4Organization of the Nervous System Central nervous system (CNS)Brain and spinal cordIntegration and command centerPeripheral nervous system (PNS)Paired spinal and cranial nervesCarries messages to and from the spinal cord and brain
5Peripheral Nervous System (PNS): Two Functional Divisions Sensory (afferent) divisionSensory afferent fibers – carry impulses from skin, skeletal muscles, and joints to the brainVisceral afferent fibers – transmit impulses from visceral organs to the brainMotor (efferent) divisionTransmits impulses from the CNS to effector organs
6Motor Division: Two Main Parts Somatic nervous systemConscious control of skeletal musclesAutonomic nervous system (ANS)Regulates smooth muscle, cardiac muscle, and glandsDivisions – sympathetic and parasympathetic
8The Cells of the Nervous System The human nervous system is comprised of two kinds of cells:Neurons: excitable cells that transmit electrical signalsGlia: Supporting cells – cells that surround and wrap neuronsThe human brain contains approximately 100 billion individual neurons.Behavior depends upon the communication between neurons.
9Fig. 2-1, p. 30 Figure 2.1: Estimated numbers of neurons in humans. Because of the small size of many neurons and the variation in cell density from one spot to another, obtaining an accurate count is difficult. (Source: R. W. Williams & Herrup, 1988)Fig. 2-1, p. 30
10Supporting Cells: Neuroglia The supporting cells (neuroglia or glial cells):Provide a supportive scaffolding for neuronsSegregate and insulate neuronsGuide young neurons to the proper connectionsPromote health and growth
11Supporting Cells: Neuroglia Glial cells make up 90 percent of the brain's cells. Glial cells are nerve cells that don't carry nerve impulses.The various glial (meaning "glue") cells perform many important functions, including: digestion of parts of dead neurons, manufacturing myelin for neurons, providing physical and nutritional support for neurons,
16Microglia and Ependymal Cells Microglia – small, ovoid cells with spiny processesPhagocytes that monitor the health of neuronsEpendymal cells – range in shape from squamous to columnar (Ciliated)They line the central cavities of the brain and spinal columnTheir apical surfaces are covered in a layer of cilia, which circulate CSF around the central nervous system. Their apical surfaces are also covered with microvilli, which absorb CSF. Ependymal cells are a type of Glial cell and are also CSF producing cells
18Oligodendrocytes, Schwann Cells, and Satellite Cells Oligodendrocytes – branched cells that wrap CNS nerve fibersSchwann cells (neurolemmocytes) – surround fibers of the PNSSatellite cells surround neuron cell bodies with ganglia
19Oligodendrocytes, Schwann Cells, and Satellite Cells Figure 11.3d, e
20Fig. 2-10, p. 35 Figure 2.10: Shapes of some glia cells. Oligodendrocytes produce myelin sheaths that insulate certain vertebrate axons in the central nervous system; Schwann cells have a similar function in the periphery. The oligodendrocyte is shown here forming a segment of myelin sheath for two axons; in fact, each oligodendrocyte forms such segments for 30 to 50 axons. Astrocytes pass chemicals back and forth between neurons and blood and among neighboring neurons. Microglia proliferate in areas of brain damage and remove toxic materials. Radial glia (not shown here) guide the migration of neurons during embryological development. Glia have other functions as well.Fig. 2-10, p. 35
21Figure 2.11: How an astrocyte synchronizes associated axons. Branches of the astrocyte (in the center) surround the presynaptic terminals of related axons. If a few of them are active at once, the astrocyte absorbs some of the chemicals they release. It then temporarily inhibits all the axons to which it is connected. When the inhibition ceases, all of the axons are primed to respond again in synchrony. (Source: Based on Antanitus, 1998)Fig. 2-11, p. 36
22Neurons (Nerve Cells) Structural units of the nervous system Composed of a body, axon, and dendritesLong-lived, amitotic, and have a high metabolic rateTheir plasma membrane function in:Electrical signaling
23Fig. 2-4, p. 32 Figure 2.4: Neurons, stained to appear dark. Note the small fuzzy-looking spines on the dendrites.Fig. 2-4, p. 32
27Figure 2.2: An electron micrograph of parts of a neuron from the cerebellum of a mouse. The nucleus, membrane, and other structures are characteristic of most animal cells. The plasma membrane is the border of the neuron. Magnification approximately x 20,000. (Source: Micrograph courtesy of Dennis M. D. Landis)Fig. 2-2, p. 31
28The Cells of the Nervous System The membrane refers to the structure that separates the inside of the cell from the outside environment.The nucleus refers to the structure that contains the chromosomes.The mitochondria are the structures that perform metabolic activities and provides energy that the cells requires.Ribosomes are the sites at which the cell synthesizes new protein molecules
