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Telencephalon (Cerebrum)

Cerebral Sulci And Gyri


There are four types of sulci: - 

  1. Axial sulci:
    Axial Sulci develop along the long axis of rapidly growing homogeneous areas. They are longitudinally enfolded. Posterior part of calcarine sulcus is axial sulcus, since visual cortex grows along it.
  2. Limiting sulci:
    Limiting sulci develop along planes separating cortical areas, which differ in functions they predominantly sub serve. Central sulcus is an example of limiting sulcus; because it separates the motor cortex in front from the sensory cortex behind Anterior part of Calcarine sulcus is also acts as limiting Sulcus, which separates the visual cortex below from the isthmus between the cingulate and parahippocampus gyri.
  3. Operculated sulci:
    Operculated sulci are similar to limiting sulcus in that it separates structurally and functionally different areas but transition occurs at the lip and not the floor. The lunate sulcus is an example of operculated sulcus, because it separates the striate and peri striate areas at the surface of occipital pole.
  4. Complete sulci:
    A sulcus that is deep enough to produce an elevation in the wall of ventricle is called complete Sulcus. Collateral sulcus and anterior part of calcarine sulci are complete, because the former produces the 'collateral eminence' in the floor of the inferior horn of lateral ventricle, and the later forms the' calcar avis' of the posterior horn of the lateral ventricle.

Functional areas of cerebral cortex




Frontal lobe has following functional areas-

  1. Primary motor cortex (area 4): Located in precentral gyrus and anterior part of paracentral lobule.
  2. Premotor cortex (areas 6, 8): Located anterior to precentral gyrus on lateral surface.
  3. Frontal eye field (area 8, 9): Located in posterior part of middle frontal gyrus.
  4. Broca's motor speech area (area 44, 45: It is located in posterior part of inferior frontal gyrus in the dominant hemisphere (left hemisphere in 90% individuals).
  5. Prefrontal area (areas 9, 10, 11, 12): Located in anterior part of frontal lobe.

Parietal lobe consists;-

  1. Primary somatosensory cortex (areas 3, 1, 2): Located in postcentral gyrus and posterior part of paracentral lobule.
  2. Somatosensory association areas: These areas are ;-
    1. Superior parietal lobule (areas 5, 7)
    2. Supramarginal gyrus (area 40)
    3. Angular gyrus (area 39)

Temporal lobe contains -

  1. Primary auditory area (areas 41, 42): Located in the anterior part of transverse temporal gyri of Heschl, and posterior part of superior temporal gyrus.
  2. Auditory association cortex (secondary auditory area or area 22): It also includes Wernicke’s speech area and is located in the posterior part of superior temporal gyrus.

Occipital lobe has-

  1. Primary visual cortex (strike area, area 17): It is present in medial surface of occipital lobe on upper lip and lower lip of calcarine sulcus.
  2. Visual association areas (secondary visual area, areas 18, 19): It includes parastriate (area 18) and peri striate (area 19) areas of occipital lobe. It also contains occipital eye field on lateral surface.

In somatosensory area and primary motor area, the whole body is represented upside down (vertical)


White matter of cerebrum


White matter of cerebrum hemisphere is divisible into 3 fiber systems-

  1. Association fibers: Connect the one functional area of one cerebral hemisphere to other in the same cerebral hemisphere.
  2. Projection fibers: Connect cerebral cortex to other parts of brain and spinal cord.
  3. Commissural fibers: Connect same areas between the two cerebral hemispheres.

Association fibers

Projection fibers

Commissural fibers

1.  Cingulum

2.  Superior longitudinal fasciculus

3.  Inferior longitudinal fasciculus

4.  Uncinate fasciculus

1.  Fimbria

2.  Fornix

3.  Corona radiata

4.  Internal capsule: It is a compact V-shaped band   of neocortical projection fibers.

1. Anterior commissure

2. Hippocampal commissure

3. Habenular commissure

4. Posterior commissure

5. Corpus callosum




Internal capsule


The internal capsule forms the main highway for the input and output fibers of the cerebral cortex. It is V-shaped on horizontal section with concavity directed laterally. The internal capsule is divided from before backwards into following parts: anterior limb, genu, posterior limb, sub lentiform part and retrolentiform part.




Part of internal


Cortical afferents

(ascending tract)

Cortical efferents

(descending tract)

Arterial supply

Anterior limb

o Anterior thalamic radiation

o Frontopontine fibers

o Corticostriate fibers

o Striate branch of anterior cerebral

o Recurrent branch of anterior

cerebral (Heubner's artery)

o Striate branch of middle

cerebral (Charcot's artery)


o Anterior part of superior

thalamic radiation

o Corticonuclear fibers

o Internal carotid artery (direct


o Recurrent branch of anterior


o Striate branch of middle cerebral

Posterior limb

o Superior thalamic radiation

o Fibres from globus pallidus

to subthalamic nucleus

o Corticospinal tract

o Corticopontine fibers

(frontopontine and


o Corticorubral tract

o Few corticostriate fibres

o Middle cerebral artery

(Lateral striate and medial striate


o Anterior choroidal artery

o Posterior cerebral artery

(posterolateral branch)

Retrolentiform part

o Optic radiation

o Posterior thalamic radiation

o Fibers from thalamus to

parietal and occipital lobe

o Parietopontine fibres

o Occipitopantine fibres

o Occipitotectal fibres

o Occipitocollicular fibres

o Posterior cerebral artery



o Auditory (acoustic)


o Internal thalamic radiation

o Temporopontine fibres

o Parietopontine fibers

o Posterior cerebral artery

o Anterior choroidal artery


Corpus callosum


Corpus callosum is the largest commissure and the largest fibre pathway of brain. It connects homotopic (symmetrical) and heterotopic (anatomically different but functionally similar) areas of two hemispheres. It connects all parts of two cortexes except lower and anterior temporal lobes which are connected by anterior commissure.




