Student
Performance Objectives - for the lecture
1.
State the functions of the nervous system.
2. Identify the 3 structural and the 3 functional types of neuron.
3. Given
a diagram of a "typical" neuron, identify and state the function of:
a. axon, axolemma, axoplasm
b.
cell membrane
c. cyton
d.
dendrites
e. myelin sheath
f.
nodes of Ranvier, internodes
g. terminal axon
fibers
h. Nissl bodies
i.
Schwann cells
j. neurilemma
4. Explain
the reason a unipolar neuron is said to have a single branched axon, and no dendrites.
5. Explain the formation of the myelin sheath in the peripheral and central nervous
systems.
6. Explain the functions of the 5 different types of neuroglial cells.
7. Define each of the following terms as related to nervous tissue: irritability,
conductivity.
8. Explain the importance of neurotubules, kinesin and dynein
within the axon as related to the process of axon transport.
9. Explain the
factors that cause the measurement of -70 mv as the potential across the cell
membrane of a resting neuron.
10. Explain what is meant by local potentials
developing along the dendrites and cyton of a neuron.
11. Explain an action
potential in terms of the ion movements occurring across the neuronal cell membrane
during depolarization and repolarization.
12. Explain the importance of membrane
pumps in the continuous, lifelong ability of neurons to maintain the resting membrane
potential (RMP).
13. Describe what a nerve impulse is.
14. Describe why
nerve impulses travel at higher velocities in myelinated nerve fibers compared
with unmyelinated nerve fibers.
15. Draw a labeled diagram of a chemical synapse.
16. Describe the transmission of a signal across an excitatory chemical synapse.
17. Explain the importance of neurotransmitter breakdown soon after synaptic transmission.
18. Explain the difference between an excitatory and inhibitory synapse on the
basis of the functioning of the receptor on the postsynaptic membrane.
19.
Explain how the nervous system codes for the quality and intensity of stimuli.
20. Explain the difference between a convergent and divergent neuronal circuit.
21. Distinguish between the central nervous system (CNS), the peripheral nervous
system (PNS), and the autonomic nervous system (ANS).
Student
Required Nervous System Items- for Laboratory Practical Examinations (items
will be added or removed at your laboratory instructor's discretion). For each
item, identify its position in a neuron model or on a microscopic slide of a neuron
and indicate its function.
Lesson
Outline
A.
Properties and Functions of the Nervous System
http://www.emc.maricopa.edu/faculty/farabee/biobk/BioBookNERV.html
1. Properties
- irritability and conductivity
a.
Irritability refers to the ability of neurons (cells of the nervous system)
to detect and to respond to a stimulus.
b.
Conductivity refers to the ability of neurons to transmit signals from
one neuron to other neurons and from a neuron to muscles and glands.
2.
Functions
a. Provide sensory
awareness through electrochemical input to the central nervous system (CNS)
from peripheral receptors. The word "peripheral" refers to parts of
the body (e.g., the skin) outside the CNS (which is defined as the brain and the
spinal cord).
b. Provide
motor responses to sensory input: e.g., reflex muscular movements, voluntary
muscular movements and glandular secretions.
c.
Analysis of information brought into the CNS or analysis of thoughts generated
within the CNS. Analysis in its highest form occurs in the brain and can result
in a long delay between a sensory input and a motor response. E.g., one may respond
to an insult years after the insult was delivered, or we may solve a problem weeks
or months after initially becoming aware of the problem.
d.
Memory - the ability to recall and integrate previous experiences into
the analysis of information.
B.
Nervous System Organization
http://training.seer.cancer.gov/module_anatomy/unit5_3_nerve_org.html
1. The nervous system is broadly organized
into the central nervous system (CNS) - brain and spinal cord, and the
peripheral nervous system (PNS) - the 12 pairs of cranial nerves and 31
pairs of spinal nerves.
2. Other broad, functional divisions
of the nervous system include the somatic nervous system (SNS) and the
antonomic nervous system (ANS).
a.
The SNS includes all neural pathways involved in the conscious sensing of stimuli
(e.g., sights, sounds, touch, taste, odor) and the responses to those stimuli
through the neural connections to the skeletal muscles.
b.
