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Lecture 6

Lecture 6 Notes.docx

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Department
Neuroscience
Course Code
NROC69H3
Professor
Rutsuko Ito

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Lecture 6 Notes
Synaptic organization of the hippocampus
1. Reasons for studying the hippocampus and associated regions of the medial temporal lobe
a. Relative simplicity of the organization makes the hippocampus a good model for
studying the mechanisms of synaptic transmission, integration, and plasticity
i. The cells in the hippocampus have a very orderly laminar organization
ii. The different cell types and dendritic fields are highly segregated
b. Hippocampus plays a critical role in the encoding and storage of explicit memories
(declarative memories for episodes and facts)
i. Balateral damage to the medial temporal lobe results in severe retrograde
amnesia (inability to retrieve old, declarative memories)
ii. Damage to the hippocampus does not affect motor learning or habit formation
1. Suggests that the brain possesses independent memory systems
c. Hippocampus is highly vulnerable to certain neurological diseases like epilepsy and
alzheimers and damage from strokes
i. First brain stucture that is affected in CO poisoning
Medial temporal lobe circuitry
1. Hippocampus is part of the medial temporal lobe
2. Medial temporal lobe consists of
a. Entorhinal cortex
b. Perirhinal cortex
c. Hippocampus
3. Hippocampus receives sensory information from the cortex via the entorhinal cortex
a. Hippocampal input from entorhinal cortex is returned to entorhinal cortex via three
serial excitatory pathways of the hippocampus
i. Transverse pathway
ii. Perforant pathway
iii. Mossy fiber pathway
Hippocampal function
1. Bilateral removal of the medial temporal lobe results in
a. Impairment of declarative / episodic memory (anterograde / retrograde)
b. Normal functioning of procedural and working memory (perhaps not working
memory?)
2. Hippocampus crucial in mediating
a. Spatial learning
b. Memory
c. Navigation
i. Implications from 'place cells' in rodent hippocampus
1. Place cells
a. Cells that are tuned (fire maximally) to certain spatial locations
in an open arena
ii. Positive correlation between posterior hippocampal volume and years of
experience as a london cab driver
3. Hippocampal role in processing of contextual information
a. E.g.
i. Hippocampal lesioned rats fail to freeze to a context that has previously been
paired with shock
ii. But show intact processing of discrete cues (e.g. tone) previously paired with
shock
1. i.e. show freezing when cues are presented alone (rather than
conditional cues)
b. In humans anxiety-associated context
i. Induces activation of the
1. Right hippocampus
2. Ventral pallidum
4. Hippocampus involved with working memory
a. Working memory tasks
i. 8-arm radial maze
1. Retrieving rewards at the end of each arm
2. Record amount of re-entry errors (more in hippocampal lesioned rats)
5. Hippocampus involved in
a. Higher order spatial perceptual processes and novelty and mismatch detection
b. Hippocampal involvement in processing timing, anxiety and inhibitory mechanisms
(impulse control)
i. Demonstrated in rats
ii. Not in humans
Septotemporal axis and the transverse plane of the hippocampus
1. Rodent hippocampus
a. Extends from the septal nuclei dorsally to the temporal lobe medially and ventrally
i. Known as the septotemporal axis (long axis of the hippocampus
b. Cross sectional cut through the hippocampus defines the transverse axis
2. The hippocampus contains both transverse and septotemporal circuitry
a. BUT it is the transverse circuitry that is mainly studied for various aspects of CNS
function
i. The main circuit projection is orthogonal to the septotemporal axis:
1. (EC-DG-CA3-CA1-Sub-EC)
a. Entorhinal cortex
b. Dentate gyrus
c. CA3 pyramidal neurons
d. CA1 pyramidal neurons
e. Subiculum ?
