Somatic and Autonomic Nervous Systems

The somatic and autonomic nervous systems differ anatomically and physiologically (Table 14.2). 

FIGURE 14.3 The henbane (Hyoscyamus niger Linne), a member of the nightshade family, whose leaves, with or without the tops, constitute the official drug Hyoscyamus and are a source of the valuable medicinal alkaloids hyoscyamine and scopolamine. 

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ANATOMIC DIFFERENCES BETWEEN THE SOMATIC AND AUTONOMIC NERVOUS SYSTEMS... 

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. 

Acetylcholine A neurotransmitter in the somatic and autonomic nervous systems principal synapses using acetylcholine include the skeletal neuromuscular junction, autonomic ganglia, and certain pathways in the brain. 

CNS—brain and spinal cord—and two separate pathways within the peripheral nervous system (PNS) for two-way communication with the peripheral organs. The PNS subdivisions are the somatic and autonomic nervous systems (Figure 11.2). The latter is further divided into sympathetic and parasympathetic divisions (Figure 11.3). 

Lower urinary-tract function is under the control of the somatic and autonomic nervous system. The latter is comprised of the sympathetic and parasympathetic nervous systems. Sympathetic nerves originate in the thoracolumbar region of the spinal cord at TIO to LI. Parasympathetic nerves arise from the sacral area of the spinal cord at the level of S2 to S4. Somatic nerves from the sacral cord course through the pelvic plexus and the pudendal nerve to the external sphincter region. 

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. 

Comparison of Somatic Nervous System and Autonomic Nervous System... 

Diabetic neuropathy is characterized by a spectrum of clinical neuropathic syndromes, which includes dysfunction of almost every segment of the somatic peripheral and autonomic nervous system. Diabetic neuropathy is classified as either mononeuropathy... 

The PNS is further divided into the somatic nervous system and the autonomic nervous system. The somatic branch of the PNS is concerned witii sensation and voluntary movement. The sensory part of the somatic nervous system sends messages to the brain concerning die internal and external environment, such as sensations of heat, pain, cold, and pressure The voluntary part of die somatic nervous system is concerned witii die voluntary movement of skeletal muscles, such as walking, chewing food, or writing a letter. 

Neurohumoral transmitters are chemicals that facilitate the transmission of nerve impulses across nerve synapses and neuroeffector junctions. Acetylcholine is a neurohumoral transmitter that is present in the peripheral autonomic nervous system, in the somatic motor nervous system, and in some portions of the central nervous system. 

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. 

Table 9.1 Distinguishing Features of Autonomic and Somatic Nervous Systems Autonomic nervous system Somatic nervous system...

Anatomically, the nervous system is divided into the central nervous system (CNS) consisting of the brain and the spinal cord and the peripheral nervous system comprised of neural cells forming a network throughout the body. The peripheral system is itself subdivided into two sections the somatic system, where control of skeletal muscles allows movement and breathing, and the autonomic system which controls the actions of smooth muscle, cardiac muscle and glandular tissues. Further subdivision of the autonomic system based on anatomical and biochemical factors creates the sympathetic and parasympathetic nervous systems. 

There are two classes of movements in the human body voluntary and involuntary. Voluntary movements are pretty clear they are the movements that we can control. Reaching for the French fries, swinging a baseball bat, turning on the TV, and typing at a computer keyboard provide obvious examples. Involuntary movements include those movements that we cannot readily control such as heart beats, vascular contraction, and movement of the gut muscles, and they basically control the internal environment of the body. Voluntary movements are controlled by the somatic nervous system. Involuntary movements are controlled by the autonomic nervous system, to which we now turn. 

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. 

In addition to the integrated participation of the peripheral nerves, central neural pathways are involved in the process. These central mechanisms interact during normal sexual activity and require complex coordination between the autonomic nervous system and the somatic outflow at the level of the spinal cord. 

The PNS is further divided into functional sections known as the autonomic and somatic systems. The autonomic nervous system is also called the involuntary system. It regulates, without conscious effort, the visceral motor and sensory organs and muscles, as well as other smooth muscle and glands. The somatic is the voluntary nervous system which... 

Studies of neuromuscular junctions of the autonomic nervous system as early as 1904 led to the suggestion that adrenaline might be released at the nerve endings. Later it was shown that, while adrenaline does serve as a transmitter at neuromuscular junctions in amphibians, it is primarily a hormone in mammals. Nevertheless, it was through this proposal that the concept of chemical communication in synapses was formulated. By 1921, it was shown that acetylcholine is released at nerve endings of the parasympathetic system, and it later became clear the motor nerve endings of the somatic system also release acetylcholine. 

