Muscles and Muscle Tissue Chapter 9

Overview of Muscle Tissue n There are three types of muscle tissue –Skeletal muscle –Cardiac muscle –Smooth muscle n These muscle tissues differ in the structure of their cells, their body location, their function, and the means by which they are activated to contract

Overview of Muscle Tissue n All muscle cells are elongated cells and are referred to as muscle fibers n Muscle contraction depends on two types of myofilaments, actin and myosin n All prefixes of myo or mys and sarco reference muscle

Skeletal Muscle Tissue n Skeletal muscle tissue appear as distinct muscle that attach to the skeletal system n It has obvious striations n It is voluntary muscle under conscious control

Cardiac Muscle Tissue n Cardiac muscle occur only in the heart n The muscle is striated but involuntary n Cardiac fibers are short, fat, branched and interconnected n Cardiac muscle cells are interlocked by intercalated discs and function as a single unit

Smooth Muscle Tissue n It is found in the walls of hollow organs such as the stomach, urinary bladder, and intestines n It has no striations n It is not subject to voluntary control

Differences in Contractions n Skeletal muscle can contract rapidly but tires easily and must be rested –Skeletal muscle contractions vary in force depending on use –Cardiac muscle contracts at a steady rate but can accelerate to cope with demand –Smooth muscle contracts in steady, sustained contractions and continues on tirelessly

Muscle Functions n Muscle performs four important functions in the body: –Producing movement –Maintaining posture –Stabilizing joints –Generating heat

Producing Movement n Movement results from muscle contraction n Skeletal muscle are responsible for all locomotion and manipulation n Allow to interact or react with your external environment n It controls eye movement, facial expression, circulation, and moves gas, liquids, and solids through organs

Maintaining Posture n Skeletal muscles are utilized constantly to maintain sitting, standing, and moving postures n Postural muscle develop to compensate for the never ending pull of gravity –Our developmental milestones as an infant are our initial victories over gravity n Curves of the spinal column are shaped by the interplay of skeletal muscle and gravity

Stabilizing Joints n Skeletal muscle provide the dynamic stability of joints n Many joints are poorly reinforced by ligaments and connective tissue n Many joints have noncomplementary surface which do not contribute to stability

Generating Heat n Muscles generate heat as they contract n The heat generated is vitally important to maintain normal body temperature n Skeletal muscle generates most of the heat because it represents 40% of body mass n Excess heat must released to maintain body temperature

Functional Characteristics n Excitability or irritability –It has the ability to respond to a stimulus n Contractility –It has the ability to shorten forcibly n Extensibility –Muscle fibers can be stretched n Elasticity –Resume its normal length after being shortened

Skeletal Muscle

Anatomy of a Skeletal Muscle n Each skeletal muscle is a discrete organ with thousands of fibers n Muscle fibers predominate the tissue but it contains, blood vessels, nerve fibers, and connective tissue

Connective Tissue Wrappings n Each muscle fiber is wrapped by fine sheath of areolar connnective called endomysium n Several fibers are gathered side by side into bundles called fascicles n Each fascicle is bound by collagen fiber layer called the perimysium

Connective Tissue Wrappings n Fascicles are bound by a dense fibrous connective tissue layer called the epimysium n The epimysium surrounds the entire muscle n External to the epimysium is the deep fascia that binds muscles into functional groups

Connective Tissue Wrappings n All the connective tissue layers are continuous with one another as well as with the tendons that join muscles to bone n When muscle fibers contract they pull these connective tissue sheaths which in turn transmit the force to the bone to be moved n Connective tissue supports each cell

Nerve and Blood Supply n Normal activity of skeletal muscle is totally dependent on its nerve and blood supply n Each skeletal muscle fiber is controlled by a nerve ending n Contracting muscle fibers use huge amounts of energy which requires a continuous supply of oxygen and nutrients n In general, each muscle is served by an artery and one or more veins

Attachments n Most muscles span joints and have at least two attachments n Origin –Attachment of a muscle that remains relatively fixed during muscular contraction –Generally a more proximal or axial location n Insertion –Attachment of a muscle that moves during muscular contraction –Generally a more distal or appendicular attachment

Attachments n Direct attachments have the epimysium attaching directly to the periosteum of the bone or perichondrium of a cartilage n Indirect attachments have the epimysium attaching to a tendon or an aponeurosis n Temporalis has both muscle attachments

The Motor Unit n Each muscle is served by at least one motor nerve which contains hundreds of motor neuron axons n As a nerve enters a muscle it branches into a number of axonal terminals, each of which forms a neuromuscular junction with a single nerve fiber n A motor neuron and all the muscle fibers it supplies is called a motor unit

The Motor Unit n When a motor neuron transmits an electrical impulse all the muscle fibers that it innervates respond by contracting n The average number of muscle fibers per unit is 150 but it ranges from 4 to several hundred

The Motor Unit n Muscles that exert very fine control have small motor units n Large muscles of locomotion and weight bearing have large motor units and as a consequence have less precise control

The Motor Unit n The muscle fibers in a unit are not clustered together but rather are spread throughout the entire muscle n Stimulation of a single unit causes a weak contraction of the entire muscle n This allows control of the intensity of the contraction

Smooth Muscles n Smooth muscle lacks the courser connective tissue seen in skeletal muscle n Small amounts of endomysium is found between smooth muscle fibers

Smooth Muscles n Smooth muscles are organized into sheets of closely apposed fibers n These sheets occur in the walls of all but the smallest blood vessels and in the walls of hollow organs of the respiratory, urinary digestive and reproductive tracts

Smooth Muscles n In most cases two sheets of muscles are present with their fibers aligned at right angle to each other n These forms the longitudinal (long axis) and circular (encircling) layer n These two layers squeeze the contents of the organ

End of Chapter Chapter 9