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Cells of Nervous System
 

Cells of Nervous System

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    Cells of Nervous System Cells of Nervous System Presentation Transcript

    • Cell Structure in the Nervous System
    • Cells & the Brain
      • All info comes from sensory receptors
      • This information is transformed in the brain into perceptions & commands for movement.
      • The complexity of the brain results from the number, not the variety, of cells
      • Brain contains ~ 100 billion nerve cells
    • Types of Cells
      • 2 classes of cells in nervous system
        • Neurons or nerve cells
        • Glial cells (glia)
      • Neurons not initially recognized as single cells because they differ from other cells:
        • much larger (axon can be 0.1 mm to 2 m)
        • different in shape
        • don't touch each other directly
        • Golgi stain
      • Glia surround neurons – from the Greek for ‘glue’
    • Glial Cells
      • There are about 1 trillion glial cells in the brain
        • 10 - 50 x more than there are neurons
      • Glial cells have perform important functions including:
        • surround & support neurons
        • separate & insulate groups of neurons
        • some produce myelin
        • some are scavengers
        • maintain concentration of K + in extracellular space
        • some take up & remove chemical transmitters
        • some guide migration of neurons during development
        • some help form an impermeable lining in capillaries & venules, creating the blood-brain barrier
    • Types of Glial Cells
      • Astrocytes
        • Most numerous in brain
        • Fill spaces between neurons
        • Serve to regulate composition of the extracellular space
      • Oligodendroglia
        • Wrap axons with myelin sheath in brain and spinal cord (Not all axons are myelinated)
        • Each oligodendrocyte wraps several axons
      • Microglia
        • Phagocytes - clean debris from dying neurons & glia
    • More Glial Cells
      • Schwann Cells
        • Wrap axons with myelin sheath outside the brain and spinal cord
        • Each Schwann cell wraps only one axon
      • Ependymal Cells
        • Line ventricles
        • Involved with secretion and absorption of cerebral spinal fluid
        • Also play a role in routing embryonic cells during development 
    • Neurons
      • Basic cells of the nervous system
      • There are about 100 billion neurons in the brain
        • more in the spinal cord, peripheral nervous system and sensory organs
      • Neurons derive form the neural tube during development
      • Neurons signal information electrically with nerve impulses
    • The Neuron Doctrine
      • Golgi stain – A reduced silver method using silver chromate that stains a few, isolated neurons in their entirety
        • Invented by Camillo Golgi
        • Santiago Ramon y Cajal used it to survey the variety of structures in vertebrate nervous systems
      • Led Cajal to propose the neuron doctrine:
        • nerve cells are the structural and functional basis of the nervous system and they must communicate with one another
        • confirmed by electron microscopy 50 years later
        • Golgi and Cajal shared a Nobel Prize
    • The Structure of a Neuron
      • 4 morphologically distinct regions:
      • • Cell Body ( soma )
      • • Axon
      • • Dendrites
      • • Presynaptic Terminals
    • The Cell Body
      • Also called soma ( somata ) - perikaryon
      • Metabolic center of the cell
        • Assembly of new membrane
      • Gives rise to dendrites & axon
      • Contains the nucleus
        • Site of DNA in chromosomes
      • Contains rough & smooth endoplasmic reticulum
        • Rough ER: Stacks of flattened membrane compartments with ribosomes attached
        • Smooth Endoplasmic Reticulum: connects RER with Golgi apparatus and serves to further process membrane proteins; serves to sequester and release calcium to control cytoplasmic composition
    • Other Elements of the Cell Body
      • Site of protein synthesis (ribosomes)
        • Free ribosomes and polyribosomes loose in cytoplasm
      • Contains the Golgi apparatus
        • Stacks of membrane compartments
        • Packaging of secretory products (neurotransmitter peptides)
      • Mitochondria
        • Site of conversion of energy from chemical bonds in food to ATP, the common energy currency of the cell
    • The Cytoskeleton
      • Superstructure of the cell – 3 components:
      • Microtubules
        • Largest - 20 nm thick-walled tubes
        • Spirals of alpha and beta tubulin molecules
        • Railroad tracks for movement of organelles, etc.
      • Neurofilaments
        • 10 nm twisted cables
        • Tend to be the most static structures of the three
      • Microfilaments
        • 5 nm double helix of actin
        • Especially common in neurites along with microtubules
        • Also found in thick meshes associated with some regions of the cell membrane
    • The Neuronal Membrane
      • A continuous sheet covering the neuron
      • Separates the cytoplasm from the extra-cellular fluid
      • Lipid bilayer with many embedded proteins
      • The embedded proteins are channels critical to neuronal function
    • Dendrites
      • Neurites which receive signals from other nerve cells (message IN )
