CHAPTER 7
SUBDIVISIONS OF THE CEREBRUM
Cerebrum – largest part of the brain
- Situated in the anterior and middle cranial fossae of the skull, occupying its whole cavity
- May be divided into two parts:
o Diencephalon – forms central core
o Telencephalon – forms cerebral hemispheres
DIENCEPHALONS
- Consists of the third ventricle and its boundaries
- Extends posteriorly where third ventricle is continuous with the cerebral aqueduct, and anteriorly as far as the interventricular foramina
- Midline structure with symmetrical right and left halves
Gross Features
Inferior surface
- Only area exposed to the surface in the intact brain
- Formed by the hypothalamic and other structures, including (from anterior to posterior):
o Optic chiasma – with optic tract on either side
o Infundibulum – with tuber cinereum
o Mammillary bodies
Superior surface
- Concealed by the fornix
o Fornix – thick bundle of fibers that originates in the hippocampus of the temporal lobe and arches posteriorly over the thalamus to join the mammillary body
- Actual superior wall is formed by the roof of the third ventricle
o Consists of ependyma
o Covered superiorly by a vascular fold of pia mater, called tela choroidea of the third ventricle
- From the roof of the third ventricle a pair of vascular processes, the choroid plexuses of the third ventricle, project downward from the midline cavity of the third ventricle
Lateral Surface
- Bounded by the internal capsule of white matter
- Consists of nerve fibers that connect the cerebral cortex with other parts of the brainstem and spinal cord
Medial Surface
- Since the diencephalons is divided into symmetrical halves by the slitlike third ventricle, it also has a medial surface
- Formed in its superior part by the medial surface of the thalamus and in its inferior part by the hypothalamus
- These two areas are separated from one another by a shallow sulcus, the hypothalamic sulcus
- Stria medullaris thalami
o Bundle of nerve fibers
o Afferent fibers to the habenular nucleus
o Forms a ridge along the superior margin of the medial surface of the diencephalons
The diencephalons can be divided into four parts:
- Thalamus
- Subthalamus
- Epithalamus
- hypothalamus
Thalamus
- Large ovoid mass of gray matter that forms the major part of the diencephalon
- It is a region of great functional importance
- Serves as a cell station to all the main sensory systems (except the olfactory pathways)
- Situated on each side of the third ventricle
Anterior end of thalamus
- Narrow and rounded
- Forms the posterior boundary of the interventricular foramen
Posterior end of thalamus
- Expanded to form the pulvinar, which overhangs the superior colliclus and the superior brachium
- Lateral Geniculate Body
o Forms a small elevation on the under aspect of the lateral portion of the pulvinar
Superior surface of the thalamus
- Covered medially by the tela choroidea and the fornix
- Laterally covered by the ependyma and forms part of the floor of the lateral ventricle,
- Lateral part is hidden by the choroids plexus of the lateral ventricle
Inferior surface
- Continuous with the tegmentum of the midbrain
Medial surface of the thalamus
- Forms the superior part of the lateral wall of the third ventricle
- Usually connected to the opposite thalamus by a band of gray matter, the interthalamic connection (interthalamic adhesion)
Lateral surface of the thalamus
- Separated from the lentiform nucleus by the very important band of white matter called, internal capsule
- The thalamus
- Very important cell station
- Receives the main sensory tracts 9excpet olfactory pathway)
- Station where much of the information is integrated and relayed to the cerebral cortex and many other subcortical regions
- Key role in the integration of visceral and somatic functions
Subthalamus
- Lies inferior to the thalamus
- Situated between the thalamus and tegmentum of the midbrain
- Craniomedially, it is related to hypothalamus
- Complex structure
- Among the collection of nerve cells found in the subthalamus are the:
o Cranial ends of the red nuclei and the substantia nigra
- Shape of a biconvex lens
- Nucleus connected with the corpus striatum and involved in the control of muscle activity
- Contains many important tracts that pass up from the tegmentum to the thalamic nuclei. Ex. Cranial ends of the medial, spinal, and trigeminal lemnisci
Epithalamus
- Consists of :
o Habenular nuclei and their connections
o Pineal gland
Habenular nucleus
- Small group of neurons situated just medial to the posterior surface of the thalamus
- Afferent fibers received from the amygdaloid nucleus in the temporal lobe through stria medullaris thalami
