LECTURE 14 CHAPTER 12 MECHANISMS OF CONTRACTION AND NEURAL CONTROL MUSCLES LECTURE 14 CHAPTER 12 MECHANISMS OF CONTRACTION AND NEURAL CONTROL

MUSCLE STRUCTURE

Remember: Skeletal muscle cells (fibers) are multinuclear and have striations - under voluntary control; but also responds to reflexes. The cells are long and are called fibers. They are cells that developed by fusing together.

STRUCTURE OF SKELETAL MUSCLE Fasciculus – many muscle fibers in a bundle that are separated by connective tissue, which forms the tendon. When a muscle fiber (cell) shortens, it stretches the connective tissue and pulls the bones. Know this Figure 12.1 The connective tissue is all contiguous. It is given different names depending on where it’s located. Know this Fasciculi is plural

You should know everything on this slide from anatomy: --- Origin/Insertion --- flexor muscles/extensor muscles --- prime mover = agonist; in a flexion, the flexor is the agonist, and the extensor is the antagonist. Know this The connective tissue is all contiguous. It is given different names depending on where it’s located. Know this

What’s Inside A Muscle Fiber (cell)? Sarcolemma – plasma membrane Sarcoplasm – cytoplasm Sarcoplasmic reticulum (SR) – modified ER that stores Ca2+ Terminal Cisternae – part of the SR containing high Ca2+ T-Tubules – infoldings of plasma membrane that contain voltage-regulated gates Myofibril –contains the myofilaments (actin and myosin)

Myofibrils contain myofilaments arranged into sarcomeres Know this, but I will not ask you about A and I band terminology

Titin Filaments and M Lines M = Midline Titin: protein that runs from the Z disc to the M line and allows elastic recoil; 27,000 – 33,000 amino acids depending on the splice isoform.

Arrangement of Thick and Thin Filaments

HOW DOES A MUSCLE WORK? A muscle works by excitation-contraction coupling. This means 1) excitation – the neuron stimulates the muscle, and then 2) the muscle contracts

Neuromuscular junction is the synapse, where ACh is released from vesicles. Motor End plate: part of the muscle membrane stimulated by the axon, causing an “end-plate potential”

Action Potential travels down somatic motor neuron – (this is from Ch Action Potential travels down somatic motor neuron – (this is from Ch. 7) -- acetylcholine will be released from synaptic vesicles Fig. 7.23

Acetylcholine binds nictonic ACh receptors – (this is from Ch Acetylcholine binds nictonic ACh receptors – (this is from Ch. 7) -- Na+ travels into the skeletal muscle cell; causes depolarization, excitation Fig. 7.26

Acetylcholinesterase degrades acetylcholine– (this is from Ch Acetylcholinesterase degrades acetylcholine– (this is from Ch. 7) -- Na+ travels into the skeletal muscle cell; causes depolarization, excitation Fig. 7.28

Neuromuscular blocking agents vs Neuromuscular blocking agents vs. Acetylcholinesterase inhibitors– (this is from Ch. 7) -- e.g. curare is used as a skeletal muscle relaxant during surgery; a neuromuscular blocking agent, it causes flaccid paralysis. -- e.g. nerve gas – acetylcholinesterase inhibitor causes spastic paralysis, the muscle stays contracted

Review: Stimulating a Muscle Fiber 1)Acetylcholine is released from the motor neuron and binds its receptor in the end plate. 2)Potentials are produced (Na+ moves in). If they are strong enough they cause nearby voltage-regulated gates to reach threshold. 3)Action potentials are generated (all-or-none) in both directions in the muscle cell. They go down the T-tube.

Stimulating a Skeletal Muscle Fiber voltage-gated Ca2+ channels in transverse tubules change shape stimulates SR Ca2+ channels in SR open. - Ca2+ diffuses out of SR through calcium release channels (ryanodine receptors).

Inside the myofibril, actin is bound by troponin and tropomyosin (inhibitory proteins) which prevent actin from binding to myosin F-actin is made of 300-400 G-actin subunits, arranged in a double row and twisted to form a helix

ROLE OF CALCIUM Tropomyosin physically blocks cross bridges. But when Ca2+ binds to troponin, troponin and tropomyosin change shape. This exposes the attachment site on actin for myosin. Myosin grabs actin, Pi is released, and there is a power stroke. Actin is pulled just a little bit.

CLOSE UP OF POWER STROKE Myosin grabs actin, Pi is released, and there is a power stroke. Actin is pulled a little (towards the center of the sarcomere). For more movement to occur, a new ATP must attach…

REVIEW OF INTERACTION OF MYOSIN WITH ACTIN Click here to watch

Sliding Filament Theory of Muscle Contraction When a muscle contracts, sarcomeres (distance between the Z-lines shortens).

Action potentials cease. Calcium release channels close Muscle Relaxation Action potentials cease. Calcium release channels close Ca2+-ATPase pumps move Ca2+ back into SR (active transport). No more Ca2+ is available to bind to troponin Tropomyosin moves to block the myosin heads from binding to actin

Motor Units Motor unit - a single motor neuron and all the muscle fibers it innervates; all the muscle fibers in a motor unit contract at once. Graded contractions – varied contraction strength due to different numbers of motor units being stimulated (recruitment): makes you able to lift more weight. More motor units gives you finer control of muscles too [e.g. eye is 1:23, neuron to muscle fibers]. Divergence