Skeletal Muscles PDF

DEPARTMENT OF HUMAN PHYSIOLOGY AND PATHOPHYSIOLOGY SCHOOL OF MEDICINE UWM OLSZTYN  An individual skeletal muscle cell contains a densely arranged parallel array of cylindrical elements called myofibrils. Myofibrils occupy about 80% of the intracellural voulme. Myofibrils are dire...

DEPARTMENT OF HUMAN PHYSIOLOGY AND PATHOPHYSIOLOGY SCHOOL OF MEDICINE UWM OLSZTYN  An individual skeletal muscle cell contains a densely arranged parallel array of cylindrical elements called myofibrils. Myofibrils occupy about 80% of the intracellural voulme. Myofibrils are directly responsible for muscle contraction. Each myofibril is essentially an end-to-end chain that consist of smaller interdigitating filaments called myofilaments, which contain both thin filaments (actin filaments) and thick filaments (myosin filaments). Under the light microscope, a longitudinal section of a skeletal muscle fibre shows cross-striation which comprises alternating dark and light bands. The arrangement of thick and thin filaments in a myofibril creates a repeating pattern of alternating: light bands (I bands) – which represent a region occupied by thin filaments dark bands (A bands) – which represent the region matching the entire length of thick filaments. The Z disks (or Z lines) are visible as a dark perpendicular lines at the center of the I band. They divide myofibrils to a smaller functional units called sarcomeres. THE SARCOMERE is defined as repeating unit between adjacent Z disks. THICK FILAMENTS are bipolar assemblies of a multiple myosin molecules.  THE THIN FILAMENT Consists of the aggregation of actin molecules (termed globular actin or G-actin) into a two-stranded helical filament called F-actin, or filamentous actin, and two regulatory, actin binding proteins: tropomyosin and troponin. - G-actin - tropomyosin - troponin - Troponin T – binds to a single molecule of tropomyosin - Troponin I – binds to actin and inhibits contraction - Troponin C binds Ca2+  SEQUENCE OF EVENTS WHICH LEAD TO MUSCLE CONTRACTION Events at neuromuscular junction convert an acetylcholine signal from a somatic motor neuron into an electrical signal in the muscular fiber. Action potentials propagate from the sarcolemma to the interior of muscle fibers along the transverse tubule network. Depolarization of the T-tubule membrane results in Ca2+ release from the sarcoplasmic reticulum at the triad.  The cross-bridge cycling mechanism just described is called the sliding filament theory because the myosin cross-bridge is pulling the actin thin filament toward the center of the sarcomere, thereby resulting in an apparent ‘sliding’ of the thin filament past the thick filament. The force of muscle contraction can be regulated by: changing the frequency of muscle cell stimulation the initial length of the muscle fiber varying the number of motor units excited within a muscle. The force of muscle contraction can be regulated by: 1. Changing the frequency of muscle cell stimulation.  In isolated muscle fiber a stimulation by a single action potential evokes a contraction called a SINGLE MUSCLE TWITCH. Action potential 100% [Ca2+] Muscle twitch 50% 0% 0 5 10 15 20 ms Depolarization and refraction phases of an action potential end when the contraction phase of a muscle twitch begins.  If multiple action potentials occur close enough in time, the multiple twitches can summate and thus greatly increase the tension developed. Because this type of tension enhancement depends on the frequency of muscle stimulation, it is referred to as FREQENCY SUMMATION. Single muscle twitches (5 Hz) Frequency summation(20 Hz)  UNFUSED (INCOMPLETE) TETANUS – occurs when the stimulation rate of the muscle fiber is not at a maximum value, and consequently the fiber relaxes slightly between stimuli. The peak of the unfused tetanus graph is bumpy, because it still contains components of single twitches. Unfused tetanus (80 Hz)  FUSED TETANUS – occurs when the stimulation rate is fast enough that the muscle fiber does not have time to relax. Instead it reaches maximum tension and remains there. The peak of the fused tetanus graph resembles a straight line. The maximal tension developed in a single muscle fiber is directly proportional to the rate at which action potentials occur in the fiber. Fused tetanus(100 Hz) The force of muscle contraction can be regulated by: 2. The initial length of a muscle fiber. TENSION DEVELOPED BY INDIVIDUAL MUSCLE FIBERS IS A FUNCTION OF FIBER LENGTH Muscle length influences tension development by determining the degree of overlap between actin and myosin filaments. The force of muscle contraction can be regulated by: 3. Varying the number of motor units excited within a muscle.  The whole assembly of muscle fibers innervated by the axon from one motor neuron is called a MOTOR UNIT. The amplitude of skeletal muscle contraction force depends, to a large extent, of the number of contracting myofibers, therefore of the number of motor units excited within a muscle. In whole skeletal muscle, the force developed may be increased by increasing the number of contracting motor units, and thus, summing the contractions of multiple fibers. This effect is known as MULTIPLE-FIBER SUMMATION or SPATIAL SUMMATION.  TYPES OF THE MOTOR UNITS I II small motor neurons large motor neurons high excitability of motor low excitability of motor neurons (they fire at low neurons (they fire at high thresholds) thresholds) fast conduction in nerve very fast conduction in fibres nerve fibres small number of the large number of the myofibres innervated by each myofibres innervated by each motor neuron motor neuron mainly red / oxidative mainly white / glycolytic myofibers myofibres  LENGTH – TENSION RELATIONSHIP IN SKELETAL MUSCLE If a stimulation causes an If a stimulation causes shortening increase in tension, but no of a muscle, while a tension shortening, the contraction is developed by muscle is constant referred to as ISOMETRIC the contraction is referred to as CONTRACTION. ISOTONIC CONTRACTION. muscle relaxes muscle contracts muscle relaxes muscle contracts  In human body pure isotonic contractions occur never, and pure isometric contractions occur rarely.  The most common type of skeletal muscle contraction in our body is a two –phase AUXOTONIC CONTRACTION.  Muscle fatigue is a temporary decline in ability of a muscle to generate force, caused by previous activity of this muscle.  FEATURES OF MUSCLE FATIGUE the latent period is longer the amplitude of muscle contraction is lower the time of muscle relaxation is longer the excitability of muscle is lower the maximal force of contraction is lower the energy stores are depleted the amplitude of muscle contraction time  CAUSES OF MUSCLE FATIGUE Depletion of energy stores. Accumulation of lactic acid in the myoplasm. The central nervous system also contributes to fatigue, especially the manner in which fatigue is perceived by individuals. First the nervous centers are fatigued then the end plate is fatigued (we can stimulate directly the sarcolemma of a muscle fiber the last thing that is fatigued is the muscle fiber itself. Photo Credit Photodisc/Photodisc/Getty Images