1. Chapter 9 Muscle Structure & Physiology Lecture 16
Visual Anatomy & Physiology First Edition Martini & Ober
Chapter 9
Muscle Structure & Physiology
Lecture 16
80 min, 36 slides

2. Lecture Overview Types, characteristics, functions of muscle
Structure of skeletal muscle
Mechanism of skeletal muscle fiber contraction
Energetics of skeletal muscle contraction
Skeletal muscle performance
Types of skeletal muscle contractions
Comparison of skeletal muscle with smooth muscle and cardiac muscle

3. Muscular System Review - Three Types of Muscle Tissues Skeletal Muscle
usually attached to bones
under conscious control (voluntary)
striated
multinucleated
Cardiac Muscle
wall of heart
not under conscious control
striated
branched
Smooth Muscle
walls of most viscera, blood vessels, skin
not under conscious control
not striated

4. Functions of Muscle Provide stability and postural tone
Fixed in place without movement
Maintain posture in space
Purposeful movement
Perform tasks consciously, purposefully
Regulate internal organ movement and volume (mostly involuntary)
Guard entrances/exits (digestive/urinary)
Generation of heat (thermogenesis)

5. Characteristics of All Muscle Tissue
Contractile
Ability to shorten with force
CANNOT forcibly lengthen
Extensible (able to be stretched)
Elastic (returns to resting length)
Excitable (can respond electrical impulses)
Conductive (transmits electrical impulses)

6. Structure of a Skeletal Muscle
Figure from: Hole’s Human A&P, 12th edition, 2010
epimysium (around muscle)
perimysium (around fascicles)
endomysium (around fibers, or cells)
Alphabetical order largest to smallest: fascicle, fiber, fibril, and filament

7. Skeletal Muscle Fiber (Cell)
Fully differentiated, specialized cell – its structures are given special names
sarcolemma (plasma membrane)
sarcoplasm (cytoplasm)
sarcoplasmic reticulum (ER)
transverse tubule
triad
cisternae of sarcoplasmic reticulum (2)
myofibril (1-2 µm diam.)
T tubules located at A band-I band junctions and encircle the myofibrils within the muscle fiber.
Figure from: Saladin, Anatomy & Physiology, McGraw Hill, 2007
Transverse tubules contain extracellular fluid ( [Na+],  [K+])
Sarcoplasmic reticulum is like the ER of other cells; but it contains  [Ca2+ ]

8. Structure of the Sarcomere
I band
A band
H zone
Z line
M line
(~ 2µm long)
Triads are placed symmetrically around the M lines, with cisternae lying over the A bands.
Figure from: Hole’s Human A&P, 12th edition, 2010
The sarcomere is the contractile unit of skeletal (and cardiac) muscle

9. Structure of the Sarcomere
‘A’ in A band stands for Anisotropic (dArk)
‘I’ in I band stands for Isotropic (LIght)
Isotropic – the same in all directions measured; anisotropic – different when measured in different directions. H band (Ger. Helle = light). M line (Ger. Mitte = middle). Z line/disk (Ger. Zwichenscheibe = intercalated line).
Zones of non-overlap: I band (thin filaments), and H zone (thick filaments)
A sarcomere runs from Z line (disk) to Z line (disk) (From ‘Z’ to shining ‘Z’!)
Figure from: Saladin, Anatomy & Physiology, McGraw Hill, 2007

10. Preview of Skeletal Muscle Contraction
Major steps:
Motor neuron firing
Depolarization (excitation) of muscle cell
Release of Ca2+ from sarcoplasmic reticulum
Shortening of sarcomeres
Shortening of muscle/CTs and tension produced
T Tubule
Sarcoplasmic reticulum
Physiology here we come!!
Figure from: Martini, Anatomy & Physiology, Prentice Hall, 2001

11. Grasping Physiological Concepts
The steps in a physiological process give you the ‘when’, i.e. tell you when things happen and/or the order in which they happen.
For each step in a process, you should MUST ask yourself the following questions - and be sure you get answers!
How? (How does it happen?)
Why? (Why it happens and/or why it’s important?)
What? (What happens?)

