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Anatomy & Physiology > Chapter 9 Muscle and Muscle Tissue > Flashcards

Chapter 9 Muscle and Muscle Tissue Flashcards

1
Q

Skeletal Muscle: Span Joints and Attachment

A

Span joints and attach to bone (directly or indirectly)

* direct attachment – epimysium of the muscle is fused to the periosteum of a bone (or perichondrium of cartilage)
* indirect attachment = tendon (rope-like) or aponeuorsis (sheet-like)
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2
Q

Skeletal Muscle: Origen

A

Origin = attachment site of muscle on bone that is *less moveable

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3
Q

Skeletal Muscle: Insertion

A

Attachment site of muscle on bone that when muscle contracts *moves bone toward the muscle’s origin

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4
Q

Skeletal Muscle: Organ

A

Each skeletal muscle is a discrete organ – skeletal muscle fibers (cells), blood vessels, nerves and connective tissue

* generally one nerve, one artery and one or two veins enter (or exit) near the center of each muscle 
* *every muscle cell is supplied with a nerve ending
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5
Q

Skeletal Muscle: Connective Tissue Sheaths (revestimento)

A

Several different connective tissue sheaths associated with each muscle
Superficial to deep:
**epimysium –dense irregular connective tissue that surrounds whole muscle
**perimysium – dense irregular connective tissue that wraps fascicles = groups of muscle cells
**endomysium – areolar connective tissue that surround each muscle cell

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6
Q

Skeletal Muscle- Connective Tissue Sheaths: Epimysium

A

Dense irregular connective tissue that surrounds whole muscle

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7
Q

Skeletal Muscle- Connective Tissue Sheaths: Perimysium

A

Dense irregular connective tissue that wraps fascicles = groups of muscle cells

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8
Q

Skeletal Muscle- Connective Tissue Sheaths: Endomysium

A

Areolar connective tissue that surround each muscle cell

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9
Q

Skeletal Myofiber: Composition

A

Skeletal myofiber = skeletal muscle cell

* long cylindrical with multiple nuclei just beneath the sarcolemma (plasma membrane)
* sarcoplasm (cytoplasm) 
* myofibrils 
* sarcomere – about 2 um long = functional unit of skeletal muscle
* sarcoplasmic reticulum (SR)  = elaborate smooth endoplasmic reticulum
* T-tubules = deep invaginations of the sarcolemma
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10
Q

Skeletal Myofiber: Sarcoplasm

A

Sarcoplasm (cytoplasm) – contains large amounts of:

* glycosomes = granules of glycogen (complex carbohydrate – repeating units of glucose)
* myoglobin = pigment that stores oxygen (similar to haemoglobin)
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11
Q

Skeletal Myofiber: Myofibrils

A

Myofibrils (100-1000) = tightly packed protein bundles that run the entire length of the cell

* mitochondria and other organelles squeezed between them
* made up of **repeating units of sarcomeres
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12
Q

Skeletal Myofiber:

Sarcomere

A

Sarcomere – about 2 um long = functional unit of skeletal muscle
= region of myofibril between two successive Z discs
-contains myofilaments = contractile fibers and their arrangement produces the striations

THIN FILAMENTS = filamentous (F) actin

* anchored to the Z discs (alpha actin)
* produce the light coloured (I) band 
* subunit for actin (actin G) has binding site for myosin

REGULATORY PROTEINS – intertwined along the actin filament

* tropomyosin – rod-shaped protein that spiral around actin core
* troponin – globular three polypeptide complex
- TnI - inhibitory unit that binds to actin
- TnT – binds to tropomyosin
- **TnC – binds calcium

THICK FILAMENTS = myosin

* forms the dark (A) band
* attached to an elastic protein (titin) that attaches to the Z-line
* each myosin molecule= 2 heavy chain and 4 light chain polypeptides attached to globular **myosin heads
- during contraction myosin heads bind to actin forming crossbridges
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13
Q

Skeletal Myofiber:

Sarcoplasmic Reticulum

A

Sarcoplasmic reticulum (SR) = elaborate smooth endoplasmic reticulum

* *stores calcium and has gated channels to allow its release
* surrounds each myofibril like a sleeve with enlarged terminal cisterns that flank each side of a T-tubule forming a triad
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14
Q

Skeletal Myofiber:

