Dr. Jacki Houghton, jhoughton@vcccd.edu ( 818)-397-2506 (TEXT ONLY)           Links:  HOME   UNIT 1   UNIT 2   UNIT 3   UNIT 4   UNIT 5   UNIT 6            Lessons: 1   2   3   4

Muscle Tissue   (download here)    brief ppt here with quiz photos

Muscle Overview

•      The three types of muscle tissue are skeletal, cardiac, and smooth

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

Muscle Similarities

•      Skeletal and smooth muscle cells are elongated and are called muscle fibers

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

•      Muscle terminology is similar

•    Sarcolemma – muscle plasma membrane

•    Sarcoplasm – cytoplasm of a muscle cell

•    Prefixes – myo, mys, and sarco all refer to muscle

Skeletal Muscle Tissues

•      Has obvious stripes called striations

•      Is controlled voluntarily (i.e., by conscious control)

•      Contracts rapidly but tires easily

•      Is responsible for overall body motility

•      Is extremely adaptable and can exert forces over a range from a fraction of an ounce to over 70 pounds

Cardiac Muscle Tissue

•      Occurs only in the heart

•      Is striated like skeletal muscle but is not voluntary

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

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

Smooth Muscle Tissue

•      Found in the walls of hollow visceral organs, such as the stomach, urinary bladder, and respiratory passages

•      Forces food and other substances through internal body channels

•      It is not striated and is involuntary

Muscle Function

•      Skeletal muscles are responsible for all locomotion

•      Cardiac muscle is responsible for coursing the blood through the body

•      Smooth muscle helps maintain blood pressure, and squeezes or propels substances (i.e., food, feces) through organs

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

Functional Characteristics of Muscles

•      Excitability, or irritability – the ability to receive and respond to stimuli

•      Contractility – the ability to shorten forcibly

•      Extensibility – the ability to be stretched or extended

•      Elasticity – the ability to recoil and resume the original resting length

Skeletal Muscle

•      Each muscle is a discrete organ composed of muscle tissue, blood vessels, nerve fibers, and connective tissue

•      The three connective tissue wrappings are:

•    Epimysium – an overcoat of dense regular CT that surrounds the entire muscle

•    Perimysium – fibrous CT that surrounds groups of muscle fibers called fascicles

•    Endomysium – fine sheath of CT composed of reticular fibers surrounding each       muscle fiber

Skeletal Muscle: Attachments

•      Muscles span joints and are attached to bone in at least two places

•      When muscles contract the movable bone, the muscle’s insertion moves toward the immovable bone – the muscle’s origin

•      Muscles attach:

•    Directly – epimysium of the muscle is fused to the periosteum of a bone

•    Indirectly – CT wrappings extend beyond the muscle as a tendon or aponeurosis

Microscopic Anatomy of a Skeletal Muscle Fiber

•      Each fiber is a long, cylindrical cell with multiple nuclei just beneath the sarcolemma

•      Fibers are 10 to 100 nm in diameter, and up to hundreds of centimeters long

•      Sarcoplasm has numerous glycosomes and a unique oxygen-binding protein called myoglobin

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


•      Myofibrils are densely packed, rodlike contractile elements

•      They make up most of the muscle volume

•      The arrangement of myofibrils within a fiber is such that a perfectly aligned repeating series of dark A bands and light I bands is evident


•      The smallest contractile unit of a muscle

•      The region of a myofibril between two successive Z discs

•      Composed of myofilaments made up of contractile proteins

•    Myofilaments are of two types – thick and thin

Myofilaments: Banding Pattern

•      Thick filaments – extend the entire length of an A band

•      Thin filaments – extend across the I band and partway into the A band

•      Z-disc – coin-shaped sheet of proteins (connectins) that anchors the thin filaments and connects myofibrils to one another

