Ch. 6 The Muscular System NOTES Page | 1 The Muscular System

Ch. 6 The Muscular System NOTES
The Muscular System

Muscles are responsible for all types of body movement

Three basic muscle types are found in the body
o
Skeletal muscle
o
Cardiac muscle
o
Smooth muscle
Characteristics of Muscles
o
Muscle cells are elongated (muscle cell = muscle
fiber)
o
Contraction of muscles is due to the movement of
microfilaments
o

All muscles share some terminology

Prefix myo refers to muscle

Prefix mys refers to muscle

Prefix sarco refers to flesh
Skeletal Muscle Characteristics:
o
Most are attached by tendons to bones
o
Cells are multinucleate
o
Striated – have visible banding
o
Voluntary – subject to conscious control
o
Cells are surrounded and bundled by connective
tissue
o
o
Connective tissue wrappings of skeletal muscle:

Endomysium – around single muscle fiber

Perimysium – around a fascicle (bundle) of fibers

Epimysium – covers the entire skeletal muscle

Fascia – on the outside of the epimysium
Skeletal muscle attachments:


Epimysium blends into a connective tissue attachment

Tendon – cord-like structure

Aponeuroses – sheet-like structure
Sites of muscle attachment

Bones

Cartilages

Connective tissue coverings
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
Smooth Muscle Characteristics:
o
Has no striations
o
Spindle-shaped cells
o
Single nucleus
o
Involuntary – no conscious control
o
Found mainly in the walls of hollow organs

Cardiac Muscle Characteristics:
o
Has striations
o
Usually has a single nucleus
o
Joined to another muscle cell at an intercalated disc
o
Involuntary
o
Found only in the heart
Function of Muscles:
 Produce movement

Maintain posture

Stabilize joints

Generate heat
Microscopic Anatomy of Skeletal Muscle:

Cells are multinucleate

Nuclei are just beneath the sarcolemma
o
Sarcolemma – specialized plasma membrane
o
Sarcoplasmic reticulum – specialized smooth
endoplasmic reticulum
o
Myofibril -- bundles of myofilaments

Myofibrils are aligned to give distinct
bands


I band = light band

A band = dark band
Sarcomere -- Contractile unit of a
muscle fiber

Organization of the sarcomere:

Thick filaments = myosin
filaments
o
Composed of the protein myosin
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o

Has ATPase enzymes
Thin filaments = actin filaments
o
Composed of the protein
actin

Myosin filaments have heads (extensions, or
cross bridges)

Myosin and actin overlap somewhat

At rest, there is a bare zone that lacks actin
filaments

Sarcoplasmic reticulum (SR) – for storage of
calcium
Properties of Skeletal Muscle Activity:

Irritability – ability to receive and respond to a stimulus

Contractility – ability to shorten when an adequate stimulus is
received

Nerve Stimulus to muscles:
o
Skeletal muscles must be stimulated by a nerve to
contract

Motor unit -- one neuron

o
Muscle cells stimulated by that neuron
Neuromuscular junctions – association site of nerve
and muscle
o
Synaptic cleft – gap between nerve and muscle

Nerve and muscle do not make contact

Area between nerve and muscle is filled with
interstitial fluid
o
Transmission of nerve impulse to muscle:

The neurotransmitter for skeletal muscle is acetylcholine

Neurotransmitter attaches to receptors on the sarcolemma

Sarcolemma becomes permeable to sodium (Na+)

Sodium rushing into the cell generates an action potential

Once started, muscle contraction cannot be stopped
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
The Sliding Filament Theory of Muscle Contraction:
o
Activation by nerve causes myosin heads (crossbridges) to attach to binding sites on the thin
filament

o
Myosin heads then bind to the next site of the thin filament
o
This continued action causes a sliding of the myosin along the actin
o
The result is that the muscle is shortened (contracted)
Contraction of a skeletal muscle:
o
Muscle fiber contraction is “all or none”
o
Within a 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
Types of Graded Responses

Twitch -- single, brief contraction (not a normal muscle function)

