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Chapter 3

B64 - Chapter 3

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University of Toronto Scarborough
Janelle Leboutillier

PSYB64 Chapter 3 Cells of the Nervous System Neurons and Glia  Nervous system is made up of two types of cells o Neurons: specialized to carry out the functions of information processing and communication o Glia: 10 – 50 glia for every neuron, variety of support functions for neurons The Structure of Neurons  Organelles: small internal structures found in animal cells that serve different functions/purposes  Cell body/soma: The main mass of a neuron, containing the nucleus and many organelles  Axons: The branch of a neuron usually responsible for carrying signals to other neurons  Dendrites: The branch of a neuron that generally receives information from other neurons Neurons Membranes  Neural membrane separates intracellular fluid of the cell’s interior from the extracellular fluid  Neural membrane is made up of a double layer of phospholipids (fatty molecules containing phosphates) o Do not dissolve in water due to fat content o Can restrain water based fluids on either side, maintain the structural integrity of the cell  Phospholipid membrane contains protein structures that maintain permeability (movement of substances across cell membrane)  Two primary types of protein structures o Ion channels o Ion pumps  Two structures are channels/pores to allow ions to follow into or out of neuron  Ion channels o Move ions passively o Ion selectivity o Ability to open and close in response to stimuli in immediate vicinity o Voltage – dependent channels: open in response to electrical status of surrounding membrane o Ligand-gated channels: open when they come in contact with specific chemicals  Typically naturally occurring chemical messengers (may be artificial sources from drugs)  Synapse: junction between two neurons  Ion pumps o Move actively with the need of energy o Ion selectivity o Sodium-potassium pumps: maintain the differences in chemical composition between intracellular and extracellular fluids  Sodium ions are sent out as potassium ions come in (3 Na for every 2 K) o Calcium pumps: moves calcium ions out of cell  Use less energy than sodium – potassium pumps  Neurotransmitters: chemical messengers The Neural Cytoskeleton  Cytoskeleton: structural support that maintains the shape of the neuron  Three types of filament/fiber that make up neural cytoskeleton o Also move elements within the cell and anchor various channel and receptor proteins in appropriate places of neural membrane PSYB64  Microtubules: largest type of cytoskeleton; hollow tubes o Movement of various materials within the cell  Anterograde transport: movement along the microtubules from the cell body to axon terminal  Retrograde transport: movement back to the cell body from the periphery of the neuron  Alzheimer’s disease: characteristic symptoms is the presence of neurofibrillary tangles consisting of a protein (tau) o In a healthy, tau connects adjacent microtubules and holds them in place  too much in Alzheimer’s disease  Neurofilaments: middle – sized filaments; most common fiber in the neuron o Structure is similar to that of hair o Runs parallel to the length of the axon and provide structural support  Microfilaments: smallest fibers; participate in changing the shape and length of the neuron branches during development in response to learning The Neural Cell Body  Nucleus: contain the DNA that directs the cell’s functions  Nucleolus: substructure of the nucleus which builds ribosomes  Ribosomes: engage in protein synthesis; produce proteins on their own and with the ER  Endoplasmic reticulum: located in cell body; both rough or smooth; o Rough: has many ribosomes on surface; smooth does not have any o Ribosomes are made at the Rough ER and then moved by Smooth ER  Golgi apparatus: receives ribosomes from smooth ER; inserts completed proteins into vesicles  Mitochondria: extract oxygen and pyruvic acid from sugar in the intracellular fluid and construct and release molecules of ATP Dendrites  Branches; each neuron has a large amount  Serve as locations at which information is received from other neurons  Greater the surface area of dendrite membrane a neuron has, larger the number of connections or synapses it can form with other neurons  At each synapse, there are special ion channels serving as receptor sites that are embedded in the neural membrane o Receptor sites interact with molecules of neurotransmitter released by adjacent neurons that float across synaptic gap (fluid – filled space between the transmitting and receiving neurons)  Dendritic spines: knobs formed by some dendrites o Provide additional locations for synapses to occur o Can change their shape based on the amount of activity occurring at the synapse (contributes to learning and memory)  Human mental retardation can be caused by abnormal dendritic spines since they play a role in learning and memory o Spines are long and thin (similar to