The Muscular System

The Muscular System ANS 215 Anatomy & Physiology of Domesticated Animals

Primary Functions • Movement • Blood • Ingesta/excreta • Gametes/conceptus • Support • Generation of body heat

Calssifications • Smooth • No striations • Individual cells are spindle shaped with centrally located nuclei • Regulated by the autonomic nervous system • Located in visceral structures • Aggregates of myofilaments are composed of actin and myosin • Filaments are not arranged in order

Smooth Muscle

Classifications • Cardiac • Found only in the heart • Regulated by the autonomic nervous system • Shows striations characterized by alternating white and dark bands • Composed of elongated, branching cells with irregular contours at junctions with other cells

Classifications • Cardiac • Boundary area where cells meet is called intercalated disk • This specialized structure facilitates transmission of nerve impulses • Each cell has a nucleus (sometimes 2) that is centrally located

Cardiac Muscle

Classifications • Skeletal • Long bundles of cells – striated • Multinucleated with nuclei at periphery of cells • Innervated by spinal or cranial nerves • Three types • red or dark (highest myoglobin and mitochondria content) • white or pale (lowest myoglobin and mitochondria content) • intermediate (characteristics between white and red fibers)

Classifications • Skeletal • Skeletal muscle comprises majority of body muscle mass • Skeletal muscle fiber can run the length of the muscle with which it is a part • Skeletal muscle fibers do not branch or anastomose

Classifications • Skeletal • Often described according to type of movement performed • Flexors – if they are located on the side of the limb towards which the joint decreases the joint angle • Extensors – if they are located on the side of the limb towards which the joint increases the joint angle • Adductors – if they pull a limb toward the median plane • Abductors – if they pull a limb away from the median plane • Sphincters – are arranged circularly to constrict body openings

Skeletal Muscle

Arrangement • Function of muscles is to contract or shorten and thereby move an object • Primary consideration for accomplishing this goal is arrangement • Examples include: • Sheets • Sheets rolled into tubes • Bundles • Rings • Cones • Discrete cells or clusters of cells

Arrangement • The effects of skeletal muscles (apart from sphincters) are noted some distance from their location • This means the contraction must be transmitted • One end must be anchored and the other attached directly to a tendon or moveable part • Accordingly, anatomic description of a skeletal muscle sometimes refers to its origin and insertion

Arrangement

Skeletal Muscle Harnessing • Harness for skeletal muscle fibers is composed of connective tissue elements • Epimysium • Perimysium • Endomysium • Continuous from the individual muscle fibers to the connective tissue structure to which it attaches and on which it exerts its contraction

Skeletal Muscle Harnessing • Connective tissue elements of skeletal muscle are: • Muscle fibers comprise a muscle bundle which are attached by their cell covering (sarcolema) to connective tissue division, endomysium • The endomysium is continuous with the connective tissue that envelopes muscle bundles, the perimysium

Skeletal Muscle Harnessing • The perimysium is continuous with the connective tissue that envelops the muscle (collection of bundles), the epimysium • Then epimysium is continuous with the tendon or aponeurosis, which can travel some distance from its attachment

Microstructure of Skeletal Muscles • Depending on diameter of the muscle fiber, there might be several hundred to several thousand myofibrils within one muscle • Each myofibril has striations or banding • Myofibrils are further broken into repetitive units (sarcomeres)

Microstructure of Skeletal Muscles • Sarcomeres contain the protein myofilaments actin and myosin • Give rise to striations by their arrangement • Sarcomeres of a myofibril are in alignment with the sarcomeres of all the other myofibrils of the muscle fiber • The Z line is located at each end of a sarcomere and is common to both sarcomeres that it separates

Microstructure of Skeletal Muscles • Actin filaments project from the Z line into the sarcomeres that it separates • Actin of two sarcomeres common to the same Z line comprise an I band

Microstructure of Skeletal Muscles • The myosin filaments are centrally located within a sarcomere, coupled with the overlap of actin filaments provide for the dark banding (A band) • There is a regular 2:1 ratio of actin to myosin

Sarcotubular System • Skeletal muscle fibers contain a network of tubules known as the sarcotubular system • Tubules are located within the muscle fiber, but outside the myofibrils

Sarcotubular System • Two tubule sets • Arranged transversely (right angles) to myofibrils • Open to the outside of fibers • contain extracellular fluid • Found at boundary of A and I bands • each sarcomere has two

Neuromuscular Junction • Each skeletal muscle fiber is innervated by a motor neuron • Axon terminal end bulb forms synapse with muscle fiber • Release of Acetylcholine (Ach) causes depolarization • One motor neuron innervates many muscle fibers • Motor unit • Motor unit ration of 1:150 means one nerve fiber supplies 150 muscle fibers • Smaller ration increases precision

Scanning Electron Micrograph of Neuromuscular Junction

Skeletal Muscle Contraction • Depolarization of muscle fibers • Acetylcholine released into synapse • Ca++ facilitates release • Parturient paresis occurs if not enough Ca++ • Opens ligand-coated Na+ channels • Causes a propagated action potential • sarcolema • T-tubule • sarcoplasmic reticulum • Cholinesterase destroys Ach

Skeletal Muscle Contraction • Depolarization of muscle fibers • Depolarization of sarcoplasmic reticulum causes Ca++ release • Ca++ diffuses to myofibrils • Ca++ initiates contraction • Ca++ pumped back into sarcoplasmic reticulum • active transport • Ca++ ATPase Pump

The Muscular System