Muscle Tissue Properties

Introduction to the Muscular System

• More that 700 muscles have been named in the human body.
• Muscle tissue is distributed almost everywhere in the body.
• Muscle tissue facilitates body movements through skeletal muscles.
• Muscle tissue promotes the movement of substances through the Gl tract through smooth muscles.
• Muscle tissue enables the movement of blood through cardiac muscle contractions.
• Muscles contract rather than flex.
• Flexion is a specific movement.

Properties of Muscle Tissue

• There are four unique properties to muscle tissue: excitability, contractility, elasticity, and extensibility.

Excitability

• Muscle cells are very responsive to input from stimuli.
• A stimulus, such as an electrical charge, triggers an electrical change across the muscle cell's plasma membrane.
• An electrical change sparks a muscle contraction.

Contractility

• Electrical changes within muscle cells generates tension with the cell, leading to a contraction.
• A shortening of the fibers pulls on bones of the skeleton or causes a movement of a specific body part.

Elasticity

• Elasticity is "the ability to return to its original length".
• A contracted muscle cell recoils to its resting length when the tension is removed.

Extensibility

• Extensibility is the capability of extending in length.
• It responds to the contraction of opposing muscle cells.
• When the biceps brachii muscle contracts and shortens, the triceps brachii muscles lengthen as an example of extensibility.

Characteristics of Skeletal Muscle Tissue

• A skeletal muscle consists of epithelial tissue, connective tissue, muscle tissue, and neural tissue.
• The sizes of fibers vary.
• A single muscle fiber can be as long as the muscle itself.
• A small skeletal muscle fiber in the toe might measure approximately 100 microns in length and 10 μm in diameter.
• Biceps brachii may have a muscle fiber of about 35 cm in length and 100 µm in diameter.

Functions of Skeletal Muscle Tissue

• Body movement is made possible through muscle tissue.
• Maintaining posture requires muscle tissue.
• Temperature regulation requires muscle tissue.
• Muscle tissue enables the storage and movement of materials.
• Support is provided by muscle tissue.

Temperature Regulation

• Energy, supplied by ATP, fuels muscle tissue contraction.
• Heat is always produced as a byproduct of energy usage.
• Heat generated from muscle contraction maintains normal body temperature.
• Exercising will cause the body to feel warmer.
• When feeling cold, muscles cyclically contract in an attempt to warm the body.

Gross Anatomy of Skeletal Muscle

• Each skeletal muscle is made up of "fascicles" with fascis meaning bundle.
• A fascicle is a bundle of muscle fibers.
• Muscle fibers contain cylindrical structures called myofibrils.
• Myofibrils are composed of myofilaments.

Connective Tissue Components of Skeletal Muscle

• There are 3 concentric layers of CT that encircle each muscle
• These layers are composed of collagen and elastin
• The individual muscle fiber is encircled by the endomysium
• Groups of muscle fibers are encircled by the perimysium
• The entire muscle itself is encircled by the epimysium
• Connective tissue components provide protection.
• They serve as a means of attachment, and blood vessels and nerve distribution sites.

Endomysium

• Endon within, mys = muscle
• Is the innermost CT layer of skeletal muscle and encircles the individual muscle fiber.
• Primarily, it is an areolar CT layer that surrounds and electrically insulates each muscle fiber.
• Reticular fibers help keep capillary beds and other fibers in place

Perimysium

• Peri = around
• Surrounds each fascicle.
• Comprised of dense irregular CT.
• Helps support the "neurovascular bundles" that branch to supply/innervate each individual fascicle.

Epimysium

• Epi = upon
• Is a layer of dense irregular CT surrounding the whole skeletal muscle.

The Fascia

• Deep Fascia – “visceral” or “muscular” fascia
• Includes the "endomysium, perimysium, epimysium".
• Separates individual muscles.
• Binds muscles with similar functions together.
• Fills space between muscles with fascia as a band or filler.
• Superficial Fascia
• Is deep to the dermis and is referred to as “subcutaneous layer".
• Composed of areolar and adipose CT
• Separates the muscle from the skin.

