Notes on the Web
Muscle Tissues (Histology and Physiology)

Bruce G. Stewart

Related Textbook Readings

Selected Images of Histological Materials From Laboratory Studies

Related Human Anatomy Coloring Book Assignments

From Human Anatomy Coloring Book
Plates Structures
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Lecture Outlines

I. General Information - Muscles

A. Muscle tissue - specialized tissue that generates force

1. about 40-50% of body weight

B. Myology - study of muscles

C. Characteristics of muscle tissue

1. excitability - ability to receive and respond to stimuli

a. stimulus - internal or external changes that are strong enough to initiate an impulse (action potential)

2. contractility - ability to generate force to shorten and thicken to do work

3. extensibility - ability to be stretched

4. elasticity - ability to regain shape

D. Functions

1. motion (both reflex and voluntary)

a. includes heart beating, etc.

2. posture maintenance - by contraction of skeletal muscles

3. heat production

a. up to 85% of body heat!

E. Major Muscle Tissue Types

1. skeletal muscle tissue

a. striated

b. voluntary

c. mainly attached to bones and moving parts of body, but also skin, other muscles, deep fascia

2. Cardiac muscle tissue

a. striated

b. involuntary

c. built in controls (involuntary nerves and hormones)

3. Smooth muscle tissue

a. helps with maintenance of internal environment

b. in walls of blood vessels, stomach and intestines; also in hair follicles

c. non-striated

d. involuntary with built in controls and/or stimulated by autonomic nervous system

II. Skeletal Muscle Tissue

A. Connective Tissue Components

1. fascia - sheet or band of fibrous, connective tissue under skin or around muscles and organs

2. superficial fascia - subcutaneous layer

a. storage of waste and fat

b. insulation

c. mechanical protection

d. pathway for nerves and vessels

3. deep fascia - lines body wall and holds muscles together

a. allows for free movement, for support for blood vessels and nerves, and for origins of some muscles

4. other coverings - fibrous connective with collagen, etc.

a. epimysium - wraps entire muscle

b. perimysium - wraps fasciculi [bundles of muscle fibers (cells)]

c. endomysium - surrounds fibers

5. tendons - cords

6. aponeurosis - sheets

7. tendon sheaths (synovial) - not actually a part of muscles, but they are critical to the function of some by lubricating high stress tendons and reducing friction

B. Nerve and Blood Supply

1. well innervated and vascularized

a. artery and vein for each nerve

b. capillaries within endomysium with each fiber typically close to 10 or more capillaries.

c. each fiber makes contact with synaptic end bulb of a nerve cell

C. Histology

1. muscle fibers (= myofibers) cellular units of muscles

a. parallel cells

b. 10-100 um in diameter

c. up to 30 cm in length!

2. muscle fiber construction

a. sarcolemma - membrane sheath

b. sarcoplasm - cytoplasm

c. multinucleate - syncytial tissue

d. sarcoplasmic reticulum - like smooth ER

  • terminal cisterns
  • transverse tubules (T-tubules)
    • extensions of sarcolemma
  • triad - one T-tubule and adjacent cisterns

e. myofibrils

  • cylindrical structures 1-2 um in diameter in fiber
  • longitudinal
  • made of myofilaments
    • thin - 6 nanometers wide
    • thick - 16nanometers wide

f. sarcomeres - compartments of myofibrils

  • z-lines - dense zones on ends
  • A (anisotropic) band - length of thick myofilaments
  • I (isotropic) band - thin myo only
  • H zone - area in "A" with only thick myofilaments
  • M-line - transverse threads that connect middle parts of adjacent thick myofilaments

g. thin myofilament structure

  • anchored at z lines
  • made of a double helix of actin protein molecules
    • each actin has a myosin binding site that forms "cross-bridge" to an actin binding site on one head of a myosin molecule in the thick filaments during actual contraction
  • tropomyosin - forms two thin strands that wrap around the actin helices of the thin filament
  • troponin - "welds" the tropomyosin to the actin helices at regular intervals and has ability to change shape in the presence of high concentrations of Ca++
    • troponin I - actin binding site
    • troponin C - Ca binder
    • troponin T - tropomyosin binder
  • the tropomyosin - troponin complex
    • extremely important in contraction physiology

h. myosin - golf club shaped protein with "shaft" but with two "heads"

