Organization (Embryogenesis) Ventricular System Blood Supply Blood-Brain-Barrier Membranous Structure (Mater)

Internal Anatomy Neurofibrology Special Systems

The BRAIN

Organization of the Brain..

Development & Architecture

During embryonic differention of the neural tube, the cranial end expands into three vesicles:

Prosencephalon (forebrain)

Mesencephalon (midbrain)

Rhombencephalon (hindbrain)

The Proencephalon further evaginates dorsally and laterally into the forebrain, the Telencephalon

The remaining portion of the Proencephalon then becomes the Diencephalon....the rostral-most aspect of the brainstem

The Mesencephalon does not change

The Rhombencephalon (hindbrain) expands rostrally to form the Metencephalon (which will become the Cerebellum and the Pons) and caudally to form the Myelencephalon (to become the Medulla of the brainstem)

Functional Divisions of the Brain**

Embryological Nomenclature		Anatomical Nomenclature
Proencephalon------------>	Telencephalon-------------->	Cerebral Cortex/Subcortical  Nuclei
	Diencephalon--------------->	Thalamus, Hypothalamus, Epithalamus
Mesencephalon------------>	Unchanged	-Midbrain
Rhombencephalon-------->	Metencephalon------------->	Cerebellum*, Pons
	Myelencephalon------------>	Medulla oblongata

** Orange Text denotes Brainstem Components

* Cerebellum is considered separate from brainstem

Schematic of Brain Embryogenesis



From deLahunta (Back to Brain)

The Ventricular System

Organization

• Cerebral spinal fluid (csf)surrounds, permeates and nourishes the entire central nervous system. This fluid is made in special sites including the choroid plexi within the ventricular system. There are four ventricles connected via "rivulets" ("aquaducts?", "canals"?).
• There are two C-shaped ventricle bodies lateral and somewhat dorsal to the corpus collosum, each in proximity to a cerebral cortical hemisphere. In addition to csf, the lateral ventriculi contain the hippocampus and fornix, which are important components of the limbic system (emotion?).
• These lateral ventriculi connect with the third ventricle then, via the aquaduct of sylvius, to the fourth ventricle. The figure below illustrates the approximate anatomical relationship of these structures.
• 

Flow of CSF

• CSF is produced via plasma ultrafiltration from vascular endothelium through basement membrane and active transport across ependymal cells of choroid plexi. Production is constant but decreases with increased osmotic pressure (not hydrostatic ---unless severely elevated hydrostatic pressure results in choroid plexus atrophy) of blood.
• Movement of CSF is "powered" by pulsatile pressure waves as blood passes through vessels of the choroid plexus system.
• Circulation is from ventricles, out aperatures emanating from lateral recesses of fourth ventricle to subarachnoid space, around cerebellum and dorsal sinuses, perivascular spaces, spinal cord.
• CSF bathes and nourishes all CNS tissues, as it penetrates deeply into parenchyma.

The Blood Supply

Ventral View: ( partial visualization of blood supply.click on Red text heading to see image...click again to hide)

Doral-Lateral-Sagital View:( partial visualization of

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blood supply.. click on Red text heading to see image...click again to hide)

The origin of the primary blood supply to the brain is the common carotid artery off the brachycephalic trunk. This divides at--or near---the hyoid bone, medial to the retropharyngeal lymph node into external and internal branches

The internal carotid runs through the deep structures of the head in a somewhat circuitous fashion to the brain. In addition, a substantial collateral blood

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source is the basilar artery. This is derived from branches of the verterbral artery system, forming the ventral spinal artery.

The internal carotid supplies several cerebral arteries, and, eventually, the arterial circle (of Willis) on the ventral surface. The circle gives rise to the rostral cerebellar artery; the caudal cerebellar artery is derived from the the basilar artery which also supplies branches to the brainstem (pons and medullar).

