The Nervous System B : The Brain

Objectives:

1. to identify the different regions of  the brain

2. to identify the different lobes of the cerebrum

3. to know the external and internal structures of the brain

4. to identify the different ventricles of the brain

5. to identify each cranial nerve and its function

THE PROTECTIVE MEANS

     The brain is well protected by being housed inside the cranium surrounded by the cranial bones. The three layers of  cranial meninges; the dura, the arachnoid and the pia maters, separate between the brain tissue and the skull. The subdural space is filled with interstitial fluid and the subarachnoid space filled with the CSF. There are three extentions from the dura mater separating between the different parts of brain; the falx cerebri located between the two cerebral hemisphere, falx cerebelli between the two cerebellar hemispheres and tentorium cerebelli located between the cerebrum and cerebellum.

THE MAIN PARTS OF THE BRAIN

     The brain consists of the following main parts: the cerebrum, the diencephalon, the brainstem and the cerebellum. The cerebrum occupies the majority of the brain, the diencephalon is composed of thalamus, hypothalamus, epithalamus and subthalamus. The brain stem is continuous with the spinal cord composed of mid brain, pons and medulla oblongata while the cerebellum is the most posterior and inferior part of the brain.

1.THE CEREBRUM

The largest part of the brain.

*. The surface layer is the cerebral cortex, 2-4mm thick and is composed of gray matter. It contains billion of neurons.

*. The cortex contains gyri ( convolutions), deep grooves called fissures and shallow grooves called sulci.

*. The cerebral white cortex lies beneath the cortex ; composed of tracts connect parts of brain with itself and other parts of the nervous system.

*. The cerebrum is nearly separated into two hemispheres by the longitudinal fissure which contains the falx cerebri. Internally it is connected by the corpus callosum, a bundle of transverse white fibers.

The lobes

*. Four lobes of the cerebrum exist on the cerebral cortex: the frontal, the parietal, the temporal and the occipital.

*. The central sulcus separates between the frontal (anterior) and parietal (posterior) lobes.

*. The lateral fissure separates between the frontal (superior) and the temporal (inferior).

*. The parieto-occipital sulcus separates between the parietal (anterior) and the occipital (posterior).

*. The fifth lobe of the cerebrum, the insula, lies deep to the parietal, frontal and temporal lobes and connot be seen from the external view.

*. The central sulcus separates between the precentral gyrus ( within the frontal lobe), which is the primary motor area and the postcentral gyrus ( within the parietal lobe), which is the primary somatosensory area.

2.THE DIENCEPHALON

Extends from the cerebrum to the brain stem, composed of:

1.The thalamus

     The largest part of the diencephalons. Consists of pair of oval shaped masses of gray matter connected by the intermediate mass of thalamus, arranged into many nuclei. Thalamus works as relay center.

2.The hypothalamus

     A small part lies inferior and anterior to thalamus. It is composed from many nuclei, the most important one is the mammillary body which can be seen in the inferior  surface of the brain just anterior to the brain.

Mainly the hypothalamus controls the pituitary gland, emotional and behavioral pattern, eating and drinking and body temperature.

3.The epithalamus

     A small structure lies superior and posterior to the thalamus. Consists of the pineal gland that secrets the melatonin to participate in setting the biological clock. The habenular nuclei also is part of the epithalamus involved in olfaction and emotional responses to odor.

3.THE INTERNAL STRUCTURES OF THE BRAIN STEM

The brain stem is composed of:

1. The medulla oblongata

The connection piece between the cerebrum and the spinal cord located at the level of the foramen magnum. It contains sensory and motor neurons tracts that pass between the brain and the spinal cord in addition to many nuclei of the last five cranial nerves.

The medulla oblongata contains two main structures:

*. The pyramid which is located at the anterior medial side of the medulla oblongata containing motor tracts.

*. The olive at the lateral side of the medulla oblongata which contains neural nuclei.

*. Contains the nucli of the cranial nerves # VIII, IX, X, XI and XII.

2. The pons

*. Can be identified as a bulged structure superior to the medulla oblongata and anterior to the cerebellum.

