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Muscle Tissue. 3 Types of Muscle Tissue. Skeletal muscle attaches to bone, skin or fascia striated with light & dark bands visible with scope voluntary control of contraction & relaxation. 3 Types of Muscle Tissue. Cardiac muscle striated in appearance involuntary control

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3 types of muscle tissue
3 Types of Muscle Tissue

  • Skeletal muscle

    • attaches to bone, skin or fascia

    • striated with light & dark bands visible with scope

    • voluntary control of contraction & relaxation

3 types of muscle tissue1
3 Types of Muscle Tissue

  • Cardiac muscle

    • striated in appearance

    • involuntary control

    • autorhythmic because of built in pacemaker

3 types of muscle tissue2
3 Types of Muscle Tissue

  • Smooth muscle

    • attached to hair follicles in skin

    • in walls of hollow organs -- blood vessels & GI

    • nonstriated in appearance

    • involuntary

Skeletal muscles
Skeletal Muscles

  • Attach to bones

  • Produce skeletal movement (voluntary)

  • Maintain posture

  • Support soft tissues

  • Regulate entrances to the body

  • Maintain body temperature

  • muscle fibers increase in size during childhood = human GH

  • testosterone also increases muscle size

  • mature muscle fibers range from 10 to 100 microns in diameter

  • typical length is 4 inches - some are 12 inches long

Properties of Skeletal Muscles

Electrical excitability

-ability to respond to stimuli by producing electrical signals

such as action potentials

-two types of stimuli: 1. autorhythmic electrical signals

2. chemical stimuli


-ability to contract when stimulated by an AP

-isometric contraction: tension develops, length doesn’t change

-isotonic contraction: tension develops, muscle shortens


-ability to stretch without being damaged

-allows contraction even when stretched


-ability to return to its original length and shape

Gross Anatomy

  • muscles are really groups of

  • fascicles

  • the fascicles are groups of muscle

  • fibers = considered to be an

  • individual muscle cell

  • each myofibril is comprised of

  • repeating units = sarcomeres

Gross anatomy
Gross Anatomy

  • muscle is wrapped in a protective fascia

  • -fascia = sheet of fibrous connective tissue that supports

  • and surrounds muscle or organs

  • a superficial fascia separates muscle from the overlying skin

  • -also known as the subcutaneous layer

  • -made up of areolar tissue and adipose tissue

  • -provides support for blood vessel and nerves

  • -the adipose tissue stores most of the body’s triglycerides

  • and provides insulation

  • muscles with similar functions are grouped and held together by layers

  • of deep fascia

  • -dense irregular connective tissue

  • -allow free movement of muscles, carries nerves, BVs

  • three layers of connective tissue surround a muscle

    • Epimysium

    • Perimysium

    • Endomysium

  • these layers further strengthen and protect muscle

  • outermost layer = epimysium

    • encircles the entire muscle

    • separates them into bundles = fascicles

  • next layer = perimysium

    • divides and surrounds groups of 10 to 100 individual

    • muscle fibers

    • give meat its “grain” because the fascicles are visible

    • both epimysium and perimysium are dense irregular connective tissue

    • extend off the muscle to become organized as dense regular connective tissue = tendon

  • when the tendon is a broad flat sheet = aponeurosis

  • penetrating the muscle fibers and separating them into myofibrils = endomysium(areolar connective tissue)

    • myofibrils are made of filaments of proteins = myofilaments

Microanatomy of skeletal muscle fibers
Microanatomy of Skeletal Muscle Fibers

  • New terminology

    • Cell membrane = sarcolemma

    • Cytoplasm = sarcoplasm

    • Internal membrane system = sarcoplasmic reticulum

  • Large, multinucleated cells

    • embryonic development – stem cells (satellite cells) differentiate into immature myoblasts which begin to make the proteins of the myofibrils and myofilaments

    • These myoblasts mature into myocytes

    • Multiple myocytes fuse to form the muscle cell (muscle fiber)

    • once fused, these muscle cells lose the ability of undergo mitosis

    • number of muscle cells predetermined before birth – they get larger as we grow

    • but satellite cells can repair damaged/dying muscle cells throughout adulthood

      • Exclusive to skeletal muscle only!!!

