Structure and Functions of the Cells of the Nervous System

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Chapter 2

• Structure and Functions of the Cells of the
  Nervous System

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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)

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Types of neurons

• Sensory neurons a neuron that detects changes
  in the external or internal envt 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
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Cells of the Nervous System Neurons
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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

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Internal structure of the neuron

• Membrane lipid bilayer creates a boundary for
  the cells 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)

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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

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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

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Supporting cells Glia

• Astrocyte
• Provide physical support
• Clean up debris (phagocytosis)
• Produce some necessary compounds
• Provide nourishment to neurons

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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
  dont aid in regrowth this way
• Also, the immune system of individuals with
  multiple sclerosis attacks only myelin produced
  by Oligos, not of Schwann cells

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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
• CVOs
• Area postrema poisons detected here in order to
  induce vomiting
• Why is this necessary?

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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

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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

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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-

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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

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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!

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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

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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

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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

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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

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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)
• Receptor activates G protein (these are called G
  protein coupled receptors, or GPCRs)
• a 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)
• Ions then enter cell to produce postsynaptic
  potential

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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)

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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)

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HW for next time

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