The Autonomic Nervous System - Essay Example

? The Autonomic Nervous System The autonomic nervous system derives its from two words, ly auto meaning self and nomo which implies govern. Therefore, the autonomic nervous system controls the involuntary activities of the smooth muscles and several undertakings o the glands. It forms a component of the peripheral nervous system and can be subdivided into two categories. These include the sympathetic and parasympathetic nervous system. In all these categories are comprised of effector neurons which connect the central nervous system top their effector organs. Each pathway comprises of a preganglionic neurone as well as a postganglionic neurone. Within the sympathetic system, the synapses between these two neurons are located near the spinal cord. On the other hand within the parasympathetic nervous system, these two neurons are located near to, or within the effector organs (Tortora & Derrickson 2009). The effects of the sympathetic and parasympathetic nervous systems normally oppose each other. They are termed as antagonists, thus is one system contracts a muscle, the other usually relaxes it. The balance between the two systems concisely regulates the involuntary activities of the organs and glands. Exceptional to note is that it is feasible to control consciously specific activities of the autonomic nervous system through training. Some examples in this include control of the anal and bladder sphincters. With regard to nerve impulse transmission process, the sympathetic nervous system stimulates effectors and produces noradrenalin as the neurotransmitter at nerve junctions. In contrast, the parasympathetic nervous system inhibits effectors and produces acetylcholine at the nerve junction, otherwise called the synapse (Tortora & Derrickson 2009). The autonomic nervous system comprises of motor neurons which innervate smooth and cardiac muscles as well as the glands. These neurons also ensure optimal environments conditions within the systems to ensure maximum support for body activities. The neurons operate via subconscious control and have viscera as most of their effectors. Within the autonomic nervous system, the preganglionic fibers release acetylcholine as the major neurotransmitter. Postganglionic fibers release norepinephrine or acetylcholine whose effects can either be stimulatory or inhibitory. The neurotransmitter effects within the autonomic nervous system on target organs are dependent on the neurotransmitter released. Additionally, such effects are dependent on the type of receptors expressed on the effector organs. The divisions of the autonomic nervous system serve similar visceral organs but cause opposite effects. These divisions exemplified by parasympathetic and sympathetic categories help in maintaining homeostasis. Precisely, the sympathetic division mobilizes the body during activity while the parasympathetic division conserves the energy within the body. The role of parasympathetic division is well illustrated when a person relaxes after taking a meal. In such a circumstance, the division plays a role of keeping the energy level consumption as low as possible. As such, the heart rate, blood pressure and respiratory rates are kept at lower concentrations. However, during such circumstances, the gastrointestinal tract activity is high as a result of digestion. The skin is warm while the pupils are constricted. On the other hand, the sympathetic division is a good depiction of fight or flight system. This division allows the regulation of activities during exercises. In such scenarios, the system reduces the flow of blood to organs while it increases the flow of blood to muscles. Its activity is illustrated by an individual who is under a threat and as such, the heart rate increases with rapid and deep breathing. Additionally, the glucose levels in the blood are high because this important sugar is released from the liver. Furthermore, the skin is cold and sweaty while the pupils of the eyes are dilated due to alertness. Parasympathetic division possesses a number of motor neurons to the effector organs that play crucial role in transmission of impulses. The occulomotor nerves innervate the smooth muscles within the eyes that cause the pupils to constrict and the lenses to unflatten hence achieve focus on objects. The facial pathway stimulates large glands that are found within the head. These glands include nasal, sublingual salivary and lachrymal glands. The glossopharyngeal pathway activates the parotid salivary gland. The vagus nerves account for almost 90% o all the preganglionic and parasympathetic fibers within the body. These nerves, two specifically, serve every organ within the abdominal and thoracic cavities. As the vagus nerves pass through the thoracic cavity, they sub branch to cardiac plexuses hence supply fibers to the heart. These functions to slow the rate of heart beat during times of low energy consumption. The parasympathetic division also has the sacral outflow comprising S2 to S4 which serve the distal half of large intestine. This sacral