Chapter 6

The Muscular System

The Muscular System

 Muscles are responsible for all types of body movement
 Three basic muscle types are found in the body
1. Skeletal muscle
2. Cardiac muscle
3. Smooth muscle

Muscle Types
 Skeletal and smooth muscle cells are elongated (muscle cell = muscle fiber)
 Contraction and shortening of muscles are due to the movement of
microfilaments
 All muscles share some terminology
o Prefixes myo- and mys- refer to “muscle”
o Prefix sarco- refers to “flesh”

Muscle Types
 Skeletal muscle
o Most skeletal muscle fibers are attached by tendons to bones
o Skeletal muscle cells are large, cigar-shaped, and multinucleate
o Also known as striated muscle because of its obvious stripes
o Also known as voluntary muscle because it is the only muscle tissue subject
to conscious control

Muscle Types
 Skeletal muscle cells are surrounded and bundled by connective tissue
o Endomysium—encloses a single muscle fiber
o Perimysium—wraps around a fascicle (bundle) of muscle fibers
o Epimysium—covers the entire skeletal muscle
o Fascia—on the outside of the epimysium

Muscle Types
 The epimysium of skeletal muscle blends into a connective tissue attachment
o Tendons—cordlike structures
 Mostly collagen fibers
 Often cross a joint because of their toughness and small size
o Aponeuroses—sheetlike structures
 Attach muscles indirectly to bones, cartilages, or connective tissue coverings

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Muscle Types
 Smooth muscle
o No striations
o Involuntary—no conscious control
o Found mainly in the walls of hollow visceral organs (such as stomach, urinary
bladder, respiratory passages)
o Spindle-shaped fibers that are uninucleate
o Contractions are slow and sustained

Muscle Types
 Cardiac muscle
o Striations
o Involuntary
o Found only in the walls of the heart
o Uninucleate
o Branching cells joined by gap junctions called intercalated discs
o Contracts at a steady rate set by pacemaker

Muscle Functions
 Whereas all muscle types produce movement, skeletal muscle has three other
important roles:
o Maintain posture and body position
o Stabilize joints
o Generate heat

Microscopic Anatomy of Skeletal Muscle

 Sarcolemma—specialized plasma membrane
 Myofibrils—long organelles inside muscle cell
o Light (I) bands and dark (A) bands give the muscle its striated (banded)
appearance

Microscopic Anatomy of Skeletal Muscle

 Banding pattern of myofibrils
o I band = light band
 Contains only thin filaments
 Z disc is a midline interruption
o A band = dark band
 Contains the entire length of the thick filaments
 H zone is a lighter central area
 M line is in center of H zone

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Microscopic Anatomy of Skeletal Muscle
 Sarcomere—contractile unit of a muscle fiber
o Structural and functional unit of skeletal muscle
 Organization of the sarcomere
o Myofilaments produce banding (striped) pattern
 Thick filaments = myosin filaments
 Thin filaments = actin filaments

Microscopic Anatomy of Skeletal Muscle

 Thick filaments = myosin filaments
o Composed of the protein myosin
o Contain ATPase enzymes to split ATP to release energy for muscle
contractions
o Possess projections known as myosin heads
o Myosin heads are known as cross bridges when they link thick and thin
filaments during contraction

Microscopic Anatomy of Skeletal Muscle

 Thin filaments = actin filaments
o Composed of the contractile protein actin
o Actin is anchored to the Z disc
 At rest, within the A band there is a zone that lacks actin filaments called the H
zone
 During contraction, H zones disappear as actin and myosin filaments overlap

Microscopic Anatomy of Skeletal Muscle

 Sarcoplasmic reticulum (SR)
o Specialized smooth endoplasmic reticulum
o Surrounds the myofibril
o Stores and releases calcium

Stimulation and Contraction of Single Skeletal Muscle Cells

 Special functional properties of skeletal muscles
o Irritability (also called responsiveness)—ability to receive and respond to a
stimulus
o Contractility—ability to forcibly shorten when an adequate stimulus is received
o Extensibility—ability of muscle cells to be stretched
o Elasticity—ability to recoil and resume resting length after stretching

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The Nerve Stimulus and Action Potential
 Skeletal muscles must be stimulated by a motor neuron (nerve cell) to contract
 Motor unit—one motor neuron and all the skeletal muscle cells stimulated by that
neuron

The Nerve Stimulus and Action Potential

 Neuromuscular junction
o Association site of axon terminal of the motor neuron and sarcolemma of a
muscle
 Neurotransmitter
o Chemical released by nerve upon arrival of nerve impulse in the axon
terminal
o Acetylcholine (ACh) is the neurotransmitter that stimulates skeletal muscle

The Nerve Stimulus and Action Potential

 Synaptic cleft
o Gap between nerve and muscle filled with interstitial fluid
o Although very close, the nerve and muscle do not make contact

The Nerve Stimulus and Action Potential

 When a nerve impulse reaches the axon terminal of the motor neuron,
Step 1: Calcium channels open, and calcium ions enter the axon terminal
Step 2: Calcium ion entry causes some synaptic vesicles to release
acetylcholine (ACh)
Step 3: ACh diffuses across the synaptic cleft and attaches to receptors on the
sarcolemma of the muscle cell

