Chapter 6 - Neural Control Mechanisms
Human Physiology, pages 153-204
I.
II.
Chapter 6 - Introduction
A.
Discuss evolution of nervous system
B.
One of two communication methods within the body
1.
Other is endocrine or hormonal
2.
Modern concept is a neuroendocrine system
3.
No evidence for a separate "mind" - all neural functions
should be attributable to functions of brain cells and
biochemistry
4.
Figure 6-1, page 154
Structure and maintenance of neurons
A.
Basic unit of the nervous system is the neuron, Figure 6-2a, page
155.
1.
Cell body - site of nucleus
2.
Dendrites - branches of cell body
3.
Axon - a single process extending from cell body
a)
Initial segment (trigger zone) also known as axon
hillock
b)
Neural signals pass through cell body to axon hillock
and then down axon - more later in chapter
c)
Axon may have branches called collaterals
d)
Axon ends in axon terminal or terminus (one
terminus, two termini)
e)
Some axons have varicosities - more later
f)
Axons vary in length from more than a meter to quite
short
(1)
Projection neurons - connect with distant
parts of the nervous system or peripheral
organs
(2)
Local circuit neurons - short and connect
only with cells in immediate vicinity
4.
Myelin - a fatty material formed from the plasma membrane
of Schwann cells during development, Figure 6-2b & c, page
155.
a)
Derived from neural crest cells in embryo and
migrate to peripheral neurons
b)
Nodes of Ranvier  spaces between myelin-forming
cells - more later
Important  the plasma membrane is primarily lipid
which is a good resistor to electrical flow - more later
d)
In the CNS the myelin-forming cells are the
oligodendroglia (a kind of glial cell)
Axon transport
a)
Materials and some organelles have to be
transported from cell body where they are
synthesized to the terminus
b)
Transported on cytoskeleton
c)
Transport can go from terminus to cell body
c)
5.
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Chapter 6 - Neural Control Mechanisms
Human Physiology, pages 153-204
III.
Functional classes of neurons, Figure 6-3, page 156, Table 6-1, page 157
A.
Afferent neurons - part outside CNS
1.
Have receptors at peripheral end, NOTE: this is a different
use of the word receptor than used previously
2.
Propagate electrochemical signals from receptor into brain
or spinal cord
3.
Do not have dendrites
B.
Efferent neurons
1.
Propagate electrochemical signals from CNS out to effector
cells such as muscle or glands
2.
Efferent neurons go to effectors!
3.
Cell body, dendrites, and a little of the axon is in the CNS most of the axon is in the periphery
C.
Interneurons - all within CNS
1.
Integrators and signal changers
2.
Ninety (99%) of all neurons
3.
Number of interneurons in a reflex arc differ depending on
the complexity
D.
For each afferent neuron entering CNS there are about 200,000
interneurons and 10 efferent neurons
E.
Synapse  the connection by which two excitable cells
communicate
Figure 6-4, page 157  illustrates presynaptic/postsynaptic
change
2.
A neuron may have thousands of presynaptic neurons
synapsing on its surface
IV.
Glial cells comprise 90% of the nervous system, the other 10% are
neurons
A.
Also known as neuroglia
B.
These sustain neurons metabolically, support them physically and
help regulate the ionic concentrations in the extracellular space
C.
Volume-wise: neurons are 50% and glial cells are 50%
1.
D.
E.
V.
Oligodendroglia  myelin of axons in the CNS
Astroglia  multiple functions
1.
Help regulate extracellular fluid
2.
Sustain neurons metabolically
3.
Migrate during development
4.
Secrete growth factors
F.
Microglia  may perform immune function in CNS
Neural Growth and Regeneration
A.
Neurons or glial develop from neural ectoderm (neuroblasts, aka
stem cells) and migrate to eventual adult site in controlled patterns
1.
Growth cone, forms the tip of each extending axon and is
involved in finding the correct route and final target for the
process
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Chapter 6 - Neural Control Mechanisms
Human Physiology, pages 153-204
a)
As the axon grows, it is guided along the surfaces of
other cells, most commonly glial cells
b)
Which route is followed depends largely on
attracting, supporting, deflecting, or inhibiting
influences exerted by several types of molecules.
Some of these molecules, such as cell adhesion
molecules (CAM), reside on the membranes of the
glia and embryonic neurons. Others are soluble
neurotropic factors (growth factors for neural
tissue) in the extracellular fluid surrounding the
growth cone or its distant target
2.
