Introduction to the Brain


 

The brain is the most sophisticated apparatus in the neuraxis. It is rostrally placed and controls other structures below it. It keeps the spinal cord under check, together with other systems in the nervous system. Thus we have the control of the autonomic nervous system lying in the hypothalamus. Also the basal ganglia controls proprioception. Other parts of the brain control emotions and behaviour while the most rostral and sophisticated apparatus in the brain helps to control the volitional actions and motor functions.

 

Brain at different surfaces

The brain is designed as a tree so that it has a stem and  hemispheres which may represent the leaves of the tree. The brain stem consist of the following

  • Medulla oblongata

  • Pons

  • Midbrain

  • Diencephalon (Higher brain stem)

Most neuroanatomists will stop at the midbrain but some will go up to the level of the diencephalon which  is then called the upper brain stem.

The hemispheres are two

  • Cerebral hemisphere

  • Cerebellar hemisphere

 

 


Medial aspect of brain showing brainstem colored

Medulla

The medulla is the lowest part of the brain stem and the brain itself. It is divided into two main parts

  •  Medulla which is also called the open medulla.

  • Lower medulla also called the closed medulla.

 The open medulla is so called because it opens into the fourth ventricle. The 4th ventricle is placed at its back. The closed medulla is so called because it is closed with a central canal within its substance just as you have in the spinal cord.

  The closed medulla is closed because it has the central canal of the medulla buried in its middle. 

The medulla has at its ventral surface, the pyramids on the two sides of the anteriomedian sulcus. Lateral to the pyramids are the olivo-pyramidal interval which form  sulci for the escape of rootlets of the hypoglossal nerve. Lateral to the olive is the  postolivary sulcus and it carries the rootlets of cranial nerves 9, 10 and 11- glossopharyngeal, vagus and accessory nerves.

Posterior aspect of the medulla has the tuberculum gracilis most lateral to the posteromedian fissure and lateral to it is the tuberculum cuneatus. The two tubercula have buried in them the nuclei gracilis and cuneatus respectively. At the back of the upper medulla is the 4th ventricle. It is covered at the back of the medulla by the inferior medullary velum and tela choroidea. The floor of he 4th ventricle is called the rhomboid fossa.

 

 


Medulla in yellow

 

Rhomboid fossa (back of medulla and pons): 1 facial colliculus 2 tubeculum acoustica 3 stria medullaris 4 median eminence 5 hypoglossal trigone 6 vagal trigone 7 tuberculum gracilis 8 tuberculum cuneatus

Pons

The pons is found only mammals and it seems to be responsible for learned coordination and balance as opposed to unlearned coordination which are basically instinctual.  The coordination which the pons helps to integrate are from the learned areas of the cerebral cortex. The tract runs from the cerebral cortex down to the cerebellum with synapses in the pons. The tract begins as the corticopontine tracts which then proceeds to the deep cerebellar nuclei. All the lobes of the cerebral cortex contribute to this. They include

  • Frontopontine

  • Temporopontine

  • Occipitopontine

  • Parietopontine

The whole tract beginning from the cortex and ending in the cerebellum after relaying in the pons may then be called corticopontocerebellar tract and they relay in the ventral aspect of the pons in the basis pontis.

Pons is therefore divided into the following parts

  • Basis pontis ( most ventral)

  • Pontine tegmentum which contains nuclear groups of cranial nerves V, VI, VII . It also contains the reticular formation of the pons.

 The most dorsal part of the pons is the fourth ventricle. The floor, which is part of the so-called rhomboid fossa because it is pyramidal in shape, contains the following structures

  • Facial colliculus, this is the swelling which has the abducent nucleus and also the internal genu of the facial nerve.

  • It also has the rostral extreme of the tuberculum acousticum. It is separated from the back of the medulla by striae medullaris.

 


Pons in red

Cross section through pons stained with Pal-Weigert method

 

Midbrain

The midbrain is dwarfed by other structures in the neuraxis. It contains the following parts-

  • Ventral crus cerebri which carries the pyramidal tract from the forebrain.

  •  Dorsal to the crus cerebri is the midbrain tegmentum. The most dorsal part of the midbrain is the tectum. This is called tectum opticum in lower animals, especially the reptiles to emphasize the role in visual reflexes. In man it contains or mediates both visual and auditory reflexes. It also contains two structures  

 

  • 1.              Superior colliculus

  • 2.              Inferior colliculus

The midbrain tegmentum also contains the nuclei of two important cranial nerves. They are

  • Oculomotor

  • Trochlear nerves

The level of the superior colliculus is the same level of the oculomotor nucleus while the level of the trochlear nerve has the inferior colliculus. The trochlear is the only cranial nerve that issues from the dorsal aspect of the brain. It then passes around the lateral aspect of the brain stem in  order to appear in the ventral aspect of the brain and enter the cavernous sinus.