Corpus callosum is around 10 cm long. It has four parts from behind forwards: Splenium, trunk (body), genu and rostrum. The splenium is the enlarged posterior end and lies 6 cm in front of occipital pole. The genu lies 4 cm behind the frontal pole.

  1. Fibres passing through rostrum connect orbital surface of both hemispheres.
  2. Fibres of genu (forceps minor) connect medial and lateral surfaces of frontal lobes.
  3. Fibres passing through the trunk (body) are projected bilaterally to wide areas of cerebral cortex, and most of them are intersected by projection fibres of corona radiata.
  4. Fibers passing through splenium (forces major) connect the occipital lobes of two hemispheres


Diencephalon (interbrain) along with telencephalon (cerebrum) constitutes the prosencephalon (forebrain). Third ventricle is the cavity of diencephalon. Diencephalon consists of thalamus, hypothalamus, metathalamus (including medial and lateral geniculate bodies), epithalamus and subthalamus.



  1. The two thalami are ovoid masses of grey matter lying close together rostrally and joined in the midline by the mass intermedia and separated only by the third ventricle. The thalamus is a sensory relay and integrative center connecting with many areas of brain, including the cerebral cortex, basal ganglia, hypothalamus and brain stem.
  2. Ascending sensory fibers, ascending through the brainstem, synapse in the thalamus and then relayed to the cerebral cortex via the internal capsule. Motor fibers descending from cerebral cortex pass to brainstem via the internal capsule but without synapsing in the thalamus.
  3. Thalamus is a collection of neurons which are organized into a large number of well-defined nuclear masses. Thalamic nuclei are divided as follows ;-
    1. Anterior
    2. Medial geniculate nucleus
    3. Dorsomedial
    4. Lateral geniculate nucleus
    5. Ventral anterior
    6. Pulvinar
    7. Ventral lateral (Intermediate)
    8. Lateral dorsal
    9. Lateral posterior
    10. Intralaminar
    11. Reticular
    12. Ventral posterior
      1. Ventral posterolateral
      2. Ventral posteromedial
  4. Every thalamic nuclei except reticular nuclei sends axon to different part of cortex. The output or reticular nuclei is mainly to other thalamic nuclei.
  5. Ventral posterior nuclei have afferents :-
    1. Ventral posterolateral: Spinal lemniscus (rostral continuation of spinothalamic tract and medial lemniscus.
    2. Ventral posteromedial: Trigeminal lemniscus and solitariothalamic tract.



The hypothalamus can be divided in the sagittal plane into anterior (supraoptic), middle (tuberal) and posterior (mammillary) regions. The hypothalamus is also divided into three regions in the coronal plane; lateral, medial and periventricular (surrounding third ventricle).


Saggital subdivision (Regions)

Coronal subdivision (zones)





Supraoptic nucleus

Lateral preoptic nucleus

Supraoptic nucleus

Medial preoptic nucleus

 Suprachiasmatic nucleus

 Anterior nucleus

 Paraventricular nucleus


Lateral hypothalamic area

Ventromedial nucleus

Dorsomedial nucleus

 Arcuate nucleus

 Paraventricular nucleus


Lateral hypothalamic area

Mammillary body

Mammillary body

 Posterior hypothalamic nucleus



Other parts of diencephalon

  1. Epithalamus consists of habenular nucleus lying in habenular trigone, pineal body, habenular commissure, posterior commissures, and stria medullaris thalami. Pineal gland helps in maintaining rhythm sleep-wake cycle by secreting melatonin.
  2. Metathalamus is considered to be a part of thalamus and consists of medial and lateral geniculate bodies, Medial geniculate body is concerned with auditory pathway and lateral geniculate body is concerned with optic (visual) pathway
    1. Medial geniculate body pathway (Auditory pathway) :-
    2. Lateral geniculate body pathway (visual pathway):-
  3. Subthalamus consists of:-
    1. Grey matter: -
      1. Cranial end of red nucleus and substantia nigra extend into it,
      2. Subthalamic nucleus
      3. Zona incerta.
    2. White matter: -
      1. Cranial end of lemnisci
      2. Dentatothalamic tract along with rubrothalamic fibres,
      3. Ansa reticularis,
      4. Fasciculus lenticularis
      5. Subthalamic fasciculus.

Mesencephalon (midbrain) and rhombencephalon (hindbrain)


Mesencephalon consists of midbrain and rhombencephalon (hindbrain) consists of metencephalon (pons and cerebellum) and myelencephalon (medulla oblongata). Midbrain, pons and medulla oblongata together form brain stem.

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