| Shepherd;
Synaptic Organization of the Brain |
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| Chapter |
Page |
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Topic |
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| Shepherd
and Koch; Synaptic Circuits |
1 |
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Synaptic Circuits |
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| Shepherd
and Koch; Synaptic Circuits |
25 |
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Pyramidal neurons are the principal neuron in all three basic types of cortex: olfactory, hippocampal,
and neocortex. |
|
24 |
| Shepherd
and Koch; Synaptic Circuits |
25 |
|
Pyramidal neurons feature a long
axon that gives off collaterals that make synapses on
targets within the neighborhood of the cell and at different distances from the cell. |
|
0 |
| Shepherd
and Koch; Synaptic Circuits |
28 |
|
Reptilian dorsal general cortex has been regarded as a model for the evolutionary
precursor of mammalian
neocortex. |
|
3 |
| Shepherd
and Koch; Synaptic Circuits |
30 |
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Rhythmic activity can be generated by two main mechanisms -- intrinsic membrane properties and synaptic circuits. |
|
2 |
| Shepherd
and Koch; Synaptic Circuits |
30 |
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Intrinsic membrane properties were first found in pacemaker neurons in central pattern generator circuits controlling breathing,
walking, and other highly stereotyped behaviors in invertebrates. |
|
0 |
| Shepherd
and Koch; Synaptic Circuits |
30 |
|
Many types of neurons in the vertebrate central nervous
system possess complex and highly nonlinear ionic conductances that allowed these cells to respond to
inputs by oscillating at
various frequencies. |
|
0 |
| Shepherd
and Koch; Synaptic Circuits |
30 |
|
High frequency oscillations
(40-60 Hz) of a large proportion
of the neuronal population in a given area appear to
be a common occurrence in awake, behaving animals. |
|
0 |
| Shepherd
and Koch; Synaptic Circuits |
32 |
|
The two common
fast-acting neurotransmitters -- excitatory (ACh) and inhibitory (GABA). |
|
2 |
| McCormick;
Neurotransmitter Actions |
37 |
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Membrane Properties and Neurotransmitter Actions |
|
5 |
| McCormick;
Neurotransmitter Actions |
38 |
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Protein macromolecules in the membrane |
|
1 |
| McCormick;
Neurotransmitter Actions |
38 |
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Because ions are unequally distributed across the
membrane, they tend to diffuse
down their concentration gradient through ionic channels. |
|
0 |
| McCormick;
Neurotransmitter Actions |
50 |
|
Ionic channels that conduct Ca2+ are present in all neurons. |
|
12 |
| McCormick;
Neurotransmitter Actions |
51 |
|
Experiments by Rodolfo Llinás and colleagues |
|
1 |
| McCormick;
Neurotransmitter Actions |
57 |
|
Gap junctions
are actual physical
connections between neighboring neurons made by large macromolecules that extend through the membranes of both
cells and contain water-filled pores. |
|
6 |
| McCormick;
Neurotransmitter Actions |
57 |
|
Gap junctions
allow for the direct exchange of ions and other small molecules between cells. |
|
0 |
| McCormick;
Neurotransmitter Actions |
58 |
|
Ephaptic interactions refer to interactions between neurons based largely upon their close physical proximity. The flow of ions into and out of one neuron
will set up local electrical currents that can partially pass through
neighboring neurons. |
|
1 |
| McCormick;
Neurotransmitter Actions |
58 |
|
In regions that possess closely spaced neuronal elements, such
as the close packing of cell bodies in hippocampus and cerebellum
or the bundling of dendrites in the cerebral cortex, there is a possibility of significant ephaptic interaction. |
|
0 |
| McCormick;
Neurotransmitter Actions |
58 |
|
Release of transmitter is triggered
by the entry of Ca2+ into the presynaptic
terminal. |
|
0 |
| McCormick;
Neurotransmitter Actions |
59 |
|
Ephaptic interactions (diagram) |
|
1 |
| McCormick;
Neurotransmitter Actions |
59 |
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Binding of
acetylcholine (Ach) to the
nicotinic postsynaptic receptor induces a conformational change in the ionic channel, thereby opening the gate and allowing ions to flow through the
pore. |
|
0 |
| McCormick;
Neurotransmitter Actions |
60 |
|
Once a neurotransmitter is released, the length of time that it is present in the synaptic
cleft is controlled by
either: (1) hydrolysis of the transmitter, (2) reuptake into the presynaptic
terminal, (3) uptake into neighboring cells, or by (4) fusion out of the
cleft. |
|
1 |
| McCormick;
Neurotransmitter Actions |
60 |
|
Neuroactive substances in the
nervous system have been classified as either 'neurotransmitters' or 'neuromodulators' according to the duration and functional implications of
their actions. |
|
0 |
| McCormick;
Neurotransmitter Actions |
61 |
|
Substances released in neurotransmitter roles cause
postsynaptic responses that are both quick in onset
(<1 msec) and relatively short in duration (tens of msec). |
|
1 |
| McCormick;
Neurotransmitter Actions |
61 |
|
Substance released in a neuromodulatory role are characterized
by prolonged duration and the ability to modulate the response of the neuron to other inputs. |
|
0 |
| McCormick;
Neurotransmitter Actions |
61 |
|
Most neurons in
the brain are under the
influence of as many as a dozen
or more neuroactive active substances. |
|
0 |
| Shepherd;
Synaptic Organization of the Brain |
77 |
|
Spinal Cord: Ventral Horn |
|
16 |
| Shepherd;
Synaptic Organization of the Brain |
121 |
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Cochlear Nucleus |
|
44 |
| Shepherd
and Greer; Olfactory Bulb |
159 |
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Olfactory Bulb |