The ANS includes all neural pathways involved in the innervation and control
of the smooth muscles of the body's internal organs (e.g., blood vessels,
digestive organs like the small intestine, reproductive organs like the uterus
in females or sperm ducts in males) and cardiac muscle (found only in the
heart).
3. At the cellular level of organization,
the nervous system is based on the fundamental cellular unit - the neuron.
It is estimated that there are about 1 trillion neurons in the nervous system.
These highly irregularly shaped cells possess the nervous system's basic properties
- irritability and conductivity.
4. Also at the cellular
level are the specialized connective tissue cells of the nervous system - the
neuroglia. There are at least 10 neuroglial cells for every one neuron in the
nervous system. They come in different types each performing different functions
that serve the activities of the neurons:
a.
Schwann cells - synthesize myelin for the nerve fibers of the PNS.
http://members.tripod.com/blustein/Schwann_Cells/schwann_cells.htm
http://www.neuro.wustl.edu/neuromuscular/lab/schcell.html
b. Oligodendrocytes - synthesize
myelin for the nerve tracts of the CNS.
c.
Astrocytes - form structurally supportive and protective barriers around
the neurons shielding them from direct contact with substances carried in the
blood. Astrocytes form the basis of the blood-brain barrier and help regulate
the chemical composition of the interstitial fluid directly bathing the neurons.
They appear to communicate with neurons electrochemically and through their secretion
of neuronal growth factors.
http://members.tripod.com/blustein/Astrocytes/astrocytes.htm
d. Microglia - protect
neurons from microbes and can remove unwanted material (dead cells) from nervous
tissue by phagocytosis.
http://www.microglia.net/microglia.htm
e. Ependyma - cells lining
the brains cavities (ventricles) that secrete and help circulate cerebrospinal
fluid.
http://en.wikipedia.org/wiki/Ependyma
3. Connective Tissues associated with the Nerves composing
the PNS:
a. Each nerve fiber
(axon) of the PNS is surrounded by Schwann cells (and possibly a myelin sheath)
surrounded by a thin layer of connective tissue, the endoneurium.
http://www.med.uiuc.edu/histo/small/atlas/objects/691.htm
b. The nerve fibers making up nerves
are arranged in groups called fascicles, with each fascicle surrounded
by a slightly thicker layer of connective tissue, the perineurium.
http://www.med.uiuc.edu/histo/small/atlas/objects/683.htm
c. All the fascicles of the
nerve are surrounded by a thicker layer of connective tissue, the epineurium.
http://www.med.howard.edu/anatomy/gas/wk2/images/image2.gif
C. Neuron Organization
http://www.people.eku.edu/ritchisong/301notes2.htm
1. Functional Cell Types
a.
Sensory (afferent) neurons - conduct nerve impulses into the CNS from peripheral
receptors connected to the body surface (skin), major sense organs (e.g., eyes),
and internal organs (e.g., pressure receptors in large arteries).
b.
Motor (efferent) neurons - conduct nerve impulses from the CNS to peripheral
effectors (e.g., skeletal muscles, smooth muscles, cardiac muscle, glandular tissue).
c. Association neurons
(interneurons) - these neurons are found only in the CNS and conduct nerve
impulses among themselves as part of the analysis and memory functions of the
nervous system. They are the basis for the delay in response between an incoming
sensory signal and an outgoing motor response. Association neurons make
up as many as 90% of the neurons of the nervous system.
2.
Structural Cell Types
a.
Multipolar neurons - these are the most commonly observed type of neuron
possessing many dendrites and a single axon. Most association neurons are multipolar.
http://psyweb.com/Physiological/Neurons/multineuron.jsp
b.
Bipolar neurons - possess a single dendrite and a single axon. They are
found in specialized locations associated with the delivery of sensory signals
into the CNS - they are found in the olfactory epithelium of the nasal passages,
the eye's retina, and in the inner ear.
c.