29Nerve Cell Body (Perikaryon or Soma) Contains the nucleus and a nucleolusIs the major biosynthetic centerIs the focal point for the outgrowth of neuronal processesHas no centrioles (hence its amitotic nature)Has well-developed Nissl bodies (rough ER)Contains an axon hillock – cone-shaped area from which axons arise
30Processes Armlike extensions from the soma Called tracts in the CNS and nerves in the PNSThere are two types: axons and dendrites
31Dendrites of Motor Neurons Short, tapering, and diffusely branched processesThey are the receptive, or input, regions of the neuron
32Axons: StructureSlender processes of uniform diameter arising from the hillockLong axons are called nerve fibersUsually there is only one unbranched axon per neuronRare branches, if present, are called axon collateralsAxonal terminal – branched terminus of an axon
33Axons: Function Generate and transmit action potentials Secrete neurotransmitters from the axonal terminalsMovement along axons occurs in two waysAnterograde — toward axonal terminalRetrograde — away from axonal terminal
34Myelin SheathWhitish, fatty (protein-lipoid), segmented sheath around most long axonsIt functions to:Protect the axonElectrically insulate fibers from one anotherIncrease the speed of nerve impulse transmission
35MyelinMyelin is about 40 % water; the dry mass of myelin is about % lipid (cholesterol and phospholipid)) and about % proteins.Some of the proteins that make up myelin are myelin basic protein (MBP), myelin oligodendrocyte glycoprotein (MOG), and proteolipid protein (PLP).The primary lipid of myelin is a glycolipid called galactocerebroside. The intertwining hydrocarbon chains of sphingomyelin serve to strengthen the myelin sheath.
36Myelin Sheath and Neurilemma: Formation Formed by Schwann cells in the PNSA Schwann cell:Envelopes an axon in a troughEncloses the axon with its plasma membraneHas concentric layers of membrane that make up the myelin sheathNeurilemma – remaining nucleus and cytoplasm of a Schwann cell
37Myelin Sheath and Neurilemma: Formation Figure 11.5a–c
39Nodes of RanvierAre gaps in the myelin sheath formed by spaces between successive oligodendrocytes (in CNS) or Schwann cells (in PNS) along the length of the axon.Nodes of Ranvier contain Na+ ion channels, and are sites where action potentials are generated by membrane depolarizations.They are the sites where axon collaterals can emerge
40Unmyelinated AxonsA Schwann cell surrounds nerve fibers but coiling does not take placeSchwann cells partially enclose 15 or more axons
41Axons of the CNS Both myelinated and unmyelinated fibers are present Myelin sheaths are formed by oligodendrocytesNodes of Ranvier are widely spacedThere is no neurilemma
42Regions of the Brain and Spinal Cord White matter (diencephalon) – dense collections of myelinated fibersGray matter – mostly soma and unmyelinated fibers
43White matterSituated between the brainstem and cerebellum, the white matter consists of structures at the core of the brain such as the thalamus and hypothalamusCertain nuclei within the white matter are involved in the expression of emotions, the release of hormones from the pituitary gland, and in the regulation of food and water intake
44White matterThe nuclei of the white matter are involved in the relay of sensory information from the rest of the body to the cerebral cortex, as well as in the regulation of autonomic (unconscious) functions such as body temperature, heart rate and blood pressure.
45Grey matterGrey matter – closely packed neuron cell bodies form the grey matter of the brain.The grey matter includes regions of the brain involved in muscle control, sensory perceptions, such as seeing and hearing, memory, emotions and speech.
46Neuron Classification Structural:Multipolar — three or more processesBipolar — two processes (axon and dendrite)Unipolar — single, short process
47Neuron Classification Functional:Sensory (afferent) — transmit impulses toward the CNSMotor (efferent) — carry impulses away from the CNSInterneurons (association neurons) — shuttle signals through CNS pathways
51Comparison of Structural Classes of Neurons Table
52Comparison of Structural Classes of Neurons Table
53Comparison of Structural Classes of Neurons Table
54Blood brain barrierThe blood-brain barrier is a mechanism that surrounds the brain and blocks most chemicals from entering.Because neurons in the brain generally do not regenerate, it is vitally important for the blood brain barrier to block incoming viruses, bacteria or other harmful material from entering.
56Fig. 2-12, p. 37 Figure 2.12: The blood-brain barrier. Most large molecules and electrically charged molecules cannot cross from the blood to the brain. A few small, uncharged molecules such as O2 and CO2 cross easily; so can certain fat-soluble molecules. Active transport systems pump glucose and amino acids across the membrane.Fig. 2-12, p. 37