Chapter 12 Flashcards
(267 cards)
1
Q
The central nervous system consists of
A
The brain and spinal cord
2
Q
The peripheral nervous system consists of
A
All nervous tissue outside of the CNS
3
Q
A nerve is
A
A bundle of hundreds or thousands of axons plus associated connective tissue and blood vessels that lie outside of the brain and spinal cord
4
Q
There are _____ pairs of cranial nerves that emerge from the ______
A
Twelve, brain
5
Q
There are _______ pairs of spinal nerves that emerge from the ________
A
31, spinal cord
6
Q
Define sensory receptor
A
A structure of the nervous system that monitors changes in the internal and external environment.
7
Q
The sensory or afferent division of the PNS
A
Conveys input into the CNS from sensory receptors in the body
8
Q
The motor or efferent division of the PNS
A
Conveys output from the CNS to its effectors (muscles, glands)
9
Q
The PNS is divided into
A
Sensory and motor divisions
10
Q
The somatic nervous system SNS
A
Conveys output from the CNS to skeletal muscles only
11
Q
The autonomic nervous system ANS
A
Conveys output from the CNS to smooth muscle, cardiac muscle, and glands
12
Q
The ANS is comprised of two branches the
A
Sympathetic division
Parasympathetic division
13
Q
Parasympathetic division of the ANS helps with
A
Rest and digest
14
Q
The sympathetic division of the ANS deals with
A
Fight or flight
15
Q
The third branch of the ANS is the
A
Enteric plexuses which regulates the muscles of the walls of the digestive tract. It
16
Q
The three main functions of the nervous system
A
Sensory function
Integrative function
Motor function
17
Q
The integrative function of the nervous system involves
A
Processing sensory information and determining appropriate motor responses
18
Q
Nervous tissue is comprised of two types of cells
A
Neurons and neuroglia
19
Q
Neurons
A
Connect the body to the brain and the spinal cord
20
Q
Neuroglia
A
Support, nourish, and protect neurons and maintain the interstitial fluid that bathes them.
21
Q
Neurons do not_______ but neuroglia do
A
Divide
22
Q
Like muscle cells neurons posses electrical excitability which means
A
They can respond to stimuli and convert it into an action potential
23
Q
Define stimulus
A
Any change in the environment that is strong enough to initiate a nerve impulse
24
Q
A nerve impulse is synonymous with an action potential which is
A
An electrical signal that propagates on the surface of a neuron
25
Neurons have three main parts
Cell body
Dendrites
Axon
26
The cell body of a neuron contains
Many normal organelles along with free ribosomes and clusters of rough ER called Nissl bodies
27
The cytoskeleton is composed of
Neurofibrils and microtubules
28
Neurofibrils are composed of bundles of __________ which provide _______
Intermediate filaments, shape and support
29
Microtubules assist in
Moving materials between the cell body and axon
30
Aging neurons contain
Lipofuscin which is a pigment that is a product of neuronal lysosomes that accumulate as a neuron ages.
31
Bumps on a neuronal plasma membrane are called
Somatic spines
32
Somatic spines
Receptor sites that bind chemical messengers from other neurons
33
Define ganglion
A collection of neuron cell bodies outside of the CNS
34
Define nerve fiber
A general term for any neuronal process that emerges from a cell body. Dendrites and axons are considered nerve fibers
35
Define dendrites
The receiving or input portions of a neuron
36
Dendritic spines
Numerous receptor sites that receive input via chemical messages from other neurons
37
What is the function of an axon
To propagate nerve impulses toward either another neuron, a muscle fiber, or a gland cell.
38
Define axon hillock
The cone shaped area where an axon connects to a cell body
39
What is the initial segment of an axon
The part of an axon closest to the axon hillock
40
What is the trigger zone?
The junction of the axon hillock and the initial segment where most action potentials arise
41
An action potential originates in the _________ from which they travel along the ________ to their destination.
Trigger zone, axon
42
What is the cytoplasm of the axon called
Axoplasm
43
The plasma membrane of an axon is called the
Axolemma
44
Side branches of an axon are called
Axon collateral
45
Axon terminals
Many fine processes at the end of an axon
46
Define synapse
The site of communication between two neurons or between a neuron and it’s effector
47
Define synaptic end bulbs
The tips of some axon terminals that swell into a bulbous structure
48
At the end of some axons are varicosities which are
A string of swollen bumps at the end of an axon
49
Both varicosities and synaptic end bulbs contain
Synaptic vesicles
50
Define synaptic vesicles
Membrane enclosed sacs that store neurotransmitters
51
Define neurotransmitters
A molecule released from a synaptic vesicle that excites another neuron, muscle fiber, or gland cell.