- Other fibers pass from hippocampal formation through the fornix
- Some reach habenular nucleus of opposite side forming habenular commissure
- Axons pass to the interpeduncular nucleus in the roof of the interpeduncular fossa, tetcum of midbrain, thalamus and reticular formation of midbrain
- Center for integration of olfactory, visceral and somatic afferent pathways
PINEAL GLAND (BODY)
- small, conical, structure attached to pineal stalk to the diencephalons
- projects backward so that it lies to the midbrain
- the base possesses a recess continuous with the cavity of the third ventricle
- superior part of the base of the stalk contains habenular commissure
- inferior part contains posterior commissure
- seen to be incompletely divided into lobules by connective tissue septa extending into the substance of the gland from the capsule
- 2 types of cells found in the gland:
o pinealocytes
o glial cells
- brain sands- concretions of calcified material progressively accumulating within the pineal gland with age
- no nerve cells
- adrenergic sympathetic fibers derived from the superior cervical sympathetic ganglia enter the gland and run in association with the blood vessels and pinealocytes
- influence activities of pituitary gland, islets of langerhans of pancreas, parathyroids, adrenals, and gonads
Functions of pineal gland
- pineal secretions actions are mainly inhibitory and either directly inhibit production of hormones or indirectly inhibit secretion of releasing factors by hypothalamus
- Pineal gland DOES NOT POSSESS A BBB
- Pineal activity exhibits a circadian rhythm influenced by light
- Gland has been found to be most active during darkness
- Probable nervous pathway from the retina
o 1st: runs to suprachiasmatic nucleus of hypothalamus
o 2nd tegmentum of midbrain
o 3rd pineal gland to stimulate its secretions
§ may include reticulospinal tract, sympathetic outflow of thoracic part of spinal cord, superior cervical sympathetic ganglion and postganglionic nerve fibers that travel to pineal gland and blood vessels
- Melatonin and enzymes present in high concentrations
o Released into blood and CSF of third ventricle and pass to anterior lobe of pit. Gland and inhibit release of gonadotrophic hormone
o In humans: Plasma melatonin level rises in darkness and falls during the day
- Regulation of reproductive function
HYPOTHALAMUS
- Part of the diencephalons that extends from the region of the optic chiasma to the caudal border of the mammillary bodies
- Lies below the hypothalamic sulcus on the lateral wall of the third ventricle
- Strategically well placed close to the limbic system, the thalamus, the ascending and descending tracts, and the hypophysis
- Microscopically, the hypothalamus is composed of small nerve cells arranged in groups or nuclei
- Physiologically, there is hardly any activity in the body that is not influenced by the hypothalamus
- Controls and integrates the functions of the ANS and ES and plays vital role in body homeostasis
- Involved in such activities as regulation of body temperature, body fluids, dives to eat and drink , sexual behavior, and emotion
Relations of the hypothalamus
- Anterior to hypothalamus is an area that extends forward from the optic chiasma to the lamina terminalis and anterior commissure, referred tp as the preoptic area
- Caudally, it merges into the tegmentum of the midbrain
- Superior
- When seen from below it is related to the following structures, from anterior to posteriot:
o Optic chiasma
o Tuber cinereum
o Infundibulum
o Mammillary bodies
OPTIC CHIASMA
- Flattened bundle of nerve fibers situated at the junction of the anterior wall and floor of the third ventricle
- Superior surface attached to lamina terminalis
- Inferiorly related to hypophysis cerebri, from which it is separated by the diaphragma sellae
- Anterolateral corners of chiasma are continuous with optic nerves and posterolateral corners with optic tracts
- Optic recess of the third ventricle, a small recess lies on its superior surface
- Fibers originating from the nasal half of each retina cross the median plane at the chiasma to enter the optic tract of the opposite side
TUBER CINEREUM
- convex mass of gray matter
- continuous inferiorly with the infundibulum
- Infundibulum
o hollow and continuous with the posterior lobe of the hypophysis cerebri
- Median eminence
o Raised part of the tuber cinereum to which is attached the infundibulum
o Neurohypophysis is formed by:
§ Median eminence
§ Imfundibulum
§ Posterior lobe (pars nervosa) of hypophysis cerebri
MAMMILLARY BODIES
- 2 small hemispherical bodies situated side by side posterior to the cinereum
- possess a central gray core of gray matter invested by a capsule of myelinated nerve fibers
