Lecture 24Lecture Outline:1. Smooth Musclea. Internal Structureb. Excitation contraction couplingc. Single unit vs multi-unit Smooth Muscle:- Internal Structure: much smaller than skeletal muscleo Poorly developed sarcoplasmic reticulumo No transverse tubuleso No troponin on thin filamentso Contain tropomyosin, but not in blocking positiono No neuromuscular junction or motor end plateo Varicosities release NT over entire smooth muscleo Sliding filament mechanism: thick and thin filaments interact to produce smooth muscle contraction- Excitation contraction coupling: relies on phosphorylation of cross bridgeo As long as phosphate binds to cross bridge, it will cycle and interact with thin filamento To stop contraction, phosphate must be cleaved from cross bridge1. Response to stimuli causes calcium to leak into cell 2. Calmodulin becomes active when extracellular calcium comes into cell3. Activation of calcium calmodulin activates calcium-colmodulin myosin light-chain kinase which attaches phosphate cross bridge4. Cycling and cross bridge activity occurs5. Myosin light chain phosphatase cleaves phosphate from cross bridge and ends cross bridge cyclingo Results in graded response depending on how much calcium made available in response to various stimuli(not all or none response)- Influences on the entry or removal of Ca 2+ from sarcoplasm:o APs in sarcolemma, some smooth muscle cells can spontaneously depolarize and fire APs Usually done rhythmically Caused by potassium leaking out faster than sodium and calcium leaking ino Neurotransmitters released by autonomic neuronso Hormones Oxytocin: receptors on uterine smooth muscle Vasopressin: receptors on vascular smooth muscle Epinephrine; receptors in airway smooth muscle Gastrin: receptors in stomach smooth muscle Release of NT will only effect smooth muscle where they have bindable receptorso Locally induced charges in the chemical composition of the extracellular fluid Oxygen, CO2, H+, K+o Passive stretch Done in blood and lymphatic vessels as a response to increased pressure- Some smooth muscle receives dual innervation from the ANSBio 312Single unit smooth muscle:- Found in walls of hollow organs, involved in peristaltic contractions- Most smooth muscle falls under this category- Spontaneous action potential induced by pacemaker (rhythmic) potentialo Spread to non-pacemaker cells through gap junctionso Individual AP stimulates multiple muscle cellsMulti-unit smooth muscle:- Found in skin, eyes (iris, ciliary body)- Very dependent on ANS- No pacemaker activity- Can contract without action potentialo Hormones or neurotransmitters can allow calcium into cell and allow cross bridge cycling without initiation from an action potential- No gap junctions, do not move in
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