Day 6: Muscle

Question 1

satellite cells

Answer 1

• post natal growth + increase in nuclei occurs by recruiting satellite cells
• undifferentiated, quiescent precursors, activated
• divide and differentiate in response to demands of growth + damage
• enclosed within external lamina of muscle fiber

Question 2

sarcopenia

Answer 2

age-related involuntary loss of skeletal muscle

inversely associated with muscle strength and endurance

potentially leading to dependency in older persons

Question 3

cachexia

Answer 3

• muscle-wasting stage of cancer, AIDS, chronic forms of kidney disease + heart failure
• body begins breaking down skeletal muscle + turns it into glucose to keep the brain functioning
• cannot be turned off or fully reversed by conventional nutritional support

Question 4

sarcomere

Answer 4

smallest unit of contraction

Question 5

sarcoplasmic reticulum

Answer 5

smooth ER, sequesters calcium

Question 6

maximum range of contraction is related to

Answer 6

muscle length

Question 7

maximum strength of contraction

Answer 7

related to muscle diameter

Question 8

red vs. white muscle fibers

Answer 8

red (type I): slow-twitch, oxidative, capable of repeated contraction without fatigue, smaller muscle (predominate in postural muscles, shoulders and back)

white (type IIB): fast-twitch, glycolytic, strong and fast, larger muscle (predominate in limb muscles)

intermediate (type IIA): fast-twitch, glycolytic-oxidative; fatigue resistant

Question 9

characteristics of cardiac muscle

Answer 9

• innervated by autonomic nervous system (visceral motor)
• influences the rate of cardiac cells’ spontaneous contraction
• no equivalent of satellite cells (tissue can’t regenerate)

Question 10

ultrastructure of cardiac muscle

Answer 10

• have one or two nuclei per cell
• intercalated discs:
• fascia adherens:
  
  • analogous to zonula adherens of epithelia, anchoring sites for sarcomere nearest the end of cells
• macula adherens (desmosome) :
  
  • ​binds desmin of sequential cells to prevent separation during contraction
• gap junctions:
  
  • ​lie parallel to long axis of myocytes
  • provide direct electrotonic communications between cells, passing the stimulus for contraction from cell to cell
• T tubules:
  
  • have larger diameters than those of skeletal muscle and are each lined by external (basal) lamina
  • each associates with one terminal cistern of the sarcoplasmic reticulum (unlike 2 in cardiac muscle) (forms a dyad, which is found at the Z line (instead of A-I junction in skeletal muscle)
    
    • calcium is bound to the negatively-charged external lamina in t-tubule, and released to sarcoplasm upon membrane depolarization

Question 11

two cardiac muscle fibers

Answer 11

• myocardial endocrine cells:
  
  • majority in atria of heart
  • granules of atrial naturietic protein in cells
    
    • ANP relseased in response to stretch of atrial wall
    • acts in kidneys and reduces body fluid and lowers blood pressure
• cardiac conducting cells:
  
  • modified for conducting signals to coordinate the cardiac cycle
    
    • more gap junctions, fewer myofilaments (myofibrils), more glycogen
    • in nodes (SA, AV): smaller than most myocytes
    • in interventricular bundles and branches (Purkinje fibers): larger and bulkier than most myocytes in the myocardium

Question 12

unique functional properties of smooth muscle

Answer 12

• able to synthesize and secrete fibers and nonfibrillar ECM
• capable of mitotic division
• plasticity: behaves more like viscous mass than rigid structure
• contraction takes >10X longer (and requires less energy) than striated muscles

Question 13

fine structure of smooth muscle

Answer 13

• central (slightly euchromatic, more than fibroblast) nucleus
• very few mitochondria, cytoplasm dominated by microfilaments
  
  • actin filaments insert into dense bodies in sarcoplasm, into dense plaques in the sarcolemma
• no t-tubules, only caveolae (like pinocytic vesicles) that bind calcium
• external lamina around each cell to connect to surrounding connective tissue
• gap junctions (nexus) in unitary type smooth muscle

Question 14

2 functional arrangments of smooth muscle

Answer 14

• unitary (single-unit, visceral)
  
  • sheet or bundle acts as synctium (due to many gap junctions)
  • few motor axon contacts
    
    • examples: walls of gut, uterus, ureters
• multi-unit
  
  • individual fibers, indepedent of each other (no gap junctions)
  • precise, fine control
  • localized contractions
  • every fiber innervated individually
    
    • examples: blood vessel wall and iris of eye

Question 15

regulation of contraction of smooth muscle

Answer 15

• myogenic
  
  • via gap junctions
  • stretch
• neurogenic (ANS)
• hormones and other signaling molecules
  
  • nitric oxide
  • oxytocin
  • endothelin
  • secretions of enteroendocrine cells (serotonic, substance P, CCK)

Question 16

muscle fascicle

Answer 16

group of muscle fibers surrounded by connective tissue carrying blood vessels and nerves

Question 17

contractile system of smooth muscle

Answer 17

• actin filaments interact with myosin polymers and the bound end of the actin is attached to the dense bodies and dense plaques by alpha-actinin.
• the dense bodies and plaques are bound together by inelastic desmin (or vimentin in vascular smooth muscle)

Question 18

intercalated disks vs. terminal bar

Answer 18

• terminal bar:
  
  • zonula occludins (tight junction)
  • zonula adherens (intermediate junction)
  • macula adherens (desmosome)
• intercalated disks:
  
  • gap junctions/nexus
  • zonula adherens (intermediate junctions)
  • macula adherens (desmosome)