•      Thin filaments do not overlap thick filaments in the lighter H zone

•      M lines appear darker due to the presence of the protein desmin

Ultrastructure of Myofilaments: Thick Filaments

•      Thick filaments are composed of the protein myosin

•      Each myosin molecule has a rodlike tail and two globular heads

•    Tails – two interwoven, heavy polypeptide chains

•    Heads – two smaller, light polypeptide chains called cross bridges

Ultrastructure of Myofilaments: Thin Filaments

•      Thin filaments are chiefly composed of the protein actin

•      Each actin molecule is a helical polymer of globular subunits called G actin

•      The subunits contain the active sites to which myosin heads attach during contraction

•      Tropomyosin and troponin are regulatory subunits bound to actin

Arrangement of the Filaments in a Sarcomere

•      Longitudinal section within one sarcomere

Sarcoplasmic Reticulum (SR)

•      SR is an elaborate smooth endoplasmic reticulum that mostly runs longitudinally and surrounds each myofibril

•      Paired terminal cisternae form perpendicular cross channels

•      Functions in the regulation of intracellular calcium levels

•      Elongated tubes called T tubules penetrate into the cell’s interior at each A band–I band junction

•      T tubules associate with the paired terminal cisternae to form triads

T Tubules

•      T tubules are continuous with the sarcolemma

•      They conduct impulses to the deepest regions of the muscle

•      These impulses signal for the release of Ca2+ from adjacent terminal cisternae

Contraction of Skeletal Muscle Fibers

•      Contraction – refers to the activation of myosin’s cross bridges (force generating sites)

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

•      Contraction ends when cross bridges become inactive, the tension generated declines, and relaxation is induced

Sliding Filament Mechanism of Contraction

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

•      In the relaxed state, thin and thick filaments overlap only slightly

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

•      Each myosin head binds and detaches several times during contraction, acting like a ratchet to generate tension and propel the thin filaments to the center of the sarcomere

•      As this event occurs throughout the sarcomeres, the muscle shortens

Sequential Events of Contraction

•      Cross bridge attachment – 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 the myosin head – energy from hydrolysis of ATP cocks the myosin head into the high energy state

Nerve Stimulus of Skeletal Muscle

•      Skeletal muscles are stimulated by motor neurons of the somatic nervous system

•      Axons of these neurons travel in nerves to muscle cells

•      Axons of motor neurons branch profusely as they enter muscles

•      Each axonal branch forms a neuromuscular junction with a single muscle fiber

Neuromuscular Junction

•      The neuromuscular junction is formed from:

•    Axonal endings, which have small membranous sacs (synaptic vesicles) that contain the neurotransmitter acetylcholine (ACh)

•    The motor end plate of a muscle, which is a specific part of the sarcolemma that contains ACh receptors that helps form the neuromuscular junction

•      Though exceedingly close, axonal ends and muscle fibers are always separated by a space called the synaptic cleft

•      When a nerve impulse reaches the end of an axon at the neuromuscular junction:

•    Voltage-regulated calcium channels open and allow Ca2+ to enter the axon

•    Ca2+ inside the axon terminal causes axonal vesicles to fuse with the axonal membrane

•    This fusion releases ACh into the synaptic cleft via exocytosis

•    ACh diffuses across the synaptic cleft to ACh receptors on the sarcolemma

•    Binding of ACh to its receptors initiates an action potential in the muscle

Action Potential

•      A transient depolarization event that includes polarity reversal of a sarcolemma (or nerve cell membrane) and the propagation of an action potential along the membrane

Contraction of Skeletal Muscle (Organ Level)

•      Contraction of muscle fibers (cells) and muscles (organs) is similar

•      The two types of muscle contractions are:

•    Isometric contraction – increasing muscle tension (muscle does not shorten)

•    Isotonic contraction – decreasing muscle length (muscle shortens during contraction)

Motor Unit: The Nerve-Muscle Functional Unit

•      A motor unit is a motor neuron and all the muscle fibers it supplies

•      The number of muscle fibers per motor unit can vary from four to several hundred

