- ✓ SMOOTH MUSCLE TYPES
- ✓ STRUCTURE
- ✓ SMOOTH MUSCLE CONTRACTION
SMOOTH MUSCLE TYPES
There are two types of smooth muscle cells.
- Visceral or single unit type.
- Multiunit type.
Single unit type has automatic myogenic activity and it is increased by stretch. These muscle cells show cell to cell propagation of electrical and mechanical activity. This type is found in the uterus, ureter and the intestine where these act as pacemakers. These cells show spontaneous depolarisation which leads to an action potential followed by repolarisation. In this type, the cells are connected to one another by gap junctions which form low resistant bridges.
Multiunit type does not have inherent automatic activity. They are activated by the motor sympathetic and parasympathetic nerves. These are mostly found in the vascular wall, iris and piloerectores. Every cell has its innervation and respond to neural stimulation and influenced by humoral factors.
The smooth muscle also contain afferent nerve endings which are stimulated by pressure or tension. Excessive tension may give rise to pain. For example, Renal colic, Biliary colic, Intestinal colic and Labour pains.
Smooth muscle or plain muscle or unstriped muscle is found in number of organs and tissues of our body namely gastrointestinal wall, gallbladder, bile duct, blood vessels, ureters, urinary bladder, uterus and bronchi and bronchiole. Smooth muscle cells are innervated by the autonomic nervous system.
STRUCTURE (FIG. II-25)
These are spindle shaped cells. Their diameter is about 5–10 µ and their length is about 50-500µ. There are no striations. Actin, myosin and tropomyosin are present. There is no troponin in the smooth muscle. These muscle proteins are irregularly shaped and randomly arranged into bundles of inter digitating thick and thin filaments. The‘T’ tubules are absent. In some cells these are represented by caveoli. There are less number of mitochondria anddepend upon glycolysis for their metabolic needs (Fig. II-25).
SMOOTH MUSCLE CONTRACTION
The average membrane potential is −50 mv. The excitation contraction coupling in smooth muscle is a slow process. Calcium initiates excitation contraction coupling. Calcium binds to calmodulin and the resulting complex activates myosin light chain kinase (MLCK). This enzyme catalyses the phosphorylation of myosin. Actin then slides on myosin producing contraction. The release of Ca++ is mediated by inositol-triphosphate (IP3). Another enzyme myosin phosphatase removes the phosphate from myosin. The amount of myosin active at any time depends on the relative activity of MLCK and phosphatase. Relaxation occurs when the intracellular Ca++ falls and the MLCK becomes inactive. At this point phosphatase removes phosphate from the myosin light chain protein and the cross bridges cycling ceases. The attached but dephosphorylated cross-bridges are called “latch” bridges. These are responsible for tone in the smooth muscle. Since the latch cross-bridges do not cycle or cycle very slowly, they do not use ATP much. They sustain the tone developed (Fig. II-26).
The events occurring during contraction and relaxation of smooth muscle
Difference between Skeletal and Smooth Muscle Contractions
- Cross-bridge cycling is slow in smooth muscle
- Cross-bridges remain attached for more time in smooth muscle
- These are called latch-bridges.
- The energy required is less and derived from glycolysis
- Slow contraction and slow relaxation
- Smooth muscle organs show the feature of receptive relaxation or adaptation.
- Smooth muscle activity is influenced by nervous, hormonal and metabolic factors.
Myosin in smooth muscle must be phosphorylated for activation of the myosin head ATPase. In the skeletal muscle phosphorylation is not necessary for activation of ATPase and the contraction is triggered by the binding of calcium to troponin.
Neuromuscular Endings in the Smooth Muscle
There is no organised neuromuscular junction in the smooth muscle as in skeletal muscle. Postganglionic autonomic nerve endings give rise to multiple branches which run along 57the muscle membrane and groove the membrane surface. These endings present a beaded appearance. These beads are called varicosities and contain transmitter vesicles. There may be even 20,000 varicosities in each neuron. On stimulation the synaptic transmitter is released from multiple points. The vesicles of one neuron make contact with vesicles of another neuron or with smooth muscle membrane and then passes on to make similar contacts with other cells. This type of synaptic contact is called synapse enpassant. Similar synaptic endings are seen in the cardiac muscle also (Fig. II-27).
