Three basic muscle types are found in the body
Skeletal muscle \n Cardiac muscle \n 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
- Prefixes myo- and mys- refer to “muscle”
- Prefix sarco- refers to “flesh”
- Skeletal muscle
- Most skeletal muscle fibers are attached by tendons to bones
- Skeletal muscle cells are large, cigar-shaped, and multinucleate
- Also known as striated muscle because of its obvious stripes
- Also known as voluntary muscle because it is the only muscle tissue subject to conscious control
- Skeletal muscle cells are surrounded and bundled by connective tissue
- Endomysium—encloses a single muscle fiber (cell)
- Perimysium—wraps around a fascicle (bundle) of muscle fibers
- Epimysium—covers the entire skeletal muscle \n blends into a connective tissue attachment
- Fascia—on the outside of the epimysium
- Smooth muscle
- No striations
- Involuntary—no conscious control
- Found mainly in the walls of hollow visceral organs (such as stomach, urinary bladder, respiratory passages)
- Spindle-shaped fibers that are unicleate
- Contractions are slow and sustained
- Cardiac muscle
- Striations
- involuntary
- Found only in the walls of the heart
- Uninucleate
- Branching cells joined by gap junctions called intercalated discs*-*
- Contracts at a steady rate set by pacemaker
Muscle Function
- Whereas all muscle types produce movement, skeletal muscle has three other important roles:
- Maintain posture and body position
- Stabilize joints
- Generate heat
Microscopic Anatomy of Skeletal Muscle
Sarcolemma —specialized plasma membrane
Myofibrils—long organelles inside muscle cell
- Light (I) bands and dark (A) bands give the muscle its striated (banded) appearance
Banding pattern of myofibrils
- I band = light band
- Contains only thin filaments
- Z disc is a midline interruption
- 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

Iband
Aband
Iband
Hzone
Zdisc
Zdisc
Sarcomere—contractile unit of a muscle fiber
- Structural and functional unit of skeletal muscle
Organization of the sarcomere
- Myofilaments produce banding(stripped pattern)
- Thick filaments = myosin filaments
- Thin filaments = actin filaments
Thick filaments = myosin filaments
- Composed of the protein myosin
- Contain ATPase enzymes to split ATP to release energy for muscle contractions
- Possess projections known as myosin heads
- Myosin heads are known as cross bridges when they link thick and thin filaments during contraction
Thin filaments = actin filaments
- Composed of the contractile protein actin
- 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

Sacromere
Zdisc
Mline
Zdisc
Thin Actin
Thick Myosin
- Sarcoplasmic reticulum (SR)
- Specialized smooth endoplasmic reticulum
- Surrounds the myofibril
- Stores and releases calcium
Stimulation and Contraction of Single Muscle Cell
- Special functional properties of skeletal muscles
- Irritability (responsiveness)—ability to receive and respond to a stimulus
- Contractibility—ability to forcibly shorten when an adequate stimulus is received
- Extensibility—ability of muscle cells to be stretched
- Elasticity—ability to recoil and resume resting length after stretching
Nerve Stimulus and Action Potential
- Skeletal muscles must be stimulated by a motor neuron (nerve cell) to contract
- Motor unit—one motor neuron and the skeletal muscle cells stimulated by that neuron
- Neuromuscular junction
- Association site of axon terminal of the motor neuron and sarcolemma of a muscle
- Neurotransmitter
- Chemical released by nerve upon arrival of nerve impulse in the axon terminal
- Acetylcholine (ACh) the neurotransmitter that stimulates skeletal muscle
- Synaptic cleft
- Gap between nerve and muscle, filled with interstitial fluid
- Although very close, the nerve and muscle do not make contact
The Nerve Stimulus and Action Potential
Mechanism of Muscle Contraction: Sliding Filament
- What causes filaments to slide?
- Calcium ions (Ca2+) bind regulatory proteins on thin filaments and expose myosin-binding sites, allowing the myosin heads on the thick filaments to attach
- Each cross bridge pivots, causing the thin filaments to slide toward the center of the sarcomere
- Contraction occurs, and the cell shortens
- During a contraction, a cross bridge attaches and detaches several times
- ATP provides the energy for the sliding process, which continues as long as calcium ions are present