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