      • Postsynaptic membrane in dendrites have molecular receptors for neurotransmitters (chemical messengers)
      • May have little bulges or pegs called dendritic spines where incoming synapses connect
      • Carry impulses from other neurons or receptors toward the cell body
      • Most neurons have several dendrites
      • Have polyribosomes suggesting some local protein synthesis in dendrites
    • The Axon
      • Main conducting unit of the neuron
      • Each neuron has only one axon
      • Axon collaterals – many axons do have branches
      • Carries impulses away from the cell body to other neurons or to effectors (message OUT )
        • Efferent - axon going away from reference neuron
        • Afferent - axon coming toward reference neuro
      • Axon hillock = the conical region at the beginning of an axon where it joins the cell body
      • Axon terminal ( terminal bouton ) – the end of the axon
    • Visualizing the Neuron
    • Myelin
      • Large axons have an insulating sheath - myelin
      • Supports, insulates & nourishes the axon and helps maintain chemical balance.
      • This sheath is made up of oligodendrocytes in the CNS & Schwann cells in the PNS.
      • Nodes of Ranvier = Gaps in the sheath which allow passage of the electrical signal through ion channels
        • Saltatory conduction
        • Transmission very fast .
    • Visualizing Myelinated Neurons
    • Presynaptic Terminals
      • Neurons don't touch; separated by a space = synaptic cleft (synapse)
        • Cell transmitting a signal = presynaptic
        • Cell receiving = postsynaptic; can be another neuron or an effector such as a muscle or gland
      • Signal within cells is electrical, but between cells is chemical (b/c no direct contact)
      • Chemical messengers which transmit the signal = neurotransmitters
      • Neurotransmitters are stored in vesicles, released from terminals at the end of the axon into the synapse
        • Terminal buttons
        • Motor endplates
    • Anatomy of a Synapse
    • Diversity of Neurons
      • Neurons can be classified in several ways:
      • Based on neurites
      • Based on dendrites
      • Based on connections/ function
      • Based on axon length
      • Based on neurotransmitter
    • Number of Neurite Processes
      • Neurons classed in 3 (4) groups by number of processes:
      • Unipolar
        • the simplest neuron
        • a single primary process with many branches
        • no dendrites from the cell body
        • common in invertebrates
      • Bipolar
        • oval shaped soma
        • 2 processes: dendrite (info in) & axon (info out)
        • most sensory neurons are bipolar
    • Number of Processes (Cont.)
      • Pseudounipolar
        • Sensory cells of touch, pressure, pain are special type of bipolar
        • 1st develop as bipolar; the 2 processes fuse to form a single axon
        • axon splits at the cell body
        • one goes to spinal cord, other to periphery (skin, joints, muscles)
      • Multipolar
        • most common type in vertebrates
        • one axon and one or more dendrite
        • vary in size and shape
    • Visualizing Cell Types
    • Classification Based on Dendrites
      • Pyramidal cells/stellate cells
      • Spiny cells/aspinous cells
    • Classification Based on Function
      • 3 groups:
      • Sensory ( afferent ) neurons
        • receive stimuli & transmit them to the central nervous system
      • Motor neurons
        • carry impulses away from the central nervous system to muscles or glands
      • Interneurons
        • link sensory to motor neurons
    • Visualizing Cell Types - 2
    • Classification Based on Neurotransmitter
      • Cholinergic neurons –
        • use acetylcholine
      • Gabanergic neurons –
        • use gamma aminobutyric acid
      • Dopaminergic neurons –
        • use dopamine
    • How Neurons Carry the Message
      • Only neurons are involved in transmission of electrical signals.
      • Within a nerve cell, message is an electrical signal = action potential
        • Cascading membrane depolarization creates the movement of the action potential as a nerve impulse
        • rapid, all or none impulses
        • in myelinated neurons, gaps at regular intervals allow regeneration of the action potential
      • Between nerve cells the message is carried chemically
    • Principles of Impulse Transmission
      • PRINCIPLE #1:
      • Information conveyed by an action potential is determined not by the form of the signal, but by the pathway the signal travels.
        • action potential for sound looks like action potential for odor
      • PRINCIPLE #2:
      • Principle of Dynamic Polarization :
      • electrical signals flow in a consistent direction
      • PRINCIPLE #3:
      • Principle of Connectional Specificity:
      • Contact is not random - cells communicate with certain targets and not others
    • Types of Signals
      • Each sensory & motor nerve cell generates 4 types of signals:
        • an input signal
        • an integration signal (trigger)
        • a conducting signal
        • an output signal
    • Functional Regions
      • Almost all neurons have 4 corresponding functional regions:
        • receptive - local input
        • integrative - trigger
        • conductile - signaling
        • secretory - output