Muscle Tissue Types: Skeletal, Cardiac, Smooth

• Muscular tissue consists of specialized cells that contract and generate force.
• The function is movement, posture, and heat generation.
• There are three tissue types: skeletal, cardiac, and smooth muscle
• Myology studies muscle structure, function, and disorders.

Skeletal Muscle Tissue

• Long, cylindrical, striated fibers are seen via microscope
• Control is voluntary, regulated by the somatic nervous system.
• Location of skeletal muscle is attached to bones through tendons.
• The function of this muscle type is to produce body movements, maintain posture, and generate heat (thermogenesis).

Cardiac Muscle Tissue

• Cardiac muscle contains branched, striated fibers with one or two nuclei.
• Cardiac muscle contains intercalated discs (gap junctions + desmosomes).
• Control is involuntary and regulated by the autonomic nervous system, and hormones
• Location is exclusively in the heart.
• Cardiac muscle pumps blood.

Smooth Muscle Tissue

• The structure is spindle-shaped, with non-striated fibers and a single nucleus.
• Control is involuntary and regulated by the autonomic nervous system, hormones, and local factors.
• It is located in the walls of hollow organs, including the iris of eyes, airways in lungs, blood vessels, intestines, gallbladder, urinary bladder, stomach and skin/hair follicles
• This muscle moves substances through internal organs via peristalsis and regulates vessel diameter.

Relation to other structures

• Skeletal muscles attach to bones through tendons.
• Muscles pull on bones to create movement.
• Muscles cross joints and movement occurs through contraction and relaxation.

Skeletal and Cardiac Muscle tissue

• Cardiac muscle is involuntary and autorhythmic, unlike skeletal.
• Autorhythmicity allows the ability to contract autonomously due to pacemaker cells that generate action potentials.

Smooth muscle

• Both cardiac and smooth muscle are involuntary and rely on the autonomic nervous system for regulation.
• Both can undergo sustained contractions.

Producing Body Movements

• Skeletal muscles work with bones and joints to create movement.

Stabilizing Body Positions

• Postural muscles keep the body upright through sustained contractions for storage and prevents outflow from hollow organs.

Storing and Moving Substances

• Smooth muscle regulates organ functions through peristalsis.

Generating Heat

• Muscle contractions aid maintenance of body temperature (thermogenesis).
• Thermogenesis contracts tissue to produce heat.
• Shivering is the involuntary contractions of skeletal muscles and can increase the rate of heat production

Electrical Excitability

• Excitability is a property of both muscle and nerve cells.
• The electrical excitability enables the ability to respond to certain stimuli by generating electrical signals called action potentials.
• Action potential(impulses) referred to as muscle action potentials, those in nerve cells are called nerve action potential

Stimuli

• Autorhythmic electrical signals arise in the muscular tissue itself, in the heart's pacemaker.
• Chemical stimuli, such as neurotransmitters released by neurons, hormones in the blood and changes in pH.

Contractility

• Contractility enables the ability of muscular tissue to contract fully when stimulated

Extensibility

• Extensibility allows the ability of muscular tissue to stretch without damage, though the connective tissue limits the range.
• Smooth muscle endures the greatest amount of stretching

Elasticity

• Elasticity is the ability of muscular tissue to return to its original length/shape after contraction or extension

Skeletal Muscle Fibers Composition

• Skeletal muscles are organs that consist of hundreds to thousands of muscle fibers (myocytes).
• Myocytes' long, elongated shapes allow contraction

Connective Tissue Components

• The components surrounds and protects muscular tissue
• The subcutaneous layer/hypodermis is composed of areolar connective tissue and adipose tissue.
• The layer provides for nerves, blood vessels and lymphatic vessels to enter and exit

Adipose Tissue

• Adipose stores most of the body’s triglycerides, is an insulating layer (reduces heat loss) and protects muscles from trauma

Fascia

• Irregular connective tissue which lines body wall and supports and surrounds muscles and other organs.

Three Layers of Connective Tissue

• Epimysium is the outer layer which surrounds the entire muscle and is a dense irregular connective tissue.
• Perimysium surrounds groups of 10 to 100+ muscle fibers called fascicles and is also dense irregular connective tissue.
• Endomysium penetrates each fascicle and separates individual muscle fibers from one another and is mostly reticular fibers.
• The three layers extend beyond fibers to form a ropelike tendon linking the muscle to the periosteum of a bone.