T-tubules

A

T-tubules = deep invaginations of the sarcolemma

* increases the surface area of the myofiber and T-tubular contains extracellular fluid
* conduct nerve impulses to the deepest regions of the muscle cell 
* contain voltage sensor that trigger the opening of gated calcium channels on the SR
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15
Q

Skeletal Myofiber:

Sarcomere

A

THIN FILAMENTS = filamentous (F) actin

* anchored to the Z discs (alpha actin)
* produce the light coloured (I) band 
* subunit for actin (actin G) has binding site for myosin
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16
Q

Skeletal Myofiber:

Sarcomere

A

REGULATORY PROTEINS – intertwined along the actin filament

* tropomyosin – rod-shaped protein that spiral around actin core
* troponin – globular three polypeptide complex
- TnI - inhibitory unit that binds to actin
- TnT – binds to tropomyosin
- **TnC – binds calcium
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17
Q

Skeletal Myofiber:

Sarcomere

A

THICK FILAMENTS = myosin

* forms the dark (A) band
* attached to an elastic protein (titin) that attaches to the Z-line
* each myosin molecule= 2 heavy chain and 4 light chain polypeptides attached to globular **myosin heads
- during contraction myosin heads bind to actin forming crossbridges
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18
Q

Sliding Filament Model of Contraction

A

*in relaxed muscle fibers – actin and myosin overlap only and the ends
*contraction = formation of crossbridges between myosin head and actin filament
-shortening occurs when crossbridges generate enough tension to exceed the forces opposing shortening
-ratchet-like formation, release and reattachment of crossbridges cause thin filament slide past the thick ones
actin and myosin filaments overlap to a greater degree

-Z lines of sarcomere get closer together and overall – myofibril shortens
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19
Q

Excitation/Contraction Coupling

A
  • sequence of electrical and mechanical events that lead to contraction
    • electrical events requires nerve stimulus and interaction with the skeletal muscle cell at the neuromuscular junction
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20
Q

Excitation/Contraction Coupling:

Somatic Motor Neurons

A

SOMATIC MOTOR NEURONS (part of somatic -voluntary- nervous system)
*cell bodies reside in brain or spinal cord and axons extend to muscle cells
*branched endings (terminals) at the end of each axon
*neuromuscular junction (or motor end plate) = specific connection between:
#one **axon terminal
contains synaptic vesicles filled with the neurotransmitter, acetylcholine (ACh)

	#and **sarcolemma of one myofiber (skeletal muscle cell)
	contains millions of **ACh receptors
	#separated by the synaptic cleft – very small physical space (50-80 nm) filled with extracellular fluid
	contains **acetylcholinesterase = enzyme that breaks down ACh
#**myasthenia gravis = autoimmune disease that destroys ACh receptors
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21
Q

Excitation/Contraction Coupling

  • Somatic Motor Neurons diseases:
    Myasthenia gravis
A 
#**myasthenia gravis = autoimmune disease that destroys ACh receptors
	*symptoms include: drooping upper eyelids, difficulty swallowing and talking, and generalized muscle weakness
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22
Q

Excitation/Contraction Coupling:

Generation of action potential across the sarcolemma

A

Generation of action potential across the sarcolemma

* myofiber has negative resting membrane potential (very polarized -90 mV)
* action potential (AP) = all-or-none series of predictable and repeatable electrical changes lasting only a few miliseconds

#generation of an end plate potential

#depolarization – generation and propagation of action potential

#repolarization – restoring sarcolemma to initial polarized state

#refractory period = time during which cell cannot be stimulated again

*propagation of AP along sarcolemma and down T-tubule
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23
Q

Generation of an end plate potential

Excitation/Contraction Coupling: Generation of action potential across the sarcolemma

A

Generation of an end plate potential

* binding of ACh to its receptor open Na+ channel (ligand-gated channel)
* Na+ enters and produces local depolarization = end plate potential (interior less negative than before
24
Q

Depolarization

Excitation/Contraction Coupling: Generation of action potential across the sarcolemma

A

Depolarization – generation and propagation of action potential
*spread of end plate potential to neighbouring areas triggers opening of voltage-gated Na+ channels
*THRESHOLD = voltage at which AP generated (positive feedback mechanism)
AP initiated in one spot – propagates – moves along length of sarcolemma
depolarization wave triggers AP at new location

25
Q

Repolarization

Excitation/Contraction Coupling: Generation of action potential across the sarcolemma

