Muscle Overview

Types and Functions of Muscle Tissue

Muscle tissue is essential for movement and force generation in the human body. Muscles convert chemical energy from ATP into mechanical energy, enabling locomotion and other vital functions.

• Types of Muscle Tissue: skeletal, cardiac, and smooth.

• These types differ in structure, location, function, and means of activation.

• Skeletal muscle is responsible for voluntary movements and is attached to bones.

• Cardiac muscle is found only in the heart and contracts involuntarily.

• Smooth muscle is located in the walls of hollow organs and is also involuntary.

Muscle Similarities

Common Features of Muscle Cells

Despite their differences, all muscle cells share certain structural and functional characteristics.

• Muscle fibers: Skeletal and smooth muscle cells are elongated and referred to as muscle fibers.

• Muscle contraction depends on two kinds of myofilaments: actin and myosin.

• Muscle terminology is similar across types:

  • Sarcolemma: muscle plasma membrane

  • Sarcoplasm: cytoplasm of a muscle cell

  • Prefixes myo-, mys-, and sarc- all refer to muscle

Skeletal Muscle Tissues

Structure and Function

Skeletal muscle tissue is packaged in muscles that attach to and cover the bony skeleton. It is responsible for voluntary movements and overall body mobility.

• Controlled voluntarily (by conscious control)

• Adaptable and can exert forces ranging from a fraction of an ounce to over 70 pounds

Cardiac Muscle Tissue

Characteristics and Function

Cardiac muscle tissue is found only in the heart. It is striated like skeletal muscle but contracts involuntarily.

• Contracts at a steady rate set by the heart's pacemaker

• Neural controls allow the heart to respond to changes in bodily needs

Smooth Muscle Tissue

Location and Function

Smooth muscle tissue is found in the walls of hollow visceral organs (e.g., stomach, urinary bladder, respiratory passages). It helps regulate the passage of substances and is not striated.

• Involuntary contraction

• Can contract in the absence of nervous system stimulation via cells of Cajal

Muscle Function

Roles of Muscle Tissue

Muscles are responsible for all locomotion and many other functions.

• Cardiac muscle circulates blood

• Smooth muscle maintains blood pressure, squeezes or propels substances (e.g., food, feces) through organs

• Muscles also maintain posture, stabilize joints, and generate heat

Functional Characteristics of Muscles

Properties of Muscle Tissue

Muscle tissue exhibits several key functional characteristics:

• Excitability (Irritability): Ability to receive and respond to stimuli

• Contractility: Ability to shorten forcibly when stimulated

• Extensibility: Ability to be stretched or extended

• Elasticity: Ability to recoil and resume original resting length

Skeletal Muscle Structure

Organization and Connective Tissue Wrappings

Skeletal muscle is a discrete organ composed of muscle tissue, blood vessels, nerve fibers, and connective tissue.

• Connective tissue sheaths reinforce the muscle and prevent it from bursting during strong contractions.

• Epimysium: Overcoat of dense regular CT surrounding the entire muscle

• Perimysium: Fibrous CT surrounding groups of muscle fibers called fascicles

Skeletal Muscle: Nerve and Blood Supply

Innervation and Vascularization

Each muscle is served by one nerve, an artery, and one or more veins.

• Each muscle fiber is supplied with a nerve ending that controls contraction

• Continuous delivery of oxygen and nutrients via arteries is required

• Wastes are removed by veins

Skeletal Muscle: Attachments

Muscle-Bone Connections

Muscles span joints and are attached to bones in two ways:

• Directly: Epimysium of muscle fused to periosteum of bone

• Indirectly: CT wrappings extend beyond muscle as tendon or aponeurosis

• Ligaments: Attach muscle to bones at joints; they cross the joint

Microscopic Anatomy of a Skeletal Muscle Fiber

Cellular Structure

Muscle fibers are long, cylindrical cells with multiple nuclei just beneath the sarcolemma.

• Fibers are 10–100 μm in diameter and up to hundreds of centimeters long

• Each cell is a syncytium produced by fusion of embryonic cells

• Contains glycosomes (glycogen storage) and myoglobin (oxygen-binding pigment)

• Fibers contain usual organelles, myofibrils, sarcoplasmic reticulum, and T tubules

Myofibrils

Structure and Banding Pattern

Myofibrils are densely packed, rod-like contractile elements that make up most of the muscle cell volume.

• Exhibit aligned repeating series of dark A bands and light I bands

• Other organelles are squeezed between myofibrils

Sarcomeres

Functional Unit of Muscle

The sarcomere is the smallest contractile unit of muscle, defined as the region between two successive Z discs.

