The role of the brain in addiction - Essay Example

Introduction Addiction may be defined as “habitual psychological and physiological dependence on a substance or practice beyond ones voluntary control.” The various neurotransmitters like dopamine and reward pathways linked to the limbic system play an important role in addiction. Sensory transduction The process by which sight, sound and smell is converted into electrical signals in a form that can be interpreted by the nervous system is called sensory transduction. In this process, the external stimulus is recognized by a protein embedded in the lipid membrane of the sensory receptor, which “changes conformation, either directly producing an electrical signal (as for touch receptors in the skin or for hair cells in the ear and vestibular system) or triggering an enzymatic cascade and a change in the concentration of an intracellular second messenger that generates the electrical response (as in the eye and nose).” (Fain G, 2003) The Special Senses Photoreception and Signal Transmission:  The photoreceptor cells of the retina are the cones and rods. Light stimulus causes changes in the visual chemical of the cones and rods producing a receptor potential. This passes through the bodies of the rods and cones and acts at the synapses to induce a signal in the bipolar cells. This signal is then transmitted to the ganglion cells. The axons of the ganglion cells leave the eyeball posteriorly to become the optic nerve. The two optic nerves enter the cranial cavity and join to form the optic chiasma. In the optic chiasma, the axons from the nasal (medial) halves of the retinas cross over to the opposite sides. From the optic chiasma on either side of the brainstem, the optic tracts continue. The optic tracts relays information to the LGB (lateral geniculate body) of the thalamus. From here, information is carried to the occipital lobes of the cerebral cortex, where the information is perceived as conscious vision. Due to the crossover, the right visual field is perceived within the left hemisphere, and the left visual field is perceived within the right hemisphere (Lesson assignment, n.d.) The nervous pathways for sound The neuron associated with the hair cells of the organ of Corti (located in the basilar membrane of the inner ear) carries the sound stimulus to the brainstem. Through a series of connections, the signal reaches Brodmanns area number 41, in the temporal lobe of the brain where the stimulus is perceived as sound (Lesson assignment, n.d.) Equilibrium A number of continuous inputs inform the brain about the body posture. The semicircular ducts of the internal ear provide one of the input system. In addition to this input, the other inputs are; the proprioceptive sense, which monitors the condition of the muscles of the body, receptors associated with the joint capsules, the integument, etc., and vision (Lesson assignment, n.d.) Sensorimotor integration Sensorimotor integration can be considered as a fundamental aspect of human behaviour and is necessary to perform some of the basic activities of daily life. The frontal lobe can be considered as one of the critical structures involved in sensorimotor integration. “Different regions of the frontal lobe receive segregated cortical inputs from a variety of cortical areas of sensory significance.” (Miller B, Cummings J, 1998, p.54.) Chronobiology Chronobiology is a field of biology that examines time-related phenomena in living organisms. These cycles eating, sleeping, mating, hibernating, migration, and cellular regeneration. Of all rhythms, the most important rhythm is the circadian rhythm, which refers to the 24-hour daily biological cycle. The other rhythms are, the infradian rhythms, which are long-term cycles (Examples are: annual migration, reproduction cycles in certain animals or the monthly menstrual cycle of human females). Ultradian rhythms are short cycles, having periods of less than 24 hours (Example, 90-minute REM cycle in sleep). Tidal rhythms, are observed in marine life, and follow the transition from high to low tide and back. (Wikipedia, 2005.) Normal Sleep and Sleep Disorders “Normal sleep is divided into non–rapid eye movement (NREM) and rapid eye movement (REM) sleep. The stages of sleep are stage I (light sleep), stage II, stages III and IV (deep or delta-wave sleep), and REM sleep; NREM sleep comprises stages I-IV. Sleep is an active process that cycles at an ultradian rhythm of about 90 minutes.” (EMedicine. 2005.) The International Classification of Sleep Disorders has classified sleep disorders into four main categories: (HelpGuide, 2005) 1. Dyssomnias are disturbances in the amount, timing, or quality of sleep resulting in excessive daytime sleepiness or insomnia. 2. Parasomnias are disorders of partial arousal or disorders that interfere with sleep stage transitions. Abnormal events occur during sleep. 3. Sleep disorders due to medical/psychiatric disorders. 