52
Describe slow axonal transport
It moves materials about 1-5 mm per day it moves the axoplasm in one direction only, from the cell body toward the axon. It supplies new axoplasm to developing or regenerating axons and replenishes axoplasm in growing and mature axons.
53
Describe fast axonal transport
Moves materials 200-400mm per day. It uses proteins as motors to move materials in both directions.
54
Anterograde fast axonal transport
Moves organelles and synaptic vesicles from the cell body toward the axon terminals.
55
Retrograde fast axonal transport
Moves materials from the axon toward the cell body to be degraded or recycled
56
What are the three structural classifications of neurons
Multipolar
Bipolar
Pseudo unipolar
57
Multipolar neurons
Usually have several dendrites and one axon. Most neurons of the brain and spinal cord are multipolar
58
Bipolar neurons
Have one main dendrite and one axon. Found in the retina of the eye, the inner ear, and the olfactory area of the brain.
59
Pseudo-unipolar neurons
Have dendrites and an axon that fuse together. Commonly found in sensory receptors. The cell bodies are located in the ganglia of spinal and cranial nerves.
60
Purkinje cells are located in
The cerebellum
61
Pyramidal cells are found in
The cerebral cortex
62
Three functional classifications of neurons
Sensory neurons
Motor neurons
Interneurons
63
Sensory neurons
Once an appropriate stimulus activates a sensory neuron the neuron forms a nerve impulse which is conveyed to the CNS.
64
Motor neurons
Convey nerve impulses away from the CNS through the PNS to effectors.
65
Interneurons
Are involved with interpretation of incoming sensory information and eliciting a motor response. Heavily involved in integration.
66
Neuroglia do not
Generate or propagate action potentials
67
Gliomas
Brain tumors originating from neuroglia. Usually aggressive and malignant.
68
Which types of neuroglia are found only in the CNS
Astrocytes
Oligodendrocytes
Microglia
Ependymal cells
69
Astrocytes
Have many processes and are the largest most numerous neurogli
70
5 functions of Astrocytes
1 they are strong and can support neurons
2 protects neurons from harmful elements of blood
3 assists in embryonic development
4 maintain the chemical environment suitable for action potential generation
71
What is the function of oligodendrocytes?
The processes of oligodendrocytes form and maintain the myelin sheath around CNS axons.
72
Define myelin sheath
A multilayered lipid and protein covering around some axons that insulates them and increases the speed of nerve impulse conduction
73
Define microglial cells
Small cells with slender processes. They function as phagocytes.
74
Ependymal cells
Produce, monitor and assist with the circulation of cerebrospinal fluid.
75
What types of neuroglia are locate in the PNS
Schwann cells and satellite cells
76
Schwann cells
Form myelin sheath around axons in the PNS and participate in axon regeneration
77
Satellite cells
Surround the cell bodies of neurons of PNS ganglia providing structural support and regulate the exchange of materials between cell bodies and interstitial fluid.
78
What is the neurolemma
The outer nucleated cytoplasmic layerof a schwann cell that completel surrounds the myelin sheath.
79
what does the neurolemma do?
aids in the regrowth and regeneration of an axon
80
What is a nucleus in neurology
a cluster of neuronal cell bodies in the CNS
81
What is a tract in neurology
a bundle of axons in the CNS
82
White matter is composed mainly of
myelinated axons
83
gray matter is composed of
neuronal cell bodies
dendrites
unmyelinated axons
axon terminals
neuroglia
84
What types of electrical signals are used by neurons to communicate
Graded potentials
Action potentials
85
Graded potentials are used for _________
Action potentials are used for___________
Short distance communication
long distance communication
86
when an action potential occurs in a neuron it is called
a nerve action potential
87
When you touch an object with your finger
1a graded potential develops in a sensory receptor in the skin of the finger
2The graded potential causes the axon to form a nerve impulse which travels to the CNS and releases a neurotransmitter at a synapse with an interneuron
3The neurotransmitter stimulates the interneuron to form a graded potential in its dendrites and cell body
4 In response to the graded potential, the axon of the interneuron forms a nerve impulse. The nerve impulse travels along the axon, which results in neurotransmitter release at the next synapse with another interneuron.
5 The process in number four continues until the interneurons in the cerebral cortex, the outer part of the brain, are activated, perception occurs and you are able to feel the smooth surface
88
define upper motor neuron
a type of motor neuron that synapses with a lower motor neuron farther down in the CNS in order to contract a skeletal muscle.
