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How Human brain senses and works

The sensory nervous system is involved with the reception and processing of sensory information. This information is received through the cranial nerves, through tracts in the spinal cord, and directly at centers of the brain exposed to the blood. The brain also receives and interprets information from the special senses (vision, smell, hearing, and taste). Mixed motor and sensory signals are also integrated.

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From the skin, the brain receives information about fine touch, pressure, pain, vibration and temperature. From the joints, the brain receives information about joint position. The sensory cortex is found just near the motor cortex, and, like the motor cortex, has areas related to sensation from different body parts. Sensation collected by a sensory receptor on the skin is changed to a nerve signal, that is passed up a series of neurons through tracts in the spinal cord. The posterior column–medial lemniscus pathway contains information about fine touch, vibration and position of joints. Neurons travels up the back part of the spinal cord to the back part of the medulla, where they connect with “second order” neurons that immediately swap sides. These neurons then travel upwards into the ventrobasal complex in the thalamus where they connect with “third order” neurons, and travel up to the sensory cortex. The spinothalamic tract carries information about pain, temperature, and gross touch. Neurons travel up the spinal cord and connect with second-order neurons in the reticular formation of the brainstem for pain and temperature, and also at the ventrobasal complex of the medulla for gross touch.

Vision is generated by light that hits the retina of the eye, which is transformed into a nerve signal by receptors and sent ultimately to the visual cortex in the occipital lobe. Vision from the left visual field lands on the right side of each retina (and vice versa) and passes through the optic nerve until some information changes sides, so that all information about one side of the visual field passes through tracts in the opposite side of the brain. The nerves reach the brain at the lateral geniculate nucleus, and travel through the optic radiation to reach the optic cortex. Hearing and balance are both generated in the inner ear. The movement of liquids within the inner ear is generated by motion (for balance) and transmitted vibrations generated by the ossicle bones (for sound). This creates a nerve signal that passes through the vestibulocochlear nerve. From here, it passes through to the cochlear nuclei, the superior olivary nucleus, the medial geniculate nucleus, and finally the auditory radiation to the auditory cortex. Smell is generated by receptor cells in the epithelium of the olfactory mucosa in the nasal cavity. This information passes through a relatively permeable part of the skull to the olfactory nerve. This nerve transmits to the neural circuitry of the olfactory bulb from where information is passed to the olfactory cortex. Taste is generated from receptors on the tongue and passed along the facial and glossopharyngeal nerves into the solitary tract in the brainstem. Some taste information is also passed from the pharynx into this area via the vagus nerve. Information is then passed from here through the thalamus into the gustatory cortex.

The brain influences blood pressure and controls the rate of breathing, mainly by centres in the medulla.and pons. The vasomotor center of the medulla causes arteries and veins to be somewhat constricted at rest, as well as influencing the heart rate. It does this by influencing the sympathetic nervous system and parasympathetic via the vagus nerve. Information about blood pressure is generated by stretch receptors in the arch of aorta and passed to the brain along the vagus nerve, and about the stretch of the carotid sinus via the a nerve joining with the glossopharyngeal nerve. This information travels up to the solitary nucleus. Signals from here influence the vasomotor nucleus to adjust vein and artery constriction accordingly. The respiratory centres of the medulla and pons control respiration, by generating a signal that is passed down the spinal cord, along the phrenic nerve to the diaphragm and other muscles. The respiratory centre is split into three parts. A dorsal respiratory group causes the desire to breath in (inhalation) and receives sensory information directly and from the body. The pneumotaxic center influences the duration of each breath. A ventral respiratory influences breathing during periods of heavy exertion (expiration). The respiratory center directly senses blood carbon dioxide and pH. Information about blood oxygen, carbon dioxide and pH levels are also sensed on the walls of arteries in the aortic arch and carotid bodies. This information stimulates chemoreceptors in the arteries, which pass information again via the vagus nerve and glossopharyngeal nerves to the respiratory nucleus. High carbon dioxide, an acidic pH, or low oxygen stimulate the respiratory centre. The desire to breathe in is also affected by stretch receptors in the lungs which, when activated, prevent the lungs from overinflating by transmitting information to the respiratory centres via the vagus nerve. The brain, especially the hypothalamus, is heavily involved in regulating multiple bodily functions. The diencephalon include the neuroendocrine regulation, regulation of the circadian rhythm, control of the autonomic nervous system, regulation of fluid homeostasis, and food intake. The circadian rhythm is controlled by two main cell groups in the rostral and caudal hypothalamus. The rostral hypothalamus includes the suprachiasmatic nucleus, which through gene expression cycles generates roughly 24 long clock, and Ventrolateral preoptic nucleus. The caudal hypothalamus contains orexinergic neurons that control arousal through their projections to the ascending reticular activating system.The hypothalamus controls the pituitary gland through the release of peptides such as oxytocin, and vasopressin, as well as dopamine into the median eminence. The hypothalamus influences the autonomic nervous system, through ascending projections into autonomic cell groups in the brain stem. Through the autonomic projections, the hypothalamus is involved in regulating functions such as blood pressure, heart rate, breathing, sweating, and other homeostatic mechanisms. The hypothalamus also plays a role in thermal regulation, and when stimulated by the immune system, is capable of generating a fever. The hypothalamus is influenced by the kidneys. When blood pressure falls, the renin released by the kidneys stimulate the hypothalamus to elicit drinking behavior. The hypothalamus also regulated food intake through autonomic signals, and hormone release by the digestive system.

