Brain stem: Nuclei and tracts

Medulla

• Transitional zone between spinal cord and the medulla.• Dorsal expansions of gray matter at the level of pyramids

decussation, form the gracile and cuneate nuclei. • Ventral gray horn includes first cervical, spinal root of

accessory nerve. Bundles of fibers traversed that pass from the pyramids to the lateral corticospinal (CS) tracts.

• The dorsal gray horns of the spinal cord are replaced by spinal trigeminal nuclei.

• Above the pyramidal decussation (DP), the medulla has entirely different structures than that in spinal cord.

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Medial lemniscus system

• Gracile fasciculus: touch, proprioception from lower limb (ipsilateral) ends in gracile nucleus. Cuneate fasciculus from upper limb and terminates in cuneate nucleus. There is a point to point projection of the fibers, which served as the anatomical basis for reorganization of the source of a stimulus.

• Axons from gracile nucleus and cuneate nucleus form the internal arcuate fibers (IA), cross the midline in the decussation of the medial lemniscus then becomes medial lemniscus. It then runs to the ipsilateral ventral posterior nucleus of thalamus, from there then project to the primary somesthetic cortex of the parietal lobe.

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Spinothalamic & Spinotectal Tracts

• Spinothalamic tract conveys pain, temperature, from the contralateral side of the body

• Spinotectal: same as spinothalamic, conveys somesthetic information to superior colliculus and reticular formation of the midbrain.

• The two tracts merge to form spinal lemniscus. Spinal thalamic fibers continue to the ventral posterior nucleus of the thalamus (see previous slides)

Spinoreticular fibers

• conveys sensory data from skin and internal organs• It has different route from spinal cord up to thalamus and

cerebral cortex.• 1).medial lemniscus system

– to ventral posterior thalamic nuclei then somatosensory area of the cerebral cortex.

• 2). neospinothalamic system– prominent in mammals, from lamina I, IV-VI (tract cells) in the

dorsal horn. It has no collateral fibers to the reticular formation. It ends in ventral posteriolateral division of ventral posterior nucleus of thalamus.

• 3). Paleospinothalamic– prominent in all vertibrates. It sends collateral branches to

reticular formation, diversed projections.

Spinocerebellar Tracts

• Carry proprioceptive signals, dorsal (uncrossed), ventral (crossed).

• Dorsal spinocerebellar fibers (L3 above, ) enter the inferior cerebellar peduncle and the ventral spinocerebellar (lumbarsacral region, L-S) continues through pons and enters the cerebellum by superior cerebellar peduncle. Both carry unconscious proprioception to cerebellum (see previous slides).

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Accessory (external) cuneate nucleus

• Lateral to cuneate nucleus, fibers form these nuclei, cuneocerebellar fibers, enter cerebellum via inferior cerebellar peduncle. Served as supplemental pathway for proprioceptive information from upper limbs.

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Inferior olivary complex

• Several groups of neurons receive afferent information from different sources and then project them to cerebellum. These neurons are called precerebellar nuclei, which include components of inferior olivary complex. Inferior olivary nucleus is the largest component.

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Components

• Inferior olivary complex includes– 1). inferior olivary nucleus– 2). medial accessory olivary nucleus– 3). dorsal accessory olivary nucleus

Olivocerebellar fibers

• originate from inferior olivary complex, cross in the midline, enter the inferior cerebellar peduncle. Their function is to maintain equilibrium and the stereotyped movements of postural changes and locomotion

DESCENDING TRACTS

• Corticospinal tract:

• Originate from frontal and parietal lobes, fibers that cross over in the decussation of pyramids (lateral corticospinal tract). Uncrossed fibers (15%, Ventral corticospinal tract), cross at spinal cord level. Major function: motor control

Tracts originate from midbrain

• 1). Central tegmental tract: – arises from ipsilateral red nucleus, terminates in

inferior olivary complex, relay signals• 2). Axons from contralateral red nucleus

(rubrospinal tract)– terminates C2 in human

• 3). tectospinal tract:– originates in superior colliculus, cross the midline

• 4). tectobulbar fibers: – from superior colliculus, ends in reticular formation of

pons and medulla, involves eye movements.

Nuclei of Cranial Nerves

• Hypoglossal nucleus – Innervate tongue muscles

• Nucleus ambiguus– Of vagus nerve, innervate muscles of soft

palate, pharynx, larynx, upper esophagus

• Dorsal nucleus – Vagus’ largest parasympathetic nucleus

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Dorsal Pons (Tegmentum)

• Pons can be divided into:– Ventral (Basal)– Tegmental (dorsal)

Acsending tracts

• Medial lemniscus– Twisted when it leaves medullar and enters

pons. So the sequence is neck, arm, trunk, and leg from medial to lateral.