*. Consists of tracts and nuclei of three cranial nerves: V, VI and VII.

3. The midbrain

*. Extends from the pons inferiorly to the diencephalons superiorly.

*. Contains the following structures:

         *cerebral peduncles consist of motor fibers

         *corpora quadrigeminina consists of the superior colliculi and inferior colliculi.

         *nuclei of the cranial nerves # III and IV.

4.THE CEREBELLUM

*. The second large part of the brain

*. Lies in the most posterior inferior aspects of the cranial cavity

*. Separated by the transverse fissure and the tentorium cerebelli from the cerebrum

*. Composed of two cerebellar hemispheres connected by  the vermis

*. The cortex is made of the gray matter and called the folia

*. The center is made of the white matter called the arbora vitae

*. Connected to other parts of the brain by:

     *. The superior cerebellar peduncles; connection with mid brain

     *. The middle cerebellar peduncles; connection with the pons

     *. The inferior cerebellar peduncles; connection with medulla oblongata

THE CEREBROSPINAL FLUID

*. The cerebrospinal fluid (CSF) is a clear, colorless fluid.

*. It protects the brain and the spinal cord  against chemical and physical injuries and carries oxygen,  glucose and other needed chemicals from the blood to the neurons and neuroglia.

*. There are four CSF filled cavities within the brain called ventricles;

         *. A lateral ventricle is located in each hemisphere of the cerebrum. The lateral ventricles are separated by the septum pellucidum and connected to the third ventricle by the interventricular foramen.

         *. The third ventricle is a narrow cavity along the midline superior to the hypothalamus and between the right and left halves of the thalamus. Connected to the fourth ventricle by the cerebral aqueduct.

         *. The fourth ventricle is between the brain stem and the cerebellum. Continues with the central canal inferiorly. Also connected to the subarachnoid space through three opening ; one medial aperture and paired lateral apartueres.

*. CSF is formed by the filtration from the capillaries network called the choroids plexus (inside the ventricle) and circulates through the subarachnoid space, ventricles and central canal.

*. Most of CSF is absorbed by the arachnoid villi of the superior sagittal blood sinus at the same rate of CSF synthesis, thereby maintaining constant CSF volume and pressure.

THE CRANIAL NERVES

     There are twelve (12) pairs of nerves that extends from the brain to other tissue exist in the head, neck and shoulder mainly.

*. The cranial nerves emerge from the ventral side of the brain except# 4

*. The cranial nerves contain mixed fibers except #1 and #2

Number of the nerve	location	Name of nerve	Main function
I	frontal lobe	OLFACTORY	Pure sense of olfaction
II	Occipital lobe	OPTIC	Pure sense of vision
III	Midbrain	OCULOMOTOR	Muscles of eyeball
IV	Midbrain posteriorly	TROCHLEAR	Muscles of eyeball
V	Pons	TRIGEMINAL	Muscles of mastication
VI	Pons	ABDUCENS	Muscles of eyeball
VII	Pons	FACIAL	Facial muscles expression
VIII	Pons and cerebellum	VETIBULOCOCHLEAR	Hearing and balance
IX	Medulla oblogata, posteriorly	GLOSSOPHARYNGEAL	Sense and movement of tongue and pharynx muscles
X	Medulla oblogata, posteriorly	VAGUS	viscera
XI	Medulla oblogata, posteriorly	ACCESSORY	Muscles of neck and shoulder
XII	Medulla oblongata, anteriorly	HYPOGLOSSAL	Muscles of the tongue

*. The olfactory nerve emerges from the inferior surface of the cerebral cortex of the frontal lobe.

*. The two optic nerves cross with each other forming the optic chiasm superior to the pituitary gland.

*. #III and IV can be seen between the midbrain and the pons.

*. #V lies at the lateral sides of the pons.

*. #VI, VII, VIII from the medial to the lateral side at the border between the pons and medulla oblongata.

*. #IX, X and XI from the medulla oblongata posterior to the olive nucleus.

*. #XII between the olive and the pyramid.