Muscles and Muscle Tissue 9 Part A

The three types of muscle tissue are skeletal, cardiac, and smooth Muscle Overview The three types of muscle tissue are skeletal, cardiac, and smooth These types differ in structure, location, function, and means of activation

Muscle terminology is similar Muscle Similarities Skeletal and smooth muscle cells are elongated and are called muscle fibers Muscle contraction depends on two kinds of myofilaments – actin and myosin Muscle terminology is similar Prefixes – myo, mys, and sarco all refer to muscle

Skeletal Muscle Tissue Has obvious stripes called striations Is controlled voluntarily (i.e., by conscious control) Contracts rapidly but tires easily Is responsible for overall body motility Is extremely adaptable and can exert forces ranging from a fraction of an ounce to over 70 pounds

Cardiac Muscle Tissue Occurs only in the heart Is striated like skeletal muscle but is not voluntary Contracts at a fairly steady rate set by the heart’s pacemaker Neural controls allow the heart to respond to changes in bodily needs

Smooth Muscle Tissue Found in the walls of hollow visceral organs, such as the stomach, urinary bladder, and respiratory passages Forces food and other substances through internal body channels It is not striated and is involuntary

Functional Characteristics of Muscle Tissue Excitability, or irritability – the ability to receive and respond to stimuli Contractility – the ability to shorten forcibly Extensibility – the ability to be stretched or extended Elasticity – the ability to recoil and resume the original resting length

Skeletal muscles are responsible for all locomotion Muscle Function Skeletal muscles are responsible for all locomotion Cardiac muscle is responsible for coursing the blood through the body Smooth muscle helps maintain blood pressure, and squeezes or propels substances (i.e., food, feces) through organs Muscles also maintain posture, stabilize joints, and generate heat

The three connective tissue sheaths are: Skeletal Muscle Each muscle is a discrete organ composed of muscle tissue, blood vessels, nerve fibers, and connective tissue The three connective tissue sheaths are: Endomysium – fine sheath of connective tissue composed of reticular fibers surrounding each muscle fiber Perimysium – fibrous connective tissue that surrounds groups of muscle fibers called fascicles Epimysium – an overcoat of dense regular connective tissue that surrounds the entire muscle

Skeletal Muscle Figure 9.2 (a)

Skeletal Muscle: Nerve and Blood Supply Each muscle is served by one nerve, an artery, and one or more veins Each skeletal muscle fiber is supplied with a nerve ending that controls contraction Contracting fibers require continuous delivery of oxygen and nutrients via arteries Wastes must be removed via veins

Skeletal Muscle: Attachments Most skeletal muscles span joints and are attached to bone in at least two places When muscles contract the movable bone, the muscle’s insertion moves toward the immovable bone, the muscle’s origin Muscles attach: Directly – epimysium of the muscle is fused to the periosteum of a bone Indirectly – connective tissue wrappings extend beyond the muscle as a tendon or aponeurosis

Microscopic Anatomy of a Skeletal Muscle Fiber Sarcoplasm has numerous glycosomes and a unique oxygen-binding protein called myoglobin Fibers contain the usual organelles, myofibrils, sarcoplasmic reticulum, and T tubules PLAY InterActive Physiology®: Muscular System: Anatomy Review: Skeletal Muscle Tissue

Myofibrils are densely packed, rodlike contractile elements They make up most of the muscle volume The arrangement of myofibrils within a fiber is such that a perfectly aligned repeating series of dark A bands and light I bands is evident

Myofibrils Figure 9.3 (b)

The smallest contractile unit of a muscle Sarcomeres The smallest contractile unit of a muscle The region of a myofibril between two successive Z discs Composed of myofilaments made up of contractile proteins Myofilaments are of two types – thick and thin

Sarcomeres Figure 9.3 (c)

Myofilaments: Banding Pattern Thick filaments – extend the entire length of an A band Thin filaments – extend across the I band and partway into the A band Z-disc – (connections) that anchors the thin filaments and connects myofibrils to one another