Unipolar neurons - possess a single branched axon. They are the afferent
neurons that bring sensory information into the spinal cord. Their cytons
lie in the dorsal root ganglia and one of their axonal branches extends all the
way out to a peripheral receptor. The other axonal branch extends into the dorsal
horn of gray matter in the spinal cord. Both branches of a unipolar neuron's axon
possess a myelin sheath.
http://www.wisc-online.com/objects/index_tj.asp?objid=AP11804
3. Neuronal Organelles
a.
The cyton or cell body of a neuron has most of the organelles seen in other
cells including a nucleus, cell membrane, endoplasmic reticulum, Golgi, mitochondria,
lysosomes and a cytoskeleton. However many of these organelles are modified to
perform specialized functions and these will be emphasized below. One organelle
lacking from neurons is the centriole apparently due to the inability of mature
neurons to undergo mitosis. However some regions of the brain (notably the hippocampus)
contain stem cells capable of producing new neurons by mitosis of these stem cells.
b. Nissl bodies - these
were noted by early microscopists as deeply staining portions of the neuronal
cyton (cell body). They are masses of rough endoplasmic reticulum involved in
the synthesis of neuronal proteins. They occur in clumps that are bounded from
the rest of the cytoplasm by neurofibrils - portions of the cytoplasmic
cytoskeleton. Although mature neurons don't divide they are extremely metabolically
active, as evidenced by the amount of Nissl substance they contain. It is easy
to understand this in that the protein synthesis occurring in the cyton must supply
the cyton itself with needed materials and also the long neuronal fibers that
extend outward from the cyton.
http://education.vetmed.vt.edu/Curriculum/VM8054/Labs/Lab3/Examples/exnissl.htm
c. Dendrites - extensions
of the cyton that are generally short and numerous. They possess many synapses
on their surface and conduct signals, emanating from other neurons, into the cyton.
Dendrites, like the cyton, have no myelin sheath.
d.
Axon - a single extension of the cyton arising from a slightly swollen
area on the cyton called the axon hillock. The axon may vary in length
from a few millimeters to a meter and carries nerve impulses away from the cyton
toward other neurons, muscles or glands. Its membrane is called the axolemma
and its contents are referred to as axoplasm.
(1)
Axons of the PNS have a cellular covering of neuroglial cells - the Schwann cell
sheath or neurilemma. If an axon is myelinated, the Schwann cell wraps
itself around the axon, as much as 100 times, with each Schwann cell sequentially
occupying an adjacent portion of the length of the axon. The tight Schwann cell
wrappings, encircling the axon, are called the myelin sheath. The myelin
sheath speeds up nerve impulse conduction along the axon. Areas covered by
myelin are called internodes. Areas between Schwann cell- wrapped regions
possess no myelin and are called Nodes of Ranvier. Unmyelinated axons in
the PNS are still covered by Schwann cells, just without the multiple coils of
membranous wrapping.
http://adam.about.com/reports/000456.htm
(2) Axons in the CNS may also
have a myelin sheath but it forms from a different type of neuroglial cell than
the Schwann cell of the PNS. The oligodendrocyte forms myelin sheaths in the CNS.
It is believed that oligodendrocytes do not support regeneration of neurons, whereas
Schwann cells do: this may be the reason that neuronal regeneration after damage
is possible in the PNS (peripheral nerves), but is not commonly observed in the
brain or spinal cord.
(2)
Axons contain components of the cytoskeleton called neurotubules that serve
as guidewires for the transport of organelles and chemicals from the cyton to
the axon termination areas (anterograde transport) and from these terminal
areas back to the cyton (retrograde transport). This overall process of
movement of materials within the axon is called axon transport or axon
flow. The power for this movement along the neurotubules is based on the activity
of two motor proteins: kinesin powers anterograde movements and dynein
powers retrograde movements.
(3)
Each axon ends by forming tiny branches that have little swellings called synaptic
knobs that form synapses with other neurons, muscle fibers or the cells
of glands.
4. Neuronal bioelectrical effects
http://highered.mcgraw-hill.com/sites/0072841869/student_view0/chapter12/animations__english_.html#
a. Sodium-Potassium
Pump Action: a pump (called the sodium-potassium pump) in the cell membrane
of a neuron uses the energy of ATP to pump potassium ions into the neuron and
pump sodium ions out of the neuron. Chloride ions follow sodium and phosphates
and negatively charged intracellular proteins tend to pair up with potassium.