f. Entorhinal cortex
2. Therefore any transverse slice of the hippocampus contains most of the
main circuitry with intact neurons and synaptic connections
b. Cresyl violet stain along the transverse plane shows a very defined laminar structure
with visible layers where rows of pyramidal or granule cell bodies are arranged
Transverse pathways
1. The hippocampus forms a principally unidirectional network with
a. Input from the entorhinal cortex (EC) that
b. forms connections with the dentate gyrus (DG) and CA3 pyramidal neurons via the
perforant path (lateral and medial PP)
i. CA3 neurons also receive input from the DG via the mossy fibers (via the mossy
fiber pathway; MF)
ii. CA3 neurons send axons to CA1 pyramidal cells via the schaffer collateral
pathway (SC)
iii. CA3 neurons send axons to CA1 pyramidal cells in the contralateral
hippocampus via the associational commisural pathway (AC)
1. CA1 neurons also receive inputs direct from the perforant path and
send axons to the subiculum (Sb)
a. Neurons in the subiculum send main hippocampal output back
to the EC (forming a loop)
Principal neurons
1. There are two types of principal neurons in the hippocampus
a. Principal neurons in the dentate gyrus
i. Granule cells
1. Have small spherical cell bodies that are arranged 4-6 cells thick
2. Monopolar (their dendrites emanate only from the top of the cell body,
extending into the molecular layer)
3. Axons of granule cells are called mossy fibers (due to the appearance of
their synaptic terminals)
b. Principal neurons in the CA1 / CA3 / Subiculum
i. Pyramidal cells
1. Cell bodies that are arranged 3-5 cell bodies thick in the pyramidal cell
layer of the CA1 and CA3
2. Have elaborate dendritic trees that extend in both directions
a. Multipolar

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Description
Lecture 6 Notes Synaptic organization of the hippocampus 1. Reasons for studying the hippocampus and associated regions of the medial temporal lobe a. Relative simplicity of the organization makes the hippocampus a good model for studying the mechanisms of synaptic transmission, integration, and plasticity i. The cells in the hippocampus have a very orderly laminar organization ii. The different cell types and dendritic fields are highly segregated b. Hippocampus plays a critical role in the encoding and storage of explicit memories (declarative memories for episodes and facts) i. Balateral damage to the medial temporal lobe results in severe retrograde amnesia (inability to retrieve old, declarative memories) ii. Damage to the hippocampus does not affect motor learning or habit formation 1. Suggests that the brain possesses independent memory systems c. Hippocampus is highly vulnerable to certain neurological diseases like epilepsy and alzheimers and damage from strokes i. First brain stucture that is affected in CO poisoning Medial temporal lobe circuitry 1. Hippocampus is part of the medial temporal lobe 2. Medial temporal lobe consists of a. Entorhinal cortex b. Perirhinal cortex c. Hippocampus 3. Hippocampus receives sensory information from the cortex via the entorhinal cortex a. Hippocampal input from entorhinal cortex is returned to entorhinal cortex via three serial excitatory pathways of the hippocampus i. Transverse pathway ii. Perforant pathway iii. Mossy fiber pathway Hippocampal function 1. Bilateral removal of the medial temporal lobe results in a. Impairment of declarative / episodic memory (anterograde / retrograde) b. Normal functioning of procedural and working memory (perhaps not working memory?) 2. Hippocampus crucial in mediating a. Spatial learning b. Memory c. Navigation i. Implications from 'place cells' in rodent hippocampus 1. Place cells a. Cells that are tuned (fire maximally) to certain spatial locations in an open arena ii. Positive correlation between posterior hippocampal volume and years of experience as a london cab driver 3. Hippocampal role in processing of contextual information a. E.g. i. Hippocampal lesioned rats fail to freeze to a context that has previously been paired with shock ii. But show intact processing of discrete cues (e.g. tone) previously paired with shock 1. i.e. show freezing when cues are presented alone (rather than conditional cues) b. In humans anxiety-associated context i. Induces activation of the 1. Right hippocampus 2. Ventral pallidum 4. Hippocampus involved with working memory a. Working memory tasks i. 8-arm radial maze 1. Retrieving rewards at the end of each arm 2. Record amount of re-entry errors (more in hippocampal lesioned rats) 5. Hippocampus involved in a. Higher order spatial perceptual processes and novelty and mismatch detection b. Hippocampal involvement in processing timing, anxiety and inhibitory mechanisms (impulse control) i. Demonstrated in rats ii. Not in humans Septotemporal axis and the transverse plane of the hippocampus 1. Rodent hippocampus a. Extends from the septal nuclei dorsally to the temporal lobe medially and ventrally i. Known as the septotemporal axis (long axis of the hippocampus b. Cross sectional cut through the hippocampus defines the transverse axis 2. The hippocampus contains both transverse and septotemporal circuitry a. BUT it is the transverse circuitry that is mainly studied for various aspects of CNS function i. The main circuit projection is orthogonal to the septotemporal axis: 1. (EC-DG-CA3-CA1-Sub-EC) a. Entorhinal cortex b. Dentate gyrus c. CA3 pyramidal neurons d. CA1 pyramidal neurons e. Subiculum ? f. Entorhinal cortex 2. Therefore any transverse slice of the hippocampus contains most of the main circuitry with intact neurons and synaptic connections b. Cresyl violet stain along the transverse plane shows a very defined laminar structure with visible