Autonomic nervous system tissues

The afferent division carries sensory information toward the CNS and the efferent division carries motor information away from the CNS toward the effector tissues (muscles and glands). The efferent division is further divided into two components (1) the somatic nervous system, which consists of motor neurons that innervate skeletal muscle and (2) the autonomic nervous system that innervates cardiac muscle, smooth muscle, and glands. 

Figure 9.1 The autonomic nervous system and its effector organs. The efferent pathways of this system consist of two neurons that transmit impulses from the CNS to the effector tissue, preganglionic neuron (solid line), and postganglionic neuron (dashed line). As illustrated, most tissues receive nervous input from both divisions of the ANS the sympathetic and the parasympathetic.

Adrenal medulla. Derived from neural crest tissue, the adrenal medulla forms the inner portion of the adrenal gland. It is the site of production of the catecholamines, epinephrine and norepinephrine, which serve as a circulating counterpart to the sympathetic neurotransmitter, norepinephrine, released directly from sympathetic neurons to the tissues. As such, the adrenal medulla and its hormonal products play an important role in the activity of the sympathetic nervous system. This is fully discussed in Chapter 9, which deals with the autonomic nervous system. 

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. 

The autonomic nervous system (ANS) modifies contractile activity of both types of smooth muscle. As discussed in Chapter 9, the ANS innervates the smooth muscle layer in a very diffuse manner, so neurotransmitter is released over a wide area of muscle. Typically, the effects of sympathetic and parasympathetic stimulation in a given tissue oppose each other one system enhances contractile activity while the other inhibits it. The specific effects (excitatory or inhibitory) that the two divisions of the ANS have on a given smooth muscle depend upon its location. 

Psychological stress may influence the immune system by activation of the hypothalamic-pituitary-adrenal (HPA) axis and the sympathetic-adrenal-medullary axis (SAM). The well-described innervation of primary and secondary lymphoid tissues by the autonomic nervous system also has been implicated in stress-related modulation of the immune response. These pathways operate by producing biological mediators that interact with and affect cellular components of the immune system.13... 

The term central nervous system is sometimes used as a synonym for the brain, but it also includes the spinal cord. Indeed, the word system implies the entirety of the tissues working together to achieve a single function. Usage, however, has validated its division into the central and peripheral nervous systems, and even the subdivision of the latter into the autonomic nervous system and the voluntary nervous system. 

While the dose-limiting toxicity for vinblastine usually is leukopenia, that for vincristine is most commonly neurotoxicity (58). Prominent manifestations of neurotoxicity are loss of the Achilles tendon reflex, paresthesias, loss of muscle strength (e.g., in the foot and wrist), and ataxia. Constipation and abdominal pain may occur and are thought to result, at least in part, from actions on the autonomic nervous system. Leukopenia and stomatitis are possible effects of vincristine treatment, but they occur relatively infrequently. Alopecia occurs with vincristine at a frequency comparable to that observed with vinblastine, and vincristine also is a potent tissue irritant. Vincristine may produce a syndrome of inappropriate secretion of antidiuretic hormone, and some manifestations of neurotoxicity, such as seizures, have been considered to be due to electrolyte disturbances associated with the relative excess of the antidiuretic hormone (58). 

The 1998 OECD test guidelines for the oral 28-/90-day studies (see Table 4.12) examine a number of simple nervous system endpoints, e.g., clinical observations of motor and autonomous nervous system activity, and histopathology of nerve tissue. It should be recognized that the standard 28-/90-day tests measure only some aspects of nervous system stmcture and function, while other aspects, e.g., learning and memory and sensory function is not or only superficially tested. Primarily the standard 28-/90-day tests are intended as a screening for neurotoxicity and depending on the results, further testing may be needed. 

The nervous system is divided into two parts the central nervous system (CNS) and the peripheral nervous system (PNS). The CNS consists of the brain and spinal cord. The PNS consists of all afferent (sensory) neurons, which carry nerve impulses into the CNS from sensory end organs in peripheral tissues, and all efferent (motor) neurons, which carry nerve impulses from the CNS to effector cells in peripheral tissues. The peripheral efferent system is further divided into the somatic nervous system and the autonomic nervous system. The effector cells innervated by the somatic nervous system are skeletal muscle cells. The autonomic nervous system innervates three types of effector cells (1) smooth muscle, (2) cardiac muscle, and (3) exocrine glands. While the somatic nervous system can function on a reflex basis, voluntary control of skeletal muscle is of primary importance. In contrast, in the autonomic nervous system voluntary control can be exerted, but reflex control is paramount. 

Many visceral organs are innervated by both divisions of the autonomic nervous system. In most instances, when an organ receives dual innervation, the two systems work in opposition to one another. In some tissues and organs, the two innervations exert an opposing influence on the same effector cells (e.g., the sinoatrial node in the heart), while in other tissues opposing actions come about because different effector cells are activated (e.g., the circular and radial muscles in the iris). 

B. Skeletal muscle is innervated by the somatic nervous system. All other choices are tissues that are innervated by the autonomic nervous system. 

The nervous system is conventionally divided into the central nervous system (CNS the brain and spinal cord) and the peripheral nervous system (PNS neuronal tissues outside the CNS). The motor (efferent) portion of the nervous system can be divided into two major subdivisions autonomic and somatic. The autonomic nervous system (ANS) is largely independent (autonomous) in that its activities are not under direct conscious control. It is concerned primarily with visceral functions such as cardiac output, blood flow to various organs, and digestion, which are necessary for life. The somatic subdivision is largely concerned with consciously controlled functions such as movement, respiration, and posture. Both systems have important afferent (sensory) inputs that provide information regarding the internal and external environments and modify motor output through reflex arcs of varying size and complexity. 

The eye is a good example of an organ with multiple autonomic nervous system (ANS) functions, controlled by several autonomic receptors. As shown in Figure 6-9, the anterior chamber is the site of several autonomic effector tissues. These tissues include three muscles (pupillary dilator and constrictor muscles in the iris and the ciliary muscle) and the secretory epithelium of the ciliary body. 

The SNS and PNS counteract and thereby balance each other in the regulation of the autonomic functions of the body. Should one system become inoperable the other will dominate. Table 5 lists the responses of various tissue and organs to SNS and PNS stimulation. Figure 24 summarizes the details of the autonomic nervous system. 

CRITICAL ASSESSEMENT OF THE METHOD In general pharmacological studies during anesthesia should be assessed appropriately due to the possible interaction between the test compound and the used anesthetic as well as due to the reduced tone of the autonomic nervous system. Enteral administration of the candidate compound should be avoided, because enteral absorption of the test compound might be reduced due to the impaired intestinal motility during anesthesia. With respect to the effect of the aesthetic compound itself on intermediary metabolism the barbiturate pentobarbital sodium is the most inert anesthetic and does not cause alterations of metabolic blood and tissue parameters. In contrast, e.g. urethane as well as isoflurane (inhalation aesthetic) influences by itself substantially metabolic parameters over time (hours). 

The first glucose sensors to be discovered were the pancreatic -cells in the islets of Langerhans these manufacture the hormone insulin and release it into the blood when glucose concentration rises. Islet tissue also contains a-cells, which manufacture the antagonistic hormone glucagon. Insulin secretion is a complex process, and the islet cells receive additional signals from the gut and the autonomic nervous system, which modulate the insulin release to match the food that has been eaten. 

---