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