Smooth Muscle Physiology PDF

1.08 Smooth muscle PHYSIOLOGY Maria Carmelita Vera Cruz, MD 1ST BIMONTHLY 14 September 2021...

1.08 Smooth muscle PHYSIOLOGY Maria Carmelita Vera Cruz, MD 1ST BIMONTHLY 14 September 2021 TRANS 07 ○ Attached to the cell membrane, other dense bodies scattered SMOOTH MUSCLE PHYSIOLOGY all over Types: multi-unit, single-unit ○ Dense bodies linked to other dense bodies by a scaffolding of protein bridges ○ Lots of actin, sparse myosin ○ dense bodies - same role as Z disc in skeletal muscle Smooth Muscle Contraction Characteristics Skeletal - rapid contraction Smooth - prolonged, tonic contraction, lasting for hours, even days ○ A. slow cycling of cross-bridges (attachment to actin, release from actin) Much, much slower in smooth muscles than in skeletal muscles Longer time that actin and myosin are attached to each other ○ B. very low energy requirement, very economical ○ C. slow onset, prolonged/sustained duration ○ D. force of contraction - greater than skeletal muscle probably due to the longer time actin is attached to myosin ○ E. percentage of shortening of muscle - can shorten far greater percentage of its length and still maintain almost full force of contraction, smooth muscle very “flexible” / ○ 1. Multi-unit adjustable Discrete muscle fibers, each fiber operating entirely ○ F. latch mechanism - once smooth muscle has developed independently of the others, innervated by single nerve full contraction, the degree of activation of the muscle can be ending (like skeletal muscles) reduced to far less than the initial level and yet the muscle Most important characteristics: each fiber can contract will still maintain its full strength of contraction independently of the others Energy consumed to maintain contraction - miniscule Examples: ciliary muscles of the eye, iris of the eye, Importance: piloerector muscles Maintain prolonged tonic contraction in smooth muscle ○ 2. Single-unit / unitary / syncytial / visceral for hours and hours with very little use of energy A whole mass of fibers (hundreds to thousands contract Also very little excitatory signal is required from nerve together as a single unit) fibers or hormonal sources. Fibers aggregated into sheets or bundles Cause of latch phenomenon: prolonged attachment of Cell membranes adherent to each other at multiple points myosin cross-bridges to actin Many GAP JUNCTIONS ○ G. stress-relaxation of smooth muscle - ability to return to Examples: found in the walls of most viscera in the body, its original force of contraction seconds or minutes after it like the gut, bile ducts, ureters, uterus, blood vessels has been elongated or shortened Gap junctions Importance: allows a hollow organ to maintain ○ Intimate connections between cells thru which ions can flow approximately the same amount of pressure inside its freely from one cell to the next so that APs travel from one lumen regardless of length of muscle fibers fiber to the next → entire muscle mass contracts together Regulation of contraction by Ca++ Chemical basis: SMOOTH MUSCLE SKELETAL MUSCLE actin actin Myosin myosin calcium-activated calcium-activated ATP needed ATP needed No troponin With troponin Physical basis: Dense bodies - where actin filaments attach Steps: TRANS (7) CORE 1 of 2 1.08 Smooth muscle ○ A. Ca++ bind with calmodulin ○ Remember: calcium channels open more slower than ○ B. calmodulin-Ca++ combination joins with and activates sodium channels → slow action potentials of smooth muscle myosin kinase (a phosphorylating enzyme) Two roles of Ca++ in smooth muscle contraction ○ C. One of the light chains (called the regulatory chain of each ○ To cause depolarization : by the entrance of Ca++ into cell, myosin head becomes phosphorylated in response to the the cell depolarizes myosin kinase. When the light chain is phosphorylated, the ○ To remove the inhibition, exposing the active sit, allowing the head has the capability of binding with the actin filament. myosin head to attach to the active site - power stroke. ○ D. Power stroke occurs. Slow Wave Potentials (pacemaker waves) Source of Calcium Some smooth muscle cells = self-excitatory SR in smooth muscles APS arise within the muscle itself without an extrinsic stimulus ○ Rudimentary, not very well developed Associated with a slow wave rhythm of the action potential ○ Main source of calcium for contraction is the ECF Slow wave - NOT an action potential Caveoli - rudimentary analogs of the t-tubule system ○ Does not spread over the entire muscle membrane ○ Found in some smooth muscle cells ○ Localized ○ Small invaginations of the membrane ○ Importance: can initiate an action potential If slow wave reaches -35 mV (the threshold in smooth muscle How do the Ca++ enter the cell to cause depolarization and action potential develops and spread over the entire muscle) contraction? ○ 1. Diffusion - smooth muscle fibers are very small compared Stimulation by stretch to skeletal muscle, calcium can easily diffuse through Stretch - visceral (single-unit) smooth muscle is stretched membrane sufficiently, spontaneous action potentials are usually ○ 2. Hormone-gated calcium channels generated ○ Results from a combination of the normal slow wave Calcium potentials plus a decrease in negativity of the membrane ○ Ca++ concentration in the ECF - very important in smooth potential caused by the stretch itself muscle contraction ○ Example: when the gut is overstretched by intestinal ○ Very low Ca++ in the ECF → muscle contraction almost contents, a local automatic contraction often sets up ceases peristalsis wave that moves the contents away from the ○ Force of contraction is highly dependent on ECF calcium overfilled intestine concentration Smooth muscle contraction without action potentials The Calcium pump 1. Local tissue factors - local environment control ○ What causes relaxation? ○ Example: factors that cause vasodilation ​calcium pump - pumps calcium ions out of the smooth ○ A. lack of oxygen in local tissues muscle fiber back into the extracellular fluid or into a ○ B. excess carbon dioxide sarcoplasmic reticulum ○ C. increased hydrogen ion concentration Pump is very slow-acting ○ D. increased lactic acid Prolonged duration of contraction in smooth muscle ○ E. Increased potassium ions ○ F. Decreased body temperature Neural and hormonal control of smooth muscle contraction 2. Hormones that affect contraction: ○ NE, epi, Ach, angiotensin, vasopressin, oxytocin, serotonin, Neural control histamine ○ Diffuse junctions ○ A hormone causes contraction when the muscle cell fiber ○ Varicosities contains hormone-gated excitatory receptors for a particular ○ Contact junctions hormone The neurotransmitters: acetylcholine, norepinephrine ○ A hormone cause inhibition instead of contraction if the Type of receptor - determines whether the smooth muscle will membrane contains inhibitory receptors rather than be excited or inhibited by the neurotransmitter that arrives. excitatory receptors Resting membrane potential : -50 to -60 mV Action Potential of visceral smooth muscle 1. Spike potential - similar to skeletal AP, occurs in unitary smooth muscle, duration is 10 to 50 milliseconds ○ Can be elicited by electrical stimulation, hormones, transmitter subs, spontaneous generation in the muscle itself 2. Action potential with plateau - onset is similar to typical spike potential but instead of rapid repolarization, there is a plateau or a delay in repolarization for several hundred to several thousand milliseconds Importance of Ca++ channels in generating smooth muscle action potentials Smooth muscle cell membranes - have more voltage-gated calcium channels than skeletal muscle bu with very, very few sodium channels ○ Flow of calcium ions into the cell thru the channel → mainly responsible for the action potential PHYSIOLOGY 2 of 2