Chapter 2: Overview of the Brain Notes

Brain Overview

Basic Brain Formations

  • From a phylogenetic perspective, the encephalon contains the following formations:
    • Mielencephalon (medulla oblongata)
    • Metencephalon (pons Varolio and cerebellum)
    • Mesencephalon (midbrain)
    • Diencephalon (diencephalon)
    • Telencephalon (cerebral hemispheres)

Cerebral Hemispheres

  • The telencephalon consists of two hemispheres united at the base by white matter formations:
    • corpus callosum
    • cerebral trigon (fornix)
    • anterior and posterior white matter commissures.
  • In the upper part, the hemispheres are free, intersected by the interhemispheric fissure.
  • The hemispheres have three faces:
    • external or convex, which relates to the cranial vault.
    • medial - facing the interhemispheric fissure.
    • basal - related to the base of the skull.
  • The faces of the hemispheres are furrowed by fissures:
    • Rolando's fissure (central).
    • Silvius fissure (lateral).
    • parieto-occipital fissure (least deep).
  • These fissures divide the hemispheres into 4 lobes: frontal, parietal, temporal, occipital.
  • There is also a lobe called the insula, which is not visible on the surface of the hemispheres, being located in the depth of the lateral Sylvius fissure.
External Convex Face of Cerebral Hemispheres
  • Gyri of the cerebral hemispheres:
    • Frontal lobe: superior frontal gyrus, middle frontal gyrus, inferior frontal gyrus, precentral gyrus.
    • Parietal lobe: superior parietal lobule, inferior parietal lobule, supramarginal gyrus, angular gyrus, postcentral gyrus.
    • Temporal lobe: superior temporal gyrus, middle temporal gyrus, inferior temporal gyrus.
    • The occipital lobe includes the occipital pole.
Medial Face of Hemispheres
  • Located on the medial face:
    • superior frontal gyrus
    • paracentral lobule
    • precuneus and cuneus formations
    • parieto-occipital gyrus
    • cingulate gyrus
    • parahippocampal gyrus
    • dentate gyrus
    • uncus formation
    • lateral and medial occipitotemporal gyri.
Basal Face of Hemispheres
  • Located on the basal face:
    • orbital gyri
    • straight gyri
    • olfactory bulbs
    • olfactory tracts
    • inferior temporal gyri
    • parahippocampal gyrus
    • uncus formation
    • isthmus of the cingulate gyrus
    • lateral and medial occipitotemporal gyri
    • occipital pole.
Brodmann Areas
  • Brodmann areas are numbered regions of the cerebral cortex that have been associated with different functions.

Overall Organization of the Nervous System

  • The overall organization of the nervous system from a topographical perspective:
    • Central Nervous System (CNS).
    • Peripheral Nervous System (PNS).
Central Nervous System (CNS)
  • Integrates and controls the entire nervous system.
  • Receives information (afferents) from the environment.
  • Processes the information received.
  • Provides signals (efferents) for carrying out response reactions.
Peripheral Nervous System (PNS)
  • Connects the central nervous system with the organs and tissues of the body.
  • The somatic nervous system consists of:
    • 12 pairs of cranial nerves.
    • 31 pairs of spinal nerves.
Autonomic Nervous System (ANS)
  • Controls all the autonomous functions (activity of internal organs) of the body and consists of:
    • Sympathetic ANS: prepares the body for "fight or flight."
    • Parasympathetic ANS: creates the conditions for the body to relax after exertion.

Meninges of the Neuraxis

  • The entire CNS is covered by meninges, containing 3 membranes:
    1. Dura mater – peripheral fibroelastic layer made up of two layers, with a protective role.
    2. Arachnoid – middle transparent membrane that sends bridges to the pia mater, forming numerous spaces called areolae filled with cerebrospinal (cerebrospinal) fluid.
    3. Pia mater – elastic membrane, very fine, made of connective tissue, which is abundantly vascularized. This membrane molds to the neuraxis, and at the level of ventricle III it forms dilatations called choroid plexuses, which contain specific cells that secrete cerebrospinal fluid. The pia mater mainly has a nutritional role.
Spaces of the Meninges
  • Between the cranial box and the dura mater - epidural space.
  • Between the dura mater and the arachnoid – subdural space.
  • Between the arachnoid and pia mater – subarachnoid space.

Cerebral Ventricles: Cerebrospinal Fluid and the Blood-Brain Barrier

  • In addition to nervous tissue formations, the CNS also contains cerebral ventricles, of which there are four:
    • 2 lateral ventricles
    • 3rd ventricle (V3)
    • 4th ventricle (V4).
  • The first two ventricles are the largest, shaped like horns, located in the cerebral hemispheres.
  • V3 is located at the fusion between the hemispheres and normally has a diameter of 6-7 mm.
  • V3 is connected to V4 through the cerebral aqueduct Sylvius.
  • V4 is located at the level of the pons Varolio and cerebellum, has a rhomboid shape, and the cerebrospinal fluid is transported to the ependymal canal located in the center of the spinal cord.
Cerebrospinal Fluid
  • The amount of cerebrospinal fluid in humans is 120-150ml, most of which is in the subarachnoid space, and 20-40 ml in the ventricles.
  • Cerebrospinal fluid:
    • transparent and colorless.
    • Chemical structure: glucose, proteins, mineral salts, hormones, etc.; does not contain enzymes and antibodies.
  • Functions of cerebrospinal fluid:
    1. Regulates osmotic pressure in the brain.
    2. Has a protective function, acting as a hydraulic cushion for nervous tissue.
    3. Serves as a nutritive medium for the CNS.
    4. Eliminates disintegration products into the lymphatic and venous system.
Blood-Brain Barrier
  • The blood-brain barrier is a membrane formed from the capillaries of blood vessels and neuroglial cells.
  • It is located between the blood environment and the cerebrospinal fluid.
  • It was discovered after investigations of both cerebral environments, where the substances and their concentrations were too different.
  • It has been found that iodine compounds, salts of nitric acid, salts of salicylic acid, penicillin, streptomycin, etc. do not cross the blood-brain barrier.
  • Substances such as alcohol, drugs, and toxins easily cross this barrier.