Brainstem: Anatomy, Function, and Treatment Notes

Brainstem Introduction

Brainstem is the part of brain formed by the medulla oblongata, pons, and midbrain. It forms:

1. The pathway for ascending and descending tracts between brain and spinal cord
2. The location of many important centers for the 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 the spinal cord. Medulla forms the main pathway for the ascending and descending tracts of the spinal cord. It also has many important centers which control the vital functions.

Read And Learn More: Medical Physiology Notes

1. Respiratory Centers: Dorsal and ventral group of neurons form the medullary respiratory centers which maintain normal rhythmic respiration.
2. Vasomotor Center: This center controls blood pressure and heart rate.
3. Deglutition Center: This center regulates the pharyngeal and esophageal stages of deglutition.
4. Vomiting Center: It induces vomiting during irritation or inflammation of Gl tract
5. Superior and Inferior Salivatory Nuclei: The nuclei control the secretion of saliva.
6. Cramai Nerve Nuclei:
   • The nuclei of the 12th, 11th, 10th, and some nuclei of the 8th and 5th cranial nerves are located in the medulla oblongata. The 12th cranial (hypoglossal) nerve controls the movements of the tongue.
   • The 11th cranial (accessory) nerve controls the movements of the shoulder 10th cranial (vagus) nerve controls almost all the vital functions in the body, viz. cardiovascular system, respiratory system, Gl, 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 the 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. The medial and inferior vestibular nuclei extend into the medulla.
   • All the medullary centers and nuclei of cranial nerves are controlled by the higher centers situated in the cerebral cortex and hypothalamus.

Pons

Pons: Pons forms a bridge between the medulla and the midbrain. The Functions of Pons

1. The axons of pontine nuclei join to form the middle cerebellar peduncle or the brachium pontis. Pons forms the pathway connecting the cerebellum with the cerebral cortex
2. The pyramidal tracts pass through the pons
3. The medial lemniscus is joined by the fibers of the 10th, 9th, 7th, and 5th cranial nerves in pons
4. The nuclei of 8th, 7th, 6th and 5th cranial nerves are located in pons
5. It contains the pneumatic and apneustic centers for regulation of respiration
6. Pons also contains the vestibular nuclei, which are already mentioned in medulla oblongata.

Midbrain

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

1. Tectum
2. Cerebral peduncles.

 

1. Tectum: Tectum is formed by two structures:
   1. Superior colliculus
   2. Inferior colliculus.
      1. Superior Colliculus
         • It is a small structure and is an important center for reflexes. Through the tectospinal tract, the superior colliculus controls the movements of the eyes, head, trunk, and limbs in response to visual impulses.
         • The efferent fibers from the superior colliculus going to the nucleus of 3 cranial (oculomotor) nerves cause constriction of pupil during light reflex.
         • Thus, it forms the center for light reflex. Superior colliculus also receives afferents from the optic tract, which helps in the integration of optical and postural reflexes.
      2. Inferior Colliculus: It consists of single layer of neurons to which the lateral lemniscus (auditory fibers) synapses. The inferior colliculus is the center for auditory reflexes. Stimulation of this also produces reflex vocalization.
2. Cerebral Peduncles: Cerebral peduncles include
   1. Basis peduncle
   2. Substantia nigra
   3. Tegmentum which includes red nucleus.
      1. Basis Pedunculus: It consists of pyramidal tract fibers in the middle, temporopontine fibers laterally, and frontopontine fibers medially.
      2. Substantia Nigra: It is situated below the red nucleus. Substantia nigra is considered as one of the components of basal ganglia.
      3. Tegmentum: It lies dorsal to substantia nigra and is actually the upward continuation of the reticular formation in the pons. Tegmentum comprises three decussations and red nucleus.
         • Decussations in tegmentum:
           1. Superior cerebellar peduncle: It is formed by fibers between the cerebellum and other parts of CNS, The fibers are predominantly efferent fibers from the dentate nucleus of cerebellum and few fibers are from other cerebellar nuclei such as nucleus globosus and nucleus emboliformis.
           2. Forel’s decussation: It is due to the crossing of rubrospinal tracts from either side.
           3. Mynert’s decussation: It is due to the crossing of medial longitudinal bundle formed by the efferent fibers of 3rd, 4th, and 6th cranial nerves.

Red Nucleus: Red nucleus is a large oval or round mass of gray matter extending between the superior colliculus and hypothalamus.

Parts of red nucleus:

1. Red nucleus has two parts
   1. Nucleus magnocellularis, which is formed by large cells. Fibers from this form the rubrospinal and retrobulbar tracts
   2. Nucleus parvocellularis, which is formed by smaller cells. The fibers from this form are mainly the rubroreticular tract.
2. Connections of red nucleus
   1. Afferent connections: Red nucleus receives fibers from
      1. Nucleus parvocellularis receives fibers from motor cortex (area 6) – corticofugal fibers.
      2. Nucleus magnocellularis receives fibers from motor cortex (area 6) – pallidorubral fibers
      3. Nucleus magnocellularis receives fibers from dentate nucleus (of the opposite side) – cerebellorubral or dentatorubral tract.
   2. Efferent connections: Red nucleus sends efferent fibers to various parts of brain and spinal cord.
      1. Rubrospinal tract to the spinal cord
      2. Rubrobulbar tract to medulla
      3. Rubroreticular fibers to reticular formation
      4. Rubrothalamic tract to lateral ventral nucleus of thalamus
      5. A rubro-olivary tract to inferior olivary nucleus
      6. Fibers to nuclei of the 3rd, 4th, and 6th cranial nerves.
   3. Functions of red nucleus
      1. Control of muscle tone: Because of its connections with the cerebellum, vestibular apparatus, and skeletal muscle, red nucleus plays an important role in facilitating the muscle tone.
      2. Control of complex muscular movements: Red nucleus cents els the complex muscular movements. It plays an important role in the integration of various impulses received from many important areas of the brain.
      3. Control of righting reflexes: Red nucleus is the center for all the righting reflexes except optical righting reflexes.
      4. Control of movements of the eyeball: Through its efferent connections with nuclei of 3rd, 4th, and 6th cranial nerves, the 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 skilled muscular movements by its connections with the spinal cord and cerebral cortex.