1. Muscles and Muscle Tissue
Back to aponeurosis

2. Types of Muscle Tissue
Skeletal muscle is associated with the bony skeleton, and consists of large cells that bear striations and are controlled voluntarily.
Cardiac muscle occurs only in the heart, and consists of small cells that are striated and under involuntary control.
Smooth muscle is found in the walls of hollow organs, and consists of small elongated cells that are not striated and are under involuntary control.

3. Muscle Functions
Muscles produce movement by acting on the bones of the skeleton, pumping blood, or propelling substances throughout hollow organ systems.
Muscles aid in maintaining posture by adjusting the position of the body with respect to gravity.
Muscles stabilize joints by exerting tension around the joint.
Muscles generate heat as a function of their cellular metabolic processes.
Functional Characteristics of Muscle Tissue
Excitability, or irritability, is the ability to receive and respond to a stimulus.
Contractility is the ability to contract forcibly when stimulated.
Extensibility is the ability to be stretched.
Elasticity is the ability to resume the cells’ original length once stretched.

4. Gross Anatomy of a Muscle Organ
Origin, belly and insertion
Made up of thousands of muscle fibers bundled in connective tissue coverings which contains many blood vessels and a motor nerve ending for each muscle fiber
Connective Tissue Wrappings (deep fascia)
Deep fascia surrounds and penetrates muscle
Epimysium surrounds the muscle organ
Fascia surrounds groups of muscles forming compartments
Perimysium surrounds the fascicles
Endomysium surrounds each muscle fiber
The deep fascia is interconnected to the subcutaneous fascia and the subserous fascia
The deep fascia connects to bones via tendons & aponeuroses

5. Skeletal Muscle Return Muscles attach:
Directly – epimysium of the muscle is fused to the periosteum of a bone
Indirectly – connective tissue wrappings extend beyond the muscle as a tendon or aponeurosis
Skeletal Muscle
Return

6. Muscle Classification: Functional Groups
Skeletal muscles work together or in opposition
Muscles only pull (never push)
As muscles shorten, the insertion generally moves toward the origin
Whatever a muscle (or group of muscles) does, another muscle (or group) “undoes”
Prime movers – provide the major force for producing a specific movement
Antagonists – oppose or reverse a particular movement
Synergists -add force to a movement or reduce undesirable or unnecessary movement
Fixators – synergists that immobilize a bone or muscle’s origin

7. Arrangement of Fascicles
Parallel – fascicles run parallel to the long axis of the muscle
Fusiform – spindle-shaped muscles
Pennate – short fascicles that attach obliquely to a central tendon running the length of the muscle
Convergent – fascicles converge from a broad origin to a single tendon insertion
Circular – fascicles are arranged in concentric rings

8. Naming Skeletal Muscles
Location of muscle – bone or body region associated with the muscle
Shape of muscle – e.g., the deltoid muscle (deltoid = triangle)
Relative size – e.g., maximus (largest), minimus (smallest), longus (long)
Direction of fibers – e.g., rectus (fibers run straight), transversus, and oblique (fibers run at angles to an imaginary defined axis)
Number of origins – e.g., biceps (two origins) and triceps (three origins)
Location of attachments – named according to point of origin or insertion
Action – e.g., flexor or extensor, as in the names of muscles that flex or extend, respectively

9. Microscopic Anatomy of a Skeletal Muscle Fiber
Myofibrils - 80% of the cell
Surrounded by sarcoplasmic reticulum (stores Ca++)
contains transverse tubules which surface at the sarcolemma
Made up of myofilaments
actin -thin
myosin – thick
Each fiber is a long, cylindrical cell with multiple nuclei just beneath the sarcolemma (plasma membrane - supported by dystrophin)
Sarcoplasma - specialties
Glycogen
Myoglobin
mitochondria

10. Sarcomere -smallest contractile unit
Striations - caused by arrangement of myosin and actin
I band - light
A band - dark
Z line to Z line (one sarcomere)
Molecular composition of myofilaments
myosin
cross bridges (heads contain ATPases)
elongated tail proteins
actin -double stranded helix
Troponin
Tropomyosin
Titin - function