  • bundled in parallel with heads out to form the thick filaments
  • heads - one head forms cross-bridge to myosin binding sites on actin, and the other head has ATP binding site (on other head) to link energy-releasing reactions to physical change in shape of protein heads (for power stroke)

Internet Resource for study of muscle histology and physiology: Get Body Smart - Muscle Physiology Animations

3. Contraction

a. Sliding filament Theory

  • myosin cross-bridges pull on thin myofilaments v slide them toward H-zone
  • sarcomere shortens but not filaments

b. Neuromuscular Junction

  • neuron delivers stimulus
  • axon - up to 91 cm long
  • bundle forms nerve
  • motor neuron - neuron to muscle
  • detailed structure
    • telodendria - branched end of axon
    • motor end plate - sarcolemma next to axon
    • neuromuscular junction (= myoneural junction)
    • synaptic end bulbs - expanded telodendria
      • synaptic vesicles - contain neurotransmitters
  • synaptic trough - invaginated area of sarcolemma
    • synaptic cleft - space between bulb and trough
    • subneural clefts - folds of trough
      • increase receptor area
  • acetylcholine (ACH)
    • released at cleft from synaptic vesicles when nerve action potential arrives
    • alters Na+ and K+ ions in post-synaptic neuron and results in a muscle action potential spreading on that neuron
      • this ultimately leads to muscle contraction in manner described later in our lectures

4. motor unit

a. motor neuron with all muscle fibers it stimulates

  • 150 fibers/neuron average

b. range 2000/fiber in gastrocnemius to 10/fibers in extrinsic eye muscles

c. recruitment

  • way of resting some while maintaining tension in muscle

D. Physiology of Contraction

1. Conditions prior to contraction

a. Ca2+ ions low in sarcoplasm but high in sarcoplasmic reticulum

b. ATP conc. high including attachments to ATP-binding sites of myosin cross-bridges

c. myosin cross bridges blocked from actin by tropomyosin-troponin complex on actin and by ATP bound to myosin cross-bridges

2. Initiation of Contraction

a. nerve action potential arrives at synaptic end bulb

  • causes release of a little Ca2+ ions to synaptic end bulb
    • this causes release of acetylcholine from vesicles into synaptic cleft

b. (ACH) binds with receptor sites on sarcolemma of muscle fiber

  • causes membrane proteins (acetylcholine-gated ion channels) to open and allow rapid influx at cations (esp. Na+)

c. muscle action potential results

d. muscle action potential travels along sarcolemma and through transverse tubules

e. action potential causes sarcoplasmic reticulum to release Ca2+ ions into sarcoplasm around micro filaments

  • through calcium release channels

f. Ca2+ combines with troponin C and causes structural change

g. moves troponin and tropomyosin to side

h. exposes myosin-binding sites on actin!

i. ATP on myosin splits (ADP + P) and energy causes myosin cross-bridges to bind to actin

3. Contraction Proper

a. ADP + P release from myosin and myosin changes orientation

b. myosin cross bridge moves toward H zone (=power stroke)

c. actin thin filaments slide past thick myofilaments

d. ATP combines again and causes repeat

e. Z lines are pulled closer

  • up to 50% of distance of sarcomere

4. Relaxation

a. acetylcholine digested by acetylcholinesterase

b. Ca2+ actively transported into sarco. reticulum

c. troponin-tropomyosin complex reattached to actin

d. ATP breaks myosin cross-bridge/actin bonds

5. Note: rigor mortis due to lack of ATP which is needed to break cross-bridge bonds and release the thick and thin filaments from each other

E. Energy for Contraction and Relaxation

1. Existing ATP

a. ATP --> ADP + P

b. enough to last only 5-6 seconds of vigorous exercise

2. Phosphagen System

a. kicks in after existing ATP is depleted

b. phosphocreatine

  • high energy molecule found in muscle tissue
  • 2-3 x's conc of ATP
  • process:
    • phosphocreatine --> creatine + phosphate + energy
    • energy + ADP + P ---> ATP

c. provides approximately 15 seconds max for short bursts (e.g. 100m dash)

3. Glycogen - Lactic Acid System

a. kicks in after phosphage system

b. glycogen converted to glucose

c. requires Ca2+ and calmodulin, etc.

