Musculoskeletal System Physiology PDF

SMU-210 Page 125 Faculty Of Medicine – Sohag University  SMU-210 1 Functional structure of skeletal muscles  The human body contains > 400 skeletal muscles (40 % of B. wt) while smooth & cardiac muscles are (10% of B. wt.)  Skeletal muscles...

SMU-210 Page 125 Faculty Of Medicine – Sohag University  SMU-210 1 Functional structure of skeletal muscles  The human body contains > 400 skeletal muscles (40 % of B. wt) while smooth & cardiac muscles are (10% of B. wt.)  Skeletal muscles are under voluntary control (somatic N.S.)  Skeletal muscle contraction depends on its nerve supply. Functions: Locomotion. Heat production. Maintain postuer & stability. Help venous return.  Each fiber contains a number of myofibrils each is formed of myofilaments. Muscle  fascicles  fibers  myofibrils  myofilaments Structure of the muscle fiber Myofibrils:  Thread-like structures that contain 2 types of interdigitating protein filaments. Thick filaments → the protein (myosin) Thin filaments → the protein (actin)  This arrangement gives the striated appearance of the skeletal muscles.  Myofibrils are divided into functional units (sarcomeres) by Z lines → attach all myofibrils together in a single muscle fiber. Page 126 Faculty Of Medicine – Sohag University  SMU-210 Structure of the myofibril Banding pattern (cross striations): by L/M A band (dark area) I band (light area) Lies: in the center of the sarcomere Lies: on either side of the Z line Contains: thick & thin filaments interdigitating Contains: thin filaments (actin) only (myosin) & (actin) Its center (H zone): cotains only thick (myosin) filaments only Cross bridges:  Projections from the thick (myosin) filaments towards the thin (actin) filaments.  Plays a fundamental role in muscle contraction. 1) Characteristics of the contractile filaments Myosin filament  The two heavy chains wrap spirally around each other to form the tail (body) of the myosin molecule and share in the formation of the head.  The four light chains present in myosin head (two to each head).  The head contains: 1. Actin binding site. 2. Catalytic site (for ATPase enzyme).  The junction between the head and the tail is called the arm. Page 127 Faculty Of Medicine – Sohag University  SMU-210  2 hangs (flexible points) are present: 1. One where the arm leaves the body of the myosin filament. 2. The other where the head attaches to the arm. Actin filament The actin filament is composed of three protein components: actin, Tropomyosin and troponin. 1. Actin:  The backbone of the actin filament is a double strand of F-actin protein molecule.  Attached to it several molecules of ADP. It is believed that these ADP molecules are the active sites on the actin filaments with which the cross bridges of the myosin filaments interact to cause muscle contraction. 2. Tropomyosin: These molecules are wrapped spirally around the sides of the F-actin. Function: In the resting state, the tropomyosin molecules cover the active sites of the actin strands, preventing attraction between the actin and myosin filaments. 3. Troponin:  Attached intermittently along the sides of the tropomyosin.  These are actually complexes of three loosely bound protein subunits: a) (Troponin I): inhibit the interaction between actin & myosin. b) (Troponin T): binds with tropomyosin. c) (Troponin C): for calcium ions that initiate the contraction process.  When the muscle fiber is contracted, the length of the sarcomere is about 2 μm. At this length, the actin filaments completely overlap the myosin filaments, and the tips of the actin filaments are just beginning to overlap one another. 2) The sarcotubular system “The more rapidly contracting is the muscle, the more the sarcoplasmic reticulum is extensive & developed”. The sarcotubular system is formed of a T-tubular system & the sarcoplasmic reticulum. Page 128 Faculty Of Medicine – Sohag University  SMU-210 The T-tubular system  It is an internal invagination of the cell membrane.  It is present in the junction between the light & dark bands in the skeletal muscles.  The space between the 2 layers of the membrane is an extension of the extracellular space. Function: conduct the action potential from the surface of the muscle to the muscle fibrils inside. The sarcoplasmic reticulum  Longitudinal tubules parallel to the muscle fibrils and surrounding them.  Its 2 ends are dilated and contain Ca+2 and called “terminal cisterns”. Function: Ca+2 storage & release. The arrangement of the T-tubules with the terminal cisterns on either side is called triad. The transverse (T) tubule The sarcoplasmic reticulum (SR) It is an invagination on the surface of It forms a network of membranous channels muscle membrane at the junction of A surround each myofibril & run parallel with & I bands it Conducts the action potential to the SR It has a high conc. Of Ca+2 (essential for muscle contraction Contains Extracellular fluid The ends of SR expand to form terminal The T tubule membrane contains a cisternae which contacts with the T tubules voltage sensitive (dihydropyridine = on either side of sarcomere by foot process DHP) receptor that opens the ryanodine The SR membrane contains a protein called Ca+2 release channels on the SR (ryanodine receptor) that contains the foot membrane process & a Ca+2 release channel Page 129 Faculty Of Medicine – Sohag University  SMU-210 The Motor End Plate 2 (Neuromuscular Junction)  There is no cytoplasmic continuity between the nerve and the muscle.  At the motor end plate, the neurilemmal sheath becomes continuous with the sarcolemma and the axon loses its myelin sheath.  The axon breaks into several branches each end in a dilatation called the synaptic knobs (sole foot).  The muscle membrane is invaginated to enclose the synaptic knob and it is called the synaptic gutter.  The space between the knob and the sarcolemma is called the synaptic cleft.  The muscle membrane at the bottom of the synaptic gutter is greatly folded.  The synaptic knob contains many vesicles filled with acetyl choline and many mitochondria.  At the synaptic cleft there is ECF & the enzyme cholinesterase. Page 130 Faculty Of Medicine – Sohag University  SMU-210 Properties of neuromuscular transmission 1) Unidirectional: from the nerve to the muscle not the reverse. 2) Delay: of about 0.5 msec. which is the time needed for release of acetyl choline until the generation of action potential. 3) Easily fatigued: when depletion of the transmitter takes place. 4) Blocked by curare. 5) Mg+2 prevents the release of the transmitter at the MEP  decrease the neuromuscular transmission. Page 131 Faculty Of Medicine – Sohag University