U N I T II Textbook of Medical Physiology, 11th Edition GUYTON & HALL Copyright © 2006 by Elsevier, Inc. Contraction and Excitation of Smooth Muscle
Copyright © 2006 by Elsevier, Inc. CLASSIFICATION OF THE MUSCULAR TISSUE Non-striated muscle (smooth muscle) Striated muscle (cardiac, skeletal muscle)
Copyright © 2006 by Elsevier, Inc. SMOOTH MUSCLE No cross striations Location: in the walls of blood vessels, hollow viscera Function: controls the size of the lumen. Shape: each fiber is fusiform in shape. Diameter: 1-5 um Length: um No well developed sarcotubular system. Caveolae are present. Nucleus: single, rod- shaped nucleus located in the centre of each cell.
Copyright © 2006 by Elsevier, Inc. Types of Smooth Muscle MULTI-UNIT Smooth Muscle UNITARY Smooth Muscle ( syncytial/visceral )
Copyright © 2006 by Elsevier, Inc. MULTI-UNIT AND UNITARY SMOOTH MUSCLES
Copyright © 2006 by Elsevier, Inc. MULTI-UNIT AND UNITARY SMOOTH MUSCLES Location Type of NM Junctions Arrangement Control Membrane potential.
MULTI-UNIT AND UNITARY SMOOTH MUSCLES 1: LOCATION : UNITARY: Wall of GIT, ureter, bile duct, uterus and large blood vessels. MULTIUNIT: Ciliary muscle of eye, Iris of eye and pilo-erector muscle, vas deferens
MULTI-UNIT AND UNITARY SMOOTH MUSCLES 2: ARRANGEMENT: UNITARY: Sheaths or bundles with gap junctions in between, so act as single unit. If one part is excited whole is excited.(functional syncytium) MULTIUNIT: Individual muscle fibers. Each fiber has insulating outer membrane with glycoprotein and collagen consistency. Independently each muscle fiber is excited.
Copyright © 2006 by Elsevier, Inc. MULTI-UNIT AND UNITARY SMOOTH MUSCLES 3: TYPE OF NM JUNCTION: UNITARY: Diffuse junction. No contact with nerve fiber. Neurotransmitter is released near the fiber. MULTIUNIT: Contact junction. One of the autonomic nerves form junction with muscle fiber and releases neurotransmitter like nor-epinephrine
Copyright © 2006 by Elsevier, Inc. MULTI-UNIT AND UNITARY SMOOTH MUSCLES 4: CONTROL: UNITARY: Mainly non-nervous stimuli (hormonal stimuli like oxytocin and serotonin). Nervous stimuli are less important Show spontaneous contractions. MULTIUNIT: Mainly through nervous stimuli. Does not show spontaneous contraction
MULTI-UNIT AND UNITARY SMOOTH MUSCLES Types of potential in unitary smooth muscles 5: MEMBRANE POTENTIAL: UNITARY: 3 types: Spike potential, Action potential with Plateau and spikes superimposed on Slow wave. MULTIUNIT: action potential is not produced. Only localized depolarization in response to excitation.
Contractile mechanism in smooth muscle CHEMICAL BASIS FOR Smooth Muscle Contraction: Contain both actin and myosin Doesn’t contain the normal troponin complex
Copyright © 2006 by Elsevier, Inc. Contractile mechanism of smooth muscle fiber E/M shows that actin and myosin are present in the cytoplasm. Actin: diamter is 5-7 nm Myosin: diameter nm actin/myosin ratio: 15/1 Desmin intermediate filaments are also present in the cytoplasm. (dia= 10 nm) Dense bodies: intermediate filaments cross over one another to form darkly staining patches.
Copyright © 2006 by Elsevier, Inc. EXCITATION- CONTRACTION COUPLING OF SMOOTH MUSCLE FIBERS
Copyright © 2006 by Elsevier, Inc. EXCITATION-CONTRACTION COUPLING OF SMOOTH MUSCLE FIBERS
Copyright © 2006 by Elsevier, Inc. Contraction-Relaxation – myosin based regulation Ca 2+ calmodulin MLCK MLC active (phosphorylated) MLC inactive (dephosphorylated) Relaxation Contraction MLCP MLCK, myosin light chain kinase MLCP, myosin light chain phosphatase
Copyright © 2006 by Elsevier, Inc. Mechanisms That Increase Intracellular Ca 2+ Concentration in Smooth Muscle
Copyright © 2006 by Elsevier, Inc. Comparison of Smooth muscle contraction and Skeletal muscle contraction Slow cycling of the myosin cross-bridges Low energy requirement to sustain smooth muscle contraction Slowness of onset of contraction and relaxation of smooth muscle contraction Maximum force of contraction is often greater in smooth muscle than in skeletal muscle
Copyright © 2006 by Elsevier, Inc. “LATCH” Mechanism facilitates prolonged holding of contractions of smooth muscle Stress-Relaxation of smooth muscle
Copyright © 2006 by Elsevier, Inc.