So, neurons are bathed in a sodium chloride solution on the outside and a potassium
phosphate solution on the inside. Other ions are present and are important for
certain activities, but these ions are most important for the membrane potentials.
For each ATP molecule utilized, the sodium-potassium pump pulls 2 potassium ions
into the cell and throws 3 sodium ions out of the cell.
b.
Permeability of the cell membrane: the cell membrane permits the diffusion
of relatively large amounts of potassium ions out of the cell and only a small
amount of sodium ions into the cell. These diffusive movements are simply due
to these ions moving down their concentration gradients after their active transport
by the sodium-potassium pump..
c.
Development of the Resting Potential:
http://distance.stcc.edu/AandP/AP/AP1pages/nervssys/unit10/resting.htm
The net charge on either side of the neuronal cell membrane is called the resting
potential and is experimentally measured as about -70 millivolts (as measured
from the inside where it is negative). This resting membrane potential (RMP)
is due mainly to the the outward diffusion of potassium ions that result
in a net positive charge developing on the outside of the cell membrane (potassium
carries a single positive electrical charge) and a net negative charge on the
inside of the cell membrane (due to the negative phosphate ions and proteins left
inside when potassium diffused out of the cell). The permeability properties of
the membrane do not allow phosphates or proteins to follow potassium when it diffuses
out of the cell. If the outward diffusion of potassium ions were the only influence
on the RMP, it would be about -90 mv. But there are 2 other influences:
(1) The
unequal pumping action of the sodium-potassium pump: the pump brings only
2 positively charged potassium ions into the cell for each 3 positively charged
sodium ions it ejects. The result is that the pump tends to make the RMP about
-3mv lower than would be if the pump were not working. So if the outward diffusion
of potassium and the sodium-potassium pump were the only factors at work in the
establishment of the RMP, then the RMP value would be about -93 mv.
(2) Inward diffusion of positively charged sodium ions: some small amount
sodium ions do diffuse into the cell and their positive charges neutralize some
of the negative charges (left behind after potassium diffused out) bringing -90
mv to a less negative value. This effect of sodium is not as strong as might be
expected because some negatively charged chloride ions accompany sodium when it
diffuses into the neuron. So the net result of potassium diffusing out of the
neuron, sodium and chloride diffusing into the neuron, and the action of the sodium-potassium
pump is -70 mv.
d. Local
potentials, action potentials, and the generation of a nerve impulse (1)
Local potentials: The dendrites and the cyton of a single neuron in the
central nervous system may have as many as 10,000 synapses on its combined surface
area. A signal arriving at any one of those synapses may be stimulatory or inhibitory.
Stimulatory means that the signal opens a ligand-gated channel (the neurotransmitter
is the ligand that opens the gate) allowing sodium ions to enter the neuron.
Inhibitory means that the signal opens a ligand-gated channel allowing potassium
ions to leave the neuron. So, in every millisecond of existence in the nervous
system, tens of thousands of signals influence the electrical charge on the dendrites
and cyton of every neuron. These are referred to as local potentials. The
net result of all the stimulatory and inhibitory signals (local potentials), each
based on either stimulatory or inhibitory neurotransmitters attaching to the neuronal
cell membrane, reaches the area of the axon hillock - the area of the cyton where
the axon begins. This region is the first part of the neuron to possess voltage-gated
ion channels (not the ligand-gated channels of the dendrites and cyton). If the
signal is beyond the threshold value, meaning that the RMP has been changed from
-70 to about -55 mv, then an action potential occurs.
(2)
Action Potentials:
http://www.blackwellpublishing.com/matthews/channel.html
http://alpha.furman.edu/~einstein/general/neurodemo/ap.htm
An action potential occurs when a voltage-gated ion channel opens
and positively charged sodium ions diffuse into the axon changing the membrane
potential from -70 to zero and even higher, often reaching +35 mv. We say that
the membrane has been depolarized at that spot. It occurs in about 1/2
of a millisecond. Then the sodium gate closes and the usual outward diffusion
of potassium occurs and the membrane potential returns to the resting potential
of -70 and may go lower to -73 as there is often a temporary overshoot in outward
diffusion of potassium. This return to the RMP is called repolarization.
