Biol 3455 Lecture Muscle Tissue

What does Muscle Tissue have

muscle cells specialized for contraction

Without muscle tissue

nothing in the body would move, and the body itself could not move.

3 types of Muscle tissue

skeletal, cardiac, smooth

Common Properties of Muscle Tissue

excitability, contractility, extensibility, elasticity

Excitability

responsiveness or irritability, is the ability to receive

and respond to a stimulus

What do muscle tissue respond to

chemical/electrical stimulus from a nerve cell with a change in membrane potential

Contractility

ability to shorten forcibly when stimulated

Extensibility

ability of muscle to be stretched

Elasticity

ability of a muscle to recoil (spring back) to its resting length.

What contributes to natural elasticity of muscle tissue

Connective tissue sheaths

skeletal muscle tissue

packaged into the skeletal muscles, organs that attach to and cover the bony skeleton

- voluntary muscle bc it is subject to conscious control

- responsible for overall body mobility. It can contract rapidly, but it tires easily and must rest after short periods of activity.

Skeletal muscle fibers

the longest muscle cells and have obvious stripes called striations.

Skeletal muscles

organs composed mainly of skeletal muscle tissue, but they also contain connective tissues, blood vessels, and nerves.

• attach directly or indirectly to bones

Single Muscle Fiber

each cell in skeletal muscle

3 layers of CT in skeletal muscle

epimysium, perimysium, endomysium

Role of Connective Tissue Sheaths

support each cell and reinforce and hold together the muscle, preventing the bulging muscles from bursting during exceptionally strong contractions.

Epimysium

- "overcoat" of dense irregular connective tissue that surrounds the whole muscle.

- separates muscle from surrounding tissues

- blends with the deep fascia that lies between neighboring muscles or the superficial fascia deep to the skin

- define the muscle's volume and prevents friction

between neighboring muscles.

Perimysium

the muscle fibers are grouped into fascicles that resemble bundles of sticks.

▪ Surrounding each fascicle is a layer of dense irregular connective tissue

Endomysium

surrounds individual skeletal muscle cells, called muscle fibers, and loosely interconnects adjacent muscle fibers.

What does elastic connective tissue layer in endomysium contain?

(1) Capillary networks (the smallest blood vessels) that supply

blood to the muscle fibers.

(2) Myosatellite cells, stem cells that help repair damaged muscle

tissue.

(3) Nerve fibers that control the muscle

Tendon

the collagen fibers of the epimysium, perimysium, and endomysium come together to form either a bundle

What happens when tendons and aponeuroses contact the bone

the collages fibers extend into the bone matrix providing a firm attachment

what do EPimysium, perimysium, and endomysium contain

nerves and blood vessels that supply the muscle fibers

Role of vascular network

delivers the necessary oxygen and nutrients and carries away the metabolic wastes generated by active skeletal muscles.

When do skeletal muscles contract

when the central nervous system stimulates them.

role of Axons in muscle contraction

nerve fibers extending from neurons, penetrate the epimysium, branch through the perimysium, and enter the endomysium to innervate individual muscle fibers.

why are skeletal muscle called voluntary muscles

we have voluntary control over their contractions

Neuromuscular junciton

Chemical communication between a synaptic terminal of the neuron and a skeletal muscle fiber occurs at a site

What happens at the neuromuscular synapse?

synaptic terminal of the neuron is bound to the motor end plate of the skeletal muscle fiber.

Motor end plate

specialized area of the muscle cell membrane within a neuromuscular synapse.

Organizational Levels of Skeletal Muscle

Muscle > Fascicle > Muscle Fiber (cell)> Myofibril > Sarcomere> Myofilament

Muscle

consists of hundred to thousands of muscle cells + ct wrappings

Fascicle

discrete bundle of muscle cells segregated from the rest of the muscle by a connective tissue sheath

Muscle FIber

elongated multinucleate cell with striated appearance

Myofibrils

rodlike contractile elements that occupy most of the muscle cell volume

Sarcomere

contractile unit, composed of myofilaments made up of contractile proteins

Sarcolemma

cell membrane of a skeletal muscle fiber

What does the Sarcolemma surround

Sarcoplasm/cytoplasm

Why does the sarcolemma have a transmembrane potential

bc more negative charges on its cytoplasmic side and more positive charges on its extracellular side.

