Muscle
• functions
• voluntary control and reflex action
– move body parts
– coordination of antagonistic muscles
• balance at rest
– stabilize joints during locomotion
• composition
– 75 % water
– 22 % protein
– 7 % lipids & inorganic salts
• most species muscle 40-45 % of BW
• Thoroughbred - 55 % BW : other horses 45 %
Structure of Muscle
• epimysium - outer connective tissue covering, surrounds
the entire muscle
• fasciculi - bundles of fibers
• perimysium - sheath of connective tissue surrounding
fasciculi
• muscle fiber - inside fasciculi
• endomysium - connective tissue surrounding muscle fiber
Muscle Fiber
• sarcolemma - plasma membrane surrounding muscle fiber
– fuses with tendon, which inserts into bone
• sarcoplasm - gelatin-like substance between myofibrils,
(muscle fibers cytoplasm)
– contains dissolved proteins, minerals, fats, and
organelles
– contains large quantity of stored glycogen
– contains myoglobin
– transverse tubules
• T tubules- extensions of sarcolemma, laterally through
muscle fiber
– transmit nerve impulse from sarcolemma to each
myofibril
– pathway for glucose, oxygen and ions from
extracellular fluids
• sarcoplasmic reticulum (SR) -longitudinal network found
within muscle fiber
– storage site for calcium, for muscle contraction
• myofibril - contractile element of skeletal muscle
– 100’s - 1000’s in one fiber
– sarcomere - subunit of myofibril (striations)
– functional unit of myofibril
– actin filaments
– myosin filaments
• actin and myosin - 3000 actin : 1500 myosin / myofibril
• myosin
– thick filament
– double stranded with myosin heads
• actin
– thin filament contains active site for attachment of
myosin head
– actin
• backbone of filament
– tropomyosin
• tube shaped protein, twists around actin
– troponin
• attached to actin and tropomyosin
Fiber Action
• nerve impulse to nerve endings
– release ACh
– binds to receptors on sarcolemma
– charge transmitted length of muscle fiber
• depolarizes membrane (action potential)
– impulse travels through T tubules and SR
• release stored calcium
• calcium binds to troponin
• troponin lifts tropomyosin off actin
– freeing binding sites
–
–
–
–
myosin heads attach to actin (ATPase splits ATP)
conformational change
drag actin and myosin in opposite directions
heads release , reattach to a new active site further on
actin
• end of muscle action
– calcium is actively pumped out of sarcoplasm back to
SR
– requires energy
• internal membrane and energy pathways regulate Ca &
ATP for contraction
– cramping
• weak Ca pump
• energy exhausted to run Ca pump
Fiber Type
• contractile and metabolic properties
– contractile - speed of contraction
– metabolic - ability to use different pathways
• contraction
– all or nothing
– amount of contraction = number of fibers stimulated
• twitch - cycle of contraction and relaxation
– varies with type of fiber
• ST - slow twitch - SO -type I
• FT - fast twitch - FOG & FG - type IIA & type IIB
• no conversion of type
• can utilize either pathway, better adapted to one or another
• overall muscle types
– ST
• appear flatter
• Arabian
– FT
• appear bulky
• QH
• force = proportional to cross section area
– FT more forceful
– sacrifice efficiency for force
– contractile ability
• small animals: 3-5 fold difference
• horse: 10 fold difference
• muscle biopsy and NMRI
• fiber type variation within horse
– front legs - less FT than hind legs
– deep middle gluteal - less FT & higher ST
• structural
– superficially - less ST & more FT
• speed and force
• generally higher % of FT (FOG & FT)
– 70-90 %
• variation of breeds
– QH
• higher % FT and glycolytic enzyme activity
– Heavy hunter
• higher % ST and oxidative enzyme activity
– Thoroughbred
• relatively high oxidative enzyme activity