Myofilaments: Banding Pattern Thin filaments do not overlap thick filaments in the lighter H zone M lines appear darker due to the presence of the protein desmin (filament guide)

Myofilaments: Banding Pattern Figure 9.3 (c, d)

Contraction of Skeletal Muscle Fibers Contraction – refers to the activation of myosin’s cross bridges (force-generating sites) Shortening occurs when the tension generated by the cross bridge exceeds forces opposing shortening Contraction ends when cross bridges become inactive, the tension generated declines, and relaxation is induced

Length-tension relationship at 70% to 130% of resting length muscles can generate most tension because maximum number of cross bridges can be made below 70% or more than 130% resting length results in little generation of tension because of cross bridges overlap too much or not all

Contraction of Skeletal Muscle (Organ Level) Contraction of muscle fibers (cells) and muscles (organs) is similar The two types of muscle contractions are: Isometric contraction – increasing muscle tension (muscle does not shorten during contraction) Isotonic contraction – decreasing muscle length (muscle shortens during contraction)

Motor Unit: The Nerve-Muscle Functional Unit A motor unit is a motor neuron and all the muscle fibers it supplies The number of muscle fibers per motor unit can vary from four to several hundred When a motor unit fires all the fibers attached contract completely. (all or nothing response) Muscles that control fine movements (fingers, eyes) have few fibers per motor unit

Motor Unit: The Nerve-Muscle Functional Unit Figure 9.12 (a)

Motor Unit: The Nerve-Muscle Functional Unit Large weight-bearing muscles (thighs, hips) have motor units with 100’s of muscle fibers Muscle fibers from a motor unit are spread throughout the muscle; therefore, contraction of a single motor unit causes weak contraction of the entire muscle Some motor units have larger fibers than other motor units. Muscles start with motor units with small fibers

The three phases of a muscle twitch are: A muscle twitch is the response of a muscle to a single, brief threshold stimulus The three phases of a muscle twitch are: Figure 9.13 (a)

Graded Muscle Responses Graded muscle responses are: Variations in the degree of muscle contraction Required for proper control of skeletal movement Responses are graded by: Changing the frequency of stimulation Changing the strength of the stimulus

Muscle Response to Varying Stimuli A single stimulus results in a single contractile response – a muscle twitch Frequently delivered stimuli (muscle does not have time to completely relax) increases contractile force – wave summation Figure 9.14

Muscle Response to Varying Stimuli More rapidly delivered stimuli result in incomplete tetanus If stimuli are given quickly enough, complete tetanus results Figure 9.14

Muscle Response: Stimulation Strength Threshold stimulus – the stimulus strength at which the first observable muscle contraction occurs Beyond threshold, muscle contracts more vigorously as stimulus strength is increased Force of contraction is precisely controlled by multiple motor unit summation This phenomenon, called recruitment, brings more and more muscle fibers into play

Stimulus Intensity and Muscle Tension Figure 9.15 (a, b)

Treppe: The Staircase Effect Staircase – increased contraction in response to multiple stimuli of the same strength Contractions increase because: There is increasing availability of Ca2+ in the sarcoplasm Muscle enzyme systems become more efficient because heat is increased as muscle contracts PLAY InterActive Physiology®: Muscular System: Contraction of Whole Muscle

Summation Additional motor units contract before initial ones relax----form of graded response of whole muscle Treppe--stair case effect intial stimuli do not produce as much shortening as subsequent stimuli of same strength (They add up)

Treppe: The Staircase Effect Figure 9.16

Types of muscle contractions All or None principle Applies to individual fibers a subthreshold stimulus will generate no contraction a threshold stimulus will generate a maximal contraction for all muscle fibers contained in a motor unit stimuli above threshold will activate more motor units and generate more tension in muscle but will not result in greater contraction on any one fiber

Characteristics of muscle contraction Can contract in graded fashion As more motor units contract or existing motor units contract more rapidly, tension increases strength of stimulus determines number of motor units activated

Strength of muscle contraction Muscle tone--- a few alternating batteries of motor units are kept active, stabilizing posture and keeping muscles ready to respond to further stimulation Number of motor units stimulated size of muscle