|
38 |
| Shepherd
and Greer; Olfactory Bulb |
170 |
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Intrinsic neurons of all later
than projection neurons. |
|
11 |
| Shepherd;
Synaptic Organization of the Brain |
205 |
|
Retina |
|
35 |
| Llinás; Cerebellum |
255 |
|
Cerebellum |
|
50 |
| Llinás; Cerebellum |
255 |
|
Cerebella cortex receives two
types of afferents -- climbing
fibers and mossy fibers, and generates a single output
system, the axons of
Purkinje cells. |
|
0 |
| Llinás; Cerebellum |
255 |
|
Cerebellum as
a whole is connected to the rest of the central
nervous system by three
large fiber bundles, the cerebellar
peduncles. |
|
0 |
| Llinás; Cerebellum |
256 |
|
Integrative properties of dendritic
trees. |
|
1 |
| Llinás; Cerebellum |
256 |
|
Intrinsic excitability of the Purkinje cell and of the cerebellar nuclear cell membrane; the crystal-like organization of the synaptic connectivity of the cerebellar cortex. |
|
0 |
| Sherman
and Koch; Thalamus |
289 |
|
Thalamus |
|
33 |
| Sherman
and Koch; Thalamus |
289 |
|
Thalamus is
most highly developed in mammals and especially so in primates. |
|
0 |
| Sherman
and Koch; Thalamus |
290 |
|
Schematically three-dimensional
view of right thalamus with many of its major nuclei. (Diagram) |
|
1 |
| Sherman
and Koch; Thalamus |
295 |
|
Major input to thalamus originates among layer 6 pyramidal cells of the cortex. There seems to be at least an order of magnitude more corticothalamic axons than thalamocortical ones. |
|
5 |
| Sherman
and Koch; Thalamus |
295 |
|
Each cortical axon innervates many
thalamic neurons. |
|
0 |
| Sherman
and Koch; Thalamus |
295 |
|
Strong reciprocity exists in thalamocortical
connections. |
|
0 |
| Sherman
and Koch; Thalamus |
296 |
|
Corticothalamic pathway faithfully adheres to the map established in the thalamic nucleus. |
|
1 |
| Sherman
and Koch; Thalamus |
299 |
|
Roughly 25% of
the cells in most thalamic
nuclei are local interneurons. |
|
3 |
| Sherman
and Koch; Thalamus |
319 |
|
Rich array of membrane
properties of thalamic relay cells plus their complex ensemble of inputs from various
sources suggest that relay of peripheral information
to cortex is not a simple,
trivial affair. |
|
20 |
| Sherman
and Koch; Thalamus |
319 |
|
Gating and other transformations and thalamic relay. |
|
0 |
| Sherman
and Koch; Thalamus |
319 |
|
There seem to be three different response modes in thalamic neurons -- rhythmic bursting, arrhythmic bursting, and tonic
firing. |
|
0 |
| Sherman
and Koch; Thalamus |
319 |
|
Rhythmic bursting of thalamic neurons occurs during quiet or non-REM sleep, perhaps during drowsiness, and might also occur during epileptic
episodes. |
|
0 |
| Sherman
and Koch; Thalamus |
319 |
|
Arrhythmic bursting and tonic firing of thalamic
neurons occur during waketime
activity, meaning that both burst and tonic modes
can be effective relay nodes. |
|
0 |
| Sherman
and Koch; Thalamus |
321 |
|
Perhaps burst
mode is used during visual
search or maybe during periods when attention is directed elsewhere as a
sort a wake-up call for novel and potentially interesting or dangerous stimuli. |
|
2 |
| Sherman
and Koch; Thalamus |
321 |
|
Although burst firing may
be ideal for signal
detection, the nonlinear
distortion during this relay mode means that the stimulus will not
be accurately analyzed. |
|
0 |
| Sherman
and Koch; Thalamus |
321 |
|
Tonic mode,
with its more linear relay,
would permit faithful signal analysis. |
|
0 |
| Sherman
and Koch; Thalamus |
321 |
|
Brain stem and cortical
afferents to thalamus in
terms of their ability to affect response mode. |
|
0 |
| Sherman
and Koch; Thalamus |
323 |
|
Activity in brain
stem afferents is associated with more alert behavioral states. |
|
2 |
| Sherman
and Koch; Thalamus |
324 |
|
Every thalamic nucleus appears to receive afferents from layer 6 of the relevant cortical region or
regions. |
|
1 |
| Sherman
and Koch; Thalamus |
325 |
|
Layer 5 innervation pattern suggest that certain
thalamic nuclei receive their primary afferents from the cortex rather than subcortically and they relay this afferent
cortical activity to other
cortical areas. This provides the thalamus with a much more extensive role
in cortical-cortical communication. |
|
1 |
| Sherman
and Koch; Thalamus |
325 |
|
There are at least two functionally distinct corticothalamic systems. One derives from layer 6 and serves to modulate the relay properties of its thalamic target. The other derives from layer 5 and represents the primary information to be relayed by the target thalamic cells. These latter thalamic
relay cells are also under the modulatory influence
of layer 6 input, which is ubiquitous
for all thalamic nuclei. |
|
0 |
| Sherman
and Koch; Thalamus |
325 |
|
There are two types of thalamic nuclei -- one receiving its primary afferent from subcortical sources and the other, from cortical sources. |
|
0 |
| Sherman
and Koch; Thalamus |
325 |
|
First-order of
thalamic relay nuclei received primary afferent from subcortical sources. |
|
0 |
| Sherman
and Koch; Thalamus |
325 |
|
Higher-order
thalamic relay nuclei receive primary input from
the cortex that has already
received and acted on
information from its
first-order thalamic relays. |
|
0 |
| Sherman
and Koch; Thalamus |
325 |
|
Olfactory pathway from the olfactory bulb to olfactory
paleocortex is analogous to a first-order relay without a thalamic
component. |
|
0 |
| Sherman
and Koch; Thalamus |
325 |
|
Olfactory pathway to olfactory
paleocortex may have evolved
before thalamus and neocortex. |
|
0 |
| Sherman
and Koch; Thalamus |
326 |
|
Thalamocortical interactions permit a rich avenue of communication among cortical
areas. |
|
1 |
| Sherman
and Koch; Thalamus |
326 |
|
Cortical areas
can communicate through the thalamus, with some thalamocortical inputs relaying via layer 5, to another cortical area. |
|
0 |
| Sherman
and Koch; Thalamus |
326 |
|
