8

Brainstem

The brainstem is the stalk-like part of the brain which connects the spinal cord with the forebrain. From below upwards it consists of three parts: medulla oblongata, pons, and midbrain. The midbrain is continuous above with the cerebral hemispheres and the medulla oblongata is continuous below with spinal cord. Brainstem is located in the posterior cranial fossa. Its ventral surface lies on the clivus. Posteriorly, the pons and medulla are separated from the cerebellum by the cavity of the fourth ventricle.

The brainstem, like spinal cord consists of nerve fibres and nerve cells. Most of the nerve fibres in the brainstem are arranged longitudinally in the form of tracts as in the spinal cord. But the nerve cells of the brainstem do not form a single central mass of grey matter as in the spinal cord; rather most of them are aggregated to form the well defined nuclei. In addition to the well defined tracts and nuclei, the brainstem consists of diffuse system of nerve cells and nerve fibres called reticular formation. The central canal of spinal cord continues upwards in the lower part of the brainstem and then it widens and moves dorsally to form the fourth ventricle.

The brainstem serves the following four major functions:

• It provides passage to various ascending and descending tracts that connect the spinal cord to the different parts of the forebrain.

• It contains important autonomic reflex centres (vital centres) associated with the control of respiration heart rate and blood pressure.

• It contains reticular activating system which controls consciousness.

• It contains important nuclei of the last ten cranial nerves (i.e. IIIrd to XIIth).

N.B. The bilateral destructive lesions of the brainstem are fatal because of damage to ‘vital centres’ which control respiration, heart rate, and blood pressure. The impairment of reticular activating system leads to progressive loss of consciousness, followed by stupor and coma.

Medulla Oblongata

The medulla oblongata is the direct upward continuation of the spinal cord, extending from foramen magnum to the lower border of the pons. It forms the lowest part of the brainstem and lies almost vertically in the anterior part of the posterior cranial fossa between the clivus (superior surface of the basi-occiput) in front and the vallecula of the cerebellum behind.

The medulla is shaped like a truncated cone (bulb-like) hence its alternative name ‘bulb’ (cf. bulbar paralysis).

Medulla oblongata measures about 3 cm in length, 2 cm in breadth (at the widest part) and 1.25 cm in thickness).

The medulla contains vital centres which are essential for life. These are: (a) cardiac centre, (b) vasomotor centre, and (c) respiratory centre.

Medulla provides attachment to last four cranial nerves.

The lower part of the medulla like the spinal cord contains the central canal. In the upper part of the medulla this canal widens and moves dorsally to form the lower part of the fourth ventricle. Thus, the medulla is divided into a lower closed part and an upper open part.

An overview of structural components of medulla and their functions is provided in Table 8.1.

Table 8.1

Structural components and functions of the medulla oblongata

Components	Functions
Grey matter
• Nucleus gracilis and nucleus cuneatus	Relay conscious proprioceptive sensations to the thalamus
• Olivary nuclei	Relay information associated with voluntary muscle movement to the cerebellum
• Vital centres
– Cardiac centre	Regulates heart rate and force of contraction
– Vasomotor centre	Regulates distribution of blood flow in vessels
– Respiratory centre	Regulates respiratory movements
• Nuclei of last four cranial nerves (IX, X, XI and XII)	See Chapter 9
• Other nuclei	Relay ascending sensory information from spinal cord to the higher centres
White matter
• Ascending and descending tracts	Connect the spinal cord with the other parts of the brain (to and fro)

Clinical Correlation

• Since the medulla contains vital centres, an injury to it is usually fatal. The respiratory centre is particularly vulnerable to compression and poliomyelitis with subsequent respiratory failure.

• The bulbar paralysis is characterised by the paralysis of muscles supplied by last four cranial nerves arising from the medulla.

External Features

The medulla is divided into right and left symmetrical halves by anterior median fissure and posterior median sulcus (Fig. 8.1).

Fig. 8.1 External features on the anterior (ventral) aspect of the brainstem. Note the attachment of last cranial nerves.

The anterior median fissure is continuous below with the corresponding fissure on the spinal cord, and above it ends into a small triangular depression called foramen caecum, at the lower border of the pons. It is interrupted in its lower part by the bundles of fibres crossing obliquely from one side to the other, the decussation of pyramids.

