Nervous System
Chapter 9
Pages 211-257
Chapter 9 Wordbytes
1.
2.
3.
4.
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8.
9.
10.
af- = toward
arachn- = spider
astro- = star
auto- = self
dendro- = tree
di- = 2, through
ef- = away from
enter- = intestines
epen- = above
-ferrent = carried
encephalo- = brain
12. gangli- = swelling
13. -glia = glue
14. mening- =membrane
15. micro- = small
16. neuro- = nerve
17. –oid = similar to
18. oligo- = few
19. peri- = around
20. somat- = body
11.
Nervous System Overview



Master controller and communicator for the
body
Responsible for all behavior
3 functions:



Sensory input monitors changes
inside/outside of body
Integration processes and interprets, then
decides what should be done
Motor output causes a response in effector
organs
Organization—2 main parts:
1.
Central Nervous System (CNS) = brain
and spinal cord

2.
Interprets incoming sensory info. and
dictates motor responses
Peripheral Nervous System (PNS) =
nerves from brain & in spinal cord



INPUT-Afferent or Sensory division
OUTPUT- Efferent or Motor division
Subdivided: Somatic (SNS—from CNS to
skeletal muscles=voluntary) & Autonomic
(ANS—regulate smooth & cardiac muscle,
glands=involuntary)
Major structures:
Histology


Highly cellular—densely packed & tightly
intertwined
2 types of cells:
1.
Neuron= nerve cell
•
2.
Specialized for signal carrying & information
processing
Neuroglia cells support, nourish & protect
neurons
•
Neuroglia critical for homeostasis of interstitial
fluid around neurons
Supporting cells (Neuroglia)
~ half the volume of CNS
 Cells smaller than neurons
 Can multiply and divide and fill in
brain areas
 Do not conduct nerve impulses

Supporting Cells in CNS
Astrocytes most abundant and most
versatile; blood-brain barrier
 Oligodendrocytes (O lig o dendro cytes)
have fewer branches; produce insulating
myelin sheath in CNS
 Microglia ovoid cells with thorny processes;
provide defense (because immunity cells not
allowed in CNS)
 Ependymal cells squamous/columnar cells
with cilia; produce cerebrospinal fluid (CSF)

Supporting Cells in PNS
Schwann cells PNS cell support; produce
& maintain myelin sheath, regenerate
PNS axons
 Satellite cells in PNS ganglia; support
neurons in ganglia, regulate exchange of
materials between neurons and interstitial
fluid

Neuron Characteristics
They conduct nerve impulses from
one part of the body to another
 They have extreme longevity
live/function for a lifetime
 They are amitotic (a mi totic) lose
their ability to divide
 They have a high metabolic rate =
need O2 and glucose

Neuronal Structure




Cell body nucleus, cytoplasm with typical
organelles; most within CNS = protected by cranial
bones & vertebrae
Dendrites short, highly branched input structures
emerging from cell body = high surface area to
receive signals
Axon may be short or long, only one per neuron;
conducts away from cell body toward another
neuron or effector
 Emerges at cone-shaped axon hillock
Axon terminals at end of axon with synaptic
bulbs
(Neurilemma)
Figure 9.3
= impulse
direction
Pg. 216
Myelination

Axons covered with a myelin sheath



Many layered lipid & protein creating insulations
Increases speed of nerve conduction.
Formed by:
• Schwann cells in PNS
• Oligodendrocytes (O lig o dendro cytes) in CNS

Nodes of Ranvier (Ron v a)= gaps in the myelin


Nodes are important for signal conduction
Some diseases destroy myelin multiple sclerosis &
Tay-Sachs
Multiple Sclerosis

What is it?
https://health.google.com/health/ref/Multiple+sclerosis
Gray and White Matter
White matter- primarily
myelinated axons
 Gray matter- nerve cell
bodies, dendrites,
unmyelinated axons,
axon terminals &
neuroglia


Spinal cord gray matter
is centrally located
Classification of Neurons
1.
Structural according to #
of processes (Fig. 9.6):



Multipolar 3 or more;
most common, especially
in CNS
Bipolar 2 processes (an
axon and a dendrite) that
extend from opposite
sides; found in special
sense organs
Unipolar 1 process that
divides like a T; found in
ganglia in PNS
2.
Functional according to the
direction impulse travels (Fig. 9.7)

Sensory (afferent) neurons
transmit impulses from sensory
receptors toward or into the
CNS; mostly unipolar, with cell
bodies in ganglia outside CNS

Motor (efferent) neurons carry
impulses away from CNS to
muscles and glands; multipolar,
usually with cell bodies in CNS

Interneurons (association
neurons) between motor &
sensory neurons; most in CNS;
99% of neurons in body; mostly
multipolar
Neurophysiology
Neurons are highly irritable =
responsive to stimuli
 When stimulated, an electrical
impulse (action potential) is conducted
along its axon