12. Contraction of the Sarcomere
When skeletal muscle contracts: - H zones and I bands get smaller - Areas of overlap get larger - Z lines move closer together - A band remains constant
BUT – lengths of actin and myosin filaments don’t change
How can this be explained?
Figure from: Martini, Anatomy & Physiology, Prentice Hall, 2001

13. Sliding Filament Theory
Figure from: Hole’s Human A&P, 12th edition, 2010
Theory used to explain these observations is called the sliding filament theory …

14. Myofilaments Thin Filaments composed of actin
associated with troponin and tropomyosin
Thick Filaments
composed of myosin
cross-bridges
Figure from: Hole’s Human A&P, 12th edition, 2010

15. Mechanism of Sarcomere Contraction
Figure from: Hole’s Human A&P, 12th edition, 2010
When you think myosin, think mover:
1. Bind 2. Move 3. Detach 4. Reset
Ca2+  troponin
myosin  actin

16. Mechanism of Sarcomere Contraction
Figure from: Hole’s Human A&P, 12th edition, 2010
4. Reset
1. Bind
3. Detach
2. Move
Cycle repeats about 5 times/sec Each power stroke shortens sarcomere by about 1% So, each second the sarcomere shortens by about 5%
What would happen if ATP was not present? …

17. Neuromuscular Junction
site where axon and muscle fiber communicate
motor neuron
motor end plate
synaptic cleft
synaptic vesicles
neurotransmitters
The neurotransmitter for initiating skeletal muscle contraction is acetylcholine (ACh)
Synaptic cleft is about 60 – 100 nm wide.
About 50 million ACh receptors on a muscle cell. Deficiency of these leads to muscle paralysis in myasthenia gravis.
Ca2+
Ca2+ SR
Ca2+
Ca2+
Ca2+
Figures from: Saladin, Anatomy & Physiology, McGraw Hill, 2007

18. Stimulus for Contraction: Depolarization
nerve impulse causes release of acetylcholine (ACh) from synaptic vesicles
ACh binds to acetylcholine receptors on motor end plate
generates a muscle impulse
muscle impulse eventually reaches sarcoplasmic reticulum (via T tubules) and Ca2+ is released
acetylcholine is destroyed by the enzyme acetylcholinesterase (AChE)
Each action potential causes the release of the contents of about 60 synaptic vesicles (about 10,000 molecules of ACh).
Botulinum toxin (Clostridium botulinum) blocks exocytosis of ACh from synaptic vesicles. Curare binds to and blocks ACh receptors. Anticholinesterase agents, e.g., neostigmine, slow removal of ACh from the synaptic cleft. Defective ryanodine receptors (quick to open, slow to close) release too much Ca in skeletal muscle and may be stimulated to release Ca by succinyl choline or halothane anesthetics and may give rise to uncontrollable, extensive muscle contractions causing malignant hyperthermia.
Figure from: Martini, Anatomy & Physiology, Prentice Hall, 2001
Linking of nerve stimulation with muscle contraction is called excitation-contraction coupling

19. Summary of Skeletal Muscle Contraction
Relaxation
Figure from: Martini, Anatomy & Physiology, Prentice Hall, 2001

20. Modes of ATP Synthesis During Exercise
Muscle stores enough ATP for about 4-6 seconds worth of contraction, but is the only energy source used directly by muscle. So, how is energy provided for prolonged contraction?
Muscle contains about 5 millimoles of ATP and 15 millimoles of CP per Kg of tissue. This is enough to power about 1 min of brisk walking or 6 seconds of sprinting or fast swimming.
(ATP and CP)
Continual shift from one energy source to another rather than an abrupt change
Figure from: Saladin, Anatomy & Physiology, McGraw Hill, 2007

21. Energy Sources for Contraction
Figure from: Hole’s Human A&P, 12th edition, 2010
Creatine phosphate
2) Glycolysis (30 sec – 2 min)
3) Aerobic respiration
stores energy that quickly converts ADP to ATP
CP + ATP provide about seconds of energy
Creatine is obtained from the diet (milk, meat, fish) and is also made by the kidneys, liver, and pancreas. Creatine is spontaneously and slowly catabolized to creatinine, which is excreted by the kidneys (and so used as an indication of kidney function).
myoglobin stores extra oxygen