The posterior median sulcus continues below with the corresponding sulcus of the cord and is present only in the lower half of the medulla. Above its lips diverge to form the boundaries of a triangular area, the lower part of the floor of fourth ventricle.

Each half of the medulla is marked by two sulci— anterolateral and posterolateral, which are direct upward continuations of the corresponding sulci of the cord.

The anterolateral sulcus extends along the lateral border of the pyramid and along it emerges the rootlets of the hypoglossal (XII cranial) nerve.

The posterolateral sulcus lies between the olive and the inferior cerebellar peduncle and along it emerges the rootlets of glossopharyngeal (IX), vagus (X) and accessory (XI) cranial nerves.

The anterolateral and posterolateral sulci with nerve roots divide the surface of each half of the medulla oblon-gata into anterior, posterior and lateral regions like that in the spinal cord.

Features on the anterior (ventral) aspect of medulla (Fig. 8.1)

The ventral aspect of medulla presents following features:

• Pyramids. These are two elongated elevations, one on either side of anterior median fissure and are produced by the corticospinal (pyramidal) fibres. Most of these fibres about (75%) cross to the opposite side (pyramidal decussation) in the lower part of medulla and then descend as lateral corticospinal tract in the lateral white column of the spinal cord. About 20% of uncrossed fibres run downwards as anterior corticospinal tract in the anterior white column of the spinal cord; the remaining 5% run downwards along with uncrossed fibres in the lateral white column of the spinal cord (Fig. 8.2).

Fig. 8.2 Showing decussation of pyramidal (corticospinal) tracts in the lower part of medulla oblongata and location of lateral and anterior corticospinal tracts in the spinal cord. (P = pyramid.)

• Olives. These are oval elevations, posterolateral to the pyramids and are produced by an underlying mass of grey matter called inferior olivary nucleus.

• Rootlets of the hypoglossal nerve. These emerge from the anterolateral sulcus between the pyramid and the olive.

• Inferior cerebellar peduncles. These are thick bundles of fibres lying posterolateral to the olive, and attach the medulla with the cerebellum.

• Rootlets of the IXth, Xth, and XIth (cranial part) cranial nerves. These emerge through the posterolateral sulcus separating the olive from the inferior cerebellar peduncle.

Features on the Posterior (Dorsal) Aspect of Medulla (Fig. 8.3)

The dorsal aspect the medulla is well demarcated into lower closed and upper open parts.

Fig. 8.3 External features on the posterior (dorsal) aspect of the brainstem. Note the features in the floor of the fourth ventricle.

Features of the closed part

The closed part, on either side of posterior median sulcus, presents three longitudinal elevations. From medial to lateral these are: fasciculus gracilis, fasciculus cuneatus, and inferior cerebellar peduncle. The upper ends of the fasciculus gracilis and fasciculus cuneatus expand to form the gracile and cuneate tubercles respectively due to underlying nuclei of the same name.

Another elevation present lateral to cuneate tubercle, the tuber cinereum is produced by the spinal nucleus of trigeminal nerve.

Features of the open part

The open part of the medulla forms the lower part of the floor of fourth ventricle, which presents number of features like, median sulcus, hypoglossal and vagal triangles, vestibular areas, area postrema, stria medullaris, etc. (Fig. 8.3). For details see floor of fourth ventricle on page 122.

Internal Structure

As in the spinal cord, in the medulla also, the white matter surrounds the central mass of grey matter but in the latter, the grey matter is broken up into nuclei that are separated by nerve fibres. The internal structure of medulla is well appreciated by examining its transverse sections at the following four levels:

• At the level of decussation of pyramids.

• At the level of decussation of medial lemnisci.

• At the level of the olives.

• At the level just inferior to the pons.

Transverse section of medulla at the level of pyramidal decussation (the great motor decussation)

The section at this level passes through the inferior half of the medulla, and closely resembles to that of spinal cord. However, following important features are observed at this level (Fig. 8.4):

Fig. 8.4 Transverse section through the lower closed part of the medulla oblongata at the level of pyramidal decussation.

