Muscle Tissue

Overview: Muscle Tissue Types 1. Skeletal: attached to bones; striated, voluntary 2. Cardiac: forms wall of heart; striated, involuntary 3. Smooth: located in viscera; non-striated, involuntary

Functions of Muscle Tissue Body movements 2. Stabilization 3. Organ composition 4. Moving substances within body 5. Generating body heat

Muscle Tissue Properties Electrical Excitability: ability to respond action potential (impulse)

Muscle Tissue Properties Contractility = ability to shorten and thicken, generating force to do work Isometric contraction = muscle develops tension but does not shorten Ex/ Postural Muscles Isotonic contraction = the tension remains constant while the muscle shortens Ex/ Flexing Bicep

Muscle Tissue Properties Extensibility = ability to extend without damaging tissue Elasticity = ability to return to original shape after contraction or extension

Skeletal Muscle Tissue

Skeletal Muscular Tissue Each skeletal muscle is a separate organ composed of cells called fibers.

Muscle Fiber Arrangement Made up of bundles of Fasicles Fasicle Bundle of muscle fibers Muscle Fibers Muscle Cell Myofibrils Bundles of Thick and Thin Filaments Thick and Thin Filaments Made up of muscular proteins

Connective Tissue Components of Skeletal Muscle

Connective Tissue Components Fascia A sheet or band of fibrous connective tissue that is deep to the skin and surrounds muscles and other organs of the body

Connective Tissue Components Superficial Fascia Separates muscle from skin Deep Fascia Lines the body wall and limbs and holds muscles together, allows free movement of muscles

Connective Tissue Components Epimysium = covering entire muscle Perimysium = covering fascicles Endomysium = covering individual muscle fibers All are extensions of deep fascia.

Connective Tissue Components Tendons Attach muscles to bones Aponeurosis Tendon that extends as a broad, flat layer.

Muscle Attachment Origin - tendons attach muscle to a stationary bone Insertion - muscle attaches to moving bone (usually distal)

Microscopic Anatomy of Skeletal Muscle Fiber

Microscopic Anatomy of Skeletal Muscle Fiber Sarcolemma = muscle cell membrane Sarcoplasm = muscle cell cytoplasm Contains Glycogen – energy storage Myoglobin – oxygen storage

Microscopic Anatomy of Skeletal Muscle Fiber T-Tubules Tiny depressions of sarcolemma (muscle cell membrane) - Allows nerve impulses to reach muscle fibers

Microscopic Anatomy of Skeletal Muscle Fiber Myofibrils thin and thick filaments of muscle fiber Sarcoplasmic Reticulum Encircles each myofibril; stores calcium ions

Microscopic Anatomy of Skeletal Muscle Fiber Myofibrils are composed of thick and thin filaments arranged in units called Sarcomeres Motor Unit Nerve associated with a muscle fiber

Microscopic Anatomy of Skeletal Muscle Fiber Sarcomeres Basic functional units of a myofibril; show distinct dark (A band) and light (I band) areas.

Microscopic Anatomy of Skeletal Muscle Fiber 1. The darker middle portion is the A band; consisting of the thick filaments with some thin filaments overlapping the thick ones. 2. The lighter sides are I bands, which consist of thin filaments only. 3. A Z disc passes through center of I band. 4. A narrow H zone in center of each A band contains thick but no thin filaments.

Muscle Fiber Types Slow Twitch Fast Twitch Intermediate Fibers Called “Red Fibers” Contain myoglobin (stores oxygen for cellular respiration) Can contract for long periods without fatiguing Fast Twitch Called “White Fibers” Contain less myoglobin Contract rapidly; fatigue easily Intermediate Fibers These fibers have have the fast twitch speed associated with white fibers combined with a substantial oxidative capacity associated with red fibers

Muscle Proteins

Contractile Proteins Myosin Main component of thick filaments; functions as motor proteins 2. Actin Main component of thin filaments; connects to myosin for the sliding together of the filaments

Regulatory Proteins Switch contractions “on” and “off”. Tropomyosin and troponin are bound to the thin filament

Structural Proteins Keep thick and thin filaments in proper alignment; give myofibril elasticity and extensibility.