Muscle Attachments

• At the ends of a muscle, the CT layers, specifically deep fascia, merge to form a "tendon".
• Tendons attach muscle to bone, skin, or another muscle.
• Most commonly, tendons form a thick, cord like structure.
• Tendons may at times form a thin, flattened sheet called "aponeurosis".

Muscle Attachments and Bone Contraction

• Most skeletal muscles will cross a mobile joint.
• Upon muscle contraction, one of the bones remain fixed, while the other moves.
• The less mobile attachment of a muscle is called the "origin" which in the limbs, is typically more proximal.
• The more mobile attachment of a muscle is called the "insertion".

Blood Vessels and Nerves

• The epi- and perimysium facilitates an extensive network of blood vessels and nerve fibers.
• Skeletal muscles are classified as “voluntary" muscles, controlled by the "somatic nervous system," which allows voluntary movement.
• The neurons stimulating muscle contraction are called "motor neurons" and have an extension called an "axon" or "nerve fiber".
• The axon or nerve fiber runs through the epimysium and perimysium.
• A motor neuron axon "penetrates" the endomysium and transmits a nerve impulse to a muscle fiber.
• The junction between the axon and the muscle fiber is called a "neuromuscular junction".

Microscopic Anatomy of Skeletal Muscle

• Skeletal muscle cells share components with the typical cell, however many of them are named differently.
• Sarcolemma: plasma membrane
• Sarcoplasm: the cytoplasm
• Unique to muscle fibers are transverse tubules and sarcoplasmic reticulum.

Transverse Tubules

• "T-tubules" are invaginations of the sarcolemma that extend into the sarcoplasm.
• Invaginations form a network of membranous tubules around the myofibrils.
• When a nerve sends an electrical signal to the muscle fiber, the signal travels rapidly along the sarcolemma and enters the T-tubule network.

Sarcoplasmic Reticulum

• It is similar to an endoplasmic reticulum.
• It stores calcium ions (Ca2+).
• Action potentials stimulate the release of Ca2+, which initiates muscle contraction.
• It runs parallel and perpendicular to the muscle fiber: perpendicular tubes are "blind sacs" called “terminal cisternae".
• Two terminal cisternae are located on either side of a T-tubule, forming a “triad".
• Parallel tubes form a network around the myofibrils.

Satellite Cells

• Skeletal muscle fibers are multinucleated.
• During development, groups of embryonic cells called "myoblasts" fuse to form single skeletal muscle fibers.
• As myoblasts fuse, the nuclei remain; thus the nucleus of each myoblast contribute to the cell.
• Some myoblasts do not fuse with other cells.
• They remain in adult skeletal muscle as satellite cells.
• They can be stimulated to differentiate and assist in repair and regeneration.

Myofibrils and Myofilaments

• The sarcoplasm of a skeletal muscle fiber contains hundreds to thousands of cylindrical structures called myofibrils.
• Each myofibril measures 1-2 µm in diameter and extends the entire length of the muscle fiber.
• During muscle contraction, myofibrils shorten in length.
• Myofibrils consist of bundles of myofilaments, which are thread-like.

Myofilaments (Filament)

• "Thick filaments" measure about 11 nanometers in diameter.
• They are assembled from bundles of myosin protein.
• Each strand has a free globular head and an elongated tail so the tails of two proteins are intertwined.
• Myosin heads form “crossbridges” where they bind with thin filaments.
• "Thin filaments" measure 5-6 nanometers in diameter.
• They are composed of two helical strands of the protein actin.
• Two regulatory proteins are also part of the thin filament, troponin and tropomyosin.

Organization of a Sarcomere

• The sarcomere is thefunctional contractile unit of a skeletal muscle fiber.
• It is defined as the distance from one "Z disc" to next "Z disc".
• Myofibrils contain multiple sarcomeres that are lined up end to end, giving skeletal and cardiac muscle a distinctive striated appearance.

Contraction of Skeletal Muscle Fibers

• Motor neuron activity incites skeletal muscle contraction.
• Each muscle fiber is controlled by one motor neuron.
• The point where a motor neuron meets a skeletal muscle fiber is a neuromuscular junction.

Components of the Neuromuscular Junction

• Synaptic knob
• Synaptic vesicles
• Motor end plate
• Synaptic cleft
• Acetylcholine receptor
• Acetocholinestarase

The Synaptic Knob

• Nerve impulses travel down the axon and enters the synaptic knob.
• The synaptic knob is the expanded tip of an axon.
• Covers a large surface area of the sarcolemma.

Synaptic Vesicles

• The cytoplasm of the synaptic knob houses many synaptic vesicles.
• The synaptic vesicles are filled with the neurotransmitter acetylcholine

The Motor End Plate

• Is a specialized region of the sarcolemma with an abundance of folds and indentations.
• The motor end plate is covered by the synaptic knob.
• Folds and indentations increases surface area.

The Synaptic Cleft

• It is the narrow space separating the synaptic knob and the motor end plate.

AcH Receptors

• They are located in the motor end plate and is the specialized region of the sarcolemma.
• The ACh from the synaptic vesicles is released into the synaptic cleft where it binds to its AcH receptors.

Acetylcholinesterase (AChE)

• Is an enzyme residing in the synaptic cleft
• It rapidly breaks down molecules of ACh
• It prevents the continuous stimulation of ACh receptors by Ach.

The Sliding Filament Theory

• When a muscle contracts, thick and thin filaments slide past each other and the sarcomere shortens.
• The thick and thin filaments maintain their same length whether the muscle is relaxed or contracted.
• The relative positions between thick and thin filaments changes.
• The stimulation of a muscle fiber leads to muscle fiber contraction

Muscle Contraction

• A nerve impulse at a neuromuscular junction causes ACh release.
• AcH binds receptors on the motor end plate, initiating a muscle impulse.
• The muscle impulse quickly spreads along the sarcolemma and into the muscle fiber.
• This leads to calcium ions to be released into the sarcoplasm.
• Calcium ions bind to troponin, causing tropomyosin to move and expose active sites on actin.
• Myosin heads attach to the actin and they form crossbridges.
• Myosin heads go through cyclic "attach-pivot-detach-return" events as the thin filaments are pulled past the thick filaments.
• This leads to a sarcomere shortening and contraction.
• ATP is required to detach the myosin heads and complete the cyclic events.
• This process continues so long as calcium ions stay bound to the troponin.
• Calcium ions are moved back into the sarcoplasmic reticulum by ATP-driven ion pumps to reduce calcium concentration in the sarcoplasm, leading to relaxation.

Muscle Contraction - A Summary

• The events of muscle contraction have been called, “excitation-contraction coupling"
• The stimulation of a muscle fiber by a nerve impulse results in a series of events that culminates in muscle fiber contraction

Rigor Mortis

• Rigor mortis is the "stiffening" of the body observed after death because of postmortem muscle contraction.
• Postmortem muscle contraction stems from the depletion of adenosine triphosphate and a resultant development of a stable complex of actin and mysoin.
• It typically begins to develop within 2 hours after death, and the muscles are unable to relax
• Rigor mortis first affects jaw muscles, followed by the face and upper and lower extremities; and disappears in the same order.
• It usually takes 6-12 hours to develop full rigor mortis.
• Violent exercise, depleting ATP, and high body temperatures accelerate the development of the condition.
• In temperate climates, rigor persists 36-48 hours and may disappear in less than 24 hours in hot weather, and persist for several days in cold weather
• Instantaneous rigor mortis (cadaveric spasm) sometimes but rarely affects individuals in cases of sudden deaths.

Motor Units

• Motor units are composed of a single motor neuron that is comprised of all of the muscle fibers it controls.
• A motor unit typically controls some, but not all of the muscle fibers in an entire muscle-
• Many motor neurons are needed to innervate it since most muscles have high number of motor units.
• An inverse relationship exists between the size of the motor unit and the degree of control provided.
• The smaller the motor unit, the finer the control.
• The motor units of the eye innervate only 2-3 muscle fibers.
• Power-generating muscles such as those in the lower limbs can have a single motor unit innervate thousands muscle fibers.
• If a muscle fiber is stimulated, one motor unit will contract, all fibers in the single motor unit will contract, entirely.
• If greater force is needed for a task, MORE motor units are recruited/activated.
• If a task requires less force, FEWER motor units are activated.

Muscle Tone

• Muscle tone is the resting tension in a skeletal muscle, and enables maintained constant tension on tendons
• It stabilizes the position of bones and joints, and produces tension that is insufficient in moving a joint

Isometric Contraction

• Iso = same, metron = measure
• The length of the muscle remains unchanged.
• The tension produced by contraction doesn’t exceed the resistance to exceed the range of force needed to move.
• Force tension still exerted, but inadequate to move load.

Isotonic Contraction

• Iso = same, tonos = tension
• The tension produced from contraction is equal to or greater than the resistance
• Muscle fiber length changes, resulting in movement
• These are concentric, those with a shortening muscle, and eccentric contractions.

Concentric and Eccentric Contractions

• Concentric Contractions
• With contraction, muscle actively shortens
• Load is less than the tension that can be generated by the muscles involved.
• An example is biceps brachii during "flexion”.
• Eccentric Contractions:
• Contraction is tied with the muscles lengthening which
• Occurs with the biceps brachii during "extension"

Types of Skeletal Muscle

• There are 3 types of skeletal muscle fibers: slow oxidative, fast oxidative, and fast glycolytic.
• Each skeletal muscle typically contains a percentage of each of these muscle fiber types.

Fast Versus Slow Fibers

• Fast fibers have higher myosin ATPase, ATP splitting, activity than those in slow fibers do.
• Higher ATPase activity leads to more rapid ATP splitting and a faster energy for cross-bridge cycling.
• One result is a fast twitch, compared to slower twitches where splitting ATP proceeds more slowly.
• Myoglobin is a oxygen binding pigment found in skeletal muscle fibers: stores oxygen received from capillary beds.
• Mitochondria: produce large quantities of ATP via oxidative phosphorylation.

Slow Oxidative (SO) Fibers

• Are otherwise called "Type I", and contain half the diameter of other skeletal muscle fibers.
• Contain slow ATPase.
• Produce contractions that are sluggish to act in response and not strongly powered
• Muscles can stay contracted over long periods of time, fatigue-free
• ATP is primarily supplied through aerobic cellular respiration achieved through mitochondria and oxidative phosphorylization
• They also have extensive vascular supply.
• Fibers are dark red by having high counts of myoglobin (oxygen binding protein found in muscle) with appearance of "dark meat" type fibers

Fast Oxidative (FO) Fibers Types

• The second type, are the “intermediate fibers", type lla.
• They aren't the most numerous of skeletal muscle fibers.
• Of intermediate size with fast ATPase.
• Produces quick powerful contractions.
• ATP provided through aerobic respiration.
• Vascular network doesn't have as many resources like SO fibers.
• The rate of delivery for nutrients and oxygen is comparatively lower, as well as for less colored (myoglobin) than SO fibers.

Fast Glycolitic (FG)

• These are "Type IIb" or "fast anaerobic fibers".
• They represent the greatest prevalent type of skeletal muscles and are, in diameter size, the largest, also containing Fast ATPase.
• They provided high levels of power and speed.
• Contraction has a rate of only short bursts for a period of time and receives supply on ATP thru anaerobic (non-oxygen) means.
• Lack of blood in these fibers turns them colorless, hence a "white meat" texture

Fiber Distribution

• Different fiber types varies between different skeletal muscles as they are classified by type.
• Within a single motor unit, all fibers belong to the same type.
• Some muscles are without SO fibers, while some lack FG fibers .
• SO fibers mainly consist of the skeletal muscle fibers within our backs, and calf muscles.
• The eye and hand muscles have no SO fibers.
• Genetics determine the ratios of fibers, and can determine an particular individual endurance ability.
• Athletic athletes, depending on genetics, will tend to vary which sport they play

Muscle Fiber Composition Effects on Sports Performance

• Individuals with elevated quantities of SO fibers, by hereditary determinants to outperform long distance runner than elevated increased quantity with FG leg components
• Those with FG greater inherited, or increased levels have upper hand in sprinting or weight lifting competition

Skeletal Muscle Fiber Organization

• Bundles of muscles fibers (fascicles) lie "parallel” to each other within each of the following muscles.
• Organization and orientation from muscle to muscle will present four different arrangements: Circular, Parallel, Convergent and Pennate

Circular Muscles

• Muscle fibers are concentrically arranged around an opening or recess.
• Contraction by the sphincter closes the opening, or decreases the diameter of it
• Occurs within cilliary bodies of eye, to change the the area (lens and pupil)
• Typically located at entrances and exits of all internal passageways (as exhibited by the area around lips/ mouth)

Parallel Muscles

• The fascicles run parallel to its long axis.
• They have a central body (belly or gaster.
• Muscle body often shortens/ contracts but thickens (increased diameter and size.)
• The Parallel organization provides for high endurance muscle that are generally not very strong and are
• Exhibited near the sides of our midsection and the lower forearm as Biceps or Abs

Convergent Muscles Muscles

• They have Widespread muscle fibers converge on a common attachment site.
• Attachement site = Single Tenon, Tendinous Sheet, Slender Band of Collagens
• Common shape is triangular of which the versatile pulls allow the muscle to be used at most or modified.
• Contraction across all the muscle fibers is non as forceful, or as a parallel muscle but the widespread effect of muscle pull is not an efficient.

Pennate Muscles

• Tendons and muscle fibers resemble a large feather and at times have greater than one tendon extending through their body
• The arrangement of fibers that help generate tension (pull) relative to the tendon moves as far as a parallel muscle can within its tendon.
• They can generally generate more tension than a parallel muscle of the same size, for these reasons it is common with: muscle fibers that are on the same side of the tendon (Unipennate) with extensor at digitorum
• Bipennate muscle is most common on all sides
• Multipennate muscle as seen with brachii deltoid.

The Naming of Skeletal Muscles

• Skeletal muscles are named according to the following criteria:
• Muscle action
• Specific body regions
• Muscle attachments
• Orientation of muscle fibers
• Muscle shape and size
• Muscle heads or tendons of origin

Characteristics of Cardiac and Smooth Muscles

• Learning Objectives:

1. Describe the similarities and differences among the types of muscle tissue.

Cardiac Muscles

• Cardiac myocytes with individual muscle cells are configured in compact (thick bundles) in myocardium
• Striated, shorter, and thicker configuration
• Only one or two nuclei exist in the cardiac muscle
• In the cardiac muscle, Y-shaped branches are formed, which join at "intercalated discs”.
• The cardiac muscle is autorhythmic.
• Individual cells can generate an impulse without nervous stimulation (involuntary) and is also
• Linked to to rhythm of heart beat (as described later in book)

Cardiac Muscle Dependent on Calcium

• The heart also hinges upon dependency on calcium ions for contractions and are:
• Sarcoplasmic reticula (specialized smooth ER)
• Less developed/ configured less for the purposes of less calcium storage volume
• Most calcium ions stimulate contraction and occur in the Interstitial fluids
• Are aerobic by nature/ use only aerobic respiration
• Abundant cardiac myocyte/ mitochondrion volume

Smooth Muscle Characteristics

• Fibers are spindle and fusiform by having short cells
• Single, Center nucleus
• Thick filaments can be both precisely thick or short , depending
• Dense bodies - cytoskeletal atatchments around cells with Sarcoplasm
• Can be stimulated by use Calcium of interstitial fluid
• Cytoplasm calcium regulation is tied with contractive functions
• Is slow and and under involuntary control for smoothness of function

Test your knowledge of muscle tissue properties, including extensibility, elasticity, and excitability. Explore the roles of acetylcholinesterase and the neuromuscular junction. Match muscle tissue properties with their descriptions.