•      Muscles that control fine movements (fingers, eyes) have small motor units

•      Large weight-bearing muscles (thighs, hips) have large motor units

•      Muscle fibers from a motor unit are spread throughout the muscle; therefore, contraction of a single motor unit causes weak contraction of the entire muscle

Muscle Tone

•      Muscle tone:

•    The constant, slightly contracted state of all muscles, which does not produce active movements

•    Keeps the muscles firm, healthy, and ready to respond to stimulus

•      Spinal reflexes account for muscle tone by:

•    Activating one motor unit and then another

•    Responding to activation of stretch receptors in muscles and tendons

Isotonic Contractions

•      In isotonic contractions, the muscle changes in length (decreasing the angle of the joint) and moves the load

•      The two types of isotonic contractions are concentric and eccentric

•    Concentric contractions – the muscle shortens and does work

•    Eccentric contractions – the muscle contracts as it lengthens

Isometric Contractions

•      Tension increases to the muscle’s capacity, but the muscle neither shortens nor lengthens

•      Occurs if the load is greater than the tension the muscle is able to develop

Muscle Fiber Type: Functional Characteristics

•      Speed of contraction – determined by speed in which ATPases split ATP

•    The two types of fibers are slow and fast

•      ATP-forming pathways

•    Oxidative fibers – use aerobic pathways

•    Glycolytic fibers – use anaerobic glycolysis

•      These two criteria define three categories – slow oxidative fibers, fast oxidative fibers, and fast glycolytic fibers

Muscle Fibers: Speed of Contraction

•      Slow oxidative fibers contract slowly, have slow acting myosin ATPases, and are fatigue resistant

•      Fast oxidative fibers contract quickly, have fast myosin ATPases, and have moderate resistance to fatigue

•      Fast glycolytic fibers contract quickly, have fast myosin ATPases, and are easily fatigued

Smooth Muscle

•      Composed of spindle-shaped fibers with a diameter of 2-10 m and lengths of several hundred m

•      Lack the coarse CT sheaths of skeletal muscle, but have fine endomysium

•      Are organized into two layers (longitudinal and circular) of closely apposed fibers

•      Found in walls of hollow organs (except the heart)

•      Have essentially the same contractile mechanisms as skeletal muscle


•      When the longitudinal layer contracts, the organ dilates and contracts

•      When the circular layer contracts, the organ elongates

•      Peristalsis – alternating contractions and relaxations of smooth muscles that mix and squeeze substances through the lumen of hollow organs

Innervation of Smooth Muscle

•      Smooth muscle lacks neuromuscular junctions

•      Innervating nerves have bulbous swellings called varicosities

•      Varicosities release neurotransmitters into wide synaptic clefts called diffuse junctions

MUSCLE CHEAT SHEET  (download here)

Muscle – surrounded by epimysium 

Muscles are made of Fascicles – surrounded by perimysium 

Fascicle is made of muscle cells called muscle fibers 

A muscle fiber or cell, is surrounded by a connective tissue called endomysium 

Inside of a muscle cell, or fiber are parallel (lenghthwise) structures called myofibrils 

Inside of a muscle fiber are many nuclei and mitochondria. 

Inside of a muscle fiber and surrounding each myofibril is a specialized smooth  (SER) endoplasmic reticulum called a sarcoplasmic reticulum 

The sarcoplasmic reticulum has the special function of spreading an action potential very quickly. 

The sarcoplasmic reticulum is a series of channels which run parallel as well as perpendicular to the myofibril.  The perpendicular channels are called terminal cisterns and lay over the junction between each A band and adjacent I band.   

The sarcoplasmic reticulum and the terminal cisterns store large quantities of calcium ions which are require for muscle contraction. 

T-Tubules are deep invaginations of the sarcolemma (plasma membrane) that run between each pair of terminal cisterns. 