• Composed of myofilaments made up of contractile proteins

• Myofilaments are of two types: thick and thin

Myofilaments: Banding Pattern

Arrangement and Function

Thick filaments extend the entire length of the A band; thin filaments extend across the I band and partway into the A band.

• Coin-shaped sheet of proteins (connectins or alpha actinins) anchors thin filaments and connects myofibrils

• Thick and thin filaments overlap in the A band

• Desmin protein appears darker due to its presence

Ultrastructure of Myofilaments: Thick Filaments

Myosin Structure

Thick filaments are composed of the protein myosin.

• Each myosin molecule has a rod-like tail and two globular heads

• Two interwoven, heavy polypeptide chains

• Two smaller, light polypeptide chains called cross bridges

Ultrastructure of Myofilaments: Thin Filaments

Actin Structure

Thin filaments are chiefly composed of the protein actin.

• Actin is a helical polymer of globular subunits called G actin

• G actin contains the active sites to which myosin heads attach during contraction

• Tropomyosin and troponin are regulatory subunits bound to actin

Sarcoplasmic Reticulum (SR)

Calcium Regulation

The SR is an elaborate smooth endoplasmic reticulum that regulates intracellular calcium levels.

• SR forms cross channels called terminal cisternae

• Functions in the regulation of intracellular calcium levels

• T tubules penetrate the cell's interior at each A band–I band junction

• T tubules associate with paired terminal cisternae to form triads

T Tubules

Impulse Conduction

T tubules conduct impulses to the deepest regions of the muscle, ensuring rapid signal transmission for contraction.

• Signal for the release of Ca2+ from adjacent terminal cisternae

Contraction of Skeletal Muscle Fibers

Mechanism of Contraction

Contraction refers to the activation of myosin's cross bridges (force-generating sites).

• Shortening occurs when tension generated by cross bridges exceeds forces opposing shortening

• When cross bridges become inactive, tension declines and relaxation is induced

• Muscle fibers routinely shorten by 30–35%

Sliding Filament Mechanism of Contraction

How Muscles Shorten

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

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

• Myosin heads bind and detach several times during contraction, acting like a ratchet to generate force and propel thin filaments to the center of the sarcomere

• As sarcomeres shorten, the muscle shortens

Role of Ionic Calcium (Ca2+) in the Contraction Mechanism

Calcium's Role in Muscle Contraction

Calcium ions are crucial for the regulation of muscle contraction.

• At low intracellular Ca2+ concentration, myosin cross bridges cannot attach to binding sites on actin

• At higher Ca2+ concentrations, Ca2+ binds to troponin, causing a conformational change that moves tropomyosin away from actin's binding sites

• This permits contraction (sliding of thin filaments by myosin cross bridges) to begin

Sequential Events of Contraction

Steps in Muscle Contraction

• Cross bridge formation: myosin cross bridge attaches to actin filament

• Working (power) stroke: myosin head pivots and pulls actin filament toward M line

• Cross bridge detachment: ATP attaches to myosin head and the cross bridge detaches

• Cocking of myosin head: energy from hydrolysis of ATP cocks the myosin head into the high energy state

Regulation of Contraction

Requirements for Muscle Contraction

For a skeletal muscle to contract, the following must occur:

• Be stimulated by a nerve ending

• Propagate an electrical current, or action potential, along its sarcolemma

• Increase in intracellular Ca2+ levels, the final trigger for contraction

• Linking the electrical (neurological) signal to the contraction is called excitation-contraction coupling

Additional info:

• ATP is the direct energy source for muscle contraction. It is regenerated by creatine phosphate, anaerobic glycolysis, and aerobic respiration.

• Muscle fatigue occurs when ATP production cannot keep pace with ATP use, leading to physiological inability to contract.

• Oxygen debt refers to the extra amount of oxygen needed for restorative processes after muscle activity.

• Muscle tone is the constant, slightly contracted state of muscles, important for posture and joint stability.

• Motor unit consists of a motor neuron and all the muscle fibers it supplies; fine control muscles have small motor units, large muscles have large motor units.

• Muscle twitch is the response of a muscle to a single brief stimulus.

• Graded muscle responses are required for proper control of skeletal movement and are influenced by stimulus frequency and strength.

• Isotonic contractions result in muscle shortening and movement; isometric contractions increase tension without changing muscle length.

• Smooth muscle contracts via peristalsis and can undergo hyperplasia (increase in cell number).

• Age-related changes include loss of muscle mass (sarcopenia) and decreased regenerative ability, but regular exercise can reverse some effects.

• Developmental aspects include improved neuromuscular coordination with age and differences in muscle mass between males and females due to sex hormones.