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The Muscular System

Slide 6.1

Muscles are responsible for all types of body movement – they contract or shorten and are the machine of the body
Three basic muscle types are found in the body

Skeletal muscle
Cardiac muscle
Smooth muscle

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Characteristics of Muscles

Slide 6.2

Muscle cells are elongated �(muscle cell = muscle fiber)
Contraction of muscles is due to the movement of microfilaments
All muscles share some terminology

Prefix myo refers to muscle
Prefix mys refers to muscle
Prefix sarco refers to flesh

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Skeletal Muscle Characteristics

Slide 6.3

Most are attached by tendons to bones
Cells are multinucleate
Striated – have visible banding
Voluntary – subject to conscious control
Cells are surrounded and bundled by connective tissue = great force, but tires easily

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Connective Tissue Wrappings of�Skeletal Muscle

Slide 6.4a

Endomysium – around single muscle fiber
Perimysium – around a fascicle (bundle) of fibers

Figure 6.1

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Connective Tissue Wrappings of�Skeletal Muscle

Slide 6.4b

Epimysium – covers the entire skeletal muscle
Fascia – on the outside of the epimysium

Figure 6.1

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Skeletal Muscle Attachments

Slide 6.5

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

Slide 6.6

Has no striations
Spindle-shaped cells
Single nucleus
Involuntary – no conscious control
Found mainly in the walls of hollow organs
Slow, sustained and tireless

Figure 6.2a

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Cardiac Muscle Characteristics

Slide 6.7

Has striations
Usually has a single nucleus
Joined to another muscle cell at an intercalated disc
Involuntary
Found only in the heart
Steady pace!

Figure 6.2b

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Function of Muscles

Slide 6.8

Produce movement
Maintain posture
Stabilize joints
Generate heat

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Microscopic Anatomy of Skeletal�Muscle

Slide 6.9a

Cells are multinucleate
Nuclei are just beneath the sarcolemma

Figure 6.3a

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Microscopic Anatomy of Skeletal�Muscle

Slide 6.9b

Sarcolemma – specialized plasma membrane
Sarcoplasmic reticulum – specialized smooth endoplasmic reticulum

Figure 6.3a

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Microscopic Anatomy of Skeletal Muscle

Slide 6.10a

Myofibril

Bundles of myofilaments
Myofibrils are aligned to give distrinct bands

I band = �light band
A band = �dark band

Figure 6.3b

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Microscopic Anatomy of Skeletal Muscle

Slide 6.10b

Sarcomere

Contractile unit of a muscle fiber

Figure 6.3b

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Microscopic Anatomy of Skeletal Muscle

Slide 6.11a

Organization of the sarcomere

Thick filaments = myosin filaments

Composed of the protein myosin
Has ATPase enzymes

Figure 6.3c

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Microscopic Anatomy of Skeletal Muscle

Slide 6.11b

Organization of the sarcomere

Thin filaments = actin filaments

Composed of the protein actin

Figure 6.3c

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Microscopic Anatomy of Skeletal Muscle

Slide 6.12a

Myosin filaments have heads (extensions, or cross bridges)
Myosin and �actin overlap �somewhat

Figure 6.3d

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Properties of Skeletal Muscle Activity (single cells or fibers)

Slide 6.13

Irritability – ability to receive and respond to a stimulus
Contractility – ability to shorten when an adequate stimulus is received

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Nerve Stimulus to Muscles

Slide 6.14

Skeletal muscles must be stimulated by a nerve to contract (motor neruron)
Motor unit

One neuron
Muscle cells stimulated by that neuron

Figure 6.4a

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Nerve Stimulus to Muscles

Slide 6.15a

Neuromuscular junctions – association site of nerve and muscle

Figure 6.5b

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Nerve Stimulus to Muscles

Slide 6.15b

Synaptic cleft – gap between nerve and muscle

Nerve and muscle do not make contact
Area between nerve and muscle is filled with interstitial fluid

Figure 6.5b

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Transmission of Nerve Impulse to Muscle

Slide 6.16a

Neurotransmitter – chemical released by nerve upon arrival of nerve impulse

The neurotransmitter for skeletal muscle is acetylcholine

Neurotransmitter attaches to receptors on the sarcolemma
Sarcolemma becomes permeable to sodium (Na⁺)

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Transmission of Nerve Impulse to Muscle

Slide 6.16b

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

Slide 6.17a

Activation by nerve causes myosin heads (crossbridges) to attach to binding sites on the thin filament
Myosin heads then bind to the next site of the thin filament

