A P Chapter 8 Notes The Muscular System Essay

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The Muscular System
Muscles are responsible for body movement
There are 3 basic types of muscles: skeletal, cardiac, and smooth

A. Function of Muscles
1. Produce movement
2. Maintain posture
3. Stabilize joints
4. Generate heat

B. Characteristics of Muscles
Are elongated (a muscle cell = a muscle fiber)
Contraction of muscles is due to the movement of microfilaments
All share the same terminology
Myo- = mucle
Mys- = muscle
Sarco- = flesh
Most attached by tendons to bones
Cells are multinucleated
Striated: have visible bonding
Voluntary: subject to conscious control
Cells are surrounded and bundled by connective tissue = great force, but tires easily

C. Connective Tissue Wrappings of Skeletal Muscle
Endomysium: around single muscle fiber
Perimysium: around a fascicle (bundle) of fibers
Epimysium: covers entire skeletal muscle
Fascia: on the outside of the epimysium

D. 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

E. Smooth Muscle Characteristics
Has no striations
Spindle-shaped cells
Single nucleus
Involuntary
Found mainly in the walls of hollow organs
Slow, sustained and tireless

F. Cardiac Muscle Characteristics
Has striations
Usually a single nucleus
Joined to another muscle cell at an intercalated disc
Involuntary
Found only in the heart
Steady pace

G. Microscopic Anatomy of Skeletal Muscle
Cells are multinucleated
Nuclei are just beneath the sarcolemma
Sarcolemma: specialized plasma membrane
Sarcoplasmic reticulum: specialized smooth endoplasmic reticulum
Myofibril: bundles of microfilaments
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 filament = myosin filaments
Composed of the protein myosin
Has ATPase enzymes
Thin filament = actin filaments
Composed of the protein actin
Myosin filaments have heads (extensions, cross bridges)
Myosin and actin overlap somewhat

H. Properties of Skeletal Muscle Activity (single cells or fibers)
Irritability: ability to receive and respond to a stimulus
Contractibility: ability to shorten when an adequate stimulus is received

I. Nerve Stimulus to Muscles
Skeletal muscles must be stimulated by a nerve to contract (motor neuron)
Motor unit
One neuron
Muscle cells stimulated by that neuron
Neuromuscular junctions: association site of nerve and muscle
Synaptic cleft: gap between nerve and muscle
Nerve and muscle do not make contact
Are between nerve and muscle is filled with interstitial fluid

J. Transmission of Nerve Impulse to Muscle
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+)
Na+ rushing into the cell generates an action potential
Once started, muscle contraction cannot be stopped

K. The Sliding Filament Theory of Muscle Contraction
Activation by nerve causes myosin heads (cross bridges) to attach binding sites of the thin filament
Myosin heads then bind to the next side of the filament
This continued action causes a sliding of the myosin along the actin
The result is that the muscle is contracted

L. Contraction of a Skeletal Muscle
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

M. Muscle Response to Strong Stimuli
Muscle force depends upon the number of fibers stimulated
More fibers contracting results in greater muscle tension
Muscle can continue to contract unless they run out of energy

N. Energy for Muscle Contraction
Initially, muscle used stored
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