Paraphrasing and proofreading the content -

The rat anococcygeus muscle is present deep within the pelvis of the coccygeal vertebrae, in the midline of pelvic cavity. The pair of muscles is found to be lying behind and to either side of the colon specifically. Joining these muscles forms a ventral bar which is known to be few centimetres from the anus, at the region of the terminal colon.A physiological function of the anococcygeus muscle has yet to be elucidated.

For pharmacological studies this muscle is very useful as its certain properties make it suitable to give a much better response to drugs than other muscles in the rat body. The special property include: the muscle being in pair, as it can used for control and test preparations as well as the arrangement of the muscle i.e. some smooth muscles can be circular, helical or longitudinal making the response to drug difficult to interpret but rat anococcygeus muscle are in parallel running in the same direction giving a simple response to drug that can easily be interpreted.

The ancoccygeous muscle is innervated by motor sympathetic fibers (where the transmitter is norepinephrine) and consists of a dense adrenergic innervation, it has no cholinergic innervation, however it does have non-adrenergic and non-cholinergic, NANC inhibitory innervation, the inhibitory transmitter is recognized as nitric oxide (NO). L- arginine is involved in the release of nitric oxide, certain drugs have been developed to act as competitive inhibitors of nitric oxide synthase where they have the ability to selectively block endothelium dependent relaxation. L-arginine has a similar molecular structure to these drugs and an example of such drug is L-NAME. Tetrodoxin (TTX) is also used as an inhibitor of NO synthase.

The purpose of this experiment was to observe and evaluate the responses of rat anococcygenus muscle to drugs such as acetylcholine and noradrenaline, as well as the responses to stimulation of nerves by electrical field stimulation by contraction and relaxation.

Discussion

In vitro rat anococcygeus is an appropriate smooth muscle preparation that is useful both for teaching and research purposes.

Acetylcholine (Ach) was used to ................. . Ach is released from the presynaptic neuron into the synaptic cleft and once it reaches the synaptic gap Ach binds to the presynaptic receptors. Presynaptic inhibition or activationcauses presynaptic cholinergic neurons auto modulation. Ach can bind to postsynaptic receptors and activate them causing a cholinergic response. Acetylcholinesterase is an enzyme which acts on the acetylcholine and triggers its degradation into choline and acetyl coenzyme A, hence ending its effect.  The two important families of Ach receptors include the muscarinic receptors (G-protein receptors) and the nicotinic receptors (ligand gated ion channels).

The diagram above shows both sympathetic and parasympathetic branches being modulated at the preganglionic level by acetylcholine (neurotransmitter). Ach is allowed to bind to the nicotinic receptors at the autonomic ganglia which in turn trigger the release of norepinephrine; however this is only possible when the sympathetic synapse is stimulated. Acetylcholine binding to tissue muscarinic receptors will produce a parasympathetic or cholinergic response. There are five subclasses of muscarinic receptors and activation of the M3 receptors at the smooth muscle level produces responses on a variety of organs.

Tyrosine synthesizes norepinephrine as a precursor and it's packed into synaptic vesicles. It is released into the synaptic cleft where it acts on the alpha-adrenergic receptors to cause a contraction of the smooth muscle and this is followed by the signal termination, occurring from either the degradation of norepinephrine or by the uptake by surrounding cells.

cGMP-Coupled Signal Transduction

A third mechanism that is very important in regulating vascular smooth muscle tone is the nitric oxide (NO)-cGMP system. Vascular endothelial cells normally produce NO, which diffuses from endothelial cells to adjacent smooth muscle cells where it activates guanylyl cyclase leading to increased formation of cGMP and vasodilation. The precise mechanisms by which cGMP relaxes vascular smooth muscle is unclear; however, cGMP can activate a cGMP-dependent protein kinase, inhibit calcium entry into the vascular smooth muscle, activate K+ channels, and decrease IP3.

Acetylcholine (ACh), whether released by cholinergic autonomic nerves or exogenously administered, binds to muscarinic receptors on the vascular endothelium (muscarinic receptors in coronary vessels), which stimulates the formation and release of NO as described above to produce vasodilation.

For pharmacological and physiological studies to evaluate the involvement of adrenergic neurotransmission guanethedine is regularly used. It is taken up into adrenergic neurons and, then it blocks the nerve conduction at the preterminal axons, which inhibits the re-uptake of noradrenaline into the nerve endings TTX completely blocks the sodium channels? And L-NAME NO synthase inhibitors are partially involved in the inhibition; this suggests that they occur from the activation of nicotinic acetylcholine receptors.

Since the effect of both NO synthase inhibitors was of the same magnitude, we can exclude any inhibitory effect of L-NAME on cholinergic neural responses, as has been suggested previously.