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University of Toronto Scarborough
Janelle Leboutillier

Chapter 3: Cells of the Nervous System Neurons and Glia  2 types of cells, neurons and glia  Neuron is specialized to carry out the functions of info processing and communication  The glia serve a variety of support functions for neurons  10-15 glia for every neuron The Structure of Neurons  Many of the organelles are found in the soma or cell body  Neurons are different from other cells in that they have axons and dendrites which are used to communicate with other cells  Neuron membranes o The neural membrane must separate the intracellular fluid of the cell’s interior from the extracellular fluid surrounding the neuron o The neural membrane is made up of a double layer of phospholipids which do not dissolve water and because of this property the neural membrane is able to restrain the water based fluids on either sides, maintain the structural integrity of the cell o Within this layer, there are protein structures that control its permeability. The primary types of protein structures are ion channels and ion pumps o Ion channels allow ions to move passively whereas the ion pumps are active o Both ion channels and ion pumps show selectivity in ions (the amino acids that make up the ion channels or pump determines which ions will be allowed to pass through) o Voltage-gated channels open and close in response to the electrical status of adjacent areas of membrane o Ligand-gated channels open when they come in contact with specific chemicals which are typically naturally occurring chemical messengers but can be drugs from artificial sources o Most important pumps in neurons are sodium-potassium pumps and calcium pumps o Sodium-potassium pumps help maintain the differences in chemical composition between the intracellular and extracellular fluids and they do a “prisoner exchange” across the neural membrane by sending 3 sodium out while taking 2 potassium in o Calcium pumps do a similar job but they do not exchange, they pump calcium out  The Neural Cytoskeleton o The cytoskeleton provides structural support to the neuron o 3 types of filaments make up the skeleton… 1. Microtubules – largest of the 3; responsible for movement of various material in the cell; anterograde movement (soma to axon terminal) and retrograde movement (periphery to soma); implicated in development of Alzheimer’s; normally, tau protein connects the microtubules but in Alzheimer’s, tau levels become too high and they disconnect from the microtubules and get tangled causing neurofibrillary tangles (hinders cell’s ability to signal and maintain structure), leading to neuronal death 2. Neurofilaments – most common in the neuron; run parallel to the length of the axon and provide structural support 3. Microfilaments – smallest; may participate in changing he shape and length of the structures during development since they are located in the branches of the neurons  The Neural Cell Body o Specialized to participate in communication of the neuron o Nucleus – contains DNA that directs the cell’s functions and contains the nucleolus o Nucleolus – builds ribosomes which engage in protein synthesis and produce proteins either on their own or with the endoplasmic reticulum o Endoplasmic reticulum – divided into rough (because of ribosomes) and smooth o Golgi apparatus – inserts the completed proteins into vesicles o Mitochondria – extract oxygen and pyruvic acid from sugar in the ICF and construct and release molecules of ATP which is the major source of energy for the neuron  Dendrites o Serve as locations at which info from other neurons is received o The greater the surface area of the dendritic membrane a neuron has, the larger the number of connections it can form with other neurons o There are special ion channels on the dendrite synapse that serve as receptor sites which interact with neurotransmitters that are released by adjacent neurons in the synaptic gap o Some dendrites form dendritic spines that provide additional locations for synapses to occur and they are able to change their shape based on the amount of activity occurring at the synapse (contributes to learning and memory) o Abnormal dendritic spines are an underlying cause of some types of human mental retardation in which the spines become long and thin o The spines of people with retardation fail to mature normally in response to learning  The Axon o Responsible for carrying neural messages to other neurons o AP arise in the axon hillock and are then transmitted down the length of the axon o Myelin insulation allows for rapid signal transmission even in spite of the smaller axon diameter o Local circuit neurons are neurons that have axons that barely extend at all from the cell body and that communicate with adjacent cells (analogous to local telephone calling area) o Projection neurons have very large axons (analogous to a long-distance telephone system) o The ends of many neurons are divided into branches called collaterals o At the end of each axon collateral is the axon terminal which contains large numbers