II. Neurological Neuroanatomy

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II. Neurological Neuroanatomy

• Lecture 1

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A. Protection and Nourishment

• In addition to the bony protective housing of the
  brain and spinal cord, we have a layered system
  of membranes called the meninges that cover them.
• Within certain layers of these meninges, there is
  a cushioning layer of fluid called cerebrospinal
  fluid (CSF).

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1. The Meninges

• The meninges are coverings that run continuously
  around the spinal cord and brain.
• Those associated specifically with the cord are
  known as the spinal meninges.
• Those associated specifically with the brain are
  known as the cranial meninges.

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1. The Meninges

• The outer cranial and spinal meninx is called the
  dura matertough mother.
• The dura mater forms a tube from the level of the
  second sacral vertebra to the foramen magnum,
  where it is continuous with the dura mater of the
  brain.

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1. The Meninges

• In the spinal meninges, between the dura mater
  (3) and the wall of the vertebral canal is the
  epidural space (4), which is filled with fat,
  connective tissue and blood vessels.

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1. The Meninges

• The spinal epidural space serves as padding
  around the cord and also as the site for
  injection of anesthetics.

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1. The Meninges

• Unlike the spinal dura mater, the cranial dura
  mater is marked by complex folds that divide the
  contents of the cranial cavity into different
  cerebral subdivisions.

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1. The Meninges

• These folds are the falx cerebri, between the
  cerebral hemispheres, and the tentorium
  cerebelli, between the superior surface of the
  cerebellum and the inferior occipital lobes.
• These folds actually serve to brace the brain
  against rotary displacement.

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1. The Meninges

• The cranial dura mater (green) actually consists
  of two layers that are closely united except in
  certain regions where they separate to form the
  venous sinuses (dark blue).

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1. The Meninges

• The thicker, outer layer, the endosteal layer,
  adheres tightly to the cranial bones and serves
  as periosteum (for bone growth, repair, and
  nutrition).
• The thinner, inner layer, the meningeal layer,
  corresponds to the spinal dura mater.

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1. The Meninges

• Under normal circumstances, there is not an
  epidural space between the skull (cranium) and
  the dura mater.
• In this slide, a bleed from a dural vessel
  resulted in a epidural hematoma.

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1. The Meninges

• The middle meninx is the arachnoid (purple),
  lying deep to the dura.

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1. The Meninges

• The arachnoid mater is a delicate connective
  tissue membrane that does not contain blood
  vessels.

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1. The Meninges

• This membrane forms a tube inside the spinal dura
  mater, and it bridges over the sulci of the brain
  (it does not dip into those shallow grooves).

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1. The Meninges

• In some areas, fingerlike projections of the
  arachnoid, called arachnoid villi, push into the
  dural venous sinuses, especially the superior
  sagittal sinus, to form the arachnoid
  granulations where cerebrospinal fluid diffuses
  into the blood stream.

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1. The Meninges

• The inner meninx is known as the pia mater, or
  delicate mother.
• This transparent fibrous membrane adheres closely
  to the surface of the spinal cord and brain.
• It contains numerous blood vessels.

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1. The Meninges

• It also fuses with the ependyma of the ventricles
  to form the choroid plexuses of the ventricles.

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1. The Meninges

• Between the arachnoid mater and the pia mater is
  the subarachnoid space.
• This is the area through which cerebrospinal
  fluid circulates.

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1. The Meninges

• In addition to CSF, cerebral arteries and veins,
  as well as cranial nerves (spinal nerves) pass
  through this space.

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2. The Ventricular System

• The ventricles are located deep in the brain and
  are derived embryologically from the central
  canal.

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2. The Ventricular System

• Cerebrospinal fluid is formed within the
  ventricles by the choroids plexuses.
• It circulates through the ventricular system,
  into the subarachnoid space, before it is removed
  by the arachnoid villi into the venous system.

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2. The Ventricular System

• There is a lateral ventricle in each cerebral
  hemisphere that communicates with third
  ventricle, located medially, by a narrow, oval
  opening, called the interventricular foramen
  (left and right foramina of Monro).

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2. The Ventricular System

• The cerebral aqueduct (aqueduct of Silvius)
  courses from the 3rd ventricle down the length of
  the midbrain and connects with 4th ventricle,
  dorsal to the pons and medulla, but ventral to
  the cerebellum.

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2. The Ventricular System

• From the roof of the 4th ventricle, CSF exits the
  ventricular system to enter subarachnoid space of
  the brain and spinal cord.
• These openings are called the median aperture of
  Magendie (arrow) and the two lateral apertures of
  Lushka.

