The brain Parts 3 hindbrain

The hind-brain is located in the back of the cranial cavity an area known as the posterior fossa. It lies below a fold of dura mater called the tentorium cerebelli.

It consists of pons, cerebellum and the medulla oblongata. The word brain stem includes midbrain, pons and medulla oblongata.

The pons is situated in front of the cerebellum. It is connected to the midbrain by the cerebral peduncles.

Below it continues as the medulla but is separated from it by a groove. The ventral surface is convex from side to side and made up of fibers running from side to side forming small ridges.

In the midline of pons there is a vertical groove which lodges an artery called the basilar artery which is formed by the union of the right and left vertebral arteries.

The vertebral arteries start in the neck, pass up through the openings in the transverse processes of the cervical vertebrae, enter the skull through the foramen magnum and join together at the lower end of pons to form the basilar artery.

Pons contains the nuclei of the V, VI, VII and VIII cranial nerves.

The cerebellum is the largest part of the hindbrain and weighs approximately 150 grams. It is situated behind the pons and the medulla and the fourth ventricle lies in between the cerebellum behind and pons and medulla in front.

It consists of a central constricted portion called vermis and two expanded structures called cerebellar hemispheres. The surface of the cerebellum has multiple grooves.

A sagittal section through the cerebellar hemisphere shows a central mass of white matter surrounded by grey matter. In the middle of the white matter is a mass of grey matter called the dentate nucleus.

The cerebellum is connected to the other parts of the brain by three pairs of nerve bundles called projection fibers. The superior cerebellar peduncles connect it to the cerebrum, the middle cerebellar peduncles connect it to the pons and the inferior cerebellar peduncles connect it to the medulla.

The fibers of the superior peduncle are derived mainly from the dentate nucleus. Some fibers also arise from the cortex. The fibers cross the midline and go to the opposite side. They end in the red nucleus, thalamus and the nuclei of occulomotor nerve.

The middle cerebellar peduncles consist of fibers coming from the pontine nuclei of the opposite side.

The inferior cerebellar peduncle contains fibers from the spinal cord, medulla and fibers from the vestibular division of the eighth cranial nerve.

Many fibers connect the two hemispheres of the cerebellum. In each hemisphere there are fibers which connect various parts of that hemisphere.

It can be seen that the cerebellum is connected to many parts of the nervous system. This is essential as it is involved in coordination of voluntary movements, balance and muscle tone.

The commands for motor actions are not initiated by the cerebellum. The commands coming from the cerebral cortex are modified by the cerebellum to make the movements accurate.

Diseases of cerebellum result in staggering gait, inability to perform rapid alternative movements, slow slurred speech, abnormal eye movements called nystagmus and decreased muscle tone.

A person with cerebellar disease will find it very difficult to walk along a straight line. If asked to touch the tip of nose with the tip of a finger the person either falls short of the target or overshoots.

The medulla oblongata (spinal bulb) is a continuation of the pons and the lower end is continuous with the spinal cord. It is about 3 centimeters long.

The lower part of the medulla is closed and contains the central canal which continues into the spinal cord. The upper part is open and contributes to the floor of the fourth ventricle.

There are midline grooves on the anterior and posterior surfaces of the medulla which divide the closed part into two symmetrical halves. The posterior groove exists only in the closed part.

Each half is further subdivided into three parts called anterior, middle and posterior parts by grooves called antero-lateral and postero-lateral grooves. These parts are continuous with those of the spinal cord.

The anterior part situated in between the midline anterior fissure and the antero-lateral fissure is called the pyramid. The sixth cranial nerve (abducent) emerges between the pyramid and the lower border of pons.

The pyramid contains motor fibers from the brain to the medulla (corticobulbar) and spinal cord (corticospinal). Approximately two thirds of these fibers cross to the opposite side (pyramidal or motor decussation) and descend into the spinal cord as the lateral cerebrospinal fasciculus.

The remaining fibers which do not cross the midline continue as the anterior cerebrospinal fasciculus.

The middle part is situated between the antero-lateral and postero-lateral grooves. Its upper part contains an oval mass called the olive which contains a nucleus called the inferior olivary nucleus.

The fibers from the olive constitute the olivocerebellar tract and cross the midline to enter the opposite cerebellar hemisphere through the inferior cerebellar peduncle. The lower part is continuous with the lateral funiculus of the spinal cord and is of the same width.

The posterior part is behind the postero-lateral groove. The lower part contains two tracts called fasciculus gracilis situated medially and the fasciculus cuneatus situated laterally.

The fasciculus gracilis carries sensory information from the lower half of the body while the fasciculus cuneatus carries sensory impulses from the arms and trunk.

The fasciculus gracilis ends at the upper end of the posterior part in a nucleus called nucleus gracilis and the fasciculus cuneatus ends in a nucleus at the upper end of the posterior part of the medulla called the nucleus cuneatus.

From these nuclei second order fibers arise. Some of them enter the cerebellum through the inferior cerebellar peduncle. But most of the fibers cross the midline (this is called sensory decussation and is above the motor decussation mentioned above) and forming a band of fibers called the medial lemniscus terminate in the thalamus.

The medulla contains the nuclei of the cranial nerves glossopharyngeal, vagus, accessory and hypoglossal.

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