Smooth Muscle Physiology

Muscular System Functions Body movement (Locomotion) Maintenance of posture Respiration –Diaphragm and intercostal contractions Communication (Verbal and Facial) Constriction of organs and vessels –Peristalsis of intestinal tract –Vasoconstriction of b.v. and other structures (pupils) Heart beat Production of body heat (Thermogenesis)

Properties of Muscle Excitability: capacity of muscle to respond to a stimulus Contractility: ability of a muscle to shorten and generate pulling force Extensibility: muscle can be stretched back to its original length Elasticity: ability of muscle to recoil to original resting length after stretched

Types of Muscle Skeletal –Attached to bones –Makes up 40% of body weight –Responsible for locomotion, facial expressions, posture, respiratory movements, other types of body movement –Voluntary in action; controlled by somatic motor neurons Smooth –In the walls of hollow organs, blood vessels, eye, glands, uterus, skin –Some functions: propel urine, mix food in digestive tract, dilating/constricting pupils, regulating blood flow, –In some locations, autorhythmic –Controlled involuntarily by endocrine and autonomic nervous systems Cardiac –Heart: major source of movement of blood –Autorhythmic –Controlled involuntarily by endocrine and autonomic nervous systems

Connective Tissue Sheaths Connective Tissue of a Muscle –Epimysium. Dense regular c.t. surrounding entire muscle Separates muscle from surrounding tissues and organs Connected to the deep fascia –Perimysium. Collagen and elastic fibers surrounding a group of muscle fibers called a fascicle Contains b.v and nerves –Endomysium. Loose connective tissue that surrounds individual muscle fibers Also contains b.v., nerves, and satellite cells (embryonic stem cells function in repair of muscle tissue Collagen fibers of all 3 layers come together at each end of muscle to form a tendon or aponeurosis.

Nerve and Blood Vessel Supply Motor neurons –stimulate muscle fibers to contract –Neuron axons branch so that each muscle fiber (muscle cell) is innervated –Form a neuromuscular junction (= myoneural junction) Capillary beds surround muscle fibers –Muscles require large amts of energy –Extensive vascular network delivers necessary oxygen and nutrients and carries away metabolic waste produced by muscle fibers

Muscle Tissue Types

Smooth Muscle Fusiform cells One nucleus per cell Nonstriated Involuntary Slow, wave-like contractions

Smooth Muscle Cells are not striated Fibers smaller than those in skeletal muscle Spindle-shaped; single, central nucleus More actin than myosin No sarcomeres –Not arranged as symmetrically as in skeletal muscle, thus NO striations. Caveolae: indentations in sarcolemma; –May act like T tubules Dense bodies instead of Z disks –Have noncontractile intermediate filaments

Smooth Muscle Grouped into sheets in walls of hollow organs Longitudinal layer – muscle fibers run parallel to organ’s long axis Circular layer – muscle fibers run around circumference of the organ Both layers participate in peristalsis

Smooth Muscle Is innervated by autonomic nervous system (ANS) Visceral or unitary smooth muscle –Only a few muscle fibers innervated in each group –Impulse spreads through gap junctions –Who sheet contracts as a unit –Often autorhythmic Multiunit: –Cells or groups of cells act as independent units –Arrector pili of skin and iris of eye

Smooth Muscle Cell

Smooth Muscle Contraction: Mechanism

Smooth Muscle Relaxation: Mechanism

Excitation-Contraction Coupling: (below)

Single-Unit Muscle

Properties of Single-Unit Smooth Muscle –Gap junctions –Pacemaker cells with spontaneous depolarizations –Innervation to few cells –Tone = level of contraction without stimulation –Increases/decreases in tension –Graded Contractions No recruitment Vary intracellular calcium –Stretch Reflex Relaxation in response to sudden or prolonged stretch

Multi-Unit Muscle

Multi vs. Single-Unit Muscle

Comparisons Among Skeletal, Smooth, and Cardiac Muscle

Disorders of Muscle Tissue Muscle tissues experience few disorders –Heart muscle is the exception –Skeletal muscle – remarkably resistant to infection –Smooth muscle – problems stem from external irritants