11. Contraction of a Muscle Fiber
Sliding filament model
Which filament moves?
In what direction? 8.

12. CONTRACTION OF A MUSCLE FIBER
Role of Ca++ in contraction
Where is the binding site for the cross bridge?
How is covered up during relaxation?
How is it exposed for contraction?
Excitation Contraction Coupling
Binding sites exposed (Ca++ present)
Crossbridges form when myosin heads attach using ATP energy
Myosin crossbridges bends pulling on actin using energy from ATP
Crossbridges detach when ATPase hydrolyses new ATP
Ca++ are pumped back into ___________ __________
sacroplasmic reticulum

13. CONTRACTION REGULATION
(neuromuscular junction)
1. At the axon ending a nerve impulse causes the release of ____________ from the synaptic _________.
acetylcholine
vesicles
(Acetylcholine is a ______________)
neurotransmitter
2. Acetylcholine crosses the
______ _____ and binds to
___ _________ of the
_____ ___ ____ on the sarcolemma.
synaptic cleft
ACh receptors
motor end plate
Na+
3. This opens _____ channels and ____ enter the muscle cell.
Na+
4. Na+ enters the cell and the membrane becomes __________ generating an ______ _________
depolarized
action potential
5. The __- _______ carries the action potential through out the cell causing the release of ______from the _________ ___________.
T - tubules
Ca++
sarcoplasmic reticulum
6. Which then stimulates the _______ _______ action.
Sliding filament

14. Sliding filament model of contraction.
Myosin heads pull on actin
actin moves towards middle of sarcomere
How is this controlled?
Myosin must attach to actin.
Achieved by an increase in Ca+2 in the sarcoplasm

15. a-The SR & cisternae release Ca+2 into the sarcoplasm
b- Ca+2 binds to Troponin C
c- The Ca+2 activated Troponin undergoes a shape change that moves tropomyosin away from the myosin- binding site on the actin.

16. The contraction cycle now begins by:
Myosin head splits ATP & becomes energized
2) Mysosin head attaches to the myosin binding site on actin (since Ca+2 is present)
3) Myosin head releases the ADP & P, pulls the thin filament toward the center. (power stroke)
4) As the myosin heads bind the ATP, the crossbridges (heads) detach.

17. Each of the 400-600 myosin heads in one thick filament attaches & detaches about 5 times per second.
Not all the crossbridges are bound at the same time.

18. Resting Membrane Potential
Why does the nerve impulse cross the synapse?
Outside of the cell membrane the electrical charge is positive
Inside the cell the membrane has a negative charge. + - + + + - + - - - -
The predominant extracellular ion is Na+
The predominant intracellular ion is K+
The sarcolemma is relatively impermeable to both ions
All the above facts establish a resting membrane potential
Initially, this is a local electrical event called end plate potential (synapse)
Later, it ignites an action potential that spreads in all directions across the sarcolemma and down the T-Tubules
Resting Membrane Potential

19. Excitation-Contraction Coupling

20. CONTRACTION REGULATION cont. (neuromuscular junction)
Meanwhile back at the junction
The membrane is ___________ to accept another stimulus (refractory period)
____ leaves the cell for a quick repolarization.
repolarized K+
Cholinesterase decomposes ____and removes it from _____________
ACh
ACh receptor
Na+-K+ pump returns ions to _______ membrane potential conditions
resting
In this process where are the 3 places ATP is necessary?
1. To move myosin crossbridges
2. To return Ca++ to sacroplasmic reticulum
3. For the Na+K+ pump to return muscle membrane to resting potential.
Review Matching

21. Motor Unit: 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
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
Muscles that control fine movements (fingers, eyes) have small motor units
Large weight-bearing muscles (thighs, hips) have large motor units

22. Motor unit recruitment:
not all the MU of a muscle are simultaneously recruited.
As  force is required for a task
 # MU are recruited (activated).

23. Two ways that we produce graded movements-not jerky.
1) MU recruitment
2)  frequency of stimulation (APs of nerve & thus muscle cell)

24.  Frequency of stimulation
Muscle twitch: response of a MU to a single AP of its motor neuron
Wave summation: if another AP occurs before the muscle has had time to fully relax
 force of contraction

25.   force of contraction
Unfused tetanus: stimulate at a rate of x/sec, partial relaxation &
  force of contraction
Fused tetanus: stimulation rate of x/sec, no relaxation between twitches
   force of contraction
All of the above can be at least partially due to Ca+2 build up in sarcoplasm (hasn’t gotten a chance to get pumped back into SR)