89
define lower motor neuron
a type of motor neuron that directly supplies skeletal muscle fibers.
90
Suppose that you want to use the pen to write a letter. The nervous system would respond in the following way
1 A stimulus in the brain causes a graded potential to form in the dendrites and cell body of an upper motor neuron,The graded potential subsequently causes a nerve impulse to occur in the axon of the upper motor neuron, followed by neurotransmitter release.
2The neurotransmitter generates a graded potential in a lower motor neuron The graded potential triggers the formation of a nerve impulse and then release of the neurotransmitter at neuromuscular junctions formed with skeletal muscle fibers that control movements of the fingers.
3The graded potential triggers the formation of a nerve impulse and then release of the neurotransmitter at neuromuscular junctions formed with skeletal muscle fibers that control movements of the fingers.
91
define membrane potential
an electrical potential difference (voltage) across the plasma membrane.
92
Resting membrane potential
Membrane potential in electrically excitable cells
93
What is an electrical current
a flow of charged particles along a wire
94
define Electrochemical gradient
a concentration (chemical) difference plus an electrical difference
95
Four types of ion channels
leak channels, ligand-gated channels, mechanically gated channels, and voltage-gated channels:
96
Describe Leak channels
Gated channels that randomly open and close.
97
Where are leak channels typically found
Found in nearly all cells, and dendrites, cell bodies, and axons of all types of neurons.
98
Describe Ligand gated ion channels
Gated channels that open in response to binding of ligand (chemical) stimulus.
99
Where are ligand gated channels found
Dendrites of some sensory neurons such as pain receptors and dendrites and cell bodies of interneurons and motor neurons.
100
Describe Mechanically gated Ion channels
Gated channels that open in response to mechanical stimulus (such as touch, pressure, vibration, or tissue stretching).
101
Where are Mechanically gated channels found
Dendrites of some sensory neurons such as touch receptors, pressure receptors, and some pain receptors
102
Describe voltage gated ion channels
Gated channels that open in response to voltage stimulus (change in membrane potential).
103
where are voltage gated channels found
axons of all types of neurons
104
Why does resting membrane potential exist
because of a small buildup of negative ions in the cytosol along the inside of the membrane, and an equal buildup of positive ions in the extracellular fluid (ECF) along the outside surface of the membrane
105
Define electrode
are devices that conduct electrical charges.
106
What is a voltometer
a device which detects the electrical difference (voltage) across the plasma membrane
107
a cell that exhibits a membrane potential is said to be
polarized
108
Resting membrane potential arises because of three factors which are
1Unequal distribution of ions in the ECF and cytosol
2Inability of most anions to leave the cell.
3Electrogenic nature of the Na+–K+ ATPases
109
Why cant most anions leave the cell
because they are attached to nondiffusible molecules such as ATP and large proteins
110
How does the electrogenic nature of the sodium potassium pumps contribute to resting membrane potential
by pumping out Na+ as fast as it leaks in. At the same time, the Na+–K+ ATPases bring in K+. However, the potassium ions eventually leak back out of the cell as they move down their concentration gradient. Recall that the Na+–K+ ATPases expel three Na+ for each two K+ imported (see Figure 3.10). Since these pumps remove more positive charges from the cell than they bring into the cell, they are electrogenic, which means they contribute to the negativity of the resting membrane potential.
111
Define graded potential
a small deviation from the resting membrane potential that makes the membrane either more polarized (inside more negative) or less polarized (inside less negative).
112
Define a hyperpolarizing graded potential
a small deviation from the resting membrane potential that makes the membrane more polarized (inside more negative)
113
Describe a depolarizing graded potential
a small deviation from the resting membrane potential that makes the membrane less polarized (inside less negative)
114
a graded potential occurs when
a stimulus causes mechanically gated or ligand-gated channels to open or close in an excitable cell’s plasma membrane
115
Where do most graded potentials occur
mainly in the dendrites and cell body of a neuron.
116
graded potentials vary in amplitude based on
the strength of the stimulus (Figure 12.16). They are larger or smaller depending on how many ligand-gated or mechanically gated channels have opened (or closed) and how long each remains open.
117
Describe decremental conduction
The opening or closing of ion channels alters the flow of specific ions across the membrane, producing a flow of current that is localized, which means that it spreads to adjacent regions along the plasma membrane in either direction from the stimulus source for a short distance and then gradually dies out as the charges are lost across the membrane through leak channels. This mode of travel by which graded potentials die out as they spread along the membrane is called decremental conduction.
118
Define Summation
the process by which graded potentials add together.
