Muscular system

Muscle

Muscle—a Latin word for “little mouse”

Muscle is the primary tissue in the:

Heart (cardiac muscle tissue)

Walls of hollow organs (smooth muscle tissue)

Skeletal muscle

Makes up nearly half the body’s mass

Properties of Muscle Tissue

Contractility

Myofilaments are responsible for shortening of muscles cells

Actin and myosin are two type of myofilaments

Excitability

Nerve signals excite muscle cells, causing electrical impulses to travel along the sarcolemma

Properties of Muscle Tissue

Extensibility

Contraction of a skeletal muscle stretches the opposing muscle

Smooth muscle is stretched by substances within that hollow organ

Food in stomach; urine in urinary bladder

Elasticity

Recoils after being stretched

Terminology Specific to Muscle Tissue

Myo and mys—prefixes meaning “muscle”

Sarco—prefix meaning “flesh”

Sarcolemma—plasma membrane of muscle cells

Sarcoplasm—cytoplasm of muscle cells

Functions of Muscle Tissue

Produce movement

Skeletal muscle—attached to skeleton

Moves body by moving the bones

Smooth muscle—squeezes fluids and other substances through hollow organs

Functions of Muscle Tissue (continued)

Open and close body passageways

Sphincters function as valves

Open to allow passage of a substance

Contract to close the passageway

Functions of Muscle Tissue (continued)

Maintain posture and stabilize joints

Enables the body to remain sitting or standing

Muscle tone helps stabilize many synovial joints

Heat generation

Muscle contractions produce heat

Helps maintain normal body temperature

When core temperature becomes
too cold, “shivering” occurs to
generate heat

Types of Muscle Tissue

Skeletal muscle tissue

Packaged into skeletal muscles

Makes up 40% of body weight

Cells are striated

Skeletal muscle is innervated by voluntary division of the nervous system

Types of Muscle Tissue

Cardiac muscle tissue

Occurs only in the walls of the heart

Cells are striated

Contraction is involuntary

Smooth muscle tissue

Occupies the walls of hollow organs

Cells lack striations

Innervated by involuntary division of the nervous system

Skeletal Muscle

Each muscle is an organ

Consists mostly of muscle tissue

Skeletal muscle also contains:

Connective tissue

Blood vessels

Nerves

Gross Anatomy of a Skeletal Muscle

Connective tissue and fascicles

Sheaths of connective tissue bind a skeletal muscle and its fibers together

Epimysium—dense irregular connective tissue surrounding entire muscle

Perimysium—surrounds each fascicle
(group of muscle fibers)

Endomysium—a fine sheath of connective tissue wrapping each muscle cell

Gross Anatomy of a Skeletal Muscle

Connective tissue sheaths are continuous with tendons

When muscle fibers contract, pull is exerted on all layers of connective tissue are tendon

Sheaths provide elasticity and carry blood vessels and nerves

Figure 10.1  Connective tissue sheaths in skeletal muscle: epimysium, perimysium and endomysium.

Gross Anatomy of a Skeletal Muscle

Nerves and blood vessels

Each skeletal muscle supplied by branches of

One nerve

One artery

One or more veins

Nerves and vessels branch repeatedly

Smallest branches serve individual muscle fibers

Gross Anatomy of a Skeletal Muscle

Muscle attachments

Most skeletal muscles run from one bone to another

One bone will move; other bone remains fixed

Origin—less movable attachment

Insertion—more movable attachment

Figure 10.3  Muscle attachments (origin and insertion).

Gross Anatomy of a Skeletal Muscle

Muscle attachments (continued)

Muscles attach to origins and insertions by connective tissue (CT)

Fleshy attachments—CT fibers are short

Indirect attachments—CT forms a tendon or aponeurosis

Bone markings present where tendons meet bones

Tubercles, trochanters, and crests

Microscopic and Functional Anatomy of Skeletal Muscle Tissue

The skeletal muscle fiber

Fibers are long and cylindrical

Are huge cells—diameter is 10–100 µm

Length—several centimeters to dozens of centimeters

Each cell formed by fusion of embryonic cells

Cells are multinucleate

Nuclei are peripherally located

Figure 10.4b  Microscopic anatomy of the skeletal muscle fiber (cell).

