Smooth muscle Smooth Muscle A fusiform fibers B

Smooth muscle

Smooth Muscle • A. fusiform fibers • B. centrally located nucleus • Few mitochondria. • Z-lines are replaced by dense bodies. • Sustained contraction. • Single unit and multiunit smooth muscles.

• Located in the blood vessels, the respiratory tract, the iris of the eye, the gastro-intestinal tract • The contractions are slow and uniform • Functions to alter the activity of various body parts to meet the needs of the body at that time • Is fatigue resistant • Activation is involuntary Sport Books Publisher 3

• Myofilaments of the cells are not parallel as they are in skeletal and cardiac muscle • Symmetrical patterns of thick and thin filaments not evident • No troponin complex • Contraction is longer and variable and produce less tension. • Types • Single unit and multi unit smooth muscles.

Types of Smooth Muscle: Single Unit • The cells of single-unit smooth muscle, commonly called visceral muscle: • Large sheets and low resistance bridges. Contract rhythmically as a unit (as one) Are electrically coupled to one another via gap junctions Often exhibit spontaneous action potentials Are arranged in opposing sheets and exhibit stressrelaxation response – Contractile response independent of its innervations – Wall of hollow viscera e. g. GIT, uterus and urinary bladder – –

• MULTI UNIT Individual units. Non syncytial Has its own nerve supply Contract in response to stimulus Irregular tetanic contraction. Location: iris , ciliary muscles of eye, pilomotor muscle of skin. • Does not respond to stretch. • • •

Electrical properties of visceral smooth muscle

Ephaptic Conduction • Smooth muscle cells are linked electrically via gap junctions so they can stimulate each other causing a wave like action potential • This is also referred to as a FUNCTIONAL SYNCYTIUM

Mechanical properties • Irregular contraction independent of nerve supply. • Partial contraction • Myogenic • Peak contraction reached about 500 msec.

Plasticity • Stress relaxation property of smooth muscle • allows muscle to adjust to stretching without putting undo pressure on contents of organ • a constant state of partial contraction

Response to Stretch • Smooth muscle exhibits a phenomenon called stress-relaxation response in which: – Smooth muscle responds to stretch only briefly, and then adapts to its new length – The new length, however, retains its ability to contract – This enables organs such as the stomach and bladder to temporarily store contents

Smooth Muscle Regulation • Innervated by autonomic nervous system • Sympathetic stimulation- relaxation of GIT and contraction of vascular smooth muscle • Neurotransmitter are acetylcholinecontraction of GIT muscle , uterus. . • Hormones important as epinephrine and oxytocin • Atropine –relaxation • Estrogen and progesterone. • Cold , stretch stimulate contraction • Hypoxia , hypercapnia- relaxation of smooth muscle.

Smooth Muscle Fibers Compared to skeletal muscle fibers • shorter • single nucleus • elongated with tapering ends • myofilaments randomly organized • no striations • lack transverse tubules • sarcoplasmic reticulum not well developed 14

Smooth muscle contraction • Smooth muscle contraction is not controlled by the binding of Ca 2+ to the troponin complex as it is in cardiac and skeletal muscles • Calmodulin = intracellular second messenger that binds Ca 2+

Smooth Muscle Contraction: Mechanism Figure 12 -28: Smooth muscle contraction

Smooth Muscle Relaxation: Mechanism Figure 12 -29: Relaxation in smooth muscle

Cardiac Muscle and Heart Function • Fibers are branched; connect to one another at intercalated discs. The discs contain several gap junctions • SR is less abundant than in skeletal muscle, but greater in density than smooth muscle • Sarcolemma has specialized ion channels • Fibers are not anchored at ends; allows for greater sarcomere shortening and lengthening

• There are different types of cardiac muscle cells ranging from the pacemaker cells in the sinoatrial node to the atrial and ventricular cells that produce the contraction of the heart chambers Pacemaker Cells Have unstable resting membrane potential (pacemaker potential) that slowly drifts upwards until it reaches a threshold and activates and action potential

p i n e p h r i n e , n o r e p i n e p h r i n e ( L e v o p h e d ) ) i n c r e a s e s l o w i n w a r d C a Phase 0: Activation of fast Na+ channel-- initial depolarization; Phase 1: Partial repolarization; K+ efflux Phase 2: Ca 2+ entry with continued K+ efflux = "plateau phase". Initial Ca 2+ influx through slow L- type Ca 2+ Phase 3 : This phase is dominated by K+ efflux, i. e. repolarization. Phase 4 : This phase is between action potentials) 2 + c u r r e n t s a m e c h a n i s m b y w h i c h s y m p a t h o m i m e t i c

REFRACTORY PERIOD OF SKELETAL AND CARDIAC MUSCLE

Excitation-contraction coupling

3 types of muscle Smoot 11/2/2020 Cardiac Skeletal 23