26. Muscle Twitch
A muscle twitch is the response of a muscle to a single, brief threshold stimulus
The three phases of a muscle twitch are:
Latent period – first few milliseconds after stimulation when excitation- contraction coupling is taking place
Period of contraction – cross bridges actively form and the muscle shortens
Period of relaxation – Ca2+ is reabsorbed into the SR, and muscle tension goes to zero

27. Graded Muscle Responses
Graded muscle responses are:
Variations in the degree of muscle contraction
Required for proper control of skeletal movement
Responses are graded by:
Changing the frequency of stimulation
More rapidly delivered stimuli result in incomplete tetanus
If stimuli are given quickly enough, complete tetanus results
Changing the strength of the stimulus
Threshold stimulus – the stimulus strength at which the first observable muscle contraction occurs
Beyond threshold, muscle contracts more vigorously as stimulus strength is increased
Force of contraction is precisely controlled by multiple motor unit summation
This phenomenon, called recruitment, brings more and more muscle fibers into play
Starting length of the muscle - optimal length-tension relationship
The relative size of the muscle – the bulkier the muscle, the greater its strength

28. Contraction of Skeletal Muscle (Organ Level)
The two types of muscle contractions are:
Isometric contraction –Tension increases to the muscle’s capacity, but the muscle neither shortens nor lengthens
Maintain posture
Isotonic contraction – the muscle changes in length (decreasing the angle of the joint) and moves the load
Concentric contractions – the muscle shortens and does work
Eccentric contractions – the muscle contracts as it lengthens
as in setting down a load without dropping it

29. Muscle Tone & Treppe Muscle tone:
Is 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
Lack of nerve stimulation causes muscles to become flaccid and atrophy.
Spinal reflexes account for muscle tone by:
Activating one motor unit and then another
Responding to activation of stretch receptors in muscles and tendons
Treppe: The Staircase Effect -increased contraction in response to multiple stimuli of the same strength
Contractions increase because:
There is increasing availability of Ca2+ in the sarcoplasm
Muscle enzyme systems become more efficient because heat is increased as muscle contracts

30. Muscle Metabolism: Energy for Contraction
ATP is the only source used directly for contractile activity
As soon as available stores of ATP are hydrolyzed (4-6 seconds), they are regenerated by:
The interaction of ADP with creatine phosphate (CP), no oxygen required
Anaerobic glycolysis – lactic acid formation, no oxygen required
Aerobic respiration – oxygen required
 ATP --->ADP + P1
Heat Production
Only 40% of the energy released in muscle activity is useful as work
The remaining 60% is given off as heat
Dangerous heat levels are prevented by radiation of heat from the skin and sweating

31. 3 Sources of ATP in muscle:
1) creatine phosphate (3 - 6x more plentiful than ATP, only found in muscle) donates a phosphate to ADP.
ATP & creatine phosphate provide energy for 15 sec of maximal activity.

32. 2) Anaerobic (no O2) glycolysis.
Glucose (from blood or glycogen breakdown in muscle) is converted to pyruvic acid (get 2 ATP molecules) and then to lactic acid.
Provides energy for sec of maximal activity
Both 1 & 2 occur in cytoplasm.

33. Muscle Metabolism: Anaerobic Glycolysis
When muscle contractile activity reaches 70% of maximum:
Bulging muscles compress blood vessels
Oxygen delivery is impaired
Pyruvic acid is converted into lactic acid
The lactic acid:
Diffuses into the bloodstream
Is picked up and used as fuel by the liver, kidneys, and heart
Is converted back into pyruvic acid by the liver (Cori Cycle)

34. 3) Aerobic (O2 required)
Pyruvic acid (from glucose)
Amino acids
Fatty acids
generate ATP (36 per glucose molecule), CO2, H2O, heat.
Provides ATP for activities longer than 40 sec
Takes place in mitochondria

35. Muscle Fatigue & Oxygen Debt
Muscle fatigue – the muscle is in a state of physiological inability to contract
Muscle fatigue occurs when:
ATP production fails to keep pace with ATP use
There is a relative deficit of ATP, causing contractures
Lactic acid accumulates in the muscle
Ionic imbalances are present - Na+-K+ pumps cannot restore ionic balances quickly enough
SR is damaged and Ca2+ regulation is disrupted
Intense exercise produces rapid muscle fatigue (with rapid recovery)
Low-intensity exercise produces slow-developing fatigue
Oxygen Debt - the extra amount of O2 needed for a muscle to return to a resting state:
Oxygen reserves must be replenished
Lactic acid must be converted to pyruvic acid – inc. ATP
Glycogen stores must be replaced
Cramp

