Chapter 10: Muscles and Muscle Tissue
Chapter Objectives
OVERVIEW OF MUSCLE TISSUE
1. Describe the three key functions of muscle.
2. Describe the four special properties of muscle tissue.
SKELETAL MUSCLE TISSUE
3. Describe the difference between a fascicle, a myofiber, a myofibril and a myofilament.
4. Explain what is meant by muscle atrophy and hypertrophy.
5. Describe and give the functions of the four kinds of proteins found in myofibrils.
6. Describe the different bands found in a sarcomere.
7. Explain how the arrangement of the thick and thin myofilaments forms the observed
sarcomere structural patterns of the myofibril.
8. Discuss the connection between the sarcolemma and the Transverse tubules.
9. Describe and give the function and location of the sacroplasmic reticulum and the
terminal cisternae.
10. List the three layers of connective tissue that surrounds skeletal muscle and give a
function for the tissue.
THE NEUROMUSCULAR JUNCTION
11. Show the general features of the neuromuscular junction that allows signals coming from
the brain to be conveyed across the gap between the neuron motor cell and the
sarcolemma of the muscle cell.
12. Describe the effects of certain drugs, bacterial toxins and disorders on the function of the
neuromuscular junction (NMJ).
CONTRACTION AND RELAXATION OF SKELETAL MUSCLE FIBERS
13. Describe the conditions inside a skeletal muscle cell at rest in regards to ATP, Calcium,
and the state of the myofilaments.
14. Explain why the process of muscle contraction is called Excitation-Contraction Coupling.
15. Describe what is happening in a skeletal muscle cell during the Sliding Filament
Mechanism in regards to ATP, Calcium and myofilament function.
16. Illustrate the progressive overlap of the thick and thin filaments as they pull the Z disc
toward the center of the sarcomere, and the result on the length of the fibril, fiber, and
muscle.
17. Define a power stroke cycle and explain the steps of the cycle. Include the factors that
maintain continuous cycling of muscle contraction.
18. Describe the condition called rigor mortis and its cause.
MUSCLE METABOLISM
19. Describe energy use in muscle cells and list the three sources for ATP regeneration in
muscle cells.
20. Discuss the relative durations of time that each form of ATP production provides for
sustained activity.
21. Describe the use of creatine phosphate in muscles to produce ATP.
22. Define oxygen debt and discuss the purpose of the elevated use of oxygen after exercise.
CONTROL OF MUSCLE TENSION
23. Define the All or None Principal
24. Define a motor unit and indicate the relationship between a motor neuron, the number of
muscle fibers it innervates, and the strength of contraction by the motor unit.
25. Discuss the effects of increasing the number of active motor units on contraction force.
26. Define muscle tone and note how it normally works in body posture maintenance.
27. Discuss the activities that occur in each period of a twitch contraction as shown in a
myogram.
28. Define Refactory period.
29. Define twitch, treppe and tetanus and explain the conditions that cause them.
30. Compare and contrast isotonic and isometric contractions.
TYPES OF SKELETAL MUSCLE FIBERS
31. Discuss the slow oxidative fibers based on their cellular components, fatigue resistance,
duration of contraction and ATPase reaction rate.
32. Discuss the fast glycolytic based on their cellular components, fatigue resistance,
duration of contraction and ATPase reaction rate.
33. Discuss the intermediate fatigue resistant fibers based on their cellular components,
fatigue resistance, duration of contraction and ATPase reaction rate.
34. Examine how various types of exercise can induce changes in the fibers in skeletal
muscle.
SMOOTH MUSCLE TISSUE
35. Compare skeletal and smooth muscle cells with respect to organelles, cytoskeleton, and
contractile filaments.
36. Contrast the source and nature of differences in contraction between smooth and skeletal
muscles.
CARDIAC MUSCLE TISSUE
37. Compare and contrast differences in cardiac and skeletal muscle cell structure and
physiology.