Myofibrils and Sarcomeres

Striations result from internal structure of myofibrils

Myofibrils

Are long rods within cytoplasm

Make up 80% of the cytoplasm

Are a specialized contractile organelle found in muscle tissue

Are a long row of repeating segments called sarcomeres (functional unit of skeletal muscle tissue)

Myofibrils and Sarcomeres

Basic unit of contraction of skeletal muscle

Z disc (Z line)—boundaries of each sarcomere

Thin (actin) filaments—extend from Z disc toward the center of the sarcomere

Thick (myosin) filaments—located in the center of the sarcomere

Overlap inner ends of the thin filaments

Contain ATPase enzymes

Figure 10.4c-d  Microscopic anatomy of the skeletal muscle fiber (cell).

Mechanism of Contraction

Two major types of contraction

Concentric contraction—muscle shortens to do work

Eccentric contraction—muscle generates force as it lengthens

Muscle acts as a “brake” to resist gravity

“Down” portion of a pushup is an example

Mechanism of Contraction

Sliding filament mechanism

Explains concentric contraction

Myosin head attach to thin filaments at both ends of a sarcomere

Then pull thin filaments toward the center of the sarcomere

Thin and thick filaments do not shorten

Initiated by release of calcium ions

Powered by ATP

Figure 10.7  Changes in striations as skeletal muscle contracts.

The Role of Titin

Titin—a springlike molecule in sarcomeres

Resists overstretching

Holds thick filaments in place

Unfolds when muscle is stretched

Sarcoplasmic Reticulum and T Tubules

Sarcoplasmic reticulum

A specialized smooth ER

Interconnecting tubules surround each myofibril

Some tubules form cross-channels called terminal cisterns

Cisterns occur in pairs on either side of a
t tubule

Sarcoplasmic Reticulum and T Tubules

Sarcoplasmic reticulum

Contains calcium ions—released when muscle is stimulated to contract

Calcium ions diffuse through cytoplasm

Trigger the sliding filament mechanism

T tubules—deep invaginations of sarcolemma

Triad—T tubule flanked by two terminal cisterns

Figure 10.5  Sarcoplasmic reticulum and T tubules in the skeletal muscle fiber.

Innervation of Skeletal Muscle

Motor neurons innervate skeletal muscle tissue

Neuromuscular junction is the point where nerve ending and muscle fiber meet

Axon terminals

Located at ends of axons

Store the neurotransmitter – Acetylcholine (ACh)

Synaptic cleft

Space between axon terminal and sarcolemma

Figure 10.8  The neuromuscular junction.

Figure 10.9  Motor units.

Types of Skeletal Muscle Fibers

The muscle fiber types are categorized based on how:

they manufacture energy (ATP)

quickly they contract

Slow oxidative fibers

produce ATP aerobically

Fast glycolytic fibers

produce ATP anaerobically by glycolysis

Muscle Actions and Interactions

Prime mover (agonist)

Has major responsibility for a certain movement

Antagonist

Opposes or reverses a movement

Synergist—helps the prime mover

By adding extra force

By reducing undesirable movements

Fixator

A type of synergist that holds a bone firmly in place

Arrangement of Fascicles in Muscles

Skeletal muscles—consist of fascicles

Fascicles—arranged in different patterns

Fascicle arrangement—tells about action of a muscle

Arrangement of Fascicles in Muscles

Types of fascicle arrangement

Convergent

Origin of the muscle is broad

Fascicles converge toward the tendon of insertion

Example—pectoralis major

Arrangement of Fascicles in Muscles

Types of fascicle arrangement

Parallel—fascicles run parallel to the long axis of the muscle

Straplike—sternocleidomastoid

Fusiform—biceps brachii

Arrangement of Fascicles in Muscles

Types of fascicle arrangement

Pennate

Unipennate—fascicles insert into one side of the tendon

Bipennate—fascicles insert into the tendon from both sides

Multipennate—fascicles insert into one large tendon from all sides

Arrangement of Fascicles in Muscles

Circular

Fascicles are arranged in concentric rings

Surround external body openings

Sphincter—general name for a circular muscle

Examples

Orbicularis oris and orbicularis oculi

Figure 11.1  Patterns of fascicle arrangement in muscles.

Muscles of Mastication and Tongue Movement

Four main pairs of muscles involved in mastication

Innervated by mandibular division of the trigeminal nerve (cranial nerve V)

Prime movers of jaw closure

Masseter and temporalis

Side-to-side movement

Pterygoid muscles

Compression of cheeks

Buccinator muscles

Figure 11.10a-b  Muscles of mastication and tongue movement, left lateral view.