The complex of the T tubule flanked by two terminal cisterns at the A-I junction is called a triad.  The T tubules go deep into the muscle fiber to ensure that the deeper myofibrils contract at the same time and the more superficial ones. 

Contractile units along each myofibril are called a sarcomere. 

Sarcomeres have a very distinct structure which allows them to contract. 

The anchoring proteins are called Z discs.  Attached to the Z discs are actin filaments and titin filaments which are attached to the myosin filaments.  Actin and myosin interact to contract the sarcomere.







This is your Muscle Tissue Video Lecture

STUDY GUIDE: You will need to know the terms on the handout that begin with epimysium etc.

There may be diagrams of histology so you need to know the structures down to the components of the sarcomere

There will be a matching section containing vocabulary from the notes on the left and below the videos like: isotonic, isometric excitability, elasticity, spasm antagonists, fixators etc. etc.

There will be a matching section where I will give you the name of a muscle and you will have to identify the INSERTION.

The final section will be a list of muscles and you will identify their actions from a list.  You will have to pick ALL actions performed by that muscle that appear in the list.




Shapes of Muscles by fiber direction

The muscles of the body can be difficult to remember, as their names are often long and confusing. The key to learning the muscles is to understand the basic naming conventions, once you see the patterns, it will be much easier to remember.

Here are some of the basics of naming muscles

Deltoid - shaped like a triangle
Orbicularis - orbit, circular muscle
Major/Minor - large/small or sometimes upper and lower
Vastus - large
Dorsi or Dorsal - backside
Infra / Supra - lower and upper
Longis / Brevis - long/ short (brief) 
Medialis / Lateralus - medial (toward the inside), lateral (toward the outside)

Some muscles are named for the region or the bone they are attached to, for example:

biceps femoris - two headed muscle attached to the femur
extensor carpi radialis longus - long muscle that runs the length of the radius (bone) to the carpals (wrist bones) that extends the fingers

Muscle Movement Classification

  1. Agonist

  2. A muscle that causes motion.
  3. Antagonist

  4. A muscle that can move the joint opposite to the movement produced by the agonist.
  5. Target

  6. The primary muscle intended for exercise.
  7. Synergist

  8. A muscle that assists another muscle to accomplish a movement.
  9. Stabilizer

  10. A muscle that contracts with no significant movement to maintain a posture or fixate a joint.

Brief Notes:

Gross Anatomy:

3 concentric layers of connective tissue

    1. epimysium dense irregular connective tissue; surrounds  entire skeletal muscle; separates the muscle from surroundings
    2. perimysium divides the muscles into compartments (containing bundles of fibers called fasicles);  contains blood vesicles and nerves
    3. endomysium surrounds each muscle fiber; binds fibers to each other

-          satellite cells between endomysium & muscles fibers; repairs damaged muscle fibers

Tendon - collagen fibers of epimysium, perimysium, and endomysium attach muscle to bone and other tissue

Neuromuscular Junction site of chemical communication between synaptic terminal of a neuron and skeletal muscle fibers 


  • -          sarcolemma cell membrane

  • -          sarcoplasm cytoplasm

  • -          differ from regular cells:

  • a. muscle cells (fibers) long 100mm

  • b. multinucleatedΰ 100; embryonic cells fuse to create muscle cells; myoblasts that don’t fuse become satellite cells in adulthood

  • c. contain indentations in the membrane forming tubule network; transverse or T. tubule conducts electric impulse

Myofibrils (100-1000) located in the sarcoplasm

  • -          length extends entire length of muscle

  • -          shortenΰcause contractions

  • -          surrounded by mitochondria & glycogen molecules

  • -          myoglobin reddish pigment similar to hemoglobin stores O2 until needed for mitochondria

Sarcoplasmic Reticulum membrane sleeve that helps control contractions of individual myofibrils

Myofibrils consist of myofilaments (bundles):

