
Consists of circuits of neurons and
supporting cells
› A Neuron is a nerve cell; the fundamental unit of
the nervous system, having structure and
properties that allow it to conduct signals by
taking advantage of the electrical charge
across its cell membrane
› In the simplest animals with a nervous system (ex.
cnidarians), the neurons controlling the
contraction and expansion of their
gastrovascular cavity are arranged in diffuse
nerve nets

More complex animals have nerve nets as well as
nerves, which are bundles of fiber-like extensions of
neurons
› Ex. Sea stars have a nerve net in each arm,
connected by radial nerves to a central nerve
ring. This organization is better suited than a
diffuse nerve net for
› controlling more
› complex movements.
http://www.cartage.org.lb/en/themes/Sciences/Life
Science/GeneralBiology/Physiology/NervousSystem/
NervousSystems/nervsys_1.gif

Greater complexity of nervous systems and more
complex behavior evolved with cephalization
› Clustering of neurons in a brain near the front
end in animals with elongated, bilaterally
symmetrical bodies
http://img.sparknotes.com/101s/bi
ology/20-1.jpg

A small brain and longitudinal nerve cords
constitute the simplest clearly defined
central nervous system (CNS)
› Simplest nervous system exhibited in flatworms,
such as the planarian
› In more complex invertebrates, behavior is
regulated by more complicated brains and
ventral nerve cords containing segmentally
arranged clusters of neurons called ganglia
Nerves that connect the CNS with the rest
of an animal’s body make up the
peripheral nervous system (PNS).
 Nervous system organization correlates with
animal’s lifestyle

http://img.sparknotes.com/101s/bi
ology/20-1.jpg
http://www.saburchill.com/images04/2
91007005.jpg
http://upload.wikimedia.org/wikibooks
/en/e/e4/Horse_nervous_system_labell http://www.cartage.org.lb/en/themes/Sciences/LifeScience/Ge
neralBiology/Physiology/NervousSystem/NervousSystems/nervsys
ed.JPG
_1.gif

Three stages of processing of information by
the nervous systems: sensory input,
integration, and motor output
› Sensory neurons transmit info from sensors that
detect external stimuli, internal conditions, or
muscle tension
› This info is sent to the CNS, where interneurons
integrate (analyze&interpret) the sensory input
› Motor output leaves the CNS via motor neurons,
which communicate with the effector cells
(muscle cells or endocrine cells)
 Example: Reflexes
http://www.youtube.com/watch?v=Q
mNQdLkkJHM&feature=related

Most of a neuron’s organelles are located in
the cell body.
› Two types of extensions from cell body:
 Dendrites- highly branched extensions that receive
signals from other neurons
 Axon- a typically much longer extension that transmits
signals to other cells
 Axon hillcock- conical
region of an axon where
it joins the cell body,
typically the region
where the signals
that travel down
the axon are
generated
http://science.kennesaw.edu/~jdirnber/Bio2108/Le
cture/LecPhysio/48_05NeuronStructure_L.jpg


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Many axons are enclosed by a layer called the myelin sheath
Near its end, an axon usually divides into several branches, each
of which ends in a synaptic terminal
› Snapse- the site of communication between a synaptic
terminal and another cell
Information is passed from the transmitting neuron to the
receiving cell by means of chemical messengers called
neurotransmitters
The complexity of a neuron’s shape is reflects the number of
synapses it has with other neurons
http://science.kennesaw.edu/~jdirnber/Bio2108/Le
cture/LecPhysio/48_05NeuronStructure_L.jpg

Gila are supporting cells that are essential for the structural
integrity of the nervous system and for the normal functioning
of the neurons
› Astrocytes- provide structural support for neurons and
regulate the extracellular concentrations of ions and
neurotransmitters
 Helps create the blood-brain barrier, which restricts the
passage of most substances into the CNS
http://porpax.bio.miami.edu/~cmallery
/150/neuro/c7.48.7.astrocytes.jpg


Radial Glia- form tracks along which newly formed neurons
migrate from the neural tube (the structure that gives rise to the
CNS)
› Both radial glia and astrocytes can act as stem cells,
generating neurons and other glia
Oligodendrocytes- glia that form the myelin sheaths around the
axon in the CNS
› Schwann cells- same thing, but in the PNS
› Myelin sheath provides
› electrical insulation of the axon
http://4.bp.blogspot.com/_TFshpEsf4xM
/R2aQJvKZSXI/AAAAAAAAADc/2Iiso91
w2r0/s400/motor%2Bneruone.bmp

All cells have an electrical potential
difference (voltage) across their plasma
membrane.
› This voltage is called the membrane
potential
 In neurons, the membrane potential is typically
between -60 and -80 mv (millivolts) when the
cell is not transmitting signals. The (-) indicates
that the inside of the cell is negative relative to
the outside.

The membrane potential of a neuron that
is not transmitting signals is called the
resting potential
› Resting potential depends on the ionic
gradients that exist across the plasma
membrane.
› Example: In mammals, the extracellular fluid
has a sodium ion concentration of 150 mM
(millimolar). In the cytosol, the Na
concentration is 15 mM. Therefore, the Na
concentration gradient is 150/15 = 10.
› (Outside concentration/Inside Concentration)

When the electrical gradient exactly
balances the concentration gradient, an
equilibrium is established.
› The magnitude of the membrane voltage at
equilibrium is called the equilibrium potential
(Eion) , and is given by the Nernst equation
› Eion = 62 mV [log ([ion] outside/ [ion] inside)]
 The Nernst equation applies to any membrane
that is permeable to a single type of ion.

The resting potential results from the
diffusion of K and Na ions channels that
are always open.

Neurons also have gated ion channels, which open or
close in response to three kinds of stimuli..
› Stretch-gated ion channels- are found in cells that
sense stretch and open when the membrane is
mechanically deformed
› Ligand-gated ion channels are found at synapses
and open or close when a specific channel when a
specific chemical binds to the channel
› Voltage-gated ion channels- are found in axons and
open or close when the membrane potential
changes

Gated ions are responsible for generating the
signals of the nervous system

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

Action Potential- the reversal and restoration across the
plasma membrane of a cell, as an electrical inpulse
passes along it (depolarization and repolarization).
A stimulus strong enough to produce a depolarization
that reaches the threshold triggers the action potential
When an impulse passes along the neuron, sodium and
potassium ions diffuse across the membrane through
voltage-gated ion channels
The electrical potential is initially reversed and then
restored. This is called an action potential.
Look in AP book for detail on pg. 1018 & Allot book pg.
53
Youtube video:
http://www.youtube.com/watch?v=SCasruJT-DU