MEDULLA OBLONGATA PONS MIDBRAIN
MEDULLA OBLONGATA
MEDULLA OBLONGATA The brain stem consists of the medulla oblongata, pons and midbrain. It is sited in the posterior cranial fossa, and its ventral surface lies on the clivus. The medulla oblongata between the pyramidal decussation and the lower border of the pons represents the transition from the spinal cord to the brain. The anterior median fissure, which is interrupted by the pyramidal decussation , and the anterolateral sulcus on each side extend up to the pons.
MEDULLA OBLONGATA The anterior funiculi thicken below the pons to form the pyramids . Lateral to them on each side bulge the olives . Medulla oblongata and pons together form the hindbrain , also known as rhombencephalon, named after this fossa. The medulla oblongata extends from just above the first pair of cervical spinal nerves to the lower border of the pons. It is approximately 3 cm in length and 2 cm in diameter at its widest.
MEDULLA OBLONGATA The ventral surface of the medulla is separated from the basilar part of the occipital bone and apex of the dens by the meninges and occipito-axial ligaments. Caudally, the dorsal surface of the medulla occupies the midline notch between the cerebellar hemispheres. Caudally, the ventral median fissure is interrupted by the obliquely crossing fascicles of the pyramidal decussation .
MEDULLA OBLONGATA Rostrally, it ends at the pontine border in the foramen caecum . Immediately lateral to the ventral median fissure there is a prominent elongated ridge, the pyramid , which contains descending pyramidal, or corticospinal, axons. The lateral margin of the pyramid is indicated by a shallow ventrolateral sulcus. From the ventrolateral sulcus emerges, in line with the ventral spinal nerve roots, a linear series of rootlets which constitute the hypoglossal nerve . The abducens nerve emerges at the slightly narrowed rostral end of the pyramid, where it adjoins the pons.
MEDULLA OBLONGATA Caudally the pyramid tapers into the spinal ventral funiculus. Lateral to the pyramid and the ventrolateral sulcus there is an oval prominence, the olive , which contains the inferior olivary nucleus. Lateral to the olive is the posterolateral sulcus. The glossopharyngeal, vagus and accessory nerves join the brain stem along the line of this sulcus, in line with the dorsal spinal nerve roots.
MEDULLA OBLONGATA Some brain stem cell groups are the nuclei of cranial nerves CN III – CN XII : they are concerned with the sensory, motor and autonomic innervation of the head and neck. Other autonomic fibres that arise from the brain stem are distributed more widely via the vagus nerve CN X . The brain stem also contains the reticular formation , that extends throughout its length, and is continuous caudally with its spinal counterpart.
MEDULLA OBLONGATA Some reticular nuclei are referred to as vital centres since they are concerned with regulation of cardiac and respiratory activities; other parts of the reticular formation are involved in the regulation of muscle tone, posture and reflex activities. The brain stem is the site of termination of numerous ascending and descending fibres and is traversed by many others: • spinothalamic tract (spinal lemniscus), • medial lemniscus, • trigeminothalamic tracts all ascend through the brain stem to reach the thalamus .
MEDULLA OBLONGATA MEDIAL STRUCTURES OF MEDULLA 1. The hypoglossal nucleus of CN XII 2. The medial lemniscus, which contains crossed fibers from the gracile and cuneate nuclei 3. The pyramid (corticospinal fibers)
MEDULLA OBLONGATA LATERAL STRUCTURES OF MEDULLA 1. The nucleus ambiguus (CN IX, X, and XI) 2. The vestibular nuclei (CN VIII) 3. The inferior cerebellar peduncle, which contains the dorsal spinocerebellar, cuneocerebellar, and olivocerebellar tracts 4. The lateral spinothalamic tract (spinal lemniscus) 5. The spinal nucleus and tract of trigeminal nerve
MEDULLA OBLONGATA In the caudal medulla, the corticospinal and dorsal column - medial lemniscal pathways - send axons across the midline. The nucleus gracilis and nucleus cuneatus give rise to axons that decussate in the caudal medulla (the crossing axons are the internal arcuate fibers), which then form and ascend in the medial lemniscus. The corticospinal (pyramidal) tracts , which are contained in the pyramids, course ventromedially through the medulla.
MEDULLA OBLONGATA Most of these fibers decussate in the caudal medulla just below the crossing of axons from the dorsal column nuclei, and then travel down the spinal cord as the (lateral) corticospinal tract. The olives are located lateral to the pyramids in the rostral two-thirds of the medulla. The olives contain the convoluted inferior olivary nuclei. The olivary nuclei send climbing (olivocerebellar) fibers into the cerebellum through the inferior cerebellar peduncle. The olives are a key distinguishing feature of the medulla.
