Smooth Muscle

Dr. Niranjan Murthy H L Asst. Prof., Dept. of Physiology Sree Siddhartha Medical College & Hospital, Tumkur

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Plain muscle-- non-striated Involuntary Visceral Autonomic

TYPES 1) Multi-unit smooth muscle: discrete smooth muscle fibers.  innervated by single nerve ending.  predominantly neural regulation.  iris, ciliary muscle, etc 2) Unitary smooth muscle: aggregated into sheets or bundles.  syncytial smooth muscle.  gap junctions.  walls of viscera & blood vessels.

ULTRASTRUCTURE • Made up of actin and myosin filaments • Actin, 5-10times that of myosin • Actin filaments are attached to dense bodies • Myosin filaments have “sidepolar” cross-bridgesbridges on either sides hinge in opposite directions

NMJ of smooth muscle • Varicosities- enlargements devoid of schwann cells and containing vesicles • Noradrenergic neurons have up to 20,000 varicosities per neuron each 5μm apart • One neuron innervate many effector cells • Diffuse junctions and contact junctions • Synapse en passant • Neurotransmitters- Ach, Noradrenaline,etc • Excitatory or inhibitory junctional potentialsdepends on the type of receptor • Cholinergic as well as noradrenergic fibers

POTENTIALS IN SMOOTH MUSCLE Resting membrane potential: -50 to -60mv AP in unitary smooth muscle:• Spike potentials similar to that of skeletal muscle  duration of 10-50ms • AP with plateau repolarization is delayed  duration of several 100 to a 1000ms  prolonged contraction seen in uterine & certain vascular smooth muscle

Role of Ca2+ ions Responsible for AP Slower and longer opening of Ca2+ channels Same Ca2+ is responsible for contraction Source of Ca2+ ions 1. Through cell membrane- mostly from ecf 2. Sarcoplasmic reticulum- separate sarcoplasmic tubules lie near cell membrane. caveoli are small invaginations of cell membrane-represent rudimentary analogue of t-tubules

Spontaneous generation of AP • Often associated with slow waves • Seen in GIT smooth muscles • Slow waves are caused due to waxing and waning of Na+ -K+ pump • Slow waves are also called pacemaker waves Junctional potential:• Depolarization of multi-unit smooth muscle without AP generation • Electrotonic conduction • Fibers are too small to generate AP

MECHANISM OF SM CONTRACTION • Smooth muscle doesn’t contain troponin • Calmodulin acts as regulatory protein • Myosin kinase and myosin phosphatase play a major role

Influx of Ca2+ from ECF

4 Ca2+ ions bind to calmodulin

Ca2+-calmodulin complex Activates myosin kinase

Myosin kinase with ATP phosphorylates Regulatory chain of myosin head

Myosin head develops Binding capacity Cycle of attachment & Detachment- contraction

reduced Ca2+ levels

Activation of myosin phosphatase

Dephosphorylation of Regulatory light chain

Attachment-detachment Cycle stops

Relaxation

COMPARISON OF SMOOTH MUSCLE & SKELETAL MUSCLE CONTRACTION • Cycling of cross-bridges is slower in smooth muscle. This is due to far lesser ATPase activity. • 1/10 to 1/300 times less energy is required for SM to sustain same tension of contraction as in skeletal muscles. • Slow onset of contraction & relaxation. Total contraction time of 1-3sec which is 30 times as long as that of skeletal muscle • Force of contraction is 4-6kg/cm2 in SM and 34kg/cm2 in skeletal muscle.

Latch mechanism:After development of full contraction, the degree of activation can be reduced to far less than initial levels with maintenance of full force of contraction. It can maintain prolonged tonic contractions with little energy.

Stress-relaxation:When visceral unitary smooth muscle is stretched, the tension increases initially. But, within few seconds, tension returns to original level. Closely related to latch phenomenon- when muscle is stretched, latch phenomenon resists. Myosin head detaches and attach further along actin filament. Tension comes back to normal as the number of cross-bridges involved will be almost the same. Reverse stress-relaxation

FACTORS AFFECTING SM CONTRACTION WITHOUT APS 1. Local tissue factors- oxygen, carbon dioxide, pH 2. Hormones- catecholamines, acetyl choline, angiotensin, vasopressin, serotonin, histamine

PROPERTIES OF SMOOTH MUSCLE 1. 2. 3. 4. 5.

Excitability Conductivity Contractility Tonicity Plasticity