BRAIN STEM

Brainstem Performs spinal cord-like functions for the head Contains LMN for head muscles Initial processing of general afferent information concerning head

Conduit function Ascending tracts reach thalamus and cerebellum Descending tracts reach spinal cord Many tracts do not run through straight- frequent involvement of relay nuclei

Cranial nerve functions Cranial nerves are head’s equivalent of spinal nerves Involved in olfaction, sight, hearing, equilibrium and taste Olfactory → telencephalon Optic → diencephalon

The rest project to or emerge from brainstem Collection of sensory and motor nuclei related to cranial nerve function at various levels in brainstem

Integrative function Complex motor patterns Regulation of cardiovascular and respiratory activity Some regulation of consciousness [function of reticular formation in central core of brainstem]

Much of this is accomplished by thereticular formation, which forms the central core of the brainstem

Medulla The medulla is vaguely scoop The "handle" corresponds to the caudal or closed portion, containing a central canal continuous with that of the spinal cord. The open portion of the scoop corresponds to the rostral or open medulla, in which the central canal expands into the fourth ventricle.

The apex of the V-shaped caudal fourth ventricle, where it narrows into the central canal, is called the obex The longitudinal grooves on the surface of the spinal cord continue into the medulla, more clearly at some levels than at others. They divide the surface of the caudal medulla and part of the rostral medulla into a series of columns that completely encircle it

The anterior median fissure is briefly interrupted by the pyramidal decussation at the junction between spinal cord and brainstem, but then it continues rostrally to the edge of the pons, separating the two pyramids

The pons Pons is the Latin word for "bridge," and this portion of it (called the basal pons) looks like a bridge interconnecting the two cerebellar hemispheres. It is not, however, a direct interconnection. Rather, many of the fibers descending in each cerebral peduncle synapse in scattered nuclei of the ipsilateral half of the basal pons.

These nuclei in turn project their fibers across the midline, after which they funnel into the middle cerebellar peduncle (brachium pontis) and finally enter the cerebellum

Transverse planes defining the subdivisions of the brainstem

The Internal Structure of the Brainstem Reflects Surface Features and the Position of Long Tracts The corticospinal and spinothalamic tracts have consistent locations throughout the brainstem The three major longitudinal pathways (corticospinal tract, poterior columns, and spinothalamic tract) can be followed systematically through the brainstem

Two of the three stay in more or less the same location throughout the brainstem. Corticospinal fibers travel in the most ventral part of the brainstem, traversing the cerebral peduncle, basal pons, and medullary pyramid. At the spinomedullary junction, most of the fibers in the pyramids decussate and form the lateral corticospinal tracts

The spinothalamic tract at all levels of the brainstem is in or near the anterolateral corner of the tegmentum, similar to its position in the spinal cord. The posterior columns terminate in the posterior column nuclei (nucleus gracilis and nucleus cuneatus) of the medulla

Locations of the corticospinal tract, medial lemniscus, spinothalamic tract, and reticular formation in the caudal and rostral medulla (A, B), caudal pons and midpons(C, D) and caudal and rostral midbrain (E, F)

Efferent fibers from these nuclei decussate in the medulla to form the medial lemniscus, which reaches the thalamus

Caudal/ “closed” medulla Extends from caudal edge of pyramidal decussation [spinomedullary junction] to obex [caudal end of 4 th ventricle] looks somewhat like the spinal cord. Part of the anterior horn is still present caudally, as are structures similar to Lissauer's tract and part of the posterior horn

The latter two are actually the spinal tract and spinal nucleus of the trigeminal nerve. These are the head's equivalent of Lissauer's tract and the substantia gelatinosa (i.e., they deal with pain, temperature, and some tactile information,

Fasciculi gracilis and cuneatus continue into the caudal medulla but are gradually replaced by the posterior column nuclei (nucleus gracilis and nucleus cuneatus). Nucleus cuneatus begins and ends a bit rostral to nucleus gracilis

Postsynaptic fibers leave these two nuclei in a ventral direction and arch across the midline to form the contralateral medial lemniscus, a vertically oriented band of fibers These decussating fibers are part of the collection of internal arcuate fibers and are sometimes called the sensory decussation.

Caudal medulla –spinomedullary junction- level of pyramidal decussation

Throughout the medulla, the medial lemniscus is organized so that fibers representing cervical segments are most posterior Adjacent to nucleus cuneatus and embedded in fasciculus cuneatus is the lateral (or external) cuneate nucleus This is the upper extremity equivalent of Clarke's nucleus, and the axons of these cells join the posterior spinocerebellar tract in the inferior cerebellar peduncle at a slightly more rostral level.

Caudal medulla, just caudal to the obex..

Level of sensory decussation

The prominent pyramids and their decussation are located most anteriorly in the caudal medulla. Each pyramid consists of corticospinal fibers that originated in ipsilateral cerebral cortex and are (mostly) bound for the contralateral anterior horn.