Other cortical areas, via their layer 6 outputs, can modify the thalamic route of corticocortical communication. |
|
0 |
| Sherman
and Koch; Thalamus |
328 |
|
Relay functions served by thalamus is not limited to transferring peripheral information to cortex;
certain thalamic nuclei
serve chiefly to relay information from cortex to cortex; This role may prove crucial to cortico-cortical
communications. |
|
2 |
| Wilson; Basal Ganglia |
329 |
|
Basal Ganglia |
|
1 |
| Wilson; Basal Ganglia |
329 |
|
Basal ganglia
have no direct connections
with either the sensory or motor organs. |
|
0 |
| Wilson; Basal Ganglia |
329 |
|
Basal ganglia
diseases produce severe
deficits of movement. Parkinson's disease, movements are more difficult to make. Huntington's
disease, useless and unintended movements interfere
with intended ones. |
|
0 |
| Wilson; Basal Ganglia |
329 |
|
Only voluntary, purposive movements affected, reflexive movements relatively unaffected. |
|
0 |
| Wilson; Basal Ganglia |
329 |
|
Anatomical connections of the basal
ganglia link it to elements of the sensory, motor, cognitive, and motivational apparatus of the brain. |
|
0 |
| Wilson; Basal Ganglia |
329 |
|
Major structures of the basal ganglia are the caudate nucleus, putamen,
globus substantia nigra, and subthalamic nucleus. |
|
0 |
| Wilson; Basal Ganglia |
329 |
|
Two largest sources of input to the basal ganglia, the cerebral cortex and the thalamus. |
|
0 |
| Wilson; Basal Ganglia |
329 |
|
Most of the input to the basal ganglia from other brain structures arrives in the neostriatum, which consists of the caudate nucleus, putamen, and nucleus accumbens. |
|
0 |
| Wilson;
Basal Ganglia |
330 |
|
Within the caudate
nucleus and the putamen, inputs from sensory, motor; and
association cortical areas converge with inputs from the thalamic
intralaminar nuclei, dopaminergic
inputs from the substantia nigra pars compacta, and serotoninergic inputs from the dorsal raphe nucleus. Connections
arising from the limbic cortex and hippocampus are formed in a third structure, the nucleus
accumbens. |
|
1 |
| Wilson; Basal Ganglia |
331 |
|
Three input structures of the basal ganglia (caudate nucleus, putamen, nucleus accumbens.) |
|
1 |
| Wilson;
Basal Ganglia |
332 |
|
Input fibers to the neostriatum arise primarily from the cerebral cortex, the intralaminar
nuclei of the thalamus, the dopaminergic neurons of the substantia nigra
(pars compacta), the serotoninergic neurons of the dorsal raphe nucleus, and
the basolateral nucleus of the amygdala. Less numerous inputs also arise from
the external segment of the globus pallidus and from the substantia nigra,
pars reticulata. |
|
1 |
| Wilson; Basal Ganglia |
345 |
|
The neostriatum is unusual in that the principal neurons, as well as many of
the interneurons, are inhibitory. The principal neuron of the
globus pallidus and substantia nigra is also inhibitory. Excitatory influences in the basal ganglia arise mostly from incoming fibers.
|
|
13 |
| Wilson; Basal Ganglia |
345 |
|
Neostriatal spiny cells fire very rarely and in episodes that last for only about 0.1 to
3 sec. |
|
0 |
| Wilson; Basal Ganglia |
346 |
|
Cells in the globus pallidus and
substantia nigra fire tonically at very high
rates. Their tonic firing produces a constant inhibition of neurons in the thalamus and superior
colliculus. |
|
1 |
| Wilson; Basal Ganglia |
347 |
|
Firing of spiny neostriatal neurons can cause a transient pause in tonic
inhibition, releasing thalamic and superior colliculus neurons to respond to excitatory inputs that would otherwise be subthreshold. |
|
1 |
| Wilson; Basal Ganglia |
347 |
|
The neostriatum acts to disinhibit neurons in the thalamus and superior colliculus; interneurons of the neostriatum help regulate the duration, strength, and spatial pattern of the
disinhibition. |
|
0 |
| Wilson; Basal Ganglia |
373 |
|
Cells in globus
pallidus and in substantia
nigra pars reticulata have very
high rates of tonic
activity. Fire rhythmically at a rate determined by membrane characteristics. |
|
26 |
| Wilson; Basal Ganglia |
373 |
|
Inhibition
exerted by (usually silent) neostriatal spiny
neurons during their brief
episodes of friring causes
a momentary decrease in the rate of the tonic firing. |
|
0 |
| Wilson; Basal Ganglia |
373 |
|
Fast-firing cells of the globus
pallidus and substantia
nigra are GABAergic neurons, which exert a tonic inhibition on the target cells of the thalamus and superior colliculus. |
|
0 |
| Wilson; Basal Ganglia |
373 |
|
Overall, excitation of neostriatal spiny neurons leads to a disinhibition of the otherwise suppressed activity of neurons in
the thalamus and superior colliculus. |
|
0 |
| Haberly; Olfactory Cortex |
377 |
|
Olfactory Cortex |
|
4 |
| Haberly; Olfactory Cortex |
380 |
|
In the neocortex, subtle differences in cytoarchitecture and connections reflect functional specialization. |
|
3 |
| Johnston;
Hippocampus |
417 |
|
Hippocampus |
|
37 |
| Johnston;
Hippocampus |
417 |
|
Hippocampus is
one of the most thoroughly
studied areas of the
mammalian central nervous system. |
|
0 |
| Johnston;
Hippocampus |
417 |
|
Hippocampal formation includes the dentate gyrus, the hippocampus, the subiculum,
presubiculum and parasubiculum, and the entorhinal cortex. |
|
0 |
| Johnston;
Hippocampus |
417 |
|
The dentate
gyrus, hippocampus, and subiculum have a single cell layer with less
cellular or acellular layers located above and below it. The other parts of the hippocampal
formation have several
cellular layers. |
|
0 |
| Johnston;
Hippocampus |
417 |
|
A patient known by his initials, H.M., underwent bilateral hippocampal removal for the
treatment of intractable epilepsy. He was left with a permanent loss of the ability to encode new information
into long-term memory. |
|
0 |
| Johnston;
Hippocampus |
417 |
|
In addition to memory studies, the hippocampus is also of interest because
