Lecture 15 - Basic Structure of Skeletal Muscle Flashcards
What is the basics of muscle structure?
a. Muscles are made up of bundles (fascicles) of fibres
b. Each fibre is made up of myofibrils
Why do myofibrils appear striated?
Because of the alternating dark and light bands in register
What are myofibrils
a. Occupy 80% of the fibre volume
b. 100s and 1000s of myofibrils each in each fibre
c. Approx. 1-2 um in diameter
d. Maintained in transverse register across the cell giving rise to the striation pattern
Sarcomeres
contractile component
connective tissue/tendon
series-elastic component
Basic steps of muscle contraction
a. NMJ AP generation travels down t-tubule release of Ca2+ Ca2+ binding to troponin
b. Influx of Ca2+, triggering the exposure of binding sites on actin
c. The binding of myosin to actin
d. The power stroke of the crossbridge, which results in the crossbridge disconnecting from actin
e. The bind of ATP to the cross budge Which results in the crossbridge disconnecting from actin
f. The hydrolysis of ATP, which leads to the re-energizing and repositing of the crossbridge
g. Cessation of AP reuptake of Ca2+ into SR relaxation
Basic steps of excitation contraction coupling
a. Somatic motor neuron releases Ach at neuro-muscular junction
b. Net entry of Na+ through Ach receptor-channel initiates a muscle action potential
c. Action potential in t-tubule alters conformation of DHP receptor
d. DHP receptor open Ca2+ release channels in sarcoplasmic reticulum and Ca2+ enters cytoplasm’s
e. Ca2+ binds to troponin, allowing strong actin-myosin binding
f. Myosin heads execute power stroke
g. Actin filament slides toward centre of sarcomere
the relaxed state
a. Myosin head is unable to bind to actin molecule
b. Tropomyosin blocks the myosin binding site on the actin molecule
Length-tension relationship – active force
a. Three major points
b. Ascending limb - (sarcomere length short; overlap not optimal between actin and myosin; max. active force compromised)
c. Plateau Region - (sarcomere length is optimal; overlap optimal between action and myosin; ideal of max. force)
d. Descending limb - (sarcomere length long/stretched; little or no overlap between actin and myosin; max. force compromised.)
Length-tension relationship – passive force
a. Passive – force – just stretching the muscle (and not stimulating it) will pull on force transducer
b. Therefore, the more the muscle is stretched, the higher the ‘passive force’ will be
c. But, the more the muscle is stretched, the less overlap there will be between myosin and actin and so ‘active force’ will decrease accordingly
Force-velocity relationship
a. The greater the lead, the less the muscle shortens and the shorter the duration of contraction
b. The greater the load, the slower the contraction
Muscle power
a. Power = load x velocity
b. Power will be zero when there is no load on the muscle and when the load is so heavy it cannot be moved at all
Muscles for purpose – why is there different architectures?
a. The arrangement of fibres relative to the axis of force generation
b. Fibre length is never (or rarely) the same as muscle length
c. Most fibres insert obliquely into the tendon
i. Resembles a feather arrangement
ii. Called ‘pinnation’ or ‘pennation’
d. More fibres can be packed in
i. Increasing the effective cross-sectional area (CSA) of the muscle
What are the types of muscle actions?
a. Shortening (or miometric, concentric)
b. Lengthening (or plyometric or eccentric)
c. Isometric (or fixed, same length)
shortening muscle action
If the force developed but the muscle is greater than the load on the muscle
Lengthening muscle action
When the load on the muscle is greater than the force developed by the muscle the muscle is stretched producing a lengthening action
Isometric muscle action
When the force developed by the muscle and the load are equivalent, or the load is immoveable, a fixed-end or isometric action occurs
Fibre to nerve ratio
a. Extraocular muscles i.e. eye muscles = low F:N because fine and delicate movement
b. Quadriceps = high F:N because they are required for heavy work
motor unit
a motor neuron and all the muscle fibres that it innervates
isometric twitch
response to a single electrical stimulus
optimum muscle length (L0)
length at which maximum twitch is recorded
summation
staircase effect whereby twitch responses “add” or “sum” together in response to repeated electrical stimulation (relates to the recruitment of motor units or the frequency-force relationship)
tetanus
the muscle response following stimulation of a frequency sufficient to cause fusion
Frequency-force relationship:
plot of stimulation frequency vs isometric force response
Maximum isometric force (P0 ):
maximum tetanic force response taken from the plateau of the frequency-force relationship
Henneman’s size principle
a. Rm is high and conduction velocity will be low
b. Rm is low and conduction velocity will be high
c. A given level of excitatory input will produce more depolarization of the smallest axons because of the smaller membrane surface area
Smooth muscle features
a. Has narrow, tapered rod-shaped cells
b. Has nonstriated, uninucleate fibres
c. Occurs in walls of internal organs and blood vessels
d. In involuntary
Cardiac muscle features
a. Has striated, tubular, branched, uninucleate fibres
b. Occurs in walls of heart
c. Is involuntary
Skeletal muscle features
a. Has striated, tubular, multinucleated fibres
b. Is usually attached to skeleton
c. Is voluntary
- Smooth muscle “latching”
a. Low level of Ca2+ inside the cell (tonic contraction response) can lead to prolonged and forceful contraction of the smooth muscle
b. Energy efficient