outflow also serves the reproductive organs, urinary bladder and the ureters. The sympathetic outflow of the motor neurons is more complex compared to the parasympathetic division. This division innervates a majority of organs and it innervates superficial and visceral regions. It arises from the spinal cord specifically between segments T1 through to L2. Pathways to the head have preganglionic fibers that serve the skin as well as the head. These fibers stimulate dilator muscles of the iris, stimulate the tarsus muscle of the eye and inhibit nasal and salivary glands. Caffeine including its fellow xanthenes, theophylline and theobromine are the most consumed drugs in the world, having surpassed nicotine and alcohol or universality and popularity. Caffeine, normally linked to coffee is also major ingredient of tea, as is theophylline. Additionally, it forms an ingredient component in chocolate as theobromine. Additionally, caffeine may be found in kola nuts, yerba mate and guarana seeds which are found Africa and South America respectively. Moderate doses of caffeine enhance alertness, mood and physical stamina. However, these benefits come with an increase in tolerance with use and withdrawal symptoms of fatigue, severe headache and dysphoria. The cognitive, affective and behavioral consequences of caffeine consumption are generally positive for moderate doses. Nevertheless, this can lead to clinical diagnoses exemplified by anxiety disorders of physical tremor, cognitive impairment or even twitching. Due to its common usage and the lack of apparent danger associated with it, caffeine is generally not even considered as a drug. Certainly, as a result of its ubiquity it is considered as part of the modern society despite its nature as a psychoactive drug (Winston, Hardwick & Jaberi, 2005). According to international standards of chemical names, caffeine is referred to as 1, 3, 7-trimethylxanthine and has a chemical formula of C8H10N4O2. Caffeine is a member of the purine family of compounds implying that it possesses a double ringed crystalline organic base, C5H4N4. When consumed orally, caffeine is rapidly and completely absorbed into the bloodstream through the gastrointestinal tract. Caffeine is extensively broken down in the liver into three major metabolites including paraxanthine, theobromine and theophylline. Each of these resulting products has its own unique effects within the body. There is also individual variation in the rate of metabolism. The effects of this group of chemicals are exemplified by the stimulation of the central nervous system and diuresis. Additionally, these chemicals exhibit stimulation of cardiac muscles and relaxation of the smooth muscle. Initially, caffeine stimulates the central nervous system mat the level of the cerebral cortex and medulla and only afterwards affects the spinal cord usually at higher doses (Echeverri, Montes, Cabrera, Galan, & Prieto, 2010). Caffeine is one of the most well known chemicals in the external environment among natural chemicals that is capable of invading the synaptic cleft. Caffeine shows affinities for wide range of receptors embedded within the synaptic membranes and internal calcium store. Additionally, caffeine displays affinity or cytoplasmic phsophodiesterases (PDEs), enabling it to modify synapses. At the synapses, or nerve junctions, local synaptic potentials are generated by synaptic inputs. As the local synaptic potentials are put together in summation transiently within space, neurons have the capability of integration of the signals. Furthermore, synapses can portray disparities in the efficiency of synaptic transmission and trigger morphological changes according to the activity. The release of excitatory transmitter is more strongly inhibited by adenosine compared to the release of inhibitory neurotransmitters. Blockade of adenosine receptors by caffeine can therefore occasionally lead to over activity at the excitatory synapses. Caffeine enhances excitatory postsynaptic potentials which are under the mediation of antagonism by presynaptic adenosine receptors. The principal cellular site of action of caffeine is the adenosine receptor. Preferential target site of caffeine that have been identified are adenosine receptor subtypes of A1 and A2A. A1 receptors are widely expressed all through the brain, whereas A2A receptors are found majorly in dopaminergic-rich areas. Adenosine is an endogenous purine nucleoside that commonly exerts inhibitory effects within the central and peripheral nervous systems. Such effects include suppression of the motor activity, inhibition of the excitatory neurotransmitters and inhibitory effects to gastric secretion. Caffeine is a non-selective competitive A1 and A2A receptor antagonist. Therefore, caffeine produces a wide range of effects in both peripheral and central nervous systems and that opposite to adenosine effects. Caffeine is consequently considered to act predominantly on the A1 receptors within the cortical regions. As such, this substance positively influences presynaptic release of neurotransmitters via the