The middle cerebral artery--a large terminal branch of the internal carotid artery--gives rise to the choroid artery, supplying the choroid plexus of the lateral ventricles. In addition, divisions of the cerebral artery supply the lateral suface if the cerebral hemispheres and the deeper tissues, including the basal nuclei

The Blood-Brain-Barrier (BBB)

• Brain capillary epithelium is unfenestrated and cells are joined via tight junctions
• Communication and movement of substances from blood to brain requires specific and exquisite transport mechanisms
• Disruption of the BBB occurs with: inflammation, trauma, hypertension, space-occupying masses, hyperosmotic conditions
• Absence of a BBB (normallly) occurs in the Circumventricular Organs:( red is clickable toggle image!)
  • Area Postrema:
    •  noxious substances in blood are "scensed" here (the chemoreceptor trigger zone) and pathways from the crtz to the vomit centers result in vomition.
    • mediation of learned taste aversions originates here.
    • in the dog, this portion of the brain may mediate the pressor effects of angiotensin
  • Neurohypophysis: the posterior pituitary
  • Pineal: is involved in the regulaton of circadian and seasonal behavioral/hormonal cycles
  • Medial Eminence: somehow involved in neuroendocrine feedback mechanism
  • Subfornical Organ and Organ vasculasum of the lamina terminalis: are involved in the angiotensin recognition-blood pressure regulation system and participate in fluid homeostasis.
  • Subcommissural Organ: function is incompletely understood.

So..What's the "Mater"?

The brain is encased in a tri-membranous structure consisting of an outer dura mater, an

inner pia mater and a delicate reticular-fibrous network in between these: the arachnoid. These continue over the spinal cord and nerve roots. They can be further described:

• Dura

• Is, itself, bi-layered
  • an outer, periosteal layer that is adherent to the inner boney cranial vault
  • an inner meningeal layer that is line with flat cells
• The periosteal portion is rich in blood vessels and nerves
• The meningeal layer separates and reflects from the periosteal layer at certain sites to form large venous sinuses and to facilitate division of the cerebral hemisheres (falx cerebri) and the latter from the cerebellum (tentorium cerebelli)

Pia

• A thin, vascular membrane consisting of intima pia (inner portion) and epipia (superficial portion)
• These are adherent to and generally tightly follow the conformation of each sulcus and gyrus of the brain
• The intima pia is composed of fine elastic fibers.
  • it's nutrition is derived primarily from cerebral spinal fluid (CSF)
  • fine blood vessels in the brain lie on the surface of the intima (in the spinal cord, these vessels are more closely related to the epipial layer)
  • at sites where blood vessels enter or leave the brain, the intima invaginates to accomodate
• The epipial layer is a meshwork of collagen fibers that are continuous with the arachnoid.

Arachnoid

• A delicate membrane between the dura and pia mater layers that is composed of reticular trebeculae network
• Together with the pia...is known as the leptomeninges
• The subarachnoid space varies in size...narrow over the cerebral hemishperes (except over sulci), larger over the brainstem
• The largest separations of pia and arachnoid are called cisternae; the largest...between the medulla and the cerebellum (the cerbellomedullary cisterna)...receives CSF fromt he ventricular system via medial and lateral foramina of the fourth ventricle
• Arachnoid "granulations" are protuberances (villi) through the meningeal layer of the dura. Thes are thought to be a major site of transfer of CSF from the subarachnoid space to the venous system.

Internal Anatomy

Grey Matter:

Nuclei: regions in which cell bodies are grouped together. Associated with the brain stem and spinal cord, the alar plate, represents the dorsal aspect of the primitive neural tuble and contains columns of afferent sensory fibers and cell bodies (GSA, SSA, SVA, GVA). The ventral aspect of the neural tube, the basal plate, contains cell bodies and fibers associated with efferent pathways (GVE, GSE, SVE)

Cortices: layers or sheets of cell bodies located on the exterior ("bark"). Is especially abundant in the cerebral cortex (among the gyri and sulci...or in some species...the smooth surface) and the cerebellum.

Reticular Formation: a loosely organized network of cell bodies and fibers (mixed grey and white matter) comprising the central core from rostral medulla to the midbrain.

