Introduction The neurons of the cerebral cortex Study methodology Laminae (layers)

 Hemisphere surface (cerebral cortex) is called pallium having variable depth/thickness (15–45mm) from one area to another. - thicker in the summit of the gyli, and thinner on the depth of the sulci.  The pallium has laminar (layers) arrangement which associate with blood vessels – angioarchtecture.  Neurons are forming a microarchtecture by presenting different types of cell morphologies  These morphologies are  i. Pyramidal cells  ii. Non-pyramidal cells called stellate or granule cells -They are spiny or non-spiny.

 Cells are also subdivided basing on their sizes, shapes and neuritic arrays.

 This section is showing cells in different orientation as they are seen in the neuropil.

 1. Pyramidal cell: This is a flask-shaped or triangular cell body with single apical or multiple basal dendrities. - Form most of the majority of projection neurons.  2. Spiny stellate cells: most of these are multipolar cells. - Numerous in the lamina IV. - They have relatively small sized cell bodies and spiny dendrites. - Their axons ramify within the gray matter in a vertical plane.  3. Non spiny stellate cells: These are numerous group of interneurons (association neurons). - They have different shapes like basket, chandelier, double bouquet, neuroglial forms bipolar or fusiform and horizontal cells.

..  This is a histological slide of the cerebrum. Numerous pyramidal shaped neurons are present within perineuronal spaces. These cells have large vesicular nuclei. The small cells seen only as nuclei are glial cells. The stringy pink background is the neuropil. At high power, a small blood vessel surrounded by a clear Virchow-Robin space is seen.

Chandelier design Bouquet design

 4. Glial cells: This is a group of supporting cells in the NS which are associating with the neurons. - They include i. astrocytes ii. oligodendrocytes iii. ependymal cells iv. microgliae. - They are occupying the neuropil part of the cerebral cortex.

Normal astrocytes Cortical astrocytes – fluorecence technique

 Study of neurons in the cerebral cortex involving staining techniques is known as artechitectonics. - study by staining the cell bodies arrangements is termed cytoarchtechtonics. - the use of myelin stained material is termed myeloarchtectonics. - the use of stained pigment distribution is termed as pigmentoarchtectonics.

 The pallium has six distinctive layers which may occasionally be appreciated by the naked eyes. The layers are lying parallel to the surface.  LAYER I: This is the molecular or plexiform layer. - it contains scattered horizontal cells and the apical dendrites of all pyramidal neurons of the cerebral cortex.  LAYER II: External granular lamina. - it contains small neuronal cell bodies (pyramidal and non pyramidal neurons).  LAYER III: External pyramidal lamina. - it contains pyramidal cells of different sizes.

 LAYER IV: Internal granular layer. - this is the narrowest of the cellular laminae - it contains densely parked small round pyramidal cells  LAYER V: Internal pyramidal lamina. - it contains the larger pyramidal cells and scattered non- pyramidal cells.  LAYER VI: The multiform layer. - it contains variety of shapes eg. Pyramidal, spindle, ovoid and many other shaped somata. - the cells are small to medium sized. - this layer blends with the white matter.

Introduction The neurons (cells) of the cerebellar cortex Important fibres

The cerebellum is a smaller region in the lower part of the brain. For the large region of the brain.  it plays an important role in motor control.  it may also be involved in some cognitive function such as attention and language.  it regulates fear and pleasure responses,  it has movement-related functions which does not initiate movement, but it contributes to co- odination, precision, and accurate timing.

 It receives input from sensory system of the spinal cord and from other parts of the brain and integrates these inputs to fine-tune motor activity.  Cerebellar damage produces disorders in fine movement, equilibrium, posture, and motor learning. [It is extremely important to remember that cerebellum is able to perform everyday voluntary (done with purpose and intent) tasks such as walking and writing. It is also essential to being able to stay balanced and upright. Patients who have suffered from damaged cerebellums often struggle with keeping their balance and maintaining proper muscle coordination]

 Cerebellar substance is divided into outer cerebellar cortex (gray matter) and inner cerebellar medullar (white matter).  The gray matter is composed of three layers i. Molecular cell layer ii. Purkinje cell layer iii. Granular cell layer  The white matter comprises of neuronal fibres mostly myelinated.