How are individual skeletal muscle fibers created

groups of embryonic cells called myoblasts fuse together to create individual skeletal muscle fibers

Single Myoblast

Each nucleus in a skeletal muscle fiber reflects the contribution of

What does each skeletal muscle fiber contain

hundreds of nuclei just inside the sarcolemma

Myosatellite cells

myoblasts do not fuse with developing muscle fibers but remain in adult skeletal muscle tissue

What happens when the skeletal muscle is injured

myosatellite cells may differentiate and assist in the repair and regeneration of the muscle.

What does Sarcoplasm contain

glycosomes and myoglobin

Glycosomes

granules of stored glycogen that provide glucose during muscle cell activity for ATP production.

Myoglobin

a red pigment that stores oxygen.

2 sets of intracellular tubules in skeletal muscle fibers

1. Sarcoplasmic reticulum

2. T Tubules

T Tubules

Deep indentations in the sarcolemmal surface form a network of

narrow tubules that extend into the sarcoplasm.

What do sarcolemma and T Tubules conduct

electrical impulses,

called action potentials, to stimulate muscle fiber contraction.

Sarcoplasmic Reticulum

surrounds each myofibril membrane complex similar to the smooth endoplasmic reticulum of other cells.

• storage and release site of calcium ions and plays an

  essential role in controlling individual myofibril contract

Terminal Cisternae

On each side of a T (transverse) tubule, the tubules of the SR enlarge,

fuse, and form expanded chambers

Triad

combination of a pair of terminal cisternae plus a T tubule

How are ions actively transported from cytosol into terminal cisternae

Calcium pump

What begins muscle contraction

Release calcium ions into sarcomeres

Myofibrils

sarcoplasm of a skeletal muscle fiber contains hundreds to

thousands of fine cylindrical fibers

• 1–2 μm in diameter and as long as the entire cell.

What is responsible for skeletal muscle fiber contraction?

active shortening of myofibrils

Myofilaments

protein filaments that make up myofibrils

2 types of filaments

thin(Actin) and thick (Myosin)

Contractile Proteins in Thick/Thin Filament

actin and myosin

Regulatory proteins in Thick/Thin Filaments

Tropomyosin/Troponin

Accessory Proteins of thick/Thin Filaments

titin and nebulin

Myosin Molecule

two heavy and four light polypeptide chains, and has a rodlike tail attached by a flexible hinge to two globular heads.

Myosin tail

2 intertwined helical polypeptide heavy chains

Cross bridge

thin and thick filament link during contraction

Globular Actin

bear the active sites to which the myosin

heads attach during contraction.

F Actin/Filamentous

G actin subunits are polymerized into long actin filaments

Tropomyosin

Polypeptide strands of tropomyosin, a rod-shaped protein,

spiral about the actin core and help stiffen and stabilize it.

▪ It is a rope-like protein made of two twisted chains (an

alpha-helical coiled-coil)

Troponin

globular three polypeptide complex.

Tnl

TnT

TnC

Tnl

inihibitory subunit that binds to actin

TnT

binds to tropomyosin and helps position it on actin

TnC

binds Calcium ions

Nebulin

extends along the F-actin strand in the

cleft between the rows of G-actin molecules

• holds the F-actin strand together

Dystrophin

links the thin filaments to the integral proteins of the sarcolemma

• crucial role in maintaining the structural integrity of

muscle fibers and preventing muscle damage.

▪ This connection helps to stabilize muscle fibers and prevent

them from tearing.

Duchenne Muscular Dystrophy DMD

A genetic condition that occurs when the DMD gene fails to produce dystrophin.

