examples of multi unit smooth muscles arc in the (i) nictitating membrane, and (ii)

examples of multi unit smooth muscles arc in the (i) nictitating membrane, and (ii) pilomotor muscles in the hair follicle. [Because of complete absence of the spontaneous activity and because of the fact that it is very well developed the nictitating membrane of cat is a common experimental tool for the pharmacologists for studies of various drugs on autonomic nerves.smooth muscles]. However, some smooth muscles exist, which cannot properly fit into any one of these two types. Best example is arteriolar smooth muscles, which like the unitary, smooth muscles show autorhythmicity but shows some properties of multiunit muscles too. This type of overlapping of characteristics is seen in some other smooth muscles too Neuro effector junction As already stated earlier in this section, atypical motor end plate is absent in the junctional legion of the supplying autonomic nerve and the smooth muscle Instead, what is found is stated below: The smooth muscles are supplied by post ganglionic autonomic nerve fibers. As a rule both the sympathetic and parasympathetic fibers supply the muscle. The junctional region between the motor autonomic nerve and the smooth muscle may best be called, 'neuro effector junction'. [The term 'neuro muscular junction'should be reserved for the junctional region of the skeletal muscle and somatic nerve this ensures clarity]. The post ganghonic fiber (which is non myelinated) divides into many terminal branches. An individual branch supplies one muscle cell. Some smooth muscle fibers (cells) do not receive any nerve supply at all. In the smooth muscles of the eye (iris/ciliary muscles) each muscle cell is supplied by one nerve twig whereas many smooth cells of uterus are not directly connected with nerve fiber In such uterine cells the action potential (AP) spreads through the gap junctions That is the innervated cell gets a stimulus via its nerve supply develops an AP the AP enters (via gap junction) to a neighboring smooth cell which is not innervated the neighboring cell is stimulated. As the nerve teiminal approaches the smooth muscle cell to be supplied, it develops some swellings called Varicosities'. These swellings are due to the presence of vesicles -a-within the nerve terminal The vesicles are filled with the neurotransmitters. Between the nerve terminal and the muscle membrane, there exists a gap (cf. skeletal muscle neuromuscular junction). the width of the gap being between 20 nm to 100 nm. Throughout the vvhole surface of the sarcolemma of the smooth muscle numerous receptors are present. These receptors can combine with the chemical transmitter and act accordingly. When an AP develops in the supplying nerve the AP proceeds down the nerve towards its neuro effector junction (NEJ). As it passes over the zone of the varicosities, the vesicles rupture releasing the neurotransmittiter---->combination of the neurolransmitter with the receptor occurs -.-> AP on the sarcolemma of the smooth muscle develops -----> contraction of the muscle follows. Mechanism of contraction of the smooth muscles The major steps are same as those of skeletal muscle. There is development of cross bridge followed by sliding of the actin filaments over the myosin filaments and the muscle shortens. However, there are some differenc es too: (i) the amount of actin and myosin/gm of smooth muscle is only some 10% of that of the skeletal muscles. Because of this the smooth muscle can never contract violently and exert great force, as can be done by the skeletal muscles. (ii) the sarcoplasmic reticulum (L system of tubules) are very poorly developed in the smooth muscles During the excitation contraction coupling. Ca++ ions are provided by the L tubules in the skeletal muscle but here (because of the fact that the L tubules are practically not found at all). Ca++ have to be provided from the outside (i. e. the EOF) via the T tubules. So Ca++ must be present in the ECF and in sufficient quantity to ensure a satisfactory contraction of the smooth muscle. [This can be shovvn in the isolated organ bath experiments shovvn to the students If the bathing fluid (Tyrode/Ringer Lock) contains even a slightly lower concentration of Ca++ the contraction fails to develop in the smooth muscle] (iii) the speed of the smooth muscle contraction is much slower than that of the skeletal muscle. Pacemaker The student may, at this stage, recall properties of the heart muscles (chap.2. sec. V). Normally SA node (and abnormally other areas of the junctional tissues) acts as the pace maker. That is. in SA node an AP can develop spontaneously (the term spontaneous means, 'wthout stimulation') and this AP can spread to other cells. Such pace making cells exist also in the smooth muscles, and at rest, develop 'pace maker potential' a phenomenon which is same as thediastohc depolarization of the cardiac muscle (chap. 2, section V). In short during rest as is the case of all other cells, these pace maker cells are -ve inside and +ve outside and has a resting membrane potential (rmp) of about say. -70 mv. But during rest, spontaneous (and mild) depolarization, occurs in these cells, as a result of which the resting membrane potential (rmp) drops, (i. e. moves towards zero) and attains a value of say -50mv. When the rmp thus reaches -50 mv. which is the critical level firing begins and an AP develops General properties of the smooth muscles By now, it should be clear that smooth muscles (SM) are rather heterogenous tissues and there are different tvpes of the SMs, e.g. single unit multiple unit etc. having rather