4. Proposed sleep disorders are sleep problems for which there is not enough information available to positively establish them as distinct disorders. Examples include, short sleepers, long sleepers, subwakefulness syndrome, fragmentary myoclonus, sleep hyperhydrosis, menstrual-associated sleep disorder, pregnancy-associated sleep disorder, terrifying hypnagogic hallucination, sleep-related laryngospasm, and sleep choking syndrome. Mechanisms of stress There are two types of instinctive stress response, the short-term “Fight-or-Flight” response and the long-term “General Adaptation Syndrome”. The “Fight-or-Flight” response is a basic survival instinct mediated by the hormone adrenaline. Some of the effects are an increase in the heart rate and blood pressure and increased sweating. The general adaptation syndrome and burnout is a long-term effect of exposure to stress. The general adaptation syndrome operates in response to long-term exposure to stress. There are three phases; in the alarm phase, the body reacts to the stress. In the resistance phase, the body adapts to stress and the resistance to the stress increases. Finally, in the exhaustion phase as resistance is exhausted, the body’s resistance to stress declines (MindTools, 2005.) The most common sleep disorders are sleep apnoea, restless legs syndrome (RLS) and periodic limb movement in sleep (PLMS), narcolepsy, and parasomnias. (HelpGuide, 2005.) Emotion “Emotion is an aspect of a human beings mental state, normally based in or tied to the persons internal (physical) and external (social) sensory feeling.” (Wikipedia, 2005). They encompass feelings like love, hate, courage, fear, joy, sadness, pleasure etc. Some of the physical responses to emotion include the following; “shame by warmth in the upper chest and face, fear by a heightened heartbeat and increased muscle tension. Sadness by a feeling of tightness in the throat and eyes, and relaxation in the arms and legs. Desire can be accompanied by a dry throat and heavy breathing.”(Wikipedia, 2005) Brain plasticity “Brain plasticity refers to the changes that occur in the organisation of the brain, and in particular changes that occur to the location of specific information processing functions, as a result of the effect of experience.”(Wikipedia, 2005.) Two types can be considered. In positive plasticity, the organism can recover to normal levels of performance even after a brain injury. In negative or maladaptive plasticity an excessive level of neuronal growth leading to spasticity or tonic paralysis, or an excessive release of neurotransmitters in response to injury, which could kill nerve cells, can be considered as examples. (Wikipedia, 2005.) Brain Chemicals Serotonin is produced by nuclei in the median raphe of the brain stem. Serotonin acts as a inhibitor of pain pathways in the spinal cord and controls a person’s mood and even cause sleep. Another important chemical in the brain is GABA (gamma-amino-butyric-acid), which is secreted by the nerve terminals in the spinal cord, cerebellum, basal ganglia, and many areas of the cortex. GABA causes inhibition. Norepinephrine is secreted by neurons in the brain stem, hypothalamus and postganglionic neurons of sympathetic nervous system. It helps to control the overall mood and activity of the mind. At the postganglionic neurons, it may excite some organs or inhibit others. Dopamine is secreted by the neurons that originate in the substantia nigra. The effect of dopamine is usually inhibition. (Guyton, AC 1986.) The Limbic System “The limbic system is a group of brain structures that are involved in various emotions such as aggression, fear, pleasure and also in the formation of memory. The limbic system affects the endocrine system and the autonomic nervous system. It is comprised of several subcortical structures located around the thalamus.” (Nationmaster.com, n.d.). The structures in the limbic system and their functions are: a. Hippocampus- involved in the formation of long-term memory. b. Amygdala- involved in aggression and fear. c. Cingulate gyrus fornicate gyrus. d. Archicortex. Hypothalamus- controls homeostasis and is connected to the pituitary gland and thus regulates the endocrine system. Mechanisms of Addiction Tolerance: can be defined as the reduced response to a drug following repeated exposures to the drug. Sensitisation: an exaggerated response to a drug because of prior exposure to that drug Withdrawal: symptoms (usually aversive) that accompany the abrupt cessation of drug use. The Biological Basis of Addiction Although personality, social, and genetic factors may be important factors, the pharmacological activation of brain reward systems is largely responsible for producing a drugs potent addictive properties. “Nonpharmacological factors are likely to be important in influencing initial drug use and in determining how rapidly an addiction develops.” (Addiction Science Network, 2000). Mesolimbic Dopamine Neurons Dopamine is a neurotransmitter, which has a significant role in the regulation of mood and affect as well as in the motivation and reward processes. Although the brain has numerous dopamine systems, the mesolimbic dopamine system is the most important for motivational processes. By enhancing the mesolimbic dopamine activity, some addictive drugs produce their potent effects on behaviour. Cells in the mesolimbic dopamine system generate action potentials at a constant slow rate, releasing small amounts of dopamine into the synaptic cleft. This may be responsible for maintaining normal affective tone and mood. Some forms of clinical depression may result from unusually low dopamine levels (Addiction Science Network, 