24. Smooth muscle tissue. Structural, ultrastructural, chemical and functional characteristics.

Smooth muscle tissue is unstriated and involuntary, it has cellular type of organisation, and originates from mesenchyme. Their cells are elongated, spindle–like cells and are connected by intercellular contacts; desmosomes, nexus. The nucleus is centrally located.

It is found in the blood, lymph vessels & internal viscera. Its function is to generate force to contract muscle for vital activities including peristalsis in the gut and emptying of the urinary bladder. 

Smooth muscle tissue consists of mono-nucleated cells that are elongated and tapered. They are organised into sheets, arranged in parallel bundles. In order to achieve the closest packing, adjacent cells must overlap and are bound by loose connective tissue.

Their cytoplasm is called sarcoplasm that contains 3 sets of filaments. 

These include, thick-myosin filaments, thin-actin filaments and intermediate filaments such as desmin. 

The sarcoplasm also consists of dense bodies that are scattered around the cell and link intermediate filaments (desmin) to the sarcolemma. This allows the intermediate filaments to harness the force generated.

The sarcolemma is the plasma membrane of the muscle fibre which is enveloped by the external lamina, an outer layer of glycoproteins and reticular fibres.

The sarcolemma and its transverse tubules, play a role in initiation of contraction.

Caveolae are also found & provide a large surface area for the release of calcium ions which are required for contraction.

When calcium ions enter muscle cells, calcium binds to calmodium which enables myosin to interact with actin. Smooth muscle tissue does not contain troponin so cross bridges form right away with the help ATP.

Smooth muscle cells have two types of intercellular junctions; zonula adherens that provide adhesion and anchor the cell during contraction and gap junctions that provide electrical coupling.

Smooth muscle is regulated by the autonomic nervous system. Axons containing synaptic vesicles come into contact with the sarcolemma of the smooth muscle cells. The synaptic vesicles contain a neurotransmitter. The presence of gap junctions allows the spread of excitation to adjacent cells therefore contractions are synchronised. 

Smooth muscle contraction is slower and can resist fatigue for much longer than the other types of muscle tissue.