Chapter Lecture Notes
Functions of Skeletal Muscle
Movement of body
Posture maintenance
Heat production - 85% of body heat is generated by skeletal muscle
25 - 40 % of energy from nutrients is converted to ATP by cellular respiration
60 - 75 % of energy from nutrients is converted to heat
Properties of Skeletal Muscle
Excitability - ability to receive and respond to a stimulus
stimulus = change in environment strong enough to initiate an electrical signal, action
potential
In skeletal muscle, stimulus is usually acetylcholine (ACh, neurotransmitter)
Contractility - ability to shorten (can shorten up to 50% of resting length)
Extensibility - ability to stretch (can stretch up to 20% of resting length)
Elasticity - ability to return to original length after contraction
Histology
Common root words for muscle
sarco = fleshy
myo = muscle
Large → Small: fascicle → myofiber → myofibril → myofilament (Table 10.3)
Muscle consists of elongated cells called muscle fibers or myofibers (Fig 10.1 & 10.2)
A bundle of these fibers is called a fascicle
Inside of a muscle cell there are small cylinders called myofibrils which may number several 100
to several 1000/cell (Fig 10.3)
Exercise increases myofibril production (hypertrophy); lack of exercise decreases myofibrils
(atrophy)
Each myofibril consists of myofilaments (proteins) (Fig 10.3, 10.4 and Table 10.2 & 10.3)
thick myofilaments = myosin
thin myofilaments = actin, troponin, tropomyosin
Sarcomere - myofilaments don't extend entire length of muscle fiber; they are stacked into
compartments called sarcomeres (Fig 10.3 & Table 10.1)
Sarcomeres are the functional unit of a skeletal muscle (contractile unit)
Parts of a sarcomere
A band - myosin + overlapping actin
I band - only actin, troponin, tropomyosin
Z disc – through center of I band
H zone - only myosin
M line – middle of sacromere
Sarcomere extends from Z disc to Z disc - 2 I bands / sarcomere
Sarcoplasm – cytoplasm
Sarcolemma - cell membrane (Fig 10.2)
T tubule - tubular invagination of sarcolemma that surrounds each myofibril at the A band - I
band junction
Sarcoplasmic reticulum (SR) - smooth endoplasmic reticulum that stores Ca2+
Each myofibril is surrounded by SR
Terminal cisternae – dilated end sacs of the SR
Triad - 2 terminal cisternae + 1 T tubule located at the A band - I band junction
Entire muscle is wrapped in fibrous connective tissue which is continuous with tendons that
insert skeletal muscle into periosteum of bone
Connective tissue holds muscle together as well as serving to transmit blood vessels and
nerves to inner muscle cells
Connective tissue layers of skeletal muscle (Fig 10.1)
Epimysium - outside of entire muscle
Can be called deep fascia (sheet of fibrous connective tissue) as compared to
superficial fascia also known as subcutaneous layer
Perimysium - divides muscle cells into fascicles (bundles)
Endomysium - covers individual muscle cells
Neuromuscular Junction
A synapse or site of communication between a neuron and muscle is usually named a
neuromuscular junction (NMJ) (Fig 10.9)
Structure
Neuromuscular junction = axon terminal (synaptic end bulb) of a motor neuron +
synaptic cleft + motor end plate (sarcolemma under motor neuron)
One per muscle fiber and usually in middle
Function
Acetylcholine (Ach) is released from the axon terminal (synaptic end bulb) of the motor
neuron by exocytosis
Ach diffuses across the synaptic cleft to the motor end plate
Ach binds to receptors on the motor end plate
Ach binding initiates a change in the electrical state of the muscle cell called an action
potential
Problems at the NMJ
Curare - binds to ACh receptors in skeletal muscle membrane
Competes with ACh but does not stimulate the ACh receptor
Muscle paralysis
Myasthenia gravis - antibodies destroy ACh receptors
Muscle paralysis
Botulism (from the bacteria Clostridium botulinum) - toxin inhibits ACh release
Muscle paralysis
A dilute solution of botulinum toxin (Botox) can be injected into a muscle that is in
spasm to help it relax
Tetanus - (from the bacteria Clostridium tetani) - this anaerobic bacteria produces a toxin that
blocks an inhibitory neurotransmitter in the central nervous system
Causes muscle spasms and painful convulsions
Tetanus shots immunize against the toxin
Organophosphates (in some pesticides) - inhibits acetylcholinesterase
Acteylcholinesterase – enzyme that inactivates acetylcholine
Muscle spasms
Sliding Filament Mechanism
Sliding Filament Mechanism means: myosin (thick myofilaments) cross bridges pull actin (thin