Normally, after growth and projection of the axons, many of
the newly formed neurons and synapses degenerate. As
many as 50 to 70 percent of neurons die by apoptosis in
some regions of the developing nervous system! Not known
why.
3.
Division of neuron precursors is largely complete before
birth, and after early infancy new neurons are formed at a
slower pace to replace those that die
4.
Severed axons can repair themselves and regain significant
function provided that the damage occurs outside the central
nervous system and does not affect the neuron's cell body.
a)
After repairable injury, the axon segment now
separated from the cell body degenerates.
b)
The proximal part of the axon (the stump still
attached to the cell body) then gives rise to a growth
cone, which grows out to the effector organ so that
in some cases function is restored
5.
Severed axons within the central nervous system attempt
sprouting, but no significant regeneration of the axon occurs
across the damaged site, and there are no well-documented
reports of significant function return.
6.
In humans, spinal injuries typically crush rather than cut the
tissue, leaving the axons intact.
a)
In this case, a primary problem is self-destruction
(apoptosis) of the nearby oligodendroglia
b)
When these cells die and their associated axons
lose their myelin coat, the axons cannot transmit
information effectively
7.
Researchers are attempting a variety of measures to provide
an environment that will support axonal regeneration in the
central nervous system. They are creating tubes to support
regrowth of the severed axons,
VI.
Basic Principles of Electricity
A.
Molecules may have a positive charge, negative charge or no
charge (neutral) and macromolecules may have both + and - with a
net of one or the other or neutral
B.
Electrical definitions
1.
Opposite charges attract
2.
Similar charges repel
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Chapter 6 - Neural Control Mechanisms
Human Physiology, pages 153-204
a)
3.
4.
The energy required to separate opposite charges or
bring similar charges together is called potential or
potential difference
b)
The unit of energy is voltage, a thousands of a volt is
a millivolt (1000 mV = 1 volt)
c)
Figure 6-5, page 160
Movement of charged particles creates a current
Resistance is the hindrance to current
Ohm’s Law
I = V/R
V = voltage, R = resistance, I = current
VII.
The Resting Membrane Potential
A.
All cells measured thus far exhibit a potential difference between the
inside of the cell and the outside - this is called the resting
membrane potential and it is negative inside
1.
Use analogy with battery
2.
Measured with a voltmeter (microvoltmeter), Figure 6-6 a &
b, page 161
3.
Distribution of ions across cell membrane, Table 6-2, page
162
Ion
Na+
ClK+
4.
5.
6.
Extracellular
150
110
5
Intracellular
15
10
140
The resting membrane potential is determined mainly by two
factors
a)
The differences in ion concentration of the
intracellular and extracellular fluids
b)
The permeabilities of the plasma membrane to the
different ion species
c)
Figure 6-7, page 161
Diffusion potentials
a)
Sodium and potassium are most important in resting
membrane potential
b)
Figure 6-8, page 162; membrane is impermeable to
sodium ion but allows potassium to flow freely
(1)
The net result is a diffusion potential created
by the concentration difference of potassium
and the electrochemical potential
(2)
The equilibrium potential is achieved when
the electrical force is equal and opposite to
the chemical gradient
(3)
Figure 6-9, page 163; potential created by
only sodium ion being freely diffusible
Nernst equation → E = 60 log10 [C]o / [ C]I (for any ion)
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Chapter 6 - Neural Control Mechanisms
Human Physiology, pages 153-204
7.
B.
Appendix D, page 791, Goldman equation for more than one
ion determining potential
The resting membrane potential
1.
A potential difference can be realized in a resting nerve cell
under the following conditions
a)
Potassium ion is much greater inside cell than
outside
b)
Cell membrane is 50 to 75 times more permeable to
potassium than to sodium
c)
Forces acting on potassium ion  Figure 6-10, page
164
Forces acting on sodium ion  Figure 6-11, page
164
2.
Leakage can occur through membrane so that the actual
potential difference is not at the theoretical potassium
equilibrium potential
a)
The Na-K-ATPase pump compensates, Figure 6-12,
page 165
b)
Three sodiums out for every two potassiums back in
 known as an electrogenic pump but is usually a
rather small contribution to the potential difference
3.