layers where rows of pyramidal or granule cell bodies are arranged Transverse pathways 1. The hippocampus forms a principally unidirectional network with a. Input from the entorhinal cortex (EC) that b. forms connections with the dentate gyrus (DG) and CA3 pyramidal neurons via the perforant path (lateral and medial PP) i. CA3 neurons also receive input from the DG via the mossy fibers (via the mossy fiber pathway; MF) ii. CA3 neurons send axons to CA1 pyramidal cells via the schaffer collateral pathway (SC) iii. CA3 neurons send axons to CA1 pyramidal cells in the contralateral hippocampus via the associational commisural pathway (AC) 1. CA1 neurons also receive inputs direct from the perforant path and send axons to the subiculum (Sb) a. Neurons in the subiculum send main hippocampal output back to the EC (forming a loop) Principal neurons 1. There are two types of principal neurons in the hippocampus a. Principal neurons in the dentate gyrus i. Granule cells 1. Have small spherical cell bodies that are arranged 4-6 cells thick 2. Monopolar (their dendrites emanate only from the top of the cell body, extending into the molecular layer) 3. Axons of granule cells are called mossy fibers (due to the appearance of their synaptic terminals) b. Principal neurons in the CA1 / CA3 / Subiculum i. Pyramidal cells 1. Cell bodies that are arranged 3-5 cell bodies thick in the pyramidal cell layer of the CA1 and CA3 2. Have elaborate dendritic trees that extend in both directions a. Multipolar b. One originating from the base of the pyramidal shaped cell body (basal dendrites) i. Projects outward into the stratum oriens c. One projecting from the apex of the cell body (apical dendrites) i. Inward into the stratum radiatum ii. CA3 neurons can have variable dendritic lengths 1. Those located close to the DG have shorter dendritic lengths 2. Those located closer to the CA1 have longer dendritic lengths iii. CA1 neurons 1. Have more regular dendritic trees 2. Are longer on average than CA3 neurons 3. Receive ~ 30,000 excitatory inputs and ~1700 inhibitory inputs 4. The distribution of synaptic inputs along the dendritic shaft: a. Tuft i. Excitatory inputs from extrinsic sources 1. EC 2. Thalamus b. Apical dendrites i. Excitatory inputs from local sources c. Basal dendrites i. Inhibitory (GABAergic) inputs from local interneurons Physiological properties Intracellular response 1. Stimulation of any of the hippocampal pathways (PP / MF / SC) generates a characteristic sequence of synaptic response a. Excitation b. Biphasic inhibition i. Fast component 1. GABAa receptors ii. Slow component 1. GABAb receptors iii. Due to feedback and feedforward inhibition iv. Serves to 1. Dampen the effect of afferent excitation 2. Narrows the time window in which presynaptic activity can trigger a spike in pyramidal neurons v. Enables neurons to be precise coincidence detectors vi. Allow high fidelity transfer of timing information between brain regions 2. Notable differences in the firing properties of the different principal neurons in the hippocampus a. CA1 and DG neurons i. Can fire repetitively ii. Show spike frequency adaptation and slow afterhyperpolarization b. CA3 neurons i. Fire in short bursts of 5-10 APs with declining amplitudes 1. Burst activity is thought to be important in explaining the seizure susceptibility of the hippocampus 2. Entrains CA3 neuronal output to a theta rhythm that is observed during a. Behavioral exploration b. Vigilance c. Spatial learning Extracellular response 1. Pyramidal cells and their dendrites a. Have very orderly laminar organization b. All are in the same orientation c. All receive synaptic inputs from the same area (layer) 2. These properties make the hippocampus highly amenable to electrical field (extracellular) recordings which represent the summed responses from a number of neurons in the vicinity of the recording electrode a. Informative because the time course of the field potential is roughly equal to the time course of the underlying synaptic current 3. Stimulation of schaffer collaterals in the CA1 a. A field recording electrode placed in the stratum radiatum i. First record a brief negative transient (fiber volley) 1. Reflects current moving into the dendrites (away from the electrode) 2. The size of this deflection indicates the amount of post-synaptic depolarization in the neuronal population ii. Followed by a slower but larger negative potential called pEPSP 1. Reflects the summed activity across a population of neurons 2. Current moving into the dendrites during pEPSP will exit the neurons near the cell body layer a. Field electrode in the stratum pyramidale would record a positive potential iii. If depolarization is sufficient to trigger action potentials in the population of cells 1. Then a population spike is generated in the stratum pyramidale 2. This is a negative deflection as current moves down the axon (away from electrode) 3. The size of the population spike indicates the number of cells and the synchrony with which these cells fire an action potential The perforant pathway from the entorhinal cortex into the hippocampus 1. The main input from the entorhinal cortex to the adjacent hippocampus a. Layer II neurons that project from the PP axons i. PP axons bifurcate and synapse repeatedly on granule cell dendrites within the dentate gyrus (DG), CA3 dendrites, and CA1 ii. The PP has two parts that terminate on distinct regions within the granule cell dendritic field (molecular layer) and CA3 dendritic field 1. Medial PP a. Terminates in a layer more proximal to the pyramidal cell soma 2. Lateral PP b. Layer III neurons that project from PP axons i. Distinct populations of CA1
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