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Human Structure > Histology: Muscle > Flashcards

Histology: Muscle Flashcards

1
Q

Define muscle tissue.

A
  • Aggregates of specialized, elongated cells, arranged in parallel, that have primary role of contraction
    • Voluntary locomotion (Skeletal Muscle)
    • Heart Function (Cardiac Muscle)
    • Involuntary Motility & Vasodilation/Vasocontriction (Smooth Muscle)
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2
Q

Skeletal Muscle Cells

A
  • Muscle fiber = muscle cell
  • Sarcolemma = plasma membrane
    • T- tubules: (A-I Junction)
  • Sarcoplasm = cytoplasm
  • Sarcoplasmic Reticulum = smooth ER
    • Reservoir for calcium ions
    • Terminal cisterna = expanded regions of SR adjacent to T tubules
    • Triad = association of SR flanking t tubule
  • Sarcomere = functional unit of muscle cell
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3
Q

Skeletal Muscle Organization

A
  • Myofilaments = actin & myosin organized into sarcomere
  • Myofibrils = many sarcomeres end to end
  • Muscle Fiber = collections of myofibrils
  • Fascicles = bundles of muscle fibers
  • Muscle = collections of fascicles enclosed within CT and attached to bone via tendon
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4
Q

Connective Tissue of Muscle

A
  • For transduction
  • Contains blood vessels, nerve, lymphatics
  • Epimysium:
    • Dense, irregular CT
    • Surrounds entire muscle
  • Perimysium:
    • Dense, irregular CT
    • Surrounds fascicles
  • Endomysium:
    • Loose (reticular fibers)
    • Surrounds individual muscle fibers
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5
Q

Skeletal Muscle LM

A
  • Long, multinucleated cells
    • Elongated nuclei located peripherally along fibers
  • Striations evident in longitudinal section
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6
Q

Devleopment of Skeletal Muscle

A
  • Mesenchymal myoblats fuse to form myotubes with many nuclei
  • Myotubes differentiate to muscle fibers
  • Hypertrophy in response to increased load

Satellite Cells:

  • Reserve progenitor cells
  • Proliferates to form new muscle fibers following injury
    • Limited regeneration capacity
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7
Q

Sarcomere

A

Repetitive functional unit of contractile apparatus

  • Myosin Filaments = thick
    • Occupy A Band
  • Actin Filaments = thin
    • Occupy I Band
    • Overlap myosin filaments
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8
Q

Sarcomere Bands

A

A Band

  • Anisotropic (dark) in polarized light
  • Myosin filaments
  • Overlap

I Band

  • Isotropic (light) in polarized light
  • Actin filaments

H Band

  • Central region of A band
  • Only myosin filaments
  • Gets shorter when actin overlap myosin

M Line

  • Center of A Band
  • Thick filaments linked

Z Line

  • Where thin filaments are linked
  • Mark ends of sarcomere
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9
Q

Myosin

A
  • Thick Filaments
  • Consists of a tail (anchored at M line) and a head (pointed towards outside of sarcomere)
    • Head contains binding sites for actin and ATP
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10
Q

Actin

A
  • Double-stranded helix of actin monomers
  • Regulatory proteins intertwined with actin strand:
    • Troponin:
      • ​Ca2+ binding protein to initiate muscle contraction
    • Tropomyosin
      • ​Sits in groove of actin filaments to block myosin binding sites
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11
Q

Neuromuscular Junction

A
  • Motor neuron contacts a muscle fibers
  • Synaptic vesicles contain Acetylcholine (ACh)
  • ACh receptors on junctional folds (increases surface area)
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12
Q

Mechanism of Contraction

A
  1. Action Potential reaches fiber and ACh is released into synaptic cleft
  2. ACh binds receptors on sarcolemma resulting in localized depolarization and activation of voltage-gated Na+ channels
    • Results in widespread depolarization
  3. Depolarization of T tubules results in Ca2+ release from terminal cisternae of SR
  4. Ca2+ binds troponin
    • Troponin changes shape, moving tropomyosin
  5. Myosin head can bind actin to initiate cross-bridge cycle
  6. Myosin heads pivot to move thin filaments
  7. ATP binds myosin head resulting in myosin head release
    • Absence of ATP, results in sustained muscle contraction (e.g. rigor mortis after death)
    • ATP hydrolyzed to ADP + Pi and myosin head resets
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13
Q

Mechanism of Relaxation

A
  1. Ca2+ transported back into sarcoplasmic reticulum
  2. Tropomyosin moves back to re-cover myosin binding sites
  3. Filaments passively slide back to relaxed state
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14
Q

Sarcomere Shortening

A
  • A Band stays the same length during shortening
  • H Zone gets smaller (completely disappears)
  • I Band gets smaller (from actin/myosin overlap)
  • Z discs moves closer together
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15
Q