Muscle cell anatomy
Muscle Cell Anatomy

  • Transverse tubules

    • ingrowths of sarcolemma

    • Carry electrical impulses deep into the fiber

  • Myofibrils within sarcoplasm of the fiber

    • Contain a“skeleton” of protein filaments (myofilaments) organized as Sarcomeres

  • Myofilaments form the myofibrils

    • Thin filaments (actin, troponin, tropomyosin)

    • Thick filaments (myosin)

Microanatomy of Skeletal Muscle Fibers

  • muscle fibers are bound by a plasma membrane = sarcolemma

  • thousands of tiny invaginations in this sarcolemma called T or transverse

  • tubules - tunnel in toward the center of the cell

  • -T tubules are open to the outside of the fiber

  • - filled with interstitial fluid

  • - action potentials generated in the neuron travel along the sarcolemma

  • and the T tubules

  • - allows for the even and quick spread of an action potential deep into the cell

  • the cytoplasm is called a sarcoplasm

  • -substantial amounts of glycogen - can be broken into glucose

  • -contains myoglobin - binds oxygen needed for muscle ATP

  • production

  • -support multiple myofibrils – structures for contraction

Microanatomy of Skeletal Muscle Fibers

  • contractile elements of the myofibrils = myofilaments

  • -2 microns in diameter

  • -comprised of primarily actin or myosin

  • -give the muscle its striated appearance

  • fibers also have a system of fluid-filled membranes = sarcoplasmic

  • reticulum

  • -encircles each myofibril

  • -similar to the endoplasmic reticulum of other cells

  • -have dilated end sacs = terminal cisterns

  • -stores calcium when at rest - releases it during contraction

  • -release is triggered by an AP

The Proteins of Muscle

  • Myofibrils contain two kinds of myofilaments

    • Thin

    • Thick

  • these Myofilaments are built of 3 kinds of protein

    • contractile proteins

      • myosin and actin

    • regulatory proteins which turn contraction on & off

      • troponin and tropomyosin

    • structural proteins which provide proper alignment, elasticity and extensibility

      • titin, myomesin, nebulin, actinin and dystrophin

      • Dystrophin – connects myofibrils to sarcolemma

        • -transmits tension along muscle

      • Actinin – part of Z-line

      • Titin – connects myosin to Z-line and M-line

        • Role in recovery after being stretched

      • Nebulin – forms core of the actin chain/thin filament

M line

Sarcomere Structure

  • sarcomere = regions of myosin (thick myofilament) and actin (part of thin myofilament)

  • bounded by the Z line

  • thin filaments project out from Z line – actin attaches via actinin (structural protein)

  • thick filaments lie in center of sarcomere - overlap with thin filaments and connect to them

  • via cross-bridges

  • myosin/thick filament only region = H zone

  • myosin/thick filaments are held in place by the M line proteins at the center and titin at

  • the Z-line

  • thin filament only region = I band

  • length of myosin/thick filaments = A band

  • contraction = “sliding filament theory”

  • -thick and thin myofilaments slide over each other and sarcomere shortens

Contraction the sliding filament theory
Contraction: The Sliding Filament Theory

  • Contraction:

    • Active process

    • Elongation is passive

    • Amount of tension produced is proportional to degree of overlap of thick and thin filaments

  • SF Theory: 1954

    • Explains how a muscle fiber exerts tension

    • Four step process

      • Active sites on actin

      • Crossbridge formation

      • Cycle of attach, pivot, detach, return

      • Troponin and tropomyosin control contraction

Contraction the sliding filament theory1
Contraction: The Sliding Filament Theory

