The muscular system

The Muscular System

There are 3 types of muscles

A. Function of Skeletal Muscles

• Produce movement– Muscle pulls tendons to move the skeleton

• Maintain posture and body position– Continuous muscle contraction

• Guard entrances and exits– Encircle openings to digestive and urinary tracts.

Control swallowing, defecation and urination

• Maintain body temperature– Energy from contraction is converted to heat

Skeletal Muscle

• Bundles are formed by:

• epimysium epi = upon

• perimysium peri = around

• endomysium end = within

Terms

• Plasmalemma = Sarcolemma

• Sarcoplasm = Cytoplasm

• Sarcoplasmic Reticulum = Endoplasmic Reticulum

B. Anatomy of Skeletal Muscles - Gross Anatomy

Surrounds muscle

Divides muscle into compartments, each contain a bundle of muscle fibers called fascicle

Bundle of muscle fibers

Surrounds each muscle fiber, and tie adjacent fibers together

All three layers attach muscle to bone

Myofibrils

• Cylinder as long as entire muscle fiber

• Each fiber contains 100s to 1000s

• Responsible for contraction• When myofibrils contract

the whole cell contracts• Consist of proteins

– Actin – thin filaments– Myosin – thick filaments

Connective Tissue Sheaths

• Connective Tissue of a Muscle– Epimysium. Dense regular c.t. surrounding entire muscle

• Separates muscle from surrounding tissues and organs• Connected to the deep fascia

– Perimysium. Collagen and elastic fibers surrounding a group of muscle fibers called a fascicle

• Contains b.v and nerves

– Endomysium. Loose connective tissue that surrounds individual muscle fibers

• Also contains b.v., nerves, and satellite cells (embryonic stem cells function in repair of muscle tissue

• Collagen fibers of all 3 layers come together at each end of muscle to form a tendon or aponeurosis.

B. Anatomy of a Skeletal Muscle – Blood Vessels and Nerves

• Muscle contractions require energy– Blood vessels deliver oxygen and nutrients to

produce energy(ATP)

• Muscle contractions are under stimulation from the CNS(central nervous system)

Sarcomere

• Smallest functional unit of muscle fiber

• Each myofibril contains 10,000 sarcomeres end to end

• Interaction between thick and thin filaments cause contraction

• Banded appearance

Sarcomeres: Z Disk to Z Disk

• Sarcomere - repeating functional units of a myofibril

– About 10,000 sarcomeres per myofibril, end to end

– Each is about 2 µm long

• Differences in size, density, and distribution of thick and thin filaments gives the muscle fiber a banded or striated appearance.

– A bands: a dark band; full length of thick (myosin) filament

– M line - protein to which myosins attach– H zone - thick but NO thin filaments– I bands: a light band; from Z disks to ends

of thick filaments• Thin but NO thick filaments• Extends from A band of one sarcomere to

A band of the next sarcomere

– Z disk: filamentous network of protein. Serves as attachment for actin myofilaments

– Titin filaments: elastic chains of amino acids; keep thick and thin filaments in proper alignment

Microanatomy – Sarcolemma and T-Tubules

•Very large cells

•100’s of nuclei •Cell membrane

•pores open to T-tubules

•Network of narrow tubules

•filled with extracellular fluid

•form passageways through muscle fiber

Sarcoplasmic Reticulum

• Specialized form of SER• Tubular network around

each myofibril

• In contact with T-Tubule

Sarcoplasmic Reticulum (SR)• SR is an elaborate, smooth endoplasmic

reticulum – runs longitudinally and surrounds each myofibril– Form chambers called terminal cisternae on either

side of the T-tubules

• A single T-tubule and the 2 terminal cisternae form a triad

• SR stores Ca++ when muscle not contracting– When stimulated, calcium released into sarcoplasm– SR membrane has Ca++ pumps that function to

pump Ca++ out of the sarcoplasm back into the SR after contraction

Thick and Thin Filaments

• Thin– twisted actin molecules– Each has an active site where

they interact with myosin– Resting – active site covered by

tropomyosin which is held in place by troponin

• Thick– Myosin– Head attaches to actin during

contraction– Can only happen if troponin

changes position, moving tropomyosin to expose active site

Muscle Fiber Anatomy• Sarcolemma - cell membrane

– Surrounds the sarcoplasm (cytoplasm of fiber)• Contains many of the same organelles seen in other cells• An abundance of the oxygen-binding protein myoglobin

– Punctuated by openings called the transverse tubules (T-tubules)• Narrow tubes that extend into the sarcoplasm at right angles

to the surface• Filled with extracellular fluid

• Myofibrils -cylindrical structures within muscle fiber– Are bundles of protein filaments (=myofilaments)

• Two types of myofilaments1. Actin filaments (thin filaments)2. Myosin filaments (thick filaments)

– At each end of the fiber, myofibrils are anchored to the inner surface of the sarcolemma

– When myofibril shortens, muscle shortens (contracts)

Structure of Actin and Myosin

Myosin (Thick)

Myofilament

• Many elongated myosin molecules shaped like golf clubs.

• Single filament contains roughly 300 myosin molecules

• Molecule consists of two heavy myosin molecules wound together to form a rod portion lying parallel to the myosin myofilament and two heads that extend laterally.

• Myosin heads1. Can bind to active sites on the

actin molecules to form cross-bridges. (Actin binding site)

2. Attached to the rod portion by a hinge region that can bend and straighten during contraction.

3. Have ATPase activity: activity that breaks down adenosine triphosphate (ATP), releasing energy. Part of the energy is used to bend the hinge region of the myosin molecule during contraction

Sliding Filament Model of Contraction

• Thin filaments slide past the thick ones so that the actin and myosin filaments overlap to a greater degree

• In the relaxed state, thin and thick filaments overlap only slightly

• Upon stimulation, myosin heads bind to actin and sliding begins

Sliding Filament Model of Contraction

• Each myosin head binds and detaches several times during contraction, acting like a ratchet to generate tension and propel the thin filaments to the center of the sarcomere

• As this event occurs throughout the sarcomeres, the muscle shortens

InterActive Physiology®: Muscular System: Sliding Filament TheoryPLAYPLAY

Sliding Filaments and Cross Bridges

• Sarcomere contraction – Sliding Filament Theory– Thin filaments slide

toward center of sarcomere

– Thick filaments are stationary

– Myosin head attaches to active site on actin (cross bridge)

– Pull actin towards center, then detaches

Questions

• How would severing the tendon attached to a muscle affect the ability of the muscle to move a body part?

• Why does skeletal muscle appear striated when viewed through a microscope?

• Where would you expect the greatest concentration of calcium ions in resting skeletal muscles to be?

Control of Muscle Fiber Contraction

Under control of the nervous system

1. Electrical signal travels to the muscle.2. Electrical signal spreads over entire

sarcolemma, down t-tubules to sarcoplasmic reticulum

3. Sarcoplasmic reticulum releases massive amounts of calcium

4. Increase in calcium – sarcomeres contract

The Contraction Cycle• Step 1

– Ca+ binds to troponin, changing the tropomyosin position, in turn exposing active site on actin

• Step 2– Myosin head attaches to

actin

• Step 3– Pulling of actin towards

center of sarcomere

• Step 4– Detachment of cross

bridge

• How would a drug that interferes with cross-bridge formation affect muscle contraction?

• What would you expect to happen to a resting skeletal muscle if the sarcolemma suddenly became very permeable to calcium ions?

Questions