Lecture 5- The Brain

Question 1

Purpose of the meninges

Answer 1

Provide physical support

Question 2

Name the meninges

Answer 2

Dura Mata
Arachnoid Mata
Pia Mata

Question 3

Dura Mata

Answer 3

Dense, firm layer of collagenous connective tissue

Outer layer consists of collagen and elastic fibers and a smooth inner surface of simple squamous epithelium

Question 4

Spinal dura mata

Answer 4

A tube that extending from foramen magnum to S2, pierced by spinal nerves
Separated from wall of spinal canal by epidural space containing adipose tissue and a venous plexus

Question 5

Cranial dura mata

Answer 5

Firmly attached to periosteum from which it received small blood vessels
Characterized by the dural reflections, flax cerebri, tentorium cerebelli and dural venous sinuses

Question 6

Leptomeninges constituents and features

Answer 6

Pia mata and arachnoid mata together
Numerous trabeculae passing between the two layers
Pia mata adheres to parenchyma of CNS, therefore cannot be removed or manipulated
Surfaces of both layers are covered by simple squamous epithelium

Question 7

Leptomeninges development

Answer 7

Initially develop as a single layer from the mesoderm surrounding the embryonic brain and spinal cord
Fluid-filled spaces form within the layer and join to form the subarachnoid space

Question 8

Leptomeningial trabeculae

Answer 8

Numerous passing between the two layers

Delicate strands of connective tissue with squamous epithelial cells on their surfaces

Question 9

Zonulae occludentes

Answer 9

Tight junctions that connect adjacent arachnoid epithelial cells, preventing exchange of large molecules between the blood in the dural vasculature and CSF
No tight junctions between pial cells, therefore free exchange of macromolecules between CSF and CNS tissue

Question 10

Types of cortical tissue

Answer 10

Paleocortex of the olfactory system
Archicortex of the hippocampal formation
Neocortex in the remainder of the cerebral cortex

Question 11

Layers of the cerebral cortex

Answer 11

Numbers differ according to region
Up to 5 layers in paleocortex, with superficial ones more distinct
No more than 3 layers in the archicortex

Question 12

Neocortex

Answer 12

6 layers always present at some stage of embryonic or fetal development
Layers not often discernable in adults
Impregnation technique used to decipher histological description of cell
Nissl stain for cell density
Myelin stain for distribution of axons and synapses

Question 13

Cortical neuron classification criteria

Answer 13

Morphology: shape of neural processes (eg. pyramidal cells)
Function: projection neurons that connect the cerebral cortex to entirety of its distal intracortical, subcortical or subcerebral targets, or interneurons that make local connections within the cortex
Spiny or aspiny (smooth): spiny have dendrites with dendritic spines, mostly excitatory.
Smooth are interneurons that are mainly inhibitory

Question 14

Pyramidal cells

Answer 14

Projection neurons of the cerebral cortex
Excitatory, glutamatergic neurons
Connect cerebral cortex to the entirety of its distal intracortical, subcortical and subcerebral targets
Make up vast majority of cortical neurons
Spiny

Question 15

Pyramidal cell characteristics

Answer 15

Prominent apical dendrite orientated towards pial surface of the cortex
Basal dendrites
Both apical and basal dendrites have dendritic spines
Spines are small outgrowths of the dendrite that resembles a thorn
Majority of synaptic contacts received are on the dendritic spines rather than directly on the dendrite shaft or cell body

Question 16

Cortical interneurons

Answer 16

Represent approximately 20-30% of all cortical neurons

Make local connections within the cortex that can span multiple layers

Question 17

Classes of cortical interneurons

Answer 17

Spiny pyramidal and stellate cells

Aspiny and nonpyramidal cells

Question 18

Spiny interneurons

Answer 18

Excitatory glutamatergic neurons locates in 4th layer
Receive sensory inputs from thalamus
Inhibitory GABAergic neurons located in all layers of the cortex

Question 19

Aspiny interneurons

Answer 19

Main inhibitory component of neocortical circuits

Finely modulate activity of projection neurons

Question 20

Layers of neocortex

Answer 20

Molecular layer
External granular layer
External pyramidal layer
Internal granular layer
Internal pyramidal layer
Multiform or fusiform layer

Question 21

Layer 1: Molecular layer

Answer 21

Most superficial, few nerve cells but many dendritic and axonal processes in synaptic interaction
Consists predominantly of terminal branches of dendrites and axons, which give a “molecular” appearance in sections stained for nerve fibers
Histologically very clear

Question 22

Layer 2: External granular layer

Answer 22

Contains many small neurons, which establish intracortical connections
Densely packed of small stellate cells and small pyramidal cells whose apical dendrites terminate in the molecular layer and whose axons are sent to lower cortical layers

Question 23

Layer 3: External pyramidal layer

Answer 23

Contains medium-sized neurons giving rise to association and commissural fibers
Medium to large pyramidal cells whose apical dendrites extend to layer 1
Axons of these cells function as association or intracortical association fibers
Easily identifiable histologically

Question 24

Layer 4: Internal granular layer

Answer 24

The site of termination of afferent fibers from specific thalamic nuclei
Received thalamocortical connections, especially from the specific thalamic nuclei
The most prominent in the primary sensory cortices

Question 25

Layer 5: Internal pyramidal layer

Answer 25

Origin of projection of fibers to extracortical targets such as the basal ganglia, brainstem and spinal cord
Contains Betz cells in the primary motor cortex, which project fibers into the pyramidal tract
Consists of large pyramidal cells
Major source of outflow fibers particularly to motor areas of brainstem, basal ganglia and spinal cord
Consistently identifiable histologically

Question 26

Layer 6: Fusiform or Multiform layer

Answer 26

Contains association and projection neurons
Composed of mixture of spindle-shaped cells, pyramidal cells and stellate cells
Dendrites ascend to various cortical levels
Axons enter the white matter as short association fibers or ascend to other cortical layers
However, cortico-thalamic outflow usually originates from pyramidal cells in this layer