Repolarization takes about 1/2 of a millisecond. So an action potential is
a depolarization followed by a repolarization and takes a total of about 1 millisecond.
(3)
Nerve Impulses (signals):
http://www.blackwellpublishing.com/matthews/actionp.html
When an action potential occurs at or near the axon hillock, the next area of
the axon, just distal to the axon hillock, now goes through its own action potential.
And then the area next to that one has an action potential and so on down the
axon all the way to the axon terminals with no reduction in the intensity of the
action potentials. The reason for this propagation of the action potential is
that as sodium ions diffuse into the axon during the depolarization portion of
the first action potential near the axon hillock, some of these sodium ions diffuse
over to the next (adjacent) part of the axon and open the voltage-gated ion channels
that are found there (and all along the axon). The stimulation of the voltage-gated
ion channel then lets sodium ions enter, followed by potassium ions exiting and
we have just generated a new action potential - sodium diffusing in followed by
potassium diffusing out. And the process continues all the way down the length
of the axon. So we see that what we call a nerve impulse, or bioelectrical
signal, is a series of action potentials occurring sequentially down the length
of the axon.
e. Nerve
impulse propagation down unmyelinated and myelinated fibers
(1)
The nerve impulse moves down unmyelinated nerve fibers slowly with each region
of the axon going through depolarization and repolarization cycles utilizing voltage-gated
sodium channels. http://w3.ouhsc.edu/human_physiology/presentation/V-gchan.gif
In this process, regions only fractions of a millimeter apart must each depolarize
and repolarize as the signal is propagated. The result is a nerve impulse that
travels at a rate of only about 2 meters/sec (about 4 miles per hour).
(2)
The nerve impulse moving down a myelinated nerve fiber moves at much higher velocity
- at between 100 and 120 meters/sec (about 220 - 265 miles/hour). The reason is
what is called saltatory conduction: the points of depolarization and repolarization
are not every adjacent spot on the axon, but only at nodes of Ranvier which
are about 1 mm apart along the length of the axon.
http://www.brainviews.com/abFiles/AniSalt.htm When sodium enters
at one node of Ranvier during depolarization, it rapidly diffuses, inside the
neuron, to the next node of Ranvier where it opens the voltage-gated sodium
channel and a new depolarization-repolarization cycle occurs. So the impulse
jumps along the axon from one node of Ranvier to the next (like a smooth, flat
stone skipping across the water of a smooth lake; saltation means jumping along).
D. Synapses
1. In general, synapses are points
of communication between neurons. The space separating one neuron from the next
one, the synaptic cleft, is only about 20-40 millimicrons in width. The
majority of synapses in the nervous system are chemical synapses that operate
through the release of neurotransmitters from the presynaptic neuron that stimulate
or inhibit the postsynaptic neuron. However, about 5% of synapses are electrical
synapses involving gap junctions that permit direct flow of ions from one
cell into the next. These electrical synapses allow more rapid communication between
cells.
2. Overall Structure - The presynaptic
neuron of chemical synapses possesses a synaptic knob filled with synaptic
vesicles attached to the cytoskeleton that are moved into and out of position
to discharge their neurotransmitters through the presynaptic membrane into
the synaptic cleft. The postsynaptic neuron possesses proteins on its postsynaptic
membrane which serve as receptors for ligand-gated channels to permit
movements of ions into and out of the postsynaptic membrane.
3.
Calcium's Role:
http://lessons.harveyproject.org/development/nervous_system/cell_neuro/synapses/release.html
An electrical signal traveling down a nerve fiber reaches the axon terminal and
causes the opening of voltage-gated calcium channels permitting diffusion of calcium
ions from the surrounding fluid to enter the synaptic knob. The electrical
signal itself ends. The calcium that enters the synaptic knob causes the synaptic
vesicles already positioned on the presynaptic membrane to discharge their content
of neurotransmitters into the synaptic cleft. With continued electrical signaling
and continued entry of calcium ions into the presynaptic neuron, discharged synaptic
vesicles move off the presynaptic membrane and new "loaded" synaptic
vesicles are moved into position as they slide along the cellular cytoskeleton
into position on the presynaptic membrane.