A

Repolarization – restoring sarcolemma to initial polarized state

* voltage-gated Na+ channels close and  voltage-gated K+ channels open
* Na+ no longer enters, and K+ leaves returning cell to negative charged conditions
* Na+ - K+ ATPase – restores appropriate ionic conditions
26
Q

Propagation of AP along sarcolemma and down T-tubule

Excitation/Contraction Coupling: Generation of action potential across the sarcolemma

A

Propagation of AP along sarcolemma and down T-tubule

* changes the shape of the **voltage-sensitive (DHP receptors) proteins in T-tubule
* interaction of T-tubule with SR at triads triggers the **opening of calcium channels on SR
27
Q

Excitation/Contraction Coupling:

Increased intracellular calcium

A

Increased intracellular calcium

* release of Ca++ from SR and entry of into the sarcoplasm =  intracellular event that couples the AP with muscle contraction
* *calcium binds to troponin – and inhibitory proteins on actin moved out of the way
- shape change allows troponin to roll off tropomyosin the binding sites for the myosin heads on the actin filament
* *myosin head binds to actin and crossbridge cycling begins
- requires maintained elevated intracellular Ca++ levels and ATP
28
Q

Excitation/Contraction Coupling:

Calcium removal terminates contraction

A

Calcium removal terminates contraction

*calcium pumped back into SR and inhibitory proteins block myosin binding site on actin

29
Q

Rigor Mortis

Excitation/Contraction Coupling:
Calcium removal terminates contraction

A
  • *rigor mortis – muscle stiffness following death
    • cross bridge detachment is ATP driven
    • as cells begin to die, cannot pump out calcium so crossbridges form
    • ATP synthesis ceases so crossbridges cannot release
    • muscles begin to stiffen 3-4 hours after death, peak rigidity at 12 hours then dissipates over next 48-60 hours as muscle proteins breakdown
30
Q

Factors affecting muscle contraction

A
  • motor unit
    • muscle twitch
    • graded muscles responses
    • frequency of the stimulation = wave or temporal summation
    • strength of the stimulus = motor unit recruitment
    • muscle tension = force exerted on an object by contracting muscle
    • muscle tone
    • load = opposing force exerted on muscle by the weight of the object to be moved
    • force of muscle contraction
    • isometric contraction
    • isotonic contraction
    • energy supply
      - direct phosphorylation
      • anaerobic glycolysis
        - aerobic respiration
    • exercise
31
Q 
Factors affecting muscle contraction
# Motor unit
A

Motor unit = one motor neuron and all the muscle fibers it innervates (supplies)

* number of myofibers per motor unit may be as few as four – for muscles that exert fine control (muscles of fingers or eyes) or up to several hundred – for weight-bearing (postural) muscles
* muscle fibers in a single motor unit are not clustered but are spread throughout muscle 
- stimulation of a single motor unit causes a weak contraction of the entire muscle
32
Q 
Factors affecting muscle contraction
#muscle twitch
A

Motor unit’s response to a single action potential

33
Q 
Factors affecting muscle contraction
#graded muscles responses
A

Not isolated twitches but smooth muscle contractions that vary in strength as different demands are placed on them and are altered by changing the:

FREQUENCY OF THE STIMULATION = wave or temporal summation
*single AP produces a twitch
*if multiple AP’s arrive one after the other at the neuromuscular junction, sequential twitch contractions can begin before the tension from the previous contraction has subsided
-Due to release of new calcium from SR before the calcium level from the previous release has returned to normal – i.e. the level of intracellular calcium increases
-Contractions add together and as frequency of delivery closer and closer together – get quivering contraction = unfused or incomplete tetanus
-(Complete or fused) tetanus = sustained fused contraction – reaches maximum tension with no sign of relaxation
Prolonged tetanus leads to muscle fatigue = physiological inability to contract even though muscle still receiving stimuli - tension drops to zero
Attributed to ionic imbalances and calcium regulation and release

STRENGTH OF THE STIMULUS = motor unit recruitment

* subthreshold stimuli - no observable contractions
* threshold stimulus - observable contractions appear
* maximal stimulus – strongest stimulus that increases contractile force
* size principle – motor units with the smallest muscle fibers activated first as controlled by most excitable motor neurons; largest motor units with large course muscle fibers controlled by largest and least excitable (highest threshold) neurons
34
Q

