Chapter 7 Muscular System PDF

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CHAPTER 7 MUSCULAR SYSTEM 7.1 FUNCTIONS OF THE MUSCULAR SYSTEM 3 TYPES OF MUSCLE TISSUE 1. 2. 3.

SKELETAL CARDIAC SMOOTH

MAJOR FUNCTIONS OF THE MUSCULAR SYSTEM 1. 2. 3. 4. 5. 6. 7.

MOVEMENT OF THE BODY - contraction - responsible for the overall movement MAINTENANCE OF POSTURE - constantly maintain tone RESPIRATION - muscles of the thorax PRODUCTION OF HEAT- when skeletal muscles contract heat is given off as the by-product COMMUNICATION – CONSTRICTION OF ORGANS AND VESSELS – CONTRACTION FO THE HEART – propelling blood all parts of the body

7.2 CHARACTERISTIC OF SKELETAL MUSCLE 1.

SKELETAL MUSCLE – associated connective tissue; 40% of body weight Striated muscle because transverse bands

FUNCTIONAL CHARACTERISTICS 1.

2. 3. 4.

CONTRACTILITY - ability of skeletal muscle to shorten with force They lengthen passively Either gravity or the contraction of an opposing muscle produces a force that pulls on the shortened muscle, causing it to lengthen EXCITABILITY - from nerves; capacity of skeletal muscle to respond to stimulus EXTENSIBILITY – muscle stretch ; After a contraction, skeletal muscles can be stretched to their normal resting length and beyond to a limited degree ELASTICITY - ability to recoil to heir original resting - after they have been stretched

SKELETEAL MUSCLE STRUCTURE CONNECTIVE TISSUE COVERINGS OF MUSCLE EPIMYSIUM – MUSCULAR FASCIA – connective tissue sheet surrounding the skeletal muscle Each whole muscle is subdivided by a loose connective tissue called the PERINISIUM into numerous muscle bundles called muscle FASCICULI. Each fascicle is then subdivided by a loose connective tissue called the ENDOMYSIUM, into separate muscle cells, called MUSCLE FIBERS

MUSCLE FIBER STRUCTURE A muscle fiber is a single cylindrical fiber, with several nuclei located at its periphery SARCOLEMA – the cell membrane of the muscle fiber; where the muscle nuclei of the muscle fiber are located TRANSVERSE TUBULES, T TUBLUES - tubelike invaginations on the surface of the sarcolemma ; tubelike extend inward into the muscle fiber. -

They connect the Sarcolemma to the sarcoplasmic reticulum

SARCOPLASMIC RETICULUM - T tubules are associated with a highly organized smooth endoplasmic reticulum

-

Has a relatively high concentration of Ca 2+ , which plays a major role in muscle contraction

SARCOPLAMS- the cytoplasm of muscle fiber MYOFIBRILS - threadlike structures that extend from one end of the muscle fiber to the end. TWO LAYERS OF MYOFIBRILS (PROTEIN FIBERS) A. B.

Actinmyofilaments – Myosin They are both arranged in highly orderd, repeating units called SARCOMERE which are joined end-to-end to form the myofibrils

ACTIN AND MYOSIN MYOFILAMENTS ACTIN MYOFILAMENTSS- or thin filaments, are made up of three components: ACTIN , TROPONIN, AND TROPOMYOSIN. The ACTIN STRANDS, which resembles two minute striands of pearls twisted together, have attachment sites for the myosin myofilaments. TROPONIN molecules are attached at specific intervals along the actin myofilaments. (these molecules have binding sites for CA2+.) TROPOMYOSIN - Filaments are located along the groove between the twisted strands of actin myofilaments subunits The tropomyosin filaments block the myosin myofilament binding sites on the actin myofilaments in an unstimulated muscle. In other words , if no Ca2+ is present , the tropomyosin filament cover the attachment sites on the actin myofilament. However, when Ca2+ is present, it binds to troponin, which causes the tropomysin filaments to expose the attachment sites on the actin myofilaments. MYOSIN MYOFILAMENTS, or thick myofilaments, resemble handles of minute golf clubs, the end that looks like a golf club is called MYOSIN HEAD. Three important properties: (1) The head can bind to attachment sites on the actin myofilaments (2) They can bend and straighten during contraction (3) They can breakdown atp releasing energy

