3- Muscle Physiology- Pt 4.docx

- **Excitation-contraction Coupling: General Info** - Excitation-contraction coupling is the overall process which transforms a nerve impulse into a muscle contraction. - The T-Tubules are associated with the sarcoplasmic reticulum. - The **sarcoplasmic reticulum** is...

- **Excitation-contraction Coupling: General Info** - Excitation-contraction coupling is the overall process which transforms a nerve impulse into a muscle contraction. - The T-Tubules are associated with the sarcoplasmic reticulum. - The **sarcoplasmic reticulum** is used for calcium storage. - The sarcoplasmic reticulum is composed of terminal cisternae (which are large chambers), and long longitudinal tubules (which surround all surfaces of the contracting myofibrils). - **DHPR (dihydropyridine receptors)** are voltage sensing proteins. - DHPR are mechanically coupled to the RyRs (ryanodine receptors). - **RyRs** **(ryanodine receptors)** are calcium channels on the sarcoplasmic reticulum membrane. - Once a muscle contraction is initiated, the contraction will continue as long as the calcium ion concentration remains high. - **SERCA1a** **(sarcoplasmic reticulum calcium ATPase)** is a calcium pump that is continuously active. - SERCA1a is located in the walls of the sarcoplasmic reticulum and works by pumping calcium back into the sarcoplasmic reticulum. - **Excitation-contraction Coupling: Steps** - Step 1: The action potential scatters across the sarcolemma from the motor end plate. - Step 2: The action potential travels through the T-Tubule and fast depolarization penetrates the cell. - Step 3: The action potential causes a conformational change in the DHPR located on the T-Tubule. - Step 4: RyRs calcium channels open. - Step 5: Calcium rapidly diffuses out of the sarcoplasm. - Step 6: A muscle contraction is initiated. - Step 7: SERCA1a will remove the calcium ions from the myofibrillar fluid, after the contraction has occurred. - **Walk Along Theory (of muscle contractions): General Info** - Before a muscle contraction begins, the myosin head is in a low energy configuration. - Myosin head bound to ATP is considered to be in a **low energy configuration.** - The Myosin head has an ATPase activity where it can cleave ATP into ADP + P, which remain bound to the myosin head. - When the myosin head is bound to ADP and a phosphate group (P), it is considered to be in a **"cocked position".** - Myosin head bound to ADP is considered to be in a **high energy configuration.** - **Walk Along Theory (of muscle contractions): Steps** - Step 1: The Myosin head binds to ATP (putting it in a low energy configuration). - Step 2: The Myosin head cleaves ATP into ADP + P, causing the myosin head to cock into a high energy configuration. - Step 3: Calcium binds to troponin, opening the actin binding site. - Step 4: The Myosin head binds to actin, forming the **cross bridge**. - Step 5: Cross bridging triggers Myosin to release the phosphate group, leaving just ADP attached. - Step 6: This triggers a **power stroke**, which is when the myosin head moves from a high energy state to a low energy state, causing actin to be pulled closer to the M-line. During this, the myosin heads bend towards the center of the sarcomere (called a **micro-contraction**). - Step 7: ADP detaches from myosin. During this, myosin is still attached to actin (and is considered to be in a **rigor state**). - Step 8: ATP binds to the myosin head group. - Step 9: Myosin detaches from actin.