Anatomy of spinal cord & vertebral column

Brainstem

Introduction and functions:

o   Medulla oblongata, pons and midbrain in posterior cranial fossa. (Some also include diencephalon: Everett & A K Dutta)

Functions:

o   Contains brain stem homologues of spinal cell groups, nuclei of cranial nerves III–XII (innervation of the head and neck & autonomic fibers of X) and brain stem reticular formation - continuous caudally with spinal reticular formation.

o   Reticular nuclei: vital centers (cardiac and respiratory activities), essential for cerebral cortical arousal & consciousness, and regulation of muscle tone, posture and reflex activities.

o   Site of termination of ascending and descending fibers and is traversed by many others. The spinothalamic tract (spinal lemniscus), medial lemniscus (dorsal columns) and the trigeminothalamic tracts ascend through the brain stem to reach the thalamus, corticospinal projections descend through the brain stem and corticobulbar projections end within it.

o   Central canal extends into the caudal half of the medulla, migrating progressively more dorsally until it opens out into the lumen of the fourth ventricle. This divides the medulla into a closed part, which contains the central canal, and an open part, which contains the caudal half of the fourth ventricle.

Gross features, relations

o   Extends from upper border of arch of atlas (Pair of C1 nerve roots, midpoint of dens) to lower end of mammillary body, mostly in infratentorial compartment,

o   Connected to cerebellum by 3 pairs of peduncles & separated by 4^th ventricle.

o   Relations: ant clivus and foramen magnum

o   Post: notch b/w cerebellar hemispheres, separated by 4^th ventricle and sup & inf medullary velum from cerebellum.

o   Sides: petrous part of temporal bone upto IAM

o   Anterior view & Posterior view (see diagrams)

4^th ventricle (see diagram of brainstem dorsal view)

o   Also known as rhomboid fossa, formed by dorsal surfaces of pons and open part of medulla oblongata.

o   Median sulcus divides into symmetrical halves.

o   Sulcus limitants divide each half into median eminence and vestibular areas (lat, med, sup & inf groups of med vestibular nu).

o   Striae medullaris (arcuate nu -> ICP -> contralateral cerebellum) runs from central sulcus to ICP.

o   Facial Colliculus (produced by int genu of motor fibers of VII) lies on medial eminence above stria medullaris, hypoglossal triangle (XII Nu & Nu intercalatus) lies below it.

o   Vagal triangle (dorsal nu of X) lies lower to stria medullaris.

o   Sup & Inf fovea lie at junction of sulcus limitants and periphery.

o   Funiculus separans separate area posttrema from vagal trigone. Area posttrema does not have BBB and CTZ, which contains neurons that act as receptors for vomiting reflex.

o   Just above sup fovea lies locus ceruleus, an area with bluish discoloration & contains pigmented neurons known as substantia ferruginea rich in melanin. ? Special group of reticular nuclei. Rich in nor-epinephrine, efferents ascend through median forebrain bundle & terminate monosynaptically in cerebral & cerebellar cortices, midbrain colliculi, thalamus & hypothalamus. Exert important role in induction of paradoxical sleep where EEG is desynchronized & REM. (see A K Pg 436)

o   4^th ventricle below Striae medullaris is called ‘calamus scriptorius’ or pen nib.

Development:

Midbrain & closed part of medulla retain the primitive form of neural tube, in pons & open part of medulla lateral wall of neural tube spreads outwards due to acute ventral pontine flexure & roof plate is thinned out and stretched. Eventually floor of 4^th ventricle becomes rhomboid and alar lamina comes to lie lateral to basal lamina separated by sulcus limitants. Cerebellar rudiments develop as dumbbell shaped rudiments in rhomboid lips of alar lamina and grow over stretched roof plate. Other cells of rhomboid lips migrate ventrally over basal part of pons & form pontine nu, some migrate caudally to form arcuate nu and those remain at lips form Ponto-bulbar body – that lies in dorsolat surface of ICP, receives cortical fibers and sends efferents to contralat cerebellum via ICP.