Chloride ion
a)
In most cells chloride ion moves passively with the
concentration and/or electrical gradient and do not
contribute to determination of the resting membrane
potential
b)
Some cells can actively pump chloride and this can
contribute to resting membrane potential
4.
Most of the negative charge in neurons is contributed to by
the negatively charged proteins - they do not diffuse through
the membrane readily or at all
VIII.
Graded Potentials and Action Potentials
A.
Two kinds of "disruptions" can occur in resting membrane potential graded potentials and action potentials
1.
Terms - Table 6-3, page 166
2.
Figure 6-13, page 165
a)
Depolarized - potential toward zero
b)
Polarized - potential away from zero, repolarized
refers to re-establishing the resting membrane
potential
c)
Hyperpolarized - potential difference greater than
original
d)
d)
B.
C.
Overshoot  inside of cell becomes positive
Recording depolarization  Figure 6-14, page 167
Graded (receptor potentials, synaptic potentials, end-plate
potentials and pacemaker potentials)
1.
Can be a depolarization or hyperpolarization, Figure 6-15,
page 167
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Chapter 6 - Neural Control Mechanisms
Human Physiology, pages 153-204
2.
D.
Graded response - amplitude varies with conditions of the
initiating events
3.
Is conducted decrementally i.e., amplitude decreases with
distance
4.
Graded response; can be summed temporally or spatially
5.
Has no threshold (explain with action potential)
6.
Has no refractory period (explain with action potential)
7.
Duration varies with initiating conditions (explain with action
potential)
8.
Initiated by stimulus (receptor), by neurotransmitter
(synapse) or spontaneously (electrical)
9.
Leakage of charge, Figure 6-16, page 167
Action potentials - Figure 6-17 A & B, page 168
1.
Rapid alteration or transient change in the resting membrane
potential is known as an action potential
a)
Cells that undergo such change are termed
excitable - mainly neurons and muscle cells
b)
The action potential can also be propagated
(conducted) long distances - later
2.
Ionic basis of the action potential - "ionic flux theory" by
Hodgkin, Huxley and Eccles (won Nobel Prize in 1963)
a)
An electric current opens sodium gates (channels)
and sodium ion flows down its concentration and
electrical gradient - the influx of sodium ion causes
depolarization
b)
When the potential passes zero and approaches
about +35 mV then the sodium gates close and
potassium gates open - the efflux of potassium ion
repolarizes the cell (after hyperpolarization)
c)
THE NA-K PUMP DOES NOT IMMEDIATELY REESTABLISH THE RESTING MEMBRANE
POTENTIAL!
(1)
The resting membrane potential is
reestablished by the efflux of potassium
(2)
If the Na-K pump is poisoned by oubain
action potentials can still occur on a neuron
for a long time
(3)
The pump acts only after many action
potentials have been elicited and an
imbalance in the ion distribution occurs very few ions are involved in a single action
potential
3.
Differences between Voltage-Gated Sodium and Potassium
Channels
a)
Sodium channels open faster in response to a given
voltage change
b)
Once activated, sodium channels close more rapidly
c)
Sodium channels inactivet, cycling through an
inactive phase
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Chapter 6 - Neural Control Mechanisms
Human Physiology, pages 153-204
4.
5.
6.
7.
Mechanism of ion-channel changes
a)
The basis of the configurational changes in the
gates or channels is not known
b)
In the case of sodium ion it is a positive feedback
mechanism, Figure 6-189, page 169
c)
Channels that respond to changes in membrane
potential are termed voltage-sensitive channels
d)
Some nonneural cells are depolarized by the influx
of Ca2+
e)
Local anesthetics can prevent action potentials by
preventing the channels from "opening" and
"closing" - include novocaine & xylocaine
f)
Table 6-4, page 170 → Differences between
voltage-gated sodium and potassium channels
Threshold, Figure 6-19, page 170
a)
The membrane potential at which the net movement
of ions across the membrane first changes from
outward to inward is the threshold
b)
The stimulus strength is called the threshold
stimulus
c)
Subthreshold stimuli can occur but can’t elicit an
action potential - they can summate, however
d)
A suprathreshold stimuli does not elicit a larger
action potential since action potentials exhibit what
is called ALL-OR-NONE, that is they have the same
amplitude regardless of stimulus strength as long as
the stimulus strength is at threshold or above
e)
The information conveyed by action potentials is
analogous to an FM radio, frequency modulated,
rather than an AM radio, amplitude modulated since the amplitude doesn’t change only the
frequency and/or rate can convey information (THIS
IS AN IMPORTANT CONCEPT!)