of its high seizure susceptibility. It has the lowest seizure threshold of any brain region. Most patients with epilepsy have seizures that involve the hippocampus, and these seizures are often the most
difficult to control
medically. |
|
0 |
| Johnston;
Hippocampus |
418 |
|
Portions of the hippocampal formation, particularly the entorhinal cortex, are prime targets for
the pathology associated with Alzheimer's disease, and the hippocampus is very
vulnerable to the effects
of ischemia and anoxia. |
|
1 |
| Johnston;
Hippocampus |
420 |
|
The principal neurons in the dentate gyrus are the granule cells, and in the hippocampus they are the pyramidal neurons. |
|
2 |
| Johnston;
Hippocampus |
420 |
|
The pyramidal
cell layer of the hippocampus has been divided into three regions designated CA1, CA2,
CA3 based on the size and
appearance of the neurons. |
|
0 |
| Johnston;
Hippocampus |
421 |
|
Vast majority of interneurons in the dentate gyrus and hippocampus have locally restricted target regions, lack spines, and are GABAergic. |
|
1 |
| Johnston;
Hippocampus |
423 |
|
Hippocampal interneurons with cell bodies in or near the pyramidal cell layer can be
classified into three groups on the basis of their synaptic targets -- axo-axonic cells, basket cells, and bistratified cells. |
|
2 |
| Johnston;
Hippocampus |
423 |
|
Axo-axonic cells synapse onto the initial segments of pyramidal neurons and thus exert a strong control over action
potential initiation. |
|
0 |
| Johnston;
Hippocampus |
423 |
|
Basket cells
synapse onto the somata of pyramidal neurons. Each basket cell can make multiple contacts onto a pyramidal
neuron, forming what looks
like a "basket"
into which the soma sits. |
|
0 |
| Johnston;
Hippocampus |
423 |
|
Bistratified cells make synaptic contacts onto apical and
basal dendrites of pyramidal neurons. |
|
0 |
| Johnston;
Hippocampus |
423 |
|
Mutual inhibitory connections among these interneurons. |
|
0 |
| Johnston;
Hippocampus |
423 |
|
Mutual inhibitory connections synchronize interneurons, producing oscillations at various frequencies, including theta (5 Hz) and gamma (40
Hz) frequencies. |
|
0 |
| Johnston;
Hippocampus |
423 |
|
Many GABAergic
interneurons also contain and release neuroactive peptides. |
|
0 |
| Johnston;
Hippocampus |
423 |
|
Interneurons
whose post-synaptic targets are exclusively other interneurons. |
|
0 |
| Johnston;
Hippocampus |
423 |
|
Basic circuitry of the hippocampal formation has been known since the time of Ramon y Cajal (1911). |
|
0 |
| Johnston;
Hippocampus |
424 |
|
Unidirectional progression of excitatory pathways that link each region of the hippocampal formation -- the trisynaptic circuit. |
|
1 |
| Johnston;
Hippocampus |
424 |
|
Entorhinal cortex is considered the starting point of the trisynaptic circuit since much of the sensory information that reaches the hippocampus enters through
the entorhinal cortex. |
|
0 |
| Johnston;
Hippocampus |
424 |
|
Neurons located in layer II of
the entorhinal cortex give rise to
a pathway, the perforant path, that projects through (perforates) the
subiculum and terminates both in the dentate gyrus and in the CA3 field of the hippocampus. |
|
0 |
| Johnston;
Hippocampus |
424 |
|
Dentate gyrus
is the next step in the progression of connections, and it gives rise to the mossy fibers that
terminate on the proximal dendrites of the CA3 pyramidal cells. |
|
0 |
| Johnston;
Hippocampus |
425 |
|
The CA3 pyramidal cells project heavily to other levels of CA3 as well as to CA1. The projection to CA1 is typically called the Schaffer collateral projection. |
|
1 |
| Johnston;
Hippocampus |
425 |
|
Information entering the entorhinal cortex from a particular cortical area can traverse
the entire hippocampal circuit through the
excitatory pathways and ultimately be returned to
the cortical area from which it originated. The transformations that take place through
this traversal are presumably essential for enabling the information to be stored as long-term memories. |
|
0 |
| Johnston;
Hippocampus |
427 |
|
The major
input to the dentate gyrus is from the entorhinal
cortex. |
|
2 |
| Johnston;
Hippocampus |
428 |
|
Subcortical inputs to the dentate gyrus originate mainly from the septal nuclei, supramamillary region
of the posterior hypothalamus, and several monoaminergic
nuclei in the brainstem, especially the locus coeruleus and raphe
nuclei. |
|
1 |
| Johnston;
Hippocampus |
428 |
|
Dentate gyrus
receives a particularly prominent noradrenergic
input, primarily from the locus
coeruleus. |
|
0 |
| Johnston;
Hippocampus |
428 |
|
Serotonergic
projection originates from the raphe nuclei. |
|
0 |
| Johnston;
Hippocampus |
430 |
|
Many of the cells in the raphe nuclei that
project to the hippocampal formation appear to be nonserotonergic. |
|
2 |
| Johnston;
Hippocampus |
430 |
|
Dentate gyrus
receives a lighter and diffusely distributed dopaminergic
projection that arises
mainly from cells located in the ventral tegmental
area. |
|
0 |
| Johnston;
Hippocampus |
430 |
|
Dentate gyrus
does not project to other brain regions. Within the hippocampal formation, it
only projects to CA3 via the mossy fibers. |
|
0 |
| Johnston;
Hippocampus |
431 |
|
CA3 pyramidal cells give rise to highly collateralized
axons that distribute fibers both within the hippocampus (to CA3, CA2, CA1), to the same fields in
the contralateral hippocampus (the commissural projections), and subcortically to the lateral septal nucleus. |
|
1 |
| Johnston;
Hippocampus |
431 |
|
CA3 cells,
especially those located proximally in the field, and CA2 cells contribute a small number of
collaterals that innervate the polymorphic layer of the dentate gyrus. |
|
0 |
| Johnston;
Hippocampus |
431 |
|
All of the CA3 and CA2 pyramidal cells give rise to highly divergent projections to all portions of the hippocampus. The projections to CA3 and CA2 are
typically called the associational connections, and the CA3 projections to the CA1 field are called the Schaffer collaterals. |
|
0 |
| Johnston;
Hippocampus |
