Brain Stem
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Introduction
The brainstem regulates many basic functions, including respiration, heart rate, blood pressure, and visceral activity. It includes the midbrain, pons, and the medulla oblongata, which is continous with the spinal cord.
The brainstem contains tracts that contains tracts that connect the cortex, cerebellum, and spinal cord, as well as important autonomic nuclei. It serves as the origin of almost all the cranial nerves.
White (tracts) and gray (nuclei) matter are distributed throughout the brainstem, with patches of gray scattered throughout.
Pons
The anterior pons is dominated by the basal pons, which looks like a bridge (pont, en francais) between the two cerebellar hemispheres. The middle cerebellar and superior cerebellar peduncles attach the pons to the cerebellum. Middle Cerebellar Peduncle is the primarily connection between pontine nuclei and cerebellum. The Superior Cerebellar Peduncle is...
The trigeminal nerve (CN V) enters the middle of the pons. The abducent nerve (CN VI) leaves the anterior pons medially at its connection with the medulla. The facial nerve (CN VII) enters and leaves the pons in two parts lateral to the abducens nerve. The vestibular nerve (CN VIII) enters the pons in two parts as well, lateral to the facial nerve.
Tracts and Nuclei
The corticopontine fibres and pontine nuclei originate in many areas of the cortex, communicating intention to make skilled movements; these fibres terminate on ipsilateral pontine nuclei before crossing the midline and entering the middle cerebellar peduncle.
The abducens nucleus is...
The facial nucleus is an important sensory nucleus of the face, receiving signals regarding fine touch of tongue etc. It's motor functions include the muscles of mastication - temporalis, masseter, tensor tympani, others. Smaller muscle groups are also involved the - stapedius and posterior digastric.
The mesencephalic nucleus is involved in proprioreception; cell bodies are in CNS after passing through the trigeminal ganglion.
Signals to cerebellum, principal nucleus, and reticular formation.
Neurons pass through the facial ganglion, where sensory pseudounipolar cell bodies lie. The genu of facial nerve, along 4th ventricle, forms facial colliculus. It exits at the boundary between the pons and medulla.
The superior and lateral vestibular nuclei controls head and eye movements in a reflex fashion through CN III, IV, and VI and the medial longitudinal fasciculus. Signals arrive from the vestibular system in the four vestibular nuclei in the pons.
The trapezoid body is involved in auditory processing. It is a structure of crossed fibres from trochlear nucleus and is in the same relative position as medial lemniscus.
The medial lemniscus, carrying sensory information from the body, takes on a transverse orientation en route through the midbrain to the ventral posterolateral nucleus in the thalamus.
The anterior spinocerebellar tract leaves the midbrain through the superior cerebellar peduncle
Midbrain
The midbrain is an important conduit to the cortex, also containing several important cranial nerve nuclei. It is an extension if the pons inferiorly. Superiorly, it makes direct connection with the thalamus, the internal capsule, and the basal ganglia (I think...)
The cerebral aqueduct runs through the midbrain, separating the posterior tectum from the anterior tegmentum.
The midbrain is characterized by the paired superior and inferior colliculi on its posterior surface and the cerebral peduncles (basis pedunculi) on its anterior surface.
The oculomotor (III) nerve emerges between the two cerebral peduncles. The trochlear (IV) nerve leaves the midbrain-pons junction just below the inferior colliculus.
The periaqueductal gray (PAG) is an area of gray matter surrounding the cerebral aqueduct, if you can believe it. It is an important part of the decending pain control system.
The superior colliculus coordinates head and eye movements during tracking. It contains the Edinger-Wesphal nucleus adjacent to oculomotor nucleus, with parasympathetic neurons for pupillary constriction. Its inputs include the retinal ganglion cells via optic tract. Somatosensory input is via spinomesencephalic tract. Outputs are the lateral geniculate nucleus of thalamus, the tectobulbar pathway, with lower motor neurons of CN III ,IV, and VI; the tectospinal tract, with LMNs of cervical spinal cord; and the reticular formation.
The inferior Colliculus is an auditory relay. The inferior colliculi are paired bumps on the posterior midbrain. Its input is the auditory pathway; outputs are via the inferior brachium, to the medial geniculate nucleus in the thalamus.
The cerebral peduncles are large collections of fibres on the anterior midbrain, receiveing information from the internal capsule and transmitting it to the medullary pyramids. They include the corticospinal tract and motor fibres from corticopontine tract.
