Chapter 11Muscles

Structure Muscles cells

◦generate force and movements used to regulate the internal environment, and they also produce movements in the external environment. Ex. skeletal muscle, smooth muscle, and

cardiac muscle

Types of Muscle Tissues Skeletal muscles

◦ its contraction is responsible for supporting and moving the skeleton.

◦ contraction of skeletal muscle is initiated by impulses in the neurons to the muscle and is usually under voluntary control

Types of Muscle Tissues Smooth muscles

◦ Surround various hollow organs and tubes, Ex. stomach, intestines, urinary bladder, uterus, blood vessels, and airways in the lungs.

◦ The autonomic nervous system, hormones, autocrine/paracrine agents, and other local chemical signals control smooth muscle contraction. Some smooth muscles contract autonomously, however, even in the absence of such signals.

◦ Smooth muscle is not normally under voluntary control.

Types of Muscle TissuesStraited

(Cardiac) Muscle◦ Muscle of the

heart◦ Contraction

propels blood through the circulatory system

Skeletal musclesMuscle fiber

◦ A single skeletal muscle cell

◦ Formed by fusion of undifferentiated, mononucleated cells, known as myoblasts into single cylindrical, multinucleated cell.

StructuresSkeletal muscle differentiation is

completed around the time of birthContinue to increase in size during

growth from infancy to adult, but no new fibers are formed from myoblasts.

If skeletal-muscle fibers are destroyed after birth as a result of injury, they cannot be replaced.

New fibers can be formed, however, from undifferentiated cells known as satellite cells◦Does not restore a severely damaged

muscle to full strength.

StructuresMuscle

◦refers to a number of muscle fibers bound together by connective tissue.

Tendons◦Collagen fibers that links the

muscles and bones◦Located at each end of the muscles

StructuresStriated muscles

◦ Under light microscope, is a series of light and dark bands perpendicular to the long axis of the fiber

◦ ex. Cardiac musclesMyofibrils

◦ cylindrical bundles with thick and thin filaments in the cytoplasm

◦ Cytoplasm of fiber is filled with myofibrils

Skeletal-muscle fibers viewed through a light microscope. Each bracket at the left indicates one muscle fiber. Arrowindicates a blood vessel containing red blood cells.

MyofibrilsSarcomere

◦ One repeating unit of thick and thin filament in the myofibril

Thick filaments◦ Composed of

contractile protein myosin

◦ Found in middle of sarcomere where their order parallel produces wide, dark A bands

Thin filaments ◦ Composed of

contractile protein actin

◦ As well as Troponin and Tropomyosin (regulation contraction)

Structures Each sarcomere contains 2

sets of thin filaments◦ One end of each thin

filament is anchored to a network of interconnecting protein called Z line

◦ Other end overlaps a portion of the thick filament

I bands◦ Light bands that lies

between the ends of the A bands of two adjacent sarcomeres and contains those portions of the thin filaments that do not overlap the thick filaments.

H zone ◦ Narrow light band in

the center of the A band

◦ Only thick filaments, specifically their central parts, are found in the H zone

M line◦ Narrow, dark band

in the center of the H zone

◦ Corresponds to proteins that link together the central region of the thick filaments

M line

Structures Titin

◦A protein that functions as a molecular spring which is responsible for the passive elasticity of muscle

◦Extends from Z line to M line◦linked to both the M-line proteins and

the thick filaments

StructuresCross bridges

◦Projections that bridges the space between overlapping thick and thin filaments

◦portions of myosin molecules that extend from the surface of the thick filaments toward the thin filaments

◦During contraction, the cross bridges make contact with the thin filaments and exert force on them

Molecular Mechanisms of Contraction

Contraction-turning on of the force-generating sites( cross bridges) in a muscle fiber.

Relaxation- force generation is turned off and tension declines.◦the emotional state of low tension.

Contraction and Relaxation

The sliding filament theory is the explanation for how muscles produce force (or, usually, shorten).  It explains that the thick and thin filaments within the sarcomere slide past one another, shortening the entire length of the sarcomere. 

Sliding filament mechanism

Actin- globular proteins composed of a single polypeptide that polymerizes with other actins to form two intertwined helical chains.

Actin-thin filament.Myosin-thick filament.

Is the sequence of events that occur between the time a cross bridge binds to a thin filament, moves, and then is set to repeat the process.

Cross bridge cycle

1. 1. attachment of the cross bridge to the thin filament.

2. 2. movement of cross bridge, producing tension on the thin filament.

3. 3. detachment of the cross bridge from the thin filament.

4. 4. energizing of the cross bridge for it to repeat the cycle.

Cross bridge cycle

1. (ATP hydrolysis) provides energy for cross-bridge movement

2. ATP binding tom myosin breaks the link formed between actin and myosin during cycle, allowing the cycle to be repeated.

Roles of ATP

Roles of Troponin, Tropomyosin, and Calcium in contraction.

Tropomyosin and troponin are proteins that prevents cross bridges from interacting with actin in resting muscle fiber.

Calcium triggers contraction by reaction with regulatory proteins that in the absence of calcium prevent interaction of actin and myosin.

Introduction

Is a complex of three regulatory proteins that is essential to muscle contraction.

Troponin is attached to the protein tropomyosin and lies within the groove between actin filaments in muscle tissue. 

Troponin

Troponin C-binds to calcium ions to produce a conformational change in TnI

Troponin T-binds to tropomyosin, interlocking them to form a troponin-tropomyosin complex

Troponin I-binds to actin in thin myofilaments to hold the troponin-tropomyosin complex in place

Individual subunits serve different functions:

Is an actin-binding protein that regulates actin mechanics.

Chains of tropomyosin molecules are arranged end to end along the actin filament.

It inhibits contraction by blocking the interaction of actin and myosin, except when influenced by troponin.

Tropomyosin 

Muscle contraction is regulated by calcium ions, which will change thin filament into an activated state by binding to troponin.

Removal of calcium from troponin reverses the process, turning off contractions.

Calcium