Chapter 9

Muscular System

Skeletal Muscle

Each skeletal muscle is an organ made up of skeletal muscle fibers, connective tissue coverings, blood vessels, and nerve fibers

Structure of a Skeletal Muscle

Outside In

• Each skeletal muscle is then covered by an outer, very tough fibrous layer of CT called deep fascia– The deep fascia may extend past the length of the

muscle (tendon or aponeurosis), and attach that muscle to a bone, cartilage or muscle

• Each skeletal muscle is covered by a second layer of dense, fibrous CT called epimysium

A skeletal muscle is composed of a variety of tissues

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Muscle

Bone

Tendon

Fascia(covering muscle)

Epimysium

Skeletal Muscle cont.

• Skeletal muscles are formed from bundles of fascicles – Each fascicle is wrapped in a third layer of CT

made of collagen called perimysium

• Fascicles are formed from bundles of muscle fibers– Each muscle fiber (cell) is wrapped in a thin,

delicate (fourth) layer of CT called endomysium– Cell membrane= Sarcolemma– Cytoplasm= Sarcoplasm

Muscle fibers (cells)

Muscle fiber

Sarcolemma

Nucleus Sarcoplasmicreticulum

Fascicles

Fascicle

Axon of motor neuron

Blood vessel

A skeletal muscle is composed of a variety of tissues

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Muscle

Bone

Tendon

Fascia(covering muscle)

EpimysiumPerimysium

Endomysium

Fascicle

Skeletal Muscle cont.

• Each muscle fiber contains many threadlike structures called Myofibrils. – Myofibrils play an important role in muscle

contraction

• They consist of two types of Protein Filaments– Thick filament= Myosin– Thin filament= Actin

Thick and thin filaments

Filaments

Myofibrils

Myofibril

Muscle fibers (cells)

Muscle fiber

Sarcolemma

Nucleus Sarcoplasmicreticulum

Fascicles

Fascicle

Axon of motor neuron

Blood vessel

A skeletal muscle is composed of a variety of tissues

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Muscle

Bone

Tendon

Fascia(covering muscle)

EpimysiumPerimysium

Endomysium

Myofibril

• Within the sarcoplasm (cytoplasm) of a muscle fiber, there are two specialized membranous organelles (“little organs”)

• Sarcoplasmic reticulum (SR) – Network of membranous channels that surrounds

each myofibril and runs parallel to it– Similar endoplasmic reticulum in other cells – SR has high concentrations of calcium ions

compared to the sarcoplasm (maintained by active transport calcium pump)

– When stimulated by muscle impulse, membranes become more permeable to calcium ions and calcium diffuses out of SR and into sarcoplasm

Skeletal Muscle Fibers

Skeletal Muscle Fibers cont.

• Transverse tubules (TT)– set of membranous channels that extends into the

sarcoplasm as invaginations continuous with muscle cell membrane (sarcolemma)

– TTs are filled with extracellular fluid and extend deep into the cell

– Each TT runs between two enlarged portions of SR called cisternae

– These structures form a triad near the region where actin and myosin overlap

Skeletal Muscle Fibers cont.

• SR and TT are involved in activating the muscle contraction mechanism (discussed in greater detail later).

• Because one TT is associated with two SR they are termed the Triad

Skeletal Muscle Fibers cont.

• The organization of think and thick filaments within muscle fibers produces light and dark bands (striations) characteristic of skeletal muscle fibers

• The striations form a repeating pattern of units called sarcomeres. – Functional unit of muscle

• Myofibrils are made from sarcomeres in a row, end-to-end.

Sarcomere characteristics• I bands=light area =

thin filaments alone• A bands=dark area =

overlapping of thick and thin filaments

• Z lines=Sarcomeres meet one another

Sarcomere characteristics cont.

• H zone=Lighter area in A bands with only thick filaments

• M line=darker area with proteins to hold thick filaments in place

Molecules Involved in Contraction

Filaments

• Thick filaments = protein myosin

• Thin filaments = protein actin

Other

• Tropomyosin

• Troponin

Molecules Cont.

