BIOL 2457 1st Edition Lecture 19Outline of Last Lecture The skeletal system: joints Outline of Current LectureMuscle tissues Contraction and Relaxation of Skeletal Muscle The Contraction Cycle The onset of contraction begins with the SR releasing calcium ions into the muscle cell The calcium ions bind to actin opening the myosin binding sites The contraction cycle consists of 4 steps ATP hydrolysis Reorients and energizes the myosin head Formation of cross-bridges Myosin head attaches to the myosin-binding site on actin Power stroke During the power stroke the crossbridge rotates, sliding the filaments Detachment of myosin from actin As the next ATP binds to the myosin head, the myosin head detaches from actin The contraction cycle repeats as long as ATP is available and the Ca2+ level is sufficiently high Continuing cycles apply the force that shortens the sarcomere Excitation–Contraction Coupling An increase in Ca2+ concentration in the muscle starts contraction A decrease in Ca2+ stops it Action potentials causes Ca2+ to be released from the SR into the muscle cell Ca2+ moves tropomyosin away from the myosin-binding sites on actin allowing cross-bridges to form The muscle cell membrane contains Ca2+ pumps to return Ca2+ back to the SR quickly Decreasing calcium ion levels As the Ca2+ level in the cell drops, myosin-binding sites are covered and the muscle relaxes Length–Tension Relationship The forcefulness of muscle contraction depends on the length of the sarcomeres When a muscle fiber is stretched there is less overlap between the thick and thin filaments and tension (forcefulness) is diminished When a muscle fiber is shortened the filaments are compressed and fewer myosin heads make contact with thin filaments and tension is diminished The Neuromuscular Junction Motor neurons have a threadlike axon that extends from the brain or spinal cord to a group of muscle fibers Action potentials arise at the interface of the motor neuron and muscle fiber Synapse Where communication occurs between a somatic motor neuron and a muscle fiber Synaptic cleft Gap that separates the two cells Neurotransmitter Chemical released by the initial cell communicating with the second cell Synaptic vesicles Sacs suspended within the synaptic end bulb containing molecules of the neurotransmitter acetylcholine (Ach) Motor end plate The region of the muscle cell membrane opposite the synaptic end bulbs Contains acetylcholine receptors Nerve impulses elicit a muscle action potential in the following way (1) Release of acetylcholine Nerve impulse arriving at the synaptic end bulbs causes many synaptic vesicles to release ACh into the synaptic cleft (2) Activation of ACh receptors Binding of ACh to the receptor on the motor end plate opens anion channel Allows flow of Na+ to the inside of the muscle cell (3) Production of muscle action potential The inflow of Na+ makes the inside of the muscle fiber more positively charged, triggering a muscle action potential The muscle action potential then propagates to the SR to release its stored Ca2+ (4) Termination of ACh activity Ach effects last only briefly because it is rapidly broken down by acetylcholinesterase (AChE) Botulinum toxin Blocks release of ACh from synaptic vesicles May be found in improperly canned foods A tiny amount can cause death by paralyzing respiratory muscles Used as a medicine (Botox®) Strabismus (crossed eyes) Blepharospasm (uncontrollable blinking) Spasms of the vocal cords that interfere with speech Cosmetic treatment to relax muscles that cause facial wrinkles Alleviate chronic back pain due to muscle spasms in the
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