EXSS 276 1st Edition Lecture 19 Outline of Last Lecture I. Types of Muscle TissueII. Functions of Muscle TissueIII. PropertiesIV. Skeletal MuscleOutline of Current LectureI. SarcomereII. Pharmacology of NMJIII. The Motor UnitIV. All or None ResponseV. Muscle ToneCurrent LectureI. Sarcomerea. The contractile unitb. Consists of:i. Z disc: regions of dense material that separate sarcomeres; actin attachmentii. A band: darker middle part of sarcomere; extends length of the filaments;actin and myosiniii. I band: lighter area; only contains thin filaments; actiniv. H zone: center of A band; only contains thick filaments; myosinv. Zone of overlap: outer parts of A band; thick and thin filaments lie side bysidevi. M line: supporting proteins which hold thick filaments together at middle of the H zone; myosin attachmentThese notes represent a detailed interpretation of the professor’s lecture. GradeBuddy is best used as a supplement to your own notes, not as a substitute.vii.c. Neuromuscular Junction (NMJ)i. Region of synaptic contact between somatic motor neuron and skeletal muscle fiber1. Separated by synaptic cleft (chemical)2. Motor neuron side: synaptic bulbs on motor neuron’s axon (houseNT’s)3. Muscle fiber side: region of sarcolemma called motor end plateii. ACh is neurotransmitteriii. Where muscle action potentials ariseiv. End of axon nears the surface of a muscle fiber at its motor end plate region (remain separated by synaptic cleft or gap)v. What do you need on motor end plate for ACh?1. Receptors2. Binding of two ACh molecules and end of motor plate opens ion channel 3. Na+ can then flow across membrane4. Inflow changes charge on inside of cell (makes it more positive)5. Influx brings closer to triggering action potential (threshold of -70 mV) 6. SR releases calcium, troponin changes shape, moves tropomyosin out of the way for myosin to bindvi. How do we terminate ACh activity?1. Binding is brief2. AChE: breaks down AChII. Pharmacology of NMJa. Botulinum toxin (Botox)i. Blocks release of NT at NMJ so muscle contraction cannot occurb. Curare (plant poison from poison arrows)i. Causes muscle paralysis by blocking ACh receptorsii. Used to relax muscle during surgeryIII. The Motor Unita. One motor neuron and all the skeletal muscle cells (fibers) it innervatesb. Each skeletal muscle is supplied by a nerve, artery and two veinsc. Each motor neuron supplies multiple muscle cellsd. Each muscle cell is supplied by one motor neuron terminal branch and is in contact w/ one or two capillariese. Recruitment: increasing number of active motor unitf. Motor units in a whole muscle fire asynchronouslyi. Some fibers are active others are relaxedii. Delays fatigueiii. Produces smooth muscular contraction rather than series of jerky movementsg. Precise movements require smaller contractionsi. Motor units must be smaller (less fibers/nerves)h. Large motor units active when large tension neededIV. All or None Responsea. For motor unit to be recruited into activity, motor nerve impulse must meet or exceed thresholdb. If threshold not met, no fibers in that unit actc. Two mechanisms for increasing forcei. Motor unit summationii. Twitch summationV. Muscle Tonea. Involuntary contraction of small number of motor units (alternately active and inactive in constantly shifting pattern)i. Keeps muscles firm even though relaxedii. Does not produce movementb. Essential for maintain posture (head upright)c. Important in maintaining blood pressurei. Tone of smooth muscles in walls of blood