431 |
|
Highly ordered
and spatially distributed
pattern of projections from CA3 to CA3 and from CA3 to CA1. |
|
0 |
| Johnston;
Hippocampus |
432 |
|
Each CA3
neuron makes contacts with many CA1 pyramidal cells. It has been
estimated that a single CA1
neuron may be innervated by
more than 5,000
ipsilateral CA3 pyramidal cells. |
|
1 |
| Johnston;
Hippocampus |
432 |
|
Projections
from CA3 to CA1 terminate as asymmetric, axospinous synapses located
on the apical and basal dendrites of the CA1 pyramidal cells. The sizes and shapes of the spines and presynaptic profiles in
this region are quite variable and may be related to the physiological
efficacy of the synapses in CA1. |
|
0 |
| Johnston;
Hippocampus |
432 |
|
CA3-to-CA3 associational and CA3-to-CA1 Schaffer collateral projections are both divergently distributed along the septotemporal axis. |
|
0 |
| Johnston;
Hippocampus |
432 |
|
Single CA3 and CA2 pyramidal
cells give rise to highly
arborized axonal plexuses that distribute to as much as 75% of the septotemporal extent of the ipsilateral
and contralateral CA1 fields. |
|
0 |
| Johnston;
Hippocampus |
432 |
|
Total length
of the axonal plexus from single CA3 neurons can be as long as 150-300 mm, and a single CA3 cell may contact as many as 30,000 to 60,000 neurons in the ipsilateral hippocampus. |
|
0 |
| Johnston;
Hippocampus |
432 |
|
The only
sizable subcortical projection from CA3 is to
the lateral septal nucleus. |
|
0 |
| Johnston;
Hippocampus |
432 |
|
Some CA3
fibers cross in the ventral hippocampal commissure to
innervate the homologous region of the contralateral lateral septal nucleus. |
|
0 |
| Johnston;
Hippocampus |
432 |
|
Essentially all of the CA3 cells give rise to projections both to CA1
and to the lateral septal nucleus. |
|
0 |
| Johnston;
Hippocampus |
433 |
|
CA3 field
receives inputs from the noradrenergic nucleus locus coeruleus. |
|
1 |
| Johnston;
Hippocampus |
435 |
|
Highly structured and organized
arrangement of synaptic pathways makes the hippocampus ideal for studying synaptic actions in vivo or in
vitro. Single-shock electrical stimulations to the perforant path, mossy fibers, or Schaffer collaterals
result in a characteristic sequence of excitation followed by inhibition in the appropriate target neurons. |
|
2 |
| Johnston;
Hippocampus |
438 |
|
Electrophysiological behavior of the different neurons in the hippocampus is variable. |
|
3 |
| Johnston;
Hippocampus |
438 |
|
Dentate granule and CA1 pyramidal neurons can fire repetitively at up to several hundred Hz. |
|
0 |
| Johnston;
Hippocampus |
438 |
|
CA3 pyramidal neurons tend to fire in short bursts of 5-10 action potentials. |
|
0 |
| Johnston;
Hippocampus |
438 |
|
Bursting properties of CA3 hippocampal neurons are thought to be important for explaining the seizure susceptibility of the hippocampus. |
|
0 |
| Johnston;
Hippocampus |
438 |
|
A prominent feature of hippocampal neurons firing repetitively is that the frequency of action potentials declines or
accomodates during the train, and there is a slow afterhyperpolarization
(AHP) at the end of the
train. |
|
0 |
| Johnston;
Hippocampus |
438 |
|
Both the frequency
accomodation and the slow
AHP result in part from the activation of potassium
channels by the influx of calcium ions during the train. |
|
0 |
| Johnston;
Hippocampus |
438 |
|
Presynaptic mechanisms, including the quantal hypothesis for transmitter release and the role of presynaptic calcium, have been studied directly at both mossy
fiber and Schaffer
collateral synapses. |
|
0 |
| Johnston;
Hippocampus |
439 |
|
Because excitatory synapses and some
inhibitory synapses terminate on dendrites, the physiology and biophysics of synaptic transmission is complicated by
the properties of dendrites. |
|
1 |
| Johnston;
Hippocampus |
439 |
|
Basic sequence of synaptic transmission begins with an action potential in the presynaptic axon that elicits Ca2+ influx into the bouton,
and through a number of poorly understood steps, neurotransmitter is released into the cleft from transmitter-containing vesicles in the presynaptic terminal. |
|
0 |
| Johnston;
Hippocampus |
439 |
|
Transmitter molecules diffuse across the synaptic cleft and bind to specific receptors on the postsynaptic neuron, opening ion channels. |
|
0 |
| Johnston;
Hippocampus |
439 |
|
Single boutons may have as few as 1 active zone in some Schaffer collateral boutons to as many as 37 active zones on some of the largest mossy fiber
terminals. |
|
0 |
| Johnston;
Hippocampus |
439 |
|
A prominent
theory is that one vesicle per action potential is
released at each active zone with a mean probability of about 1 release every fourth action potential. |
|
0 |
| Johnston;
Hippocampus |
439 |
|
Vesicles can sometimes release their transmitter spontaneously in the absence
of a presynaptic action potential. |
|
0 |
| Johnston;
Hippocampus |
439 |
|
Inhibitory neurons can fire
repetitively at rates much higher than is typical for excitatory neurons. |
|
0 |
| Johnston;
Hippocampus |
439 |
|
Excitatory input to inhibitory interneurons may trigger a high-frequency train of action potentials in the
interneurons, leading to longer-lasting
transmitter release and a
longer-lasting inhibition of the post-synaptic neuron than from the excitatory response. |
|
0 |
| Johnston;
Hippocampus |
439 |
|
Major excitatory neurotransmitter in the hippocampus is glutamate. |
|
0 |
| Johnston;
Hippocampus |
440 |
|
Major inhibitory neurotransmitter in the hippocampus is GABA . |
|
1 |
| Johnston;
Hippocampus |
442 |
|
It was once believed that
inhibitory synapses were primarily on the cell bodies of pyramidal neurons.
There is now much evidence that GABAergic synapses occur both on the cell bodies as well as throughout the dendritic tree. |
|
2 |
| Johnston;
Hippocampus |
442 |
|
Neuromodulatory
neurotransmitters acting pre-
and/or postsynaptically that under some conditions can be considered inhibitory.