MEDULLA OBLONGATA The spinothalamic tract and the descending hypothalamic fibers course together in the lateral part of the medulla below the inferior cerebellar peduncle and near the spinal nucleus and tract of CN V. Clinically, even small lesions can destroy vital cardiac and respiratory centres, disconnect forebrain motor areas from brain stem and spinal motor neurones. Irreversible cardiac and respiratory arrest follows complete destruction of the neural respiratory and cardiac centres in the medulla.
MEDULLA OBLONGATA MEDULLARY LEVEL CN IX , the glossopharyngeal, and CN X , the vagus nerve — of its several nuclei, one supplies the muscles of the pharynx and larynx; the vagus nerve is primarily a parasympathetic nerve to the organs of the thorax and abdomen. • CN XI , the spinal accessory nerve, innervates some of the muscles of the neck. • CN XII , the hypoglossal nerve, supplies motor fibers to the muscles of the tongue.
MEDULLA OBLONGATA CRANIAL NERVE NUCLEI SPINAL NUCLEUS OF CN V The spinal nucleus of the trigeminal nerve (CN V) is located in a position analogous to the dorsal horn of the spinal cord. Central processes from cells in the trigeminal ganglion conveying pain and temperature sensations from the face enter the brain stem in the rostral pons but descend in the spinal tract of CN V and synapse on cells in the spinal nucleus. SOLITARY NUCLEUS The solitary nucleus receives the axons of all general and special visceral afferent fibers carried into the CNS by CN VII, IX, and X. These include taste, cardiorespiratory, and gastrointestinal sensations carried by these cranial nerves. Taste and visceral sensory neurons all have their cell bodies in ganglia associated with CN VII, IX, and X outside the CNS.
MEDULLA OBLONGATA CRANIAL NERVE NUCLEI NUCLEUS AMBIGUUS The nucleus ambiguus is a column of large motoneurons situated dorsal to the inferior olive. Axons arising from cells in this nucleus course in the CN IX and CN X. In the CN X , these fibers supply muscles of the soft palate, larynx, pharynx, and upper esophagus. A unilateral lesion will produce ipsilateral paralysis of the soft palate causing the uvula to deviate away from the lesioned nerve and nasal regurgitation of liquids, weakness of laryngeal muscles causing hoarseness, and pharyngeal weakness resulting in difficulty in swallowing. uvulA Away CN X
MEDULLA OBLONGATA CRANIAL NERVE NUCLEI DORSAL MOTOR NUCLEUS OF CN X These visceral motoneurons of CN X are located lateral to the hypoglossal nucleus in the floor of the fourth ventricle. This is a major parasympathetic nucleus of the brain stem, and it supplies preganglionic fibers innervating terminal ganglia in the thorax and the foregut and midgut parts of the gastrointestinal tract.
MEDULLA OBLONGATA CRANIAL NERVE NUCLEI HYPOGLOSSAL NUCLEUS The hypoglossal nucleus CN XII is situated near the midline just beneath the central canal and fourth ventricle. This nucleus sends axons into the hypoglossal nerve to innervate all of the tongue muscles except the palatoglossus. The lesion of the hypoglossal nerve: tongue pointing toward same (affected) side on protrusion. Tongue Towards CN XII
MEDULLA OBLONGATA CRANIAL NERVE NUCLEI THE ACCESSORY NUCLEUS The accessory nucleus CN XI is found in the cervical spinal cord. The axons of the spinal accessory nerve arise from the accessory nucleus, pass through the foramen magnum to enter the cranial cavity, and join the fibers of the vagus to exit the cranial cavity through the jugular foramen. As a result, intramedullary lesions do not affect fibers of the spinal accessory nerve. The spinal accessory nerve supplies the sternocleidomastoid and trapezius muscles. The lesion of the spinal accessory nerve – drooping shoulder.
MEDULLA OBLONGATA CRANIAL NERVE NUCLEI The rootlets of the glossopharyngeal (CN IX) and vagus (CN X) nerves exit between the olive and the fibers of the inferior cerebellar peduncle. The hypoglossal nerve (CN XII) exits more medially between the olive and the medullary pyramid.
PONS
PONS PONS The pons is located between the medulla (caudally) and the midbrain (rostrally). The cerebellum overlies the pons. On the ventral surface of the brain stem, the transition between medulla and pons is clearly demarcated by a transverse sulcus. Laterally, in a region known as the cerebellopontine angle , the facial CN VII, vestibulocochlear CN VIII and glossopharyngeal CN IX nerves emerge.
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