The Neuroscience on the Web Series:
CMSD 620 Neuroanatomy of Speech, Swallowing and Language

CSU, Chico, Patrick McCaffrey, Ph.D.

Chapter 6. The Midbrain, Pons, Medulla and Reticular Formation

The Midbrain (mesencephalon)

The mesencephalon is the most superior part of the brainstem. It is divided into an anterior and a posterior section by the Aqueduct of Sylvius which connects the third and fourth ventricles. Motor tracts, including the fibers of the pyramidal system, pass downward on the midbrain's anterior surface. Sensory axons, including those of the spinothalamic tract also ascend, along the front of the midbrain behind the motor tracts.

The corpora quadrigemina, which is located on the posterior surface of the midbrain, is composed of two superior colliculi and two inferior colliculi. The superior colliculi are part of the visual system, relaying input from the optic tract to the lateral geniculate bodies of the thalamus. The inferior colliculi are part of the auditory pathway and send information to the medial geniculate bodies of the thalamus.

Several important nuclei are located in the midbrain, including the red nuclei, the substantia nigra, and the nuclei of cranial nerves III and IV.

The red nuclei connect the midbrain to the cerebellum and to the inner ear. It is also an important part of the extra pyramidal tract. The cerebellum compares input from muscles and joints with motor output from the cortex and relays subsequent adjustments to the cortex through the red nucleus and thalamus.

The substantia nigra is a group of dark-colored, dopaminergic cells. Lesions here cause Parkinson's disease

CN III is the oculomotor nerve.

CN IV is the trochlear nerve.

Both of these cranial nerves provide innervation for motor movements of the eyes.

The cerebral peduncles (cruz cerebri) are two very large bundles of axons which are a continuation of the efferent projections within the internal capsule. They begin and end within the brain stem. The efferent projections include cortico bulbar (cortex to brain stem) and cortico spinal (cortex to spinal cord) axons. Webb, Adler and Love, 2008.

The Pons

The pons, which is also part of the brain stem, is inferior to the midbrain and superior to the medulla. Its posterior border is separated from the cerebellum by the aqueduct of Sylvius, and more inferiorly, by the fourth ventricle. Motor and sensory tracts traverse the anterior surface of the pons. The sensory fibers are located behind the motor fibers.

The nuclei of cranial nerves V and VI are located in the pons. CN V, or the trigeminal, sends motor messages to the jaw and receives sensory messages from the teeth, tongue, and parts of the face. CN VI, or the abducens, provides motor innervation to the eye.

The motor nucleus of cranial nerve VII, the facial nerve, is located on the border of the pons and medulla. The upper part of the nerve innervates the muscles of facial expression including the eye lids, forehead and the lips The lower part innervates the voluntary muscles of the face below the eyelids. Cortico-bulbar (pyramidal) fibers provide contralateral and ipsa innervation (bilateral) to the muscles of the upper face but only contralateral (unilateral) innervation to the lower face. Additionally, facial paralysis due to a pyramidal lesion will not permit voluntary control of the muscles but these paralyzed muscles will respond to emotional expression. This is due to extrapyramidal nonvolitional control. An example is the natural smile created by the basal ganglia, and a smile for the camera, or Biden's smile during the debate with Palin. I can't decide whether Palin's wink was deliberate or involuntary.

The Medulla Oblongata

The medulla is the most inferior part of the brain stem. The cell bodies of several cranial nerves are found there. Tthe cell body with it's nucleus, and the axon and axonal branches are all part of the lower motor neuron.

Lower Motor Neuron Cell Bodies

CN IX, the glossopharyngeal nerve

CN X, the vagus nerve

CN XI, the spinal accessory nerve

CN XII, the hypoglossal nerve

Because the nuclei of the vagus nerve are found in the medulla, it is a center for circulation and respiration. It is also quite important to swallowing. It controls muscles of the pharynx, larynx and velum.

(Note: Most of the cranial nerves important for speech and swallowing are located in the medulla.)

The Reticular Formation

The reticular formation is a set of interconnected nuclei that are located throughout the brain stem. Its dorsal tegmental nuclei are in the midbrain while its central tegmental nuclei are in the pons and its central and inferior nuclei are found in the medulla.

The reticular formation has two components:

The ascending reticular formation is also called the reticular activating system. It is responsible for the sleep-wake cycle, thus mediating various levels of alertness. This part of the reticular system projects to the mid-line group of the thalamus, which also plays a role in wakefulness. From there, information is sent to the cortex.

The descending reticular formation is involved in posture and equilibrium as well as autonomic nervous system activity. It receives information from the hypothalamus. The descending reticular formation also plays a role in motor movement.

Interneurons of the reticular formation receive some of the cortico-bulbar fibers from the motor cortex. It is those fibers that innervate the three cranial nerves involved in eye movement. Other cortico-bulbar fibers innervate cranial nerves directly. The descending reticular nuclei in the brain are involved in reflexive behavior such as coughing, chewing, swallowing and vomiting.

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Other courses in the Neuroscience on the Web series:
CMSD 636 Neuropathologies of Language and Cognition | CMSD 642 Neuropathologies of Swallowing and Speech

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