of mitochondria and synaptic vesicles that contain chemical messengers o The vast majority of CNS neurons and PNS neurons are myelinated o Myelin is formed by certain types of glia that wrap themselves around segments of axon but doesn’t cover the entire length of an axon o The axon hillock remains completely uncovered and between each myelin segment there is a node of Ranvier o Large diameter axons have thicker myelin and greater distances between nodes of Ranvier o Myelin allows human axons to be smaller in diameter without sacrificing transmission speed and space is saved. Myelin also reduces the energy requirements of neurons by decreasing the amount of work done by sodium- potassium pumps and the myelin segments wrap so tightly around axons that there is no ECF between the myelin and axon membrane (no need for myelin sheath) o In myelinated axons, ion channels are only in the axon hillock and nodes of Ranvier whereas in the unmyelinated axons, the channels are located along the entire length of the axon o Fewer ions move through the ion channels of a myelinated axon membrane than through an unmyelinated axon membrane of the same length during signaling Structural Variations in Neurons  Another strategy of categorizing neurons is according to the number of branches they have extending from the cell body  Unipolar neurons – have single branch from the cell body; invertebrate nervous system; in vertebrate, may be part of sensory systems and autonomic nervous system  Bipolar neuron – have 2 branches (axon and dendrite); play important roles in sensory systems; Ven Economo Neuron is a type of bipolar neuron found in the anterior cingulate cortex and are designed to provide fast intuitive assessments of complex situations  Most common neuron type is the multipolar neuron – have many branches and can be further classified into shape o Ex. Pyramidal cells, Purkinje cells Functional Variations in Neurons  Sensory neurons receive info from the outside world; vision, hearing, touch, taste, smell  Motor neurons transmit commands from the CNS to the muscles and glands  Interneurons are specialized for neither sensory nor motor functions but act as a bridge between them Glia  Support activity of neurons  Macroglia are the largest, microglia are the smaller varieties  Macroglia o Astrocytes  Provide support functions to neurons (structural matrix for neurons)  Form connections with blood supply of bran and that allows them to transfer glucose and other nutrient to the neurons and contribute to the BBB  Surround and isolate area of synapse to keep the neurotransmitters from moving outside a restricted area  They digest dead neurons  Important role in learning and memory which involves the reorganization of synaptic connections between neurons  Important role in chemical signaling and influence adjacent neurons to release glutamate and ATP (exciting and suppressing activity of neighboring cells)  Glutamate is excitatory and if too much is released by the glia, neurons will die o Oligodendrocytes and Schwann Cells  Supply myelin covering to insulate axon fibers  Provide myelin in CNS  Schwann cells supply the myelin for PNS  Oligodendrocytes contribute to the structural stability of the brain and spinal cord  Schwann cells provide single myelin segment on one peripheral axon (takes many Schwann cells to myelinate a peripheral nerve)  In multiple Sclerosis (MS), nervous system is demyelinated which causes neural signaling that does not work properly  Schwann cells help guide the growth of damaged axons whereas oligodendrocytes lack this ability  Microglia o Serve as brain’s cleanup crew to digest the debris in the brain such as any molecules related to cell damage or from any injury o Uncontrolled activation of microglia can damage the brain though with the release of substances that actually cause inflammation o Micoroglia can also sometimes ingest the healthy cells in the vicinity of the damaged ones The Generation of the Action Potential 1. Development of electrical signal , action potential, in axon hillock of presynaptic cell 2. At axon terminal, process switches from electrical to chemical signaling 3. Presynaptic neuron releases neurotransmitter from terminal and the neurotransmitter floats in the synaptic gap waiting for the postsynaptic neuron 4. Postsynaptic neuron can take the message and send it along The Ionic Composition of the Intracellular (ICF) and Extracellular Fluids (ECF)  ECF is characterized by large concentrations of Na and Cl ions but small concentration of K ions (similar to seawater)  ICF contains large numbers of K ions and low concentration of Na and Cl and large proteins that are negatively charged  Electrical environment inside the neuron is more negative than the outside (diff. of -70mV) o Resting membrane potential (RMP) The Movement of Ions  AP result from the movement of ions  Diffusion (tendency of molecules to distribute themselves equally within a medium) moves molecules along a concentration gradient from areas of high conc. to low conc.  There is more K inside the cell so
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