22. Oxygen Debt (The Cori Cycle)
Oxygen debt – amount of extra oxygen needed by liver to convert lactic acid to glucose, resynthesize creatine-P, and replace O2 removed from myoglobin.
when oxygen is not available
glycolysis continues
pyruvic acid converted to lactic acid (WHY?)
liver converts lactic acid to glucose
Figure from: Hole’s Human A&P, 12th edition, 2010
(The Cori Cycle)

23. Muscle Fatigue Inability to maintain force of contraction
Commonly caused by
decreased blood flow
ion imbalances
accumulation of lactic acid
relative decrease in ATP availability
decrease in stored ACh
Cramp – sustained, involuntary contraction
It seems that most forms of exercise-associated cramps result from an abnormally sustained activity of the nerve cells in the spinal cord which control skeletal muscle, the alpha motor neurons. Fatigue appears to be the central factor in exercise associated muscle cramps (EAMC). Fatigue enhances the input to the alpha motor neurons from the main receptors in the muscles (muscle spindles) and inhibits the input from the receptors in their tendons (Golgi tendon organs) that signal tension. Because the spindle signals excite alpha motor neurons, while those from tendon organs are inhibitory, these fatigue effects can combine to promote uncontrolled activity in the relevant regions of the spinal cord. It is a common experience that cramp may be precipitated by contraction of the muscle from an already shortened position, and this of course is when the tension signal from its tendon is weakest.

24. Length-Tension Relationship
Figure from: Martini, Anatomy & Physiology, Prentice Hall, 2001
Maximum tension in striated muscle can only be generated when there is optimal overlap between myosin and actin filaments

25. Muscular Responses Threshold Stimulus
minimal strength required to cause contraction in an isolated muscle fiber
Recording a Muscle Contraction (myogram)
latent period
period of contraction
period of relaxation
refractory period
all-or-none response
A single twitch
Typical latent period is about 2 ms. Entire twitch lasts about 70 – 100 msec.
An individual muscle fiber (cell) is either “on” or “off” and produces maximum tension at that resting length for a given frequency of stimulation

26. Treppe, Wave Summation, and Tetanus
Wave (Temporal) Summation
Treppe (10-20/sec)
Little/no relaxation period
Complete Tetanus
(>50/sec)
Incomplete Tetanus
(20-30/sec)
Tetany is a sustained contraction of skeletal muscle
Figure from: Martini, Anatomy & Physiology, Prentice Hall, 2001

27. Treppe, Wave Summation, and Tetanus
all involve increases in maximum tension generated in a muscle fiber after re-stimulation
The difference among them is WHEN the muscle fiber receives the second and subsequent stimulations:
Treppe – stimulation immediately AFTER a muscle cell has relaxed completely.
Wave Summation – Stimulation BEFORE a muscle fiber is relaxed completely
Incomplete tetanus – partial relaxation between stimuli
Complete tetanus – NO relaxation between stimuli

28. Motor Unit
single motor neuron plus all muscle fibers controlled by that motor neuron
** Contraction in a single muscle fiber is an “all or none” phenomenon (but remember that the tension will not always be maximal)
Motor units differ in the number of muscle fibers innervated by a single motor neuron, e.g., in the eye 3-6 muscle fibers/neuron while the gastrocnemius has about 1,000 muscle fibers/neuron.
Figure from: Hole’s Human A&P, 12th edition, 2010

29. Recruitment of Motor Units
recruitment - increase in the number of motor units activated to perform a task
whole muscle composed of many motor units
as intensity of stimulation increases, recruitment of motor units continues until all motor units are activated

30. Sustained Contractions
smaller motor units recruited first
larger motor units recruited later
produces smooth movements
muscle tone – continuous state of partial contraction

31. Types of Contractions concentric – shortening contraction
isotonic – muscle contracts and changes length
eccentric – lengthening contraction
isometric – muscle “contracts” but does not change length
Muscles can generate about 3-4 Kg/cm2 (about 50 lbs/sq in.). Quadriceps has as much as 16 sq. in. of muscle -> about 800 lbs of tension.
Figure from: Hole’s Human A&P, 12th edition, 2010