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Language functions are generally localized to Wernicke’s area and Broca’s area. Wernicke’s area is at the posterior part of the superior temporal gyrus of the dominant half of the brain, and seems to be responsible for creation and interpretation of spoken thought. Broca’s area is located in the prefrontal cortex and prefrontal cortex, most commonly on the left side of the brain, and is responsible for the creation of motor activity responsible for speaking. These two areas are connected by the arcuate fasciculus. Areas of the cerebellum, basal ganglia and areas of the motor cortex related to the face and larynx also play a role in coordinating and regulating muscle movements during speech. There has been substantial debate over these pathways. The study of how language is represented, processed, and acquired by the brain is neurolinguistics, which is a large multidisciplinary field drawing from cognitive neuroscience, cognitive linguistics, and psycholinguistics. This field originated from the 19th-century discovery that damage to different parts of the brain appeared to cause different symptoms: physicians noticed that individuals with damage to a portion of the left inferior frontal gyrus now known as Broca’s area had difficulty in producing language (Broca’s aphasia) whereas those with damage to a region in the left superior temporal gyrus, now known as Wernicke’s area, had difficulty in understanding it.

Each hemisphere of the brain interacts primarily with one half of the body: the left side of the brain interacts with the right side of the body, and vice versa. The developmental cause for this is uncertain. Motor connections from the brain to the spinal cord, and sensory connections from the spinal cord to the brain, both cross sides in the brainstem. Visual input follows a more complex rule: the optic nerves from the two eyes come together at a point called the optic chiasm, and half of the fibers from each nerve split off to join the other. The result is that connections from the left half of the retina, in both eyes, go to the left side of the brain, whereas connections from the right half of the retina go to the right side of the brain. Because each half of the retina receives light coming from the opposite half of the visual field, the functional consequence is that visual input from the left side of the world goes to the right side of the brain, and vice versa. Thus, the right side of the brain receives somatosensory input from the left side of the body, and visual input from the left side of the visual field.

The left and right sides of the brain appear symmetrical, but they function asymmetrically. For example, the counterpart of the left-hemisphere motor area controlling the right hand is the right-hemisphere area controlling the left hand. There are, however, several important exceptions, involving language and spatial cognition. The left frontal lobe is dominant for language. If a key language area in the left hemisphere is damaged, it can leave the victim unable to speak or understand, whereas equivalent damage to the right hemisphere would cause only minor impairment to language skills.

A substantial part of current understanding of the interactions between the two hemispheres has come from the study of “split-brain patients”—people who underwent surgical transection of the corpus callosum in an attempt to reduce the severity of epileptic seizures. These patients do not show unusual behavior that is immediately obvious, but in some cases can behave almost like two different people in the same body, with the right hand taking an action and then the left hand undoing it. These patients, when briefly shown a picture on the right side of the point of visual fixation, are able to describe it verbally, but when the picture is shown on the left, are unable to describe it, but may be able to give an indication with the left hand of the nature of the object shown.

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Emotions are generally defined as two step multicomponent processes involving elicitation, followed by psychological feelings, appraisal, expression, autonomic responses, and action tendencies. Attempts to localize basic emotions to certain brain regions have been controversial, with some research finding no evidence for specific locations corresponding to emotions, and instead circuitry involved in general emotional processes. The amygdala, orbitofrontal cortex, mid and anterior insula cortex and lateral prefrontal cortex, appeared to be involved approach related emotions, while weaker evidence was found for the ventral tegmental area, ventral pallidum and nucleus accumbens in incentive salience.Others, however, have found evidence of activation of specific regions, such as the basal ganglia in happiness, the subcallosal cingulate cortex in sadness, and amygdala in fear.

Executive functions
Executive functions is an umbrella term for various cognitive processes and sub-processes, that allow for the control of thought and behavior. These functions include the ability to filter information, or attention, the ability to manipulate working memory, the ability to switch tasks, response inhibition, and the ability to determine the relevance of information. The prefrontal cortex plays a significant role in executive functions. Neuroimaging during tasks testing executive function, such as stroop task and memory tasks, have found that cortical maturation of the prefrontal cortex correlates with executive function in children. Future planning involves activation of the dorsolateral prefrontal cortex, anterior cingulate cortex, angular prefrontal cortex, right prefrontal cortex, and supramarginal gyrus. Working memory manipulation involves the DLPFC, inferior frontal gyrus, and areas of the parietal cortex. Response inhibition involves multiple areas of the cortex, including the inferior frontal gyrus, and ventrolateral prefrontal cortex. Task shifting doesn’t involve specific regions of the brain, but instead involves multiple regions of the prefrontal cortex and parietal lobe.