d. glycolysis releases 2 ATP/glucose

e. pyruvic acid converted to lactic acid so glycolysis can continue in absence of O2

f. provides approximately 30-40 sec. of max muscle activity (e.g. 400-m dash)

4. Aerobic System

a. Pyruvic acid enters mitochondria

b. process (Krebbs cycle and electron transport chain) pyruvic acid + O2 --> CO2 + H2O + energy (34 ATP)

c. also - metabolism of lipids, amino acids, etc.

d. sustained activity

F. All-or-none Principle - threshold stimulus --> total contraction only for individual fibers.

G. Kinds of Contractions

1. Twitch contraction

a. follows threshold stimulus

b. rapid, jerky contraction

c. Twitch Contraction Myogram - draw a myogram (label the x and w axes) of a muscle twitch contraction based on lecture or your textbook using the following phases and time frames:

  • latent period
    • use 10 milliseconds in your graphing
      • due partly to time delay between release of calcium from SR and its alteration of troponin
  • contraction period
    • use 50 milliseconds in your graphing
    • cross-bridge activity with power strokes
  • relaxation period
    • use 40 milliseconds for your graphing
    • occurs during active transport of Ca2+ back into SR.

d. refractory period

  • time required for muscle to be able to respond to stimulus
    • 5 millisec in skeletal
    • 300 millisec in cardiac

2. Other muscle fiber contraction patterns (draw myograms of each in the boxes provided)

a. wave summation

  • stimulus received before full relaxation

b. tetanus

  • allows smooth sustained contractions
    • incomplete
    • complete
   

3. treppe

a. strength is higher in progressively subsequent contractions up to a maximum

b. "warmup" phenomenon is due to treppe

4. isotonic vs. isometric type contractions

a. isotonic - shortening occurs

b. isometric - tension constant but no shortening

H. Muscle Tension

1. related to such things as

a. frequency of stimuli

b. number of fibers contracting at a given time

c. tension from stretched elastic elements

d. length of myofibers affects tension from contraction

  • highest tension at maximum thin filament and thick filament overlap (more force due to more myosin heads pulling on thin filaments)
  • none at 175% + of length since thin filaments are completely pulled away from thick filaments

I. Muscle Tone

1. state of sustained partial contraction

2. maintains posture, etc.

3. tension monitored by muscle spindles

4. disorders

a. hypotonia

  • flaccid muscles symptoms
  • can result from inactivity,
  • nervous disorders, etc
  • flaccid paralysis

b. hypertonia

  • spasticity and rigidity
  • spastic paralysis
  • due to nervous disorders, etc.

c. muscle atrophy vs. hypertrophy

J. Types of Skeletal Muscle Fibers

1. Color - red vs. white

a. red - has more myoglobins (an O2 storing pigment)

  • smaller diameter
  • more mitochondria and capillaries

b. white - less myoglobin

  • more SR

2. velocity of Contraction

a. Type I - slow twitch

  • red fibers
  • slow but steady ATP splitting
  • resistant to fatigue
  • common in postural muscles

b. Type IIB - fast twitch (glycolytic)

  • white but
  • lots of glycogen
  • can do anaerobic met.
  • can split ATP rapidly/contr. fast
  • fatigue easily
  • common in arm

c. Type IIA - fast twitch (oxidative)

  • rare in humans
  • characteristics of both type I and IIB

III. Cardiac

A. General Structure

1. one centrally-located nucleus per fiber

2. more sarcoplasm and mitochondria larger and more numerous

3 action/myosin arrangement similar but not in discrete myofibrils

4. SR is well-developed and t-tubules a little different (due to lack of discrete myofibrils)

5. myofibers branch and interconnect within networks

6. two distinct networks, atrial and ventricular, separated from each other by connective tissue insulation

a. intercalated discs exist between cardiac fibers in the same network

b. disc areas have high concentrations of desmosomes "welds" that help hold the fiber together and gap junctions that conduct muscle action potentials from one fiber to the next without stopping

B. Physiology

1. rhythmic contractions; requires large amounts of O2 and ATP.