Structural Proteins Titin = helps sarcomere return to its resting length after contraction. 2. Myomesin = forms M line 3. Nebulin = helps maintain alignment of thin filaments in the sarcomere 4. Dystrophin = reinforces sarcolemma and helps transmit the tension generated by the sarcomeres to the tendons

Contraction and Relaxation of Skeletal Muscle Fibers

Sliding Filament Theory During muscle contraction, myosin cross-bridges pull on thin filaments, causing them to slide inward toward the H Zone Z discs come toward each other and the sarcomere shortens but the thick and thin filaments do not change in length. The sliding filaments and shortening of sarcomeres causes the shortening of the whole muscle fiber and ultimately the entire muscle.

The Contraction Cycle At the beginning of contraction, the sarcoplasmic reticulum releases calcium ions, which bind to troponin and cause the troponin- tropomysium complex. This uncovers the myosin- binding sites on actin. When the binding sites are “free” the contraction cycle begins.

Excitation and Coupling Phase An increase in calcium ion concentration in the cytosol starts contraction; a decrease stops it. The muscle action potential releases calcium ions from the sarcoplasmic reticulum that combine with troponin, causing it to pull on tropomyosin to change orientation, thus exposing myosin-binding sites on actin and allowing the actin and myosin to bind together.

Excitation and Coupling Phase Calcium ions remove the contraction inhibitor and allows contraction of the muscle fiber Calcium ion active transport pumps return calcium ions to the sarcoplasmic reticulum

Summary: Sliding Filament Theory

The Neuromuscular Junction

The Neuromuscular Junction Muscle action potentials arise at the neuromuscular junction (NMJ), the synapse between a somatic motor neuron and a skeletal muscle fiber. Synapse A region of communication between two neurons or a neuron and a target cell.

The Neuromuscular Junction Synapses separate cells from direct physical contact. Neurotransmitters bridge the gap. The neurotransmitter at an NMJ is acetycholine (Ach).

Muscular Responses

Muscle Twitch Cycle of contraction and relaxation of a muscle fiber. When a muscle fiber contracts, it will contract completely Called “All-or-none” response Latent Period Time between stimulus and muscle contraction Contraction Phase Muscle contracts Refractory Period Relaxation phase in which muscle cannot respond

Sustained Contractions Even when a muscle is at rest, its fibers maintain muscle tone. Example: back muscles that control posture

Recruitment The process of increasing the number of active motor units. Prevents fatigue and helps provide smooth muscular contraction rather than jerky motions. Aerobic training builds endurance and anaerobic training builds muscle strength.

Muscle Tone A sustained partial contraction of portions of a relaxed skeletal muscle. Essential for maintaining posture.

Striated and Involuntary Cardiac Muscle Tissue Striated and Involuntary

Cardiac Muscle Tissue Cardiac muscle tissue is found only in the heart wall. Fibers are arranged similarly to skeletal muscle fibers. Connect to adjacent fibers via intercalated disks.

Cardiac Muscle Tissue Contract when stimulated by own autorythmic fibers. This continuous rhythmic activity is a major physiological difference between cardiac and skeletal muscle tissue.

Non-Striated and Involuntary Smooth Muscle Non-Striated and Involuntary

Smooth Muscle – Two Types Visceral Found in walls of hollow viscera and small blood vessels. Multiunit Found in large blood vessels, large airways, arrector pilli muscles, and the iris of the eye.

Regeneration of Muscle Tissue

Regeneration of Skeletal Muscle Tissue Skeletal muscle fibers cannot divide and have limited powers of regeneration. The number of new fibers formed is minimal. Extensive repair results in fibrosis, the replacement of muscle fibers with scar tissue.

Regeneration of Cardiac Muscle Tissue Cardiac muscle fibers cannot divide or regenerate. Once the heart is injured, it cannot repair itself.

Regeneration of Smooth Muscle Tissue Smooth muscle fibers have limited capacity for division or regeneration. Example: Repair of blood vessels Vessels are composed of smooth muscle

Aging and Muscle Tissue At 30 years of age, there is a progressive loss of skeletal muscle, which is replaced by fat. There is also a decrease in maximal strength and a slowing of muscle reflexes.

Rigor Mortis Ca++ pumps run out of ATP Ca++ cannot be removed Continuous contraction Eventually tissues break down

Muscle Atrophy Decrease in size of muscle fibers Disuse atrophy - bedridden individuals, casts Denervation atrophy loss of nerves and muscle function

Muscular Diseases Muscular Dystrophy Myasthenia gravis Loss of muscle fibers Linked to young males (ages 3-5) Lacking certain protein thus allows too much Ca+ into cell this leads to cell death and replacement with scarring Myasthenia gravis autoimmune disease antibodies bind to ACh receptors atrophy of muscle fibers drugs that keep levels of ACh high are used