119
when a graded potential occurs in the dendrites or cell body of a neuron in response to a neurotransmitter, it is called a
postsynaptic potential
120
the graded potentials that occur in sensory receptors are termed
Receptor Potentials
121
The amplitude of a graded potential depends on
the stimulus strength. The greater the stimulus strength, the larger the amplitude of the graded potential.
122
Define Action potential
a sequence of rapidly occurring events that decrease and reverse the membrane potential and then eventually restore it to the resting state.
123
An action potential inside of a neuron is called
a nerve impulse
124
The two phases of a nerve impulse are
a depolarizing phase and a repolarizing phase
125
During the depolarizing phase
the negative membrane potential becomes less negative, reaches zero, and then becomes positive.
126
During the repolarizing phase
the membrane potential is restored to the resting state of −70 mV
127
Following the repolarizing phase there may be an
after-hyperpolarizing phase,
128
during the afterhyperpolarizing phase
the membrane potential temporarily becomes more negative than the resting level
129
What causes the depolarizing phase
The first channels that open, the voltage-gated Na+ channels, allow Na+ to rush into the cell
130
What causes the repolarizing phase of an action potential
voltage-gated K+ channels open, allowing K+ to flow out
131
what causes the hyperpolarizing phase
the voltage-gated K+ channels remain open after the repolarizing phase ends.
132
A nerve impulse occurs in the membrane of the axon of a neuron when depolarization reaches a certain level termed the
threshold
133
A nerve impulse will not occur in response to a
subthreshold stimulus
134
Define subthreshold stimulus
a weak depolarization that cannot bring the membrane potential to threshold.
135
Threshold stimulus
a stimulus that is just strong enough to depolarize the membrane to threshold
136
Several nerve impulses will form in response to a
suprathreshold stimulus
137
defin suprathreshold stimulus
a stimulus that is strong enough to depolarize the membrane above threshold
138
Describe the all or none principle
a nerve impulse is generated in response to a threshold stimulus but does not form when there is a subthreshold stimulus
139
define refractory period
The period of time after nerve impulse begins during which an excitable cell cannot generate another nerve impulse in response to a normal threshold stimulus
140
During the absolute refractory period
even a very strong stimulus cannot initiate a second nerve impulse
141
Graded potentials do not have
a refractory period
142
Define relative refractory period
the period of time during which a second nerve impulse can be initiated, but only by a larger-than-normal stimulus
143
To communicate information from one part of the body to another
nerve impulses in a neuron must travel from where they arise at the trigger zone of the axon to the axon terminals
144
nerve impulses are not
decremental
145
a nerve impulse keeps its strength as it spreads along the membrane. This mode of conduction is called
propogation
146
propogation depends on
positive feedback
147
it is not the same nerve impulse that propagates along the entire axon. Instead
the nerve impulse regenerates over and over at adjacent regions of membrane from the trigger zone to the axon terminals
148
Inflow of sodium ions (Na+) causes the _________
outflow of potassium ions (K+) causes the __________
Depolarizing phase
Repolarizing phase
149
Define anesthetics
drugs that induce partial or total loss of sensation, especially pain.
150
How do local anesthetics work
They block transmission of nerve impulses to pain centers in the central nervous system by inhibiting the opening of voltage-gated Na+ channels.
151
What are the two types of propogation
continuous conduction
saltatory conduction
152
Continuous conduction occurs in
unmyelinated axons and in muscle fibers
153
saltatory conduction occurs in________ and takes advantage of ___________
myelinated axons, increased numbers of ion channels in myelinated gaps
154
saltatory conduction has two advantages
1 It is much faster
2 it is more energy efficient
155
What three factors affect the speed of propogation of a nerve impulse
1 amount of myelination
2 axon diameter (larger diameter is faste)
3 temperature
156
Describe A Fibers
A fibers are the largest diameter axons (5–20 μm) and are myelinated. A fibers have a brief absolute refractory period and conduct nerve impulses at speeds of 12 to 130 m/sec (27–290 mi/hr). The axons of sensory neurons that propagate impulses associated with touch, pressure, position of joints, and some thermal and pain sensations are A fibers, as are the axons of motor neurons that conduct impulses to skeletal muscles.
157
Describe B fibers
B fibers are axons with diameters of 2–3 μm. Like A fibers, B fibers are myelinated and exhibit saltatory conduction at speeds up to 15 m/sec (34 mi/hr). B fibers have a somewhat longer absolute refractory period than A fibers. B fibers conduct sensory nerve impulses from the viscera to the brain and spinal cord. They also constitute all of the axons of the autonomic motor neurons that extend from the brain and spinal cord to the ANS relay stations called autonomic ganglia.
