Structure of smooth muscle

Smooth muscle cells are small and spindle shaped (thin and elongated; see Table 12.1). Similar to skeletal muscle, the contractile apparatus in smooth

Table 12.1 Comparison of Skeletal and Smooth Muscle

Feature

Skeletal muscle

Multiunit smooth muscle

Single unit smooth muscle

Location

Attached to bones; openings of

Large blood vessels; eyes;

Walls of hollow organs of digestive,

some hollow organs (sphincters)

hair follicles

reproductive, and urinary tracts;

small blood vessels

Thick filaments

Myosin

Myosin

Myosin

Thin filaments

Actin, tropomyosin, troponin

Actin, tropomyosin

Actin, tropomyosin

Filament arrangement

Sarcomeres

Diamond-shaped lattice

Diamond-shaped lattice

Microscopic appearance

Striated

Smooth

Smooth

Control

Voluntary

Involuntary

Involuntary

Innervation

Somatic nervous system

Autonomic nervous system

Autonomic nervous system

Contraction

Neurogenic

Neurogenic

Myogenic

Role of nervous system

Initiate contraction

Initiate contraction

Modify contraction

Morphology

Large, cylindrical

Small, spindle-shaped

Small, spindle-shaped

Transverse tubules

Yes

No

No

Sarcoplasmic reticulum

Well developed

Very little

Very little

Source of calcium

Sarcoplasmic reticulum

Extracellular fluid (most);

Extracellular fluid (most);

sarcoplasmic reticulum

sarcoplasmic reticulum (some)

(some)

Site of calcium binding

Troponin

Calmodulin

Calmodulin

Function of calcium

Reposition troponin/tropomyosin

Phosphorylate and activate

Phosphorylate and activate myosin

to uncover myosin binding sites

myosin to bind with actin

to bind with actin

on actin

Regulation of tension

Alter number of contacting motor

Alter number of contracting

Alter intracellular Ca++

development

units; frequency of nerve

muscle cells; alter

concentration

stimulation

intracellular Ca++

concentration

Length-tension relationship

Narrow

Broad

Broad

muscle consists of thick filaments composed of myosin and thin filaments composed of actin. However, in contrast to skeletal muscle, these filaments are not organized into sarcomeres. As such, this muscle has no striations, resulting in a "smooth" appearance.

Because there are no sarcomeres in smooth muscle, there are no Z lines. Instead, the actin filaments are attached to dense bodies. These structures, which contain the same protein as Z lines, are positioned throughout the cytoplasm of the smooth muscle cell as well as attached to the internal surface of the plasma membrane. Myosin filaments are associated with the actin filaments, forming contractile bundles oriented in a diagonal manner. This arrangement forms a diamond-shaped lattice of contractile elements throughout the cytoplasm. Consequently, the interaction of actin and myosin during contraction causes the cell to become shorter and wider.

The action potential easily penetrates all regions of these small cells. Therefore, smooth muscle does not have transverse tubules. Furthermore, smooth muscle cells have very little sarcoplasmic reticulum, so intracellular storage of calcium is limited. Instead, the calcium needed for contraction is obtained primarily from the extracellular fluid. The influx of Ca++ ions through their channels in the cell membrane stimulates the release of a small amount of Ca++ ions from the sarcoplasmic reticulum.

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