Myofibrils

• Contractile elements made of sarcomeres

Sarcoplasmic Reticulum (SR)

• A fluid system of membranous sacs encircling each myofibril which stores calcium ions (Ca²⁺) essential for contraction.

Muscular Hypertrophy

• Increase in muscle fiber size due to resistance training; enlargement of fibers after birth

Fibrosis

• Replacement of muscle tissue with scar tissue reduces function.

Muscular Atrophy

• Loss of muscle mass due to inactivity, aging, or disease.

Terminal Cisterns

• Dilated end sacs of the sarcoplasmic reticulum

Triad

• Formed by a transverse tubule and 2 terminal cisterns on either side

Filaments

• Small protein structures within myofibrils

Sarcomeres

• Functional units of myofibril and of contractile structures between two Z discs.

Z Discs

• Narrow, plate-shaped regions separating one sarcomere from the next that serves as attachment sites for thin filaments (actin).

A Band

• The dark middle of the sarcomere which contains the length of thick filaments (myosin).

I Band

• The lighter region of the sarcomere and holds only thin filaments, located between two adjacent sarcomeres and bisected by the Z disc.

H Zone

• The center of the A band which contains only thick filaments, visible during muscle relaxation; disappears during as filaments slide past each other.

M Line

• Protein structure in the H zone that holds filaments together at the middle of the sarcomere, stabilizing their positions.

Skeletal Muscle Fiber Proteins

• Contractile proteins generate force during muscle contractions
• Myosin: a contractile protein in thick filaments with a tail and heads that bind to actin during contraction.
• Actin: a contractile protein in thin filaments with myosin-binding sites for muscle contraction.

Regulatory Proteins

• Proteins for switching muscle contraction on and off

Tropomyosin

• Regulatory protein that blocks Myosin-binding sites when muscle is relaxed.
• Troponin is a regulatory protein that moves Tropomyosin to allow muscle contraction.

Structural Proteins for Filament Alignment

• Titin: a protein that stabilizes thick filaments, gives elasticity
• α-Actinin: a protein in Z discs that connects thin filaments & titin.
• Myomesin: the substance forming the M line and links filaments.
• Nebulin: Wraps thin filaments, anchoring them to Z discs.
• Dystrophin: Links thin filaments to membrane.

Sliding Filament Mechanism

• Describes how muscle contraction occurs
• States that actin slide past thick filaments (myosin), and shortening the sarcomere
• Myosin heads attach to actin filaments and pull them toward the M line which requires ATP.
• Thick and thin filaments remain the same-only their overlap changes.

Changes During Contraction

• The I band narrows.
• The H zone disappears.
• The A band remains unchanged.
• The Z discs move closer together

The Contraction Cycle

• The 4 repeating steps allow the movement of filaments toward the center of the cycle: ATP Hydrolysis, Cross-Bridge Formation, Power Stroke, and Detachment of Myosin from Actin.

Contraction Cycle steps:

• Myosin heads split ATP via Hydrolysis: energizes the myosin heads, forming cross-bridges strengthening the bond between myosin and actin.
• Power stroke: Myosin heads pivot which releases ADP.
• The cycle repeats if (Ca²⁺) remains present

Excitation-Contraction Coupling

• The sequence of events converts a nerve impulse (electrical signal) into a muscle response; steps are action potential propagation, calcium release, Troponin-Tropomyosin shift, cross-bridge formation, and calcium reuptake/relaxation.

Rigor Mortis

• Postmortem muscle stiffening due to Ca leakage.
• Myosin binds to actin, but ATP production stops.
• Unable to detach, keeping muscles rigid lasts about 24 hours.

Length-Tension Relationship

• Force a muscle fiber can generate which depends on the initial length of the sarcomere before contraction.
• Optimal sarcomere length maximizes tension with maximal overlap.

NMJ/Neuromuscular Junction

• Synapse of the motor neuron and muscle fiber.
• Structure: Synaptic End Bulb (Neuron Side), Synaptic Cleft, and Motor End Plate (Muscle Side)

Electrical Signal Translation

• ACh receptors initiate a muscle action potential.
• A nerve impulse triggers the opening of voltage-gated Ca2+ channels, which cause synaptic vesicles to release ACh into synaptic cleft as it depolarizes the muscle fiber.
• Action potential triggers influx of Na+ which leads to excitation-contraction coupling and spreads along the T-tubules.