Figure 6.7

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The Sliding Filament Theory of Muscle Contraction

Slide 6.17b

This continued action causes a sliding of the myosin along the actin
The result is that the muscle is shortened (contracted)

Figure 6.7

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The Sliding Filament Theory

Slide 6.18

Figure 6.8

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Contraction of a Skeletal Muscle

Slide 6.19

Muscle fiber contraction is “all or none”
Within a skeletal muscle, not all fibers may be stimulated during the same interval
Different combinations of muscle fiber contractions may give differing responses
Graded responses – different degrees of skeletal muscle shortening, rapid stimulus = constant contraction or tetanus

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Muscle Response to Strong Stimuli

Slide 6.22

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

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Energy for Muscle Contraction

Slide 6.23

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

After this initial time, other pathways must be utilized to produce ATP

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Energy for Muscle Contraction

Slide 6.24

Direct phosphorylation

Muscle cells contain creatine phosphate (CP)

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

Figure 6.10a

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Energy for Muscle Contraction

Slide 6.26a

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

Figure 6.10b

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Energy for Muscle Contraction

Slide 6.26b

Anaerobic glycolysis (continued)

This reaction is not as efficient, but is fast

Huge amounts of glucose are needed
Lactic acid produces muscle fatigue

Figure 6.10b

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Energy for Muscle Contraction

Slide 6.25

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

Figure 6.10c

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Muscle Fatigue and Oxygen Debt

Slide 6.27

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

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Types of Muscle Contractions

Slide 6.28

Isotonic contractions

Myofilaments are able to slide past each other during contractions
The muscle shortens

Isometric contractions

Tension in the muscles increases
The muscle is unable to shorten

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Muscle Tone

Slide 6.29

Some fibers are contracted even in a relaxed muscle
Different fibers contract at different times to provide muscle tone
The process of stimulating various fibers is under involuntary control

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Muscles and Body Movements

Slide 6.30a

Movement is attained due to a muscle moving an attached bone

Figure 6.12

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Muscles and Body Movements

Slide 6.30b

Muscles are attached to at least two points

Origin – attachment to a immoveable bone
Insertion – attachment to an movable bone

Figure 6.12

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Effects of Exercise on Muscle

Slide 6.31

Results of increased muscle use

Increase in muscle size
Increase in muscle strength
Increase in muscle efficiency
Muscle becomes more fatigue resistant

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Types of Ordinary Body Movements

Slide 6.32

Flexion – decreases angle of joint and brings two bones closer together
Extension- opposite of flexion
Rotation- movement of a bone in longitudinal axis, shaking head “no”
Abduction/Adduction (see slides)
Circumduction (see slides)

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Body Movements

Slide 6.33

Figure 6.13

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Left: Abduction – moving the leg away from the midline

Above – Adduction- moving toward the midline

Right:

Circumduction: cone-shaped movement, proximal end doesn’t move, while distal end moves in a circle.

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Types of Muscles

Slide 6.35

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

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Naming of Skeletal Muscles

Slide 6.36a

Direction of muscle fibers

Example: rectus (straight)

Relative size of the muscle

Example: maximus (largest)

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Naming of Skeletal Muscles

Slide 6.36b

Location of the muscle

Example: many muscles are named for bones (e.g., temporalis)

Number of origins

Example: triceps (three heads)

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Naming of Skeletal Muscles

Slide 6.37

Location of the muscles origin and insertion

Example: sterno (on the sternum)

Shape of the muscle

Example: deltoid (triangular)

Action of the muscle

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

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Head and Neck Muscles

Slide 6.38

Figure 6.14

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Trunk Muscles

Slide 6.39

Figure 6.15

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Deep Trunk and Arm Muscles

Slide 6.40

Figure 6.16

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Muscles of the Pelvis, Hip, and Thigh

Slide 6.41

Figure 6.18c

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Muscles of the Lower Leg

Slide 6.42

Figure 6.19

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Superficial Muscles: Anterior

Slide 6.43

Figure 6.20

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Superficial Muscles: Posterior

Slide 6.44

Figure 6.21

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Disorders relating to the Muscular System

Muscular Dystrophy: inherited, muscle enlarge due to increased fat and connective tissue, but fibers degenerate and atrophy
Duchenne MD: lacking a protein to maintain the sarcolemma
Myasthemia Gravis: progressive weakness due to a shortage of acetylcholine receptors