NEURO DEPTH 💗
ANA 303: NEUROHISTOLOGY: CEREBELLUM AND CEREBELLUM
Grey and White Matter
Definition and Appearance: Sections through the spinal cord or brain exhibit regions of different colors:
Whitish regions: Represent white matter
Darker grey regions: Represent grey matter
Composition:
Grey Matter: Contains neuron cell bodies, dendrites, and axons that either start at or end on the cell bodies.
Unmyelinated Fibres: Most fibers found within grey matter are unmyelinated.
White Matter: Comprises predominantly myelinated fibres, with the reflective light from myelin accounting for its whitish appearance.
Neuroglia and Blood Vessels: Present in both grey and white matter.
Arrangement:
In the spinal cord and brainstem, white matter is found externally, while grey matter forms discrete masses within them.
In the cerebrum and cerebellum, there is a thin, extensive layer of grey matter on the surface called the cortex.
Beneath the cortex lies white matter interspersed with nuclei, which are defined as groups of neuron cell bodies.
Cerebellum Overview
Structure
The cerebellum, or small brain, resides in the posterior cranial fossa and features:
A superficial layer of grey matter, the cerebellar cortex.
Numerous fissures on its surface, subdividing it into segments.
Subdivisions of the Cerebellum
The cerebellum consists of:
Vermis: Central part near the midline.
Lateral Hemispheres: Two sides of the vermis.
Surfaces: Recognized by superior and inferior surfaces:
Superior Aspect: Lacks distinct separation between the vermis and hemispheres.
Inferior Aspect: Hemispheres separated by a deep depression termed the vallecula, with the vermis located at the depth of this depression.
Fissures and Folia
A series of fissures run parallel on the cerebellum’s surface, creating leaf-like bands termed folia.
Characteristic arbor-vitae (tree of life) appearance can be seen when sections are cut at right angles to the folia's long axis.
Lobes of the Cerebellum
The cerebellum has lobes separated by deeper fissures:
Anterior lobe: Located anterior to the primary fissure.
Posterior lobe (or middle lobe): Situated between the primary and posterolateral fissures.
Flocculonodular lobe: The remaining part, involved in vestibular functions.
Vermis Components: Composed of:
Lingula (in the anterior lobe),
Central lobule (in the anterior lobe),
Culmen (in the anterior lobe),
Declive, Folium, Tuber, Pyramis (Pyramid), and Uvula (in the middle lobe),
Nodule (in the flocculonodular lobe).
Grey Matter of the Cerebellum
The cerebellum's grey matter forms a thin layer covering the white matter core, referred to as the cerebellar cortex.
Within the white matter core are cerebellar nuclei, categorized as follows:
The dentate nucleus.
The emboliform nucleus.
The globose nucleus.
The fastigial nucleus.
White Matter of the Cerebellum
Each cerebellar hemisphere is centered by white matter.
Cerebellar Peduncles: Composed of white matter fibres connecting to the brainstem.
Medullary Velum: White matter connects the two hemispheres and forms parts of the roof of the fourth ventricle including:
Superior medullary velum (upper part)
Inferior medullary velum (lower part).
Structure of the Cerebellar Cortex
The cerebellar cortex features a uniform structure:
Molecular layer: Most superficial layer.
Purkinje cell layer: Middle layer, containing Purkinje cells.
Granular layer: Deep layer resting on white matter.
Neurons of the Cerebellar Cortex
Purkinje Cells: Large, flask-shaped neurons characterized by a single dendrite forming an intricate tree in the molecular layer; positioned parallel to adjoining dendritic trees.
Granule Cells: Small, numerous neurons filling the granular layer. They send axons to the molecular layer, forming parallel fibres which synapse with Purkinje cells.
Golgi Cells: Large star-shaped neurons in the granular layer, with elaborate dendrites in both the granular and molecular layers, involved in glomerular synapses.
Cerebrum Overview
The surface of the cerebral hemispheres is characterized by:
A series of grooves or sulci separated by bulging areas called gyri.
The cerebral cortex is a thin layer of grey matter that follows the contours of the sulci and gyri and extends into their depths.
Internal Structure of the Cerebrum
Beneath the cortex, the bulk is made up of white matter housing important grey matter masses:
Thalamus and Hypothalamus located laterally to the third ventricle.
Corpus Striatum: Comprising Caudate Nucleus and Lentiform Nucleus. The thin grey layer between these nuclei and the insula is called the claustrum.
Collectively referred to as basal nuclei or basal ganglia.
Internal Capsule
A significant white matter region known as the internal capsule exists between the thalamus and caudate nucleus medially, and the lentiform nucleus laterally. This region is crucial for major ascending and descending tracts.
The corona radiata term designates white matter radiating from the upper internal capsule to the cortex.
Interconnections of Cerebral Hemispheres
Cerebral hemispheres are interconnected by fibres forming commissures, the most significant being the corpus callosum.
Types of Neurons in the Cerebral Cortex
Pyramidal Cells:
Triangular bodies; most abundant in the cortex (2/3 of neurons). Dendrites arise from apex and basal angles—forming multiple synapses vertical through cortex thickness.
Stellate Cells:
Smaller multipolar neurons, comprising 1/3 of cortical neurons, with short axons terminating within the cortex. Dendritic orientation is typically vertical but can be horizontal.
Other Cell Types: Include Fusiform, Basket, Horizontal, Neuragliaform, Cells of Martinoti.
Layers of the Cerebral Cortex**
The cerebral cortex stratifies into six layers according to microscopic preparation:
Plexiform (molecular) layer: Predominantly fibres with few cells.
External Granular layer: Predominantly stellate cells.
Pyramidal Cell layer: Characterized by pyramidal neurons.
Internal Granular layer: Comprised mainly of stellate cells.
Ganglionic layer: Predominantly pyramidal cells; includes giant pyramidal cells of Betz.
Multiform layer: Variable in cell size and shape.
Conclusion
These anatomical observations reveal the complex organization of the cerebellum and cerebral cortex, each critical for their respective physiological functions.