SARCOMERES is the basic structural and functional unit of skeletal muscle because it is the smallest portion of skeletal muscle capable of contracting. The separate components of the sarcomere can slide past each other , causing the sarcomeres to shorten. When the sarcomeres shorten, the myofibrils shorten, which is the ultimate cause of contraction of the muscle fiber during a contraction. Each sarcomere extends from one z disk to an adjacent Z disk. Each Z DISK is a network of protein fibers forming an attachment site for actin myofilaments. The arrangement of the actin and myosin myofilaments in sarcomeres gives the myofibril a banded appearance. A llight I BAND , which myofilaments spans each z disk and ends a the myosin myofilaments. A darker , central region in each sarcomers called an A BAND extends the length of the myofin filaments. Te actin and myosin myofilaments overlap for some distance at both ends of thee A band. In the center of each sarcomere is a second light zone called the HZONE which consists only of myosin myofilaments. The myosin filaments are anchored in the center of the sarcomere at a dark-staining band called the M LINE. The alternating I bands and A bands of the sarcomeres are responsible for the striation in skeletal mscle fibers observed throught the microscope. It is the close association of the sarcomeres, the T tubules and the sarcoplasmic reticulum that enables a nerve stimulus to initiate contraction of the muscle fiber.

Sarcomere definition

A sarcomere is the functional unit of striated muscle. This means it is the most basic unit that makes up our skeletal muscle. Skeletal muscle is the muscle type that initiates all of our voluntary movement. Herein lies the sarcomere’s main purpose. Sarcomeres are able to initiate large, sweeping movement by contracting in unison. Their unique structure allows these tiny units to coordinate our muscles’ contractions.

The image depicts skeletal muscle fiber.

In fact, the contractile properties of muscle are a defining characteristic of animals. Animal movement is notably smooth and complex. Dexterous movement requires a change in muscle length as the muscle flexes. This calls for a molecular structure that can shorten along with the shortening muscle. Such requisites are found in the sarcomere.

Upon closer inspection, skeletal muscle tissue gives off a striped appearance, called striation. These “stripes” are given off by a pattern of alternating light and dark bands corresponding to different protein filaments. These stripes are formed by the interlocking fibers that comprise each sarcomere. Tubular fibers called myofibrils are the basic components that form muscle tissue. However, myofibrils themselves are essentially polymers, or repeating units, of sarcomere. Myofibrils are fibrous and long, and made of two types of protein filament that stack on top of each other. Myosin is a thick fiber with a globular head, and actin is a thinner filament that interacts with myosin when we flex.

Depicted is a basic illustration of skeletal muscle’s underlying components, down to the sarcomere.

Sarcomere structure When viewed under a microscope, muscle fibers of varied lengths are organized in a stacked pattern. The myofibril strands, thereby actin and myosin, form bundles of filament arranged parallel to one another. When a muscle in our body contracts, it is understood that the way this happens follows the sliding filament theory. This theory predicts that a muscle contracts when filaments are allowed to slide against each other. This interaction, then, is able to yield contractile force. However, the reason the sarcomere structure is so crucial in this theory is that a muscle needs to physically shorten. Thus, there is a need for a unit that is able to compensate for the lengthening or shortening of a flexing muscle. The sliding filament theory was first posited by scientists who had used high-resolution microscopy and filament stains to observe myosin and actin filaments in action at various stages of contraction. They were able to visualize the physical lengthening of the sarcomere in its relaxed state, and the shortening in its contracted state. Their observations led to the discovery of sarcomere zones.