The Nerve Stimulus and Action Potential

Step 4: If enough ACh is released, the sarcolemma becomes temporarily more
permeable to sodium ions (Na+)
o Potassium ions (K+) diffuse out of the cell
o More sodium ions enter than potassium ions leave
o Establishes an imbalance in which interior has more positive ions
(depolarization), thereby opening more Na + channels

The Nerve Stimulus and Action Potential

Step 5: Depolarization opens more sodium channels that allow sodium ions to
enter the cell
o An action potential is created
o Once begun, the action potential is unstoppable
o Conducts the electrical impulse from one end of the cell to the other

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 Step 6: Acetylcholinesterase (AChE) breaks down acetylcholine into acetic acid
and choline
o AChE ends muscle contraction
o A single nerve impulse produces only one contraction

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The Nerve Stimulus and Action Potential
 Cell returns to its resting state when:
1. Potassium ions (K+) diffuse out of the cell
2. Sodium-potassium pump moves sodium and potassium ions back to their
original positions

Mechanism of Muscle Contraction: The Sliding Filament Theory

 What causes filaments to slide?
o Calcium ions (Ca2+) bind regulatory proteins on thin filaments and expose
myosin-binding sites, allowing the myosin heads on the thick filaments to
attach
o Each cross bridge pivots, causing the thin filaments to slide toward the center
of the sarcomere
o Contraction occurs, and the cell shortens
o During a contraction, a cross bridge attaches and detaches several times
o ATP provides the energy for the sliding process, which continues as long as
calcium ions are present

Contraction of a Skeletal Muscle as a Whole

 Graded responses
o Muscle fiber contraction is “all-or-none,” meaning it will contract to its fullest
when stimulated adequately
o Within a whole skeletal muscle, not all fibers may be stimulated during the
same interval
o Different combinations of muscle fiber contractions may give differing
responses
o Graded responses—different degrees of skeletal muscle shortening

Contraction of a Skeletal Muscle as a Whole

 Graded responses can be produced in two ways
o By changing the frequency of muscle stimulation
o By changing the number of muscle cells being stimulated at one time

Contraction of a Skeletal Muscle as a Whole

 Muscle response to increasingly rapid stimulation
o Muscle twitch
 Single, brief, jerky contraction
 Not a normal muscle function

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Contraction of a Skeletal Muscle as a Whole
 Muscle response to increasingly rapid stimulation (continued)
o In most types of muscle activity, nerve impulses are delivered at a rapid rate
o As a result, contractions are “summed” (added) together, and one contraction
is immediately followed by another

Contraction of a Skeletal Muscle as a Whole

 Muscle response to increasingly rapid stimulation (continued)
o When stimulations become more frequent, muscle contractions get stronger
and smoother
o The muscle now exhibits unfused (incomplete) tetanus

Contraction of a Skeletal Muscle as a Whole

 Muscle response to increasingly rapid stimulation (continued)
o Fused (complete) tetanus is achieved when the muscle is stimulated so
rapidly that no evidence of relaxation is seen
o Contractions are smooth and sustained

Contraction of a Skeletal Muscle as a Whole

 Muscle response to stronger stimuli
o Muscle force depends upon the number of fibers stimulated
o Contraction of more fibers results in greater muscle tension
o When all motor units are active and stimulated, the muscle contraction is as
strong as it can get

Providing Energy for Muscle Contraction

 ATP
o Only energy source that can be used to directly power muscle contraction
o Stored in muscle fibers in small amounts that are quickly used up
o After this initial time, other pathways must be utilized to produce ATP

Providing Energy for Muscle Contraction

 Three pathways to regenerate ATP
1. Direct phosphorylation of ADP by creatine phosphate
2. Aerobic pathway
3. Anaerobic glycolysis and lactic acid formation

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Providing Energy for Muscle Contraction
 Direct phosphorylation of ADP by creatine phosphate (CP)—fastest
o Muscle cells store CP, a high-energy molecule
o After ATP is depleted, ADP remains
o CP transfers a phosphate group to ADP to regenerate ATP
o CP supplies are exhausted in less than 15 seconds
o 1 ATP is produced per CP molecule

Providing Energy for Muscle Contraction

 Aerobic respiration
o Supplies ATP at rest and during light/moderate exercise
o A series of metabolic pathways, called oxidative phosphorylation, use oxygen
and occur in the mitochondria
o Glucose is broken down to carbon dioxide and water, releasing energy (about
32 ATP)
o This is a slower reaction that requires continuous delivery of oxygen and
nutrients

Providing Energy for Muscle Contraction

 Anaerobic glycolysis and lactic acid formation
o Reaction that breaks down glucose without oxygen
o Glucose is broken down to pyruvic acid to produce about 2 ATP
o Pyruvic acid is converted to lactic acid, which causes muscle soreness
o This reaction is not as efficient, but it is fast
o Huge amounts of glucose are needed

