Lecture 12Outline of Last Lecture I. Nervous TissueII. Neuronsa. General information b. Parts of the neuronc. Axonsd. Synapses e. Dendrites f. Grouping of neuron cell bodiesIII. Supporting CellsOutline of Current LectureI. MyelinII. NervesIII. Organization of the CNSIV. Development of the CNSV. Development of the BrainCurrent Lecturec. Supporting cells in the PNS. In the peripheral nervous system, the supporting cells are satellite cells and Schwann cells. i. Satellite Cells: lie around the neuron cell bodies in ganglia, an arrangement resembling satellites (moons) around a planetii. Schwann Cells: lie around the peripheral nerve fibers 1. Also known as neurolemmocytesiii. all nerve fibers in the PNS are associated with Schwann cells, whether these fibers are myelinated or not IV. Myelina. Myelin: a fat and protein substance (lipo-protein) that surrounds the thicker nerve fibersof the body, as a myelin sheathi. Occurs on the thicker, large-diameter fibers 1. Myelin functions to speed the rate of impulse conduction 2. A large diameter has this same function3. Myelin + large diameter = characterize the rapidly conducting axonsii. Formed by Schwann Cells 1. Each Schwann cell contributes a segment of myeliniii. Myelin segments are separated by nodes of Ranvier (neurofibral nodes) which are areas of bare nerve fiberb. In the CNS, myelin is formed by the neuroglial cells called oligodendrocytes i. a tubular segment of myelin occurs at the end of eac cell process of the oligodendrocyte1. nodes of Ranvier are present btwn the segments of myelin BIOL_315 1nd Edition2. a single oligodendrocyte contributes myelin segments to several different axonsa. contrasts PNS< wjere each Schwann cell associates with only one axonc. Unmyelinated fibers:i. 2 classes of nerve fibers, those with myelin and those without myelinii. Fast-conducting fibers are the myelinated ones iii. Thinner, slowly-conducting fibers are not myelinatediv. Unmyelinated fibers in the PNS , lack the concentric membranes that form myelin but are still wrapped by Schwann cells1. Several unmyelinated fibers associate with each Schwann cell.v. In the CNS, unmyelinated fibers are separated from one another by neuroglial cells V. Nervesa. Nerve: a collection of nerve fibers in the peripheral nervous systemi. Nerves are only in the PNS and no nerves occur inside the brain or spinal cordii. Most nerves consist of axons of motor neurons and peripheral processes of sensory neurons, and contain both myelinated and unmyelinated fibersb. The nerve fibers within nerves are like muscle fibers within muscles, they are arranged inbundles called fasciclesi. Distally the fascicles branch off the main nerve to supply different regions of the bodyii. Usually contain both motor and sensory fibersiii. 3 kinds of connective tissue occur in and around nerves:1. Epineurium: around the whole nerve2. Perineurium: around and between the fascicles in the nerve3. Endoneurium: around the fibers within the nerve fasciclesMake sure you know the different between a nerve, nerve fiber, a nerve fascicle, and a neuron.Part 2: General Organization of the Central Nervous SystemI. Introductiona. Ventral half of the spinal cord is related to motor functionsb. Dorsal half is related to sensory functionsc. Dorsal interneurons and ventral motor neurons are each subdivided into somatic and visceral partsi. Somatic  innervation of the outer body (skin, and skeletal muscle)ii. Visceral  innervation of the inner body and of smooth muscle and glandsd. Sensory half of the gray matteri. Somatic sensory area1. Farthest dorsally2. Receive input from sensory neurons that carry information from the outer (somatic) part of the bodyii. Visceral sensory area1. Ventral to somatic sensory area2. Receive input from sensory neurons that monitor visceral sensations iii. Visceral motor area1. Ventral areas of the gray matter innervate the visceral effectors iv. Somatic motor area1. Contains cell bodies of motor neurons that innervate the somatic muscles of the bodyv. From dorsal to ventral in the gray matter of the spinal cord are the somatic sensory, visceral sensory, visceral motor, and somatic motor areas in that ordervi. Remember: the two visceral areas sensory and motor, lie next to each other in the center of the gray matter e. Spinal Cordin i. Involved in innervation of the body from the neck down II. Development of the CNSa. General i. Neural tube consists of a layer of epithelium, the cells of which are called neuroepithelial cellsii. Neural crest1. Formed from ectoderm2. Lies outside the neural tube and dorsal to it iii. Neuroepithelial cells multiple and the newly generated cells migrate outward forming the Mantle layer of neuroblast cells (pre-neurons)1. The mantle layer will develop into the gray matter of the spinal cord and brain2. Mantle cells sprout axons that grow outside the mantle layera. Run longitudinally along the spinal cordb. All the axons compromise the marginal layer 3. These axons form communications between different layers of the spinal cord and brain4. Most of the axons in this marginal layer eventually become myelinated, giving this external zone a white appearance 5.  the marginal layer becomes the white matter of the spinal cord and brainiv. As the mantle layer thickens more in some regions than in othersv. There are dorsal and ventral thickened zones, called alar and basal plates, respectivelyvi. The boundary btwn these plates is indicated by an angular groove in the central canal called the sulcus limitansvii. Cell bodies in the alar plate become the interneurons of the sensory part of the gray matterviii. Cell bodies of the basilar plate form the motor neurons 1. The axons of which grow out of the spinal cord to innervate the muscles and glands of the bodyAlar and basal plates form the dorsal interneurons and the ventral motor neurons, in the CNS.b. Roof and floor plates: the i. Roof plate: extreme dorsal part of the mantle laterii. Floor plate: extreme ventral part of the mantle laterc. Neurological cells migrate outward into both the gray and white parts of the spinal cord and brain, intermingling with the newly formed neurons thered. A layer of neuroepithelial cells remains to line the hollow interior of the spinal cord and brain i. Which are then called ependymal cellse. Sensory neuronsi. The sensory neurons come from neural crest cells outside the CNS 1. Each neural crest cell sends one long process to the body’s periphery or