THE BRAIN

T

he Human brain is by far the most complex structure to be ever build. Its complexity has dazzled majority of the people since last century and is still the topic of research.

The adult brain weighs about 1.3 – 1.5 kg and is approximately 2% of or total body weight, with Surface area of approximately 1120 cc in males and 1130 cc in females. In spite of its small size it contains millions and billions of neurons that carry out daily functions of the body and may be create someone like Einstein whose brain is still preserved for research purposes.

The basic functional unit of brain is called as a neuron. It is a specialised type of cell that has the capacity to conduct and transfer electrical impulses, but once damaged can take years to regenerate.

There are various types of neurons and are classified according to –

A)  The number of dendrites/processes

B)   The length of axons

According to number of processes they are

1.    Unipolar

2.    Bipolar

3.    Pseudounipolar

4.    Multipolar

According to length of axons they are

1.    Golgi type 1(Long Axon)

2.    Golgi type 2(Short Axon)

Fig 1.0 Types of neuron, based on number of processes.

Note: The bipolar neuron is not to be associated with the Bipolar condition. Bipolar condition or disorder is psychological and is characterised by fluctuant changes in the mood of person from happy to being depressed. And has certain criteria where you can assess yourself (will be discussed later).

The brain is made up of three components

1.    Forebrain

2.    Mid brain

3.    Hindbrain

All of these parts are evolved from a particular embryological tissue which after development leaves certain parts behind and are called as the ventricles and aqueducts 

of the brain and the spine (discussed further).

The Forebrain is made up of the following structures –

1.    Cerebrum

2.    Basal ganglia

3.    Hippocampus

4.    Thalamus

5.    Hypothalamus

6.    Pineal Body

7.    Infundibulum

The Midbrain consists of

1.    Crus cerebri

2.    Tegmentum

3.    Tectum

The Hindbrain consists of

1.    Pons

2.    Cerebellum

3.    Medulla oblongata

Fig 1.1 Brain and its parts

The brain has 3 predominant functions

1.    Motor

2.    Sensory

3.    Association

The Brain is located inside the skull, because of its shape, the skull is modified inside to accommodate the lobes and the structures.

Fig 1.2 Superior view of Skull (Source: Pearson Education, Inc, publishing as Pearson Benjamin cummings).

The brain is also divided according to lobes it has –

1.    Frontal lobe

2.    Temporal lobe

3.    Parietal lobe

4.    Occipital lobe

There is a hidden lobe called insula which is present in between the temporal lobe and parietal lobe and is situated underneath the parietal lobe.

The brain is thrown into folds that increase the surface area. The grooves are called sulci (singular: Sulcus) and folds are called gyri (singular: Gyrus).

The two halves of the brain are separated by the Longitudinal fissure and falx cerebri, creating a right and left cerebral hemisphere and the cerebral hemispheres are separated from the cerebellar hemispheres by Tentorium cerebelli (a fold of dura matter).

Fig 1.3 Sulci, gyri and longitudinal fissure

Fig 1.4 Showing tentorium cerebelli and Falx cerebri.

The Brain is covered with 3 layers of tissue called as the Meninges. It is this layer that is inflamed and thus causing meningitis.

The three meninges are –

1.    Dura Matter

2.    Arachnoid Matter

3.    Pia Matter

The Dura Matter being the outer most, lies below the skull bone and carries the sinuses of the brain (venous drainage pathways). The dura is made of 2 layers where the outermost layer being the supportive and attaches to the skull and the inner one covering the brain and the other two meninges.

Fig 1.5 Showing dura, tentorium and sinuses in cadaver (posterior view).

The Arachnoid matter runs along the whole surface of the brain covering it entirely but not going deep; where as the Pia matter runs deep down into each and every sulcus and carries the blood supply.

Fig 1.6 Meninges simplified (source Neuroscientifically challenged – YouTube).

As mentioned earlier, the Brain has sulci and gyri, there are a few which are predominant and are used as landmarks to study all the important parts of the brain.

The main sulcus used as a landmark is Central Sulcus which separates the frontal lobe and the parietal lobe.

Another sulcus used as landmark is the lateral sulcus which separates the Frontal and parietal lobes from the temporal lobe,

The other sulcus present are the parieto-occipital sulcus, occipito-temporal sulcus, superior and inferior sulci, superior and inferior temporal sulci, etc.

The image below shows the two main sulci used as landmark.

Fig 1.7 Sulci of brain (blue colour indicating the main sulci) [source – Sobotta’s textbook and atlas of human anatomy]

The main gyri used as landmarks and are of prime importance because they have functional importance to study are –

1.    Precentral gyrus

2.    Postcentral gyrus

3.    Superior temporal gyrus

The precentral gyrus is located just in front of the central sulcus and contains the motor functions of the body. The postcentral gyrus is located just behind the central sulcus and contains the sensory functions of the body. Superior temporal gyrus is located just below the lateral sulcus and has the auditory functions of the body.

The figure shows the gyri mentioned above.

 

Fig 1.8 Gyri of brain.

The area that connects the two halves of the brain is called as corpus callosum.

