the brain and cranial nerves

• The brain and spinal cord develop from the ectodermal neural tube
• Brain development


  1. Anterior part of neural tube expands

  2. Constrictions appear, creating three primary brain vesicles: prosencephalon, mesencephalon, rhombencephalon

  3. Prosencephalon and rhombencephalon subdivide further, forming secondary brain vesicles
• Adult brain structures derived from brain vesicles

  • Telencephalon (cerebrum and lateral ventricles)
  • Diencephalon (thalamus, hypothalamus, epithalamus)
  • Mesencephalon (midbrain and aqueduct of midbrain)
  • Metencephalon (pons, cerebellum, upper 4th ventricle)
  • Myelencephalon (medulla oblongata, lower 4th ventricle)

  
• Major parts of the adult brain

  • Brainstem
  • Cerebellum
  • Diencephalon
  • Cerebrum

  
• Brainstem


  • Continuous with spinal cord, consists of medulla oblongata, pons, midbrain
• Cerebellum


  • Posterior to brainstem

  
• Diencephalon


  • Superior to brainstem, consists of thalamus, hypothalamus, epithalamus
• Cerebrum


  • Largest part of brain, supported on diencephalon and brainstem

  
• The cranium and cranial meninges surround and protect the brain

  
• Layers of cranial meninges

  • Dura mater (outer)
  • Arachnoid mater (middle)
  • Pia mater (inner)
• Dura mater


  • Has two layers (periosteal and meningeal) that separate to enclose dural venous sinuses
• There is no epidural space around the brain
• Blood vessels enter brain tissue along the surface and are sheathed by pia mater
• Dura mater has three extensions that separate parts of the brain: falx cerebri, falx cerebelli, tentorium cerebelli
• Cerebrospinal fluid (CSF)


  Clear, colorless liquid that protects the brain and spinal cord
• CSF-filled cavities in the brain (ventricles)

  • Lateral ventricles
  • Third ventricle
  • Fourth ventricle
• Functions of CSF


  • Mechanical protection
  • Chemical protection
  • Circulation
• Brain blood flow is mainly via internal carotid and vertebral arteries, and dural venous sinuses drain into internal jugular veins
• The brain represents 2% of total body weight but consumes 20% of oxygen and glucose used by the body
• Blood-brain barrier (BBB)


  Tight junctions between endothelial cells of brain capillaries and thick basement membrane, allowing selective passage of substances
• Trauma, toxins, and inflammation can cause breakdown of the BBB
• Circulation of CSF


  1. Formed in choroid plexuses of lateral ventricles

  2. Flows into 3rd ventricle through interventricular foramina

  3. More CSF added by choroid plexus in 3rd ventricle

  4. Flows through aqueduct of midbrain into 4th ventricle

  5. Exits 4th ventricle into subarachnoid space around brain and spinal cord

  6. Reabsorbed into venous sinuses
• Blood-brain barrier


  Prevents the passage of helpful substances as well as those that are potentially harmful
• Moving drugs past the blood-brain barrier

  1. Drug is injected in a concentrated sugar solution

  2. High osmotic pressure of the sugar solution causes the endothelial cells of the capillaries to shrink

  3. This opens gaps between their tight junctions, making the BBB more leaky

  4. Allowing the drug to enter the brain tissue
• Cerebrospinal fluid (CSF)


  Fluid formed in the choroid plexuses of each lateral ventricle
• Circulation of CSF


  1. Flows into the third ventricle through two narrow, oval openings (interventricular foramina)

  2. More CSF is added by the choroid plexus in the roof of the third ventricle

  3. Flows through the aqueduct of the midbrain (cerebral aqueduct) into the fourth ventricle

  4. Choroid plexus of the fourth ventricle contributes more fluid

  5. CSF enters the subarachnoid space through three openings in the roof of the fourth ventricle

  6. CSF circulates in the central canal of the spinal cord and in the subarachnoid space around the surface of the brain and spinal cord
• Reabsorption of CSF


  1. CSF is gradually reabsorbed into the blood through arachnoid villi, fingerlike extensions of the arachnoid mater that project into the dural venous sinuses

  2. CSF is reabsorbed as rapidly as it is formed by the choroid plexuses
• Blood-cerebrospinal fluid barrier


  • Formed by tight junctions of ependymal cells covering the choroid plexuses
  • Permits certain substances to enter the CSF but excludes others, protecting the brain and spinal cord from potentially harmful blood-borne substances
• Abnormalities in the brain can interfere with the circulation of CSF from the ventricles into the subarachnoid space, causing hydrocephalus (excess CSF accumulation in the ventricles)
• Hydrocephalus can be relieved by draining the excess CSF, such as through endoscopic third ventriculostomy (ETV)
• Medulla oblongata


  • Contains all sensory (ascending) tracts and motor (descending) tracts that extend between the spinal cord and other parts of the brain
  • Contains nuclei that regulate vital body functions like heartbeat, blood vessel diameter, and breathing rhythm
  • Contains nuclei that control reflexes like vomiting, swallowing, sneezing, coughing, and hiccupping
• Pyramids


  Bulges on the anterior aspect of the medulla formed by the large corticospinal tracts that pass from the cerebrum to the spinal cord
• Decussation of pyramids


  Crossing of 90% of the axons in the left pyramid to the right side, and 90% of the axons in the right pyramid to the left side, explaining why each side of the brain controls voluntary movements on the opposite side of the body
• Olive


  Oval-shaped swelling lateral to each pyramid, containing the inferior olivary nucleus which regulates the activity of cerebellar neurons
• Gracile and cuneate nuclei


  Nuclei in the posterior medulla that receive sensory information from the spinal cord and relay it to the thalamus
• Medial lemniscus


  Band of white matter that extends through the medulla, pons, and midbrain, carrying sensory information from the gracile and cuneate nuclei to the thalamus
• Pons


  • Consists of ventral region with pontine nuclei that form a synaptic relay station, and a dorsal region with tracts connecting parts of the brain
  • Connects parts of the brain with one another, including the right and left sides of the cerebellum
• Transverse plane


  Anatomical plane that divides the body into superior and inferior parts
• Figure 14.6 shows the internal anatomy of the medulla oblongata

  
• Pyramids of the medulla


  • They contain the large motor tracts that run from the cerebrum to the spinal cord