• Thick filaments = protein myosin – rod-like tail (axis) that terminates in two globular

heads or cross bridges– Cross bridges interact with active sites on thin

filaments

• Thin filaments = Primarily the protein actin– coiled helical structure (resembles twisted strands

of pearls):– Tropomyosin = rod-shaped protein spiraling

around actin backbone to stabilize it

– Troponin = complex of polypeptides:– one binds to actin – one that binds to tropomyosin – one that binds to calcium ions

• Both tropomyosin and troponin help control actin's interaction with myosin during contraction

Molecules Cont.

Skeletal Muscle Contraction

Neuromuscular Junction

• Neuromuscular Junction (NMJ) = the site where a motor nerve fiber and a skeletal muscle fiber meet (also called a synapse or synaptic cleft)– In order for a skeletal muscle to contract, its fibers

must first be stimulated by a motor neuron

Neuromuscular Junction cont.

• Motor End Plate = the specific part of a skeletal muscle fiber's sarcolemma directly beneath the NMJ

• Neurotransmitter = chemical substance released from a motor end fiber, causing stimulation of the sarcolemma of muscle fiber; acetylcholine (ACh)

• Synaptic cleft – small space between neuron and muscle

Motor Unit• Motor Unit =

one motor neuron and many skeletal muscle fibers

Stimulus for Contraction

• The function of skeletal muscle is to move bones of the skeleton under voluntary control.

• Contraction of a skeletal muscle fiber is a complex interaction of several cellular and chemical constituents.

• The final result is a movement whereby actin and myosin filaments slide past one another.

– The muscle fiber shortens and pulls on its attachments.

Introduction

Stimulus for Contraction

• The process begins when a nerve impulse is initiated by the brain, travels down the spinal cord, into a motor neuron, which branches into many motor nerve fibers/endings

• The neuron meets the muscle at the neuromuscular junction

• Neurotransmitter (Acetylcholine) is released into the NMJ (via exocytosis)

Stimulus for Contraction cont.

• Acetylcholine diffuses across the NMJ and creates an electrical signal (similar to a nerve impulse) at the motor end-plate (sarcolemma)– The electrical signal is created by the movement

of ions – It must reach a certain strength for contraction to

be stimulated

• The muscle impulse travels over the surface of the skeletal muscle fiber and deep into the muscle fiber by means of the Transverse Tubules– This instigates the process of muscle contraction

Excitation Contraction CouplingBig Picture

• The muscle impulse reaches the sarcoplasmic reticulum, which releases calcium ions into the cytosol

• Calcium binds to troponin, moving tropomyosin and exposing myosin binding sites on actin filament

• Cross-bridges (linkages) form between actin and myosin

• Actin filaments are pulled inward by myosin cross-bridges

• The muscle fiber shortens as contraction occurs

Sliding Filament Theory

• states that muscle contraction involves the sliding movement of the thin filaments (actin) past the thick filaments (myosin)– Resulting in shortening of sarcomeres

• A relaxed muscle cell, overlapping of thick and thin filaments is only slight

• Changes in muscle during contraction: – The distance between the Z-lines of the

sarcomeres decreases– The I-Bands (light bands) shorten

• The A-Bands move closer together, but do not diminish in length.

When a skeletal muscle contracts

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A band

Sacromere

Z line Z line

Actinfilaments

Myosinfilaments

1 Relaxed

When a skeletal muscle contracts

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A band

Sacromere

Z line Z line

Actinfilaments

Myosinfilaments

1 Relaxed

2 Contracting

When a skeletal muscle contracts

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A band

Sacromere

Z line Z line

Actinfilaments

Myosinfilaments

1 Relaxed

2 Contracting

3 Fully contracted

Cross-bridge Cycling• when calcium ions are present, the myosin

binding sites on actin are exposed• myosin cross-bridge attaches to actin binding

site • myosin cross-bridge pulls thin filament• ADP and phosphate released from myosin• new ATP binds to myosin

Cross-Bridge Cycling cont.

