The sliding filament theory, Lecture notes of Biology

Download The sliding filament theory and more Lecture notes Biology in PDF only on Docsity! The Sliding Filament Theory & Excitation Contraction Coupling (Text Pg 40 - 43) There are Two Other Key Proteins Located On Actin 1. Tropomyosin • Strand like protein wrapped around actin filament that blocks the myosin binding sites on the actin 2. Troponin • Globular shaped molecules, that sit on top of tropomyosin. • Have calcium ion (Ca2+) binding sites (Very Important) The Sliding Filament Theory! • Our muscles contract when the myosin heads binds to actin (crossbridge formation) causing it to slide overtop of itself. • The length of the thin and the thick filaments have not changed. • Rather, the myofilaments slide over top of each other. • Hence the name “Sliding Filament Theory” Two Problems: o The binding sites on actin are blocked by tropomyosin!! o Myosin needs energy to bind and move the actin. How do we “Un-block” Actin? • Calcium Ions (Ca2+) bind to Troponin causing tropomyosin to move and reveal the binding sites. o Calcium is a regulatory molecule for muscular contraction. How does a muscle know when to release calcium? • Calcium is released when the cell becomes depolarized. o A resting muscle cell is “polarized” o When an action potential from the motor neuron arrives, the cell becomes depolarized (due to acetylcholine). o This wave of depolarization is transported to the interior of the muscle fibre via the transverse tubules (T-Tubules). Where do the calcium ions come from? • Calcium ions (Ca2+) come from the Sarcoplasmic Reticulum when the cell becomes depolarized. Where does the energy for muscular contractions come from? • Myosin gets its energy from a molecule called Adenosine Triphosphate (ATP - the energy currency of the cell). • ATP allows two things to occur: o It “energizes” myosin to bind to Actin o It provides the energy necessary for the “power stroke” (myosin head swivels causing actin to slide overtop of myosin). ATP + H2O → ADP + P + Energy The Relationship Between Myosin and ATP! • The myosin filaments have pivoting heads that swivel and attach to the binding sites on Actin only when ATP is present. o This is called a crossbridge (Myosin bound to Actin). o The energy released causes the power stroke (Myosin head swivels). • Some of the energy is also released as heat (metabolic by-product). When do the Crossbridges break? • The crossbridge will only break under two circumstances: a) Another ATP binds to Myosin b) When the Ca2+ ions return to the sarcoplasmic reticulum (tropomyosin returns to its inhibitory position) What Happens To The Sarcomere During Muscular Contractions? • Actin filaments on either end of the sarcomere approach each other from opposing ends and meet in the center (H-zone gets smaller). • One power stroke results in a very small displacement of the filaments and produces a small amount of force. • Crossbridge cycling occurs very rapidly to produce larger movements and greater forces. What Happens When We Relax Our Muscles? • When the muscles relax: o Ca2+ ions return to the sarcoplasmic reticulum. o Tropomyosin slides back over the binding sites on Actin breaking the crossbridges o The Actin filaments slide back to their original position.