraction cycle traction. It is the link between the action potential generated in the sarcolemma and the start of a muscle contraction. 1 - Myosin heads hydrolyse ATP and become reorientated and energised. 2 - Myosin heads bind to actin, forming cross bridges. 3 - Myosin heads rotate toward the centre of the sarcomere (power stroke). 4 - As myosin heads bind ATP, the cross bridges detach from actin. The contraction cycle continues if ATP is available and Ca2+ level in the sarcoplasm in high. Overview 1 - Nerve impulse arrives at axon terminal of motor neuron and triggers release of acetylcholine (ACh). 2 - ACh diffuses across synaptic cleft, binds to its receptors in the motor end plate, and triggers a muscle action potential (AP). 3 - Acetylcholinesterase in synaptic cleft destroys ACh, so another muscle action potential does not arise unless more ACH is release from motor neurone. - Muscle AP travelling along T tubule opens Ca2+ release channels in the sarcoplasmic reticulum (SR) membrane, which allows calcium ions to flood into the sarcoplasm. 5 - Ca2+ binds to troponin on the thin filament, exposing the binding sites for myosin. 6 - Contraction: power strokes use ATP; myosin heads bind to actin, swivel, and release; thin filaments are pulled toward centre of sarcomere. 4/6 7 - Ca2+ release channels in SR close and Ca2+ active transport pumps use ATP to restore low level of calcium ions in sarcoplasm. 8 - Troponin-tropomyosin complex slides back into position where it blocks the myosin binding sites on actin. 9 - Muscle relaxes. Relaxation - Acetylcholinesterase breaks down ACh in synaptic gap. - Muscle action potential stops. - Ca2+ release channels close. - Active transport pumps Ca2+ back into sarcoplasmic reticulum. - Tropomyosin-troponin complex recovers binding site on actin. Myasthenia Gravis - A progressive autoimmune disease. - Damages ACh receptors at neuromuscular junction. - More common in women 20 to 40 years old. - Has a possible link to thymus tumours. Symptoms of Myasthenia Gravis - Double vision - Droopy eyelids - Swallowing difficulties - Progresses to paralysis of respiratory muscles Treatment of Myasthenia Gravis Steroids that reduce antibodies that bind to ACh receptors and inhibitors of acetylcholinesterase Muscular dystrophies - Inherited, muscle-destroying diseases. - Sarcolemma tears during muscle contraction. - Mutated gene on X chromosome. - Males almost exclusively - Appears by age 5. - By age 12, may not be able to walk. 5/6 - Degeneration of individual muscle fibres produces atrophy of skeletal muscle. - Most common form is Duchenne muscular dystrophy. 6/6