Medulla oblongata

o   Length = 3cm breadth = 2cm thickness = 1.25 cm

o   From pontomedullary junction to upper border of atlas.

o   Ant: basilar part of occipital bone, basilar venous plexus & 4^th part of vertebral A. post: inf cerebellar notch, 4th ventricle, inf vermis

o   Ventral surface: ant median fissure, pyramids (somatotopy: foot lateral), Olive & ICP. CN at Ponto medullary junction VI, VII, NI, VIII (Vestibular component ventromedial); behind olives IX, X & cranial part of XI, b/w pyramid and olive: XII. In midline junction of pyramid and olive: foramen caecum (post), in lower part decussating pyramidal fibers. Ant external arcuate fibers seen below olives (arcuate nu -> ICP)

o   Dorsal surface: Open part: floor of 4^th ventricle; Closed part: post median sulcus, gracile tubercle, cuneate tubercle (clava), tuberculam cinerium (due to spinal tract & nu of V, guide for trigeminal tractotomy) & ICP. Pontomedullary jn corresponds with medullary stria. Gracile tubercles of 2 sides connected by V shaped sheet of ependyma which forms lowest part of roof of 4^th ventricle (obex)

Pons (Bridge) / Metencephalon

·       2.5 cm long, from pontomedullary junction to midbrain.

·       Ant: basilar A, rests on clivus,

·       Ventral / basilar surface: demarcation point from MCP is CN V (large sensory laterally and small motor medially). In midline at junction of midbrain – ant fovea. Long groove in midline for basilar A. Phylogenetically ventral convexity of pons grows progressively along with growth of cerebrum & cerebellum. Lateralmost part of pontomedullary junction on ventral surface is called Cerebello-pontine angle and is meeting point of pons, medulla & cerebellum – SOL of this point produces disturbance of hearing, equilibrium and VII CN palsy.

·       Dorsal surface: cranial part of 4^th ventricle separates from medullary velum and cerebellum.

Midbrain

·       Length = 2cm; Shortest segment of brain stem, retain primitive structure of neural tube, cavity is aqueduct

·       Passes through tentorial notch from pons to diencephalon.

·       Surface of each crus cerebri crossed by from above downwards optic tract, basal vein, PCA & SCA. CN III & IV pass b/w CN III & IV.

·       Interpeduncular fossa, pierced by central br of PCA (post perforated substance0

Cross sections

1.     Medulla oblongata at the level of pyramidal decussation (closed part)

Salient features:

·       Similar to spinal cord, Nu gracilis & nu cuneatus start appearing in dorsal columns, spinal tract & nu of V appears in place of substantia gelatinosa of dorsal horn.

·       Ventral and dorsal spinocerebellar tracts, spinothalamic tract, central canal, Rubrospinal, vestibulospinal and tectospinal tracts remain as it is.

·       Key feature: lower end of pyramid seen, pyramidal (corticospinal fibers) cross to contralateral side isolating anterior horn from central grey mass (known as supraspinal nu, give origin to ventral root of C1 & spinal part of XI CN, rostrally in line with NA)

2. TS of medulla oblongata at the level of sensory (lemniscal decussation, closed part)

Salient features

·       Key features: Nu gracilis, Nu cuneatus and spinal nu of V completely separate from central grey matter. Dorsal column nu – efferents decussate ant-medially on contralateral side and form medial lemniscus – internal arcuate fibers. Somatotopic organization in dorsal columns – head laterally; in medial lemniscus head posteriorly.

·       Central / periaqueductal grey matter consist of from post to ant: NTS, Dorsal nu of X and XII nu (+ perihypoglossal nu like nu intercalatus & nu of roller, no clear function may contribute to efferents to cerebellum)

·       Nu ambiguous lies in reticular formation.

·       Accessory cuneate nu seen dorsolateral to cuneate nu (C1-C7 counterpart of Clarke’s column, receive some fibers from dorsal spinocerebellar tract, efferent to cerebellum via post ext. arcuate fibers and inf cerebellar peduncle).

·       Pyramids: larger, lat corticospinal tracts not seen

·       Arcuate nu seen ant ventral end of pyramids – caudally displaced pontine nu, receive Cortico-pontine fibers and efferent to cerebellum via ant ext. arcuate fibers and stria medullaris.

·       Inf olivary nucleus starts appearing.

·       Medial longitudinal bundle b/w central grey and pyramids.

·       Unchanged: ventral & dorsal spinocerebellar tracts, spinothalamic tracts, Rubrospinal, tectospinal, vestibuolospinal.

2.     TS of medulla at open part (mid level of inf olivary nu): divide into 3 zones of pyramid, olive & ICP

Salient features

·       Central canal migrates posteriorly and widens to form 4^th ventricle.

·       Nu from medial to lateral XII, Dorsal nu of X, NTS; nu ambiguous lies slightly anteriorly in reticular formation and spinal nu of V lateral to it.