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. 

The nervous and endocrine systems control an extensive number of functions in the body. The nervous system is divided into the central nervous system and the peripheral nervous system. The peripheral nervous system is further divided into the somatic nervous system (a voluntary system innervating skeletal muscles) and the autonomic nervous system (an involuntary system innervating smooth muscle, cardiac muscle, and glands). 

The somatic nervous system is composed of sensory afferents and motor efferents and serves to perceive external states and to modulate appropriate body responses. The autonomic nervous system (ANS), together with the endocrine system, controls the milieu interieur. It adjusts internal organ functions to the changing needs of the organism. The ANS operates largely autonomously, beyond voluntary control, at the subconscious level. Its central components reside in the hypothalamus, brain stem, and spinal cord. The ANS has sympathetic and parasympathetic branches. Both are made up of afferent, mainly in the vagus nerve, and efferent fibers. 

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. 

Q3 Somatic symptoms presented in this case are dry mouth, tachycardia and sweating. Psychological symptoms are tension, apprehension, irritability, restlessness and difficulty in concentrating. The symptoms usually result from overactivity in part of the autonomic nervous system or increased tension in skeletal muscles. 

Functional The somatic nervous system is responsible for coordinating voluntary body movements (i.e. activities that are under conscious control). The autonomic nervous system is responsible for coordinating involuntary functions, such as breathing and digestion. 

Somatic versus autonomic. The somatic nervous system comprises functions that are conscious - conscious sensations such as touch, temperature, pain etc., and voluntary movements. Conversely, the autonomic nervous system deals with unconscious sensory input such as blood pressure, blood oxygen and carbon dioxide lev-els and the likewise unconscious regulatory responses to it. 

In all s mapses, the presynaptic cell will always be a neuron. Postsynaptic cells can be either neurons, striated or smooth muscle cells, or gland cells (Figure 7.2a). In the case of skeletal muscle, the presynaptic neuron will be part of the somatic nervous system. In contrast, neurons that project to the heart muscle will be part of the autonomic system - none of us can voluntarily change the heartbeat. While in many synapses the presynaptic and postsynaptic membranes are in close apposition, thus ensuring rapid action, this is not necessarily the case in the effector synapses of the autonomic nervous system, which frequently do not have extremely time-critical missions. 

As we have seen [in a previous chapter], acetylcholine occurs in s niapses in both the somatic and the autonomic nervous system. The nicotinic acetylcholine receptor is found in the motor endplate of the skeletal muscle, and in both the sympathetic and the parasympathetic ganglia of the peripheral autonomic system. Muscarinic acetylcholine receptors are found at the endings of all secondary neurons within the parasympathetic part of the peripheral autonomous system. In addition, acetylcholine receptors of both types also occur in the brain. Drugs with a useful degree of selectivity for each of these targets are available and used in practical medicine. Selectivity is based on two principles ... 

Pyriminil toxicity occurs primarily because it inhibits NADH ubiquinone oxidoreductase activity of complex I in mammalian mitochondria resulting in preferential toxicity to high-energy-demanding cells such as nerves and pancreatic jS-cells. However, pyriminil may also act as a nicotinamide antagonist and interfere with the synthesis of NADH/NADPH, furthering neural and jS-cell toxicity. Inhibition of mitochondrial respiration in nerves causes somatic, autonomic, and central nervous system neuropathies while inhibition in jS-cell causes an immediate, irreversible insulin-dependent diabetes mellitus condition. Pyriminil also acts as a noncompetitive inhibitor of rat acetylcholinesterase. 

In diabetic patients, the incidence of clinically manifested deficits in peripheral nervous system function increases with duration of disease and is approx 50% after 25 yr of disease. The resulting diabetic neuropathies comprise a group of distinct disorders that can affect both somatic and autonomic nerves, the most common of which is symmetric sensory polyneuropathy. Clinical signs of overt human diabetic neuropathy include decreases in nerve conduction velocity and action potential amplitude and in resistance to ischemic conduction failure. These abnormalities may be accompanied by sensory deficits and, in some cases, severe pain. In diabetes of long standing, morphological deterioration is evident, and both nerve fiber loss and segmental demyelination may occur. 

Before investigating two more detailed examples of potential misuse, it is necessary first to describe something of the structure of the human nervous system. This is divided into the central nervous system (brain and spinal cord) and the peripheral nervous system.46 Information from peripheral sense organs is received via afferent pathways and processed within the central nervous system. Output from the central nervous system is sent via efferent pathways to the somatic nervous system (muscles) and to the autonomic nervous system (heart, gut, glands, etc.). 

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