Action potential underlies all functional
activities of the nervous system
Action Potentials
Action potentials = nerve impulses
 Require a membrane potential


electrical charge difference across cell
membrane – like a battery
Ion Channels allow ions to move by diffusion
= current
 If no action potential then resting cell has
resting membrane potential

Ion Channels

Allow specific ions to diffuse across
membrane

Move from high concentration to low
or toward area of opposite charge
Leakage channels
 Gated channels- require trigger to open
 Voltage- Gated channels respond to a
change in membrane potential

Resting Membrane Potential
Leakage channels
 Cytosol high in K+ & interstitial fluid high in
Na+ (sodium –potassium pumps)
 Leakage lets K+ through easily and Na+
poorly
 inside is negative relative to outside
 actual value depends on the relative leakage
channel numbers

Figure 9.4
Graded Potentials
Short-lived, local changes to
membrane potential
 Cause current flows that decrease
with distance
 Magnitude varies with strength of
stimulus

Action Potential (AP)
Generated by neurons and muscle
cells
 Series of active events
 Channels actively open & close
 Some initial event is required to reach
a voltage threshold (~ = - 55 mv)
 Stimulus = any event bringing
membrane to threshold

Action Potential

Resting state


Depolarizing phase

membrane potential rises and becomes
positive
Repolarizing phase

voltage-gated channels closed
potential restored to resting value ( PNa,
 PK)
Undershoot

Potassium permeability continues
Figure 9.5
Active Events
Stimulus to reach threshold
 Na+ channel opens=>
 Na+ ions enter=>
 positive potential=>
 Causes K+ channel opening =>
 repolarization

All- or –None Phenomenon
This sequence is always the same
 If threshold then the same size of changes
occur no larger or smaller APs
 Stimulus must reach threshold to start
 After one AP there is a short period before
next can be triggered= absolute refractory
period each AP is a separate, all-or-none
event; enforces one-way transmission of AP

Conduction of Nerve Impulses
Each section triggers next locally
 Refractory period keeps it going the right
direction
 unmyelinated fiber- continuous conduction
 With myelin- saltatory conduction



Can only be triggered at nodes of Ranvier
Myelinated fibers faster & larger neurons faster
Figure 9.6a
Figure 9.6b
The Syanpse

Synapse (to clasp or join)- junction that mediates
information transfer from 1 neuron to another or
from a neuron to an effector cell

Axodendritic or axosomatic synapses – most
synapses occur between the axonal ending of a
neuron and the dendrites or cell body of other
neurons
Synaptic Transmission –
Electrical synapse
Sequence of events at synapse
 Triggered by voltage change of the Action
Potential
 Sending neuron = presynaptic
 Receiving neuron = postsynaptic
 Space between = synaptic cleft
 Neurotransmitter carries signal across cleft

Events at Synapse –
Chemical synapse
AP arrives at presynaptic end bulb=>
 Opens voltage gated Ca2+ channels=>


Ca2+ flows into cell
increased Ca2+ concentration =>
 exocytosis of synaptic vesicles=>
 Neurotransmitter released into cleft
 Diffuse across and bind to receptors in
postsynaptic cell membrane

Synaptic Transmission
Binding at receptors
 Chemical trigger of ion channels
 May depolarize or hyperpolarize postsynaptic
cell membrane
 If threshold reached at axon hillock then
postsynaptic cell action potential results

Synaptic Transmission
Finally the neurotransmitter must be removed
from the cleft Diffusion away
 Destroyed by enzymes in cleft
 Transport back into presynaptic cell
 Neuroglia destruction

Figure 9.7
Neurotransmitters
AcetylCholine (Ach)- common in PNS
 Biogenic amines - Norepinephrine (NE), Dopamine (DA),

serotonin, Histamine

Amino Acids
Glutamate, Aspartate, gamma aminobutyric
acid (GABA), glycine
Neuropeptides – endorphins
 Novel Messengers - ATP/ Nitric oxide (NO)/
Carbon monoxide (CO)

Development of Neurons
P. 422-424
 Neuroblasts
 Growth cone
 Programmed cell death

Web sites:
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http://www.sciencecases.org/chin/chin.asp
http://www.pbs.org/wgbh/nova/sciencenow/3204/01.html
http://www.getbodysmart.com/ap/nervoussystem/menu/menu.h
tml
http://www.bbc.co.uk/science/humanbody/body/interactives/3dji
gsaw_02/index.swf?startPosition=nervous
http://learn.genetics.utah.edu/units/addiction/reward/madneuro
n.cfm
http://www.gpc.edu/~bbrown/peril/neurons/level1.htm