·       Lower end of Inf vestibular nu seen dorsomedial to ICP.

·       CN X emerges from dorsal nu, NTS and NA from b/w ICP and Olives. CN XII emerges b/w olives and pyramids.

·       Inferior cerebellar peduncle appears.

·       Olivary complex seen: Inf olivary Nu + Dorsal nu + Medial nu

·       Medial long fasciculas, tectospinal tract and medial lemniscus seen in midline.

3.     TS of Pons at the level of Facial Colliculus (lower part of Pons)

Salient Features

·       Anterior basilar part containing pontine nu, corticonuclear and corticopontine fibers and posterior tegmentum.

·       Inf and middle cerebellar peduncles seen.

·       Dorsal & ventral cochlear nu seen dorsal and ventral to INP.

·       Vestibular nu seen dorsally in floor of 4^th ventricle.

·        Close to midline, dorsal to ventral: Medial long bundle, tectospinal and Rubrospinal tract and medial lemniscus (ML rotates by 90 deg. From medulla – Somatotopic organization head medial.)

·       At junction of tegmentum and basilar part is Trapezoid body – decussating fibers of Cochlear nu -> MGB.

·       VI & VII cranial nerves arise from nu, CN VII loops around VI nu to form facial Colliculus in floor of 4^th ventricle – neurobiotaxis.

·       Spinal lemniscus (lat spinothalamic tract) appears lateral to medial lemniscus.

·       Spinal nu and tract of V – lateral to spinal lemniscus.

4.     TS of pons at upper part

Salient features

·       Basilar part remains unchanged, changes in tegmental part.

·       ICP disappears; SCP & MCP can be seen.

·       CN V appears from ventral side from motor, sensory and mesencephalic nu – point of demarcation of pons and MCP.

·       Close to midline – dorsal to ventral: M T R and medial lemniscus

·       Trapezoid body

·       All four lemnisci appear MTSL from medial to lateral close to basilar part in tegmentum.

TS of Midbrain general features

·       4^th ventricle continues as cerebral aqueduct of Sylvius.

·       Part post to aqueduct – tectum; ant – cerebral peduncles, in it from dorsal to ventral tegmentum, substantia nigra (subcortical center for extrapyramidal system, broader medially, pars compacta: efferent ant and pars reticulata post: afferent) and crura cerebri (corticopontine, corticonuclear and corticopontine fibers)

·       Tectum has sup and inf colliculi (corpora quadrigemina) separated by cruciate sulcus.

6. TS of midbrain at lower level (Inferior Colliculus)

Salient features: red nucleus is absent

·       Tectum: Inf colliculi into which lateral lemniscus from cochlear nu terminates, efferent to MGB via inf brachium.

·       Periaqueductal grey matter; Mesencephalic nu of V laterally and CN IV nu ant. CN IV emerges, decussate dorsal to periaqueductal grey and emerge from vertical limb of cruciate sulcus.

·       Pretectal nu lateral to periaqueductal grey – center for ocular reflexes, gives tectospinal tract.

·       Lemnisci: MTSL in periphery

·       Close to midline: M T DSC R

·       Reticular formation

TS of midbrain at upper part (superior Colliculus)

·       Tectum: Sup colliculi, efferent to LGB via sup brachium.

·       Periaqueductal grey matter; Mesencephalic nu of V laterally and CN III nu ant. CN III emerges, does not decussate and emerge from interpeduncular fossa.

·       Pretectal nu lateral to periaqueductal grey – center for ocular reflexes, gives tectospinal tract.

·       Lemnisci: MTS in periphery; L terminate in inf Colliculus

·       Close to midline: Dorsal tegmental decussation of Mynert (Tectospinal + Tectonuclear) and ventral tegmental decussation of Forel (Rubrospinal)

·       Red nucleus

·       Reticular formation

Olivary nu complex: superior group of olivary nu & inf group (main, medial & dorsal)

Medial longitudinal fasciculus

Red Nucleus

Reticular formation

Blood supply of brainstem: supplied by vertebral A + Basilar A and their branches.

a.     Medulla oblongata: branches of vertebral, ant & post spinal, PICA and basilar A which enter along ant med fissure and post med sulcus. Vs supplying central substance enter along with rootlets of CN IX, X, XI and XII. Additional supply is from pial plexus of same A.

b.     Pons: basilar A, AICA and PICA. Direct br from basilar A enter along ventral median groove, others enter along CN V, VI, VII, VII and from pial plexus.

c.     Midbrain: PCA, SCA and basilar A, crura by Vs entering their med and lat sides. Med Vs also supply sup-med part of tegmentum including CN III nucleus; lat Vs - lat part of crus and tegmentum. Colliculi by 3 Vs on each side from PCA and SCA. Additional supply to crura, colliculi and their peduncles comes from post-lat group of central br of PCA.

