Biology 325 Fall 2004 The Central Nervous System I The brain A Basic pattern of CNS organization note cerebral hemispheres diencephalon brainstem and the cerebellum in cross section generally have a central cavity surrounded by a gray matter core nuclei external to which is white matter myelinated fiber tracts cerebral hemispheres and cerebellum have an outer bark or cortex of gray matter B Ventricles of brain CSF fluid filled cavities within brain continuous with each other and central canal of spinal cord lined with ependymal cells 1 Lateral ventricles c shaped w in cerebral hemispheres 2 Third ventricle in diencephalon communicates with lateral ventricles via interventricular foramen 3 Fourth ventricle lies dorsal to pons and superior medulla continuous with central canal of spinal cord SC three openings lateral apertures 2 and median aperture connect it to subarachnoid space C Protection of the brain brain protected by skull membranes meninges and a watery cushion CSF 1 Meninges three connective tissue membranes just external to CNS organs besides protection of CNS also enclose blood vessels and venous sinuses and contain CSF a Dura mater double layered membrane outer layer is periosteum of skull periosteal layer inner layer meningeal layer forms outermost brain covering continues down SC as dural sheath the two layers are fused together except in areas where they form dural sinuses b Arachnoid mater forms loose brain covering does not dip into sulci separated from dura by subdural space beneath arachnoid have wide subarachnoid space filled with CSF also contains largest vessels serving brain arachnoid villi project into dural sinuses CSF drainage c Pia mater delicate connective tissue membrane richly invested with blood vessels only meninx that clings tightly to brain and follows its every convolution d Note in some places dura mater extends inward to form flat septa that anchor brain to skull include falx cerebri falx cerebelli tentorium cerebelli 2 Cerebrospinal fluid CSF found in and around brain and spinal cord forms liquid cushion helps nourish brain produced by choroid plexuses clusters of capillaries enclosed by layers of ependymal cells found in the roofs of ventricles a filtrate of plasma the composition of which is finely tuned by permeability properties of ependymal cells and ion pumps in their membranes CSF fills ventricles continuous with subarachnoid space and central canal drained by arachnoid villi 3 The blood brain barrier helps maintain stable environment of brain formed by tight junctions of brain capillary endothelium very impermeable D Cerebral hemispheres CHs most superior part of brain separated from each other by median longitudinal fissure and from cerebellum by transverse fissure note gyri and sulci on surface divided into five lobes frontal parietal temporal occipital and insula lobes central sulcus delineates frontal and parietal lobes forms precentral gyrus and postcentral gyrus parieto occipital sulcus lateral sulcus and insula frontal section of brain outer cortex internal white matter basal nuclei 1 Cerebral cortex perception communication memory understanding appreciation initiation of voluntary movements gray matter neuron cell bodies dendrites no fiber tracts contains three kinds of functional areas motor areas sensory areas association areas each hemisphere concerned with sensory motor functions of opposite side of body hemispheres symmetrical in structure but not function no functional area acts alone a Motor areas located in posterior part of frontal lobes control voluntary motor function i primary motor cortex PMC in precentral gyrus pyramidal cells giving rise to long axons corticospinal pyramidal tracts allow conscious control of movements of skeletal muscle ii supplementary motor area programming of complex movements iii premotor cortex PC anterior to precentral gyrus frontal lobe controls learned motor skills of repetitive nature iv Brocca s area located in one hemisphere special motor speech area v Wernicke s area language comprehension vi Frontal eye field controls voluntary movements of the eyes b Sensory areas not confined to a single lobe concerned with conscious awareness of sensation i primary somatosensory cortex PSSC in postcentral gyrus neurons receive information from somatic sensory receptors and proprioreceptors to identify the body region being stimulated spatial discrimination ii somatosensory association area SSA posterior to PSSC many connections with it integrates and analyzes somatosensory inputs into comprehensive evaluation of what is being felt iii visual areas iiia primary visual cortex PVC largest of cortical sensory areas receives visual information originating in retinas iiib visual association area surrounds PVC interprets evaluates visual input in light of past experiences iv auditory areas iva primary auditory cortex PAC input from cochlear receptors of inner ear ivb auditory association area integration perception of sound stimulus v olfactory cortex OC input from olfactory receptors part of rhinencephalon vi gustatory cortex perception of taste stimuli c Association areas each sensory area as seen has nearby association area with which it communicates these latters in turn communicate with motor cortex and other association areas to analyze recognize and act on sensory input these other association areas are i prefrontal cortex PFC intellect cognition personality ii parietal temporal occipital association area receives input from all sensory association areas integration of all sensory input iii limbic association area motivation emotion memory 2 Cerebral white matter provides for communication between areas of cerebral cortex and areas of cerebral cortex and lower CNS centers largely composed of myelinated fibers bundled into large tracts a commissural fibers connect corresponding areas of two hemispheres b association fibers connect adjacent gyri within a single hemisphere or adjacent cortical lobes c projection fibers run vertically fibers connecting cortex to lower brain or spinal cord centers 3 Basal nuclei islands or gray matter deep within cerebral hemispheres includes corpus striatum caudate nucleus and lentiform nucleus and amygdala have extensive inputs from entire cerebral cortex from other subcortical nuclei and each other via relays through thalamus basal nuclei project to premotor and prefrontal cortices influence motor movements however no direct access to motor pathways E Diencephalon