Chapter 12 Muscular System

Points to Ponder What are the three types of muscle tissue? What are the functions of the muscular system? How are muscles named and what are the muscles of the human body? How are skeletal muscles and muscle fibers structured? How do skeletal muscles contract? How do skeletal muscle cells acquire ATP for contraction? What is rigor mortis? What are some common muscular disorders? What are some serious muscle diseases? How do the skeletal and muscular system help maintain homeostasis? How are these 2 systems related to other systems in maintaining homeostasis?

Muscle Tissue Skeletal muscle Cardiac muscle Smooth muscle Voluntary striated muscle controlled by nerves of the central nervous system Multinucleated and tubular Attacked to skeleton Cardiac muscle Involuntary striated muscle Uninucleated, tubular, and branched Intercalated disks contain gap junctions spread contractions quickly throughout the heart wall Cardiac fibers relax completely between contraction Prevent fatigue Smooth muscle Involuntary nonstriated muscle Uninucleated Cells arranged in parallel lines, forming sheets Located in the walls of hollow internal organs Slower to contract, sustain prolonged contractions does not fatigue easily

Review: 3 types of muscle tissue 12.1 Overview of the muscular system Review: 3 types of muscle tissue

Functions of skeletal muscles Support the body by allowing us to stay upright Allow for movement by attaching to the skeleton Help maintain a constant body temperature - Contraction causes ATP to break down, releasing heat Assist in movement in the cardiovascular and lymphatic vessels via muscular contractions Protect internal organs and stabilize joints Muscles pad bones Tendons hold bones together at joints

How are skeletal muscles arranged? 12.1 Overview of the muscular system How are skeletal muscles arranged? Attachments: Tendon – connective tissue that connects muscle to bone Muscles covered with fascis that extends beyond the muscle and becomes the tendon Origin – attachment of a muscle on a stationary bone Insertion – attachment of a muscle on a bone that moves Muscle contraction pulls on the tendon at its insertion causing the bone to move Action: Nervous system does not stimulate a single muscle, it stimulates an appropriate group of muscles 1. Prime mover – muscle that does most of the work 2. Antagonistic – muscles that work in opposite pairs - biceps brachii and triceps brachii are antagonist 3. Synergistic – muscles working in groups for a common action - assist the prime mover, enhances action

An example of muscle arrangement

How to Name Skeletal Muscles Size Maximus: largerst; Minimus: smallest Vastus: huge; Longus: long; Brevis: short Shape Deltoid: triangular (Greek letter delta is Δ) Trapezius: trapezoid; Latissimus: wide; Terres: round Location Frontalis overlies the frontal bone External and internal obliques; pectoralis; gluteus; brachii

How to Name Skeletal Muscles Direction of muscle fiber Rectus abdominus (rectus means straight) Transverse: across; oblique: diagonal Attachment Brachioradialis: attached to the brachium and radium Sternocleidomastoid: attached to sternum, clavicle, and mastoid Number of attachments the biceps brachii has two attachments Action extensor digitorum extends the digits adductor (to midline), flexor (flexes), levator (lift)

Muscle fibers/cells Terminology for cell structure The plasma membrane is called the sarcolemma Transverse tubules penetrate cell to contact with sarcoplasmic reticulum The cytoplasm is called the sarcoplasm Contains glycogen (store energy) and myoglobin (binds oxygen) needed for muscle contraction The SER of a muscle cell is called the sarcoplasmic reticulum and stores calcium

Skeletal Muscle Fibers Sarcoplasm contains networks of SER called sarcoplasmic reticulum (SR) Sarcoplasmic Reticulum: Function: store calcium and help transmit action potential to myofibril SR forms chambers attached to T-tubules Concentrate Ca2+ (via ion pumps) Release Ca2+ into sarcomeres to begin muscle contraction All calcium is actively pumped from sarcoplasm to SR (SR has 1000X more Ca2+ than sarcoplasm)

Skeletal Muscle Fibers Terminology for structure within a whole muscle Muscle fibers are arranged in bundles called fascicles Myofibrils are a bundle of myofilaments that run the length of a fiber Myofilaments are proteins (actin and myosin) that are arranged in repeating units Sarcomeres are the repeating units of actin and myosin found along a myofibril

