Lecture: 3 Dr. Eyad M. Hussein Anatomy and Physiology of the Brain Stem Dr. Eyad M. Hussein Ph.D of Neurology Consultant in Neurology Department, Nasser Hospital, Assistant Professor, Faculty of Medicine, Islamic University Faculty of Dentistry, University of Palestine

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Morphological Subdivisions of the Brain The Cerebrum: formed of right and left cerebral hemisphere. 2. The Cerebellum: below the posterior part of the cerebrum. 3. The Brain Stem.

The Brain Stem The brain stem occupies the posterior cranial fossa. The brain stem lying infront of the cerebellum and fourth ventricle. The brain stem formed of the following parts (from downward): 1. Medulla oblongata. 2. Pons. 3. Midbrain.

The Medulla Oblongata The medulla oblongata is the lower part of the brain stem (3 cm). Extent: Above: it is continuous with pons. Below: it is continuous with the spinal cord at the foramen magnum.

Parts of the Medulla Oblongata Closed Medulla: it is the lower half of the medulla, as it encloses a central canal continuous with that of spinal cord. Open Medulla: it is the upper half of the medulla, as it opens into the fourth ventricle.

External Features of the Medulla Oblongata A. Anterolateral Surface: The anterior median fissure. The pyramid: formed by the pyramidal (corticospinal) tract. Pyramidal decussation. The olive: formed by the inferior olivary nucleus. The anterolateral sulci: gives exit to the hypoglossal nerves. The posterolateral sulci: gives exit to the glossopharyngeal, vagus and cranial accessory nerves. The inferior cerebellar peduncle.

B. The posterior surface of the medulla oblongata: 1. The posterior surface of the upper half (open medulla): from medial to lateral: a. Posterior median fissure. b. Inferior fovea. c. Hypoglossal trigone (triangle). d. Vagal trigone. e. Vestibular trigone. 2. The posterior surface of the lower half (closed medulla): from medial to lateral: b. Gracile tract. c. Cuneate tract.

Internal Structures of the Medulla Oblongata The main nuclei of the medulla oblongata: 1. Gracile nucleus: proprioceptive and fine touch from the lower 1/2 of the body. 2. Cuneate nucleus: proprioceptive and fine touch from the upper 1/2 of the body. 3. Inferior olivary nucleus: extrapyramidal function. 4. Inferior salivary nucleus: parasympathetic function via the glossopharyngeal nerve.

5. Spinal descending nucleus of trigeminal nerve: pain and temperature sensations from the face and scalp via the trigeminal nerve. 6. Solitary nucleus: taste sensations via the facial, glossopharyngeal and vagus nerves. 7. Nucleus ambiguous: motor function of the glossopharyngeal, vagus and cranial accessory nerves. 8. Dorsal nucleus of vagus nerve: parasympathetic via the vagus nerve. 9. Hypoglossal nucleus: motor function of the tongue via the hypoglossal nerve.

The Pons Extent: it extends from the medulla oblongata below to midbrain above and lies infront of the cerebellum and fourth ventricle.

External Features of the Pons A. The anterior surface of the pons: presents the following features: 1. The basilar groove: for basilar artery. 3. The middle cerebellar peduncle. 4. The trigeminal nerve. 5. The abducent nerve. 6. The facial nerve. 7. The vestibule-cochlear nerve.

B. The posterior surface of the pons: presents the following features: 1. The median longitudinal sulcus: the middle line. 2. The medial eminence: for facial nucleus. 3. The facial colliculus: it produced by the facial nucleus. 4. The medullary stria: transverse nerve fibers which separate the posterior surface of pons from that of medulla oblongata. 5. Superior fovea. 6. Vestibular trigon.

Internal structures of the pons Pontine nuclei: they form part of the cortico-ponto- cerebellar pathway. 2. Transverse fibers: pontocerebellar fibers. 3. Longitudinal fibers: which include pyramidal and cortico-pontine fibers. 4. Nuclei of the trigeminal nerve: a. Motor nucleus. b. Sensory nuclei. 5. Nucleus of the abducent nerve. 6. Nuclei of the facial nerve: Motor nucleus. b. Superior salivary nucleus: parasympathetic function.

7. Nuclei of the vestibulocochlear nerve. 8. Lateral lemniscus: for auditory sensation. 9. Spinal lemniscus: it is a band of ascending fibers carrying pain, temperature and crude touch from the opposite side of the body below the head. 10. Trigeminal lemniscus: it is a band of ascending fibers carrying pain, temperature and touch and proprioception from the opposite side of the face and scalp. 11. Medial lemniscus: for deep sensation and fine touch from the opposite side of the body below the head.