blockade of A1 receptors (Winston, Hardwick & Jaberi, 2005). The cyclic AMP cascade is an important intracellular signaling pathway playing a vital role in the modulation ad expression of neural function in the nervous system. Activation of membrane receptors coupled to G-proteins initializes the operation of membrane bound AC and cyclic AMP production. This molecule acts as the second messenger in the pathway that plays a crucial role in protein phosphorylation an gene expression. These cyclic AMP cascades are negatively impacted by PDEs which breakdown the cyclic AMP hence turns off the pathways. Caffeine depresses PDE activity, which in turn leads to an intracellular accumulation of cyclic AMP molecules. Consequently, the activities of cyclic AMP signaling pathways are greatly enhanced. In this case, caffeine acts as a stimulant. Of a major significance in the stimulating effects of caffeine within the central nervous system, caffeine indirectly enhances dopamine activity. It is able to do this through competitive antagonism of adenosine receptors that are co-localized and functionally interact with dopamine. Adenosine receptors can form functional receptor heteromers with dopamine receptors such as A1-D1 and A2A-D2. By competitively blocking the adenosine receptors, caffeine increases is plasmatic concentration. Consequently, this has the effect o increasing the systemic effects of this substance within the vascular cells. At systemic level, adenosine triggers the chemoreceptor distributed throughout the circulation. This leads to a general increase in the sympathetic tone with a gradual increase in peripheral vascular resistance. Additionally, this effect increases the concentration of circulating catecholamines and rennin secretion. Generally by blocking the adenosine receptors, there is the release of neurotransmitters including acetylcholine, noradrenalin or dopamine. This inhibition leads to symptoms associated with migraines and headaches. The action of adenosine depends on the type of receptor it stimulates and the type of tissue or cell in which it is found. The local vascular effects of adenosine are primarily vasodilation of the different beds. This effect is directly dependent on A2a receptors which are mainly in high concentrations within the vascular tissue (Echeverri, Montes, Cabrera, Galan, & Prieto, 2010). One of the most extensive tissues in the human body is the endothelium, which additionally forms an autonomic and functional blockade covering the arterial walls. This wall is usually permeable and selective forming a continuous soft tissue which is uninterrupted. The endothelial tissues synthesize and release a broad spectrum of vasoactive substances intervening in the regulation of vascular smooth muscle cell. This is achieved via an interaction between vasoconstrictor substances such as rennin and angiotensin as well as vasodilator substances. Caffeine acts directly on endothelial cells stimulating the production of nitric oxide which has an autocrine effect thus acting on the same cell to increase calcium ions. In the endothelial endoplasmic reticulum, the activity of rynodine receptor is stimulated by caffeine. Consequently, the concentrations of calcium ions and adenine nucleotides increase. Caffeine also plays an inhibitory role against the inositol triphosphate (IP3) compound which stimulates calcium ions production from the sarcoplasmic reticulum and is indispensable for contraction. This inhibitory effect on the IP3 channel by caffeine is antagonized by supplements of ATP. Caffeine also acts directly on the myosin light chain kinase and on the actin and myosin interaction. This slightly inhibits myosin light chain phosphorylation and contraction (Tortora & Derrickson 2009). At many synapses, repetitive synaptic occurrences can lead to the production of long term changes especially on the efficiency of the synapse. In tandem with the patterns of temporal coincidence, intensity of the pre and post synaptic activities an location, synaptic efficiency can be depressed or potentiated. This can be over a long term period and hence can be termed as long term depression or long term potentiation respectively. The nervous system is able to counteract this inhibitor effect of caffeine by releasing more adenosine. Furthermore, the system is capable o compensating the interference by increasing the number of adenosine receptors on the neuron surfaces. Lastly, the nervous system increases the affinity of the receptor while in tandem also decreasing the rate at which adenosine molecules are removed (Winston, Hardwick & Jaberi, 2005). References Tortora & Derrickson (2009) Principles of Anatomy and Physiology. Echeverri, D., Montes, F., Cabrera, M., Galan, A., & Prieto, A. (2010). Caffeine’s VascularMechanisms of Action. International Journal of Vascular Medicine , 1-10. Winston, A., Hardwick, E., & Jaberi, N. (2005). Neuropsychiatric effects of caffeine. Advances in Psychiatric Treatment , 432-439. http://learn.genetics.utah.edu/content/addiction/reward http://learn.genetics.utah.edu/content/addiction/drugs Read More