These include norepinephrine, serotonin, dopamine, and neuroactive peptides. |
|
0 |
| Johnston;
Hippocampus |
442 |
|
Perforant Pathway can be separated into two groups of
fibers, the lateral and medial perforant paths. Opioid peptides influence the induction of longterm
potentiation in the lateral perforant path. |
|
0 |
| Johnston;
Hippocampus |
442 |
|
Hilar Pathways -- physiology of
the hilar region is poorly understood, compared with the rest of the
hippocampus. |
|
0 |
| Johnston;
Hippocampus |
442 |
|
Mossy Fibers. Mossy fiber boutons are among the largest synapses of the mammalian central nervous system, surpassed only
by certain synapses in the cochlear nucleus. At each bouton there are multiple active zones (up to 37) resulting in multiple release sites for neurotransmitter. The boutons
contain large amounts of Zn2+ and opioid peptides that are co-released with the main neurotransmitter glutamate. |
|
0 |
| Johnston;
Hippocampus |
442 |
|
Mossy fibers terminate on the proximal dendrites of CA3 pyramidal cells. |
|
0 |
| Johnston;
Hippocampus |
443 |
|
Recurrent Pathways. One of the hallmarks of the CA3 region is the prominent, recurrent excitatory
connections among the pyramidal
neurons. This recurrent
pathway is glutamatergic, and excitatory, and represents a form of positive feedback that makes the CA3 region inherently unstable. In combination with the intrinsic
bursting properties of CA3 neurons; subtle increases in the ratio of excitation/inhibition in this
region can result in epileptiform activity, which is characterized by spontaneous
and synchronous, rhythmic firing among large numbers of neurons. |
|
1 |
| Johnston;
Hippocampus |
444 |
|
Schaffer Collaterals are probably the best-studied synaptic
pathway in the hippocampus. Each Schaffer collateral axon synapses onto thousands
of CA1 pyramidal neurons, but usually with only one or two synaptic contacts per neuron. |
|
1 |
| Johnston;
Hippocampus |
444 |
|
Axons of the CA1
pyramidal neurons also form a recurrent excitatory pathway that synapses back onto other CA1 neurons,
although it is much sparser and weaker than that in CA3. |
|
0 |
| Johnston;
Hippocampus |
444 |
|
Synaptic Plasticity - most of the excitatory synapses in the hippocampus exhibit various forms of use- or activity-dependent synaptic plasticity. |
|
0 |
| Johnston;
Hippocampus |
444 |
|
Short-term plasticities are facilitation, post-tetanic
potentiation, and depression. They range in duration from hundreds
of milliseconds to several
minutes. |
|
0 |
| Johnston;
Hippocampus |
445 |
|
Long-term Plasticities -- There are several forms of synaptic
plasticities at glutamatergic, excitatory synapses
in hippocampus that have
durations of from 30 min to hours, days, or weeks. They are called short-term
potentiation and depression (STP and STD) and long-term
potentiation and depression (LTP and LTD). |
|
1 |
| Johnston;
Hippocampus |
445 |
|
LTP was first
described by Bliss and colleagues (Bliss and Lomo, 1973; Bliss and Gardner-Medwin, 1973) and is probably the most intensely studied of all the synaptic plasticities because of
its presumed role in learning and memory. |
|
0 |
| Johnston;
Hippocampus |
445 |
|
LTP is
typically induced by giving
one or more high-frequency (25-200 Hz) stimulus
trains to a synaptic
pathway, such as the perforant
path, mossy fibers, or Schaffer
collaterals. This period of high-frequency stimulation trains is
called the induction phase. |
|
0 |
| Johnston;
Hippocampus |
445 |
|
The induction
phase of LTP is followed by an expression phase, during which a stimulus is amplified some 50-100%.
Characteristically, an induction
phase of only a few seconds
to 1 min is much shorter
than the subsequent expression phase, which may last
up to several days. |
|
0 |
| Johnston;
Hippocampus |
445 |
|
Maximum duration of expression
phase is difficult to ascertain, but LTP in hippocampus
is unlikely to be permanent. |
|
0 |
| Johnston;
Hippocampus |
445 |
|
LTP has associative properties; synapses may
exhibit LTP only when they are active at the same time as other synapses. These and other properties make LTP a
candidate mnemonic device. |
|
0 |
| Johnston;
Hippocampus |
445 |
|
LTD represents
a long-term depression of a
synaptic response, induced by the low-frequency
stimulation (1-5 Hz) of a
synaptic pathway. LTD can last from 30 min to an
hour or more. |
|
0 |
| Johnston;
Hippocampus |
445 |
|
Most theories for learning involve strengthening of specific synaptic pathways at the expense of others, and the existence of an LTD-like phenomenon has long been
theorized. |
|
0 |
| Johnston;
Hippocampus |
445 |
|
At many synapses LTP and LTD are dependent on the activation of NMDA
receptors. A requirement for the induction of LTP is that there must be
a sufficient increase in
the intracellular Ca2+ concentration near the stimulated
synapses. This occurs by the influx of Ca2+ ions through NMDA receptors and/or voltage-gated Ca2+ channels. |
|
0 |
| Johnston;
Hippocampus |
445 |
|
At mossy fiber
synapses and perhaps at
lateral perforant-path synapses, LTP is facilitated by the release of opioid peptides. |
|
0 |
| Johnston;
Hippocampus |
446 |
|
STP and STD occur when the stimulation during induction is of insufficient intensity or duration to
induce LTP and LTD. STP and STD have lower induction
thresholds than their longer-term counterparts. |
|
1 |
| Johnston;
Hippocampus |
446 |
|
It is not clear whether STP and STD are just shorter-term
versions of LTP and LTD or
if they have separate mechanisms. Nonetheless, they share characteristics,
such as a dependence on a rise in intracellular Ca2+ concentration in the postsynaptic neuron, and at some synapses, a requirement for NMDA receptor activation.
|
|
0 |
| Douglas; Neocortex |
459 |
|
Neocortex |
|
13 |
| Douglas;
Neocortex |
459 |
|
Human brain is
three times as large as might be expected for a primate of equivalent weight.
