Sliding Filament Model & Recruitment

Myofibrils

• Contractile elements – sarcomeres
• Contractile proteins

• Thin – Actin, Troponin, Tropomyosin
• Thick – Myosin

Thin Filament

•Troponin – binds to Ca²⁺

•Tropomyosin – follows troponin

•Actin – wants to bind with myosin

Regulatory Proteins
“Guards”

Thick Filament

• Myosin – wants to bind to

actin

• Actin-binding sites
• ATP-binding sites

Contraction

• Ca²⁺ binds to troponin
  •Changes shape
  •Changes tropomyosin as well

• Actin & Myosin will be free to bind
  •Form cross bridge formation
  •Regulated by ATP & Ca²⁺ availability

“Guards” are no longer “on duty”

Recruitment

Motor Unit

• Motor neuron + muscle fibers

• Small: control fine movements
• Large: control large muscles

Muscle
Contractions

• Vary in strength
• Frequency of Stimulation: Temporal

Summation

• Strength of Stimulation: Recruitment

• Controls force of contraction more

precisely

• Anatomy: Motor Unit (neuron +

muscle fiber)

• Physiology: Force of Contraction

• ↑ # Motor units, ↑

Tension

Threshold

• Below: no muscle response
• At threshold: recruit motor

units

• Slowly excite more & more

motor units

• Maximal stimulus reached

• Above: no further increase

• Saturation