12.2 Skeletal muscle fiber contraction

The sarcomere Made of two protein myofilaments 12.2 Skeletal muscle fiber contraction The sarcomere Made of two protein myofilaments Myosin: are the thick filaments shaped like a golf club Globular head allows for cross-bridges Actin: are the thin filaments Two other proteins are present Tropomyosin and troponin These filaments slide over one another during muscle contraction

Actin Myosin

Regions of the Sarcomere A-band: - whole width of thick filaments, looks dark microscopically M line: at midline of sarcomere - Center of each thick filament, middle of A-band - Attaches neighboring thick filaments H-zone: - Light region on either side of the M line - Contains thick filaments only Zone of overlap: - ends of A-bands - place where thin filaments intercalate between thick filaments (triads encircle zones of overlap)

Regions of the Sarcomere I-band: - Contains thin filaments outside zone of overlap - Not whole width of thin filaments Z lines/disc: - the centers of the I bands

Sliding Filaments Figure 10–8

Sliding Filament Theory Contraction of skeletal muscle is due to thick filaments and thin filament sliding past each other not compression of the filaments H-zones and I-bands decrease width during contraction Zones of overlap increase width Z-lines move closer together A-band remains constant Sliding causes shortening of every sarcomere in every myofibril in every fiber Overall result = shortening of whole skeletal muscle

The beginning of muscle contraction: The sliding filament model 12.2 Skeletal muscle fiber contraction The beginning of muscle contraction: The sliding filament model Nerve impulses travel down motor neurons to a neuromuscular junction Acetylcholine (ACh) is released from the neurons and bind to the muscle fibers This binding stimulates fibers causing calcium to be released from the sarcoplasmic reticulum

Skeletal Muscle: Neuromuscular Junction Figure 10–10a, b (Navigator)

Muscle contraction continued… 12.2 Skeletal muscle fiber contraction Muscle contraction continued… Released calcium combines with troponin, a molecule associated with actin This causes the tropomyosin threads around actin to shift and expose myosin binding sites Myosin heads bind to these sites forming cross-bridges ATP bind to the myosin heads and is used as energy to pull the actin filaments towards the center of the sarcomere = contraction now occurs ATP is bind to myosin ATP is split to ADP and 2 Phosphates ADP and P remain on the myosin heads - Heads attach to an actin filament forming cross bridges ADP and P is released, cross-bridges bend sharply - Power stroke pulls action filament toward the center of the sarcomere ATP molecule binds again to myosin to break cross bridge

12.2 Skeletal muscle fiber contraction

What role does ATP play in muscle contraction and rigor mortis? 12.2 Skeletal muscle fiber contraction What role does ATP play in muscle contraction and rigor mortis? ATP is needed to attach and detach the myosin heads from actin After death muscle cells continue to produce ATP through fermentation and muscle cells can continue to contract When ATP runs out some myosin heads are still attached and cannot unattach = rigor mortis Body temperature and rigor mortis helps to estimate the time of death

Tension Production For a single muscle fiber contraction is all–or–none: as a whole, a muscle fiber is either contracted or relaxed

Resting Length Greatest tension produced at optimal resting length Optimal resting length = Optimum overlap Overlap determines the number of pivoting cross-bridges Figure 10–14

Frequency of Stimulation Twitch = single contraction due to a single neural stimulation, 3 phases: Latent period: post stimulation but not tension Action potential moves across the sarcolemma Ca2+ is released Contraction phase: peak tension production - Ca2+ bind - Active cross bridge formation Relaxation phase: decline in tension Ca2+ is reabsorbed Cross bridges decline

Terms in whole muscle contraction Motor unit – a nerve fiber and all of the muscle fibers it stimulates Muscle twitch – a single contraction lasting a fraction of a second Summation – an increase in muscle contraction until the maximal sustained contraction is reached Tetanus – maximal sustained contraction Tone – a continuous, partial contraction of alternate muscle fibers causing the muscle to look firm

Physiology of skeletal muscle contraction 12.3 Whole muscle contraction Physiology of skeletal muscle contraction

Total Number of Muscle Fibers Stimulated Each skeletal muscle has thousands of fibers organized into motor units Motor units = all fibers controlled by a single motor neuron Axon branches to contact each fiber Number of fibers in a motor unit depends on the function Fine control: 4/unit (e.g. eye muscles) Gross control: 2000/unit (e.g. leg muscles) Fibers from different units are intermingled in the muscle so that the activation of one unit will produce equal tension across the whole muscle