4. Role
of Receptor Sites: the neurotransmitter diffuses across the synaptic cleft
and attaches to receptors on the postsynaptic membrane. The receptors open ligand-gated
channels and ions diffuse into and/or out of the postsynaptic neuron depending
on the nature of the neurotransmitter and the nature of the receptor.
5.
Neurotransmitter and receptor site variations:
a.
Neurotransmitters: over 100 chemicals have been identified acting as neurotransmitters.
Acetylcholine was discussed in the muscular system chapter and also operates
in many regions of the brain in both excitatory and inhibitory capacities. Also
identified are such small molecules as excitatory amino acids and inhibitory
amino acids. Another group of small chemicals are also found called monoamines
such as epinephrine, norepinephrine, dopamine, histamine and serotonin. There
are also a group of large molecules acting as neurotransmitters - the neuropeptides,
such as endorphins and enkephalins.
b.
Receptors: Postsynaptic membrane receptors are highly varied in their responsiveness
to neurotransmitters. They determine the ultimate effect of the neurotransmitter.
For example, acetylcholine stimulates skeletal muscle but inhibits cardiac muscle
because the receptor in skeletal muscle permits ion movements that depolarize
the postsynaptic membrane, while the receptor in cardiac muscle permits ion movements
that hyperpolarize the postsynaptic membrane (meaning that it becomes even more
positive on the outside and more negative on the inside making it less likely
to depolarize).
6. Operation of an excitatory synapse
that utilizes acetylcholine as the neurotransmitter:
http://w3.ouhsc.edu/human_physiology/presentation/L-gchan.gif
After
release of acetylcholine (Ach) from the presynaptic membrane and its diffusion
across the synaptic cleft, it attaches to Ach receptors on the postsynaptic membrane.
This attachment opens ligand-gated channels in the postsynaptic membrane and sodium
and potassium ions briefly diffuse through the membrane - sodium ions diffuse
in and then potassium ions diffuse out, both through the postsynaptic membrane. The
inward diffusion of sodium ions depolarizes the membrane. If the effect of this
local potential at the synapse is strong enough to cause the membrane potential
at the region of the axon hillock of the postsynaptic neuron to reach the threshold
value (about -55 mv), then the axon of the postsynaptic neuron will fire a
nerve impulse. Cholinesterase found in the synaptic cleft breaks down Ach
into acetic acid and choline, neither of which is capable of attaching to receptor
sites. Choline is reabsorbed by the presynaptic membrane and utilized to synthesize
more Ach. Cholinesterase inhibitors prevent the breakdown of Ach and result
in spastic paralysis; they are the basis for many insecticides.
http://www.uoguelph.ca/GTI/urbanpst/cholin_t.htm
7. Operation of inhibitory synapses: some inhibitory
synapses result in the opening of ligand-gated channels that permit potassium
to leave the cell; others open channels that permit chloride to enter the cell.
In either case, the result is a hyperpolarization of the membrane (opposite of
depolarization) that makes subsequent depolarization less likely to occur.
http://home.comcast.net/~john.kimball1/BiologyPages/E/ExcitableCells.html#Hyperpolarization
E. Neuronal Integration
1. In general: Neuronal integration is a way of
saying "thinking" in that it involves manipulating (processing)
information, recalling facts that have been stored, and making decisions.
2. The synapse is of prime importance for neural
integration. Whether or not a neuron fires depends on the sum of the excitatory
and inhibitory postsynaptic potentials that impinge on the dendrites and cyton
of the neuron in question.
3. Neuronal Coding:
Understanding the quality of a stimulus and its intensity is dependent on neuronal
coding. Quality depends on which neurons are stimulated which results
in particular areas of the brain being signaled. There are cells in the eye (called
cones) that specifically respond to wavelengths of light corresponding to the
color red and these cells specifically signal portions of the cerebral cortex
that then cause us to see the color red. How red or the intensity of the
color is coded by the frequency of impulses (impulses/sec) reaching the
brain: the more intense the color or the more intense any stimulus is, the greater
the number of impulses reaching the brain per second.
