Neocortex: Synaptic Organization Quiz

Neocortex: Synaptic Organization

• The neocortex has a distinctive layered structure
• Three phylogenetic categories of cerebral cortex exist: archicortex, paleocortex, and neocortex
• Archicortex has 3-4 layers
• Paleocortex has 3 layers
• Neocortex has 6 layers, and is involved in complex functions like perception, learning, and intelligence
• Wilder Penfield's "Montreal procedure" involved stimulating and destroying neurons in the neocortex to treat epilepsy
• This procedure helped to identify primary sensory and motor areas
• Most of the human brain's surface is covered by association cortices, whose functions are known as cognition
• Cognition involves attending to external and internal stimuli, understanding their significance and responding appropriately
• Lower mammals have smooth cerebral cortex, which is simpler, while higher mammals and humans have more folds with well-defined temporal and larger frontal lobes
• Gyri are bumps, and sulci are valleys in the brain
• Fissures are very deep sulci, and function to increase the surface area of the brain and decrease axonal distance
• Brodmann's Areas are numbered areas in the brain, based on the cytoarchitectural organization of neurons
• Brodmann's Areas were identified using Nissl stain, but with little initial knowledge of their functional significance
• Prefrontal cortex is the highest level of cortical hierarchy involved in planning, decision-making, social skills, conscience, working memory and more.
• Prefrontal cortex, especially the dorsolateral and ventromedial areas including the orbitofrontal PFC, are involved in motivated decisions, emotions and autonomic responses, conscience and social skills.
• Prefrontal cortex develops slowly, reaching functional maturity around 15-19 years old
• Neurons in the prefrontal cortex (PFC) demonstrate persistent, or stable activity during delay periods, indicating a potential role in working memory
• Domain specificity theory refers to the idea that different regions of the PFC contribute to different types of working memories
• Different PFC regions play different roles in working memory.
• Persistent activity in the PFC is likely caused by neural connectivity and neuronal biophysics
• The organization of the neocortex shows several recurrent excitatory loop patterns and also shows "up" and "down" states, which are related to sleep and memory

PFC and Working Memory

• Different regions of the PFC contribute to different types of working memory
• Evidence indicates that dorsal PFC is involved in spatial working memory while the ventral PFC is involved in object working memory
• Neural circuits in the PFC can sustain stimulus-selective activity without external input
• Recurrent excitatory loops and local inhibition are possible neural mechanisms for sustained activity and involve rapid turning on and off (around 100ms).

Neocortical Principal cells

• Principal cells (pyramidal cells, Stellate cells, approximately 80% of the neurons): Release glutamate.
• Pyramidal cells are the main output neurons in the cortex; they are large and conical with apical and basal dendrites.
• Granule cells are small and round, and found primarily in layer 4, which receives input from thalamus
• The primary function is to receive input from the thalamus and to make connections onto other neurons in the cortex.

Neocortical GABAergic Cells

• These are local neurons (approximately 20%) that release GABA, an inhibitory neurotransmitter
• There are many different subtypes (bitufted, double bouquet, small basket, large basket, chandelier, and neurogliaform)

Neocortical Canonical Organization

• The circuitry of cortical regions has common features:
  • Primary source of input
  • Primary output target
  • Columnar (radial connections) and horizontal connections
• Thalamus is a major source of input to layer 4
• Cortico-cortical (primarily Layer 2/3) and cortico-cortical (via white matter) connections are primary connections among pyramidal cells
• Layer 5 and 6 pyramidal cells mostly form outputs from the cortex to other cortical areas, the thalamus, other cortical regions, and subcortical structures

Neocortical Afferents (Inputs)

• Thalamus provides input to layer 4, which is considered the main input to the neocortex
• Cortico-cortical projections are the major internal projections in the neocortex, originating from layers 2/3 and often targeting layer 2/3

Neocortical Efferents (Outputs)

• The majority of axons remain within the cortex (90-99%)
• Cortico-cortical projections send output signals to other cortical areas
• Layer 5/6 pyramidal cells target the thalamus, sub-cortical structures, and superior colliculus

Cortical Microcircuit

• Recurrence and feedforward excitation are crucial for persistent activity, involving a balance of excitation and inhibition

Information Processing in Neocortex

• Grandmother cell versus distributed coding theory: which is correct?
• Updated models of sensory information processing show thalamus -> layer 4 -> layer 2/3-> layers 5/6
• Deep cortical layer respond to thalamic inputs

Functional Cortical Columns

• The primary visual cortex consists of hypercolumns for analyzing regions of the visual field (e.g., ocular dominance, orientation columns, and blobs)
• Functional columns can also be found in regions other than primary sensory and motor cortices

Persistent Activity

• Persistent activity is likely correlated with specific circuit configurations that support persistent memory
• There is evidence of multiple recurrent excitatory loops participating in persistent activity
• The model of local circuit recurrent excitation can be useful to understand persistent activity in the cortex by demonstrating feedback loops, recurrent excitation loops, and inhibitory mechanisms that are responsible for the functionality of networks

Vertical Organization - Cell Columns

• "Barrel" cortex has distinctly organized regions of cells in primary sensory cortex, reflecting input from whiskers

Test your knowledge on the synaptic organization of the neocortex, including its layered structure and phylogenetic categories. Explore the key functions and significance of this complex brain region, and learn about the implications of Wilder Penfield's Montreal procedure in treating epilepsy. Understand how the neocortex contributes to cognition and sensory processing.