Skeletal Muscle Fiber Characterization

A

Characterized by:

  • Speed of contraction
  • Enzymatic Velocity (how quickly ATP broken down)
  • Metabolic Profile (how cell makes ATP)

3 Types of Muscle Fibers:

  1. Type I: Slow Oxidative Fibers
    • ​Small red fibers
  2. Type IIa: Fast Oxidative-Glycolytic Fibers
  3. Type IIb: Fast Glycolytic Fibers
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16
Q

Type I

A
  • Slow twitch; fatigue resistant
    • Slowest enzymatic velocity (Myosin ATPase reaction is slowest)
  • Small, appear dark red (lots of myoglobin, an oxygen bonding protein)
  • Many mitochondria (ATP produced via oxidative phosphorylation)
  • Seen in postural muscles of back
  • High proportion in endurance athletes
17
Q

Type IIa

A
  • Fast twitch, fatigue resistant
  • Intermediate sized
  • High amount of myoglobin and many mitochondria (Oxidative phosphorylation is main source of ATP)
  • Contain large amounts of glycogen (capable of anaerobic glycolysis)
  • Main fiber in leg muscles
  • Higher proportion in mid-distance sprinters
18
Q

Type IIb

A
  • Fast twitch, fatique prone
  • Large fibers, light pink/white
  • Few mitochondria, little myoglobin
  • Highest amount of glycogen (ATP produced via anaerobic glycolysis)
  • Fastest myosin ATPase velocity
  • Rapid contraction and precise movement (eye mm. and digits)
  • Higher proportion in short distance sprinters and weight lifters
19
Q

Motor Unit

A
  • A single axon and all of the muscle fibers in contact with its branches
  • Contract in unison

Fine Movements:

  • Low-innervation ratio (e.g. eye mm. 1 nerve:3 fibers)

Larger Muscle Groups:

  • High-innervation ratio (postural back muscles 1 nerve: 100s fibers)
20
Q

Proprioceptors

A
  • Provide information about stretch, tension, body position
  • Where our limbs are in space

Muscle Spindles:

  • In center of muscle belly
  • Encapsulated, intrafusal muscle fibers
  • Detect changes in muscle length via afferent neuron

Golgi Tendon Organ

  • Detects changes in tension within tendons
  • Enclose sensory axons at myotendinous junction
21
Q

Cardiac Muscle

A
  • Heart, Superior and Inferior Vena Cavas, Pulmonary Veins
  • Involuntary (contraction is intrinsic and spontaneous)
  • Striated fibers
  • Short, branched muscle cells
    • Connected via intercalated discs
  • Cells contain one nucleus, centrally located
  • T tubules larger than sekeltal muscles (located at Z line)
    • SR less well organized (diads)
  • Many mitochondria
22
Q

Cardiac Muscle Growth and Repair

A
  • Can enlarge by hypertrophy
  • Poor capacity for regeneration
    • Dead cardiac muscle cells are replaced with connective tissue
23
Q

Intercalated Discs

A
  • Interfaces between adjacent cells
    • Communication via gap junctions
  • Irregular stair-step pattern

3 Different Junctions:

  1. Fascia Adherens (adherens juctions)
    • E-cadherin proteins anchor thin filaments of terminal sarcomere to sarcolemma
    • Transverse Areas
  2. Desmosomes (macula adherens)
    • Transverse Areas
  3. Gap Junctions
    • Ionic continuity between cells to coordinate contraction
    • Longitudinal Areas
24
Q

Smooth Muscle

A
  • Non-striated
  • Slow, steady contraction (blood vessels, GI, respiratory, urinary, reproductive)
  • Involuntary (controlled by ANS & hormones)
  • Elongated, spindle-shaped fibers
  • Single, centrally located nucleus

Image:

  • Longitudinal SM on left; cross-section right
25
Q

Smooth Muscle Cells

A
  • Adjacent cells attached via gap junctions and dense bodies:
    • Dense bodies bind to intermediate filaments
    • Also anchor actin (functionally similar to Z discs)
  • Rudimentary SR, no T tubules (no diads, triads)
  • Greatest capacity for growth and repair
    • Cells regenerate via mitosis
    • Hypertrophy (size) and Hyperplasia (number)
26
Q

Smooth Muscle Contraction

A
  • Actin and myosin crisscross sarcoplasm obliquely
    • Actin filaments insert into dense bodies
  • No troponin (instead, calmodulin)
    • Calmodulin phosphorylates myosin light-chain kinase, allowing myosin heads to bind actin
  • Contraction shorts cell
    • Nucleus becomes corkscrew shaped (image)

Human Structure (30 decks)

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