2. autorhymicity (intrinsic stimulation) in three types of highly specialized cells in “conduction system”

a. Sinuatrial (SA) node cells

  • Extracellular Ca++ influx instead of Na+ results in depolarization of the sarcolemma
  • Cycles at about 70-80 times/minute

b. Atrioventricular (Av) node cells

  • Is not directly connected to SA node
  • Cycles at about 50 times/minute
  • Is stimulated by atrial myocardium due to faster cycle of SA node

c. Purkinje fibers

  • Reach papillary muscles before rest of lower ventricles
  • Communicates with lower ventricles earlier than sides of ventricles
  • Cycle at a lower rate than nodes

3. contractions 10-15X longer due to slower moving action potentials and prolonged Ca++ delivery to filaments

4. long refractory period

5. regulatory stimulation and inhibition of heart occurs via the autonomic nervous system, specifically from the medulla oblongata; mostly communicate to the SA and Av nodes but also to the myocardium of the ventricles

a. cardioacceleratory center (sympathetic nervous system)

b. cardioinhibitory center (parasympathetic nervous system via the vagus nerve)

c. these centers modify the base rate of the heart and can influence the force of contraction

d. these centers receive regulatory input from the hypothalamus of the brain which, among many other things, monitors homeostasis in the body

IV. Smooth Muscle

A. General Structure

1. fibers v5-10um x 30-200um long

a. shorter than skeletal

b. tapered

c. single nucleus

d. thick and thin myofilaments but no sarcomeres

  • 10-15 thin/1 thick (2:1 in skeletal)

e. intermediate filaments anchor to dense bodies

2. Dense bodies - serve similar purpose as Z lines

a. bundles of intermediate fibers attach to dense bodies

b. thick and thin filaments transmit force to intermediate fibers

3. Caveolae

a. serve purpose like T-tubules of skeletal muscle

4. Types

a. visceral (single-unit) muscle tissue

  • small arteries; veins and hollow viscera
  • have conducting gap junctions
  • waves of contractions

b. multiunit smooth muscle tissue

  • individual fibers have own motor units
  • walls of large arteries, airways to lungs arrector pili, intrinsic eye muscles (in iris)

B. Physiology

1. duration of contraction/relaxation 5-500x's that of skeletal muscle fibers

a. lack of T-tubules slows the process of stimulating the SR; calveoli still do it, but just slower

b. have different Ca2+ binding mechanism that is slower

c. Ca2+ move more slowly out of SR

d. allows long-term tone (e.g. in stomach, urinary bladder, blood vessels); that is, a steady tension can be maintained for a long period of time in the smooth muscle tissue

Laboratory Exercise - Histology of Major Muscle Tissue Types

At approximately this point in the lectures, we will turn our attention to the microscopic study of muscle tissues. You will examine the following specific slides:

Lecture Outline (continued)

V. Regeneration of Muscle Tissue

A. Skeletal Muscle

1. special cells called satellite cells can replace skeletal muscle fibers on an individual cell-by-cell basis and in a very limited way

a. dormant until needed

b. not sufficient to compensate for tissue damage or broad scale degeneration (such as due to fat cell replacement as a result of inactivity and aging)

2. skeletal muscle fibers cannot divide after about year one of life

3. healing by formation of fibrous scar tissue which provides no contractibility but does allow other live muscle fibers to transmit their force

B. Cardiac

1. no mitotic ability

2. no satellite cells

3. healing by scar tissue only, hence an injured myocardium can show visible evidence of areas killed during non-fatal heart attacks. You will look a demonstration slide in lab of such a case.

C. Smooth

1. some individual cells (e.g. in uterus) can divide by mitosis

2. can also arise from stem cells called pericytes

3. smooth muscle has a higher but still limited power of regeneration compared to the vast majority of tissues in the human body

VI. Homeostasis and Muscle Tissue Health

A. Oxygen Debt

1. after strenous use of muscles, the body needs oxygen to:

a. metabolize accumulated lactic acid

b. replenish ATP, phosphocreatine and glycogen stores

c. re-oxygenize hemoglobin (in blood) and myoglobin (in muscle fibers)

d. replace air in lungs and fluids with fresh oxygen-rich air

2. Maximal O2 Uptake

a. maximum rate ability of a person for aerobic catabolism of pyruvic acids

b. related to variables like gender (male>female), age (maximum rate of a person is around age 20 years), body size (larger body masses demand more O2 assuming healthy vascularized tissues ), training (long-term aerobic exercise increases efficiency of O2 delivery to tissues by greatly increasing capillary beds to skeletal muscle fiber; size of skeletal muscle fibers also can increase)

c. maximum O2 rate affects the need for lactic acid production and the resulting oxygen debt

