The Somatic System

Regarding the types of fibers acted upon by capsaicin, Szolcsanyi (1976, 1977) has shown that close arterial injections of the substance cause excitation exclusively of the slowest-conducting unmyelinated C2 fibers all others including Aap, A8, and C fibers remained unaffected. After desensitization with capsaicin, when this compound was no longer able to activate C fibers (Porszasz and Jancso, 1959), action potentials in these fibers could still be elicited by electrical stimulation of peripheral nerves, demonstrating that desensitization is not simply conduction block. Szolcsanyi 

More detailed analysis has been conducted regarding the influence of capsaicin on the electrical properties of various types of neurons. God-fraind et al. (1980), recording from rat and chick sensory neurons in culture, found that capsaicin prolongs the duration of action potentials evoked by intracellular current injection. Salt and Hill (1980) investigated the effect of microiontophoretically applied capsaicin on trigeminal nucleus caudalis and cerebellar neurons in the rat and cat. The predominant response in the trigeminal nucleus was one of excitation or potentiation of amino acid-induced excitation while cerebellar neurons were either inhibited or unaffected. 

Compared with untreated animals, however, there was a significant reduction in the proportion of neurons which responded to heat pulses of 45°C. 

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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. 

Although the nervous system is often discussed in terms of peripheral and central components, it should be regarded as a highly integrated whole in which the central nervous system (brain and spinal cord) plays a critical information gathering and processing role. The peripheral nervous system is often divided into the autonomic and somatic components. The somatic system controls the voluntary functions of the body, like those of the skeletal muscles. The... 

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 preganglionic fibers terminating in the adrenal medulla, the autonomic ganglia (both parasympathetic and sympathetic), and the postganglionic fibers of the parasympathetic division use acetylcholine as a neurotransmitter (Figure 4.2). Cholinergic neurons innervate voluntary muscles of the somatic system and are also found in the CNS. 

Within the skeletal muscle. These neurons - the a-motoneurons - are not part of the autonomic but of the somatic system. 

Visceral manipulations use manual techniques (the physician s contact with the somatic system) to balance forces that create stresses on the visceral organs. 

Cholinergic neurotransmission ChEs terminate cholinergic transmission in the central nervous system (CNS), in NMJs and in the autonomic system (the parasympathetic system, somatic motor nerves and pre-ganglionic sympathetic nerves). A few sensory cells and the NMJ in nematodes also include ChEs. 

Cole et al. (1995) reported on knock-out mice with a germ line deletion of GR. They demonstrated that lack of GR leads to perinatal death, atelectasis of the lung, and lack of adrenalin synthesis. To circumvent perinatal lethality, Tranche et al. (1999) and Brewer et al. (2003) generated tissue-specific somatic deletions of GR. This allowed to characterize GR function in the CNS, the immune system, and the liver in more detail. In particular, these approaches revealed novel aspects of organ-specific glucocorticoid physiology such as anxiety-like behavior, growth control, and polyclonal T cell activation. 

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 somatic motor nervous system or voluntary nervous system consists of nerve libers that irmervate skeletal muscle motor end-plates. 

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. 

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. 

Both clinical and sub-clinical mastitis are usually accompanied by an increase in the somatic cell count (SCC a measure of white blood cells (95%) such as macrophages, segmented neutrophil granulocytes or lymphocytes) in milk. SCCs above a certain level are an indication of immune-system activation and are used as an indicator of sub-clinical , latent or chronic udder infections/mastitis (Concha, 1986 Ostensson et al., 1988), which were shown to affect milk quality, composition and shelf life. Dairy companies therefore impose SCC thresholds or reduce payments to farmers if certain thresholds are exceeded (Hamann, 2001 Urech et al., 1999). 

Besides the arterial baroreceptors, central projections from other inputs, for example cardiac mecha-no-receptors, chemo-receptors, pulmonary stretch receptors, and somatic inputs, are capable of influencing the controlling system and thereby the autonomic effectors. 

Somatic nerves originate in the CNS and terminate at the neuromuscular junction where acetylcholine is the transmitter. Nerves of the autonomic system also use acetylcholine as the neurotransmitter at the end of the preganglionic fibres within the ganglia. With few exceptions, the postganglionic sympathetic fibres secrete noradrenaline (norepinephrine) whilst postganglionic parasympathetic fibres secrete acetylcholine. 

As described in Sect. 4, the somatic signs appear to depend a major extent on central nervons system mechanisms. 

The endocannabinoid system might also be involved, since A9-tetrahydorcan-nabinol, the major active ingredient in cannabis, decreases somatically expressed withdrawal behaviors and the aversiveness of withdrawal in mecamylamine- and naloxone-precipitated nicotine abstinence (Balfour 2002). However, genetic knockout of the CBl cannabinoid receptors did not significantly affect somatically expressed withdrawal behaviors (Castane et al. 2002). 

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. 

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