The Midbrain Extent: It extends between the pons below and the diencephalon above. Connection: It connects pons and cerebellum to the subthalamic region of the diencephalon.

External features of the midbrain A. The anterior surface of the midbrain: presents the following features: 1. Two cerebral peduncles. 2. Oculomotor nerve: emerging from the medial side of the cerebral peduncle.

B. The posterior surface of the midbrain: presents the following features: 1. Four colliculi which include: a. Two superior colliculi (Visual function). b. Two inferior colliculi (Auditory function). 2. Two superior cerebellar peduncles. 3. Superior medullary velum (membrane): lies between the two superior cerebellar peduncles. 4. The trochlear nerve. C. The lateral surface of the midbrain: presents the following features: 1. Brachium of superior colliculus. 2. Brachium of inferior colliculus.

Internal Structures of the Midbrain In transverse section of the midbrain, the Sylvius aqueduct divides it into two main parts: 1. Tectum: the smaller dorsal part behind the Sylvius aqueduct, and consists of: a. Two Superior Colliculi: reflex centers of vision. b. Two Inferior Colliculi: reflex centers of hearing.

2. Two cerebral peduncles: the larger ventral part infront of aqueduct, it consists of three parts: a. Crus cerebri: the most anterior part which consists of pyramidal and corticonuclear fibers. b. Substantia Nigra: a thick lamina of gray mater. It is an extrapyramidal center. c. Tegmentum: the posterior part of the cerebral peduncle. It contains ascending tracts, nuclei of III & IV cranial nerves, reticular formation of the midbrain.

Internal Structures of the Midbrain The main nuclei of the midbrain: 1. Nucleus of oculomotor nerve. 2. Nucleus of the trochlear nerve. 3. Red nucleus: it is an important extrapyramidal centre. 4. Mesencephalon nucleus of the trigeminal nerve: for deep sensation. 5. Inferior colliculus: it is a centre for auditory reflexes. 6. Superior colliculus: it is a centre for visual reflexes. 7. Substantia nigra: it is an extrapyramidal centre. 8. Reticular formation.

Function of the Brain Stem Cardiovascular center. Respiratory and cough center. Swallowing and vomiting center. Sleep center. Center for eye movement. Reticular nuclei. Alertness and consciousness Vestibular nuclei. Conduction function. It contains the important nuclei of cranial nerves III through XII. Important as extrapyramidal function (substantia nigra).

Reticular Formation of the Brain Stem Location: in the brain stem mainly in the midbrain. Connections: the reticular formation is connected to: 1. Cerebral cortex. 2. Basal ganglia. 3. Cerebellum. 4. Spinal cord. 5. Thalamus, hypothalamus, and limbic system. 6. Nuclei of the cranial nerves.

Functions of the Reticular Formation Control the level of consciousness (wakefulness) trough the ARAS. Regulation of the stretch reflexes and muscle tone. Pain inhibition. Control of sleep. Control visceral functions (e.g. heart rate, BP, respiration, salivation, swallowing and vomiting). Serotonin production.

Conduction Function The brain stem plays important role for conduction (afferent and efferent pathway). The Four Lemnisci: Medial Lemniscus: for deep sensation and fine touch below the head. Started from Gracile and Cuneate nuclei and ends in the thalamus Lateral Lemniscus: for auditory sensation. Trigeminal Lemniscus: started from trigeminal nuclei “after decussation” and ends in the thalamus. Spinal Lemniscus: ventral and lateral spinothalamic tract ends in the thalamus.

Medial Longitudinal Bundle Origin: midbrain. Function: rotational movement of the head and eyes: Coordinates movement of the medial and lateral recti muscles of both eyes. Coordinates movements the head and eyes in response to cochlear stimuli. Coordinates movement of the facial muscles (lips), tongue, and soft palate.

Breathing Center The respiratory neurons are located bilaterally in the medulla oblongata and divided into two group: Posterior respiratory group: modulate respiratory patterns. Anterior respiratory group: coordinates the innervation of both inspiratory and expiratory muscles.

Postural Reflexes Posture is defined as the active muscular resistance to displacement body by gravity. There are two types of postural reflexes: Static Reflexes: maintain the balance during rest. Statokinetic Reflexes: maintain the balance during movement.

Center for Cranial Nerves

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