Chapter 14 (1)

• Human brain


  • Extremely complex
  • Function associated clinically with what it means to be alive or dead
  • Importance not always well understood
• Aristotle thought brain just cooled blood, but Hippocrates had more accurate view of brain's importance
• Functions of the brain to be considered

  • Motor control
  • Sensation
  • Emotion
  • Thought
• Evolution of human central nervous system shows that spinal cord has changed very little, while brain has changed a great deal
• Greatest growth in areas of vision, memory, and motor control of the prehensile hand
• Rostral


  Toward the forehead
• Caudal


  Toward the spinal cord
• Brain weighs about 1,600 g (3.5 lbs) in men, and 1,450 g (3.17 lbs) in women
• Three major portions of the brain

  • Cerebrum
  • Cerebellum
  • Brainstem

  
• Cerebrum


  • 83% of brain volume
  • Cerebral hemispheres, gyri and sulci, longitudinal fissure, corpus callosum

  
• Cerebellum


  • Contains 50% of the neurons
  • Second largest brain region (10% of brain volume)
  • Located in posterior cranial fossa

  
• Brainstem


  • Portion of the brain that remains if the cerebrum and cerebellum are removed
  • Diencephalon, midbrain, pons, and medulla oblongata

  
• Longitudinal fissure


  Deep groove that separates cerebral hemispheres
• Gyri


  Thick folds
• Sulci


  Shallow grooves/squiggly lines
• Corpus callosum


  Thick nerve bundle at bottom of longitudinal fissure that connects hemispheres
• Cerebellum


  • Also has gyri, sulci, and fissures
  • Separated from cerebrum by transverse cerebral fissure
  • About 10% of brain volume
  • Contains over 50% of brain neurons

  
• Major components of Brainstem

  • Diencephalon
  • Midbrain
  • Pons
  • Medulla oblongata

  
• Gray matter


  • Seat of neurosomas, dendrites, and synapses
  • Dull color due to little myelin
  • Forms surface layer (cortex) over cerebrum and cerebellum
  • Forms nuclei deep within brain

  
• White matter


  • Bundles of axons
  • Lies deep to cortical gray matter, opposite relationship in the spinal cord
  • Pearly white color from myelin around nerve fibers
  • Composed of tracts, or bundles of axons, that connect one part of the brain to another, and to the spinal cord
• Meninges


  • Dura mater
  • Arachnoid mater
  • Pia mater
• Cranial dura mater


  • Has two layers: outer periosteal and inner meningeal
  • Layers separated by dural sinuses
  • Pressed closely against cranial bones, no epidural space
• Arachnoid mater


  • Transparent membrane over brain surface
  • Subarachnoid space separates it from pia mater below
  • Subdural space separates it from dura mater above in some places
• Pia mater


  Very thin membrane that follows contours of brain, even dipping into sulci
• Meningitis is inflammation of the meninges, a serious disease of infancy and childhood
• Meningitis can cause swelling of the brain, enlargment of the ventricles, and hemorrhage
• Signs of meningitis include high fever, stiff neck, drowsiness, and intense headache; may progress to coma then death within hours of onset
• Meningitis is diagnosed by examining the CSF obtained by lumbar puncture (spinal tap)
• Ventricles


  • Two lateral ventricles
  • Third ventricle
  • Fourth ventricle
• Interventricular foramen


  Tiny pore that connects lateral ventricles to third ventricle
• Cerebral aqueduct


  Runs through midbrain and connects third to fourth ventricle
• Choroid plexus


  Spongy mass of blood capillaries on the floor of each ventricle
• Ependyma


  Type of neuroglia that lines ventricles and covers choroid plexus, produces cerebrospinal fluid
• Cerebrospinal fluid (CSF)


  Clear, colorless liquid that fills the ventricles and canals of CNS, bathes its external surface
• Brain produces and absorbs 500 mL/day of CSF, with 100 to 160 mL normally present at one time
• CSF is produced by filtration of blood plasma through capillaries of the brain, with ependymal cells modifying the filtrate
• CSF flow


  1. Secreted in lateral ventricles, flows through intervertebral foramina into third ventricle

  2. Then down the cerebral aqueduct into the fourth ventricle

  3. Third and fourth ventricles add more CSF along the way

  4. Escapes through three pores: median aperture and two lateral apertures

  5. Leads into subarachnoid space of brain and spinal cord surface

  6. Reabsorbed by arachnoid granulations into superior sagittal sinus
• Functions of CSF


  • Buoyancy: allows brain to attain considerable size without being impaired by its own weight
  • Protection: protects the brain from striking the cranium when the head is jolted
  • Chemical stability: rinses away metabolic wastes and regulates chemical environment
• Brain is only 2% of adult body weight, but receives 15% of the blood (750 mL/min)
• A 10-second interruption of blood flow may cause loss of consciousness, 1-2 minutes can cause significant impairment, and 4 minutes without blood causes irreversible brain damage