  • out of shape people have to breath faster and heart rate increases higher when, say, going up stairs than people in better cardiovascular condition. Where do you fall on this continuum between "dead<---->great shape"?
  • lung damage due to smoking lowers the maximum O2 rate of the smoker (less efficient lungs), increases the level of oxygen debt, and slows the rate of payback of oxygen debt. Again, where do you fall in the continuum of "dead<---->great shape"?

B. Muscle Fatigue is the Result of Natural Homeostatic "Imbalances"

1. fatigue (inability of muscle fibers to provide optimal force) is cause by

a. pH increases (due to lactic acid) which reduces efficiency of muscle contraction physiology

b. glycogen stores (when this readily available source of glucose is depleted, the muscle fibers cannot operate as efficiently)

2. people (older and younger) who are in better aerobic condition (e.g. from exercise programs of walking, swimming, biking, jogging, etc.) do not fatigue as easily as people who are not in good fitness shape. This may sound self-evident, but it is important to remember that it is NOT just aging that results in decline of performance. Lifestyle, including exercise, plays a major role.

C. Heat Production - Maintaining Thermoregulatory Balance

1. approximately 85% of metabolic energy activity of muscle tissues is released as heat

2. heat is produced as a concequence of contraction, relaxation, and recovery (ATP restoration)

D. Aging - Partly Natural Declines and Partly Lifestyle-related Declines in Muscle Condition and Function

1. at about 30 years of age (or earlier) muscle loss begins

a. fat levels in muscles increase and gradually replace some healthy muscle cells

  • this increase is significantly greater in low activity, overweight individuals
  • this increase is correlated with age, but negative lifestyle behaviors (smoking, sedentary lifestyle, high fat-high caloric diets, etc.) greatly accelerate the process

2. consider the short career life of professional athletes in sports like tennis; while age and experience provide advantages in any sport, loss of optimal muscle tissue performance eventually exerts its toll.

3. IMPORTANT LIFE LESSON! Do not fall into the great lying rationalization that your health situation is due to age alone! "Oh, once I reached 30, my body began to fall apart." "After my pregnancy, I was just never the same." "I just can't seem to lose weight since I have gotten older." "Since I have gotten older and started the nursing program, I just don't have time to exercise." "I am just too stressed to quit smoking; I need it just to calm down." And on and on and on. BALOGNY! The fact is that at almost any age we can improve our condition. At least admit this, even if you are not yet willing to make the lifestyle changes that would show that you care for yourself.

E. Muscle-related Disorders

1. Fibrosis

a. connective tissue hypertrophy

2. Fibromyalgia

a. non-articular rheumatic disorder

  • pain, tenderness and stiffness of muscles and soft tissues (fibrous connective components)
  • hard to diagnose, and this fact is often used by addicts (could be grandma!) to implore naive doctors into prescribing mood-altering muscle relaxers and pain medications
    • "killer" doctors do exist who indiscriminately prescribe or over-prescribe narcotic medications for this and many other conditions (we will visit this issue again in other parts of our course.
  • lifestyle behaviors (e.g. heavy drinking and other drug abuse, poor diet, and lack of exercise) can also contribute to symptoms similar to fibromyalgia

3. Muscular Dystrophies

a. various heritable diseases that result in degeneration of muscle fibers

  • Duchenne (DMD)- a type of muscular dystrophy which was the first for which a specific defective gene was identified as the cause
  • Visit this link to read more about DMD including its symptoms and genetic basis:
  • If you are curious, you can explore the OMIM site for reliable information on other types of MD and for countless other genetically-related diseases.

Reminder about Textbook Study

As with other topics, your textbook has excellent presentations of the materials on muscle tissue histology and physiology. While you should focus on the specific material in the Notes on the Web, you should always use your textbook as a resource for illustrations and for understanding content that your notes cover.

As with all materials throughout the semester, you will have opportunities to ask questions or ask that any relevant material from your assignments be discussed more thoroughly in class.

Related Internet Resources

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