4.
Neuronal circuitry:
http://cas.bellarmine.edu/tietjen/HumanBioogy/neuronal_circuits.htm
Neurons
work in groups called pools that can consist of hundreds, thousands or
millions of neurons linked through synapses. They are functionally connected through
several types of circuits. In convergent circuits, many neurons send impulses
toward one or a few neurons from which the "decision" to fire or not
to fire emanates. In divergent circuits impulses travel out from a few
neurons to many resulting in widespread responses to stimuli: this is common in
the sympathetic branch of the ANS where observing danger can result in changes
in many parts of the body - heart beat, breathing, sweating to name a few. Reberverating
circuits result in neuronal interactions repeating as in rhythmic patterns
of breathing or in continuing to think about an issue - short term memory.
5. Facilitating synaptic transmission may
be a way of partially explaining memory. If memory is a specific pathway
through the cerebral cortex involving a particular pattern of synapses, then allowing
those synapses to fire more easily will allow that particular pathway to fire
more easily as a whole. The result is that whatever that pathway signifies, comes
to mind more easily.
F. Drug Effects on Neurons - the synapse is vulnerable to many drugs. The
following examples are representative of the many substances affecting the nervous
system's chemical synapses.
1. Substances that
Enhance Signal Transmission across the Synapse
a.
Prozac (fluoxetine) - Prozac acts as an antidepressant because it blocks presynaptic
membrane reabsorption (reuptake) of the monoamine neurotransmitter, dopamine,
so that its mood-elevating effects are prolonged. Prozac is frequently referred
to as a serotonin reuptake inhibitor.
b.
MAO inhibitors - Monoamine oxidase (MAO) is an enzyme that works within
the presynaptic neuron to breakdown reabsorbed monoamine neurotransmitters, like
serotonin. Drugs that inhibit the action of MAO (the MAO inhibitors) can elevate
mood. This is because they increase the quantity of monoamines in the synapses
as they reduce the rate of breakdown of the monoamines.
c.
Cocaine - cocaine is a dopamine reuptake inhibitor. Since dopamine is a
neurotransmitter associated with providing feelings of pleasure, use of cocaine
increases pleasurable feelings. The downside of cocaine use is the development
of dependence on it for any feelings of pleasure. With continued use of cocaine,
the neurons cannot produce enough dopamine to produce pleasurable sensations under
non-drug-use conditions. One must use the drug for the small amount of dopamine
that is made to stay long enough to provide one with pleasurable sensations. The
underlying problem for the neurons is that when cocaine prevents dopamine reuptake,
the dopamine diffuses away from the neurons and is degraded by other cells. Then,
the presynaptic neurons must re-synthesize dopamine and this process does not
occur rapidly enough to provide a normal level of pleasurable sensations.
d. Caffeine enhances signal
transmission across the synapse because it structurally resembles the CNS inhibitory
neurotransmitter, adenosine. The result of drinking coffee or consuming other
foods containing caffeine is that the caffeine binds to adenosine receptor sites
but does not cause inhibition of nervous system activity (sleepiness).
2.
Substances that Stimulate Transmission across the Synapse
a.
Amphetamines stimulate dopamine and norepinephrine receptors because the
amphetamine molecule has enough of a molecular resemblance to dopamine and norepinephrine.
The
following animations summarize many of the concepts covered in this unit:
http://highered.mcgraw-hill.com/sites/0072437316/student_view0/chapter45/animations.html#
Biomedical
Terminology:
Define each
term.
axolemma
axon
axoplasm
cell membrane
cholinesterase
cholinesterase inhibitors
conductivity
convergent circuit
cyton
dendrites
depolarization
divergent circuit
dynein
irritability
kinesin
ligand gated
channels
local potentials
myelin sheath
neurilemma
neurotubules
norepinephrine
Nissl bodies
nodes of Ranvier, internodes
postsynaptic
membrane
presynaptic membrane
repolarization
resting membrane potential
Schwann cells
terminal axon fibers
voltage-gated sodium channels
Practice
Quiz
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