THE BRAIN STEM

THE BRAIN STEM

The brain stem is divided into three, roughly inch-long sections: the midbrain, pons, and medulla oblongata. The brain stem is primarily composed of gray matter surrounded by white matter fiber tracts. However, unlike the spinal cord (which is otherwise identical in composition to the brain stem), the brain stem has nuclei of gray matter embedded within the white matter. The brain stem produces rigidly programmed, automatic behaviors necessary for survival. The brain stem provides a pathway for fiber tracts running between the cerebrum and spinal cord. It is also associated with 10 of the 12 pairs of cranial nerves, making it heavily involved in the innervation of the head.

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MIDBRAIN

The midbrain is found between the diencephalon and the pons. Two cerebral peduncles are found on its ventral aspect. They are vertical pillars that appear to hold up the cerebrum. Each peduncle's crus cerebri contains a large pyramidal motor tract descending to the spinal cord. The midbrain is connected to the cerebellum dorsally by the superior cerebellar peduncles, which are also fiber tracts. The third and fourth ventricles are connected by the hollow cerebral aqueduct, which delineates the cerebral peduncles ventrally from the tectum (the midbrain's roof). The aqueduct is surrounded by the periaqueductal gray matter, which is involved in pain suppression and serves as the link between fear-perceiving amygdala and ANS pathway that controls the "fight-or-flight" response. 

As previously mentioned, gray matter nuclei are scattered throughout the surrounding white matter. The corpora quadrigemina are the largest of these nuclei. They raise four domelike protrusions on the dorsal midbrain surface. The superior colliculi (the superior pair) are visual reflex centers that coordinate head and eye movements when we visually follow a moving object, even if we don't consciously intend to look at the object. The inferior colliculi form a corresponding part of the auditory reflex from the hearing receptors of the ear to the sensory cortex. They also act in reflexive response to sound, such as in the startle reflex, which causes you to turn your head toward an unexpected sound. The bandlike substantia nigra are also embedded in each side of the midbrain white matter, deep to the cerebral peduncle. These neurons are highly responsible for releasing the neurotransmitter dopamine and are functionally linked to the basal nuclei and are usually considered part of the basal nuclear complex. Another embedded nucleus is the oval-shaped red nucleus. It is found deep to the substania nigra and acquires its reddish hue because it has a rich blood supply and a relatively high level of iron pigment within its neurons. They are embedded in a system of small nuclei scattered through the core of the brain stem called the reticular formation and are primarily responsible for acting as relay nuclei in some descending motor pathways that effect limb flexion. 

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PONS

The pons is a bulging region of the brain stem wedged between the midbrain and the medulla oblongata. Its dorsal side forms part of the anterior wall of the fourth ventricle. It is chiefly composed of conduction tracts which are oriented in two different directions. Deep projection fibers run longitudinally and complete the pathway between higher brain centers and the spinal cord. The superficial ventral fibers are oriented transversely and dorsally. These fibers connect the pons bilaterally with the two sides of the cerebellum dorsally and form the middle cerebellar peduncles. These fibers come from numerous pontine nuclei which act as relays between the motor cortex and cerebellum. Several pairs of of cranial nerves come out of the pontine nuclei, some of which are responsible for helping the medulla maintain normal rhythm of breathing. 

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MEDULLA OBLONGATA

The medulla oblongata, or medulla, is the most inferior part of the brain stem. While the exact fusion point between the medulla and spinal cord is not discernible, it is considered to be at the foramen magnum of the skull. The cavity of the fourth ventricle is formed by the central canal of the spinal cord proceeding upward into the medulla. The fourth ventricle's ventral wall is indeed formed by the medulla and the pons. Large corticospinal tracts from the motor cortex create pyramids, two longitudinal ridges on flanking the midline of the medulla's ventral aspect. Most of the fibers cross to the opposite side before descending into the spinal cord at a point known as the decussation of the pyramids. This crossing over is the cause of the contralateral control exerted by the brain over most of the body. Other visible structures on the exterior of the medulla include the olives, which are swellings caused by folds of gray matter of the underlying inferior olivary nuclei. These nuclei relay sensory information on the state of stretch of muscles and joints to the cerebellum. Various nerves exit the brain stem in this area, including nerves that synapse with the cochlear nuclei (auditory relays) and numerous nuclei of thevestibular nuclear complex, which mediates responses that maintain equilibrium. Dorsally located in the medulla are the nucleus gracilis and nucleus cuneatus, which serve as relays for general somatic sensory information to the somatosensory cortex. 

The medulla also has vital visceral motor nuclei responsible for various aspects of life:

1) Cardiovascular center: Adjusts the rate and force of heart contraction and changes blood vessel diameter to regulate blood pressure.
2) Respiratory centers: Generate the respiratory rhythm and control the rate and depth of breathing.
3) Various other centers: Control things like vomiting, hiccuping, swallowing, coughing, and sneezing. 

Many of these functions overlap with those of the hypothalamus because the hypothalamus exerts its control by relaying its instructions through medullary reticular centers.