Neuroanatomy Transcriptions - Cerebrum Part I

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:

1. Thalamus
2. Subthalamus
3. Epithalamus
4. 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 1^st: runs to suprachiasmatic nucleus of hypothalamus

o 2^nd tegmentum of midbrain

o 3^rd 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 to the hypothalamus lies the thalamus and inferolaterally is the subthalamic region

- 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 to this opening is the small posterior commissure

- Superior to the commissure is the pineal recess—which projects into the stalk of the pineal body

- Superior to the pineal recess is the small habenular commissure

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 is a two-layered fold of pia mater called tela choroidea of the third ventricle

- 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