• Spinal lemniscus– Lateral to the medial lemniscus

• Ventral spinocerebellar tract: most lateral, enters cerebellum through the superior cerebellar peduncle

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Cerebellar peduncles

• Inferior cerebellar peduncle– Inferior cerebellar peduncles enter the

cerebellum from the caudal part of the pons.

superior cerebellar

• superior cerebellar peduncle and its decussation• Superior cerebellar peduncles (mainly cerebellar efferent

fibers) enter the brain stem caudal to the inferior colliculi of the midbrain. The fibers cross the midline in the decussation of the superior cerebellar peduncles. Then most of the fibers continue to the ventral lateral nucleus of the thalamus, from there, project to motor area of the cortex in the frontal lobe. Rest of the fibers end in red nucleus and in the reticular formation. Ascending fibers of superior cerebellar peduncles include ventral spinocerebellar tract as well as fibers from the red nucleus and the mensencephalic trigeminal nucleus.

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Nuclei of Cranial Nerves

• Trapezoid body from dorsal and ventral cochlear nuclei

• Such fibers above end in superior olivary nucleus

• Fibers from dorsal cochlear nucleus and superior olivary nucleus form lateral lemniscus

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MLF

• Medial longitudinal fasciculus– From superior vestibular nucleus, terminates

in abducens. Trochlear and oculomotor nerves

– Coordinate movements of eyes with the rest of the head

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Nuclei of Cranial Nerves

• Facial motor nucleus

• Abducens nucleus

• Spinal Trigeminal Tract and Nucleus

• See previous slide

(Ventral) Basal Pons

• Corticospinal fibers pass through pons• Corticopontine fibers:

– originate from cerebral cortex, end in ipsilateral pontine nuclei. Axons from these nuclei cross the midline forming the pontocerebellar fibers and enter the cerebellum through the middle cerebellar peduncle. The information from cerebral cortex could then be transmitted to cerebellum by the relay of pontine nuclei. Precision and efficiency of voluntary movements.

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Midbrain

• subdivided into:• 1. tectum: consists of superior (Fig 14, 15) and

inferior colliculi (Fig 12, 13)• 2. basis pedunculi: mass of descending fibers• 3. substantia nigra: zone of gray matter dorsal to

the basis of pedunculi• 4. Remaining of the midbrain is called

tegmentum, which includes red nucleus, periaqueductal gray matter

• Cerebral peduncle: structure on either side of the midbrain except tectum (superior and inferior colliculi)

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Tectum and associated tracts • inferior colliculus:• fibers from lateral lemniscus enter inferior

colliculus, then fibers from inferior colliculus traverse via inferior brachium to the medial geniculate body of the thalamus. Fibers from medial geniculate then project into temporal lobe. Commissural fibers between inferior colluculi, accounting in part for the bilateral cortical projection from each ear.

• Some fibers from inferior colliculus pass forward to superior colliculus, which provides a reflex turning of head, ear in response toward unexpected sound.

Superior colliculus

• Afferent fibers to the superior colliculus are coming from occipital lobe. Corticotectal fibers come from visual cortex of occipital lobe and an area in front lobe, called frontal eye field, make up the most of superior brachium ipsilaterally.

• Efferent fibers from superior colliculus are then distributed to the spinal cord and nuclei of the brain stem. The fibers destined for the spinal cord cross to the opposite side in the dorsal tegmental decussation and continue as tectospinal fibers. Efferent fibers for brain stem are called tectobulbar fibers directed bilaterally. They distribute fibers to the pretectal area, to the accessory oculomotor nuclei and to the paramedian pontine reticular formation. From there, these neuclei project to the nuclei of oculomotor, trochlear, and abducens nervers. Other efferent fibers terminate in the reticular formation near motor nuclei of facial nerve providing reflex pathway for protective closure of eyelids when there is a sudden visual stimulus.

• The pair of superior colliculi is interconnected by the commissure of the superior colliculi.

Superior colliculus

• Corticotectal fibers reach superior colliculus via superior brachium, they provide a connection between cortex and the superior colliculus, which is responsible for voluntary and involuntary movements of eyes and head, as when rapidly shifting the direction of gaze (saccadic movement) or when following objects passing across the visual field (smooth pursuit movement). Corticotectal fibers originated from occipital lobe are also responsible for accommodation reflex.

Pretectal area

• consists of four small nuclei rostral to the lateral edge of the superior colliculus. They receive fibers from the retina via optic tract and the superior brachium, which are collateral branches destined for the lateral geniculate body of the thalamus. Efferent fibers travel to Edinger-Westphal nuclei (Oculomotor). Participate the pupillary light response.

Tegmentum

• Medial and spinal leminiscus passing by feed the thalamus.

Red nucleus and associated tracts• Afferent fibers:

– from contralateral cerebellum by superior cerebellar peduncle and its decussation and from the ipsilateral cerebral motor area via corticorubral fibers.