Human brain is not simply a scaled-up version of closest primate relative,
the Bonobo chimpanzee. Greatest expansion is in the cortical structures, particularly the cerebellum and neocortex. |
|
0 |
| Douglas; Neocortex |
459 |
|
Within the neocortex, the
expansion is uneven. In comparison with nonhuman primates of equivalent body
weight, the human association and premotor areas
have expanded relative to
the sensory areas. |
|
0 |
| Douglas;
Neocortex |
459 |
|
In all mammals the neocortex
consists of a sheet of cells, about 2 mm thick. Conventionally, it is divided into 6 layers, but in many
regions more than 6 laminae are in evidence. Each
cubic millimeter contains approximately 50,000 neurons. |
|
0 |
| Douglas;
Neocortex |
459 |
|
Three major cytoarchitectural
divisions of the neocortex. (1) granular cortex of the sensory areas, contains small, densely packed neurons in the middle layers;
(2) agranular cortex of the motor and
premotor cortical areas, (3) varying populations of granule cells, mostly 'association cortex'. Within each of
these areas there are many subdivisions, both functional and anatomical.
|
|
0 |
| Douglas;
Neocortex |
461 |
|
All three
cytoarchitectural divisions of the neocortex contain the same two basic types of neurons: those whose
dendrites bear spines (the stellate
and pyaramidal cells) and
those whose dendrites are smooth (smooth
cells). |
|
2 |
| Douglas; Neocortex |
461 |
|
Pyramidal cells form about 70% of the neurons. Smooth cells form about 20% of the neurons. |
|
0 |
| Douglas; Neocortex |
462 |
|
Spiny cells are excitatory,
whereas smooth neurons are inhibitory. |
|
1 |
| Douglas; Neocortex |
464 |
|
Spiny neurons
are called this because their dendrites bear small processes called spines, which are usually club shaped, with a head
of about 1µm diameter and a shaft,
or neck, of about 0.1 µm
diameter. |
|
2 |
| Douglas; Neocortex |
464 |
|
Major subtype
of spiny neurons are the pyramidal cells, which constitute about two-thirds of the neurons in the neocortex. |
|
0 |
| Douglas; Neocortex |
464 |
|
Pyramidal neurons are found in all cortical layers except
layer 1. |
|
0 |
| Douglas; Neocortex |
464 |
|
The most prominent feature of pyramidal neurons is an apical dendrite that may extend through all the layers of the cortex above
the soma. |
|
0 |
| Douglas; Neocortex |
464 |
|
Pyramidal cells are the major output neurons of the neocortex. |
|
0 |
| Douglas; Neocortex |
464 |
|
Pyramidal cells participate both in connections between the different
cortical areas and to subcortical
structures such as the thalamus and superior colliculus. |
|
0 |
| Douglas; Neocortex |
464 |
|
Pyramidal cells are a major
provider of excitatory input to the area in which they are found -- each pyramidal neuron has a rich collateral network that forms part of the local cortical
circuitry. |
|
0 |
| Douglas; Neocortex |
464 |
|
Proximal shafts of the dendrites of the spiny cell types are nearly devoid of spines. |
|
0 |
| Douglas;
Neocortex |
464 |
|
Spine density varies
considerably between different
types of neurons. At one extreme is the sparsely spiny neuron, which may bear fewer than 100 spines over the entire dendritic tree. These neurons form a
small subclass of the inhibitory neuron population. At another extreme are
neurons such as the Betz cell, a large pyramidal cell that is found in the motor cortex (area 4) and bears about 10,000 spines. |
|
0 |
| Douglas;
Neocortex |
464 |
|
The most
prominent pyramidal cells
in the neocortex are the Betz cells of area
4, the motor cortex. The Betz cells are very large pyramidal cells located in layer 5. Their axons
form part of the pyramidal tract that descends to
the spinal cord. |
|
0 |
| Douglas;
Neocortex |
465 |
|
Primary visual cortex has a distinct set of exceptionally large pyramidal neurons,
called the solitary cells of Meynert. These pyramidal cells, which are found
in the deep layers (5 or 6,
depending on species), project to other cortical areas and down to the midbrain structures such as the superior colliculus and the pons. |
|
1 |
| Douglas;
Neocortex |
465 |
|
Within layer 5
in the visual cortex, two types of pyramidal cells have been distinguished: (1) a thick
apical dendrite that ascends to layer 1 where it forms a terminal tuft. These neurons have a bursting discharge response. (2) a thin apical dendrite that terminates
without branching in layer
2, and has a regular
discharge. |
|
0 |
| Douglas; Neocortex |
465 |
|
Theoretical work has suggested
that the shape of the dendritic tree is a major factor in controlling the pattern of spike
output from neurons. |
|
0 |
| Douglas;
Neocortex |
465 |
|
A second group of spiny neurons, the spiny stellate neurons, are found exclusively in layer 4 of the granular
cortex. They do not have the apical dendrite characteristic of pyramidal cells. Instead, dendrites of approximately equal length radiate from the soma, giving a star-like
appearance. |
|
0 |
| Douglas; Neocortex |
465 |
|
Occasionally,
spiny stellate neurons do project to other areas, but most have axonal projections confined to the area in which they occur. |
|
0 |
| Douglas; Neocortex |
465 |
|
Smooth neurons
tend to have elongated dendritic trees, both in the radial and the tangential dimension. |
|
0 |
| Douglas; Neocortex |
465 |
|
The most prominent smooth neuron is the cortical basket cell. |
|
0 |
| Douglas; Neocortex |
465 |
|
As with basket cells in the cerebellum and hippocampus, the axons of basket cells form nests or baskets around the somata of their targets, usually pyramidal cells. |
|
0 |
| Douglas; Neocortex |
466 |
|
Thalamus projects to all cortical areas and provides input to most layers of the cortex. The densest projections are to the middle layers, where they form about 5-10% of the
synapses in those layers. |
|
1 |
| Douglas; Neocortex |
466 |
|
The main
feature of thalamic input to the cortex
is that it is highly ordered. The sensory inputs are represented centrally in a
way that their topographic arrangement in the periphery is preserved. |
|
0 |
| Douglas;
Neocortex |
466 |
|
Thalamocortical mapping is achieved by preserving the nearest-neighbor
relationships of the arrangements of the sensory or
motor elements in the periphery. Such topographic projections are a ubiquitous feature of the cortex. |
|
0 |
| Douglas;
Neocortex |
466 |
|
Precision of topographic mapping varies between
areas. Primary sensory and motor
areas usually preserve the highest detail of topography, degrading progressively through secondary,
tertiary and higher order areas of cortex. |
|
0 |
| Douglas;
Neocortex |
466 |
|
In the visual
system of the primate, the fovea
of the retina contains
the highest density of photoreceptors, represented in the cortex by 30
mm/degree of visual field. In the far periphery, the ratio falls to about
0.01 mm/degree of visual field. |
|
0 |
| Douglas; Neocortex |
467 |
|
Although the thalamus is a major source of input to the
neocortex, it is not the only one. More than 20 different subcortical
structures projecting to the neocortex have been identified. |
|
1 |
| Douglas; Neocortex |
467 |
|
Subcortical structures projecting to the neocortex include
the claustrum, locus coeruleus, basal forebrain, the dorsal and median raphe, and the pontine reticular system. |
|
0 |
| Douglas;
Neocortex |
468 |
|
Three main types of monoamine-containing cortical
afferents have been described: (1) the dopamine-positive fibers arising from
the rostral mesencephalon,
(2) the noradrenaline-containing
axons originating from the locus coeruleus, (3) the serotonin (5-HT) fibers that originate from the mesencephalic raphe nuclei. |
|
1 |
| Douglas; Neocortex |
468 |
|
Locus coeruleus, a small nucleus in the dorsal pons, projects to most of the neocortex. Neurons
synthesize norepinephrine.