myofilaments) toward H zone during contraction (Fig 10.5)
I band and H zone may disappear in a contracting muscle
A band stays same length during contraction
At rest
Calcium in SR (terminal cisternae)
Troponin-tropomyosin prevents myosin from binding to sites on actin
ATP bonded to myosin cross bridges (concentration of ATP is high in relaxed muscle)
Excitation-Contraction Coupling (Fig 10.10)
Muscle action potential will be generated at the NMJ due to neural stimulus - Ach binds to its
receptor on the motor end plate
Action potential - rapid change in electrical polarity at the plasma membrane of a muscle
or neuron
Will travel along the sarcolemma down into the T tubules
Production of AP in muscle leads to contraction; Excitation-Contraction Coupling
The action potential running through the T tubules causes the sarcoplasmic reticulum’s
terminal cisternae to release calcium into the sacroplasm
The calcium will then bind to troponin
Tropomyosin changes shape so myosin binding sites on actin are uncovered (Fig 10.7)
Myosin then can bind to actin
The calcium will also activate ATPase ability of myosin
The ATPase activity of myosin activates its power stroke movement (Fig 10.6)
Pulls actin inward (H zones and I bands narrow and may disappear; A band does not
change its length)
At end of power stroke, a new ATP binds to ATP binding site on the myosin cross bridge,
resulting in detachment of myosin from actin
Attach, pull, detach (release) = steps in contraction
If lack ATP - rigor mortis
To relax following contraction
ACh is inactivated by acetycholinesterase (from sarcolemma surface)
Calcium is actively transported back into SR
ATP is required to pump calcium back into the SR
ATP attaches to myosin cross bridge and it releases from actin
Troponin-tropomyosin covers myosin binding sites on actin preventing binding of myosin
cross bridges
ATP and Muscle Function
ATP required for
Power stroke - movement of myosin cross bridges pulls actin inward in a sarcomere
At end of power stroke, ATP binds to ATP binding site, resulting in detachment of myosin
from actin (ATP needed to release cross bridges)
Pumping Ca2+ back into terminal cisternae of the SR
Sources of ATP (Fig 10.11)
Stored ATP - lasts only 6 seconds during bursts of muscle contraction
ATP generated from creatine phosphate (CP) (CP + ADP → creatine + ATP)
Together ATP that is stored and CP provide muscle power for 10-15 sec
CP replenished during resting periods
Even as ATP and CP are being used, ATP is generated by aerobic respiration and anaerobic
respiration
Resting and slowly contracting muscles obtain bulk of ATP via aerobic respiration of
fatty acids (enters as acetyl-CoA)
Aerobic pathway: glucose + O2 → CO2 + H2O + 36ATP
In actively contracting muscles, glucose (from blood and breakdown of glycogen) is
primary fuel supply
Anaerobic pathway: glucose → lactic acid + 2 ATP
Aerobic pathway produces 20X more ATP than anaerobic respiration but takes 2 1/2
times longer
Anaerobic respiration causes oxygen debt to occur
Oxygen Debt
Oxygen Debt: Amount of oxygen needed to metabolize the accumulated lactic acid and to restore
ATP levels
Muscle fatigue is result of ATP depletion and accumulation of lactic acid
80% of lactic acid diffuses from skeletal muscle to blood and is transported to liver where
it is converted back to glucose
20% remains in muscle - processed to CO2 and H2O + ATP
Oxygen debt results in labored breathing in order to pay back the O2 debt
Lactic acid decreases pH of blood which stimulates increased respiration
All or None Principal
All or None Principle - individual muscle fibers of a motor unit will contract to its fullest extent
of its immediate ability when stimulated by a nerve impulse (action potential) of
threshold level
The principle does not apply to the entire muscle but only to motor units
Motor unit - motor neuron + all the skeletal muscle fibers it services (5 fibers to 2000
muscle fibers) (Fig 10.12)
One entire muscle has many motor units
Not all are stimulated at same time
The smaller the number of muscle fibers/motor unit, the more precise the control of
the muscle fibers
So the strength of a contraction of an entire muscle depends on
Size of load - heavier the load, stronger the contraction (pencil vs book)
Number of motor units activated
Initial length of fibers - more a muscle is stretched, to a point, the greater the contraction
(Fig 10.8)
Muscle tone - partial sustained contraction important in maintenance of posture