The figure depicts the structure of a Sarcomere. (Each zone is labeled).

They first observed that the dynamic changes that were taking place were always happening in the same spots, or zones. They noticed that one zone of repeated sarcomere, later called the “A band,” maintained a constant length during contraction. The A band has a higher content of thick myosin filament, as expected by the area’s rigidity. The A band is the area in the center of the sarcomere where thick and thin filaments overlap. This gave researchers an idea of myosin’s central location. Within the A band is the H zone, which is the area composed only of thick myosin. Essentially, the A band can be thought to include “all” of the myosin including the myosin intertwined with actin at its bulbous head. Located on each end of the sarcomere’s length is the I band. The I bands are the two regions that exclusively contain thin filament. A quick way to remember this is that the I band has “thIn, actIn” filaments. The thick filaments are located not too far from the site of the I band; but on either side, their margins delineate where the thick filaments end. Likewise, the Z lines or discs that give sarcomeres a striped appearance under a light microscope actually delineate the regions between adjacent sarcomeres. The M line, or middle division, is found right in the middle of the Z lines and contain a less important third filament called myomesin.

Filament mental shortcut:



I is a thin letter, contains only thin filaments.



H is a wider letter, contains only thick filaments.

As mentioned before, contraction happens when the thick filaments slide along the thin filaments in quick succession to shorten the myofibrils. However, a crucial distinction to remember is that the myofilaments themselves do not contract. It is the sliding action that lends them their power to shorten or lengthen.

Sarcomere function Filament sliding generates muscle tension, which is without question the sarcomere’s main contribution. This action lends muscles their physical force. A quick analogy of this is the way a long ladder can be extended or folded depending on our needs for it, without physically shortening its metal parts.

Thankfully, recent research gives us a good idea of how this sliding works. The sliding filament theory has been modified to include how myosin is able to pull on actin to shorten the length of the sarcomere. In this theory, myosin’s globular head is located close to actin in an area called the S1 region. This region is rich in hinged segments that can bend and thus facilitate contraction. The bending of S1 may be the key to understanding how myosin is able to “walk” along the length of the actin filaments. This is accomplished by myosin-actin cycling. This is the binding of the myosin S1 fragment, its contraction, and its eventual release.

When myosin and actin bind, they form extensions called “cross-bridges.” These cross-bridges can form and break with the presence (or absence) of ATP. ATP makes S1 contraction possible. When ATP binds to actin filament, it moves it into a position that exposes its myosin binding site. This allows myosin’s globular head to bind to this site to form the cross-bridge. This binding causes the phosphate group of the ATP to dissociate, and thus myosin initiates its power stroke. Myosin thus enters a lower energy state where the sarcomere can shorten. Moreover, ATP must bind myosin to break the cross-bridge, and allow myosin to re-bind actin and initiate the next spasm.

SKELETAL MUSCLE a0 ORGANIZATION OF SKELETAL MUSCLE COMPONENTS B0 ELECTRON MICROGRAPH OF SKELETAL MUSCLE, SHOWING SEVERAL SARCOMERES IN A MUSCLE FIBER. C0 DIAGRAM OF TWO ADJACENT SARCOMERES, DEPECTING THE STRUCTURES RESPONSIBLE FOR THE BANDING PATTERN

