THE BRAIN


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The brain, as complex as it is, is composed of neurons and neuroglia. Sensation, control of muscle movements, memories, emotions, and all involuntary and voluntary processing in the brain is dependent on neuronal action potentials and the neurotransmitters released at synapses. Neurotransmitters are often used only in certain areas of the brain which are involved in certain types of processing.
Dopamine is a neurotransmitter used in areas that control involuntary muscle movements (hence the tremors in Parkinson's patients when the dopamine circuits are destroyed) and in areas devoted to pleasure and sexuality. Dopamine functions in the perception and pursuit of pleasure, sex drive, and orgasm. It is involved in almost every type of addiction and dopamine treatment can decrease addiction. Its increased release is correlated with increased sexual activity and periods of increased sexual receptivity; it has been used to treat sex drive disorders.
Serotonin inhibits sex drive and orgasm, promotes contentment, causes cravings for sweets, and has been used to treat depression, obsessive-compulsive disorder, panic, and anxiety. Prozac increases serotonin levels while dieting decreases them.
Endorphins, enkalphins, and dynorphin are our brain's own opiates that reduce our sensitivity to pain (may be felt during exercise ["runner's high"] and in our response to emergencies).
Some hormones affect the activity of neurons (as neuromodulators) such as oxytocin, ADH, DHEA (dehydroepiandrosterone), PEA (phenylethylamine: a human amphetamine also contained in chocolate), prolactin, estrogen, and testosterone. These neuromodulators can affect aspects of behavior varying from cognition, susceptibility to schizophrenia, sexual drives and impotence, arousal and orgasm, sexual receptivity, parenting and affectionate behavior, and fantasy making . The sex hormones in animals (and all available evidence suggests the same in humans) not only influence many brain regions involved in sexual behaviors, they also cause the sexually dimorphic sizes in some of these same brain regions (bed nucleus of the stria terminalis, massa intermedia, anterior commissure, corpus callosum, suprachiasmotic nucleus, preoptic nucleus of the anterior hypothalamus). To what extent are brains Awired@ to be male or female?

Anatomically, the brain can be divided into the brainstem (which contains the medulla, pons, and midbrain), the diencephalon, the cerebellum, and the cerebrum.
A) BRAINSTEM
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Much of the brainstem consists of the axons (white matter) traveling to and from the cerebrum and cerebellum and it is therefore lighter in color. There are however collections of soma that process information that are organized into groups known as nuclei. The nuclei of the brainstem are diffuse and are referred to as the reticular formation
The Medulla Oblongata is primarily axon tracts although there are nuclei here which control heartbeat & blood vessel diameter (the cardiovascular center), breathing (the respiratory center), swallowing, coughing, hiccuping, salivating, vomiting, & sneezing. The nuclei for 5 cranial nerves are located here. A hard blow to back of the head can be fatal.


The Pons contains axon tracts connecting different parts of the brain, primarily the cerebrum & cerebellum. It also has nuclei that help control respiration, and the nuclei of 4 cranial nerves.

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The Midbrain has motor tracts running from the cerebrum known as cerebral peduncles, a dorsal surface called the tectum which is split into 4 areas (corpora quadrigemina: pairs of the superior & inferior colliculi) that process visual & auditory stimuli respectively and contains the nuclei for 2 cranial nerves.
B) DIENCEPHALON:
The diencephalon can divided into three main regions: the thalamus, the hypothalamus, and the pineal gland.
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The thalamus composes 80% of the diencephalon. It relays impulses to cerebrum and contains nuclei that process information (notably vision and hearing in the lateral & medial geniculate nuclei). It is involved in feelings of pain, pressure & temperature that can't be localized.
The hypothalamus is an amazing area which has more vital functions dedicated to it than any other brain region of comparable size. It controls and integrates our fight and flight and rest and repose responses; it is associated with sex specific behavior, rage, & aggression; it regulates body temperature; it controls feelings of hunger, thirst, & satiety; it influences sleep & waking patterns (controls circadian rhythms); it controls body water & electrolyte compositions; it has a role in obesity; and it is a major component of endocrine system, secreting a number of important hormones.
The pineal gland secretes the hormone melatonin which contributes to sleepiness and pigment production. The pattern of melatonin secretion helps to set the circadian rhythms of the hypothalamus. In some fish. Amphibians, and reptiles, the pineal is actually a 3rd eye, capable of detecting light. Even in birds and mammals, the pineal gland (and other regions of the brain) contain receptors for light.
A stalk known as infundibulum attaches the pituitary gland to the hypothalamus.
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CEREBELLUM


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The surface of the cerebellum is composed of gray matter in ridges known as folia and the deeper white matter (the arbor vitae) resemble the branches of a tree. There are 2 cerebellar hemispheres (halves of the cerebellum) which are connected by the vermis.
The cerebellum contains complete sensory and motor maps of the body. Sensory information indicating the position of the body and motor commands reach the cerebellum and because of this, the cerebellum knows what actions are being planned and where the body parts are at any given moment. As a result, it can corrects any motor commands. It measures position and balance. compares cerebrum=s intentions and the body's actual performance, cuts the motor outputs used to only those necessary, and predicts the future by memorizing what muscles will be used next; this allows smooth movements .
Cerebellar lesions result in decreased muscle tone, clumsy movements, the loss of balance, speech problems, and abnormal movements (from tremor to flailing arms).

