Autonomic nervous system division

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.

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. 

Muscarinic receptors are responsible for postganglionic parasympathetic neurotransmission and thus for control of a wide range of smooth muscle, cardiac muscle and secretory responses. Some responses originating in the sympathetic division of the autonomic nervous system, such as sweating and piloerection, also are mediated through muscarinic receptors. 

The physiology of the autonomic nervous system. Most viscera are innervated by both divisions of the autonomic nervous system. In a sense they are antagonistic and as such are concerned with important homeostatic mechanisms. 

Forming part of the peripheral nervous system is the autonomic nervous system which controls the glands and non-skeletal muscles that are not under conscious control. This control is provided by two parts of this system the sympathetic and parasympathetic divisions which, in general, bring about antagonistic responses. 

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. 

Excitation of the parasympathetic division of the autonomic nervous system causes release of acetylchoUne at neuro-effector junctions in different target organs. The major effects are summarized in A (blue arrows). Some of these effects have therapeutic appUcations, as indicated by the clinical uses of parasympa-thomimetics (p. 102). 

Autonomic ganglia. Ganglionic stimulation occurs in both the sympathetic and parasympathetic divisions of the autonomic nervous system. Parasympathetic activation results in increased production of gastric juice (smoking ban in peptic ulcer) and enhanced bowel motility ( laxative effect of the first morning cigarette defecation diarrhea in the novice). 

Acetylcholine is the primary neurotransmitter in the parasympathetic division of the autonomic nervous system, which mainly innervates the gastrointestinal tract, eyes, heart, respiratory tract, and secretory glands. Although its receptors are crucial for maintaining all normal functions of the body, an extremely small number of illnesses can be explained by the dysfunction of cholinergic regions of the peripheral autonomic system. 

Anatomical differences between the peripheral somatic and autonomic nervous systems have led to their classification as separate divisions of the nervous system. These differences are shown in Figure 9.1. The axon of a somatic motor neuron leaves the CNS and travels without interruption to the innervated effector cell. In contrast, two neurons are required to connect the CNS and a visceral effector cell of the autonomic nervous system. The first neuron in this sequence is called the preganglionic neuron. The second neuron, whose cell body is within the ganglion, travels to the visceral effector cell it is called the postganglionic neuron. 

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). 

Some organs are innervated by only one division of the autonomic nervous system. 

Many neurons of both divisions of the autonomic nervous system are tonicaUy active that is, they are continually carrying some impulse traffic. The moment-to-moment activity of an organ such as the heart, which receives a dual innervation by sympathetic (noradrenergic) and parasympathetic (cholinergic) neurons, is controlled by the level of tonic activity of the two systems. 

The rate of pacemaker discharge within these specialized myocytes is influenced by the activity of both divisions of the autonomic nervous system. Increased sympathetic nerve activity to the heart, the release of catecholamines from the adrenal medulla, or the exogenous administration of adrenomimetic amines will cause an increase in the rate of pacemaker activity through stimulation of -adrenoceptors on the pacemaker cells (Figure 16.3). 

Erection involves a coordinated action of the autonomic nervous system, and certain drugs may interfere with either the sympathetic division (e.g., aj- receptors) or the parasympathetic division (e.g., noncholinergic neurotransmitters). 

The answer is d. (Hardman, pp 192-193.) Nicotine is a depolarizing ganglionic blocking agent that initially stimulates and then blocks nicotinic muscular (NM) (skeletal muscle) and nicotinic neural (NN) (parasympathetic ganglia) cholinergic receptors. Blockade of the sympathetic division of the autonomic nervous system (ANS) results in arteriolar vasodilation, bradycardia, and hypotension. Blockade at the neuromuscu-... 

The human nervous system can be divided into two major functional areas the somatic nervous system and the autonomic nervous system (ANS). The somatic division is concerned primarily with voluntary function—that is, control of the skeletal musculature. The ANS is responsible for controlling bodily functions that are largely involuntary, or automatic, in nature. For instance, the control of blood pressure (BP) and other aspects of cardiovascular function is under the influence of the ANS. Other involuntary, or vegetative, functions such as digestion, elimination, and thermoregulation are also controlled by this system. 

Anatomy of the Autonomic Nervous System Sympathetic and Parasympathetic Divisions... 

Sympatholytics Various sites within the sympathetic division of the autonomic nervous system Decrease sympathetic influence on the heart and/or peripheral vasculature... 

The release of epinephrine and norepinephrine from the adrenal medulla is controlled by the sympathetic division of the autonomic nervous system. As discussed in Chapter 18, sympathetic cholinergic preganglionic neurons directly innervate this gland. An increase in sympathetic activity causes increased firing in these neurons, which in turn stimulates the release of epinephrine and norepinephrine from the adrenal medulla. 

Most blood vessels, the sweat glands, and the spleen are innervated only by one division of the autonomic nervous system. In the salivary glands, the two divisions of the autonomic nervous system supplement one another. In the bladder, bronchi, gastrointestinal tract, heart, pupil, and sex organs, the two divisions of the autonomic nervous system have opposing effects (see Figure 14.5). 

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 autonomous (independent) in that its activities are not under direct conscious control. It is concerned primarily with visceral functions—cardiac output, blood flow to various organs, digestion, etc—that are necessary for life. The somatic division is largely concerned with consciously controlled functions such as movement, respiration, and posture. Both systems have important afferent (sensory) inputs that provide sensation and modify motor output through reflex arcs of varying size and complexity. 

The autonomic nervous system lends itself to division on anatomic grounds into two major portions the sympathetic (thoracolumbar) division and the parasympathetic (craniosacral)... 

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