158
Describe C fibers
C fibers are the smallest diameter axons (0.5–1.5 μm) and all are unmyelinated. Nerve impulse propagation along a C fiber ranges from 0.5 to 2 m/sec (1–4 mi/hr). C fibers exhibit the longest absolute refractory periods. These unmyelinated axons conduct some sensory impulses for pain, touch, pressure, heat, and cold from the skin, and pain impulses from the viscera. Autonomic motor fibers that extend from autonomic ganglia to stimulate the heart, smooth muscle, and glands are C fibers. Examples of motor functions of B and C fibers are constricting and dilating the pupils, increasing and decreasing the heart rate, and contracting and relaxing the urinary bladder.
159
Where do graded potentials arise
mainly in dendrites and cell body.
160
What types of channels are involved with graded potentials?
Ligand-gated or mechanically gated ion channels.
161
Conduction of graded potentials is
decremental and permits communication only over short distances
162
describe the polarity of graded potentials
May be hyperpolarizing (inhibitory to generation of action potential) or depolarizing (excitatory to generation of action potential).
163
Is there a refractory period for graded potentials
no
164
can summation occur in graded potentials
yes
165
Describe the origin of Nerve impulses
Arise at trigger zones and propagate along axon.
166
What types of channels are involved with nerve impulses
Voltage-gated channels for Na+ and K+
167
Describe the conduction of Nerve impulses
Propagate and thus permit communication over longer distances.
168
Describe the polarity of a nerve impulse
Always consist of depolarizing phase followed by repolarizing phase and return to resting membrane potential.
169
Is there a refractory period in Nerve impulses
yes
170
can summation occur in nerve impulses
no
171
Define synapse
is a region where communication occurs between two neurons or between a neuron and an effector cell (muscle cell or glandular cell).
172
The term presynaptic neuron (pre- = before) refers to
a nerve cell that carries a nerve impulse toward a synapse
173
a Post synaptic cell
recieves a signal
174
a postsynaptic cell can be either a __________________or ____________________
postsynaptic neuron (post- = after) that carries a nerve impulse away from a synapse or an effector cell that responds to the impulse at the synapse.
175
List the three types of synapses and describe them
1axodendritic (ak′-so-den-DRIT-ik = from axon to dendrite)
2 axosomatic (ak′-sō-sō-MAT-ik = from axon to cell body)
3axo-axonal (ak′-so-ak-SON-al = from axon to axon)
176
At an electrical synapse
action potentials conduct directly between the plasma membranes of adjacent neurons through structures called gap junctions.
177
Define connexon
a tubular structure that connects the cytosol of the two cells directly
178
Electrical synapses have two advantages over chemical synapses
Faster communication
Synchronization
179
Describe why synchronization is an important advantage of electrical synapses
Electrical synapses can synchronize (coordinate) the activity of a group of neurons or muscle fibers. In other words, a large number of neurons or muscle fibers can produce action potentials in unison if they are connected by gap junctions. The value of synchronized action potentials in the heart or in visceral smooth muscle is coordinated contraction of these fibers to produce a heartbeat or move food through the digestive canal.
180
Although the plasma membranes of presynaptic and postsynaptic neurons in a chemical synapse are close, they do not touch. They are separated by the
synaptic cleft
181
How does a chmical synapse work
Thus, the presynaptic neuron converts an electrical signal (nerve impulse) into a chemical signal (released neurotransmitter). The postsynaptic neuron receives the chemical signal and in turn generates an electrical signal (postsynaptic potential).
182
what is the synaptic delay
The time required for the processes at a chemical synapse
183
What is the first step at a chemical synapse
A nerve impulse arrives at a synaptic end bulb (or at a varicosity) of a presynaptic axon.
184
What is the second step at a chemical synapse
The depolarizing phase of the nerve impulse opens voltage-gated Ca2+ channels, which are present in the membrane of synaptic end bulbs. Because calcium ions are more concentrated in the extracellular fluid, Ca2+ flows inward through the opened channels.
185
What is the third step at a chemical synapse
An increase in the concentration of Ca2+ inside the presynaptic neuron serves as a signal that triggers exocytosis of the synaptic vesicles. As vesicle membranes merge with the plasma membrane, neurotransmitter molecules within the vesicles are released into the synaptic cleft. Each synaptic vesicle contains several thousand molecules of neurotransmitter.