Refractory periods
a)
Refractory means obstinate or unwilling
b)
The absolute refractory period follows an action
potential during which another action potential can
NOT be elicited no matter how strong the stimulus
strength
c)
Following the absolute refractory period there is a
relative refractory period during which an action
potential can be elicited but at a strength of stimulus
above threshold and decreasing with time
Action potential propagation, Figure 6-20, page 172
a)
An action potential at a given "active site" is
accompanied by the flow of charged particles (ions)
- this creates a local current
b)
The local current flow can alter the sodium gates in
the adjacent "active site" and allow sodium to enter,
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Chapter 6 - Neural Control Mechanisms
Human Physiology, pages 153-204
E.
that initial influx then is followed by more gates
opening, i.e. positive feedback
c)
The action potential can then be propagated down
the axon in only one direction since the "active sites"
are refractory - explain
d)
This propagation can be greatly speeded up by the
presence of myelin, Figure 6-21, page 173
(1)
The current is prevented from influencing
the membrane by the insulating sheath
except at the nodes of Ranvier
(2)
This kind of propagation is called saltatory
conduction or "jumping" conduction
(3)
Range of conducting velocities: 1 mile/hour
in small-diameter unmyelinated neurons to
400 mile/hour in large-diameter myelinated
neurons (1 mph takes 4 sec to go head to
toe but 10 milliseconds at 400 mph)
8.
Initiation of action potentials
a)
Action potential are initiated in a variety of ways
which will be discussed in later chapters
b)
Most are preceded by a graded potential of some
kind - a pacemaker potential is an example
See Table 6-5, page 174 for comparison with action potential
SECTION C - SYNAPSES
IX.
X.
Introduction to Synapses
A.
Synapses - Functionally and anatomically specialized connections
between "excitable" cells
1.
Neural-neural
2.
Neural-muscular
3.
Neural-glandular
B.
Estimate there are 1014 synapses in the CNS
C.
Activity is transferred from axon to the next cell via a
neurotransmitter in chemical synapses
1.
Excitatory synapses
2.
Inhibitory synapses
D.
Neural relationships
1.
Convergence and divergence - Figure 6-22 page 176
2.
Diagrammatic representation of synapse – Figure 6-23, pag
176
3.
Neurons function as neural integrators
Functional Anatomy of Synapses, Figure 6-24 a, b, c, d, page 198-199
A.
Two types of synapses: electrical and chemical
a)
Chemical synapses are most common in the
nervous system
b)
There are electrical synapses which are not common
and involve gap junctions
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Chapter 6 - Neural Control Mechanisms
Human Physiology, pages 153-204
B.
C.
D.
E.
2.
Few electrical in mammals
3.
Information from here on is about chemical synapses
Chemical synapses, Figure 25, page 177
1.
Axon ends in slight swelling, the axon terminal
2.
Synaptic cleft separates two cells
3.
In the presynaptic cell are vesicles containing
neurotransmitters
a)
Action potentials arriving at terminus causes influx of
calcium ion into the presynaptic terminus
b)
This triggers exocytosis - fusing of membrane of
vesicle and plasma membrane at cleft
c)
Neurotransmitter is released and diffuses across
cleft to postsynaptic membrane or subsynaptic
membrane
d)
The neurotransmitter combines with appropriate
receptors to effect receptor-operated channels, and
G-protein receptors, Chapter 6 & 7
(1)
More than one transmitter may be released
(2)
Second transmitter is called cotransmitter
e)
Transmission is only in one direction since the
vesicles are on only one side and receptors are only
on one side
f)
The receptors on the postsynaptic cell membrane is
limited to the space right under the synapse in
normal cell
g)
The events result in a synaptic delay
Excitatory chemical synapses
1.
Transmitter "causes" depolarization, Figure 6-26, page 178
2.
Opens postsynaptic channels that are permeable to sodium,
potassium and other small positively charged ions
3.
The electrical and concentration gradients favor sodium
influx whereas potassium has opposing forces
4.
Called EPSP, Excitatory Post Synaptic Potential
a)
A graded potential with all the properties
b)
Brings axon hillock potential toward threshold
Inhibitory chemical synapses
1.