  • Actin proteins in the thin

  • filament have myosin binding sites

  • these sites are “covered up” by troponin and tropomyosin in relaxed muscle

  • removal of troponin/tropomyosin from these sites is required for contraction

  • removal of troponin/tropomyosin is done through binding of calcium to troponin

  • calcium is release from the sarcoplasmic reticulum upon the action potential

  • myosin/thick myofilament is a bundle of myosin molecules

  • myosin looks like a “golf club” with a head, a hinge region and a shaft

  • each myosin protein has a globular “head” with a site to bind and breakdown ATP (ATPase site) and to bind actin (actin binding site)

  • binding of actin and myosin binding sites = cross-bridging

Increase in Cai

Removal of troponin-tropomyosin




Sliding of actin along myosin

-for cross bridging- you will need two things:

1. calcium – uncovers the myosin binding sites on actin – “pushes aside” the troponin- tropomyosin complex

2. myosin head bound to ADP

-for contraction – i.e. pivoting of the myosin head into the M line – the myosin head must be empty

-to “reset” for a new cycle of cross-bridging – the myosin head must detach and pivot back

-the myosin head must bind ATP

-once the myosin head pivots back – the ATP is broken down to ADP – head is ready to cross-bridge again – if actin is “ready”



  • Acetylcholinesterase (AChE) breaks down ACh within the synaptic cleft

  • Muscle action potential ceases

  • Ca+2 release channels close

  • Active transport pumps Ca2+ back into storage in the sarcoplasmic reticulum

  • Calcium-binding protein (calsequestrin) helps hold Ca+2 in SR

  • Tropomyosin-troponin complex recovers binding site on the actin

Rigor Mortis

  • Rigor mortis is a state of muscular rigidity that begins 3-4 hours after death and lasts about 24 hours

  • After death, Ca+2 ions leak out of the SR and allow myosin heads to bind to actin

  • Since ATP synthesis has ceased, crossbridges cannot detach from actin until proteolytic enzymes begin to digest the decomposing cells.

Length of Muscle Fibers

  • Optimal overlap of thick & thin filaments

    • produces greatest number of crossbridges and the greatest amount of tension

  • As stretch muscle (past optimal length)

    • fewer cross bridges exist & less force is produced

  • If muscle is overly shortened (less than optimal)

    • fewer cross bridges exist & less force is produced

    • thick filaments crumpled by Z discs

  • Normally

    • resting muscle length remains between 70 to 130% of the optimum

The neuromuscular junction
The Neuromuscular Junction

  • end of neuron (synaptic terminal or axon bulb) is in very close association

  • with the muscle fiber

  • distance between the bulb and the folded sarcolemma = synaptic cleft

  • nerve impulse leads to release of neurotransmitter(acetylcholine)

  • N.T. binds to receptors on myofibril surface

  • binding leads to influx of sodium, potassium ions (via channels)

  • eventual release of calcium by sarcoplasmic recticulum = contraction

  • Acetylcholinesterase breaks down ACh

  • Limits duration of contraction

Motor units
Motor Units

  • Each skeletal fiber has only ONE NMJ

  • MU = Somatic neuron + all the skeletal muscle fibers it innervates

  • Number and size indicate precision of muscle control

  • Muscle twitch

    • Single momentary contraction

    • Response to a single stimulus

  • All-or-none theory

    • Either contracts completely or not at all

  • Motor units in a whole muscle fire asynchronously

    • some fibers are active others are relaxed

    • delays muscle fatigue so contraction can be sustained

  • Muscle fibers of different motor units are intermingled so that net distribution of force applied to the tendon remains constant even when individual muscle groups cycle between contraction and relaxation.

