The Central Nervous System : Let's Think About It

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	the cerebral white matter lies deep to the cortex it provides for communication between cerebral areas and between the cortex and lower CNS centers it consists chiefly of three types of tracts (large bundles of myelinated fibers) commissures are composed of commissural fibers and they connect corresponding areas of the two hemispheres corpus callosum anterior commissure association fibers connect different areas within the same hemisphere short ones connect adjacent gyri, long ones connect different lobes projection fibers connect the cerebral hemispheres to lower CNS or spinal cord centers the internal capsule is formed at the upper limit of the brain stem and beyond that point, the projection fibers fan out into the cerebral white matter to form the corona radiata
	basal nuclei of the cerebrum basal nuclei (formerly referred to as basal ganglia) are "islands" of gray matter found within the cerebral white matter the caudate nucleus and the lentiform nucleus (composed of the putamen and the globus pallidus) make up the main mass and are collectively called the corpus striatum also involved is the amygdala (amygdaloid nucleus), which sits on the "tail" of the caudate nucleus -- it is involved in the limbic system basal nuclei receive inputs from the entire cerebral cortex, each other and other subcortical nuclei basal nuclei influence muscle movements by relaying information through the thalamus to the premotor cortex and the prefrontal cortex, but they have no direct access to motor pathways the precise role of the basal nuclei has not been ascertained due to their location role in motor control is probably fairly complex possibly have roles in cognition impairment of basal nuclei seem to result in disturbances in posture and muscle tone, involuntary movements such as tremors, and abnormal slowness of movement.
	the diencephalon forms the central core of the brain and is composed of the thalamus, the hypothalamus and the epithalamus the thalamus is composed of many different nuclei, most named according to their relative location each nucleus has a functional specialty afferent impulses from all over the body synapse with at least one thalamic nucleus and virtually all inputs ascending to the cerebral cortex are funneled through the thalamic nuclei the thalamus plays a key role in mediating sensation, motor activities, and memory the hypothalamus is found just inferior to the thalamus and superior to the brain stem. It extends from the optic chiasma to the mammillary bodies, between which is the infundibulum, the stalk from which hangs the pituitary gland the hypothalamus contains many functionally important nuclei and is the main visceral control center of the body -- it is vitally important to homeostasis it is the autonomic control center (blood pressure, heart rate, contractile force, digestion, etc) it is the emotional response and behavior center (limbic system roles) it regulates body temperature (it is the thermostat) it regulates food intake (in response to changing levels of glucose and other nutrients in the blood) it regulates water balance and thirst (ADH secretion) it regulates circadian rhythms (through the SCN) it controls the endocrine system (controls the secretion of hormones by the anterior pituitary and it itself secretes ADH and oxytocin) disturbances to the hypothalamus may result in many types of homeostatic imbalances the epithalamus is the most dorsal portion of the diencephalon the pineal gland (pineal body) extends from its posterior border and is responsible for the secretion of melatonin, involved in circadian rhythms a choroid plexus is also part of this structure
	the brain stem from superior to inferior, the brain stem is composed of the midbrain, the pons and the medulla oblongata the midbrain contains the cerebral peduncles on its ventral side, a central cerebral aqueduct, and on the dorsal side, the corpora quadrigemina the peduncles contain the pyramidal (corticospinal) motor tracts and the tracts connecting the midbrain to the cerebellum the corpora quadrigemina are the largest of the midbrain's nuclei; they include visual reflex centers, the startle reflex center and auditory relays also included in the midbrain is the substantia nigra and the red nucleus the substantia nigra releases dopamine and is dark in color due to the presence of melanin the red nucleus is red due to a rich blood supply and iron pigment in the cells the pons is the "bridge" between the midbrain and the medulla the pons is chiefy composed of conduction tracts and nuclei originating certain cranial nerves (V, VI, VII) and the pneumotaxic center the medulla (oblongata) connects the brain to the spinal cord noticeable externally on the medulla are the pyramids formed by the pyramidal motor tracts the decussation of the pyramids is the area just above the medulla-spinal cord junction where the tracts cross over to innervate the opposite side of the body another noticeable "landmark" is the pair of olives, found lateral to the pyramids these nuclei relay information regarding the stretch of muscles and joints to the cerebellum important visceral nuclei found in the medulla include : cardiovascular center (contractile force, heartrate, blood pressure) respiratory centers (rate and depth of breathing) other centers (vomiting, hiccuping, swallowing, coughing, sneezing, etc.) overlap of some functions with the hypothalamus? yes -- the hypothalamus exerts control by relaying instructions through the medullary centers, which carry them out