Chapter 12 Muscular System. Points to Ponder What are the three types of muscle tissue? What are the...

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

– 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 tissue12.1 Overview of the muscular system

Functions of skeletal muscles

1. Support the body by allowing us to stay upright

2. Allow for movement by attaching to the skeleton

3. Help maintain a constant body temperature- Contraction causes ATP to break down, releasing heat

4. Assist in movement in the cardiovascular and lymphatic vessels via muscular contractions

5. Protect internal organs and stabilize joints- Muscles pad bones- Tendons hold bones together at joints

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 muscles1. Prime mover – muscle that does most of the work2. Antagonistic – muscles that work in opposite pairs

- biceps brachii and triceps brachii are antagonist3. Synergistic – muscles working in groups for a common action

- assist the prime mover, enhances action

12.1 Overview of the muscular system

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 FibersSarcoplasm 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

– Myosin: are the thick filaments shaped likea 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

1. A-band: - whole width of thick filaments, looks dark microscopically

2. M line: at midline of sarcomere- Center of each thick filament, middle of A-band- Attaches neighboring thick filaments

3. H-zone: - Light region on either side of the M line- Contains thick filaments only

4. Zone of overlap:- ends of A-bands- place where thin filaments intercalate between thick

filaments (triads encircle zones of overlap)

Regions of the Sarcomere3. I-band:

- Contains thin filaments outside zone of overlap

- Not whole width of thin filaments

4. Z lines/disc: - the centers of the I bands

Sliding Filaments

Figure 10–8

Sliding Filament TheoryContraction of skeletal muscle is due to thick filaments and thinfilament sliding past each other

– not compression of the filaments1. H-zones and I-bands decrease width during contraction2. Zones of overlap increase width3. Z-lines move closer together4. A-band remains constantSliding causes shortening of every sarcomere in every myofibril in

every fiberOverall result = shortening of whole skeletal muscle

The beginning of muscle contraction: The sliding filament model

1. Nerve impulses travel down motor neurons to a neuromuscular junction

2. Acetylcholine (ACh) is released from the neurons and bind to the muscle fibers

3. 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…4. Released calcium combines with troponin, a

molecule associated with actin5. This causes the tropomyosin threads around actin

to shift and expose myosin binding sites6. Myosin heads bind to these sites forming cross-

bridges7. 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

1. ATP is bind to myosin2. ATP is split to ADP and 2 Phosphates3. ADP and P remain on the myosin heads

- Heads attach to an actin filament forming cross bridges4. ADP and P is released, cross-bridges bend sharply

- Power stroke pulls action filament toward the center of the sarcomere

5. ATP molecule binds again to myosin to break cross bridge


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:

1. Latent period: post stimulation but not tension- Action potential moves across the sarcolemma- Ca2+ is released

2. Contraction phase: peak tension production- Ca2+ bind

- Active cross bridge formation

3. 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

12.3 Whole muscle contraction

Physiology of skeletal muscle contraction12.3 Whole 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?

• Stored in the muscle:– Glycogen– Fat

• In the blood:– Glucose– Fatty acids


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


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

Types of muscle fibers


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

– 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

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?

12.5 Homeostasis

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