Somatic nervous system

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. 

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...

Skeletal muscle comprises the largest group of tissues in the human body and accounts for up to 40% of total body weight. This type of muscle, which is innervated by the somatic nervous system, is under voluntary control. Skeletal muscle performs many important functions in the body, including ... 

Skeletal muscle is neurogenic and requires stimulation from the somatic nervous system to initiate contraction. Because no electrical communication takes place between these cells, each muscle fiber is innervated by a branch of an alpha motor neuron. Cardiac muscle, however, is myogenic, or self-excitatory this muscle spontaneously depolarizes to threshold and generates action potentials without external stimulation. The region of the heart with the fastest rate of inherent depolarization initiates the heart beat and determines the heart rhythm. In normal hearts, this "pacemaker region is the sinoatrial node. 

Vomiting is a protective reflex of high complexity and integration involving autonomic and somatic nervous system components which results in an ejection of the gastrointestinal (GI) contents. This reflex is widely distributed among animals, including some insectivores [39], but absent in laboratory rodents [40]. 

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. 

Somatic nervous system the part of the peripheral nervous system that innervates skin, joints, and muscles responsible for the control of voluntary movements. 

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. 

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

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. 

Continual dribble of fecal matter through the anus is prevented by (1) tonic constriction of the internal anal sphincter, a circular mass of smooth muscle that lies immediately inside the anus, and (2) the external anal sphincter, composed of striated voluntary muscle that both surrounds the internal sphincter and also extends distal to it the external sphincter is controlled by the somatic nervous system and therefore is under voluntary control. 

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). 

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

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 somatic nervous system differs from the autonomic system in that a single myelinated motor neuron, originating in the CNS, travels directly to skeletal muscle without the mediation of ganglia. As noted earlier, the somatic nervous system is under voluntary control, whereas the autonomic is an involuntary system. 

Summary of the neurotransmitters released and the types of receptors found within the autonomic and somatic nervous systems. [Note This schematic diagram does not show that the parasympathetic ganglia are close to or on the surface of the effector organs and that the postganglionic fibers are usually shorter than the preganglionic fibers.]... 

Sites of actions of cholinergic agonists in the autonomic and somatic nervous systems. 

Skeletal muscle twitching is due to effects at the skeletal neuromuscular junction, which is innervated by the somatic nervous system, via motor nerves. The anticholinesterase prolongs and intensifies the actions of released acetylcholine at the junction, causing fasciculation (strong, jerky contractions) of skeletal muscle. Normally at the skeletal neuromuscular junction, the released acetylcholine is rapidly hydrolysed by cholinesterases to choline and acetate. This allows repolarization of the muscle membrane to occur following initial stimulation. In the presence of anticholinesterases the acetylcholine remains at the junction for a very prolonged period and produces repeated twitching of the muscle fibres via nicotinic receptors. 

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. 

Ach Acetylcholine is the chemical responsible for transmitting impulses betw een neurons in the central and peripheral (parasympathetic and somatic) nervous systems. It is the chemical that allows nerve cells to communicate with one another. 

The somatic nervous system controls skeletal muscle movement through motor neurons. Alpha motor neurons extend from the spinal cord and terminate on indi-... 

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