Transmitter "causes" hyperpolarization, Figure 6-27, page
179
2.
Opens postsynaptic channels that are permeable to
potassium, chloride and other small positively charged ions sodium channel are not effected
3.
Called IPSP, Inhibitory Post Synaptic Potential
a)
A graded potential with all the properties
b)
Brings axon hillock potential away from threshold
4.
Cells that actively transport chloride can hyperpolarize by
increasing chloride permeability or an IPSP
Inactivation
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Chapter 6 - Neural Control Mechanisms
Human Physiology, pages 153-204
1.
The neurotransmitter must be removed from cleft after
performing its "role"
2.
Transmitter can be inactivated enzymatically
3.
Transmitter can diffuse away from the site
4.
Transmitter can be actively transported back into the axon
terminus or into glial cells in some cases
XI.
Activation of the Postsynaptic Cell
A.
The normal situation is not one-on-one transmission but rather
many presynaptic neurons, both excitatory and inhibitory, that
interact to produce action potentials
1.
Figure 6-28, page 179 - shows interaction
2.
Exhibit temporal and spatial summation occurs, Figure 6-29,
page 180
3.
Different parts of neuron can have different thresholds
4.
Figure 6-30, page 180 - Excitatory and inhibitory current
flows
a)
B.
C.
Excitatory  positive charges flow into cell body
b)
Inhibitory  positive charges flow out of cell
Integration
1.
In CNS may have influence of local currents on neighboring
unmyelinated neurons
2.
Can have influence from other substances, both "normal"
and "abnormal" binding at receptors
Synaptic Strength
1.
The synapse can be effected by any event that effects the
amount of transmitter released - more transmitter, more
depolarization or hyperpolarization
2.
Factors that can determine synaptic effectiveness, Table 66, page 183
a)
Presynaptic factors
(1)
Availability of neurotransmitters
(a)
Availability of precursor molecules
(b)
Amount (or activity) of the ratelimiting enzyme in neurotransmitter
synthesis pathway
(2)
Axon terminal membrane potential
(3)
Axon terminal residual calcium
(4)
Activation of membrane receptors on
presynaptic terminal
(a)
Presynaptic (axon-axon) synapses
(b)
Autoreceptors
(c)
Other receptors
(5)
Certain drugs and diseases
b)
Postsynaptic factors
(1)
Immediate past history of electrical state of
postsynaptic membrane, that is, facilitation
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Chapter 6 - Neural Control Mechanisms
Human Physiology, pages 153-204
or inhibition from temporal or spatial
summation
(2)
Effects of other transmitter-type chemicals
acting on postsynaptic neuron
(3)
Certain drugs or diseases
3.
Action of other neurons that form synapses on the axon
terminus (presynaptic inhibition or facilitation), Figure 6-31
page 181
4.
Modification of Synaptic Transmission by Drugs and
Disease, Figure 6-32, page 182
a)
Agonists - create similar response than normal
transmitters
b)
Antagonists - block normal action
XII.
Neurotransmitters and Neuromodulators
A.
Neuromodulators are chemicals that act on neurons but are not
quite transmitters - very vague in defining
1.
Have long term effects
2.
Amplify or dampen the effectiveness of ongoing synaptic
activity by altering the neurons at nonsynaptic sites, i.e.,
extrasynaptic receptors
3.
Some neuromodulators can be hormones
B.
Four classes of neurotransmitters or neuromodulators and one
miscellaneous category, Table 6-7, page 205
1.
Many of these chemicals do act also as hormones and
paracrines
2.
Generally speaking axons release only one transmitter and
only that transmitter, there are exceptions in the number of
transmitters released from a single axon
3.
A single transmitter may produce EPSPs at one synapse but
IPSPs at another - i.e. the response is a property of the
subsynaptic membrane receptors and not the messenger
C.
Acetylcholine (Ach)
1.
Probably most common transmitter, certainly the best known
and first described - found in neural-muscular synapses and
in brain
2.
Two types of Ach receptors
a)
Nicotinic - receptors respond to nicotine
(1)
Example of a receptor that contains the
channel
(2)
Receptor is an allosteric protein which
changes configuration in presence of Ach
b)
Muscarinic - receptors respond to muscarine
(1)
Couple with G-proteins
(2)
Alter enzyme activity and ion channels
3.