Factors affecting muscle contraction

#graded muscles responses:
FREQUENCY OF THE STIMULATION
A

FREQUENCY OF THE STIMULATION = wave or temporal summation
*single AP produces a twitch
*if multiple AP’s arrive one after the other at the neuromuscular junction, sequential twitch contractions can begin before the tension from the previous contraction has subsided
-Due to release of new calcium from SR before the calcium level from the previous release has returned to normal – i.e. the level of intracellular calcium increases
-Contractions add together and as frequency of delivery closer and closer together – get quivering contraction = unfused or incomplete tetanus
-(Complete or fused) tetanus = sustained fused contraction – reaches maximum tension with no sign of relaxation
Prolonged tetanus leads to muscle fatigue = physiological inability to contract even though muscle still receiving stimuli - tension drops to zero
Attributed to ionic imbalances and calcium regulation and release

35
Q 
Factors affecting muscle contraction
#graded muscles responses: STRENGTH OF THE STIMULUS
A

STRENGTH OF THE STIMULUS = motor unit recruitment

* subthreshold stimuli - no observable contractions
* threshold stimulus - observable contractions appear
* maximal stimulus – strongest stimulus that increases contractile force
* size principle – motor units with the smallest muscle fibers activated first as controlled by most excitable motor neurons; largest motor units with large course muscle fibers controlled by largest and least excitable (highest threshold) neurons
36
Q 
Factors affecting muscle contraction
#Muscle tension
A

Muscle tension = force exerted on an object by contracting muscle

* muscle tone = presence of slight contraction in muscle even when relaxed
- helps stabilize joints and maintain posture
37
Q 
Factors affecting muscle contraction
#Load
A

Load = opposing force exerted on muscle by the weight of the object to be moved

38
Q 
Factors affecting muscle contraction
#Force of muscle contraction
A

Force of muscle contraction influenced by:

* number of motor units recruited
* muscle fiber size
* frequency of stimulation
* degree of stretch on muscle = length-tension relationship
39
Q 
Factors affecting muscle contraction
#Isometric contraction
A

Isometric contraction = contraction in which muscle tension develops but load is not moved – i.e. muscle does not shorten
*increasing muscle tension = measure for isometric contractions

40
Q 
Factors affecting muscle contraction
#Isotonic contraction
A

Isotonic contraction = contraction where muscle tension overcomes load and muscle shortens

* concentric contraction – muscle shortens and does work
* eccentric contraction – muscle generates force as it lengthens
- important in deceleration activities or counteracting gravity effects (climbing steps)
* amount of muscle shortening = measure of isotonic contractions
41
Q 
Factors affecting muscle contraction
#Energy supply
A

Energy supply

- source and availability of ATP
* *direct phosphorylation – transfer of phosphate from creatine phosphate(CP) to ADP using creatine kinase

* *anaerobic glycolysis – formation of ATP via catabolism of glucose in the cytoplasm of the cell and in the absence of oxygen – forms lactic acid as waste product
- poor efficiency but ATP produce 2.5 x faster than aerobic respiration

* *aerobic respiration – catabolism of glucose, glycogen, fatty acids (and amino acids) to ATP in (cytoplasm and) mitochondria in the presence of oxygen
- 95% of ATP used comes from aerobic respiration
42
Q 
Factors affecting muscle contraction
#Energy supply: direct phosphorylation
A

**direct phosphorylation – transfer of phosphate from creatine phosphate(CP) to ADP using creatine kinase

43
Q 
Factors affecting muscle contraction
#Energy supply: anaerobic glycolysis
A
  • *anaerobic glycolysis – formation of ATP via catabolism of glucose in the cytoplasm of the cell and in the absence of oxygen – forms lactic acid as waste product
    • poor efficiency but ATP produce 2.5 x faster than aerobic respiration
44
Q 
Factors affecting muscle contraction
#Energy supply: aerobic respiration
A
  • *aerobic respiration – catabolism of glucose, glycogen, fatty acids (and amino acids) to ATP in (cytoplasm and) mitochondria in the presence of oxygen
    • 95% of ATP used comes from aerobic respiration
45
Q 
Factors affecting muscle contraction
#Exercise
A

Exercise
*activities that require a surge of power but last only a few seconds rely entirely on ATP and CP stores

* aerobic endurance = length of time a muscle can continue to contract using aerobic pathways
    * anaerobic threshold = point at which muscle metabolism converts to anaerobic glycolysis
46
Q 
Factors affecting muscle contraction
#Exercise: aerobic endurance
A

