Three basic muscle types are found in the body

Skeletal muscle
Cardiac muscle
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
    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

• 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

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

Contraction of Skeletal Muscle as a Whole

• Graded responses

  • Muscle fiber contraction is “all-or-none,” meaning it will contract to its fullest when stimulated adequately

  • Within a whole skeletal muscle, not all fibers may be stimulated during the same

  • Different combinations of muscle fiber contractions may give differing responses

• Graded responses—different degrees of skeletal muscle shortening

• Graded responses can be produced in two ways

  • By changing the frequency of muscle stimulation

  • By changing the number of muscle cells being stimulated at one time

• Muscle response to increasingly rapid stimulation

  • Muscle twitch

    • Single, brief, jerky contraction

    • Not a normal muscle function

  • In most types of muscle activity, nerve impulses are delivered at a rapid rate

  • As a result, contractions are “summed” (added) together, and one contraction is immediately followed by another

  • When stimulations become more frequent, muscle contractions get stronger and smoother

  • The muscle now exhibits unfused (incomplete) tetanus

  • Fused (complete) tetanus is achieved when the muscle is stimulated so rapidly that no evidence of relaxation is seen

  • Contractions are smooth and sustained

• Muscle response to stronger stimuli

  • Muscle force depends upon the number of fibers stimulated

  • Contraction of more fibers results in greater muscle tension

  • When all motor units are active and stimulated, the muscle contraction is as strong as it can get

Energy for Muscle Contraction

• ATP

  • Only energy source that can be used to directly power muscle contraction

  • Stored in muscle fibers in small amounts that are quickly used up

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

• Three pathways to regenerate ATP

  1. Direct phosphorylation of ADP by creatine phosphate (CP)

  2. Aerobic pathway

  3. Anaerobic glycolysis and lactic acid formation

• 1. Direct phosphorylation of ADP by creatine phosphate (CP)—fastest

  • Muscle cells store CP, a high-energy molecule

  • After ATP is depleted, ADP remains

  • CP transfers a phosphate group to ADP to regenerate ATP

  • CP supplies are exhausted in less than 15 seconds

  • 1 ATP is produced per CP molecule

• 2. Aerobic respiration

  • Supplies ATP at rest and during light/moderate exercise

  • A series of metabolic pathways, called oxidative phosphorylation, use oxygen and occur in the mitochondria

  • Glucose is broken down to dioxide and water, releasing energy (about 32 ATP)

  • This is a slower reaction that requires continous delivery of oxygen and nutrients

• 3. Anaerobic glycolysis and lactic acid formation

  • Reaction that breaks down glucose without oxygen

  • Glucose is broken down to pyruvic acid to produce about 2 ATP

  • Pyruvic acid is converted to lactic acid, which causes muscle soreness

  • This reaction is not as efficient, but it is fast

  • Huge amounts of glucose are needed

Types of Muscle Contractions (Isotonic & Isometric)

• Isotonic contractions

  • Myofilaments are able to slide past each other during contractions

  • The muscle shortens, movement occurs

  • Example: bending knee, lifting weights, smiling

• Isometric contractions

  • Muscle filaments are trying to slide, but the muscle is pitted against an immovable object

  • Tension increases, but muscles do not shorten

  • Example: pushing your palms together in front of you

Muscle Tone

• State of continuous partial contractions

• Result of different motor units being stimulated in a systematic way

• Muscle remains firm, healthy, and constantly ready for action

Effect of Exercise on Muscles

• Exercise increases muscle size, strength, and endurance

  • Aerobic (endurance) exercise (jogging) results in stronger, more flexible muscles with greater resistance to fatigue

    • Makes body metabolism more efficient

    • Improves digestion, coordination

  • Resistance (isometric) exercise (weight lifting) increases muscle size & strength

    • Individual muscle fibers enlarge