Only a fraction of motor units are activated at any one time
Produces tautness instead of recognized contraction
Maintain tone without fatigue because there is a system of rotation; at any one moment
some fibers are contracted and others are relaxed & resting, ready to take up the
work next (motor units innervate scattered muscle cells and not clustered)
If muscle tone is not maintained, possibly due to nerve damage, the muscle has less than
normal tone, or is flaccid
If flaccidity is maintained for an extended period of time the muscle will atrophy, a
decrease in muscle mass by a decrease in myofibrils
Leg in cast
Bedridden
The opposite of atrophy is hypertrophy, an increase in diameter of muscle from
forceful muscle activity or repetitive muscle activity; there is an increase in
number of myofibrils, mitochondria, SR, blood vessels, connective tissue
Kinds of Contractions
Muscle twitch - rapid jerky response to single stimulus
Can be traced on a myogram (Fig 10.13)
Three phases: latent, contraction, relaxation
Length of twitch varies for different muscles
Short in eye muscle
Long in leg muscle
Refractory period - period of lost irritability (excitability) - short period of time during which
there is no response to additional stimuli
Short in skeletal muscle (.005 sec)
Long in cardiac (.3 sec)
Wave Summation (Treppe) - staircase phenomenon - reoccurring stimuli results in increasingly
stronger twitch contractions (Fig 10.14)
Increased heat in warmed up muscle increases speed of all reactions
Increased calcium in sarcoplasm activates sliding filament mechanism faster
Practical application: warming up of athletes
Tetanus - (not to be confused with bacterial disease of same name) - fusion of twitches (Fig
10.14)
Relaxation is not allowed to occur
Body normally delivers volleys of impulses in rapid succession, giving us smooth sustained
contractions
Voluntary contractions (normal movements) are tetanic contractions
Isotonic contraction - muscle shortens and can move a load (Fig 10.15)
Tension, force exerted by a contracting muscle on some object, has to exceed load to
move it
Tension remains constant as the muscle shortens
Isometric contractions - develops increased tension in muscle but muscle does not shorten or
there is minimal shortening (Fig 10.15)
Cross bridges formed but unsuccessful in moving thin myofilaments (cross bridges
spinning wheels on same cross bridge binding site)
Length remains about the same
Types of Skeletal Muscle Fibers
Slow twitch - fatigue resistant – red (SO) (Table 10.4)
Contract more slowly
Slower myosin ATPase
Fuel source: fatty acids
Aerobic respiration
More mitochondria
Low glycogen
High myoglobin
Myoglobin - similar to hemoglobin, hence red in color; binds O2 and acts as a reservoir
for O2 until needed by mitochondria
More capillaries
Small diameter
ex. - large postural muscles, long distance runners
Fast twitch – fatigable – white (FG)
Contract more rapidly
Faster myosin ATPase
Fuel source: glucose
Anaerobic respiration
Fewer mitochondria
High glycogen
Low myoglobin
Fewer capillaries
Large diameter
ex. – muscles of arm and legs
Intermediate – intermediate fatigue resistant – pink (FOG)
Contract more rapidly
Faster myosin ATPase
Fuel source: glucose
Anaerobic and aerobic respiration
More mitochondria
High glycogen
High myoglobin
More capillaries
Intermediate diameter
Most muscles have a combination of both types - all fibers of a motor unit are of same type
Endurance type exercises (running, swimming) can transform fast twitch - fatigable into
intermediate by producing more mitochondria and increased myoglobin
Cannot transform slow twitch to fast twitch fibers
Smooth Muscle
Smooth – Involuntary, nonstriated, visceral (Fig 10.16)
Located in walls of organs and blood vessels
Has actin and myosin but not arranged orderly into sarcomeres
Contract and relax more slowly than skeletal muscle due to arrangement of actin and myosin
and a different calcium regulation molecule (calmodulin instead of troponintropomyosin)
Contracts in waves as impulse spreads from one cell to another
ex. - propulsion of food through digestive system
Cardiac Muscle
Cardiac – Involuntary, striated (Table 10.5)
Same arrangement of actin and myosin as skeletal muscle but not organized into discrete
myofibrils
Has own intrinsic rhythm
Own electrical system
Cardiac cells can contract without nerve stimulation
Source of stimulation is specialized cells that initiate contraction, pacemaker
Nerve stimulation merely speeds up or slows down rate of contraction