• linkage between actin and myosin cross-bridge break

• ATP splits• myosin cross-bridge goes back to original

position

* As long as calcium ions and ATP are present, this walking continues until the muscle fiber is fully contracted

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Tropomyosin

Troponin complex

Actin monomers

Actin filament

Myosin filament

ADP + P ADP + P

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Tropomyosin

Troponin complex

Actin monomers

Actin filament

Myosin filament

Ca+2

Muscle contractionRelease of Ca+2 from sarcoplasmic reticulum exposes binding sites on thin filament:

Ca+2 binds to troponin complex

Tropomyosin pulled aside

Binding sites on actin filament exposed

1 Exposed binding sites on actin allow the muscle contraction cycle to occur

Ca+2 Ca+2 Ca+2

ADP + P ADP + P

ADP + P ADP + P

Ca+2 Ca+2 Ca+2

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2 Cross-bridge binds actin to myosin

ADP + P ADP + P

ADP + P ADP + P

Contraction cycle

P PADP ADP

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3 Cross-bridge pulls actin filament (power stroke), ADP and P released from myosin

ADP + P

ADP + P ADP + P

Contraction cycle

Contraction cycle

P PADP ADP

ATP ATP ATP

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ATP

4 New ATP binds to myosin, causing linkage to release

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ATP ATP ATP

5 ATP splits, whichprovides power to“cock” the myosincross-bridge

ADP + P ADP + P

Contraction cycle

Ca+2 Ca+2 Ca+2

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ADP + P ADP + P

ADP + P ADP + P

Contraction cycle

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Ca+2 Ca+2 Ca+2

Ca+2

ATP

Active transport of Ca+2 into sarcoplasmic reticulum, which requires ATP, makes myosin binding sites unavailable.

Muscle relaxation

ADP + P ADP + P

ADP + P ADP + P

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1 Exposed binding sites on actin allow the muscle contraction cycle to occur

Ca+2 Ca+2 Ca+2

ADP + P ADP + P

Contraction cycle

2 Cross-bridge binds actin to myosin

ADP + P ADP + P

P PADP ADP

3

ADP + P

Cross-bridge pulls actin filament (power stroke), ADP and P released from myosin

ATP ATP ATP

ATP

4 New ATP binds to myosin, causing linkage to release

ADP + P ADP + P

5 ATP splits, whichprovides power to“cock” the myosincross-bridge

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Tropomyosin

Troponin complex

Actin monomers

Actin filament

Myosin filament

ADP + P ADP + P

Ca+2

Muscle contractionRelease of Ca+2 from sarcoplasmic reticulum exposes binding sites on thin filament:

Ca+2 binds to troponin complex

Tropomyosin pulled aside

Binding sites on actin filament exposed

1

Ca+2 Ca+2 Ca+2

ADP + P ADP + P

Ca+2

ATP

Active transport of Ca+2 into sarcoplasmic reticulum, which requires ATP, makes myosin binding sites unavailable.

Muscle relaxation

Relaxation• Acetylcholinesterase is an enzyme present

in the NMJ• It immediately destroys acetylcholine, so the

motor end-plate is no longer stimulated – i.e. it cannot cause continuous muscle

contraction

• Calcium ions are transported from sarcoplasm back into sarcoplasmic reticulum

• Linkages between actin and myosin are broken

• The muscle fiber relaxes

Energy Sources for Contraction

The energy used to power the interaction between actin and

myosin comes from ATP

Introduction

• ATP stored in skeletal muscle lasts only about six seconds

• ATP must be regenerated continuously if contraction is to continue

• There are three pathways in which ATP is regenerated:– Coupled Reaction with Creatine Phosphate – Anaerobic Cellular Respiration (Ch. 4)– Aerobic Cellular Respiration (Ch. 4)

Coupled Reaction with Creatine Phosphate (CP)

• CP + ADP <------> creatine + ATP

• Muscle stores a lot of CP

• This coupling reaction allows for about 10 seconds worth of ATP

Oxygen Supply and Cellular Respiration

• Anaerobic Respiration– Steps are called glycolysis– Steps occur in the cytoplasm of the cell– Results in production of pyruvic acid and 2 ATP

• Aerobic Respiration (bet you thought you were done with this!)– Steps are called citric acid cycle and electron

transport chain– Oxygen is required– Steps occur in the mitochondrion of the cell– Results in CO2, water and 36ATP

