L15 - Muscular System PDF

L15- MUSCULAR SYSTEM Dr.Pugazhandhi Bakthavatchalam Assistant Professor of Anatomy and Physiology, AUACAS, American University of Antigua LEARNING OUTCOMES At the end of this session the student would be able to: Structure, function and inner action of cardiac and smooth muscle. Dif...

L15- MUSCULAR SYSTEM Dr.Pugazhandhi Bakthavatchalam Assistant Professor of Anatomy and Physiology, AUACAS, American University of Antigua LEARNING OUTCOMES At the end of this session the student would be able to: Structure, function and inner action of cardiac and smooth muscle. Differentiate between skeletal, smooth and cardiac muscle 8-Sep-24 2 CARDIAC MUSCLE: CONTRACTILE MECHANISM OF CARDIAC MUSCLE CARDIAC MUSCLE- Heart is composed of three major types of cardiac muscle 1. Atrial Muscle 2. Ventricular Muscle 3. Specialize Excitatory & Conductive Muscle o The atrial and ventricular muscle contract in much the same way as skeletal muscle, except duration of contraction is much longer. oThe specialized excitatory and conductive fibers contract weakly because they contain few contractile fibrils  Cardiac muscle fibers are striated in appearance Functional unit is called Sarcomere Fibers are branched; connect to one another at intercalated discs. The discs contain several Gap Junctions Nuclei are centrally located Abundant Mitochondria Sarcoplasmic Reticulum is less abundant than in skeletal muscle, but greater in density than smooth muscle Sarcolemma has specialized ion channels that skeletal muscle does not – voltage-gated Ca2+ channels Fibers are not anchored at ends; allows for greater sarcomere shortening and lengthening CARDIAC MUSCLE The dark areas crossing the cardiac muscle fibers are called intercalated discs; they are actually cell membranes that separate individual cardiac muscle cells from one another.  CARDIAC MUSCLE At intercalated disc cell membranes fuse with one another, form permeable “communicating” junctions (gap junctions) allow free diffusion of ions. Ions move with ease in the intracellular fluid along the longitudinal axes of the cardiac muscle fibers, so that action potentials travel easily from one cardiac muscle cell to the next Thus, cardiac muscle is a syncytium of many heart muscle cells, action potential spreads to all of them.  isometric contraction: Muscle contraction without significant shortening or change in distance isotonic contraction: Muscle contraction without significant change in the force of contraction  Interconnected by intercalated discs and form functional syncytia Within intercalated discs – two kinds of membrane junctions Desmosomes Gap junctions CARDIAC MUSCLE  CARDIAC MUSCLE Heart beats rhythmically as result of action potential, it generates by itself (Auto-rhythmicity) Two specialized types of cardiac muscle cells Contractile cells 99% of cardiac muscle cells Do mechanical work of pumping Autorhythmic cells Do not contract Specialized for initiating and conducting action potentials responsible for contraction of working cells  CARDIAC MUSCLE PROPERTIES The cardiac muscle cells are responsible for the electrical stimulation leads to mechanical function. The electro-physiologic properties of cardiac muscles are: Automaticity: Ability to spontaneously generate an electrical impulse. Excitability: Ability to respond to an electrical impulse. Conductivity: Allows transmission of electrical impulse to another cardiac cell. Contractility: Ability to contract after electrical impulse response. Rhythmicity: Ability to send electrical impulses in a regularly manner. Excitation of the heart is triggered by electrical impulse rather than neural transmitters. Contraction of the heart is triggered by elevation of intracellular calcium influx.  Autonomic nervous system modulates the frequency of depolarization of pacemaker Sympathetic stimulation (neurotransmitter); binds to b1 receptors on the SA nodal membranes Parasympathetic stimulation (neurotransmitter); binds to muscarinic receptors on nodal membranes; increases conductivity of K+ and decreases conductivity of Ca2+ SMOOTH MUSCLE Smooth muscle is considered to be much more primitive than either cardiac or skeletal muscle. Muscle striations are not visible in smooth muscle, so the sarcomere relationship of myosin to actin does not exists in smooth muscle. However, per cross sectional area smooth muscle is as strong as skeletal muscle and smooth muscle is highly resistant to fatigue.  Smooth Muscle 1. Fibers are smaller than skeletal muscle 2. Involuntary (Usually) 3. No apparent myofibrils under the light microscope - No cross striations 4. Fibers are thickest in the middle and have tapered ends 5. Centrally located single oval nucleus 6. Sarcoplasm contains thick and thin filaments a. Not in an orderly pattern b. 10 – 20 times more thin filaments than thick filaments 7. Dense bodies a. Have thin filaments attached to them b. Function is similar to Z disks c. Dispersed throughout the sarcoplasm or attached to the sarcolemma d. Thin filaments stretch from one dense body to another  Generation of Contraction 1. Sliding filament mechanism involving thick and thin filaments generates tension that is transmitted to the thin filaments 2. Does not contain Troponin complex 3. Actin and myosin pull on the dense bodies attached to the sarcolemma 4. Shortening of the smooth muscle fiber is lengthwise, the middle of the fiber thickens 5. Shortening causes bubble like expansion of the sarcolemma 6. Shortening is corkscrew like - the fiber twists in a helix as it shortens and rotates in the opposite direction as it lengthens TYPES OF SMOOTH MUSCLES Single Unit - Large aggregates of smooth muscle cells which act as a single unit. - Large aggregates of smooth muscle cells which act as a single unit. - These usually line the hollow organs such as blood vessels or the gastrointestinal tract like Stomach and intestines, uterus, urinary bladder Multi Unit - Single smooth muscle cells usually with a single nerve connection. - Single smooth muscle cells usually with a single nerve connection - Examples - Arrector pili muscles, Vas deferens and iris of the eye.  Smooth Muscle Contractile Mechanism a. Starts slower and lasts longer than striated muscle fiber, smooth muscle has a prolonged contraction - up to hours to days b. Can shorten and stretch to a greater extent than striated muscle c. Contraction is initiated by calcium influx into the sarcoplasm (Outside Calcium) D. Sarcoplasmic reticulum in smooth muscle is sparse - 3 to 5 % of cell volume E. Calcium flows into sarcoplasm from extracellular fluid F. No T-tubules in smooth muscle - Therefore, calcium movement is slow G. Smooth muscle tone - occurs due to the slow movement of calcium from the cell H. Smooth muscle has less ATPase activity and therefore, less degradation of ATP  SMOOTH MUSCLE STIMULATION Smooth muscle responds to stimulation from a number of different physiological systems. 1. Nerves 2. Hormones- Norepinephrine, epinephrine, Ach, angiotensin II, oxytocin, vasopressin, serotonin and histamine 3. Mechanical manipulation 4. Self stimulation (Automaticity)