Brain Stem Structure

Brainstem is the part of brain formed by medulla oblongata, pons and midbrain. Brainstem contains ascending and descending tracts between brain and spinal cord. It also contains many centers for regulation of vital functions in the body.

MEDULLA OBLONGATA

Medulla oblongata or medulla is the lowermost part of brain. It is situated below pons and is continued downwards as spinal cord. Medulla forms the main pathway for ascending and descending tracts of the

spinal cord. It also has many important centers which control the vital functions.

1. Respiratory Centers

Dorsal and ventral group of neurons form the medullary respiratory centers, which maintain normal rhythmic respiration.

2. Vasomotor Center

Vasomotor center controls blood pressure and heart rate.

3. Deglutition Center

Deglutition center regulates the pharyngeal and esophageal stages of deglutition.

4. Vomiting Center

Vomiting center induces vomiting during irritation or inflammation of gastrointestinal (GI) tract.

5. Superior and Inferior Salivatory Nuclei

Salivatory nuclei control the secretion of saliva.

6. Cranial Nerve Nuclei

Nuclei of 12th, 11th, 10th and some nuclei of 8^th and 5th cranial nerves are located in the medulla

oblongata. 12th cranial (hypoglossal) nerve controls the movements of tongue. 11th cranial (accessory) nerve controls the movements of shoulder and 10th cranial (vagus) nerve controls almost all the vital functions in the body, viz. cardiovascular system, respiratory system, GI system, etc. 8th cranial nerve (the cochlear division of this nerve), which has the relay in medulla oblongata, is concerned with the auditory function.

7. Vestibular Nuclei

Vestibular nuclei contain the second order neurons of vestibular nerve. There are four vestibular nuclei,

situated in the rostral part of medulla and caudal part of pons, namely superior, medial, lateral and inferior

vestibular nuclei. Medial and inferior vestibular nuclei extend into medulla. are controlled by higher centers, situated in cerebral cortex and hypothalamus.

PONS

Pons forms a bridge between medulla and midbrain.

Functions of Pons

1. Axons of pontine nuclei join to form the middle cerebellar peduncle or the brachium pontis. Pons

forms the pathway that connects cerebellum with cerebral cortex.

2. Pyramidal tracts pass through the pons

3. Medial lemniscus is joined by the fibers of 10th, 9th, 7th and 5th cranial nerves in pons

4. Nuclei of 8th, 7th, 6th and 5th cranial nerves are located in pons

5. Pons contains the pneumotaxic and apneustic centers for regulation of respiration

6. It also contains the vestibular nuclei, which are already mentioned in medulla oblongata.

MIDBRAIN

Midbrain lies between pons and diencephalon. It consists of two parts:

A. Tectum

B. Cerebral peduncles.

TECTUM

Tectum is formed by two structures:

1. Superior colliculus

2. Inferior colliculus.

1. Superior Colliculus

Superior colliculus is a small structure and is an important center for reflexes. Through tectospinal

tract, superior colliculus controls the movements of the eyes, head, trunk and limbs, in response to visual

impulses. Efferent fibers from superior colliculus going to the nucleus of III cranial (oculomotor) nerve cause constriction of pupil during light reflex. Thus, it forms the center for light reflex. Superior colliculus also receives afferents from optic tract, which helps in the integration of optical and postural reflexes.

2. Inferior Colliculus

Inferior colliculus consists of single layer of neurons to which the lateral lemniscus (auditory fibers) synapses.

2. Nucleus parvocellularis, which is formed by smaller cells. Fibers from this form mainly the rubroreticular tract.

Connections of red nucleus

Afferent connections: Red nucleus receives fibers from:

1. Nucleus parvocellularis, which receives fibers from motor cortex (area 6) – corticorubral fibers

2. Nucleus magnocellularis, which receives fibers from motor cortex (area 6) – pallidorubral fibers

3. Nucleus magnocellularis, which receives fibers from dentate nucleus (of opposite side) – cerebellorubral or dentatorubral tract.

Efferent connections: Red nucleus sends efferent fibers to various parts of brain and spinal cord:

1. Rubrospinal tract to spinal cord

2. Rubrobulbar tract to medulla

3. Rubroreticular fibers to reticular formation

4. Rubrothalamic tract to lateral ventral nucleus of thalamus

5. Rubroolivary tract to inferior olivary nucleus

6. Fibers to nuclei of 3rd, 4th and 6th cranial nerves.

Functions of red nucleus

1. Control of muscle tone: Because of its connections with cerebellum, vestibular apparatus and skeletal

muscle, the red nucleus plays an important role in facilitating the muscle tone.

2. Control of complex muscular movements:

Red nucleus controls the complex muscular movements. It plays an important role in the

integration of various impulses received from many important areas of brain.

3. Control of righting reflexes: Red nucleus is the center for all righting reflexes except optical righting

reflexes.

4. Control of movements of eyeball: Through its efferent connections with nuclei of 3rd, 4th and 6th

cranial nerves, red nucleus plays an important role in the control of ocular movements.

5. Control of skilled movements: Red nucleus plays an important role in controlling the skilled muscular

movements by its connections with spinal cord and cerebral cortex.