Microanatomy of Muscles

Anatomy & Physiology Class

Three Main Muscle Types

Objectives:

By the end of this presentation you will have the information to:

1. Describe the 3 main types of muscles.2. Detail the functions of the muscle system.3. Correctly label the parts of a myocyte (muscle cell)4. Identify the levels of organization in a skeletal muscle from

organ to myosin.5. Explain how a muscle contracts utilizing the correct terminology

of the sliding filament theory.6. Contrast and compare cardiac and smooth muscle with skeletal

muscle.

Muscle SystemMajor Functions:

1. Moving the skeletal system and posture.2. Passing food through the digestive

system & constriction of other internal organs.

3. Production of body heat.4. Pumping the blood throughout the body.5. Communication - writing and verbal

Specialized Cells (Myocytes) ~ Contractile Cells

Can shorten along one or more planes because of specialized cell membrane (sarcolemma) and specialized cytoskeleton.

Specialized Structures found in Myocytes

Sarcolemma: The cell membrane of a muscle cell

Transverse tubule: a tubular invagination of the sarcolemma of skeletal or cardiac muscle fibers that surrounds myofibrils; involved in transmitting the action potential from the sarcolemma to the interior of the myofibril.

Sarcoplasmic Reticulum: The special type of smooth endoplasmic reticulum found in smooth and striated muscle fibers whose function is to store and release calcium ions.

Myofibrils:a contractile fibril of skeletal muscle, composed mainly of actin and myosin

Multiple Nuclei (skeletal) & many mitochondria

Skeletal Muscle - Microscopic Anatomy

A whole skeletal muscle (such as the biceps brachii) is considered an organ of the muscular system. Each organ consists of skeletal muscle tissue, connective tissue, nerve tissue, and blood or vascular tissue.

Skeletal Muscle - Microscopic AnatomyEpimysium, perimysium and endomysium layers of connective tissue generally extend beyond the fleshy part of the muscle, forming a thick rope-like tendon. Fascia is a layer of thickened connective tissue that covers the entire muscle and is located over the layer of epimysium that also makes up the tendon that connects to the periosteum of the bone.

You will need to know the breakdown of the structure of a skeletal muscle from the organ down to the actin and myosin.muscle organfascicle myofiber myofibril sarcomere actin myosin

DRAW THIS

Sliding Filament Theory ~ main structures

● Myofibril: A cylindrical organelle running the length of the muscle fiber, containing Actin and Myosin filaments.

● Sarcomere: The functional unit of the myofibril, divided into I, A and H bands.● Actin: A thin, contractile protein filament, containing 'binding' sites.● Myosin: A thick, contractile protein filament, with protrusions known as

Myosin Heads.● Tropomyosin: An actin-binding protein which regulates muscle contraction.● Troponin: A protein attached to Tropomyosin.

Sliding Filament Theory ~ main structures

Thin Filament

Thick Filament

Sarcomeres

Sliding Filament Theory - Step 1

A nervous impulse arrives at the neuromuscular junction, which causes a release of a chemical called Acetylcholine. The presence of Acetylcholine causes the depolarisation of the motor end plate which travels throughout the muscle by the transverse tubules, causing Calcium (Ca+) to be released from the sarcoplasmic reticulum

Sliding Filament Theory - Step 2

In the presence of high concentrations of Ca+, the Ca+ binds to Troponin, changing its shape and so moving Tropomyosin from the binding site of the Actin. The Myosin heads can now attach to the Actin, forming a cross-bridge.

Sliding Filament Theory - Step 3

The breakdown of ATP releases energy which enables the Myosin to pull the Actin filaments inwards and so shortening the muscle. This occurs along the entire length of every myofibril in the muscle cell.

Sliding Filament Theory - Step 4

The Myosin detaches from the Actin and the cross-bridge is broken when an ATP molecule binds to the Myosin head. When the ATP is then broken down the Myosin head can again attach to an Actin binding site further along the Actin filament and repeat the 'power stroke'. This repeated pulling of the Actin over the myosin is often known as the ratchet mechanism.