White Matter:

Tracts and Pathways: are primarily efferent processes of cell bodies grouped into well demarcated areas of brain and cord ("fasciculi"). The (myelinated or unmyelinated) neuron fibers in white matter emanate from or project to cell bodies in the grey matter. These comprise the pathways-or tracts- upon which much of clinical neuroanatomical diagnostics and clinical manifestations are dependent.

Tracts can be divided as follows:

Long ascending, long decending, short, commissural*.

Clinically important representations of each of these will be provided on a linked page on this website...the cell bodies, the tracts/pathways, the clinical manifestations and diagnostic approach!

*Commissural: are bundles of axons that connect (via "tracts" or "pathways") regions of the brain accross the midline

Neurofibrology: definitions...

The types of neurons associated with neuronal functionality are categorized as follows:

Afferent:

GSA (General Somatic Afferent): these are sensory fibers associated with touch, pressure, temperature, pain and proprioception

SSA (Special Somatic Afferent): these fibers are related to the senses of vision and hearing

SVA (Special Visceral Afferent): these fibers are related to the senses of smell and taste

GVA (General Visceral Afferent): these fibers are sensory to viscera and blood vessels

Efferent

GVE (General Visceral Efferent): motor innervation to muscles of the viscera and blood vessels...associated with the autonomic system.

SVE (Special Visceral Efferent): motor innervation of skeletal muscle derived from the branchial arches...these include the jaw, face pharynx, larynx esophagus and the trapezius.

GSE (General Somatic Efferent): innervation of skeletal muscle of the trunk and limbs and non-branchial muscles of the head (extraocular and tongue)

Special Systems:

Selected systems to be decribed in limited fashion in this section are the vestibular and visual systems (which have peripheral components that connect to the brain). Other specialized systems, such as the chemoreceptor trigger zone, vomit center, hypothalamus (approximate structure and function), connections to the pituitary gland and other important special systems, e.g respiratory, cardioregulatory centers and the autonomic systems, will be described in the appropriate sections that focus upon the brainstem .. Some of this information may require review of the Tracts & Pathways portion of the Neuroanatomy discussion (also under construction, as of 10/01).

Vestibular System.: 

• General Comments:
  • comprised of a reflex mechanism for the maintenance of posture and equilibrium
  • works in conjunction with the visual pathways (which provides cues of orientation in space and position), with input from labyrinthine system located in the inner ear (see below).
  • the input from these, modulated by the cerebellum, establishes appropriate muscle and tendonous responses (to correct for changes) resulting from these perceived changes in posture and movement.
• The Labyrinthine Organs:
  • the schematic diagram crudely illustrates the important components of the labyrinthine sytem
  • the static labyrinths include the utricle and saccules; these signal the brain as to the stationary position of the head in space; the kinetic labyrinths consist of three semicircular canals and are responsible for signaling the time-related movements of head in space..this system involves adjusting the position of the eyes to preserve visual orientation during movement.
  • The physiologic/neurologic mechanism whereby this postural and equilibrium data are detected by the semicircular canals and utricle/saccule organs and transmitted to the appropriate areas of the brain is nothing less than fascinating (author's opinion)
    • semicicular canals, utricles, saccules are interconnected, fluid-filled sacs and ducts that are enclosed within the .petrous bone
    • this "membranous labyrinth" is filled with "endolymph"...a fluid that is likened to intracellular fluid
    • the space around the endolymph-ladened structures contains "perilymph", likened to cerebral spinal fluid.
    • the sensory receptors within these structures are special hairs..these derive from sensory neurons and are innervated via the vestibular portion of cranial nerve VIII.
    • With changes in position and movement, the endolymph is moved within the membranous labyrinth; this movement causes the hair to bend accordingly. The bending, depending on direction and velocity, either excites or inhibits the affector pathways of the vestibular nerve. The change in afferent impulse is transported to the central vestibular components in the brainstem and cerebellum where information is assessed and responses are transmitted rostrally, along the medial longitudinal fasciculus to the motor pathways of the extraocular muscles, resulting in the adjustment of eye movement and positions, and caudally, along appropriate tracts to faciltate or inhibit appropriate extensor and/or flexor muscle groups to maintain balance and position.

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