those of human fetuses with undeveloped dendritic spines)  Fragile X mental retardation protein (FMRP): found to be an essential protein to normal maturation of dendritic spines The Axon  Only one axon each neuron  Responsible for carrying neural messages to other neurons  Axon hillock: cone – shape segment of axon that lies at the unction of the axon and the cell body o Action potentials arise from her and then are transmitted down the length of the axon  Myelin: material that allows for rapid signal transmission; regardless of axon diameter o Not all axons are myelinated o Formed by certain types of glia that wrap themselves around axons PSYB64 o Does not cover entire length (axon hillocks are uncovered) o Allow axons to be smaller in diameter without sacrificing transmission speed o Reduced energy requirement of neurons by decreasing work of Na-K pumps o Little to no extracellular fluid between myelin and axons  Node of Ranvier: areas in between myelin segments that are bare  Axons vary in length and diameter  Local circuit neurons: neurons that have axons that barely extend to all from the cell body and that communicate with adjacent cells  Projection neurons: have very long axons  Collaterals: ends of axons that divide into many branches o Allows neurons with single axon to communicate with other cells  Axon terminal: swelling at the end of the collateral o Contains large numbers of mitochondria and synaptic vessels  Synaptic vesicles: contain chemical messengers; made from double – lipid molecule structure Structural Variations in Neurons  Unipolar neurons: single branch extending from the cell body o Invertebrate nervous systems o Sensory systems and autonomic nervous system  Bipolar neuron: have two branches extending from neural cell body (one axon, one dendrite) o Sensory systems (ex. Retina of the eye)  Von Economo neuron: special kind of neuron that was recently developed in primate evolution  Multipolar neuron: many branches extending from the cell body (one axon, numerous dendrites) Functional Variations in Neurons  Sensory neurons: specialized to receive information from the outside world o Vision, hearing, touch, taste, smell  Motor neurons: transmit commands from the CNS to muscles and glands  Interneurons: act as bridges between the sensory and motor systems Glia  Two types: micro and microglia Macroglia  Three primary types o Astrocytes: provide many support functions o Oligodendrocytes: supply myelin covering that insulates axon fibers o Schwann cells: “  Astrocytes are star shaped and are most common type of glia in the brain  Primary function is to provide structural matrix for neurons so they don’t float around  Form connections with blood supply of the brain  allows for transfer of glucose and other nutrients to neurons  Contribute to blood – brain barrier prevent toxins from entering brain from blood supply  Surround and isolate area of synapse, keeping neurotransmitters from moving out of restricted area o Collect excess neurotransmitters from synaptic gap  When CNS neurons are damaged, astrocytes move to and digest damaged neuron forming scar tissue o Prevents neural growth  Influence adjacent neurons and astrocytes by releasing glutamate and ATP  leads to excitement or suppression of neurons and astrocytes o ^Info can help manage brain diseases; astrocytes can limit glutamate release from glial cells during stroke to prevent damage  Oligodendrocytes provide myelin in CNS PSYB64  Schwann cells supply myelin for peripheral nerves exiting the brain and spinal cord  Multiple sclerosis (MS) – progressive demyelination of the nervous system leading to improper neural signaling  Schwann cells guide regrowth of damaged axons  Oligodendrocytes lack this capacity ^  Regrowth of peripheral axons makes body transplants possible – only for limbs not for spinal cord or brain damage repair Microglia  Branches of microglia reach out and sample their environments  If any molecules related to cell damage are detected they travel to the location of damage/injury and digest the debris  Uncontrolled activation can damage brain through release of substances leading to inflammation o May also digest healthy cells located near damaged cells The Generation of the Action Potential 1. Development of electrical signal (action potential) in the axon hillock of the neuron (sending or presynaptic) 2. Once action potential arrives at the axon terminal, the presynaptic neuron release neurotransmitters from its terminal 3. Neurotransmitters float across synaptic gap to the postsynaptic neuron a. Postsynaptic neuron then decides whether to send the message along  Recent studies have shown that some messages are done through chemical reactions that occur along axon instead of the use of an electrical signal  Energy for AP (action potential) comes from characteristics of intra and extracellular fluids The Ionic Composition of the Intracellular and Extracellular Fluids  Intra and extracellular fluids both contain water but have diff
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