Titin

Strands of the elastic protein titin extend from the tips of the

thick filaments to attachment sites at the Z line.

Functions of Titin

Helps keep the thick and thin filaments in proper

alignment.

▪ Aids in restoring resting sarcomere length after

contraction.

▪ It also helps the muscle fiber resist extreme stretching that

would otherwise disrupt the contraction mechanism.

Desmin

filaments extend from the Z disc and connect each myofibril to the next throughout the width of the muscle cell

What maintains muscle fiber strength during repeated cycles of contraction/relaxation

Connection of Sarcomeres to each other to form myofibrils

Role of Desmin IF

link myofibrils into bundles through the Z-discs and

connect them to specialized sites of adhesion (costameres) in the

sarcolemma

Striations

a repeating series of dark and light bands, are evident

along the length of each myofibril.

Sarcomeres

Thin and thick myofilaments are organized into repeating

functional units

• smallest functional units of the muscle fiber

• extends from one Z disc to the next Z disc

Z Discs

are composed of specialized proteins

that are positioned perpendicular to the myofilaments and serve as

anchors for the thin filaments.

A-actinin

bind actin molecules of the thin filament to titin

A band

central region of sarcomere that contains entire thick filament

Thin filaments partially overlap the thick filament on each end

• appears dark

• does not change in length during muscle contraction

I band

extend from both directions of a Z disc and are bisected by

the Z disc.

• only thin filament

When does the I band disappear

maximal muscle shortening, qhen the thin filaments are pulled

parallel along the thick filaments,

H zone

central portion of A band

only thick filaments

When do H zone diasapear

maximal muscle shortening, thin filaments are pulled past thick filament

M line

dark line formed by molecules of the protein myomesin

• attachment site for the thick filaments and keeps

  the thick filaments aligned during contraction and relaxation

  events.

Sliding Filament Theory

explains how muscles contract by describing the way thin actin filaments slide past thick myosin filaments.

What do myosin filaments use energy from ATP to do

walk” along the actin filaments with their cross bridges. This pulls

the actin filaments closer together'

movement of the actin filaments also pulls the Z lines closer

together, thus shortening the sarcomere.

How do Thin and Thick Filaments slide past one another

thin filaments moving toward the center of the sarcomere from both ends

What happens to other bands and zone when thin and thick filament slide

the H zones and the I bands narrow.

▪ The regions of overlap widen.

▪ The Z lines move closer together, shortening the sarcomere.

As muscle cell shortens

The I bands shorten.

▪ The distance between successive Z discs shortens.

▪ The H zones disappear.

▪ The contiguous A bands move closer together, but their length

does not change.

For a Skeletal muscle fiber to contract

1. The fiber must be activated, that is, stimulated by a nerve

ending so that a change in membrane potential occurs.

▪ 2. Next, it must generate an electrical current, called an action

potential, in its sarcolemma.

▪ 3. The action potential is automatically propagated along the

sarcolemma.

▪ 4. Then, intracellular calcium ion levels must rise briefly, pro-

viding the final trigger for contraction.

Somatic motor neurons

nerve cells that activate skeletal muscle fibers

• reside in brain/spinal cord

• their axons extend to muscle cells they serve

Acetylcholine

neurotransmitter that motor neurons use to control skeletal muscle contraction.

• synthesized in the cytoplasm of the motor neuron and is

  stored in synaptic vesicles near the distal end of its axon.

What happens when AP reaches end of the axon

some of these vesicles release acetylcholine into the synaptic cleft.

▪ Acetylcholine diffuses rapidly across the synaptic cleft and binds to

specific protein molecules (receptors) in the muscle fiber

membrane, increasing the membrane permeability to sodium and

potassium ions.

What generates an action potential that sweeps across the sarcolemma

▪ ACh binding opens the channel, which allows Na+ to enter the cell.

▪ The inflow of positive charges depolarizes the motor end plate