Muscle Tissue Types and Properties

• Our bodies are composed of four basic tissue types: epithelial, connective, nervous, and muscle tissues which account for 40-50% of total body weight.

Muscle Tissue Types

• Skeletal muscle is usually attached to the skeleton and diaphragm, aiding in posture and breathing; it is voluntary with limited regeneration abilities.
• Cardiac muscle is striated, branched, uninucleated fibers found in the heart's myocardium, facilitating blood circulation and respiration, is involuntary with limited repair capabilities.
• Smooth muscle has spindle shaped, nonstriated, uninucleated fibers in the walls of hollow organs that move materials through internal organs and is involuntary with high regenerative abilities.

Properties of Muscle Tissues

• Excitable, contraction occurs when stimulated by a nervous signal
• Contractile, muscle tissues shorten/generate tension when stimulated
• Elastic, tissues return to their original length after contraction or stretch
• Extensible, tissues can be stretched beyond their original length.

Muscle Tissue Functions

• Movement
• Locomotion
• Posture
• Regulating organ volumes
• Support
• Producing heat
• Peristalsis
• Blood pressure
• Pumping blood
• Protection

Connective Tissues and Skeletal Muscles

• Connective tissue layers protect and support muscle tissues and are known as fascia.
• Superficial fascia is fibrous tissue while deep fascia is more fibrous and holds muscle cells together.
• The three layers of deep fascia are epimysium, perimysium, and endomysium.
  • Epimysium is the external surface of the muscle and holds everything together
  • Perimysium surrounds one bundle of cells and holds fascicles together. A fascicle is a bundle of 10-100 muscle cells
  • Endomysium surrounds a single muscle cell/muscle fiber.

Fascicle Arrangements

• Circular fascicles are arranged in concentric rings (orbiculares oculi)
• Parallel fascicles run parallel to the long axis of the muscle (sartorius)
• Convergent fascicles have a broad origin and converge toward a single tendon of insertion (pectoralis major)
• Unipennate fascicles have all the fascicles on the same side of the tendon (extensor digitorum longus)
• Bipennate fascicles have fascicles on both sides of the tendon (rectus femoris)
• Multipennate have a tendon that branches within the muscle (deltoid)

Building Skeletal Muscle

• Muscle fiber is a muscle cell, and sarco- means flesh. Myo- means muscle.
• Sarcolemma is the plasmalemma or cell membrane of a muscle cell/fiber.
• Sarcoplasm is the cytoplasm of a muscle fiber.
• Sarcoplasmic reticulum is the Smooth Endoplasmic Reticulum (SER) of a muscle fiber that stores calcium needed for muscle contraction.
• Tendons extend from bones, leading to a tough layer of deep fascia holding the muscle together. Cells are grouped into fascicles.
• Within each fascicle, are muscle cells/fibers. Protein strands inside the cells are organized into myofibrils.
• Myofibrils are bundles of myofilaments (contractile proteins) like actin and myosin.
• Actin and myosin are arranged in repeating units called sarcomeres.
• A sarcomere is a repeating unit of organized actin and myosin along a myofibril.
• When the muscle contracts, the sarcomere shortens, and the actin and myosin filaments slide past one another.
• Myosin proteins are thick filaments shaped like a golf club, and actin are thin filaments shaped a bit like two strands of pearls.

Levels of Organization in Muscle Tissue

• Muscle consists of multiple fascicles which house many individual muscle cells. It is covered by the epimysium.
• A bundle of muscle fibers is known as a fascicle covered by the perimysium.
• A muscle fiber is an elongated, multinucleated cylindrical fiber with striations and contains myofibrils and is covered by the endomysium.
• Myofibrils are long cylindrical contractile elements with striations.
• Myofilaments are thick or thin filaments within myofibrils (actin or myosin)

The Sarcomere

• The functional unit in skeletal muscle is the sarcomere.
• The striated appearance of skeletal muscles results from the organization of thick myosin filaments and thin actin filaments into sarcomeres.

Sarcomere structures during Contraction

• Sarcomeres shorten during muscle contraction.
• A bands remain the same length
• H zones shorten
• M lines remain the same
• I bands shorten
• Z disks are pulled toward the M line

Muscle Contraction

• Muscle fiber equals muscle cell. Sacrolemma is the cell membrane of muscle cell/fiber. Sarcoplasm is cytoplasm of muscle cell/fiber
• Sarcoplasmic reticulum is the SER that stores and releases Ca2+
• Skeletal muscles contract to pull insertions toward origins when a motor neuron releases a neurotransmitter on the sarcolemma.
• Acetylcholine (ACh) is released into the synaptic cleft at the neuromuscular junction.
• ACh binds to receptors on the muscle fiber cell membrane/sarcolemma, to initiate electrical charge along the sarcolemma.
• The electrical impulse travels along the cell membrane, diving deep into the cell via transverse tubules (T-tubules) and causes calcium (Ca++) to be released from the terminal cisternae (reservoirs) of the sarcoplasmic reticulum.
• Ca++ combines with troponin, a regulatory molecule associated with actin that, in association with tropomyosin, allows myosin and actin to interact.
• When Ca++ binds to troponin, tropomyosin moves over and binding sites on actin are exposed and myosin reaches up and grabs onto actin in order to initiate contraction

Neuromuscular Junction (NMJ) Pharmacology

• Botulism blocks the release of ACh into the synaptic cleft, causing paralysis/loss of muscle contraction.
• Curare blocks ACh receptors on the motor end-plate, causing paralysis, and is used as a muscle relaxant for surgery.
• Neostigmine prevents removal of ACh from the neuromuscular synaptic cleft to strengthen muscle contractions.
• Myasthenia Gravis (case study): autoimmune disorder, loss of ACh receptors on the motor-end plate, muscles don't receive signal to contract, causes early fatigue & loss of strength.

Motor Units

• Motor Unit = 1 nerve cell + all of the muscle fibers that it controls.
• Muscle twitch is a single stimulus-contraction-relaxation sequence that lasts for a fraction of a second.
• Summation is where muscle cells must be repeatedly stimulated.
• Tetanus is the state of peak and sustained tension.
• Recruitment is increasing amount of force produced involving more motor units.
• Tone is a continuous, partial contraction of alternate muscle fibers.

Origin, Insertion, and Action

• Origin is the less mobile attachment.
• Insertion is the more mobile attachment.
• Action groups muscles according their primary actions into agonists, antagonists, & synergists.
• Agonist is the prime mover.
• Synergist assists the agonist in performing its action.
• Antagonist opposes the agonist.

Test Yourself

• Tendon is a cord or band of dense regular connective tissue.
• Aponeurosis is a broad flat sheet of dense regular connective tissue.
• Origin the less mobile attachment of a muscle
• Insertion is the more movable point of attachment for a muscle
• Antagonist is a muscle that produces an action that opposes those of the agonist (or prime mover)
• Synergist is a muscle that assists an agonist (or prime mover) in performing its action

Muscle Tissue Growth

• Hypertrophy is an increase in the size of muscle. Each fiber develops more myofibrils, which contain a larger number of myofilaments.
• Hyperplasia is an increase in muscle fibers
• Atrophy is the loss of muscle mass

Muscle Fiber Types

• Slow fibers are small in diameter, located in trunk/lower limbs, slow contractions, high resistance to fatigue/endurance, aerobic metabolism.
• Fast fibers are large in diameter with fast contractions, low resistance to fatigue/endurance, and rely on anaerobic carbs