Chapter 2 by yOm8H3

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									                Chapter 2
Structure and Functions of the
 Cells of the Nervous System
The Nervous System

   Central Nervous System (CNS)
       Brain and Spinal cord
       Encased within skull and spinal column
   Peripheral Nervous System (PNS)
       All nervous tissue located outside the brain and spinal cord (i.e. nerves
        of most of sensory organs)
Types of neurons

   Sensory neurons – a neuron that detects changes in the external or
    internal env’t and sends info about these changes to the CNS
   Motor neuron – a neuron located within the CNS that controls the
    contraction of a muscle or the secretion of a gland
   Interneuron – a neuron located entirely within the CNS
                                              Sensory neuron
    Motor neuron
                   interneuron



                             Spinal cord
        brain
Cells of the Nervous System: Neurons
Neurons

   Neuron types:
       Multipolar – a neuron with one
        axon and many dendrites
        attached to its soma
       Bipolar – a neuron with one
        axon and one dendrite
        attached to its soma
       Unipolar – a neuron with one
        axon attached to its soma; the
        axon divides, with one branch
        receiving sensory info and the
        other sending the info to the
        CNS
Internal structure of the neuron

   Membrane – lipid bilayer creates a boundary for the cell’s contents
   Nucleus – contains nucleolus and chromosomes
        Nucleolus – produces ribosomes
        Ribosomes – a cytoplasmic structure, made of protein, that serves as the site of
         production of proteins translated from mRNA
        Chromosomes – a strand of DNA, with assc. Proteins, found in the nucleus;
         carries genetic info
   Mitochondria – an organelle that is responsible for extracting energy from
    nutrients (and thus providing cells with ATP)
   Endoplasmic reticulum – contains ribosomes (rough) and provides channels
    for segregation of molecules involved in cellular processes (smooth); lipid
    molecules are made here (smooth)
   Golgi apparatus – wraps around products of a secretory cell (secretion =
    exocytosis); also produces lysosomes (breaks down waste products)
Internal structure of the neuron

   Cytoskeleton – structural support system of neuron; made of 3 kinds
    of protein strands (one of these is microtubules)
   Microtubule – involved in transporting substances from place to
    place within cell
   Axoplasmic transport – active process by which substances are
    propelled along microtubules that run the length of the axon
        Anterograde – from cell toward terminal buttons
        Retrograde – from terminal buttons towards cell body
Supporting cells: Glia

   Oligodendrocytes
        Provide support to axons by formation of
         myelin sheath
        Form a non-continuous tube of insulation
         along axon
        Bare, non-myelinated portions called
         Nodes of Ranvier
        In CNS only (Schwann cells form myelin
         in PNS)
   Microglia
        Phagocytosis
        Protect brain from invading
         microorganisms
        Primarily responsible for inflammatory
         reaction with brain damage
Supporting cells: Glia

   Astrocyte
       Provide physical support
       Clean up debris
        (phagocytosis)
       Produce some necessary
        compounds
       Provide nourishment to
        neurons
Supporting cells: Glia

   Schwann cells
       Create myelin sheath for axons in PNS
       Differences from Oligodendrocytes:
            With nerve damage, Schwann cells remove dead and dying axons, then
             help guide regrowth; Oligos don’t aid in regrowth this way
            Also, the immune system of individuals with multiple sclerosis attacks only
             myelin produced by Oligos, not of Schwann cells
Blood Brain Barrier (BBB)

   A semipermeable barrier b/t
    the blood and the brain
   Selectively permeable
   Allows for tight regulation of
    the components of ECF
   Weak BBB areas:
        CVO’s
             Area postrema – poisons
              detected here in order to
              induce vomiting
             Why is this necessary?
Communication within a neuron

   Neurons communicate through both chemical and electrical properties
   Electrical Properties of Axons
        By using microelectrodes, we see that the axon is electrically charged:
             Inside is negatively charged with respect to outside (a difference of 70 mV)
             Inside membrane of axon charge = -70 mV = membrane potential
              potential is a stored up source of energy
             Resting potential – the membrane potential of a neuron when it is not being altered by
              excitatory or inhibitory postsynaptic potentials
             Excitatory vs Inhibitory
                   Excitatory – causes action potential to happen
                   Inhibitory – inhibits action potential from occurring
             Depolarization – reduction (toward zero) of the membrane potential of a cell from
              normal resting (-70 mV); causes action potential
             Hyperpolarization – increase in the membrane potential; occurs after action potential
Communication within a neuron

   Action potential – the brief
    electrical impulse that provides
    the basis for conduction of info
    along an axon
   Threshold of excitation – the
    value of the membrane
    potential that must be reached
    to produce an action potential
Membrane potential