- Posterior to mammillary bodies lies an area of the brain that is pierced by a number of small apertures and is called posterior perforated substance --- transmit the central branches of the posterior cerebral arteries
THIRD VENTRICLE
- derived from the forebrain vesicle
- slit-like cleft between the two thalami
- communicates anteriorly with the lateral ventricles through the interventricular foramina (foramina of Monro) posteriorly with the fourth ventricle through cerebral aqueduct
- has anterior, posterior, lateral, superior, and inferior walls and lined with ependyma
Anterior wall
- Formed by a thin sheet of gray matter, lamina terminalis, across which runs the anterior commissure
- Anterior commisure
o Round bundle of nerve fibers situated anterior to the anterior columns of the fornix ---they connect the right and left temporal lobes
Posterior wall
- Formed by the opening into the cerebral aqueduct
- Superior
- Superior
- Superior
Lateral wall
- Formed by the medial surface of the thalamus superiorly and the hypothalamus inferiorly
- These two structures are separated by the hypothalamic sulcus
- Lateral wall is limited superiorly by the stria medullaris thalami
- Joined by the interthalamic connection
Superior wall or roof
- Formed by the a layer of ependyma that is continuous with the lining of ventricle
- Superior
- Vascular tela choroidea projects projects downward on each side of the midline, invaginating the ependymal roof to form the choroids plexuses of the third ventricle
o Within tela choroidea lie the internal cerebral veins
- Superiorly the roof of the ventricle is relatd to the fornix and the corpus callosum
Inferior wall or roof
- formed by the optic chiasma, tuber cinereum, infundibulum, with its funnel-shaped recess, and the mammillary bodies
- Hypophysis is attached to the infundibulum
- Posterior to these structures lies the tegmentum of the cerebral peduncles
GENERAL APPEARANCE OF THE CEREBRAL HEMISPHERES
Cerebral Hemispheres
- largest part of the brain
- Separated by a deep midline sagittal fissure, the longitudinal cerebral fissure
o The fissure contains the sickle shaped fold of dura mater, the falx cerebri and the anterior cerebral arteries
- Corpus Callosum
o Connects the hemispheres across the midline
- Tentorium Cerebelli
o Second horizontal fold of dura mater
o separates the cerebral hemispheres from the cerebellum
- Folds or gyri
o Increase surface area of the cerebral cortex maximally
o Where the surface of each cerebral hemisphere is thrown
o Separated from each other by sulci or fissures
- Lobes
o Named accdng to the cranial bones under which they lie
- Boundaries used for the division of the cerebral hemisphere into frontal, parietal, temporal, and occipital lobes:
o Central and Parieto-occipital sulci
o Lateral and calcarine sulci
MAIN SULCI
Central Sulcus
- of great importance because gyrus that lies anterior to it contains motor cells that initiate movements of the opposite side of the body
- Posterior to it lies general sensory cortex that receives sensory information from the opposite side of the body
- Indents the superior medial border of the hemisphere about 0.4 inch (1 cm) behind the midpoint
- Runs downward and forward across lateral aspect of the hemisphere, and its lower end is separated from the posterior ramus of the lateral sulcus by a narrow bridge of cortex
- Only sulcus of any length on this surface of the hemisphere that indents superomedial border and lies between two parallel gyri
Lateral Sulcus
- Deep cleft found mainly on the inferior and lateral surfaces of the cerebral hemisphere
- It consists of a short stem that divides into three rami
- The stem arises in the inferior surface and on reaching the lateral surface divides into:
o Anterior horizontal ramus
o Anterior ascending ramus
o And continues as the posterior ramus
- Insula
o An area in cortex that lies at the bottom of the deep lateral sulcus
o Cannot be seen from the surface unless the lips of the sulcus are separated
Parieto-Occipital Sulcus
- Begins on superior medial margin of the hemisphere about 2 inches (5 cm) anterior to occipital pole
- Passes downward and anteriorly on the medial surface to meet the calcarine sulcus
Calcarine sulcus
- Found on the medial surface of the hemisphere
- Commences under the posterior end of the corpus callosum
- Arches upward and backward to reach the occipital lobe, where it stops
- In some brains, however it continues for a short distance onto the lateral surface of the hemisphere
- Joined at an acute angle by the parieto occipital sulcus about halfway along its length
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