Chapter 2
Structure and Functions of the
Cells of the Nervous System
The Nervous System
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Central Nervous System (CNS)
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Brain and Spinal cord
Encased within skull and spinal column
Peripheral Nervous System (PNS)
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All nervous tissue located outside the brain and spinal cord (i.e. nerves
of most of sensory organs)
Types of neurons
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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
brain
interneuron
Spinal cord
Cells of the Nervous System: Neurons
Neurons
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Neuron types:
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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
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Membrane – lipid bilayer creates a boundary for the cell’s contents
Nucleus – contains nucleolus and chromosomes
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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
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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
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Anterograde – from cell toward terminal buttons
Retrograde – from terminal buttons towards cell body
Supporting cells: Glia
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Oligodendrocytes
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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
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Phagocytosis
Protect brain from invading
microorganisms
Primarily responsible for inflammatory
reaction with brain damage
Supporting cells: Glia
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Astrocyte
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Provide physical support
Clean up debris
(phagocytosis)
Produce some necessary
compounds
Provide nourishment to
neurons
Supporting cells: Glia
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Schwann cells
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Create myelin sheath for axons in PNS
Differences from Oligodendrocytes:
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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)
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A semipermeable barrier b/t
the blood and the brain
Selectively permeable
Allows for tight regulation of
the components of ECF
Weak BBB areas:
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CVO’s
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Area postrema – poisons
detected here in order to
induce vomiting
Why is this necessary?
Communication within a neuron
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Neurons communicate through both chemical and electrical properties
Electrical Properties of Axons
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By using microelectrodes, we see that the axon is electrically charged:
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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
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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
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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
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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)
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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
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Na+  K+
Na+  Cl-
Sodium-potassium transporter
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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.
3.
4.
5.
6.
K+ channels open, K+ leaves cell (these
open later than Na+ channels)
Na+ channels become refractory (i.e.
blocked an cannot open again until
membrane reaches resting potential), no
more Na+ can enter cell
K+ keeps leaving cell, causing inside of
cell to be positively charged, and return to
resting level
Resting potential reached (after first
overshooting past); K+ channels close,
Na+ channels ready again
Extra K+ outside diffuses away; axon
ready for next action potential!
Conduction of action potential
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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
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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:
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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
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3 types: axodendritic, axosomatic, axoaxonic
Axodendritic – occur on smooth surface of dendrite or on dendritic
spines
Anatomy of synapse:
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Presynaptic membrane – synaptic cleft – Postsynaptic membrane
In terminal button:
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Mitochondria, synaptic vesicles (small or large sacs that contain
neurotransmitter), cisternae
Synaptic vesicle production:
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Small – in Golgi apparatus in soma or in cisternae
Large – only in soma, transported trough axoplasm to terminal button
Release of neurotransmitter
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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:
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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.
2.
3.
Receptor activates G protein (these are called G protein coupled receptors, or
GPCRs)
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)
Ions then enter cell to produce postsynaptic potential
Postsynaptic potentials
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Action potential is not determined by the neurotransmitter itself, but
by the ion channels they open
Ion channel types and effects:
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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
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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
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e.g. ACh, muscle contractions, broken down by acetylcholinesterase
(AChE)
HW for next time
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Phew, that was alot!
For next class, read Ch 3, and start studying for Quiz 1