by: Joe Northcott and Joshua leware Pd.1 Muscular systems by: Joe Northcott and Joshua leware Pd.1

Vertebrate skeletal muscle Characterized by a hierarchy of smaller and smaller units Most skeletal muscles consist of a bundle of long fibers running parallel to the length of the muscle Each fiber is a single cell with multiple nuclei, reflecting its formation by the fusion of many embryonic cells

Muscle makeup A muscle fiber contains a bundle of smaller myofibrils arranged longitudinally The myofibrils, in turn, are composed of thin filaments and thick filaments

continued Thin filaments consist of two strands of actin and two strands of a regulatory protein coiled around one another Thick filaments are staggered arrays of myosin molecules

Striated muscle Skeletal muscle is also called striated muscle because the regular arrangement of the filaments creates a pattern of light and dark bands Each repeating unit is a sarcomere, the basic contractile unit of the muscle

Sliding-filament model Neither the thin or thick filaments change in length when the sarcomere shortens; rather, the filaments slide past each other longitudinally, increasing overlap The sliding of the filaments is based on the interactions between the myosin and actin molecules that make up the thick and thin filaments. The myosin head of the thick filaments undergoes a chemical process in which it forms a bridge with the actin molecules of the thin filaments, pulling it past. This process repeats in a manner not unlike that of pulling oneself along a rope

contractions Aerobic respiration Glycolysis (lactic acid fermentation)

The role of calcium and regulatory proteins Calcium ions and proteins bound to actin play a critical role in muscle cell contraction and relaxation Tropomyosin, a regulatory protein , and the troponin complex, a set of additional regulatory proteins, are bound to the actin strands of thin filaments

Calcium’s role When ca2+ accumulates in the cytosol, it binds to the troponin complex, causing the proteins bound along the actin strands to shift position and expose the myosin-binding sites on the thin filament. Thus when the calcium ions accumulate in the cytosol contraction occurs in the muscle

Calcium control Muscle contraction is caused by motor neurons triggering the release of calcium ions This regulation of calcium ion concentration is a multistep process involving a network of membranes and compartments within the muscle cell

Diseases Several diseases cause paralysis by interfering with the excitation of skeletal muscle fibers by motor neurons Amyotrophic lateral sclerosis (als) Also known as lou Gehrig’s disease Degeneration of motor neurons in the spinal cord and brainstem Als is progressive and usually fatal within 5 years after symptoms appear incurable

Nervous control of muscle tension Contraction of a whole muscle , such as the biceps in ones upper arm, is graded You can voluntarily alter the extent and strength of its contraction There are 2 basic mechanisms by which the nervous system produces graded contractions of whole muscles 1. varying the number of muscle fibers that contract 2. varying the rate at which muscle fibers are stimulated

Mechanism 1 A motor neuron and all the muscle fibers it controls constitute a motor unit The force developed by a muscle progressively increases as more and more of the motor neurons controlling the muscle are activated, a process called recruitment of motor neurons

Prolonged contraction Some muscles, especially those that hold up the body and maintain posture, are always partially contracted In such muscles, the nervous system may alternate activation among the motor units, reducing the length of time any one set of fibers is contracted This is to prevent fatigue

Mechanism 2 The second mechanism by which the nervous system produces graded whole- muscle contractions is by varying the rate of muscle fiber stimulation A single action potential produces a twitch lasting about 100 msec or less If a 2nd action potential arrives before the muscle fiber has completely relaxed, the two twitches add together, resulting in greater tension

Tetanus (not the disease) When the rate of muscle stimulation is high enough that the muscle fiber cannot relax at all between stimuli, the twitches fuse into one smooth, sustained contraction called tetanus

Types of skeletal muscle fibers Oxidative and glycolytic fibers Fast-twitch and slow-twitch fibers

Oxidative fibers Rely mostly on aerobic respiration Specialized to make use of a steady fuel supply: --includes many mitochondria, a rich blood supply, and a large amount of an oxygen-storing protein called myoglobin Myoglobin binds oxygen more tightly than hemoglobin so it can effectively extract oxygen from the blood

Glycolytic fibers Uses glycolysis as primary source of atp Larger diameter and less myoglobin than oxidative fibers Fatigue more readily

Fast and slow twitch fibers Fast-twitch fibers Develop tension 2-3 times faster than slow twitch-fibers Fast-twitch fibers are Used for brief, rapid, powerful contractions Slow-fibers, often found in muscles that maintain posture and can sustain long contractions a slow-twitch fiber muscle twitch lasts about 5 times as long as a fast-twitch fiber, this is due to calcium ions hanging around in the cytosol longer

Other types of muscle Cardiac muscle Smooth muscle

Cardiac muscle Found only in the heart and striated A long refractory period prevents summation and tetanus Do not require input from nervous system Action potentials last 20x longer than those of skeletal muscle fibers Plasma membranes of adjacent cardiac muscle cells interlock at specialized regions called intercalated disks, where gap junctions provide direct electrical coupling between the cells

Smooth muscle Found mainly in the walls of hollow organs(blood vessels and organs of the digestive tract) Lack striations due to irregularly arrayed actin and myosin filaments Contract and relax more slowly than striated muscle mechanism for calcium ion regulation is different than that of cardiac and skeletal muscle Some smooth muscle cells contract only when stimulated by autonomic nervous system