Choline+CoA in cytoplasm of axon terminus and stored in
vesicles (CoA is derived from pantothenic acid
4.
Inactivated by acetylcholinesterase
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Chapter 6 - Neural Control Mechanisms
Human Physiology, pages 153-204
5.
D.
E.
F.
Degeneration of cholinergic system in brain can lead to
Alzheimer's disease
Biogenic amines  R-NH2 (most derived from amino acid tyrosine)
1.
Catecholamines, Figure 6-34, page 206
a)
Norepinephrine and epinephrine
(1)
Receptors are adrenergic or noradrenergic
(2)
Alpha-adrenergic receptors and Betaadrenergic receptors are influenced by
different drugs
b)
Actively transported back into presynaptic cell and
this stops activity
c)
Distributed widely
d)
Slow time-courses - may act on neurons being acted
upon by other more rapidly acting transmitters
e)
Found in autonomic nervous system
f)
Dopamine - cocaine interferes with dopamine
reuptake
2.
Serotonin
a)
Produced from tryptophan in CNS
b)
Important in neural pathways controlling states of
consciousness and mood (sleep)
c)
Found also in platelets
Amino acid neurotransmitters
1.
Act directly as transmitters
2.
Include glycine, glutamate and aspartate
a)
Glycine can act as inhibitory transmitter
b)
Aspartate and glutamate are among most potent
transmitters in CNS - mostly excitatory
3.
GABA, gamma-aminobutyric acid, is not an alpha amino acid
but is an amino acid
4.
GABA is major inhibitory transmitter in the brain
Neuropeptides
1.
More than eighty-five have been thus far described in neural
tissue although they have been found and described to
function as hormones or paracrine agents in nonneural
tissue
2.
Derived from large proteins synthesized in cell body and are
attached to ribosomes
a)
Prohormones are packaged into vesicles and
transported in neuron where needed (terminus or
varicosities)
b)
When needed, the hormones are cleaved off by
peptidases
c)
Some are called polyproteins
d)
The neurons are called peptidergic and usually are
cosecreted with another type of neurotransmitter
3.
Types derived from prohormones or preprohormones
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Chapter 6 - Neural Control Mechanisms
Human Physiology, pages 153-204
a)
b)
G.
XIII.
A.
B.
XIV.
A.
B.
C.
Endorphins (endogenous morphines), dynorphins
and enkephalins are known as opioid peptides small, very short lived and for the most part are
analgesic in nature
Substance P - peptide that is in a group of
transmitters called tachykinins for neurons that have
to do with pain (nociceptive) and may be the link
between the nervous system and immunity system
Miscellaneous
1.
Nitric oxide serves as neurotransmitter (neural-neural and
neural-effector)
a)
Produced from arginine in a cell and that triggers
cGMP activity in effector cell
b)
Plays a role in learning, development, drug
tolerance, penile erection and sensory and motor
modulation among others
c)
Is found outside the CNS and PNS in the
cardiovascular and immune systems
d)
Basis of action of nitroglycerin
2.
Carbon monoxide – as above
3.
Adenosine and ATP  neurotransmitter (fast acting)
Neuroeffector Communication
Other type of synapses other than neural-neural
Covered in subsequent chapters
Structure of the Nervous System
Terms
1.
"Nerve" is a collection of many neurons
2.
A nerve in the CNS is called a pathway, tract or when
connecting, a commissure
3.
In the PNS, groups of neuron cell bodies is called a ganglia
4.
the CNS, groups of neuron cell bodies is called a nuclei
5.
Pathways, Figure 6-35, page 210
a)
Long neural pathways
b)
Multineuronal or multisynaptic pathways
Categorized anatomically and physiologically and are divided into
two main categories anatomically (CNS & PNS)
1.
Central nervous system
a)
Brain and brainstem
b)
Spinal cord
2.
Peripheral nervous system
a)
Spinal nerves
b)
Cranial nerves
Central nervous system: brain
1.
Six subdivisions - Figure 6-34, page 189
a)
Cerebrum
b)
Diencephalon
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2.
3.
4.