186
What is the fourth step at a chemical synapse
The neurotransmitter molecules diffuse across the synaptic cleft and bind to neurotransmitter receptors in the postsynaptic neuron’s plasma membrane. The receptor shown in Figure 12.23 is part of a ligand-gated channel (see Figure 12.11b); you will soon learn that this type of neurotransmitter receptor is called an ionotropic receptor. Not all neurotransmitters bind to ionotropic receptors; some bind to metabotropic receptors (described shortly).
187
What is the fifth step at a chemical synapse
Binding of neurotransmitter molecules to their receptors on ligand-gated channels opens the channels and allows particular ions to flow across the membrane.
188
What is the sixth step at a chemical synapse
As ions flow through the opened channels, the voltage across the membrane changes. This change in membrane voltage is a postsynaptic potential. Depending on which ions the channels admit, the postsynaptic potential may be a depolarization (excitation) or a hyperpolarization (inhibition). For example, opening of Na+ channels allows inflow of Na+, which causes depolarization. However, opening of Cl– or K+ channels causes hyperpolarization. Opening Cl− channels permits Cl− to move into the cell, while opening the K+ channels allows K+ to move out—in either event, the inside of the cell becomes more negative.
189
What is the last step at a chemical synapse
When a depolarizing postsynaptic potential reaches threshold, it triggers a nerve impulse in the axon of the postsynaptic neuron.
190
at a chemical synapse communication can only occur
in one direction
191
A neurotransmitter that causes depolarization of the postsynaptic membrane is
excitatory because it brings the membrane closer to threshold
192
a depolarizing post synaptic potential is called an
excitatory postsynaptic potential
193
A hyperpolarizing postsynaptic potential is termed an
inhibitory postsynaptic potential (IPSP).
194
When a neurotransmitter binds to the correct neurotransmitter receptor
an ion channel opens and a postsynaptic potential (either an EPSP or IPSP) forms in the membrane of the postsynaptic cell.
195
What are the two types of neurotransmitter receptors
ionotropic
metabotropic
196
An ionotropic receptor (ī-on-ō-TROP-ik) is
a type of neurotransmitter receptor that contains a neurotransmitter binding site and an ion channel. In other words, the neurotransmitter binding site and the ion channel are components of the same protein.
197
how does an ionotropic
In the absence of neurotransmitter (the ligand), the ion channel component of the ionotropic receptor is closed. When the correct neurotransmitter binds to the ionotropic receptor, the ion channel opens, and an EPSP or IPSP occurs in the postsynaptic cell.
198
What is a metabotropic neurotransmitter receptor
type of neurotransmitter receptor that contains a neurotransmitter binding site but lacks an ion channel as part of its structure. However, a metabotropic receptor is coupled to a separate ion channel by a type of membrane protein called a G protein
199
What are the three ways that neurotransmitters are removed from the synaptic cleft?
Diffusion
Enzymatic degradation
Uptake by cells
200
What are the two types of summation
Spacial summation
temporal summation
201
Describe spacial summation
Spatial summation is summation of postsynaptic potentials in response to stimuli that occur at different locations in the membrane of a postsynaptic cell at the same time. For example, spatial summation results from the buildup of neurotransmitter released simultaneously by several presynaptic end bulbs
202
Describe temporal summation
Temporal summation is summation of postsynaptic potentials in response to stimuli that occur at the same location in the membrane of the postsynaptic cell but at different times. For example, temporal summation results from buildup of neurotransmitter released by a single presynaptic end bulb two or more times in rapid succession
203
The sum of all of the excitatory and inhibitory effects at any given time determines the effect on the postsynaptic neuron, which may respond in the following ways:
EPSP
Nerve impulse
IPSP
204
If the net summation of EPSPs and IPSPs is a depolarization that reaches threshold,
then a nerve impulse will occur at the trigger zone of a postsynaptic neuron.
205
What is the function of dendrites
Receive stimuli through activation of ligand-gated or mechanically gated ion channels; in sensory neurons, produce generator or receptor potentials; in motor neurons and interneurons, produce excitatory and inhibitory postsynaptic potentials (EPSPs and IPSPs).
206
What is the purpose of a cell body
Receives stimuli and produces EPSPs and IPSPs through activation of ligand-gated ion channels.
207
What is the function of axon hillock and initial segment of axon
Trigger zone in many neurons; integrates EPSPs and IPSPs and, if sum is depolarization that reaches threshold, initiates a nerve impulse.
208
What is the function of an axon
Propagates nerve impulses from initial segment (or from dendrites of sensory neurons) to axon terminals in self-regenerating manner; impulse amplitude does not change as it propagates along axon.