LIMBIC SYSTEM
The limbic system includes areas of the cerebrum and brainstem (including the thalamus and hypothalamus) which link the conscious thoughts of the cerebrum to the involuntary functions of the brainstem. Specific areas are stimulated to result in:
a) emotions such as rage, pleasure, pain, sexual arousal (identified as electrical stimulation in an area in a lab animal causes a certain behavior); different regions are involved in different emotions:
-- hypothalamus: pleasure center, rage, aggression, sex specific behavior
--thalamus: pain, foreboding, loneliness; lesions here cause apathy & the loss of social
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--amygdala: pleasure, rage, fear; lesions here lead to excessive anger, fear, and indiscriminate sexual behavior; we also process fearful expressions in others here
b) drives such as hunger & thirst
c) autonomic functions tied to emotions (blushing, facial expression)
d) aspects of memory consolidation; lesions in the limbic system may affect the ability to make new memories or retrieve old ones
The limbic system is linked to olfaction (smell) and memory. Pheromones moderate many aspects of reproductive behavior in animals; smells can often trigger vivid memories.

CEREBRUM
The cerebrum is the largest area of the human brain with 3/4 of the total number of neurons. Conscious thought processes and all intellectual functions start here. Although certain functions (reading) can be assigned to certain areas, other functions (consciousness) have no one origin. The surface (the cerebral cortex) is composed of gray matter 2-4 mm thick (billions of neurons) with white matter beneath. There is additional gray matter in nuclei in the interior of the cerebrum (such as the basal nuclei made of the caudate nucleus, putamen, globus pallidus, hippocampus, and amygdala).

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The surface area for gray matter is increased threefold with folds (gyri) & fissures (sulci).
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Cerebral Hemispheres
The cerebrum is divided into two halves (hemispheres). The cerebral hemispheres are in constant contact since around 200 million axons which form the corpus callosum join the 2 and conduct about 4 billion impulses per second. Higher order functions are not evenly distributed between the 2 hemispheres:
a) The left hemisphere is more involved in reading, writing, speaking, math, and logic. It helps to control the emotions of the right hemisphere. Most people with left hemisphere dominance are right handed.
b) The right hemisphere processes sensory information, memory, space & pattern recognition, music, insight, artistic awareness, creativity. Many people with right hemisphere dominance are left handed (and are predominantly male).
Right hemisphere dominance is linked to stuttering and problems with hemisphere dominance may be involved in dyslexia. The 2 hemispheres may actually differ in size (some evidence left hemisphere larger than right in females; more equal in males). In primates (and preliminary evidence supports in humans) the somatosensory and motor cortices of the preferred hand are actually larger than those of the non-preferred hand.
A number of important areas of the cerebral hemispheres can be identified, such as:
Somatosensory Cortex:
The somatosensory cortex receives information from the general senses (touch, temperature, pressure, pain, vibration). Behind this area, the somatosensory association area interprets the stimuli. A lesion (such as that resulting from a stroke) in the somatosensory cortex prevents you from feeling that body part while a lesion in the association area prevents you from interpreting stimuli there (such as recognizing what you hold in your hand without looking).
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--Motor Cortex:
The motor cortex initiates voluntary muscle movements. In front of this, the premotor area coordinates complex movements (typing, writing, driving) that involve groups of muscles, forming a memory bank for learned movements. A lesion in the motor cortex prevents you from voluntarily moving that body region; in the premotor area, you might lose the ability to type but still be able to relearn it.
The Motor Speech center (Broca's Area) is located in left hemisphere only. It memorizes breathing and vocalization patterns required for speech. Lesions here cause speech problems and may render one unable to speak.
The areas for planning speech and interpreting speech are in the left hemisphere of most individuals; the corresponding regions of the right hemisphere are known as language affective areas which involve the expression and interpretation of the emotional content of speech.
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Special Senses
The special senses include sight, smell, taste, and hearing. This sensory information is recognized at specific areas: the visual, auditory, and olfactory cortices. After each cortex recognizes the incoming stimuli, the information is then interpreted and meaning is attached at the association areas near these cortexes (the visual association area, auditory association area, olfactory association area).
Lesions in the cortices for the special senses can obliterate that sense (cortical blindness occurs after damage to the visual cortices although the eyes and their input are still normal). Lesions in the association areas might prevent you from recognizing familiar faces or sounds, for example. The auditory association area (Wernicke's area) also translates sounds into thoughts. Those with lesions here can have trouble comprehending speech. Lesions in some areas of the visual association area can cause colorblindness; large lesions result in someone who can see but doesn=t know what they=re looking at.