Aerobic endurance = length of time a muscle can continue to contract using aerobic pathways
*prolonged activities where endurance rather than power is the goal – jogging, marathon runs, swimming, biking
*aerobic activity over time increases:
• number of capillaries surrounding muscle fibers
• number of mitochondria
• cellular content of myoglobin

47
Q 
Factors affecting muscle contraction
#Exercise: anaerobic threshold
A

Anaerobic threshold = point at which muscle metabolism converts to anaerobic glycolysis

* longer bursts of activity – tennis, soccer, 100-meter swim mostly anaerobic glycolysis
* resistance training (e.g. weight lifting) – increases muscle bulk by increasing
- number of contractile proteins (myofilaments and myofibrils) within myofibers
- some increases in number of mitochondria
- storage of glycogen
- connective tissue between cells
48
Q 
Factors affecting muscle contraction
#Exercise: disuse atrophy
A

Disuse atrophy = degeneration and loss of muscle mass resulting from immobilization of muscle

* e.g. cast, enforced bed rest, loss of neural stimulation 
* paralyzed muscle may atrophy to ¼ of initial size and lost muscle tissue replaced by fibrous connective tissue
49
Q 
Smooth muscle
#Characteristic
A
  • small, spindle-shaped cells with centrally located nucleus
    • has small amount of endomysium but no perimysium or epimysium
    • muscle present in walls of hollow organs (except heart)
    • organized in sheets usually:
      • longitudinal layer
      • circular layer
    • chemical and mechanical events of contractions are the same for cardiac and skeletal muscle and similar for smooth muscle
50
Q 
Smooth muscle
#Longitudinal layer 
#circular layer
A

Organized in sheets usually:

* longitudinal layer – runs parallel to long axis of organ and contraction shortens length of organ
* circular layer – runs around circumference and contraction constricts (narrow) lumen
51
Q 
Smooth muscle
#chemical and mechanical events of contractions
A

• chemical and mechanical events of contractions are the same for cardiac and skeletal muscle and similar for smooth muscle

* actin and myosin interact by sliding filament model
* final trigger for contraction = rise in intercellular calcium
* ATP energizes sliding process
52
Q 
Smooth muscle
#differences with smooth muscle
A

Differences with smooth muscle

* lacks highly structured neuromuscular junctions
* nerve fibers of autonomic (involuntary) motor system have **varicosities = numerous terminal bulbous swellings
* has no T-tubules but sarcolemma has multiple **caveolae = pouchlike infoldings containing large number of calcium channels
* *calcium enters from extracellular space (only small amount from SR)
53
Q

Smooth muscle
#differences with smooth muscle:
No striations and no sarcomeres

A

No striations and no sarcomeres:

* Contain thick and thin filaments but myosin filaments are shorter and organization is different
- thick and thin filaments arranged diagonally 
- have intermediate filaments (non-contractile) and dense bodies = points of attachment for thin filaments and tethered to sarcolemma

* Have fewer thick filaments but myosin heads along entire length
* *no troponin complex in thin filaments
* *calmodulin = protein in cytoplasm that binds calcium 
- interacts with **myosin light chain kinase
- **phosphorylates myosin –activating it
54
Q 
Smooth muscle
#differences with smooth muscle
A
  • *gap junctions = electrical coupling between cells that allow slow synchronized contractions
    • takes 30 times longer to contract and relax than skeletal muscle
    • can maintain same contractile tension for prolonged periods of time with less than 1% the energy cost of skeletal muscle
55
Q

Smooth muscle
#differences with smooth muscle:
Can be regulated by nerves, hormones or local chemicals, and stretch

A

Can be regulated by nerves, hormones or local chemicals, and stretch

* nerves are from the autonomic nervous system and a variety of neurotransmitters are associated with the system
- some are considered excitatory, others inhibitory and some can be both depending on what receptors are present for that neurotransmitter
* hormone signalling is often associated with G-protein mediated intracellular events
* direct response of smooth muscle to certain hormones or chemicals is probably responsible for smooth muscle tone 
* stretch provokes contraction which automatically moves substances along a tract
- increased tension persists briefly and then muscle adapts to new length and relaxes while still retaining it ability to contract (stretch-relaxation response)
- stretches much more and generates more tension than skeletal muscle stretched to a comparable extent

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