Muscle Metabolism

  • Production of ATP:

  • -contraction requires huge amounts of ATP

  • -muscle fibers produce ATP three ways:

  • 1. Creatine phosphate

  • 2. Aerobic metabolism

  • 3. Anaerobic metabolism

Creatine Phosphate

  • Muscle fibers at rest produce more ATP then they need for resting metabolism

  • Excess ATP within resting muscle used to form creatine phosphate

    • creatine – arginine, glycine and methionine

      • made by kidneys and liver

      • phosphorylated in muscle to make creatine phosphate

  • phosphorylated by the enzymecreatinekinase

    • takes a phosphate off of ATP and transfers it creatine

    • takes the phosphate off of creatine phosphate and transfers it back to ADP – to make ATP

  • Creatine phosphate: 3-6 times more plentiful than ATP within muscle

  • Sustains maximal contraction for 15 sec (used for 100 meter dash).

  • Athletes often use creatine supplementation

    • gain muscle mass but shut down bodies own synthesis (safety?)

Anaerobic Cellular Respiration

  • Muscles deplete creatine – can make ATP in anaerobically or aerobically

  • Glycogen converted into glucose first

  • ATP produced from the breakdown of glucose into pyruvic acid (sugar) during glycolysis

    • if no O2 present (anaerobic) - pyruvic converted to lactic acid which diffuses into the blood

  • Glycolysis can continue anaerobically to provide ATP for 30 to 40 seconds of maximal activity (200 meter race)

  • If O2 is present the pyruvic acid is converted into acetyl coA (aerobic)

Aerobic Cellular Respiration

  • ATP for any activity lasting over 30 seconds

    • if sufficient oxygen is available, pyruvic acid is converted into acteyl coA

    • Acetyl coA enters the mitochondria to enter the Kreb’s cycle

    • Kreb’s cycle generates NADH and FADH2 (electron carriers)

    • The electrons are carried to an enzymes located on the inner mitochondrial membrane

    • The electrons are then transported along three complexes of enzymes – ultimately transported to oxygen

      • This results in the synthesis of ATP, water and heat

  • Provides 90% of ATP energy if activity lasts more than 10 minutes

  • Each glucose = 36 ATP

  • fatty acids and amino acids can also be used by the mitochondria

    • Fatty acid = ~100 ATP

  • Sources of oxygen – diffusion from blood, released by myoglobin

Types of muscle fibers
Types of Muscle Fibers

  • Fast fibers = glycolytic

  • Slow fibers = oxidative

  • Fibers of one motor unit all the same type

  • Percentage of fast versus slow fibers is genetically determined

  • Proportions vary with the usual action of the muscle

    • - neck, back and leg muscles have a higher proportion of postural, slow oxidative fibers

    • - shoulder and arm muscles have a higher proportion of fast glycolytic fibers

Fast fibers
Fast Fibers

  • Large in diameter

  • Contain densely packed myofibrils

  • Large glycogen reserves

  • Fast oxidative-glycolytic (fast-twitch A)

    • red in color (lots of mitochondria, myoglobin & blood vessels)

    • split ATP at very fast rate; used for walking and sprinting

  • Fast glycolytic (fast-twitch B)

    • white in color (few mitochondria & BV, low myoglobin)

    • anaerobic movements for short duration; used for weight-lifting

  • Slow fibers

    • Half the diameter of fast fibers

    • Three times longer to contract

    • Continue to contract for long periods of time

      • e.g. marathon runners

Exercise induced muscle damage
Exercise-Induced Muscle Damage

  • Intense exercise can cause muscle damage

    • electron micrographs reveal torn sarcolemmas, damaged myofibrils an disrupted Z discs

    • increased blood levels of myoglobin & creatine phosphate found only inside muscle cells

  • Delayed onset muscle soreness

    • 12 to 48 Hours after strenuous exercise

    • stiffness, tenderness and swelling due to microscopic cell damage

  • Atrophy

    • wasting away of muscles

    • caused by disuse (disuse atrophy) or severing of the nerve supply (denervation atrophy)

    • the transition to connective tissue can not be reversed

  • Hypertrophy

    • increase in the diameter of muscle fibers

    • resulting from very forceful, repetitive muscular activity and an increase in myofibrils, SR & mitochondria