  • a.      actin thin filaments

  • b.      myosin thick filaments

Sarcomere smallest functional unit of the muscle fibers
    1. Z discs/ lines separate sarcomeres
    2. A-band  dark area; thick filaments overlapping thin filaments
    3. M-lines protein molecules adjoin thick filaments
    4. H-zone zone of thick filaments in the center of A-band
    5. I-band zone of thin filaments

Motor neurons neurons that stimulate muscle tissue

Synaptic vesicles membranes sacs containing neurotransmitters acetylcholine

Synaptic cleft space between axon terminal  & sarcolemma

All or none principle

-          once a threshold stimulus is reached, muscle fibers of a motor unit (motor neuron & muscle fiber) will contract to its fullest capability or not at all

Muscle atrophy & Hypertrophy:

  1. disuse atrophy not in use
  2. denervation atrophy nerve supply is cut-complete atrophy
  3. hypertrophy - increase in diameter of muscle; get increase of fibers (myofibrils) increase in mitochondria & increase in blood supply

2 types of skeletal muscle fibers:

  1. red muscle fibers high myoglobin content smaller in diameter; increase mitochondria & blood supply
  2. white fibers (fast) low myoglobin, bigger diameter; low mitochondrial content & blood supply


Type 1  slow twitch, slow oxidative, fatigue resistance, high Myoglobin, high mitochondria & blood supply, high capacity to produce ATP aerobically, red, contraction velocity is slow & very resistant to fatigue 

Type 2 fast twitch, fast glycolytic, fatigable fibers, low myoglobin, low mitochondria 7 few capillaries, high amount of  glycogen, white, generate ATP anaerobically, contraction are strong and fast-fatigable 

Type 2A(intermediate) fast twitch, fast oxidative and fatigue resistance, high myoglobin, high mitochondria & capillaries, ATP aerobic metabolism, contraction are fast, fatigue resistance, occurs infrequently 

Abnormal Muscle Contractions:

Organization of muscle fibers:

4 patterns of the fasicle arrangement

  1. parallel- parallel to the longitudinal axis of the muscle
  2. convergent- based over a large area with a common attachment site
  3. Pennate- one or more tendons run thru the body of the muscle

-          unipennate - muscle on the same side of the tendon

-          bipennate - muscle on both sides of the tendon

  1. circular or sphincter muscles- concentric around the opening

origin-where a muscle begins usually immovable

insertion- where the muscle ends, usually movable

action the contraction; muscle arrangement in opposing  pairs; flexors & extensor; abductors & adductors

primary actions:

  1. prime movers agonists, contraction are responsible for a particular movement
  2. synergist assist prime movers, prevent movement by stabilizing the joint
  3. antagonist muscle who’s action oppose the agonist  if agonist flexion, the antagonist produces extensions
  4. fixators stabilizes the origin of the prime mover so it is efficient

Names of the skeletal muscles:

  1. directional indications
  • a.      rectus - straight parallel fibers run along axis of body

  • b.      transversus- fibers that run across

  • c.      obliques- run at an angle to long axis

  1. unusual structure features ( # of insertion & origins)
    1. biceps- 2 tendons of origin
    2. triceps- 3
    3. quadriceps- 4
  2. shape
    1. trapezius- trapezoid
    2. deltoid- triangular
    3. rhomboideus- rhomboid
    4. orbicularis- circular
    5. longus- long
    6. longissimus- longest
    7. teres- long and round
    8. brevis- short
    9. magnus- big
    10. major- bigger
    11. maximus- biggest
    12. minor- small
    13. minimus- smallest
  3. location
    1. externus or superficial- visible at body surface
    2. internal or profundus- internal
    3. extrinsic- muscles that position or stabilize an organ
    4. intrinsic- operate within an organ
  4. muscle that identify their origins and insertion
    1. genioglossus chin + tongue = genio + glossus
  5. names that include flexor, extensor, retractor, indicate the primary function of the muscle
  6. except for the platysma & diaphragm all skeletal muscles include the term “muscle”