the General Interpretive (Gnostic) Area
The general interpretive or gnostic area allows individual bits of info to be put together for a new meaning. Without which you could understand "sit" and "here" but never "sit here". It is located in the left hemisphere only. The idea of Afire@ integrates seeing light, smelling smoke, feeling heat; this area works with the prefrontal cortex.

Any damage to this area causes aphasia may be severe enough to render the person unable to speak, read, or interpret speech; the area where speech is planned.


Prefrontal Cortex
The prefrontal cortex receives information from the general interpretive area and the association areas. It is the site of personality, our sense of socially acceptable behavior, creativity, conscience, judgement, abstract ideas, and foresight. It recognizes temporal relations between events and interprets events and makes predictions about the future consequences. This information may cause the limbic system to generate frustration, anxiety, etc.
If you cut the axons linking the prefrontal cortex to the rest of the brain (a frontal lobotomy), one becomes unconcerned, without emotion reactions. Tumors here result in mental and personality disorders.

CEREBROSPINAL FLUID
There is fluid (cerebrospinal fluid) which both surrounds and travels through the brain and spinal cord. It supports the weight of the brain and helps to regulate the chemical environment of the brain cells. The brain has spaces (ventricles) through which this fluid flows.
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AUTONOMIC NERVOUS SYSTEM
Conscious thoughts and plans have little affect on many essential body processes (breathing, heartbeat, gland secretion, etc.). The autonomic nervous system (ANS) regulates the homeostasis of the cardiovascular, respiratory, digestive, excretory, and reproductive systems.

CRANIAL NERVES
Just as there are spinal nerves which carry sensory information to the spinal cord and motor commands away from the spinal cord, there are cranial nerves which do the same (such as the optic nerve below, and the olfactory nerves which would come out of the olfactory bulb).

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SYMPATHETIC (DIVISION: FIGHT OR FLIGHT
The sympathetic division of the ANS increases tissue metabolism and alertedness; it is involved in exercise and response to emergencies. Some of the neurons travel to the adrenal glands (above the kidneys) where they release epinephrine (adrenaline) into the bloodstream.

EFFECTS
Many of the aspects of the fight or flight response sends more energy to muscles. The body's glucose and adipose reserves release these energy stores into the blood. The heart increases the number and strength of contractions to get muscles what they need and remove their wastes. Respiratory passageway dilate so more oxygen can be brought to muscles. There is a general increase in muscle tone (background state of contractions); a person may look tense or even shiver.
Less energy is sent to other parts of the body. The digestive tract shuts down, so that more energy & fluids are available for the muscle. The immune system receives less energy. There is a decreased production of sex hormones and decreased libido-obviously during an emergency you should not be preoccupied with fooling around.
Other preparations for the emergency may include:

--voiding of the urinary bladder or large intestine (so that we are lighter and can run faster)
--increased sweating for cooling
--blood moves away from the skin & becomes thicker as a defense against bleeding
--increased secretion of endorphins (pain killers); leads to euphoria and a disregard for danger or pain
--increased alertedness

Today's Drawbacks
The fight or flight response is a wonderful way to deal with the stresses that your ancestors faced-short-lived emergencies which involved physical exertion. Today stresses aren't limited to short-lived attacks by saber tooth tigers and modern stresses can evoke the sympathetic division for long periods of time. Unfortunately, the fight or flight response was never meant to be used for stressful situations which last for hours, weeks, or months and there can be negative physical side effects of long term stress.
As a result of the fight or flight resposne, the immune system receives less energy and it has fewer resources to monitor for cancer cells & fight microbes. High blood pressure and thickened blood makes heart attack and stroke more likely. Increased cholesterol in blood increases the risk of heart disease. While under stress people often have problems in digesting food since the digestive system's activity is decreased. Because of decreased libido and difficulties in erection/vaginal lubrication, individuals under stress often have trouble in their relationships.

PARASYMPATHETIC DIVISION; REST & REPOSE
When not under a stress, the parasympathetic division of the ANS governs our rest and repose response. This results in increased digestive gland secretion, increased nutrient absorption, increased activity of smooth muscle lining the digestive tract and digestive tract secretion, increased blood sent to kidney to filter blood to produce urine, decreased heartrate and blood pressure, and more resources available for the immune and reproductive systems.