STRUCTURE OF A MUSCLE: (A) PART OA MUSCLE ATACHED BY A TENDON TO A BONE. A MUSCLE IS COMPOSED OF MUSCLE FASCICULI, EACH SURROUNDED BY PERIMYSIUM. THE FASCICULI ARE SOMPOSED OF BUNDLES OF INDIVIDUAL ,USCLE FIBERS (MUSCLE CELLS), EACH SURROUNDED BY ENDOMYSIUM. THE ENTIRE MUSCLE IS SURROUNDED BY A CONNECTIVE TISSUE SHEATH CALLED SPIMYSIUM, OR MUSCULAR FASCIA. (B) ENLARGEMENT OF ONE MUSCLE FIBER CONTAINNG SEVERAL MYOFIBRILS. (C) A MYOFIBRIL EXTENDED OUT THE END OF THE MUSCLE FIBER, SHOWING THE BANDING PATTERNS OF THE SARCOMERES. (D) A SINGLE SARCOMER OF A MYOFIBRIL IS COMPOSED MAINLY OF ACTIN MYOFILAMENTS AND MYOSIN MYOFILAMENTS. THE Z DISKS ANCHOR THE ACTIN MYOFILAMENTS, AND THE MYOSIN MYOFILAMENTS ARE HELD IN PLACE BY THE M LINE. € PART OF A N ACTIN MYOFILAMENT IS ENLARGED. (F) PART OF A MYOSIN MYOFILAMENT IS ENLARGED.

EXCITABILITY OF MUSCLE FIBERS: Electrical properties of skeletal muscle. POLARIZED the inside of the membrane is positively charged RESTING MEMBRANE POTENTIAL – occurs because there is an uneven distribution of ions across the cell membrane

The resting membrane potential develops for three reasons: p155 1. 2. 3.

The concentration of K+ inside the cell membrane is higher than the outside the cell membrane The concentration of na+outside the cell membrane is higher than that inside the cell membrane The cell membrance is more permeable to K+ than it is to na+

1. 2. 3.

Cell membranes have a negative charge on the inside relative toa positive charge outside. (RESTING MEMBRANE POTENTIAL Action potentials are a brief reversal of the membrane charge. They are carried rapidly along the cell membrane Na+ move into cells during polarization, and K+ moves out of the cell during repolarization.

NERVE SUPPLY AND MUSCLE FIBER STIMULATION MOTOR NEURONS - specialized nerve cells that stimulate muscle to contract. Motor neurons carry action potentials to skeletal muscles. Where the neuron and muscle fiber form neuro muscular junctions. Neurons release acetlycholine, which binds to receptors on muscle cell membranes, stimulates an action potential in the muscle cell and causes the muscle to contract. Axons sens signals to branches NEURON MUSCULAR JUNCTION - (branch) junction of muscle fiber; near the n=center of muscle fiber SYNAPSE- refers to the cell to cell junction between the nerves, nerve cell, effector cell or gland MOTOR UNIT- is a small , precisely controlled muscle . (the fewer fibers there are in the motor units of a muscle, the greater control you have PRESYNAPTIC TERMINAL an enlarged axon terminal; The space between the presynaptic terminal and the muscle fiber membrane is the SYNAPTIC CLEFT; and the muscle fiber membrane is the POSTSYNAPTIC MEMBRANE. Each presynaptic terminal contains many small vesicles called SYNAPTIC VESICLES. These vesicles contain ACETHLCHOLINE ACH which functions as a NEUROTRANSMITTER a molecule release by a presynaptic nerve cell that stimulates or inhibits a postsynaptic cell.

ACETYLCHOINESTERASE breaks down acetylcholine; this enzymatic breakdown ensures that one action potential in the newron yiels only one action potential in the skeletal muscle fibers of that motor unit and only one contraction of each muscle fiber.

MUCLE CONTRACTION Contraction of skeletal muscle tissue occurs as actin and myosin myofilaments slide past one another, causing sarcomeres to shorten. The sliding of actin myofilaments past myo sin myofilaments during contraction is called SLIDING FILAMENT MODEL of muscle contraction. During contraction, neither the actin nor the myosin fibers shorten. The H zones and I bands shorten during contraction bt the A bands do not change in length.

