Skeletal muscle autonomic regulation

Although skeletal muscle comprises the bulk of muscle tissue in the body, smooth muscle is far more important in terms of homeostasis. Most smooth muscle is found in the walls of tubes and hollow organs. Contraction and relaxation of the smooth muscle in these tissues regulates the movement of substances within them. For example, contraction of the smooth muscle in the wall of a blood vessel narrows the diameter of the vessel and leads to a decrease in the flow of blood through it. Contraction of the smooth muscle in the wall of the stomach exerts pressure on its contents and pushes these substances forward into the small intestine. Smooth muscle functions at a subconscious level and is involuntary. It is innervated by the autonomic nervous system, which regulates its activity. 

Autonomic nerves can regulate coronary arteriolar tone. Acetylcholine released from postganglionic parasympathetic nerves relaxes coronary arteriolar smooth muscle via the NO/cGMP pathway in humans as described above. Damage to the endothelium, as occurs with atherosclerosis, eliminates this action, and acetylcholine is able to contract arterial smooth muscle and produce vasoconstriction. Skeletal muscle receives sympathetic cholinergic vasodilator nerves, but the view that acetylcholine caused vasodilation in this vascular bed has not been verified experimentally. Moreover, NO, rather than acetylcholine, may be released from neurons. However, this vascular bed responds to exogenous choline esters because of the presence of M3 receptors on endothelial and smooth muscle cells. 

The efferent portion of the peripheral nervous system can be further divided into two major functional subdivisions, the somatic and autonomic systems (see Figure 3.1). The somatic efferents are involved in voluntarily controlled functions such as contraction of the skeletal muscles in locomotion. The autonomic system functions involuntarily to regulate the everyday needs and requirements of the body without the conscious participation of the mind. It is composed primarily of visceral motor (efferent) neurons that innervate smooth muscle of the viscera, cardiac muscle, vasculature and the exocrine glands. 

The nervous system (Fig. 1) consists of the central nervous system (CNS with brain and spinal cord) and the peripheral system (with afferent sensory and efferent motor nerves), which regulate all aspects of bodily function and is staggering in its complexicity. Another distinction is between the somatic (sensoric system, innervation of skeletal muscles) and the autonomic nervous system (heart and blood circulation respiration motility of the gastrointestinal tract, smooth muscles of gall and urinary bladder, ureter, uterus, secretion of glands). The autonomic nervous system is further divided into a sympathetic and... 

In the peripheral nervous system, norepinephrine is an important neurotransmitter in the sympathetic branch of the autonomic system. Sympathetic nerve transmission operates below the level of consciousness in controlling physiological function of many organs and tissues of the body. The sympathetic system plays a particularly important role in regulating cardiovascular function in response to postural, exertional, thermal, and mental stress. With sympathetic activation, the heart rate is increased, peripheral arterioles are constricted, skeletal arterioles are dilated, and the blood pressure is elevated. In addition, sympathetic nerve stimulation dilates pupils inhibits smooth muscles of the intestines, bronchi, and bladder and closes the sphincters. Sympathetic signals work in balance with the parasympathetic portion of the autonomic nervous system to maintain a stable internal environment. 

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