Muscle Fatigue

• Muscle fatigue is a state of physiological inability to contract

• If no oxygen is available in muscle cells to complete aerobic respiration, pyruvic acid is converted to lactic acid, which causes muscle fatigue and soreness

• Results from a relative deficit of ATP and/or accumulation of lactic acid (which decreases pH)

Oxygen Debt

• The oxygen debt is the amount of oxygen necessary to support the conversion of lactic acid to glycogen

• needed to replenish spent glycogen stores

• oxygen not available– glycolysis continues– pyruvic acid converted to lactic acid– liver converts lactic acid to glucose

Heat Production

• Almost half of the energy released during muscle contraction is lost to heat, which helps maintain our body temperature at 37o C

• Excessive heat is lost through many negative feedback mechanisms (discussed in chapter 1) including sweating, dilation of superficial blood vessels, increased breathing rate, and increased heart rate

Muscle Responses

Threshold Stimulus

• The minimal strength of stimulation required to cause contraction

• A skeletal muscle fiber’s resting membrane potential must be depolarized from –100mV to –70mv before an impulse begins

• Therefore the threshold stimulus is +30mV

Recording a Muscle Contraction

• A myogram is a recording of a muscle contraction

• A twitch is a single contraction that lasts a fraction of a second, followed by relaxation

• The delay between stimulation and contraction is called the latent period

• A muscle fiber must return to its resting state (-100mV) before it can be stimulated again

– This is called the refractory period

All-or-Nothing Response• If a muscle fiber is brought to threshold or above, it

responds with a complete twitch• If the stimulus is sub-threshold, the muscle fiber will

not respond

Summation• When several stimuli are delivered in succession to

a muscle fiber, it cannot completely relax between contractions

– The individual twitches begin to combine and the muscle contraction becomes sustained

• In a sustained contraction, the force of individual twitches combines in a process called summation

– can lead to tetanic contractions

Tetanus• When the resulting sustained contraction lacks even

slight relaxation, it is called titanic contraction• Incomplete tetanus=the state of a muscle at maximum

tension that is not allowed to relax completely • Complete tetanus=the muscle is at maximum tension

and there is no relaxation phase at all

Motor Units

• Definition: A motor unit is a motor neuron and the many skeletal muscle fibers it stimulates

• Because the motor neuron branches into several motor nerve endings, it can stimulate many skeletal muscles fibers simultaneously, which then contract simultaneously

• The number of muscle fibers in a motor unit varies from 10-hundreds

Recruitment

• A muscle is composed of many motor units, controlled by many different motor neurons

• recruitment - increase in the number of motor units activated

• more precise movements are produced with fewer muscle fibers within a motor unit

• as intensity of stimulation increases, recruitment of motor units continues until all motor units are activated

Muscle Tone

• Sustained Contractions– Even when a muscle is at rest, a certain

amount of sustained contraction is occurring in its fibers. This is called muscle tone.

• Muscle tone is very important in maintaining posture

Types of Contractions

• isotonic – muscle contracts and changes length• concentric – shortening contraction• eccentric – lengthening contraction

• isometric – muscle contracts but does not change length

Fast and Slow Muscle Fibers

• Muscle fibers vary in contraction speed (i.e. slow or fast twitch)

• Slow-Twitch Fibers are also called red fibers– contain oxygen carrying pigment, myoglobin, receive a

rich blood supply, and contain many mitochondria– can generate ATP fast enough to keep up with breakdown– These fibers contract for long periods without fatiguing

• Fast-twitch fibers are also called white fibers– contain less myoglobin, blood, and fewer mitochondria.– contain extensive sarcoplasmic reticulum to store and

reabsorb calcium– These fibers contract rapidly, but fatigue easily due to

lactic acid accumulation

Smooth Muscle

The contraction mechanism of smooth muscle is similar to that of skeletal muscle

in that interaction occurs between actin and myosin, however the transverse tubules

and sarcoplasmic reticula are greatly reduced, and troponin is absent.