MUSLCE TWITCH - is the contraction of a muscle fiber in response to a stimulus 7.5 figure 3 phases 1. Lag phase –latent phase time between the application of stimulus and beginning of contraction 2. Contraction phase- muscle contracts 3. Relaxation – muscle relaxes During the lag phase, action potentials are produced in one or more motor neurons. An action potential travels along the axon of a motor neuron to a neuro muscular junction (7.5). once the stimulus reaches the neuro muscular junction , acetylcholine must be released from the presynaptic terminal , diffused across the synaptic cleft and bind to receptors that allow the Na+ , which initiates an action potential on the post synaptic membrane (7.6). Before the contraction phase can occur, the action potential must result in the release of Ca+ from the sarcoplasmic reticulum and the formation of cross-bridges (7.9 steps 1-2 )

The strength of muscle contraction varies from weak to strong. THE FORCE OF CONTRACTION A MUSCLE PRODUCES IS INCREASED IN TWO WAYS: 1. SUMMATION - the force of contraction of individual muscle fibers is increased by rapidly stimulating them (when the stimulus is low, there is time to relax. 2. RECRUITMENT - involves increasing the number of muscle fiber contracting TETANUS- is a sustained contraction that occurs when the frequency of stimulation if so rapid that no relaxation occurs. It shous be noted that compete tetanus is rarely achieved under normal circumstances and is ore commonly an experimentally induced muscular response.

FIBER TYPES SLOW TWITCH (type I myosin ) (aeorobic) OR FAST TWITCH(type IIa or IIb) (anaerobic) ( based on the differences in the rod portion of the myosin myofilament -

However while slower, the slow twitch fibers can sustain the contraction for longer than the fast twitch fibers. Likewise , type IIa fibers can sustain contraction longer than type IIb

ENERGY REQUIREMENTS OF MUSCLE CONTRACTION 1. Aerobic (with O2) – (slow twitch), more efficient than anaerobic, more flexible (ability to break down lipids and amino acids) 2. Anaerobic (without O2) – (fast twitch), able to produce ATP but too low to maintain activities 3. Atp production ATP PRODUCTION IN SKELETAL MUSCLE 1. 2. 3. 4.

Aerobic production of ATP during most exercise and normal condition Anaerobic production of ATP during intensive short term work Conversion of molecule called creatine phosphate to ATP Conversion of two ADP to one ATP and one AMP (adenosine monophosphate) during heavy exercise

AEROBIC RESPIRATION occurs mostly in mitochondria , requires O2 and breaks down glucose to produce ATP, CO and H2O , can also process lipids or amino acid to make ATP. ANAEROBIC does not req O2 , breaks down glucose to produce ATP and lactate -muscle cells cannot store ATP -Creatine phosphate probides a means to storing energy that can be rapidly used to help maintain adequate ATP in contracting muscle fiber. P163

Respiratory rate is still elevated for a time even after rest to compensate for Oxygen deficit The RECOVERY OXYGEN CONSUMPTION - amount of O2 NEEDED in chemical reactions that occur to: 1 convert lactate into glucose 2. Replesnish the depleted ATP and creatine phosphate stores in muscle fiber 3. replenish o2 in the lungs, blood and muscles

FATIGUE is a temporary state of reduced work capacity (otherwise structural damage) MULTIPLE MECHANISMS UNDERLYING MUSCULAR FATIGUE 1. Acidosis and ATP depletion due to either an increased ATP consumption or decreased ATP production 2. Oxidative stress, which is characterized by the buildup of excess reactive oxygen species 3. Local inflammatory reaction

OXIDATIVE STRESS – increase ROS INFLAMMATION - T lymphocytes, a type of B=WBC, migrate into heavily worked muscles. The presence of immune system intermediates increases the perception of pain, which most likely serves as a signal to protect those tissues from further damage. PHYSIOLOGICAL CONTRACTURE – (extreme muscle fatigue that muscles wont contract or relax ), occurs when there is too little ATP to bind to myosin filaments. (because ATP is impt to the myosin heads for cross bridge release between actin and myosin, the cross bridges between ...