5.
c)
Midbrain
d)
Pons
e)
Medulla oblongata
f)
Cerebellum
Cerebral ventricles
a)
Four interconnected containing cerebrospinal fluid
b)
Figure 6-36, page 191
Brainstem - stalk of brain that relays all information between
spinal cord, cerebrum and cerebellum (to cerebellum via
cerebellar peduncles)
a)
Midbrain
b)
Pons
c)
Medulla oblongata
d)
Reticular formation - also known as reticular core
and reticular activating system {RAS}, (old terms)
(1)
Absolutely essential for life
(2)
Runs length of brain stem through all three
parts
(3)
Houses cardiovascular, respiratory,
swallowing and vomiting centers discussed
later
(4)
Sends output and receives from almost all
parts of nervous system
Branches to:
(a)
Thalamus
(b)
Base of forebrain
(c)
Cerebral cortex
(d)
Connects cerebellum and spinal
cord (reticulospinal pathway)
(5)
Ten of the 12 cranial nerves have nuclei in
the reticular formation
Cerebellum - has a cortex and cerebellar nuclei (chiefly
involved with skeletal muscle function)
a)
Connected to brainstem by cerebellar peduncles
b)
Does not initiate or directly coordinate activity
c)
Receives input from muscles, joints, skin, eyes and
ears among others
d)
Acts as "quality control center", telling higher brain
centers what muscles have done
Forebrain - what is left after brainstem and cerebellum are
excluded
a)
Cerebrum - right and left cerebral hemispheres,
Figure 6-35, page 190
(1)
Hemispheres are connected by axon
bundles known as commissures, the corpus
callosum being the largest
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Chapter 6 - Neural Control Mechanisms
Human Physiology, pages 153-204
(2)
(3)
Areas within a hemisphere are connected by
association fibers
Outer portion is cerebral cortex, 3 mm thick
and is the most sophisticated part with
cortical neurons of two basic types
Pyramidal cells  (major output to
the rest of the nervous system)
(b)
Nonpyramidal cells
(c)
Brings together afferent (sensory)
information for perception and
control of the motor system
(4)
Divided into lobes - frontal, parietal, occipital
and temporal, Figure 6-41 page 214
(5)
Subcortical areas lie underneath cortex
(a)
Basal ganglia - role in movement
and some complex behavior
(b)
Substantia nigra - movement
(Parkinson's disease and dopamine)
b)
Diencephalon
(a)
Thalamus - relay and integrating
station for sensory input to cortex
and is important in motor control;
part of reticular core is found here
(b)
Hypothalamus - lies below
thalamus, very important part of
brain that is responsible for the
integration and control of many
systems
(c)
Limbic system - associated with
learning and emotional behavior; not
really a separate part of brain but
made up of connections from other
parts of brain, Figure 6-36, page
191
Central nervous system: spinal cord
1.
Slender cylinder, Figure 6-37, page 193 and Figure 6-38,
page 194
a)
Grey matter - central region filled with interneurons
b)
White matter - surrounding, color due to myelin
sheaths
2.
The interneurons form tracts and pathways
a)
Afferent neurons enter the CNS via the dorsal roots
and form the dorsal root ganglia which are the cell
bodies of the afferent neurons
b)
Efferent neurons leave the CNS via the ventral roots
(cell bodies in CNS) and form spinal nerves - spinal
nerves are in pairs from each side of the spinal cord
Major parts of brain: summary - Table 6-8, page 192
Peripheral nervous system
(a)
D.
E.
F.
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Chapter 6 - Neural Control Mechanisms
Human Physiology, pages 153-204
1.
2.
3.
Bundles of neurons are called nerves
a)
Thirty-one (31) pairs of spinal nerves
b)
Twelve (12) pairs of cranial nerves (discuss
misnaming of optic nerve here) - Table 6-9, page
195 (do not memorize the cranial nerves - that’s for
anatomy!)
