Section 1, Chapter 9Muscular System

Muscle is derived from Musculus, for “Mouse”

Functions of Muscles:1. Body movement 2. Maintain posture3. Produces heat4. Propel substances

through body5. Heartbeat

Types of muscles include:1. Smooth muscle2. Cardiac muscle3. Skeletal muscle

Imagine a mouse running beneath the

skin.

Characteristics of smooth muscles• Involuntary control• Tapered cells with a single, central nucleus• Lack striations

Smooth Muscle

• Visceral (single-unit) Smooth Muscle• Form sheets of muscle• Cells are connected by gap junctions• Muscle fibers contract as a group• Rhythmic contractions• Within walls of most hollow organs

(viscera)

• Multi-unit Smooth Muscle• unorganized cells that contract

as individual cells

• Located within the iris of eye and the walls of blood vessels

There are two types of smooth muscles

Smooth Muscle

Cardiac Muscle

• Located only in the heart

• Striated cells

• Intercalated discs

• Muscle fibers branch

• Muscle fibers contract as a unit

• Self-exciting and rhythmic

• Usually attached to bone

• Voluntary control

• Striated (light & dark bands)

• Muscle fibers form bundles

• Several peripheral nuclei

Skeletal Muscle

Coverings of Skeletal Muscle

Fascia• Dense connective tissue surrounding skeletal muscles

Tendons• Dense connective tissue that attaches muscle to bones• Continuation of muscle fascia and bone periosteum

Aponeurosis• Broad sheet of connective tissue attaching muscles to

bone, or to other muscles.

Coverings of Skeletal Muscle

Epimysium• Connective tissue that covers the entire muscle

• Lies deep to fascia

Perimysium• Surrounds organized bundles of muscle fibers, called fascicles

Endomysium• Connective tissue that covers individual muscle fibers (cells)

Figure 9.3 Scanning electron micrograph of a fascicle surrounded by its perimysium. Muscle fibers within the fascicle are surrounded by endomysium.

Organization of Skeletal Muscle

Figure 9.2Skeletal muscle

organization

FascicleOrganized bundle of muscle fibers

Muscle FiberSingle muscle cellCollection of myofibrils

MyofibrilsCollection of myofilaments

MyofilamentsActin filamentMyosin filament

Sarcolemma• Cell membrane of muscle fibers

Sarcoplasm• Cytoplasm of muscle fibers

Sarcoplasmic Reticulum• Modified Endoplasmic Reticulum• Stores large deposits of Calcium

Skeletal Muscle Fibers

sarcolemma

Skeletal Muscle Fibers

Openings into t-tubules

(Transverse)T-tubules: • invaginations of sarcolemma, extending into the sarcoplasm. Cisternae:• enlarged region of sarcoplasmic reticulum, adjacent to the t-tubules Triad • T-tubule + adjacent cisternae

Myofibrils

Figure 9.4 Organization of actin and myosin filaments

Myofibrils are bundles of actin and myosin filaments.

• Actin – thin filament• Myosin – thick filament

Striations appear from the organization of actin and myosin filaments

SarcomereA sarcomere is the functional unit of

skeletal muscle

• A sarcomere is the area between adjacent Z-lines.

• During a muscle contraction the z-lines move together and the sarcomere shortens.

Sarcomere

Figure 9.5 thin and thick filaments in a sarcomere.

Z Line is the attachment site of actin filaments (center of I bands)

Striations appear from alternate light and dark banding patterns.

I Bands (light band): consists of only actin filaments

A Bands (dark band) : consists of myosin filaments and the overlapping portion of actin filaments

Thick filaments composed of myosin proteins

During muscle contraction the heads on myosin filaments bind to actin filaments forming a Cross-bridge

Thin filamentscomposed of actin proteins

Thin filaments are associated with troponin and tropomyosin proteins

filaments

Cross-BridgesWhen a muscle is at rest, myosin heads are extended in the “cocked” position.

During a contraction, myosin heads bind to actin, forming a cross-bridge and the myosin head pivot forward (Power Stroke) and back (Recovery stroke)

Tropomyosin Blocks binding sites on actin when a muscle is at rest

Troponin Ca2+ binds to troponin during a muscle contraction.

Troponin moves repositions the tropomyosin filaments, so the myosin and actin filaments can interact.

Troponin-Tropomyosin Complex

The troponin-tropomyosin complex prevents cross-bridge formation when the muscle is at rest.