Mechanical Molecules PDF

Mechanical molecules Skeletal muscle contraction production of movement and force  Structure of skeletal muscle A skeletal muscle fiber is a long cylindrical, multinucleated cell that is filled with smaller units of filaments which are aligned parallel to the muscle...

Mechanical molecules Skeletal muscle contraction production of movement and force  Structure of skeletal muscle A skeletal muscle fiber is a long cylindrical, multinucleated cell that is filled with smaller units of filaments which are aligned parallel to the muscle fiber itself. Filaments: Myofibril Sarcomeres Myofilaments (myosin–actin)  Structure of skeletal muscle The largest of the filaments is the myofibril, composed of subunits called sarcomeres. Each sarcomere also contains filaments, known as myofilaments. There are two types of myofilaments. myosin (thicker) and actin (thinner). Sliding of the actin myofilament on the myosin chain is the basic mechanism of muscle contraction.  Sliding filament theory of muscle contraction The sarcomere, containing the contractile proteins actin and myosin, is the basic functional unit of muscle. The thinner actin chains are more abundant than the myosin myofilaments in a sarcomere. The actin myofilaments are anchored at both ends of the sarcomere and project into the interior of the sarcomere where they surround a thicker myosin myofilament. This arrangement of myosin surrounded by actin myofilaments is repeated throughout the sarcomere, filling its interior and giving the muscle fiber its characteristic striations.  Contraction results from the formation of cross-bridges between the myosin and actin myofilaments, causing the actin chains to “slide” on the myosin chain. The tension of the contraction depends upon the number of cross-bridges formed between the actin and myosin myofilaments.  True or False 1- The amount of the contractile proteins within the cells is strongly related to a muscle’s contractile force. ( T ) 2- The number of cross-bridges formed depends only on the abundance of the actin and myosin molecules ( F ) 6 The number of cross-bridges formed (tension of the contraction) depends on: 1. The abundance of the actin and myosin molecules. 2. The frequency of the stimulus to form cross- bridges.  Excitation-Contraction Coupling Every skeletal muscle fiber in every skeletal muscle is innervated by a motor neuron (nerve cell). Both neurons and skeletal muscle cells are electrically excitable (able to generate action potentials). An action potential is a special type of electrical signal that can travel along a cell membrane as a wave. 8  Contraction is initiated by an electrical stimulus (excitation signal) from the associated motor neuron  depolarization of the muscle fiber  calcium is released into the cell  binds with the regulating protein troponin actin binds with myosin contraction. Cessation of the nerve’s stimulus  ↓calcium inhibiting the cross bridges between actin and myosin The muscle relaxes. Action potentials  Excitation-Contraction Coupling All living cells have membrane potentials (electrical gradients across their membranes). The inside of the membrane is usually around -60 to -90 mV, relative to the outside. Neurons and muscle cells can use their membrane potentials to generate electrical signals. They do this by controlling the movement of charged particles (ions) across their membranes to create electrical currents. This is achieved by opening and closing specialized proteins in the membrane (ion channels). At the neuromuscular junction (NMJ), the axon terminal releases acetylcholine (Ach) ACh molecules diffuse across a minute space (synaptic cleft)  bind to the ACh- receptors in the muscle fiber a channel in the ACh receptor opens  positively charged ions pass into the muscle fiber depolarize  open voltage-gated sodium channels  Na+ enter muscle  contraction The T-tubules carry the action potential into the interior of the muscle fiberthe opening of calcium channels in the membrane of sarcoplasmic reticulum (SR) contraction.  True or False 1- The sarcomere is the basic functional unit of muscle. ( ) 2- The thinner actin chains are less abundant than the myosin in a sarcomere. ( ) 3- The tension of the contraction depends upon the number of cross-bridges formed between actin and myosin ( ) 4- Contraction is initiated by an electrical stimulus causing depolarization of the muscle fiber. ( ) 14  True or False 5- The combination of Ca++ with cadherin protein causes actin to bind with myosin ( ) 6- The neuromuscular junction is the site where a motor neuron’s terminal meets the muscle ( ) 7- The ACh in the synaptic cleft is degraded by the enzyme acetylcholinesterase (AChE) ( ) 8- The T-tubules carry the action potential into the interior of the muscle fiberthe, which triggers the release of calcium ions ( ) 15