Motor Units in a Skeletal Muscle Figure 10–17

Recruitment (Multiple Motor Unit Summation) In a whole muscle or group of muscles, smooth motion and increasing tension is produced by slowly increasing size or number of motor units stimulated Recruitment = order of activation of a motor unit Slower weaker units are activated first Strong units are added to produce steady increases in tension

Contraction Skeletal Muscle During sustained contraction of a muscle Some units rest while others contract to avoid fatigue For maximum tension, all units in complete tetanus Leads to rapid fatigue Muscle tone = maintaining shape/definition of the muscle Some units are always contracting Exercise = Increase # of units contraction  Increase in metabolic rate  Increase in speed of recruitment (better tone)

Where are the fuel sources for muscle contraction? 12.3 Whole muscle contraction Where are the fuel sources for muscle contraction? Stored in the muscle: Glycogen Fat In the blood: Glucose Fatty acids

What are the sources of ATP for muscle contraction? 12.3 Whole muscle contraction What are the sources of ATP for muscle contraction? Limited amounts of ATP are stored in muscle fibers Creatine phosphate pathway (CP) – fastest way to acquire ATP sustains a cell for seconds builds up when a muscle is resting Fermentation – fast-acting but results in lactate build up Cellular respiration (aerobic) – not an immediate source of ATP best long term source

Acquiring ATP for muscle contraction 12.3 Whole muscle contraction Acquiring ATP for muscle contraction

Muscle fibers come in two forms Fast-twitch fibers: rely on CP and fermentation (anaerobic) Lactate buildup is possible which leads to quick fatigue Designed for strength Provide explosions of energy, develop max. tension more rapidly (sprinting, weight lifiting) Light in color Few mitochondria Little or no myoglobin Fewer blood vessels than slow-twitch Motor units contain many fibers Slow-twitch fibers: Rely on aerobic respiration Tire when fuel supply is gone Designed for endurance Long distance running Biking and swimming Dark in color Many mitochondria Myoglobin Many blood vessels Provide oxygen Low maximum tension but high resistance to fatigue

12.3 Whole muscle contraction Types of muscle fibers

Health focus: Benefits of exercise 12.3 Whole muscle contraction Health focus: Benefits of exercise Increases muscle strength, endurance and flexibility Increases cardiorespiratory endurance Heart rate and capacity increase Air passages dilate so that the heart and lungs are able to support prolonged muscular activity HDL increases thus improving cardiovascular health Prevents development of plaque Proportion of protein to fat increases favorably May prevent certain cancers : colon, breast, cervical, uterine and ovarian Improve density of bones thus decreasing the likelihood of osteoporosis Enhances mood and may relieve depression

Common muscle disorders Spasms sudden, involuntary muscle contractions that are usually painful Seizure multiple spasms of skeletal muscles Cramps strong, painful spasms often of the leg and foot Strain stretching or tearing of a muscle

Common muscle disorders Sprain twisting of a joint involving muscles, ligaments, tendons, blood vessels and nerves Tendonitis inflammation of a tendon usually due to overuse (i.e. tennis elbow) Bursitis inflammation of a bursa usually from repetitive use or frequent pressure

Muscular diseases Fibromyalgia Muscular dystrophy Myasthenia gravis chronic achy muscles that is not well understood Muscular dystrophy group of genetic disorders in which muscles progressively degenerate and weaken Myasthenia gravis autoimmune disorder that attacks ACh receptor and weakens muscles of the face, neck and extremities Amyotrophic lateral sclerosis (ALS) commonly known as Lou Gehrig’s disease in which motor neurons degenerate and die leading to loss of voluntary muscle movement

Homeostasis: the skeletal and muscular systems Both systems are involved with movement that allows us to respond to stimuli, digestion of food, return of blood to the heart and moving air in and out of the lungs Both systems protect body parts Bones store and release calcium need for muscle contraction and nerve impulse conduction Blood cells are produced in the bone Muscles help maintain body temperature

Bioethical focus: Anabolic steroids? 12.5 Homeostasis Bioethical focus: Anabolic steroids? Anabolic steroids are a group of steroids that usually increase protein production Most common side effects are high blood pressure, jaundice, acne and great increased risk of cancer Abuse of these drugs may also cause impotence and shrinking of the testicles May lead to increased aggressiveness and violent mood swings Are they worth the risk? Should they be legal to use in athletics?