Muscles MUSCLE TYPES o Smooth muscle o Cardiac muscle o Skeletal muscle Three major muscle types classified by function location in the body and subcellular structure Skeletal muscle and cardiac mucle are sometimes classified together as striated muscles o Both have striation stripes that are visible through microscopy Smooth and cardiac muscles are sometimes classified toegether as involuntary muslces o Contraction and relaxation of both does not require conscious thought SKELETAL MUSCLE Responsible for body movements and posture Work in combination with skeleton o Both referred to as musculoskeletal system Connect to bones and other tissues via connective tissue tendons at each end of the muscle SKELETAL MUSCLES ARE CONNECTED TO OTHER TISSUES AT ORIGINS INSERTIONS AND ARE TYPICALLY ARRANGED AS ANTAGONISTIC PAIRS The larger end of a skeletal muscle attaches to the bone at the ORIGIN The opposite end of the skeletal muscle attaches to another bone or other tissue at the INSERTION The origin and insertion of a skeletal muscle are often on two bones connected by a joint Skeletal muscles often arrange din antagonistic pairs ex Biceps and triceps to enable active motion in both directions Not always joints o Rib muscles intercostal muscles have origins and insertions on rib bones ad contact but do not flex a joint o The diaphragm has origins on many bones during inhalation one side of the diaphragm contracts against the opposite side via insertions on a common central tendon Example of unique arrangement o The set of muscles that move the eye up down left right have origins on either bone of the skull or common connective tissue behind the eye and have insertions directly on the eye SUBSTRUCTURE OF SKELETAL MUSCLE Skeletal muscle is typically long Some cells may extend the entire length of the muscle Skeletal muscles are called fibers because of their structure size o Within each fiber are myofibrils long small diameter structures that take up much of the space in a fiber o Myofibrils are made up along their length of a series of sarcomeres Sarcomeres are the basic unit of contraction in striated muscles Responsible for microscopic striations visible by light microscopy Result from the highly organized arrays of contractile protein filaments Skeletal muscle fibers often have multiple nuclei o Early in development when skeletal muscles form muscle precursor cells myoblasts fuse to form large fibers fused cells have multiple nuclei SUBCLASSIFICATION OF SKELETAL MUSCLE TYPES Fusiform structure o The muscle fibers are nearly parallel with the long axis of the muscle o Thickest in the middle Pennate pinnate structure o The fibers are oriented at an angle to the long axis of the muscle o Ex Gastrocnemius and soleus muscles in the calf of the leg o Tendon typically extends further Skeletal muscle fibers classified by the speedof contraction and relaxation and susceptibility to fatigue during activity o Fast type II muscle fibers Fatigue resistant maintain a consistent level of force development during prolonged activity Fatiguable lose their ability to maintain force during activity o Slow type I or red muscle fibers Red because they contain copious amounts of the O2 binding protein myoglobin Some skeletal muscle can contain multiple fiber types Activity and exercise can alter fiber type composition of muscles Voluntary we can consciously decide to make specific movements o Begins at motor cortex in the brain to the CNS o Respiratory skeletal muscle are involuntary but we can make voluntary o Some skeletal muscle activation like shivering does not involve voluntary movements with them control CARDIAC MUSCLE Responsible for the systolic contraction of the heart that pumps blood throughout all tissues of the body o Heart wall muscle contracts during systole generates pressure to pump blood through vasculature Individual cardiac cell cardiac myocyte or cardiomyocyte o Smaller than most skeletal muscle cells o Elongated irregular branched shapes o Microscopic striations o Develop as individual cells so do not have large amounts of nuclei Cardiomyocytes are connected to nearby myocytes to form a syncytium plural syncytia Cardiomyocytes vary o Atrial myocytes are not exactly like ventricular myocytes Cardiac muscles are more similar to slow skeletal muscles o Fatigue resistant Functional characteristics of cardiac muscle depend on the quantity and type of muscles and metabolic proteins in a myocyte Classified as involuntary SMOOTH MUSCLE Classified as involuntary No striations Have single nuceli Reveals many bodily functions that do not require conscious input Individual cells are generally smaller than cardiac myocytes and skeletal fibers o Do not have highly organized arrays of the contractile protein filaments COMMON MUSCLE FEATURES MYOSIN ACTIN and CALCIUM ION Myosin and actin form filaments and are the major proteins responsible for muscle contraction o All muscle types contain these o Muscle myosin can form filaments with other myosin molecules o Actin can form filaments with other actin o These filaments form sarcomeres o Smooth muscle has no striations because the filaments in them are not as large or stable or organized as filaments in skeletal and cardiac muscle Cross bridge Cycle Myosin head hydrolyzes ATP and binds to actin in a biochemical o Myosin motor protein and mechanical cycle For myosin the partner is actin o o Myosin uses energy from ATP breakdown to generate force of movement when bound to actin o Products of ATP breakdown are lost when force is generated Myosins are not the same in all muscle types ELEVATION OF CALCIUM ION IN MUSCLE CELLS INITIATES CONTRACTION Each motor neuron is connected to innervates one or more skeletal muscle fibers Each skeletal muscle fiber is innervated by only one motor neuron o Together they form a motor unit the functional unit of the neuromuscular o Connection between a neuron and another cell is the synapse o An action potential an electrical signal in motor neuron releases chemical system signal at synapses o An action potential triggers contraction of all muscle fibers in the motor unit A motor unit s size is determined by the number of skeletal muscle fibers innervated by a motor neuron o All fibers in a unit have the same classification fast fatiguable fast fatigue resistant and slow fatigue resistant MOTOR UNIT SIZE AND CHARACTERISTICS ARE RELATED TO THE ORDER IN WHICH THEY ARE RECRUITED BY THE NERVOUS SYSTEM TO CONTRACT HENNEMAN S SIZE
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