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2. The Ventricular System

• The entire CNS contains 3-5 oz of CSF.
• CSF is a clear colorless liquid of watery
  consistency, containing proteins, glucose, urea,
  and salts.
• CSF serves two principle functions protection
  and circulation.

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2. The Ventricular System

• The fluid serves as a shock-absorbing medium to
  protect the brain and spinal cord from jolts that
  would otherwise cause them to crash against the
  bony walls of the vertebral and cranial cavities.
• The fluid also buoys the brain so that it
  floats in the cranial cavity.

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2. The Ventricular System

• Its circulatory function is nutritivesubstances
  filtered from the blood are delivered by the CSF
  to the brain and spinal cord.
• It also removes waste and toxic substances
  produced by the brain and spinal cord cells.

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2. The Ventricular System

• Hydrocephalus is a condition that results from
  impaired circulation or re-absorption of the CSF.
• The result is dilation of the ventricles and
  compression of nearby brain structures.

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2. The Ventricular System

• The relationship between the CSF producing
  choroid plexuses and the network of blood
  capillaries is the blood-CSF barrier that permits
  certain substances to enter the fluid but
  prohibits others.
• Infections in the CNS, neoplasm, or bleeding, can
  upset the blood-CSF barrier and produce changes
  in the composition of the CSF.

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3. The Vascular Supply

•  The basic arterial blood supply to the brain can
  be divided into an anterior and posterior system.

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3. The Vascular Supply

• The anterior system begins with the paired
  internal carotid arteries.
• They ascend in the neck and enter the base of the
  brain at the carotid canal of the temporal bone.

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3. The Vascular Supply

• Once through the temporal bone, the internal
  carotid arteries bifurcate into the anterior and
  middle cerebral arteries.
• The anterior cerebral arteries supply mostly
  medial structures in each hemisphere.

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3. The Vascular Supply

• The right and left anterior cerebral arteries
  communicate with each other via the anterior
  communicating artery.
• Each middle cerebral artery supplies most of the
  lateral surface of each hemisphere through its
  extensive collaterals.

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3. The Vascular Supply

• The posterior system begins with the paired
  vertebral arteries, each of which is a branch of
  the subclavian artery.
• The vertebral arteries ascend through the
  cervical vertebrae and enter the brain through
  the foramen magnum.

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3. The Vascular Supply

• They continue to ascend along the ventrolateral
  sides of the medulla.
• At the junction of the pons and medulla, the two
  vertebral arteries merge into a single basilar
  artery.

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3. The Vascular Supply

• The basilar artery continues to ascend to the
  level of the midbrain, where it bifurcates into
  the left and right posterior cerebral arteries.

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3. The Vascular Supply

• The posterior cerebral arteries give off numerous
  branches that collectively supply the brainstem,
  parts of the diencephalon, the cerebellum, and
  parts of the posterior cerebral hemispheres.

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3. The Vascular Supply

• Similar to what exists with the anterior cerebral
  arteries, the posterior cerebral arteries
  communicate with each other via the posterior
  communicating artery.

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3. The Vascular Supply

• The cerebral arterial circle or Circle of Willis
  is formed by the union of anterior and posterior
  cerebral arteries.

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3. The Vascular Supply

• Specifically, the posterior cerebral arteries are
  connected with the internal carotid arteries by
  the posterior communicating artery.
• The anterior cerebral arteries are connected by
  anterior communicating arteries.

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3. The Vascular Supply

• The joining of these vessels is termed an
  anastomosisend to end union of two vessels.
• The function of the cerebral arterial circle is
  to equalize blood pressure to the brain and
  provide alternative routes for blood to the brain
  should arteries become damaged.

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3. The Vascular Supply

• Blood vessels that enter brain tissue first pass
  along the surface of the brain.

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3. The Vascular Supply

• As they penetrate inward, they are surrounded by
  a loose fitting layer of pia mater.

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3. The Vascular Supply

• The space between the penetrating vessel and the
  pia mater is called the perivascular space.

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3. The Vascular Supply

• The blood-brain barrier (BBB) is the specialized
  system of capillary endothelial cells that
  protects the brain from harmful substances in the
  blood stream, while supplying the brain with the
  required nutrients for proper function.
• Unlike peripheral capillaries that allow
  relatively free exchange of substance across/
  between cells, the BBB strictly limits transport
  into the brain through both physical (tight
  junctions) and metabolic (enzymes) barriers.

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3. The Vascular Supply

• Thus the BBB is often the rate-limiting factor in
  determining permeation of therapeutic drugs into
  the brain.
• Additionally, BBB breakdown is theorized to be a
  key component in central nervous system (CNS)
  associated pathologies.