Muscle Excitation – Contraction Coupling

Chapter 16

KINE 3301Biomechanics of Human Movement

A single muscle fiber contains hundreds of myofilaments (Actin & Myosin) which are bundled together and surrounded by the muscle membrane (sarcolemma).

A muscle fiber has a diameter of 10 – 60 μm and a length of 10 – 300 mm.

• Troponin and tropomyosin are regulatory proteins.• Under resting conditions they a crossbridge from forming

between actin and myosin.• In the presence of Ca++ stops inhibiting muscular contraction by

rotating away from it’s resting position where it covers the actin binding site.

Sarcomere: The Functional Unit of the Muscle Cell

Concentric Cross-bridge Cycle

• Ca++ released from SR• Ca++ binds with troponin• Tropomyosin moves away

from binding site.• An Actin-Myosin cross-

bridge is formed.• ATP downgraded to ADP +

Pi.• Myosin does mechanical

work on Actin, Myosin arm rotates shortening the muscle fiber.

Eccentric Cross-bridge Cycle

• Ca++ released from SR• Ca++ binds with troponin• Tropomyosin moves away from binding

site.• An Actin-Myosin cross-bridge is formed.• ATP downgraded to ADP + Pi.• Myosin attempts to rotate and shorten

fiber.• External force causes the fiber to

lengthen, storing elastic energy in myosin arm.

• The cross-bridge is broken (1-4 pN) while it is still in the actively charged state, it immediately forms another cross-bridge at the next available Actin site without the need for any additional ATP.

• This “break & make” cross-bridges will continue until the length of the active state is exceeded.

Isometric Cross-bridge Cycle

• Ca++ released from SR• Ca++ binds with troponin• Tropomyosin moves away from binding site.• An Actin-Myosin cross-bridge is formed.• ATP downgraded to ADP + Pi.• Myosin attempts to rotate and shorten fiber.• External force causes the fiber to lengthen,

storing elastic energy in myosin arm.• From this lengthened position the myosin arm

rotates and shortens the fiber.

Contraction Type

Energy Utilized

Force Generated

Change in Fiber Length

Effect of Velocity on Force

Concentric 1 ATP 1 – 3 pN Shortened ~5 nm

Force decreases with increasing velocity:1 pN for fast velocity3 pN for slow velocity

Eccentric 1 ATP n × 1-4 pN,n is # of make & break A-M bonds

Lengthened ~5-20 nm

Force increases with increasing velocity: 3 pN for slow velocity4 pN for fast velocity

Isometric 1 ATP 3 pN Relatively Constant

none

Summary of Actin-Myosin Crossbridge Cycle