Rostral Medulla The rostral (open) medulla, as defined here, extends from the obex to the rostral wall of the lateral recess, where the inferior cerebellar peduncle turns posteriorly to enter the cerebellum. The rostral medulla no longer looks much like the spinal cord,

The caudal boundary (the obex) is approximately coincident with the caudal edge of the inferior olivary nucleus, a prominent structure that is responsible for the appearance of the olive as a surface swelling

Fibers leave the medially facing mouth (or hilus) of the inferior olivary nucleus, arching across the midline, and joining the contralateral inferior cerebellar peduncle. Medial to the inferior olivary nucleus is the medial lemniscus, which still has the shape of a flattened band with a dorsal-ventral axis.

Anterior to the medial lemniscus is the pyramid. Fascicles of the hypoglossal (XII) nerve emerge lateral to the pyramid in the groove between it and the inferior olivary nucleus. Posterior to the medial lemniscus, near the floor of the fourth ventricle, is a small but distinctive bundle of fibers that can be followed all the way to the midbrain

Rostral medulla, just rostral to the obex

This is the medial longitudinal fasciculus (MLF), which is involved in coordinating head and eye movements.

The spinothalamic tract remains in the anterolateral portion of the tegmentum, just above the inferior olivary nucleus, as does the anterior spinocerebellar tract. The posterior spinocerebellar tract moves posteriorly and joins the inferior cerebellar peduncle

Level of ‘sensory’ decussation

Level of pyramidal decussation

The Caudal Pons the inferior olivary nucleus ends, and the inferior cerebellar peduncle bends posteriorly and enters the cerebellum The MLF is in the same relative position as it was previously, adjacent to the midline and the floor of the fourth ventricle.

Caudal pons

The pyramidal tract becomes dispersed in the basal pons, which contains bundles of longitudinally oriented fibers, bundles of transversely oriented fibers, and pontine nuclei scattered among these bundles. Some of the longitudinally oriented fibers are those of the pyramidal tract

Fibers arising in the pontine nuclei cross the midline and form the massive middle cerebellar peduncle (brachium pontis). The spinothalamic tract and the anterior spinocerebellar tract remain in the anterolateral portion of the tegmentum..

Spinoreticular fibers related to the spinothalamic system terminate medial to the direct spinothalamic fibers in the reticular formation throughout the brainstem, as do collaterals of direct spinothalamic fibers

Mid pons

Rostral pons extends from the rostral edge of the middle cerebellar peduncle to the beginning of the cerebral aqueduct it includes parts of the basal pons and fourth ventricle but has no physical connection with the cerebellum.

Rostral pons- near pons-midbrain junction

The trigeminal nerve (V) is attached to the brainstem at a midpontine level the trochlear nerve (IV) emerges at the pons- midbrain junction The MLF is visible throughout the rostral pons, as is the basal pons

The fourth ventricle narrows as the plane of section approaches the cerebral aqueduct, and the superior cerebellar peduncle (brachium conjunctivum) becomes apparent in the wall of the ventricle. This is the major outflow from the cerebellum, projecting to the thalamus and to other structures

As the medial lemniscus moves laterally, it approaches the spinothalamic tract; from here through the midbrain, the two are adjacent. The corticospinal tract travels through the rostral pons as a series of longitudinally oriented bundles of fibers, accompanied by more numerous corticopontine bundles.

The anterior spinocerebellar tract moves posteriorly onto the surface of the superior cerebellar peduncle From here it turns caudally and enters the cerebellum, traveling "backward" along the peduncle.

Caudal midbrain essentially the part that contains the inferior colliculi. extends from the point of emergence of the trochlear nerve to the groove between the inferior and superior colliculi. The fourth ventricle has narrowed into the cerebral aqueduct

the superior cerebellar peduncles sink deeper into the midbrain tegmentum and begin to decussate, and the MLF continues on its usual course The basal pons protrudes rostrally under the tegmentum of the caudal midbrain. The inferior colliculus, a major component of the ascending auditory pathway is (literally) a prominent nuclear mass.

Medial to it, encircling the aqueduct, is a particularly pale-staining region of gray matter called, appropriately enough, the periaqueductal gray. The periaqueductal gray is part of an important descending pain-control system discussed later in this chapter In the caudal midbrain, the basal pons gives way to a cerebral peduncle on each side, through which corticospinal and corticopontine fibers travel.

Caudal midbrain- level of inferior colliculus

Rostral Midbrain The rostral midbrain contains the superior colliculi At this level the MLF is ending, decussation of the superior cerebellar peduncles is complete, and in their place a large red nucleus becomes visible on each side. Some fibers from the contralateral half of the cerebellum end here, but most continue on to the thalamus. Anterior to the red nucleus is the substantia nigra

Anterior to the red nucleus is the substantia nigra The pigmented cells characteristic of the dorsal part of the substantia nigra use dopamine for their neurotransmitter Anterior to the substantia nigra is a massive bundle of fibers commonly referred to as the cerebral peduncle.

Rostral midbrain- level of superior colliculus

This bundle consists principally of descending corticopontine and corticospinal fibers. The oculomotor nerve (III) emerges into the space between the cerebral peduncles (the interpeduncular fossa).

the medial lemniscus and the spinothalamic tract form a continuous curved band of fibers. Spinomesencephalic fibers (sometimes referred to as the spinotectal tract) that have accompanied the spinothalamic tract through the brainstem terminate in the periaqueductal gray, adjacent regions of the reticular formation, and certain portions of the superior colliculus

Components of inferior cerebellar peduncle