           Applied anatomy:

o   Damage to the brain stem is often devastating and life threatening; structurally and functionally compact region, where even small lesions can destroy vital cardiac and respiratory centers, disconnect forebrain motor areas from brain stem and spinal motor neurons, and sever incoming sensory fibers from higher centers of consciousness, perception and cognition.

o   Irreversible cardiac and respiratory arrest follow complete destruction of the neural respiratory and cardiac centers in the medulla (brain stem death); requires accurate diagnosis since it may occur in patients on life-support machines whose respiratory and cardiac functions can be artificially maintained indefinitely.

Concepts:

o   All nerve tracts run longitudinal to brainstem except CN, which run transversely, involvement of CN help, localize the level of lesion. CN III, VI, XII emerge from ventral aspect close to midline and closely related to Corticospinal tracts before its decussation; hence unilateral lesions of ventral part of brainstem manifest as crossed hemiplegia – UMN in cranial and LMN in body. CN with SVE like V, VII, IX, X & XI pass through lateral part of brainstem and closely related to spinal lemniscus, lesions of lateral part of brainstem manifest as ipsilateral palsy and sensory loss in region of that CN with contralateral loss of pain & temperature in body.

o   Side of lesion and sides effected determined by decussation of various fibers: spinothalamic tract in spinal cord, medial lemniscus in lower medulla, trigeminal lemniscus in Ponto-medullary region and pyramidal tract in lower medulla. CN: Cortico-nuclear fibers cross but efferents from nucleus do not except IV and part of VII.

o   Blood supply: Branches of vertebral & basilar A lie on ant medial groove

1.     Paramedian brs e.g. ant spinal A:  penetrate brainstem near median plane and supply medial zone on each side of parasagittal plane

2.     Short circumferential brs: supply anterolateral zone

3.     Long circumferential brs (PICA): posterolateral zone & cerebellum

Individual Lesions

Medulla

1)     Medial medullary syndrome / alternating hypoglossal hemiplegia: occlusion of ant spinal A & its Paramedian brs, effects XII CN, corticospinal tract & medial lemniscus hence -> ipsilateral LMN lesion of tongue muscles (deviated to same side of lesion SOOS), contralateral UMN palsy and contralateral loss of discriminatory senses

2)     Lateral medullary syndrome of Wallenberg / alternating hemianesthesia: occlusion of PICA, involves post lat part of medulla i.e. spinal lemniscus, Nu & tract of V & nu ambiguous causing ipsilateral loss of sensations from face, contralateral loss of pain & temperature from body (lat spinothalamic tract), ipsilateral palsy of muscles of soft palate, pharynx & larynx. If lesion extends further dorsally it affects ICP & vestibular nu ->cerebellar aynergia, Hypotonia, nystagmus & loss of equilibrium.

Cerebello pontine angle

1)     Acoustic neuroma from CN VIII can compress CN VIII, CN VII, ICP, MCP, spinal lemniscus & spinal tract of V -> manifest as

a.     CN VIII -> ipsilateral tinnitus, progressive deafness, vertigo

b.     Cerebellum: ipsilateral coarse intention tremors, dysmetria (difficult to measure or lack of coordination of movement typified by the undershoot or overshoot of intended position with the hand, arm, leg, or eye). It is a type of ataxia, adiadochokinesia (inability to perform the rapid alternating movements) & ataxia.

c.      Spinal trigeminal tract: Ipsilateral loss of pain & temperature from face & forehead

d.     Spinal lemniscus: Contralateral loss of pain & temperature from body

e.     CN VII -> ipsilateral LMN palsy of facial muscles, hyperaacusis, loss of taste from ant 2/3 of tongue.

Pons

1)     Raymond’s syndrome: Alternating abducent hemiplegia (lesion in medial caudal part of pons)

2)     Milalard Gubler syndrome: Alternating facial hemiplegia (lesion in medial caudal part of pons)

3)     Lesions in lateral part of mid pons: alternating trigeminal hemiplegia

Midbrain

1)     Weber’s syndrome: crossed III CN hemiplegia

2)     Benedicts syndrome:

3)     Parinaud’s syndrome: