Chapter 10.1-10.2 Muscular Tissue

10.1 Overview of Muscle Tissue

Different types of muscle tissue:

How do they differ? microscopic anatomy, location, and the way they’re each controlled by nervous and endocrine systems

  • Skeletal Muscle Tissue

    • location: originate and insert on many bones

    • function: move bones of skeleton voluntarily, controlled by neurons (controlled subconsciously, i.e diaphragm and posture and stabilizing muscles)

    • striated - alternating light and dark protein bands, aka striations

    • multinucleated

  • Cardiac Muscle Tissue

    • location: heart wall

    • function: contract and relax heart involuntarily, internal autorhythmicity, rate can be adjusted by hormones and neurotransmitters

    • striated, smaller fibers, branched, cells joined by intercalated disks

    • uninucleate

  • Smooth Muscle Tissue

    • location: walls of internal structures like organs, blood vessels, and airways; skin, attached to hair follicle

    • function: move things through organs/vessels/passageways involuntarily, some muscles have autorhythmicity

    • non-striated - smooth

    • uninucleate

Functions of Muscle Tissue

  • Produce Body Movements

    • skeletal muscles, bones, and joints involved

  • Stabilize Body Positions

    • skeletal muscle contractions stabilize joints

    • ex: posture, standing, sitting

  • Store and Move Substances in the Body (example):

    • contractions of ringlike smooth muscles (“sphincters”) prevent outflow of contents of a hollow organ

    • cardiac muscles contract heart

    • contraction and relaxation of smooth muscle in blood vessels helps regulate blood pressure

  • Generate Heat

    • contraction produces heat via thermogenesis to maintain body temperature

    • ex: shivering

Properties of Muscular Tissue

  • Electrical Excitability

    • ability to respond to stimuli by producing muscles action potentials (APs)

    • stimuli include:

      • autorhythmic electrical signals that occur on muscle tissue itself (ex: heart’s natural pacemaker)

      • chemical stimuli like neurotransmitters released by neurons, hormones, distributed by blood, or local pH changes

  • Contractility

    • ability to contract forcefully when stimulated by a nerve impulse

    • skeletal muscle contractions generate tension while pulling on its attachment points

    • great enough tension can overcome the resistance of an object, resulting in muscle shortening and movement (doesn’t always shorten, like holding something)

  • Extensibility

    • ability of muscle tissue to stretch within limits without being damaged

    • connective tissue in muscle limits range of extensibility, keeping it within contractile region of the muscle cells

    • smooth muscle undergoes greatest amount of stretching

    • cardiac muscle is stretched when heart fills with blood

  • Elasticity

    • ability of muscle tissue to return to its original length and shape after contraction or extension

10.2 Structure of Skeletal Muscle Tissue

Each skeletal muscle is a separate organ made up of hundreds of cells called muscle fibers (myocytes) due to elongated shapes

  • muscle fibers = muscle cells = myocytes

Components:

  • connective tissue components: subcutaneous tissue and fascia

    • function: protects muscle tissue

    • description: surrounds muscle tissue

    • importance:

    • subcutaneous tissue (hypodermis): separates muscle from skin

      • composition: areolar connective tissue, adipose tissue (stores most triglycerides —> insulation and protection from trauma)

      • function: pathway for nerves, blood vessels, and lymphatic vessels to enter/exit muscles

    • fascia

      • location: lines body wall and limbs, surrounds muscles and organs, fills spaces between muscles

      • composition: dense sheet or broad band of irregular connective tissue

      • function: supports muscles and organs, allows free movement of muscles, carries nerves and blood and lymphatic vessels

    • layers that extend from fascia

      • function: protect and strengthen skeletal muscle

      • all are continuous with the connective tissue that attaches skeletal muscle to other structures (i.e to bone/another muscle)

        • could come together to form a tendon

        • could extend as a broad, flat sheet, or an aponeurosis

      • epimysium

        • location: outer layer encircling the entire muscle

        • composition: dense irregular connective tissue

      • perimysium

        • location: surrounds groups of 10-100+ muscle fibers, separating them into muscle fascicles

        • composition: dense irregular connective tissue

        • *application: tearing off a piece of meat, it rips apart along the fascicles

      • endomysium

        • location: penetrates interior muscle fascicles, separating individual fibers from one another

        • composition: mostly reticular fibers

    • disorder: fibromyalgia

      • chronic, painful, nonarticular rheumatic disorder

      • affects the fibrous connective tissue components of muscles, tendons, and ligaments

      • key signs and symptoms

        • pain from gentle pressure at “tender points”

        • muscle pain and tenderness without pressure

        • stiffness of muscles, tendons, and surrounding soft tissues

        • fatigue, poor sleep, headaches, depression, IBS, inability to do daily activities

      • treatment

        • stress reduction, regular exercise, heat, gentle massage, PT, meds for pain, low-dose antidepressant to improve sleep

  • nerve and blood supply

    • generally, 1 artery + 2 veins accompany each nerve that penetrates a skeletal muscle

    • nerve supply

      • somatic motor neurons

        • function: stimulate skeletal muscle contraction

        • composition: the axon extends from brain/spinal cord to a group of muscle fibers, and branches to other muscle fibers

    • blood supply

      • blood capillaries

        • description: microscopic blood vessels that are abundant in muscle tissue

        • function: bring in O2 and nutrients, remove heat and waste products of muscle metabolism, delivers substances to muscle fibers that are needed for contraction

        • each muscle fiber comes into contact with 1+ capillary

  • microscopic anatomy of a skeletal muscle fiber

    • fibers can be 4-12 in. long

    • arises during embryonic development from fusion of many small mesodermal cells, aka myoblasts

    • each fiber has 100+ nuclei

    • once fusion happens, the fiber can no longer do cell division

    • number of muscle fibers is SET BEFORE BIRTH and most last for a lifetime

    • components

      • sarcolemma

        • description: plasma membrane of a muscle fiber

        • location: above nuclei of fibers, surrounds the sarcoplasm

        • function: facilitate AP transmission, barrier between extra and intra cellular compartments (defines individual muscle fibers)

      • T tubules

        • description: tube-like invaginations (folded inside out) of sarcolemma, open to outside of fiber

        • composition: filled with interstitial fluid

        • location: tunnel in from surface toward center of fiber

      • sarcolemma and T tubule arrangement ensures that muscle APs excite all parts of muscle fiber instantly

      • sarcoplasm

        • description: cytoplasm of a muscle fiber

        • composition: glycogen (used to synthesize ATP), myoglobin (red-colored protein that binds oxygen molecules that traveled from interstitial fluid, releases O2 when needed by mitochondria), mitochondria (in rows throughout fiber, intentionally close to the contractile muscle proteins that use ATP)

        • location: within sarcolemma

      • disorders

        • muscular hypertrophy

          • enlargement of existing muscle fibers

          • microscopic cause: increased production of myofibrils, mitochondria, sarcoplasmic reticulum, other organelles

          • physical cause: forceful repetitive muscular activity (i.e strength training)

          • hypertrophied muscles have more myofibrils, so they are capable of doing more forceful contractions

          • testosterone promotes further enlargement of muscle fibers

          • GH and other hormones stimulate increase muscle fiber size

        • fibrosis

          • replacement of muscle fibers by fibrous scar tissue

        • muscular atrophy

          • decreased size of muscle fibers because of a progressive loss of myofibrils

          • “disuse atrophy”: atrophy from lack of use of muscles resulting in flow of impulses is reduced, i.e bedridden individuals, people with casts - REVERSIBLE condition

          • “denervation atrophy”: nerve supply is disrupted or cut, muscle shrinks and is irreversibly replaced by scar tissue

      • myofibrils

        • contractile organelles of skeletal muscles

        • sarcoplasm is stuffed with these thread-like fibers

        • small diameter that extends entire length of muscle fiber

        • prominent striations make muscle fiber look striped/striated

      • sarcoplasmic reticulum (SR)

        • fluid-filled system of membranous sacs

        • encircle each myofibril

        • terminal cisterns: dilated end sacs of SR that butt against T tubule from both sides

        • terminal cistern — T tubule — terminal cistern = triad

        • relaxed SR stores calcium, released calcium from terminal cisterns of SR, triggering muscle contraction

      • filaments (myofilaments)

        • smaller protein structures

        • thin filaments: made of actin

        • thick filaments: made of myosin

        • both are involved in contraction

        • 2 thin filaments for every thick filament

        • arranged in compartments called sarcomeres

      • sarcomeres

        • basic functional units of a myofibril

        • extend from Z disc to Z disc

        • Z discs

          • plate-shaped regions of dense protein material

          • separate sarcomeres

        • A band

          • darker middle part of sarcomere

          • extends entire length of thick filaments

          • toward each end is a zone of overlap - where thick and thin filaments lay side by side

        • I band

          • lighter, denser area

          • contains the rest of the thin filaments, NO thick filaments

          • Z disc passes through each one

          • *letter I is thin - contains thin filaments

        • A and I bands alternate, creating striations

        • H band

          • center of each A band

          • contains thick, NOT thin filaments

          • *letter H is thick - contains thick filaments

        • M line

          • support proteins holding thick filaments together at the center of the H band

          • middle of sarcomere

        • A band is the only thing not contracting or shortening

  • muscle proteins of myofibrils

    • contractile

      • function: generate force during contraction

      • motor proteins: pull cell structures to move, convert ATP to mechanical energy (motion)

        • myosin: main part of thick filaments, 2 golf clubs twisted together

          • tails: thinner end, point toward M line, neighboring tails are parallel to each other to form the shaft of the thick filament

          • heads: have 2 binding sites = actin-binding site and ATP binding site (also functions as ATPase), project outward from shaft in a spiral to extend to 1 of the thin filaments

        • actin

          • main part of thin filament

          • individual molecules join to create an actin filament, twisted in a helix

          • contains a myosin-binding site for a myosin head can attach

        • myosin bound to actin

    • regulatory

      • function: switch contraction process on/off

      • troponin

        • holds tropomyosin in place

        • when Ca2+ binds to troponin, it does a conformational change that moves tropomyosin away from the myosin-binding sites on actin

      • tropomyosin

        • blocks myosin from binding to actin relaxed muscle by covering the myosin-binding sites on actin

    • structural

      • function: keep thick and thin filaments aligned, give elasticity and extensibility, link myofibrils to sarcolemma and extracellular matrix

      • titin

        • most abundant of these, big, spans half of sarcomere

        • connects Z disc to M line to stabilize position of the thick filament

        • elastic - can return to normal state after extreme stretching

        • maintains central location of A bands

      • nebulin

        • long, non-elastic, wrapped around entire length of thin filament

        • anchors thin filaments to Z discs, regulates length of thin filaments during development

      • dystrophin

        • creates attachment between thin filaments of sarcomere to connective tissue matrix surrounding muscle fibers