VPB 121: Veterinary Physiology - II (2+1)

ACTION POTENTIAL IN SMOOTH MUSCLE

Slow wave rhythm

• Some smooth muscles are self excitatory ie; elicite AP with out an extrinsic stimuli; due to waxing and waning of pumping of Na⁺ outward – if rapid sudden decrease in electronegativity, if slow gradual decrease in electronegativity and once reaches – 35 mv causing action potential. ie., most of the peaks of slow waves, one or more AP occurs leading to rhythmical contraction of smooth muscle (Pacemaker waves). This type of activity is prominent in gut, ureter etc.
• Excitation of visceral smooth muscle by stretch also possible, here slow wave potential and decrease of membrane potential caused by stretch are the causes. Eg., gut loaded by ingesta --------> stretch --------> peristalsis; Uterus at full term -------> delivery.
• In case of multi unit smooth muscle, depolarization is cased by neurotransmitters contraction with out generation of its own action potential. Because the fibers are two small to generate an action potential where as in visceral smooth muscle 30 to 40 smooth muscle fibers getting depolarized simultaneously ie., self propagating AP ensures. Yet even without an AP in the multiunit smooth muscle fibers the local depolarization (caused by NT) spreads itself electronically over the entire fiber and it is all that is needed to cause the muscle contraction.
• Mechanism of Ca²⁺ in smooth muscle contraction: An effective troponin system is lacking Ca²⁺ itself excites contraction in smooth muscle fibers by activating ATPase activity of myosin head.
• Local tissue factors and hormones causing smooth muscle contraction with out AP generation: In fact probably half or most of all smooth muscle contraction is initiated not AP but by stimulatory factors like non – nervous and non-AP stimulatory factors like local factors & hormones. Eg., Contraction of met-areterioles, pre-capillary sphincters where virtually nervous connection is not there (smaller the size, little or no nervous supply), local changes like lack of O₂,excising of CO₂, excess H⁺ cause vasodilatation. Hormones such as nor-epinephrine, epinephrine, Ach, angiotensin, vasopressin, oxtocycin, serotonin, histamine can cause vaso-constriction/dilatation depending upon the receptes in the smooth muscle (Excitatory receptor or Inhibitory receptor).
• Some of the hormones viz., nor-epinephrine, vasopressin and angiotensin have powerful excitatory effect causing spasm, of blood vessels for hours together.

Action potential in smooth muscles

• The smooth muscles have relatively lower resting membrane potential and short action potential (5 - 10 m sec.) than the skeletal and cardiac muscles. The resting potential of the smooth muscle cells range from – 50 mV to – 60 mV. The action potential is regulated by Ca⁺⁺ pump.
• Action potential in visceral smooth muscle occurs in two forms: (1) spike potential (2) action potential with plateau.
• Spike potential: Typical spike potential as in skeletal muscle occurs in smooth muscles which is elicited by electrical stimulation, hormones, transmitter substance from nerves, by stretch or spontaneously.
• Action potential with plateau occurs in visceral smooth muscle cells similar to cardiac cells to prolong the period of contraction in smooth muscles of ureters, uterus and vascular smooth muscles. In smooth muscle cells, the voltage-gated Ca⁺⁺ channels are comparatively more in number than the Na⁺ channels. Ca⁺⁺ generate the action potential in the smooth muscles. Since the Ca⁺⁺ channels are slow to open and close, the on set of smooth muscle contractions are slower.