Muscle Relaxation

• Acetylcholinesterase (AChE) breaks own ACh to stop the signal and allow the muscle to relax.

Electromyography (EMG)

• EMG measures muscle activity to diagnose muscle and nerve disorders showing the signals as waves and sound for assessment

Energy Pathways - ATP in Muscle Fibers

• The three main pathways are CP system, anaerobic glycolysis, and aerobic respiration.
• Creatine phosphate is for short bursts.
• Anaerobic glycolysis produces ATP without oxygen (up to 2 min).
• Aerobic respiration produces the most ATP and is for long-duration activities.

Muscle Fatigue

• Muscle decline in ability due to low ATP, ion imbalances, or buildup of lactic acid.
• Excess Post-exercise Oxygen Consumption (EPOC) (after exercise) restores ATP, removes lactic acid, and replenishes myoglobin oxygen stores.
• During the extra oxygen intake following exercise, lactic acid is converted back into glycogen within the liver.

Metabolism

• Metabolic changes occur with metabolic changes which do not fully explain extra the oxygen usage fully.
• Only some lactic acid converted to glycogen

Lactate and ATP

• Lactic acid is converted into pyruvic acid - for ATP production

Control of Muscle Tension

• Motor units are made of a single neuron plus all muscle fibers stimulate

Twitch Contraction

• Brief contraction following action potential

Twitch

• Electrical stimulation of a motor neuron/muscle fiber that causes muscle contraction (Myogram)

Latent Period

• Brief delay between stimulus and start of contraction.

Contraction Period

• Generates tension as actin and myosin filaments slide past

Relaxation Period

• Ca is pumped into reticulum, detach, and muscle decreases = 10-100ms depending on the type.

Refraction vs Frequency

• Activation causes non-response Wave summation occurs
• Multiple causes increase force of contraction
• Tetanus/sustained contraction causes individual twitches which cant be detected.

Motor Recruitment

Activation of all and more units to increase force of muscle

Anaerobic/Aerobic

• Anaerobic (weight-lifting) - build muscle mass and make stronger
• Aerobic(running) - improves cardiovascular health increase endurance

Types of Body Tones

• Body tone causes tension for weakness, involuntary
• Faccid causes limpness/loss in which muscle tone is

Definition

Hypotonia - Decrease muscle tone that lead to weakness and floppiness Flaccid paralysis - certain disorders of the nervous system and disorders that cause imbalance.

Types of Muscle Tones

Hypertonia - Excessive tone causes stiffness. Spasticity - increase tone associated during increase in pathological reflex Partial para is where muscle is spastic to paralysis

Rigidity Refer to Increase

Reflex unaffected affect as lockjaw caused by (bacterium entry + open wound).

Muscular Contractions

• Isotonic: shortening muscle with lifting weight, concentric, eccentric maintains tension while lowering
• Isometric: Length doesnt effect and maintains the same strength.

Fibers of Skeletal Muscle

Oxidative/Small - fatigue resistant, endurance Oxidative/Medium - fatigue resistant uses both sprints and aerobic Glyconic - fatigue reliance on anaerobics

Tissue

• Use dynamic for exercise.

Dynamic vs Static

• Dynamic increase the mobility
• Static improves flexibilty

Strength

Progressive overload increases strength. Anabolic steroids - builds with synthetic testosterone w/ severe side effects

Cardiac

• Location: heart, Branched, contain 1-2 nuclei.
• Intercalated desmosomes transmit with electrical energy
• Control: Involuntary, regulated by ANS/Hormones
• Function: Pump continuously
• Auto generates/sinconotricity/ larger mitocondria that is sustained.

Smooth vs tone

• Tone: found in small walls of body, and organs Contractions - can reduce stress but affect contracting.

Smooth Tone

• State of continuation to contraction. and maintain ability effectively
• Slow contractions related to cardiac.
• Limited regeneration
• Skeletal helps limited due to presence of more cells
• Regenerate limited of pericytes
• Originates The skeletal is mesoderm