209
What is the function of Axon terminals and synaptic end bulbs (or varicosities)
Inflow of Ca2+ caused by depolarizing phase of nerve impulse triggers exocytosis of neurotransmitter from synaptic vesicles
210
neurons, called neurosecretory cells,
secrete hormones
211
Neurotransmitters can be divided into two classes based on size:
small-molecule neurotransmitters and neuropeptides
212
The small-molecule neurotransmitters include
acetylcholine, amino acids, biogenic amines, ATP and other purines, nitric oxide, and carbon monoxide.
213
Describe acetylcholine
ACh is an excitatory neurotransmitter at some synapses, such as the neuromuscular junction, where the binding of ACh to ionotropic receptors opens cation channels (see Figure 12.24a). It is also an inhibitory neurotransmitter at other synapses, where it binds to metabotropic receptors coupled to G proteins that open K+ channels
214
Glutamate (gloo-TA-māt) (glutamic acid) and aspartate (as-PAR-tāt) (aspartic acid)
have powerful excitatory effects and are used mostly in thew CNS
215
Gamma-aminobutyric acid (GABA) (GAM-ma am-i-nō-bu-TIR-ik) and glycine are important```````````
inhibitory neurotransmitters.
216
ACh is usually released by
PNS neurons
217
Neurotransmitters are
chemical substances that neurons use to communicate with other neurons, muscle fibers, and glands.
218
Norepinephrine (NE) (nor′-ep-i-NEF-rin) plays roles in
arousal (awakening from deep sleep), dreaming, and regulating mood.
219
Brain neurons containing the neurotransmitter dopamine (DA) (DŌ-pa-mēn)
are active during emotional responses, addictive behaviors, and pleasurable experiences. In addition, dopamine-releasing neurons help regulate skeletal muscle tone and some aspects of movement due to contraction of skeletal muscles.
220
Norepinephrine, dopamine, and epinephrine are classified chemically as
catecholamines (kat-e-KŌL-a-mēns
221
The two enzymes that break down catecholamines are
catechol-O-methyltransferase (COMT) (kat′-e-kōl-ō-meth-il-TRANS-fer-ās), and monoamine oxidase (MAO) (mon-ō-AM-īn OK-si-dās).
222
Serotonin (ser′-ō-TŌ-nin), which is also known as 5-hydroxytryptamine (5-HT),
is concentrated in the neurons in a part of the brain called the raphe nucleus. It is thought to be involved in sensory perception, temperature regulation, control of mood, appetite, and the induction of sleep.
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The simple gas nitric oxide (NO)
is an important excitatory neurotransmitter secreted in the brain, spinal cord, suprarenal glands, and nerves to the penis and has widespread effects throughout the body.
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nitric oxide synthase (NOS)
catalyzes formation of NO from the amino acid arginine.
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Carbon monoxide CO
might protect against excess neuronal activity and might be related to dilation of blood vessels, memory, sense of smell, vision, thermoregulation, insulin release, and anti-inflammatory activity.
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neuropeptides
Neurotransmitters consisting of 3 to 40 amino acids linked by peptide bonds
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What do Neuropeptides do
bind to metabotropic receptors and have excitatory or inhibitory actions, depending on the type of metabotropic receptor at the synapse.
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Endorphins and dynorphins
have been linked to improved memory and learning; feelings of pleasure or euphoria; control of body temperature; regulation of hormones that affect the onset of puberty, sexual drive, and reproduction; and mental illnesses such as depression and schizophrenia.
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Substance P
is a neropeptide released by neurons that transmit pain-related input from peripheral pain receptors into the central nervous system, enhancing the perception of pain.
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Enkephalin and endorphin
suppress the release of substance P, thus decreasing the number of nerve impulses being relayed to the brain for pain sensations.
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agonist
An agent that binds to receptors and enhances or mimics the effect of a natural neurotransmitter
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antagonist
An agent that binds to and blocks neurotransmitter receptors
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Describe Substance P
Found in sensory neurons, spinal cord pathways, and parts of brain associated with pain; enhances perception of pain.
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Describe Enkephalins
Inhibit pain impulses by suppressing release of substance P; may have role in memory and learning, control of body temperature, sexual activity, and mental illness.
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Describe endorphins
Inhibit pain by blocking release of substance P; may have role in memory and learning, sexual activity, control of body temperature, and mental illness.
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Dynorphins
May be related to controlling pain and registering emotions.
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Describe Hypothalamic releasing and inhibiting hormones
Produced by hypothalamus; regulate release of hormones by anterior pituitary.