It is a band of Neurons that connect the right and left half of the brain. It contains –

1.    Rostrum

2.    Genu

3.    Body

4.    Tail/Splenium

Each of them connect a specific part of the brain, and if this corpus callosum was cut, there would be no communication between the two halves of the brain.

Fig 1.9 Corpus callosum.

Fig 2.0 Corpus callosum in a dissection

(note the extensive fibres that arise from the central white band called corpus callosum; joining two halves of brain) [Source: Medzzy]

On cross section of the brain, we see two different colours, the outer is called as grey matter and inner is called as white matter.

The grey matter appears grey because of presence of the granular structures inside the cell body of neurons; whereas the white matter appears white because of the axons of the cell bodies of neurons that travel to and from the brain.

There is a huge mass of grey matter present in the mid brain and is called as the Thalamus. It is approximately 4cm in length and lies on the either side of the ventricles.

It has two poles – anterior and posterior.

Posterior pole is also called as the pulvinar end.

The position of thalamus can be remembered from the, “EYE OF HOROUS” as it is almost similar as can be seen in the following image.

Fig 2.1 Thalamus location

Thalamus has 10 nuclei and are located across the surface of the thalamus. There is a Y shaped band running along the surface of thalamus which is formed by the laminae.

Fig 2.2 Thalamic nuclei and laminae.

Thalamus acts as a relay centre where information from all parts of the body is projected into the brain’s cortex via the Thalamo-cortical pathways.

The structure lying adjacent to thalamus are the ventricles.

The ventricular system is the most important system in the Central nervous system as it secretes CSF(Cerebrospinal Fluid), which acts as a nutrient rich fluid for brain and spinal cord to achieve nutrition and also to act as a shock absorber.

Fig 2.3 Ventricles of Brain.

The ventricular system comprises of –

a.     Two lateral ventricles

b.    Single 3^rd ventricle

c.     Single 4^th ventricle

All of these are connected to each other via aqueducts and channels.

These channels are –

a.     Interventricular foramen of Monroe

b.    Cerebral aqueduct

c.     Central canal

All of these secrete CSF via the choroid plexus present inside them which are nothing but a capillary network that filter the blood and their ultrafiltrate is called as CSF. This CSF circulates through the whole of Brain and spinal cord and is absorbed by small granulations present called as arachnoid granulations.

These granulations can be seen in the figure 2.4

Fig 2.4 Arachnoid granulations.

Lateral Ventricle lies in the parietal lobe. Where roof is mare by the corpus callosum, floor by thalamus, caudate nucleus, fornix and choroid plexus, and the lateral wall by the narrow area meeting the floor and roof.

There is a projection of the ventricle into the frontal lobe called as anterior horn of the lateral ventricle, and a posterior projection into the occipital lobe is called as the Posterior horn of the lateral ventricle. There is another horn projecting into the temporal lobe of the brain called as inferior horn.

Fig 2.5 Horns of Ventricle

The third ventricle is located in between the thalami, which opens posteriorly into the 4^th ventricle via the cerebral aqueduct and the lateral ventricles open into the 3^rd ventricle via the foramen of Monroe. The 4^th ventricle is located just behind the brainstem which is projected into the cerebellum.

Brainstem consists of mid brain, pons and medulla oblongata. These are located ventrally. Mid brain predominantly is made of the cerebral peduncles (pedunculus Latin for footstalk).

Fig 2.6 Brainstem

The brainstem is most important because it contains the centres for respiration, vomiting etc. Also, cranial nerves majority arise from the brainstem. This also serves as a pathway for tracts to go up and down the brain and periphery.

Mid brain has 4 bumpy structures in the posterior aspect and are called as colliculi. They are 2 superior colliculi and 2 inferior colliculi. And we can study the brainstem at cross sectional levels at these two colliculi.

Fig 2.7 Colliculi of brain.

The blood supply to the brain is by the following arteries –

a.     Anterior cerebral artery

b.    Middle cerebral artery

c.     Posterior cerebral artery

Anterior and Middle Cerebral arteries are a branch of internal carotid arteries, whereas the posterior cerebral artery is a branch of basilar artery which is intern an artery formed by the joining of the two vertebral arteries.

This conjunction at the brainstem is called as circle of Willis.

Fig 2.8 Circle of Willis.

  

This circle of Willis is an important aspect of brain’s circulation because it ensures uninterrupted blood supply.

This circle is completed by the following arteries –

a.     Anterior communicating arteries

b.    Posterior communicating arteries

Venous drainage of brain is in the following order –

a.     Veins of brain

b.    Intracranial Dural sinuses

c.     Internal jugular veins of neck

d.    Vena cava

e.     Right atrium of heart

3 main veins run along the brain –

a.     Superior cerebral vein

b.    Middle cerebral vein

c.     Inferior cerebral vein

Other veins are –

a.     Anterior cerebral veins

b.    Basal vein of Rosenthal

c.     Internal cerebral veins

d.    Great cerebral vein

e.     Basal vein

f.      Collicular vein

g.    Cerebellar veins

Fig 2.9 Sinuses of brain

Fig 3.0 Veins of brain seen in an angiogram