Sliding Filament Theory - Relaxation

This process of muscular contraction can last for as long as there is adequate ATP and Ca+ stores. Once the impulse stops the Ca+ is pumped back to the Sarcoplasmic Reticulum and the Actin returns to its resting position causing the muscle to lengthen and relax.

Insert Video HereCross Bridges

Sarcomere through contractionM- disk (line) does not move.

H-Zone shrinks

A-Band does not change length

Z-lines move toward the M-line

All sarcomeres contract so the the muscle insertion will move toward the origin.

Sarcomere through contractionAll sarcomeres contract so the the muscle insertion will move toward the origin.

Muscles can only PULL, not push at a joint. They work in antagonistic pairs.

More Facts~● Mature cells can change in size, but new cells are not formed when muscles

grow; however, cellular components within the cell can change in response to use.

● hypertrophy = structural proteins are added to muscle fibers increasing mass.● atrophy = when structural proteins are lost and muscle mass decreases. ● satellite cells = help to repair skeletal muscle fibers. They are located outside

the muscle fibers and are stimulated to grow and fuse with muscle cells under certain forms of stress. Satellite cells can regenerate muscle fibers to a very limited extent, but they primarily help to repair damage by facilitating the protein synthesis.

● Fibrosis = scar tissue that replaces muscle tissue not able to be fixed by satellite cells. Scar tissue cannot contract; therefore, damaged muscle loses strength and endurance.

Cardiac Muscle:The structure & function are very similar to skeletal muscle with a few exceptions:1. One nucleus centrally located2. Shorter than skeletal muscles3. Branched myoctes, not parallel4. Intercalated discs for faster electrical impulses because of gap junctions and desmosomes.

Two types of cardiac cells:The contractile cells are the main contracting cells that must beat as a unit, so the heart contracts as one organ.

Pacemaker cells- depolarize at set intervals, and the heart beats a steady, predictable 60 to 80 bpm at rest

Smooth MuscleNot striated, so contracts differently than skeletal and cardiac.

● no sarcomeres, but do have actin and myosin in the dense bodies attached to the sacolemma.

● Thin and thick filaments are aligned in a diagonal pattern across the cell so that contraction produces a twisting or corkscrew motion

● muscles form layers that are usually arranged so that one runs parallel to an organ and the other wraps around it - allows for peristalsis and labor and delivery

● Can produce more cells (hyperplasia)Two Types of Smooth Muscle:Single Unit: gap junctions allow for coordinated depolarization and contraction often triggered by stretching.Multiunit: Stimulated by hormones or autonomic nerves, no gap junctions

Insert all or none videohttps://youtu.be/9figSzfwW-Q?t=4m15s

Fast Twitch Muscles

Fast Twitch (Type IIB): - Quick bursts of energy● most skeletal muscles are of this type● large in diameter● use enormous reserves of glycogen rather

than oxygen-rich blood for quick energy● densely packed myofibrils● few mitochondria● generate a lot of tension● rely largely on anaerobic metabolism● fatigue rapidly● WHITE MEAT● fast twitch fibers appear in muscles needed for fine

movements, such as the small muscles of the hand and the eye

Slow Twitch MusclesSlow Twitch (Type I): -Endurance

● are smaller than fast twitch muscles● take about three times longer to contract

after receiving stimulus● many mitochondria● contain a large amount of myoglobin,

which carry oxygen to muscle fibers (similar to hemoglobin, which helps carry oxygen to blood)

● slow twitch fibers are needed for posture and movement, and in back muscles and muscles of the legs

● DARK MEAT

Intermediate Twitch Muscles

Intermediate (Type IIA):● have properties of both fast and slow twitch fibers● similar in appearance to fast twitch fibers● similar in endurance to slow twitch fibers