2000.) Heroin-Enhanced Dopamine Activity The rate of neuronal firing by dopamine cells is increased by Heroin, which produce an increase in dopamine release. Because of this enhanced dopamine activity, the heroin user experiences mood elevation and euphoria. As the pharmacological action ceases, the drug user is highly motivated to repeat the experience (Addiction Science Network , 2000.) Cocaine-Enhanced Dopamine Activity The reuptake of dopamine is inhibited by Cocaine, which increases dopamine availability in the synapse and increases dopamines action on the postsynaptic neurons. This causes mood elevation and euphoria. Since Cocaines effect is usually quite short, the user repeatedly administers cocaine to re-experience its effects. (Addiction Science Network, 2000.) Combined Heroin- and Cocaine-Enhanced Dopamine Activity Heroin and cocaine can be combined to produce even more intense dopamine activation. “The heroin increases cell firing and dopamine release, while the cocaine keeps the released dopamine in the synaptic cleft longer thereby intensifying and prolonging its effects.” (popularly known amongst users as "speed-ball.") (Addiction Science Network , 2000.) Repeated use of cocaine or heroin can deplete dopamine. “This causes normal rewards to lose their motivational significance (i.e., produce motivational toxicity).” (Addiction Science Network , 2000). The mesolimbic dopamine system also becomes more sensitive to pharmacological activation by psychomotor stimulants and by opiates. “These neuroadpative changes are probably critical for producing an addiction.” (Addiction Science Network , 2000.) Neurobiology of pleasure and addiction Pleasure can be described as a “state or feeling of happiness and satisfaction resulting from an experience that one enjoys.” (Tobias E, George BS, 2004;25(4):235-251). Pleasure can serve to promote addiction. The process is a complex neurobiological one, which relies on the reward circuitry or limbic activity. These processes involve dopaminergic signaling. Endorphins and endogenous morphinergic mechanisms may also play a role. Addictive drugs are able to act directly on the reward pathways. Reward pathways are linked to the brain’s limbic system. The euphoria induced by drugs enhances the activity of the brain’s pleasure and reward systems. The prefrontal or orbitofrontal cortices, cingulate gyrus, amygdala, hippocampus and nucleus accumbens are all involved in the reward physiology. The brains pleasure centre is interconnected with a structure known as the nucleus accumbens in the limbic system. The nucleus accumbens is involved in the "high" a person experiences from drugs. Thus, there is a biologically interconnection between pleasure, the limbic system and reward circuitry. The hippocampus makes memories of such feelings of pleasure, accessible to this circuitry. Neuroendocrine pathways of addictive behaviour It is well known that plasma concentrations of neuroendocrine peptides are modulated by alcohol intake. Recent research, however, suggest that the endocrine system not only responds passively to alcohol intake but also actively modulates alcohol intake behaviour. “The hypothalamo-pituitary-adrenocortical (HPA) axis, with low corticotrophin-releasing hormone (CRH) is being associated with more intense craving and increased probability of relapse after acute detoxification. The intensity of craving or the intensity of the alcohol withdrawal syndrome may also be modulated by leptin, β-endorphin and atrial natriuretic peptide (ANP), which indirectly regulate the HPA system.” (Kiefer F, Wiedemann K, 2004, pp. 205 – 212) Investigating Human Drug Addiction A team of researchers at The Massachusetts General Hospital and Harvard Medical School are investigating the use of functional magnetic resonance imaging (fMRI). This helps to observe the brains of cocaine users while they are injected with cocaine. FMRI shows regional changes, which accompany neuronal activity in specialized parts of the brain, by providing rapid, high quality pictures of blood flow and oxygen use. The researchers aim to relate dynamic changes in regional brain activity to subjective responses measured at the same time. They are also studying the brains of cocaine users while they have a craving for cocaine. This study will lead to a greater understanding of the psychobiological processes that cause compulsive drug use and relapse. Scientists are also doing research with a mammalian model of emotion (i.e. rodent ultrasonic vocalizations) to predict drug-related phenomena such as abuse potential, anatomical location of mediating neural substrates, and the psychological impact of withdrawal. (Panksepp J, Knutson B, Burgdorf J, 2002, 459-69.) Conclusion Although biological factors like personality, social, and genetic factors may be important, the neurotransmitters like dopamine and serotonin and the reward and pleasure pathways, which are linked to the limbic system in the brain, play a significant role in an individual developing addiction through various mechanisms. Research activities are being conducted to further document these mechanisms, which will help to gain further clarity. ************************************************************************ BIBLIOGRAPHY Fain G (2003). 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