32. Types of Skeletal Muscle Fibers
Slow Oxidative (SO)
(GO REDSOX!)
Fast Oxidative-Glycolytic (FOG)
Fast Glycolytic (FG)
Alternate name
Slow-Twitch Type I
Fast-Twitch Type II-A
Fast-Twitch Type II-B
Myoglobin (color)
+++ (red)
++ (pink-red)
+ (white)
Metabolism
Oxidative (aerobic)
Oxidative and Glycolytic
Glycolytic (anaerobic)
Strength
Small diameter, least powerful
Intermediate diameter/strength
Greatest diameter, most powerful
Fatigue resistance
High
Moderate
Low
Capillary blood supply
Dense
Intermediate
Sparse
SO fibers are smallest, FG fibers are largest, FOG fibers are intermediate in size. Within a motor unit, all of the skeletal muscle fibers are the same type.
All fibers in any given motor unit are of the same type

33. Smooth Muscle Fibers Compared to skeletal muscle fibers shorter
single nucleus
elongated with tapering ends
myofilaments organized differently
no sarcomeres, so no striations
lack transverse tubules
sarcoplasmic reticula not well developed
exhibit stress-relaxation response
Figure from: Martini, Anatomy & Physiology, Prentice Hall, 2001

34. Types of Smooth Muscle Single-unit smooth muscle
visceral smooth muscle
sheets of muscle fibers that function as a group, i.e., a single unit
fibers held together by gap junctions
exhibit rhythmicity
exhibit peristalsis
walls of most hollow organs, blood vessels, respiratory/urinary/ reproductive tracts
Multiunit Smooth Muscle
fibers function separately, i.e., as multiple independent units
muscles of eye, piloerector muscles, walls of large blood vessels

35. Smooth Muscle Contraction
Resembles skeletal muscle contraction
interaction between actin and myosin
both use calcium and ATP
both depend on impulses
Different from skeletal muscle contraction
smooth muscle lacks troponin
smooth muscle depends on calmodulin
two neurotransmitters affect smooth muscle
acetylcholine and norepinephrine
hormones affect smooth muscle
have gap junctions
stretching can trigger smooth muscle contraction
smooth muscle slower to contract and relax
smooth muscle more resistant to fatigue

36. Cardiac Muscle only in the heart
muscle fibers joined together by intercalated discs
fibers branch
network of fibers contracts as a unit (gap junctions)
self-exciting and rhythmic
longer refractory period than skeletal muscle (slower contract.)
cannot be tetanized
fatigue resistant
has sarcomeres
Figure from: Martini, Anatomy & Physiology, Prentice Hall, 2001

37. Review Three types of muscle tissue Muscle tissue is… Skeletal Cardiac
Smooth
Muscle tissue is…
Contractile
Extensible
Elastic
Conductive
Excitable

38. Review Functions of muscle tissue Muscle fiber anatomy
Provide stability and postural tone
Purposeful movement
Regulate internal organ movement and volume
Guard entrances/exits
Generation of heat
Muscle fiber anatomy
Actin filaments, tropomyosin, troponin
Myosin filaments
Sarcomere
Bands and zones

39. Review Muscle contraction Muscular responses Sliding filament theory
Contraction cycle (Bind, Move, Detach, Release)
Role of ATP, creatine
Metabolic requirements of skeletal muscle
Stimulation at neuromuscular junction
Muscular responses
Threshold stimulus
Twitch – latent period, refractory period
All or none response
Treppe, Wave summation, and tetanus

40. Review
Table from: Hole’s Human A&P, 12th edition, 2010

41. Review Muscular responses Fast and slow twitch muscle fibers
Recruitment
Muscle tone
Types of muscle contractions
Isometric
Isotonic
Concentric
Eccentric
Fast and slow twitch muscle fibers
Slow Oxidative (Type I) (think: REDSOX)
Fast Oxidative-glycolytic (Type II-A)
Fast Glycolytic (Type II-B)