| 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 |
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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. |
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0 |
| Shepherd
and Koch; Synaptic Circuits |
28 |
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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. |
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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. |
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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 |
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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 |
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Retina |
|
35 |
| Llinás; Cerebellum |
255 |
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Cerebellum |
|
50 |
| Llinás; Cerebellum |
255 |
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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 |
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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 |
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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 |
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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 |
|
|
|
|
|
|
|
|
|
|
|