• The nucleus gracilis and nucleus cuneatus appear as the narrow strip like projections from the posterior aspect of the central grey matter.

• The apex of posterior horn gets swollen up to form the nucleus of spinal tract of trigeminal nerve. It is an upward continuation of the substantia gelatinosa in the posterior grey column of the spinal cord.

• The spinal tract of trigeminal nerve is a bundle of fibres which caps over the nucleus of spinal tract of trigeminal nerve.

• Decussation of pyramidal tracts forms the most important feature of medulla at this level. About 75% fibres of pyramidal tract run backwards and laterally across the midline to reach the lateral white column of the opposite side of the spinal cord, where they run downwards as the lateral corticospinal tract. In doing so the anterior horns are detached from the central grey matter.

• Each detached anterior horn divides to form the spinal nucleus of accessory nerve and supraspinal nucleus of first cervical nerve. The nucleus of accessory nerve extends downwards up to fifth cervical spinal segment. The supraspinal nucleus gives off the efferent fibres of the first cervical nerve, and is continuous above with the nucleus of the hypoglossal nerve.

• Appearance of diffuse zone containing a network of fibres and scattered nerve cells within it the lateral white column adjacent to nucleus of spinal tract of trigeminal nerve is called reticular formation.

Transverse section of medulla at the level of sensory decussation

This section passes through the middle of medulla and displays following features (Fig. 8.5):

Fig. 8.5 Transverse section of medulla oblongata at the level of sensory decussation.

• The nucleus gracilis and nucleus cuneatus become more pronounced and are separated from the central grey matter. The fibres of fasciculus gracilis and fasciculus cuneatus occupy the broad posterior white column and terminate in these nuclei.

• The internal arcuate fibres arising from the cells of gracile and cuneate nuclei (second order sensory neurons conducting sensations of discriminative touch, position and vibration) course forwards and medially around the central grey matter and decussate with corresponding fibres of opposite side in the median plane (sensory decussation) and then turn upwards to ascend as the medial lemniscus on the opposite side close to the median plane. In this decussation the gracile fibres are medial to that of cuneate fibres.

• The internal arcuate fibres cut off the spinal nucleus and tract of trigeminal nerve from the central grey matter.

As the fibres from nucleus gracilis and nucleus cuneatus pass forwards and medially they intercross so that the most medial fibres (from the feet and leg) come to lie anteriorly in the medial lemniscus. In the medial lemniscus the body is represented with the head posteriorly and the feet anteriorly. The fibres of lemniscus relay into the corresponding thalamus.

• Immediately dorsolateral to the cuneate nucleus lies the accessory cuneate nucleus which receives the more lateral fibres (derived from the cervical segments of the cord) of the fasciculus cuneatus and gives rise to posterior external arcuate fibres conveying proprioceptive impulses to the cerebellum of the same side through inferior cerebellar peduncle.

• The separated spinal nucleus and tract of trigeminal nerve lies ventrolateral to the cuneate nucleus.

• The lower part of inferior olivary nucleus is seen.

• The pyramids lie on either side of the anterior median fissure.

• The central grey matter contains: (a) hypoglossal nucleus, (b) dorsal nucleus of vagus, and (c) nucleus of tractus solitarius. The hypoglossal nucleus occupies the ventro-medial position close to the midline in the central grey matter. The dorsal nucleus of vagus lies dorsolateral to the hypoglossal nucleus and nucleus of tractus soli-tarius lies just dorsolateral to the dorsal nucleus of vagus.

• Medial longitudinal bundle lies posterior to the medial lemniscus. It is small compact tract of nerve fibres which interconnect the IIIrd, IVth, VIth, VIIIth and spinal nucleus of XIth cranial nerve nuclei.

• Spinocerebellar and lateral spinothalamic tracts lie in the anterolateral area of lateral white column.

• Lateral and anterior spinothalamic tracts are very close to each other and collectively form spinal lemniscus.

Transverse section of medulla at the level of olives

Transverse section passes across the floor of the fourth ventricle and through the middle of olives and presents following features (Fig. 8.6):

Fig. 8.6 Transverse section of medulla at the level of olives: 1. medial longitudinal fasciculus, 2. tectospinal tract, 3. medial lemniscus. (NA = nucleus ambiguus).

