In this article we will discuss about the properties and factors affecting the activity of smooth muscles in human body.
Properties of Smooth Muscles:
Broadly speaking, properties of the smooth muscle are same as those of the skeletal muscle, with the following differences:
i. Excitability and Contractility:
Smooth muscle is less excitable. Contraction is slow, and worm-like. All the periods are longer. The latent period varies from 0.2-2.0 second; period of contraction from 0.5 to several seconds the period of relaxation is proportionately longer than that of contraction. The refractory period is also much longer.
One more peculiarity is that, a particular stimulus (electrical or mechanical) may cause either contraction or relaxation. If the muscle is relaxed, the stimulus will cause contraction; but if it is already contracted, the stimulus will relax the muscle. The effect of temperature is also different. Moderate warming causes relaxation and cooling contraction.
ii. Rhythmicity:
It has automatic inherent rhythmicity. When slightly stretched most varieties of smooth muscles will exhibit rhythmic contractions. These contractions are independent of all nerves and are purely myogenic. The ‘stretch’ acts as a stimulus causing the relaxed muscle to contract and the contracted muscle to relax. In this way as long as the ‘stretch’ is maintained, the alternate rhythmic contraction and relaxation will continue.
iii. Conductivity:
The wave of contraction is propagated along the muscle fibre but the rate of propagation is much slower. One peculiarity is that in the smooth muscle the stimulus travels from fibre to fibre through the nexus and the wave of contraction is propagated through the whole muscle sheet. The electric changes during contraction are also peculiar because, two negative potential changes are found to accompany each contraction.
iv. Tonicity:
Smooth muscle remains in a state of light tension.
Factors Affecting the Activity of Smooth Muscle:
i. Temperature:
Heat diminishes both excitability and tonus. Cold, on the other hand, increases tonus.
ii. Salt Balance:
The activity of the smooth muscle is modified by K; Ca ratio. Ca++ influences the Na+ permeability mechanism and increases the rate of rise of spike; K+ also increases the tone of the smooth muscle.
iii. pH:
Increase in pH lifts tonus and rhythmic activity decrease in it produces quiescence and relaxation.
iv. Previous State of Activity:
A stimulus which generally causes contraction will induce relaxation in a smooth muscle if it is already in a state of high tonus.
v. Hormones:
The secretion in a smooth gland has profound influence over the activity of smooth muscle. Contraction of uterus on stimulation is dependent on the state of oestrus of the organ. Acetylcholine stimulates and adrenaline depresses the tone of muscle. Posterior pituitary extract also increases tonic contraction of the uterine and intestinal muscles.