Tracts and Nuclei
The red nucleus is a large, richly vascularized nucleus. It is important in motor coordination. It acts as a relay and modifying station, receiving information from the cerebellum via the superior cerebellar peduncle (which decussates in the midbrain) and sending it to the thalamus and contralateral spinal cord via the rubrospinal tract. This is important for tone of flexor muscles. It also sends signals to the inferior olive which then return to the cerebellum
The Ventral Tegmental Area (VTA) lies medially of the subtantia nigra pars compacta, and comprises of dopaminergic neurons with projections to the prefrontal cortex (mesocortical) and parts of the limbic system, ie amygdala (mesolimbic). It is associated with reward, as well as drug addiction and schizophrenia.
The substantia nigra is an important dopaminergic mediator of movement and emotion.
Medulla
The medulla is the point where the pons meets the spinal cord. It is cone-shaped. Anteriorly, the medulla is dominated by the pyramids, carrying descending corticospinal fibres from the cortex to the spinal cord. The inferior olives lie laterally of the pyramids. Posteriorly, the fasiculus gracilis and cuneatus of the dorsal column of the spinal cord end in the nucleus gracilis (medial) and nucleus cuneatus (lateral), seen on the surface as tubercles.
The medulla is the site of decussation of the dorsal column, which carries sensory information pertaining to touch, proprioreception, and vibration. The fasciculus gracilis and cuneatus, terminate in their respective nuclei, with second order arcuate fibres arching anteriorly and medially, crossing the midline at the sensory decussation. They form medial lemniscus, which still maintains somatotopy. This is the site of disparate pathway overlap. From here, fibres ascend to the thalamus (VPl).
Tracts and Nuclei
Pyramidal tracts are on the anterior side of the brainstem. They are involved in transmission of motor signals from the cortex to the spinal cord, and are part of the corticospinal tracts; site of decussation of 85% (lateral corticospinal tracts).
The inferior olive is a large, prominent nucleus in the medulla. Input includes the motor cortex, the red nucleus (cerebellum-midbrain-medulla-cerebellum loop), the spinal cord, and the vestibular nuclei.
The olive transmits to the cerebellum via the inferior peduncle via crossed olivocerebellar fibres. These give rise to 'climbing fibres' in the cerebellum which have important effects on cerebellar output. These are glutaminergic axons with a 1:1 relationship with Purkinje cells. Climbing fibres, which have about 200 synaptic contacts each, generate massive EPSPs of about 40 mV, evoking a burst of spikes in Purkinje cells.
The nucleus ambiguus lies just above the inferior olive in the lateral medulla. It involves motor nucleus associated with CN IX, X. Outflow provides motor innervation to striated muscle of the palate, pharynx, larynx, and upper 1/3 of the esophagus; lesions here can cause dysphagia.
The dorsal motor nucleus of vagus lies subjacent to vagal trigone. It provides parasympathetic innervation to thorax, abdomen, to proximal 2/3 of transverse colon.
The solitary tract and nucleus carries visceral sensory input regarding taste (VII, IX) and visceral (IX, X) sensation. These synapse on 2nd order neurons in surrounding nucleus.
The spinal tract and nucleus of the trigeminal involves pain and temperature information for face, mouth, teeth, anterior 2/3 of tongue, scalp, and head from CN V. It also receives pain and temperature input from VII, IX, and X. It extends from CN V after entry to pons, and becomes continuous with substantia gelatinosa of the spinothalamic tract, making a continuous series of nuclei dealing with pain and temperature.
The spinothalamic tract continues superiorly, maintaining its anterolateral position just anterior to spinal tract of the trigeminal.
Cranial Nerves connected to the medulla include the glossopharyngeal (9th), the accessory (11th), and the hypoglossal nerve (12th). Lower motor neurons exit medulla between medulla and pyramids; lesions here causes the tongue to deviate ipsilaterally.
The medial longitudinal fasciculus (MLF) is involved in coordinating head and eye movements. (main article: motor control)
The spinocerebellar tract arrives from the spinal cord carrying proprireceptive information. It leaves the brainstem in the medulla, entering the cerebellum via the inferior cerebellar peduncle.
The reticular formation is in centre of medulla; intermingled in gray and white matter (main article: consciousness).
Conditions Affecting the Brainstem
Medial medullary sydrome is involves specific deficits caused by lesions affecting specific structures - a contralateral hemiparesis (pyramids), contralateral tactile and kinesthetic deficits (medial lemniscus), and ipsilateral paralysis and atrophy of tongue (hypoglossal nucleus).
Lateral medullary (Wallenberg's) syndrome can be caused by occlusion of one vertebral or PICA artery. It results in contralateral pain and temperature loss from body (spinothalamic tract), ipsilateral pain and temperature loss from face (spinal trigeminal tract), hoarseness and difficulty swallowing (nucleus ambiguus), and Horner's syndrome.
Resources and References
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