Muscle Fatigue and Oxygen Deficit

 If muscle activity is strenuous and prolonged, muscle fatigue occurs
 Suspected factors that contribute to muscle fatigue include:
o Ion imbalances (Ca2+, K+)
o Oxygen deficit and lactic acid accumulation
o Decrease in energy (ATP) supply
 After exercise, the oxygen deficit is repaid by rapid, deep breathing

Types of Muscle Contractions

 Isotonic contractions
o Myofilaments are able to slide past each other during contractions
o The muscle shortens, and movement occurs
o Example: bending the knee; lifting weights, smiling

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 Isometric contractions
o Muscle filaments are trying to slide, but the muscle is pitted against an
immovable object
o Tension increases, but muscles do not shorten
o Example: pushing your palms together in front of you

Muscle Tone
 Muscle tone
o State of continuous partial contractions
o Result of different motor units being stimulated in a systematic way
o Muscle remains firm, healthy, and constantly ready for action

Effect of Exercise on Muscles

 Exercise increases muscle size, strength, and endurance
o Aerobic (endurance) exercise (biking, jogging) results in stronger, more
flexible muscles with greater resistance to fatigue
 Makes body metabolism more efficient
 Improves digestion, coordination
o Resistance (isometric) exercise (weight lifting) increases muscle size and
strength
 Individual muscle fibers enlarge

Muscle Movements, Roles, and Names

 Follow the Five Golden Rules for understanding skeletal muscle activity (in Table
6.2, shown next)

Types of Body Movements

 Muscles are attached to no fewer than two points
1. Origin: attachment to an immovable or less movable bone
2. Insertion: attachment to a movable bone
 When the muscle contracts, the insertion moves toward the origin
 Body movement occurs when muscles contract across joints

Types of Body Movements

 Flexion
o Decreases the angle of the joint
o Brings two bones closer together
o Typical of bending hinge joints (e.g., knee and elbow) or ball-and-socket joints
(e.g., the hip)
 Extension
o Opposite of flexion

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 o Increases angle between two bones
o Typical of straightening the elbow or knee
o Extension beyond 180º is hyperextension

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Types of Body Movements
 Rotation
o Movement of a bone around its longitudinal axis
o Common in ball-and-socket joints
o Example: moving the atlas around the dens of axis (i.e., shaking your head
“no”)

Types of Body Movements

 Abduction
o Movement of a limb away from the midline
 Adduction
o Opposite of abduction
o Movement of a limb toward the midline

Types of Body Movements

 Circumduction
o Combination of flexion, extension, abduction, and adduction
o Common in ball-and-socket joints
o Proximal end of bone is stationary, and distal end moves in a circle

Special Movements
 Dorsiflexion
o Lifting the foot so that the superior surface approaches the shin (toward the
dorsum)
 Plantar flexion
o Pointing the toes away from the head

Special Movements
 Inversion
o Turning sole of foot medially
 Eversion
o Turning sole of foot laterally

Special Movements
 Supination
o Forearm rotates laterally so palm faces anteriorly
o Radius and ulna are parallel
 Pronation
o Forearm rotates medially so palm faces posteriorly

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 o Radius and ulna cross each other like an X

Special Movements
 Opposition
o Moving the thumb to touch the tips of other fingers on the same hand

Interactions of Skeletal Muscles in the Body

 Muscles can only pull as they contract—not push
 In general, groups of muscles that produce opposite actions lie on opposite sides
of a joint

Interactions of Skeletal Muscles in the Body

 Prime mover—muscle with the major responsibility for a certain movement
 Antagonist—muscle that opposes or reverses a prime mover
 Synergist—muscle that aids a prime mover in a movement or reduces
undesirable movements
 Fixator—specialized synergists that hold a bone still or stabilize the origin of a
prime mover

Naming Skeletal Muscles

 Muscles are named on the basis of several criteria
o By direction of muscle fibers
 Example: rectus (straight)
o By relative size of the muscle
 Example: maximus (largest)

Naming Skeletal Muscles

 Muscles are named on the basis of several criteria (continued)
o By location of the muscle
 Example: temporalis (temporal bone)
o By number of origins
 Example: triceps (three heads)

Naming Skeletal Muscles

 Muscles are named on the basis of several criteria (continued)
o By location of the muscle’s origin and insertion
 Example: sterno (on the sternum)
o By shape of the muscle
 Example: deltoid (triangular)
o By action of the muscle

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 Example: flexor and extensor (flexes or extends a bone)

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 Muscles of Trunk, Shoulder, Arm

Developmental Aspects of the Muscular System

 Increasing muscular control reflects the maturation of the nervous system
 Muscle control is achieved in a superior/inferior and proximal/distal direction

Developmental Aspects of the Muscular System

 To remain healthy, muscles must be exercised regularly
 Without exercise, muscles atrophy
 With extremely vigorous exercise, muscles hypertrophy

Developmental Aspects of the Muscular System

 As we age, muscle mass decreases, and muscles become more sinewy
 Exercise helps retain muscle mass and strength

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