c)
Most are myelinated, some not, however
d)
The nerves are divided into afferent and efferent Table 6-10, page 195
Peripheral nervous system: afferent division
a)
Sensory neurons with cell bodies in ganglia outside
CNS
b)
A second process, the central process, relays
information into the CNS where it branches
c)
Afferent neurons are sometimes called primary
afferents or first-order neurons
Peripheral nervous system: efferent division, divided into
somatic and autonomic nervous systems, please NOTE
autonomic not automatic - Figure 6-39, page 196
a)
Differences between somatic and autonomic
nervous systems, Table 6-11, page 195
b)
Somatic
c)
All the fibers going from the CNS to skeletal muscles
d)
Cell bodies are in groups in brain or spinal cord
e)
Leave CNS and pass, without intermediate
synapsing, to effector organs (skeletal muscle) (they eventually synapse on effector organ)
f)
The neurotransmitter is acetylcholine
g)
Are normally called motor neurons
h)
Autonomic, Figure 6-40, page 197
(1)
Innervate cardiac muscle, smooth
(involuntary) muscle, glands, and enteric
nervous system of gastrointestinal tract
(2)
Sympathetic  thoracolumbar
(a)
(b)
Figure 6-41, page 198 
Sympathetic trunk and spinal cord
The sympathetic anatomy or wiring
allows this portion of the autonomic
nervous system to act as a unit
which is, of course, adaptive
(3)
(4)
Parasympathetic  craniosacral
Have two neurons and one synapse in
periphery - the synapses are in autonomic
ganglia (sympathetic trunk is most
prominent)
(5)
Preganglion  before ganglion
postganglion  after ganglion
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Chapter 6 - Neural Control Mechanisms
Human Physiology, pages 153-204
(6)
Parasympathetic preganglionic &
postganglionic transmitter is acetylcholine
(cholinergic fibers) in all cases; the
sympathetic transmitter in preganglionic
fibers is acetylcholine and in postganglionic
cells is norepinephrine (adrenergic fibers)
with one exception, Figure 6-42, page 199,
Table 6-12, page 198
(7)
Nicotinic, muscarine, alpha-adrenergic and
beta-adrenergic receptors on autonomic
fibers, Table 6-12, page 219
i)
Dual innervation, Figure 6-44, page 218
(1)
Many organs get sympathetic and
parasympathetic innervation
(2)
some ability to control autonomic responses
is possible in humans and experimental
animals - basis of biofeedback
Do not attempt to memorize Table 6-13, pages 200 now, it will be
covered in subsequent chapters - merely review for appreciation of dual
innervation
THE BRAIN HAS TOP PRIORITY FOR OXYGEN AND GLUCOSE! 
XV.
Blood Supply, Blood-Brain Barrier Phenomena and Cerebrospinal Fluid
A.
The brain is a very fragile organ and must have a constant blood
supply with oxygen and glucose (little glycogen stores)
1.
Little glycogen stores
2.
Brain is 2% of body weight but receives 15% of blood supply
3.
Brain covering between bony tissue soft neural tissue meninges
a)
dura mater - next to bone
b)
arachnoid - middle
c)
pia mater - next to brain
B.
Blood-brain barrier – protects immunologically privileged area
1.
This is an anatomical and physiological phenomenon that
separates the brain tissue from the plasma of the blood.
a)
The key are the endothelial cells which compose the
capillary walls
(1)
They have tight junctions binding them
together
(2)
The tight junctions prevent most things from
moving from plasma into brain cerebrospinal
fluid
(3)
The astrocytes which surround the
capillaries are not a major part of the bloodbrain barrier
b)
Lipid soluble molecules cross barrier readily nicotine, ethanol, heroin
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Chapter 6 - Neural Control Mechanisms
Human Physiology, pages 153-204
c)
d)
Picture the wall of the capillary as having two unit
membranes
(1)
Toward brain is called antiluminal
membrane
(2)
Toward lumen of capillary is called luminal
membrane
(3)
The two sides of the system are not
symmetrical in the nature of the carriers that
are there for some substances
Water soluble molecules cross less readily or not at
all
Glucose  main energy source, carried by
specific membrane carrier (D-glucose was
readily transported, L-glucose was not),
there many glucose carriers available in the
endothelial membrane
(2)
For glucose, the carriers are symmetrical
and "work" down a concentration gradient no extra energy needed (facilitated diffusion)
e)
Amino acids - three categories [large neutral (10),
small neutral and basic and acidic]
f)
Metabolic blood-brain barrier - enzymes in the
endothelial cell modify substance and render them
incapable of entering brain, example is L-DOPA
g)
Effect of sugar infusion
2.
The chemical composition of the extracellular fluid of the
brain is carefully regulated
Cerebrospinal fluid - Figure 6-43, page 201
1.
Secreted by the choroid plexus
2.
Is found in the ventricles of the brain
3.
Circulates and is reabsorbed into the blood stream from the
top surface of the brain through one-way valves in large
veins
4.
It is also carefully regulated
(1)
C.
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