Tetanus (summing of contractions) – one contraction is immediately followed by another

o
The muscle does not completely return to a resting state
o
The effects are added
Unfused (incomplete) tetanus – some relaxation occurs between contractions
o

The results are summed
Fused (complete) tetanus – no evidence of relaxation before the following contractions
o
The result is a sustained muscle contraction
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Muscle Response to Strong Stimuli

Muscle force depends upon the number of fibers stimulated

More fibers contracting results in greater muscle tension

Muscles can continue to contract unless they run out of energy
Energy for Muscle Contraction
o
Initially, muscles used stored ATP for energy

Bonds of ATP are broken to release energy

Only 4-6 seconds worth of ATP is stored by muscles
o
After this initial time, other pathways must be utilized to produce ATP
o
Direct phosphorylation

Muscle cells contain creatine phosphate (CP)

o
CP is a high-energy molecule

After ATP is depleted, ADP is left

CP transfers energy to ADP, to regenerate ATP

CP supplies are exhausted in about 20 seconds
Aerobic Respiration

Series of metabolic pathways that occur in the mitochondria

Glucose is broken down to carbon dioxide and water, releasing energy

This is a slower reaction that requires continuous oxygen
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o
o
o
Anaerobic glycolysis

Reaction that breaks down glucose without oxygen

Glucose is broken down to pyruvic acid to produce some ATP

Pyruvic acid is converted to lactic acid
Anaerobic glycolysis (continued)

This reaction is not as efficient, but is fast

Huge amounts of glucose are needed

Lactic acid produces muscle fatigue
Muscle Fatigue & Oxygen Debt

When a muscle is fatigued, it is unable to contract

The common reason for muscle fatigue is oxygen debt

Oxygen must be “repaid” to tissue to remove oxygen debt

Oxygen is required to get rid of accumulated lactic acid

Increasing acidity (from lactic acid) and lack of ATP causes the muscle to contract less
Types of Muscle Contractions



Isotonic contractions
o
Myofilaments are able to slide past each other during contractions
o
The muscle shortens
Isometric contractions
o
Tension in the muscles increases
o
The muscle is unable to shorten
Muscle Tone
o
Some fibers are contracted even in a relaxed muscle
o
Different fibers contract at different times to provide muscle tone
o
The process of stimulating various fibers is under involuntary control
Muscles & Body Movements

Movement is attained due to a muscle moving an attached
bone

Muscles are attached to at least two points
o
Origin – attachment to a moveable bone
o
Insertion – attachment to an immovable bone
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Effects of Exercise on Muscle

Results of increased muscle use
o
Increase in muscle size
o
Increase in muscle strength
o
Increase in muscle efficiency
o
Muscle becomes more fatigue resistant
Types of Ordinary Body Movements

Flexion – brings two bones closer
together

Extension – moving two bones
farther apart
o
hyperextension – an
extension that is greater
than 180º

Rotation – movement of a bone around its
longitudinal axis

Abduction – moving a limb away from the
midline
o
example: fanning your fingers or toes
apart

Circumduction – the proximal end of the limb is
stationary, and its distal end moves in a circle
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Special Movements:

Dorsifelxion – flexing your foot

Plantar flexion – pointing your foot

Inversion – turn the sole of your foot medially

Eversion – turn the sole of your foot laterally

Supination – palms face anteriorly so the radius
and ulna are parallel

Pronation – palms face posteriorly so that the
two bones form an X

Opposition – the saddle joint in your palm
allows your thumb to move independently
Types of Muscles:

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 and helps prevent rotation

Fixator – stabilizes the origin of a prime mover
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Naming of Skeletal Muscles:

Direction of muscle fibers
o

Relative size of the muscle
o

Example: rectus (straight)
Example: maximus (largest)
Location of the muscle
o
Example: many muscles are named
for bones (e.g., temporalis)

Number of origins
o

Location of the muscles origin and insertion
o

Example: sterno (on the sternum)
Shape of the muscle
o

Example: triceps (three heads)
Example: deltoid (triangular)
Action of the muscle
o
Example: flexor and extensor (flexes
or extends a bone)
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Superficial muscles:
 Anterior:

Posterior:
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