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Describe angiotenson 2
Stimulates thirst; may regulate blood pressure in brain. As a hormone, causes vasoconstriction and promotes release of aldosterone, which increases rate of salt and water reabsorption by kidneys.
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Describe Cholecystokinin (CCK)
Found in brain and small intestine; may regulate feeding as a “stop eating” signal. As a hormone, regulates pancreatic enzyme secretion during digestion, and contraction of smooth muscle in gastrointestinal tract.
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Describe Neuropeptide Y
Stimulates food intake; may play a role in the stress response.
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The CNS contains billions of neurons organized into complicated networks called neural circuits which are
functional groups of neurons that process specific types of information.
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In a simple series circuit,
a presynaptic neuron stimulates a single postsynaptic neuron. The second neuron then stimulates another, and so on. However, most neural circuits are more complex.
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an arrangement, called divergence,
permits one presynaptic neuron to influence several postsynaptic neurons (or several muscle fibers or gland cells) at the same time.
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In a diverging circuit,
the nerve impulse from a single presynaptic neuron causes the stimulation of increasing numbers of cells along the circuit
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In another arrangement, called convergence
several presynaptic neurons synapse with a single postsynaptic neuron. This arrangement permits more effective stimulation or inhibition of the postsynaptic neuron.
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In a converging circuit
the postsynaptic neuron receives nerve impulses from several different sources
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in a reverberating circuit
the incoming impulse stimulates the first neuron, which stimulates the second, which stimulates the third, and so on.
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Among the body responses thought to be the result of output signals from reverberating circuits are
breathing, coordinated muscular activities, waking up, and short-term memory.
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describe the parallel after-discharge circuit
In this circuit, a single presynaptic cell stimulates a group of neurons, each of which synapses with a common postsynaptic cell.
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Parallel after-discharge circuits may be involved in
precise activities such as mathematical calculations.
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Plasticity
The capability to change based on experience
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Despite this plasticity, mammalian neurons have very limited powers of regeneration, which is
the capability to replicate or repair themselves.
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In the CNS,
little or no repair of damage to neurons occurs. Even when the cell body remains intact, a severed axon cannot be repaired or regrown.
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Neurogenesis
the birth of new neurons from undifferentiated stem cells
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What are two reasons for the lack of neurogenisis in humans
1) inhibitory influences from neuroglia, particularly oligodendrocytes, and (2) absence of growth-stimulating cues that were present during fetal development.
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About 24 to 48 hours after injury to a process of a normal peripheral neuron (Figure 12.29a), the Nissl bodies break up into fine granular masses. This alteration is called
Chromatolysis
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Wallerian degeneration
Degeneration of the distal portion of the axon and myelin sheath
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regeneration tube
The tube guides growth of a new axon from the proximal area across the injured area into the distal area previously occupied by the original axon. However, new axons cannot grow if the gap at the site of injury is too large or if the gap becomes filled with collagen fibers.
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Myelinated axons in the peripheral nervous system may be repaired if
the cell body remains intact and if Schwann cells remain active.
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Multiple sclerosis (MS)
is a disease that causes a progressive destruction of myelin sheaths surrounding neurons in the CNS.
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Epilepsy (ep′-i-LEP-sē) is characterized by ,
short, recurrent attacks of motor, sensory, or psychological malfunction
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selective serotonin reuptake inhibitors (SSRIs)
drugs that provide relief from some forms of depression.
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A person with dysthymia (dis-THĪ-mē-a) experiences
episodes of depression that alternate with periods of feeling normal.
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Guillain-Barré syndrome (GBS) (GHE-an ba-RĀ)
An acute demyelinating disorder in which macrophages strip myelin from axons in the PNS. It is the most common cause of acute paralysis in North America and Europe and may result from the immune system’s response to a bacterial infection. Most patients recover completely or partially, but about 15% remain paralyzed.
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Neuroblastoma (noor-ō-blas-TŌ-ma)
A malignant tumor that consists of immature nerve cells (neuroblasts); occurs most commonly in the abdomen and most frequently in the suprarenal glands. Although rare, it is the most common tumor in infants.
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Neuropathy (noo-ROP-a-thē; neuro- = a nerve; -pathy = disease)
Any disorder that affects the nervous system but particularly a disorder of a cranial or spinal nerve. An example is facial neuropathy (Bell’s palsy), a disorder of the facial (VII) nerve.
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Rabies (RĀ-bēz; rabi- = mad, raving)
A fatal disease caused by a virus that reaches the CNS via fast axonal transport. It is usually transmitted by the bite of an infected dog or other meat-eating animal. The symptoms are excitement, aggressiveness, and madness, followed by paralysis and death