Smooth Muscle Fibers

Multi-unit smooth muscle

• location– irises of eyes– walls of blood vessels

• Contraction is rapid and vigorous (similar to skeletal muscle tissue)

• less organized

• function as separate units – fibers function separately

Smooth Muscle Fibers cont.Visceral smooth muscle • Location = the walls of hollow organs• Contraction is slow and sustained

– Rhythmicity = pattern of repeated contractions– Peristalsis = wave-like motion that helps push

substances through passageways

• Structure:– single-unit smooth muscle– sheets of muscle fibers– fibers held together by gap junctions– random arrangement of actin and myosin filaments– Two layers of muscle surround the passageway

• inner circular layer• outer longitudinal layer

Smooth Muscle Contraction• A protein, calmodulin binds to calcium ions (no

troponin) and activates the contraction mechanism• Most calcium diffuses in to smooth muscle cells from

the extracellular fluid (reduced SR).• Norepinephrine and acetylcholine are smooth

muscle neurotransmitters• Smooth muscle slower to contract and relax• Smooth muscle more resistant to fatigue• Stretching can trigger smooth muscle contraction• Smooth muscle can change length without changing

tautness

CARDIAC MUSCLE

Will be studied in greater detail in Chapter 15

Cardiac Muscle

• Location=the heart• Anatomy:

– Striated uninuclear cells joined end-to-end forming a network

– Cell junctions are called intercalated discs

• Arrangement of actin and myosin not as organized as skeletal muscle

• Contains sarcoplasmic reticula, transverse tubules, and numerous mitochondria

• Sarcoplasmic reticulum is less developed than SR in skeletal muscle and stores much less calcium

Cardiac Muscle cont.

Physiology

• Self-exciting tissue (i.e. “Pacemaker”)

• Rhythmic contractions (60-100 beats/minute)

• Involuntary, all-or-nothing contractions

• Pumps blood to:– Lungs for oxygenation– Body for distribution of oxygen and nutrients

SKELETAL MUSCLE ACTIONS

Skeletal muscles generate a great variety of body movements. The action of a muscle primarily depends

upon the joint associated with it and the manner in which the muscle is attached on either side of that joint

Origin and Insertion

• Recall that skeletal muscles are usually attached to a fixed body part and a movable body part:– The origin of a muscle is its immovable

(anchored) end– The insertion of a muscle is the movable end of a

muscle

• When a muscle contracts and shortens, its insertion is pulled toward its origin

Skeletal Muscle Actions• Flexion = decreasing the

angle between 2 bones– Dorsiflexion = decreasing the

angle between the foot and shin

– Plantar flexion = pointing toes• Extension = increasing the

angle between 2 bones• Abduction = moving a

body part away from the midline

• Adduction = moving a body part toward the midline

• Circumduction = movement in a circular (cone-shaped) motion

• Rotation = turning movement of a bone about its long axis– (i.e. atlas/axis)

• Supination = thumbs up• Pronation = thumbs down• Inversion = sole of foot in• Eversion = sole of foot out• Elevation = lifting a body

part– (i.e. shoulder shrug)

• Depression = returning a body part to pre-elevated position

Interactions of Skeletal Muscles

• Prime Mover (agonist) = the primary muscle responsible for a movement– The biceps brachii in flexing the arm at the elbow

• Antagonist(s) = the muscle(s) in opposition to the action of the prime mover. The antagonist relaxes (or stretches) during the prime movement– The triceps brachii is the antagonist of the biceps brachii when we flex

the arm at the elbow• Synergist(s) = muscles that assist the prime mover

– The brachialis helps the biceps brachii during elbow flexion• Fixators = muscle groups that stabilize the origin of the

prime mover (i.e. hold it in place) so that the prime mover can act more efficiently – The scapula is the origin for many arm muscles, but it must be held in

place by fixator muscles in order to function in this way• serratus anterior• pectoralis minor

LIFE SPAN CHANGES

• Supplies of ATP, myoglobin, and creatine phosphate in muscle fibers begin to decline in one’s forties

• Half of one’s muscle mass has been replaced by connective and adipose tissue by age 80, and reflexes are reduced

• Exercise is the best way to maintain muscle function