   Q: Why is there a membrane potential?
   A: Result of balance between diffusion and electrostatic pressure
   Diffusion – movement of molecules from regions of high conc. To
    low conc.
   Substances (electrolytes, i.e. acid, base, or salt) dissolved in water
    split into two parts  ions (cations and anions)
        e.g. Na+, K+, Cl-
   Electrostatic pressure – the attractive force b/t atomic particles
    charged with opposite signs or the repulsive force b/t atomic
    particles charged with the same sign
        Na+  K+
        Na+  Cl-
Sodium-potassium transporter

   A protein found in the
    membrane of all cells that
    exchange Na+ for K+ (3 Na+
    out, 2 K+ in)
   Effectively keep intracellular
    conc. of Na+ low
   Ion channel – a specialized
    protein molecule that permits
    specific ions to enter or leave
    cells
The Action Potential

1.   Threshold of excitation is reached, Na+
     channels open (voltage dependent), Na+
     enters cell
2.   K+ channels open, K+ leaves cell (these
     open later than Na+ channels)
3.   Na+ channels become refractory (i.e.
     blocked an cannot open again until
     membrane reaches resting potential), no
     more Na+ can enter cell
4.   K+ keeps leaving cell, causing inside of
     cell to be positively charged, and return to
     resting level
5.   Resting potential reached (after first
     overshooting past); K+ channels close,
     Na+ channels ready again
6.   Extra K+ outside diffuses away; axon
     ready for next action potential!
Conduction of action potential

   Basic law of axonal conduction: All-or-none law, i.e. action potential,
    once started, is always finished to the end of the axon
   Rate law – variations in the intensity of the stimulus or other info
    being transmitted in an axon are represented by variations in the
    rate at which that axon fires
   Saltatory conduction – conduction of action potentials by myelinated
    axons; “jumps” from one node of Ravier to the next
Communication between neurons

   Via chemical properties
   To get info across synapse from presynaptic neuron to postsynaptic
    neuron: use of chemical neurotransmission
   Neurotransmitters produce postsynaptic potentials, either de- or
    hyperpolarizations, that affect rate law
   Neurotransmitters:
        Produced in cell
        Released by terminal buttons
        Detected by receptors on postsynaptic neuron
        Also neuromodulators (e.g. peptides) are released, but can travel farther
   Hormones, produced by endocrine glands, can affect cell activity also
    (target cells)
   All 3 attach to a receptor molecule called the binding site (lock and key); the
    chemical that attaches to the binding site is called a ligand
Structure of synapses

   3 types: axodendritic, axosomatic, axoaxonic
   Axodendritic – occur on smooth surface of dendrite or on dendritic
    spines
   Anatomy of synapse:
        Presynaptic membrane – synaptic cleft – Postsynaptic membrane
   In terminal button:
        Mitochondria, synaptic vesicles (small or large sacs that contain
         neurotransmitter), cisternae
        Synaptic vesicle production:
             Small – in Golgi apparatus in soma or in cisternae
             Large – only in soma, transported trough axoplasm to terminal button
Release of neurotransmitter

   Synaptic vesicles dock at release zone; Calcium enters
    cell via channels with arrival of action potential; Ca+
    binds with docked vesicles to open fusion pore;
    neurotransmitter molecules diffuse from vesicle through
    fusion pore into synaptic cleft
Activation of receptors

       After neurotransmitter release:
       Cross synaptic cleft to bind to postsynaptic receptors
       These receptors open neurotransmitter-dependent ion channels, 2 types:
          Ionotropic – direct method; contains binding site for neurotransmitter, which
           when activated, opens an ion channel to allow ions into cell to produce
           postsynaptic potential (see Fig 2.33 in text); effects do not last long
          Metabotropic – indirect method, long-lasting effects; contain neurotransmitter
           receptors that start a chain of chemical events: (Fig 2.34 in text)
          1.   Receptor activates G protein (these are called G protein coupled receptors, or
               GPCRs)
          2.   α subunit (attached to G protein) breaks away and binds with separate ion channel
               and opens it (Fig 2.34 a); or attaches to enzyme, which then activates second
               messenger to open ion channel (Fig 2.34 b)
          3.   Ions then enter cell to produce postsynaptic potential
Postsynaptic potentials

   Action potential is not determined by the neurotransmitter itself, but
    by the ion channels they open
   Ion channel types and effects:
        Na+ channel: influx causes EPSP
        K+ channel: efflux (out of cell) causes IPSP
        Cl- channel: influx causes IPSP
        Ca2+ channel: influx activates enzyme which has effects on
         postsynaptic neuron
   Buildup of EPSP creates action potential (depolarization)
   Buildup of IPSP inhibits action potential (hyperpolarization)
Termination of postsynaptic potentials

   Almost all NT are terminated by reuptake (transporter protein that
    moves NT molecules back into presynaptic cell)
   Also, by enzymatic deactivation, where an enzyme will break down
    the NT molecules
        e.g. ACh, muscle contractions, broken down by acetylcholinesterase
         (AChE)
HW for next time

   Phew, that was alot!
   For next class, read Ch 3, and start studying for Quiz 1

								
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