36. Muscle Fiber Types Determined by the two following factors:
Contraction rate depends on speed in which ATPases split ATP
Anaerobic (Glycolytic) or Aerobic (oxidative)
Slow-twitch Oxidative Fibers
slow acting myosin ATPases
slow contraction small diameter
always oxidative (aerobic)
resistant to fatigue
red fibers ATP, Mitochondria
capillaries
Posture, endurance, some marathon like
Fast-twitch oxidative fibers
Fast acting myosin ATPases
Fast contraction
moderate resistance to fatigue
pink to red in color
Walking, sprinting
Fast-twitch glycolytic fibers
ATPases fast or slow? dec. ATP
contraction rate? Most force
aerobic or anaerobic?
white fibers (myoglobin?) dec., mitochondria, caps
blood supply?
fatigable? Most jumping, wgt lift

37. Recruitment order SO, FOG, FG
Most muscles are a mixture of all 3, but percentage varies- genetics and type of exercise.
training
FG  FOG 
detraining
No slow in paralyzed people,
Lose fast types as we age & tend to see clustering of types in x- sections.

38. Smooth Muscle Composed of spindle-shaped fibers
Lack coarse connective tissue sheaths of skeletal muscle, but have endomysium
Organized into two layers (longitudinal and circular) of closely apposed fibers
Found in walls of hollow organs (except the heart)
When longitudinal layer contracts, the organ dilates & contracts
When the circular layer contracts, the organ elongates – rhythmic contractions
Peristalsis – alternating contractions and relaxations of smooth muscles that mix and squeeze substances through the lumen of hollow organs

39. Smooth muscle: 2 types
Single unit -most common
walls of hollow organs (except heart)- respiratory, digestive, urinary, reproductive, some blood vessels
b) cells arranged in sheets w/ gap junctions between cells- stimulate each other, contract rhythmically

40. 2) multiunit smooth muscle
large airways, large arteries, irises of eyes
b) cells are structurally independent (few gap junctions) so need more innervation

41. Smooth m cells differ from skeletal m.
Cells are smaller,(1000x shorter) tapered
2) No t-tubules, have caveoli: tiny invaginations
3) Actin & myosin are diagonal so don’t see striations

42. 4) No z lines
5) Generate more tension slower to contract/relax
6) > 1 neuron can innervate a cell &
> 1 neurotransmitter is possible. No true NMJ- varicosities

43. 7) Ca+2 binds to calmodulin (not troponin)
This Ca-calmodulin complex activates an enzyme (kinase) which puts a PO4 on the mysosin head & now it can interact with actin

44. 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
Whole sheets of smooth muscle exhibit slow, synchronized contraction
They contract in unison, reflecting their electrical coupling with gap junctions
Action potentials are transmitted from cell to cell

45. Characteristics of Smooth Muscle
Unique characteristics of smooth muscle include:
Smooth muscle tone
Slow, prolonged contractile activity
Low energy requirements
Response to stretch
Smooth muscle exhibits stress-relaxation response (compliance) in which:
Smooth muscle responds to stretch briefly, and adapts to its new length
The new length, however, retains its ability to contract
This enables organs such as the stomach and bladder to temporarily store contents
Smooth muscle has good regenerative ability
This is shown by estrogen’s effect on the uterus
At puberty, estrogen stimulates the synthesis of more smooth muscle, causing the uterus to grow to adult size
During pregnancy, estrogen stimulates uterine growth to accommodate the increasing size of the growing fetus
Autonomic nervous system and endocrine systems are major controls
Neurotransmitters and hormones?

46. Developmental Aspects
Nearly all muscle tissue develops from specialized mesodermal cells called myoblasts.
Skeletal muscle fibers form through the fusion of several myoblasts, and are actively contracting by week 7 of fetal development.
Myoblasts of cardiac and smooth muscle do not fuse but form gap junctions at a very early stage.
Muscular development in infants is mostly reflexive at birth, and progresses in a head-to-toe and proximal-to-distal direction.
Women have relatively less muscle mass than men due to the effects of the male sex hormone testosterone, which accounts for the difference in strength between the sexes.
Muscular dystrophy is one of the few disorders that muscles experience, and is characterized by atrophy and degeneration of muscle tissue. Enlargement of muscles is due to fat and connective tissue deposit.

47. References
GetBodySmart
Lumen