MOLECULAR LAYER  the outer lamina of the cortex lying directly below the pia mater, containing the cell bodies (unless the Purkinje cell layer is designated as a separate layer) and dendrites of Purkinje cells; non-myelinated axons of the granular layer cells; the cell bodies, dendrites, and axons of basket cells and superficially located stellate cells; and the dendrites of Golgi cells plus cerebellar glial cells.

◦ Basket cells are multipolar interneurons found in molecular layer that function to make inhibitory synapses and control the overall potentials of target cells. ◦ stellate cells are neurons. The three most common stellate cells are the inhibitory interneurons found within the molecular layer of the cerebellum, excitatory spiny stellate cells and inhibitory aspiny stellate interneurons.  PURKINJE CELL LAYER ◦ The large, spherical cell bodies of Purkinje cells are packed into a narrow layer (one cell thick) of the cerebellar cortex, called the Purkinje layer.

◦ Purkinje cells are among the most distinctive neurons in the brain and only cerebellar cortex neurons which send information (always inhibitory) to the outside. ◦ are distinguished by the shape of their dendritic tree and their myelinated axons project to the white matter. ◦ The dendrites branch very profusely, but are severely flattened in a plane perpendicular to the cerebellar folds. ◦ The dendrites are covered with dendritic spines, each of which receives synaptic input from a parallel fiber. ◦ Purkinje cells receive more synaptic inputs than any other type of cell in the brain.

 Purkinje cells use GABA as their neurotransmitter, and therefore exert inhibitory effects on their targets.  Purkinje cells normally emit action potentials at a high rate even in the absence of the synaptic input.  Other cells found in Purkinje layer include Bergmann cells a type of glia also known as radial epithelial cells or Golgi epithelial cells

 Bergmann cells are astrocytes in the cerebellum that have their cell bodies in the Purkinje cell layer and processes that extend into the molecular layer, terminating with bulbous end feet at the pial (vascular) surface.  Bergmann glia are also required for the pruning or addition of synapses.

 GRANULAR CELL LAYER: ◦ Cerebellar granule cells, in contrast to Purkinje cells, are among the smallest neurons in the brain. They are also easily the most numerous neurons in the brain: In humans, estimates of their total number average around 50 billion, which means that about 3/4 of the brain's neurons are cerebellar granule cells. ◦ Their cell bodies are packed into a thick layer at the bottom of the cerebellar cortex. ◦ A granule cell emits only four to five dendrites, each of which ends in an enlargement called a dendritic claw. These enlargements are sites of excitatory input from mossy fibers and inhibitory input from Golgi cells.

 The thin, unmyelinated axons of granule cells rise vertically to the upper (molecular) layer of the cortex, where they split in two, with each branch traveling horizontally to form a parallel fiber; the splitting of the vertical branch into two horizontal branches gives rise to a distinctive "T" shape.  Granule cells use glutamate as their neurotransmitter, and therefore exert excitatory effects on their targets.

 Cerebellar fibres: Mossy fiber (cerebellum)  are one of the major inputs to cerebellum. There are many sources of this pathway, the largest of which is the cerebral cortex, which sends input to the cerebellum via the pontocerebellar pathway. Other contributors include the vestibular nerve and nuclei, the spinal cord, the reticular formation, and feedback from deep cerebellar nuclei. Axons enter the cerebellum via the middle and inferior cerebellar peduncles Climbing fiber are the name given to a series of neuronal projections from the inferior olivary nucleus located in the medulla oblongata. These axons pass through the pons and enter the cerebellum via the inferior cerebellar peduncle where they form synapses with the deep cerebellar nuclei and Purkinje cells

 The climbing fibers carry information from various sources such as the spinal cord, vestibular system, red nucleus, superior colliculus, reticular formation and sensory and motor cortices. Climbing fiber activation is thought to serve as a motor error signal sent to the cerebellum, and is an important signal for motor timing. Parallel fiber: arise from granule cells in the cerebellar cortex. They form excitatory synapses onto the dendrites of Purkinje cells (the output neurons of the cerebellum) and the dendrites of the inhibitory interneurons basket cells and stellate cells of the molecular layer.

 Figure: Microcircuitry of the cerebellum. Excitatory synapses are denoted by (+) and inhibitory synapses by (-). MF: Mossy fiber. DCN: Deep cerebellar nuclei. IO: Inferior olive. CF: Climbing fiber. GC: Granule cell. PF: Parallel fiber. PC: Purkinje cell. GgC: Golgi cell. SC: Stellate cell. BC: Basket cell.