Myosin light chain Part of cross bridge energy cycle Permits binding with actin

Myosin light chain part of cross bridge energy cycle

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Contraction Uncovering of cross- bridge binding sites on actin in thin filament Phosphorylation of myosin cross bridges in thick filament Series of biochemical events Physical repositioning of troponin and tropomyosin Rise in cytosolic Ca 2+ (mostly from extracellular fluid) Rise in cytosolic Ca 2+ (entirely from intracellular sarcoplasmic reticulum) Muscle excitation Muscle excitation Skeletal muscle Smooth muscle Contraction Binding of actin and myosin at cross bridges Binding of actin and myosin at cross bridges P i Contraction in smooth vs. skeletal muscle Control on thick filament Control on thin filament
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Smooth Muscle Types Smooth muscle in different organs is variable and can be classified as: multi-unit or single-unit neurogenic or myogenic phasic or tonic (self-excitable)
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Phasic vs. Tonic Phasic smooth muscle contracts in bursts, triggered by action potentials that lead to increased cytosolic Ca 2+ Tonic smooth muscle is usually partially contracted at all times varies its contraction level in response to factors that alter cytosolic Ca 2+ concentration Rest=(-55 to -40mV)
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Multi-unit Multiunit smooth muscle is neurogenic contraction initiated only in response to stimulation by nervous system All multiunit smooth muscle is phasic , contracting only when neurally stimulated Multiunit smooth muscle is found in many places - walls of large blood vessels, in small airways of the lungs, hair follicles, iris & lens muscles of eye
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Single-unit Also called visceral smooth muscle, because it is found in the walls of hollow organs (digestive tract, urinary tract, small blood vessels) Myogenic = can initiate its own contraction Fibers that make up this type of muscle become excited and contract as a single unit functions electrically and mechanically as a single unit= functional syncytium
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Single-unit smooth muscle Action potentials spread from pacemaker cells to adjacent cells via gap junctions – whole muscle sheet contracts as a single unit Myogenic
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Time (min) Slow-wave potential Threshold potential Threshold potential Action potential Action potential Pacemaker potential Membrane potential (mV) Membrane potential (mV) Time (min) Slow-wave potential 0 0 Pacemaker potential Myogenic single-unit smooth muscle (digestive tract only) (digestive tract, uterus) Membrane potential inherently fluctuates
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Smooth muscle summarized Multi-unit neurogenic phasic Single-unit myogenic phasic tonic Most abundant – digestive system, urinary system, circulatory system Large blood vessels, bronchioles, hair follicles, iris and lens muscles of eye
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