Lecture Five

Produce movement in the body

• Constriction of organs and vessels • Cardiac contraction • Respiration • Postural maintenance • Body heat production

Skeletal, smooth, cardiac

Voluntary= skeletal Involuntary= cardiac & smooth

Striated= cardiac & skeletal Non-striated= smooth

• Cell membrane is composed of the sarcolemma and basement membrane. • Fuses with tendon which connects muscle to bone. • Assists with the transmission of action potentials along the muscle. • Transports metabolites in and out of cell.

• Specialised smooth endoplasmic reticulum in skeletal muscle, network of tubules run along and around myofibrils. • Storage site of calcium

• Extensions of the plasmalemma that pass laterally through the cell • Allow action potentials to be transmitted into the myofibrils

• Sarcoplasm • Mitochondria • Nucleus • Multi-nucleated • Myofibrils • Myofilaments

-Complex organelle composed of bundles of myofilaments.

-Contractile unit of muscle. o Z-line to Z-line -Striation of myofibrils reflects overlapping of thick and thin filaments -M-line: Myosin attachment -Titin enables the muscle to stretch and recoil

-Thick filaments are bundles of Myosin molecules -Each globular ‘head’ of the myosin has: o Myosin ATPase site o Actin binding site

-Thin filaments are paired chains of actin molecules each with a myosin binding site.

-At rest the binding sites are covered by the regulatory proteins o Troponin o Tropomyosin

-At rest, myosin and actin are unable to bind due to Troponin and Tropomyosin. o Tropomyosin covers the binding sites on actin o Troponin holds Tropomyosin in place -Troponin is the ‘lock’ that keeps binding sites inaccessible -Calcium is the ‘key’ -Ca2+ is stored in the Sarcoplasmic Reticulum -Upon release it spreads out in the sarcoplasm and binds to Troponin o Troponin initiates contraction process by moving Tropomyosin away to expose the Myosin binding sites on actin.

-Ca2+ binds to Troponin o Changes shape exposing the binding sites by moving Tropomyosin aside o Cross bridging occurs = attachment of Myosin to Actin

• Exposure of Active Sites - Calcium is released - Calcium makes it way to troponin which is then unlocked - Pulls tropomyosin to uncover the myosin binding sites on actin. • Cross-bridge Formation - When myosin binds to binding sites on actin • Power Stroke - Energy stored in the myosin head if used to cause the power stroke. • Cross-bridge Release - An ATP molecule binds to each head & causes detachment • ATP to ADP and P - A chemical reaction causes the P to detach from ATP • Recovery Stroke - Myosin head returns to resting position= energy stored in head.

-If calcium is still attached to troponin the cycle starts again -Repeated cycles of cross-bridge binding and power stroking ‘walks’ the cross bridges along the actin filaments pulling these inward o Towards M-Line -Overlap of the filaments increases and sarcomeres shorten

-Muscle action is an active process; therefore, it requires energy in the form of ATP. o Myosin contains a binding site for ATP. o In a relaxed state, the energised myosin heads are cocked back with ADP + Pi attached.

-Pi and ENERGY is released causing myosin head to pivot, pulling actin in towards centre of sarcomere; ADP is released from myosin.

-A new ATP binds to myosin head to release it from actin

-ATP is broken down by myosin ATPase to form ADP + Pi + energy to re-energise and re-position the head of myosin.

-Myosin and actin remain bound -In rigor mortis, the muscles stiffen because there is no ATP to release the bond. -When neural muscle stimulation ceases: o Ca2+ actively transported back into the sarcoplasmic reticulum using ATP o Tropomyosin will cover binding sites on actin -Muscle relaxation requires energy supply by ATP

• Power Stroke • Release of myosin from actin after power stroke • Transporting calcium back to sarcoplasmic reticulum