Activity in locus coeruleus
is involved with the arousal response induced by sensory stimuli. |
|
0 |
| Douglas;
Neocortex |
468 |
|
Raphe nuclei and
pontine reticular formation, are a complex of nuclei that contain the highest density of neurons
that synthesize serotonin.
These neurons project to all cortical areas with varying degrees of laminar specificity. |
|
0 |
| Douglas;
Neocortex |
468 |
|
The third monoamine projection
to cortex is the dopaminergic pathway.The dopaminergic projection to the frontal cortex originates from the ventral tegmental area, the rostral mesencephalic groups, and the nucleus linearis. All layers except
layer 4 receive dopaminergic input. Dopaminergic projections are strongest to the rostral cortical areas, especially the prefrontal cortex. |
|
0 |
| Douglas; Neocortex |
469 |
|
Major input
from any cortical area is
from other cortical areas. |
|
1 |
| Douglas; Neocortex |
469 |
|
Only 1:100 or even 1:1000 in white matter is involved in subcortical projection. |
|
0 |
| Douglas; Neocortex |
469 |
|
Most of the fibers in white matter are involved in intrahemispheric connections and interhemispheric connections. |
|
0 |
| Douglas; Neocortex |
475 |
|
Axons of cortical neurons do not extend more than
a few
millimeters laterally in an area. |
|
6 |
| Douglas; Neocortex |
475 |
|
Neurons with
similar functional properties are organized in 'columns' that
extend from the cortical surface to the white matter. |
|
0 |
| Douglas; Neocortex |
477 |
|
Output neurons from the cortex are generally pyramidal cells. |
|
2 |
| Douglas; Neocortex |
477 |
|
Cortico-cortical connections arise mainly from the superficial cortical layers, and the subcortical projections arise from the deep layers. |
|
0 |
| Douglas; Neocortex |
477 |
|
Within the deep
layers, there is an output
to regions that have a motor-related
function, e.g., the superior colliculus, basal ganglia, brainstem nuclei, and spinal cord. |
|
0 |
| Douglas; Neocortex |
477 |
|
Cortico-thalamic projection generally arises from the layer 6 pyramidal cells. |
|
0 |
| Douglas; Neocortex |
477 |
|
Basic circuit
for visual cortex (diagram) |
|
0 |
| Douglas; Neocortex |
484 |
|
Long term potentiation -- brief tetanic stimulation of a set of input fibers potentiates
synapses in hippocampal excitatory synapses for many hours. |
|
7 |
| Douglas; Neocortex |
485 |
|
Processes of LTP and LTD have
been studied in a number of different cortical areas. |
|
1 |
| Douglas; Neocortex |
485 |
|
In the neocortex the main excitatory neurotransmitter is the amino acid glutamate. |
|
0 |
| Douglas; Neocortex |
486 |
|
Activation of the NMDA receptor evokes a long-duration conductance change (tens of milliseconds), during which cations flow through the channel. |
|
1 |
| Douglas; Neocortex |
488 |
|
A number of chemical substances have inhibitory effects on cortical neurons, but the most dominant inhibitor appears to be GABA. |
|
2 |
| Douglas; Neocortex |
492 |
|
Establishment of the identity of cortical
neurotransmitters has been
one of the most tortuous activities of the last 40 years. |
|
4 |
| Douglas; Neocortex |
494 |
|
Surface area
of dendrites is one to two orders of magnitude larger
than that of the soma. |
|
2 |
| Douglas; Neocortex |
497 |
|
One of most prominent features
of cortical neurons is that dendritic spines. |
|
3 |
| Douglas; Neocortex |
497 |
|
Recent advances in imaging techniques make it possible to
measure calcium dynamics in individual spines. |
|
0 |
| Douglas; Neocortex |
497 |
|
Most of the synaptic
current injected into the spine
head reaches the trunk
dendrite via the spine neck. |
|
0 |
| Douglas; Neocortex |
497 |
|
Resistance to current flow through the neck is high, on the order of 100 MΩ or more. |
|
0 |
| Douglas; Neocortex |
497 |
|
Neck resistance could be used to control the efficacy of
the synapse and thus to provide a basis for a synaptic plasticity. |
|
0 |
| Douglas; Neocortex |
497 |
|
Twitching spine hypothesis -- Francis Crick (1982) proposed that a change in spine length could be achieved quickly,
by calcium activation of myosin and actin localized in the spine neck. |
|
0 |
| Douglas; Neocortex |
|
|
|
|
| Douglas; Neocortex |
|
|
|
|
| Douglas; Neocortex |
|
|
|
|
|
|
|
|
|
|