 


Midbrain

 

 

 

 

 

 

Cross section through upper midbrain

 

Diencephalon

Diencephalon is called the upper brain stem. It has on its sides structures of the basal ganglia which are embryologically and phylogenetically superior to it. The structures of the basal ganglia which surrounds the diencephalon are corpus striatum together with its division into a lateral lentiform nucleus and a medial caudate nucleus. Most of the diencephalic structures are placed at the back, i.e. dorsally and they are mainly thalamic. The diencephalon is divided into the following parts

1.              Thalamus- this is most massive and the most important. It is vital for the somesthetic system.

2.              Hypothalamus- this is the most important in the autonomic and endocrine activity of the nervous system.

3.              Epithalamus. This is the most dorsal part of the diencephalon and the most important organ within it is the pineal body or pineal gland. Surrounding the pineal body are the habenular trigones. We therefore have the following parts of the epithalamus

  • 1.              Pineal body

  • 2.              Habenular trigones

  • 3.              Epithelial roof of the 3rd ventricle

  • 4.              Striae medullaris thalami

The last structure within the diencephalon is the subthalamus. It is a very small structure which is more related to the paleocortex than the diencephalon.

The cavity of the diencephalon is the 3rd ventricle while the cavity of the midbrain is the cerebral aqueduct.

 

 

Diencephalon at the midsagittal section of brain


 

Coronal section through fixed brain at level of upper brain stem: 1 cortex 2 caudate nucleus 3 cerebral 'medulla' 4 corpus callosum 5 cingulate gyrus 6 fornix column 7 thalamus 8 internal capsule 9 lentiform nucleus 10 extreme capsule 11 claustrum 12 external capsule 13 insular cortex 14 3rd ventricle

 

Telencephalon

This is the most rostral part of the brain. It has the following parts

  • 1.              Cerebral cortex

  • 2.              Cerebral ‘medulla’. This forms the fibers which criss cross and interconnect the telencephalon with other levels of the neuraxis. We have association, projection and commissural fibers. Examples include the corona radiata, the corpus callosum, which joins the cerebral cortex of one side of the brain to the other.

  • 3.              The lowermost structure is called the basal ganglia, because it is at the basal level of the cerebrum (telencephalon). It consists of two structures

  • a.              Corpus striatum

  • b.              Amygdaloid nuclear complex (amygdala, amygdaloid body etc). The amygdala is very much related to the paleocortex more than the basal ganglia. Hence sometimes the term basal ganglia is synonymous with corpus striatum.

 The corpus striatum is divided into two structures

  • 1.              Lentiform nucleus

  • 2.              Caudate nucleus

Lentiform nucleus is made up of two structures

  • 1.              Globus pallidus (medial)

  • 2.              Putamen (lateral)

  Caudate nucleus has a head, body, tail  and it is related to the lateral wall of the 3rd ventricle.

 


Basal aspect of cerebrum- the basal ganglia: Arrow points to basal ganglia

 

 

 


Coronal section of gross brain showing basal ganglia: 1 putamen 2 globus pallidus

Cerebral cortex

Classification which takes into consideration the phylogeny of the brain has three divisions

  • Archicortex. This is the most primitive part of the telencephalon. It is also called archipallium. The following are parts of the human archicortex

  • 1.              Hippocampal formation

  • 2.              Indiseum griseum

  • 3.              Fasciolar gyrus

  • 4.              Dentate gyrus

 

  • Paleocortex. This is the smell brain It is responsible for the awareness of smell and is therefore fully developed in those animals that are macrosmatic, as opposed to microsmatic animals (which include man).

  • Neocortex. Neocortex is the most developed part of the brain in man. It is massive being formed by various parts of the cerebral cortex. It is responsible for volition, will, memory, learning, etc. What are called cognitive mental functions are highly specialized in this area. These functions are hardly noticeable in lower animals apart from man.

 


Cerebral cortex in red

Coronal section of brain stained: 1 caudate nucleus 2 globus pallidus, 3 putamen, 2 +3 lentiform nucleus 4 lateral ventricle 5 insular cortex  6 parahippocampal gyrus

 

 


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