• Efferent fibers:– axons crossing the midline in the ventral tegmental

decussation and continue through the brain stem into the lateral funiculus of the spinal cord as rubrospinal tract. Uncrossed fibers travel to the ipsilateral side terminate in the inferior olivary complex as rubroolivary fibers in the central tegmental tract. The axons from inferior olivary nucleus cross the midline as olivocerebellar fibers enter the inferior cerebellar peduncle, to the cerebellum.

Substantia Nigra

Large nucleus between the tegmentum and the basis pedunculi. The black color of the nuclei is due to the dopaminergic neurons of the pars compacta, area adjacent to the tegmentum. These neurons have neuromelanin pigments. The major afferent source to pars compacta is the striatum (associated with Parkinson’s disease, more later) and efferent fibers go back to the striatum.

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Parkinson's Disease

• Muscular rigidity, slow tremor, bradykinesia, or lack of movement (mask face, loss uncessary involuntary muscle movements, like swing arms while walking). Caused by degeneration of melanin-containing cells in substantia nigra. Treatment: Dopamine like L-Dopa , surgical removal, and transplant of adrenal cells, or fetal cells.

Ventral tegmental area • Basis pedunculi• Corticospinal fibers: middle three-fifths of the basis

pedunculi• Corticobulbar (corticonuclear) fibers are between

corticospinal and frontpontine tract, which occupies medial one fifth of the basis pedunculi. Most fibers terminate in reticular formation. Others end in motor nuclei of cranial nerves. The function of those corticobulbar fibers is to modify sensory transmission and control movement.

• Corticopontine fibers (two parts)• Front pontine tract is part of the corticopontine fibers.

The other part is called parietotemporopontine tract, mainly from parietal lobe.

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Visceral Pathways in the brain stem

• The ascending visceral pathway in the spinal cord are in the ventral and ventrolateral funiculi. They can be considered as part of the spinothalamic, and spinoreticular tracts. Data from these tracts reach reticular formation, thalamus and hypothalamus.

• Visceral information can also be transmitted by vagus and glossopharyngeal nerves to solitary nucleus. Then to hypothalamus by central tegmental tract.

Clinical consideration

• Medial Medullary Syndrome– Branches of vertebral and anterior spinal

arteries.– Structures affected: pyramidal tract, medial

lemniscus, nucleus CN XII– Contralateral arm or leg weakness– Contralateral decreased

proprioception/vibration sense– Ipsilateral tongue weakness

Medulla Vascular Territories

Medial Medullary Syndrome

• Crossed paralysis: neck down affected by opposite side but muscles innervated by cranial nerve are affected on the same side. Be aware that hypoglossal nerve here is the lower motor neuron, which innervates the same side of the tongue muscle. Different from upper motor neuron damage.

Lateral medullary syndrome (Wallenberg’s Syndrome)

• occlusion of medullary branch of posterior inferior cerebellar artery

• Areas affected:• Spinal trigeminal tract and nucleus:

– Ipsilateral loss of pain, temperature sensation in the area of trigeminal nerve

• Affected spinal lemniscus:– Contralateral loss of pain and temperature sensation

below neck (affected spinothalamic tract)• Destruction of ambiguus nuclei:

– Paralysis of the muscles of the soft palate, pharynx, and the larynx on the same side, difficulty swallowing and phonation (efferent fiber of glossopharyngeal N)

Wallenberg’s Syndrome

• Pathway to intermediolateral cell column of the spinal cord is usually also affected:

• (Sympathetic fibers affected, so parasympathetic innervation dominant )

• Horner’s syndrome:

• Small pupil, drooping upper eyelids (ptosis), warm, dry skin on same side of the lesion

Lesion of basal portion of pons or midbrain

• Occlusion of pontine artery off basilar artery. Corticospinal fibers affected, contralateral paralysis, abducens nerve affected, ipsilateral lateral rectus paralyzed, medial strabismus.

Millard Gübler Syndrome

• Damage:– Corticospinal tract (contralateral

hemiparelysis)– Motor nuclei of facial nerve (ipsilateral facial

paralysis, lower motor damage)

Foville’s Syndrome

• Damage:– Corticospinal tract partially (contralateral

hemiparelysis)– Motor nuclei abducens (ipsilateral lateral

rectus paralyzed)– Motor nuclei of facial (facial muscle paralysis)– medial rectus paralysis caused by destruction

of crossed fiber that innervate such muscle)

Weber’s syndrome

• Occlusion of branch of posterior cerebral artery

• Damage:– Corticospinal tract– Motor nucleus of oculomotor nerve (inability to

raise upper eyelid, lateral strabismus, paralysis of all eye muscles except lateral rectus and superior oblique, pupil dilation because of interruption of parasympathetic fibers)