CNS

The Cerebrum:

The cerebrum consists of convoluted cortex of gray matter + central medullary mass of white matter which conduct fibers between the different parts of the cerebral cortex to the other parts of the nervous system.The cerebral cortex consists of gray matter & its surface area is increased by its convolutions or "gyri" which are separated by fissures or "sulci"
According to the morphology 5 types of nerve cells present in the cortex:
1.Pyramidal cells:
are pyramidal shape cells having an apex directed towards the surface + cylindrical axons arises from the bases of the cells & passes down into the underlying white matter, axons of the small superficial pyramidal cells either they synapse in the deeper layer of the cortex or enter to the white matter, from the apex a thick apical dendrite passes upwards towards the surface where it has fine collateral branches, also short dendrites arise from both edges of the bases & ramify laterally into the surrounding nervous tissue.The size of pyramidal cells varies from small to large cells , the smallest cells tend to lie more superficially, the largest pyramidal cells are called Betz cells which are large upper motor neuron of the motor cerebral cortex in the precentral gyrus of the frontal lobe.





2.Stellate cells:
small neurons with short vertical axons & small short branching dendrites giving it a shape of a "star", the axon terminate on the nearby neurons. These cells are considered as Golgi type ІІ neurons and have non meylinated axons.
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3.Cells of Martinotti:
small polygonal cells with few short dendrites, their axons extend towards the surface & bifurcate to run horizontally in the superficial layer, they are present through out the levels of the cortex.

4.Fusiform cells:
spindle-shaped cells oriented at right angle to the cerebral cortex, their axons arise from the side of the cell body & pass superficially; their dendrites extend from each end of the cell body branching in to the deeper & more superficial layers.

5.Horizontal cells of "Cajal":

small spindle-shaped oriented parallel to the surface, they are least common found in the most superficial layer where their axons pass laterally to synaps with the dendrites of the pyramidal cells.
In addition the cerebral cortex contains supporting cells ( Astrocyte,Oligodendrocyte ,Microglia & Ependymal cells).




For descriptive purpose, the cerebral cortex can be divided into 6 layers vary in the type ,density & arrangement of their cells. According to the layers we have 6 layers:

1- Molecular ( plexiform ) layer : is the most superficial layer contains axons & dendrites of the cortical neurons making synaps with each other in addition to Cajal cells.

2- Outer granular layer : thin layer consists of small pyramidal + few stellate cells.

3- Outer Pyramidal cell layer : a broad layer composed mainly of moderate size pyramidal cells, the cells gradually increased in size in the deeper portion of this layer .

4- Inner granular layer : consists of densely packed stellate cells.

5- Gangilionic layer (inner pyramidal layer): large pyramidal cells + stellate + cells of Martinotti , the name originates from the huge pyramidal (Betz
cells) or (Ganglion) of the motor cortex.

6- Multiform cell layer:

contains wide variety of different morphological forms of cells, it contains numerous small pyramidal cells + cells of Martinotti + stellate cells lie superficially & fusiform cells lie in the deeper part.

The only efferent pathway are the axons of pyramidal cells tend to give branches which pass to the more superficial layers to communicate with dendrites.

Areas of Specialization:

In the motor cortex (in the precentral gyrus), there is relative absence of stellate cells in the 2nd & 4th layers .
In the sensory cortex (in the postcentral gyrus), there is a relative absence of pyramidal cells in the 3rd & 5th layers.

Cerebellum:

The cerebellum coordinates the muscular activity & maintains the posture & equillibrium , it consists of :
1 Outer cortex of gray matter.
2 Inner core of white matter containing 4 pairs of nuclei ( dentate , Globos , Festigial & Rostiform ).

The afferent & efferent fibers pass to & from the brain stem [midbrain, pons, medulla oblongata] by superior, middle & inferior cerebellar peduncles linking the midbrain, pons& medulla oblongata respectively .
The cerebellar cortex is thrown into a series of convolutions or folia "& it is composed of 3 layers :
a-Outer molecular layer: contains few neurons + large no. of unmyelinated fibers (dentritic arborazations & numerous thin axons run parallel to the long axis of the folia)+ "Stellate" & "Basket" cells
b-Inner granular layer: is highly cellular layer, packed with multipolar small neurons characterized by densely staining dark nuclei & scanty amount of cytoplasm.called "Granule cells", they have non- myelinated axons which pass outward to the molecular layer where they bifurcate to run parallel to the surface then synaps with dendrites of the purkinje cells .They considered as Golgi type ІІ nurons.
c–Purkinje cell layer: lies between the outer & inner layer consists of flask-shaped huge neurons with rounded nuclei called "Purkinje cells", they arranged in a single row & They considered as Golgi type ІІ nurons. each cell have:
1- fine axon arises from the base of the cell & extends down through the granular layer to enter the white matter where it synapse with the cerebellar nuclei.
2-extensively branched dendrites which extend to the outer molecular layer where they undergo profuse branching & finally make synapse with the axons of the granule cells.


There are 3 other types of small neurons in the cerebellar cortex have supportive function which are:
1-"Stellate" & "Basket" cells scattered in the molecular layer.
2- Golgi cells scattered in the granular layer.

The only efferent pathway from the cerebellar cortex are the purkinje cell axons which pass down through the granular layer into the white matter where they synapse with the 4 pairs of the cerebellar nuclei, while the afferent fibers enter the cerebellum from brain stem (by Mossy fibers) which passes through the white matter core to make complex connections with the granular cells in the inner granular layer, or by(Climbing fibers)which make synapse with the purkinje cell dendrites.










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PAGE 6
ervous System Dr.LUMA IBRAHIM
Lecture 2