• The central grey matter is spread over the floor of the fourth ventricle and contains the nuclei of several cranial nerves. From medial to lateral these are: hypoglossal nucleus, nucleus intercalatus, dorsal nucleus of vagus and vestibular nuclei (inferior and medial).

• The nucleus of tractus solitarius lies ventral to vestibular nuclei.

• The nucleus ambiguus lies deep within the reticular formation and gives origin to the motor fibres of IXth, Xth and XIth cranial nerves.

• On either side of midline (paramedian region), from dorsal to ventral lie: medial longitudinal fasciculus, tectospinal tract, medial lemniscus, and pyramidal (corticospinal) tract.

• The arcuate nuclei, thought to be inferiorly displaced pontine nuclei are situated on the anteromedial aspect of the pyramids. They receive fibres from the cerebral cortex and send efferent fibres to the cerebellum of the opposite side through the anterior external arcuate fibres.

• Laterally, from dorsal to ventral lie two prominent structures: (a) inferior cerebellar peduncle, and (b) inferior olivary nucleus.

    

1. The inferior cerebellar peduncle occupies posterolateral part.

2. The inferior olivary nucleus is the largest mass of grey matter, and forms the most prominent feature in the section through upper part of medulla. It presents a crumbled bag like appearance. Close to the main nucleus lies medial and dorsal accessory olivary nuclei.

Transverse section of medulla just Inferior to the pons (ponto-medullary junction)

There are no major changes in the distribution of grey and white matter in this section from that seen in the section of medulla at the level of olives. Only following features are observed:

• The lateral vestibular nucleus replaces the inferior vestibular nucleus.

• The cochlear nuclei are now visible. The dorsal and ventral cochlear nuclei lie on the dorsolateral and ventrolateral aspects of the inferior cerebellar peduncle respectively.

Blood Supply of the Medulla

The medulla is supplied by the following arteries:

• Two vertebral arteries.

• Anterior and posterior spinal arteries.

• Anterior and posterior inferior cerebellar arteries.

• Basilar artery.

Clinical Correlation

Vascular Disorders of Medulla Oblongata

The common vascular lesions involving the medulla are the thrombosis of posterior inferior cerebellar and vertebral arteries leading to lateral and medial medullary syndromes respectively.

• Lateral medullary (posterior inferior cerebellar artery) syndrome of Wallenberg

The dorsolateral part of the medulla is supplied by the posterior inferior cerebellar artery, which is usually the branch of the vertebral artery. This artery also supplies the inferior surface of the cerebellum.

Thrombosis of posterior inferior cerebellar artery therefore, affects a wedge-shaped area on the dorsolateral aspect of medulla (Fig. 8.7) and the inferior surface of the cerebellum and produces following signs and symptoms.

Fig. 8.7 The transverse section of the upper part of the medulla. The red areas indicate the sites of lesions: 1. dorsal nucleus of vagus, 2. nucleus of tractus solitarius, 3. vestibular nuclei, 4. inferior cerebellar peduncle, 5. spinal tract of trigeminal nerve, 6. spinal nucleus of trigeminal nerve, 7. descending sympathetic tract, 8. nucleus ambiguus, 9. lateral spinothalamic tract, 10. inferior olivary nucleus, 11. hypoglossal nerve, 12. pyramidal tract, 13. arcuate nucleus. (M = medial lemniscus.)

– Contralateral loss of pain and temperature sensation, in the trunk and limbs, due to involvement of spinothalamic tract.

– Ipsilateral loss of pain and temperature sensation over the face, due to involvement of the spinal nucleus and tract of trigeminal nerve.

– Ipsilateral paralysis of muscles of palate, pharynx and larynx due to involvement of nucleus ambiguous.

– Ipsilateral ataxia, due to involvement of inferior cerebellar peduncle and cerebellum.

– Giddiness, due to involvement of vestibular nuclei.

– Horner’s syndrome, due to involvement of descending sympathetic pathway in the reticular formation of medulla.

• Medial medullary syndrome (Dejerine’s anterior bulbar syndrome; Fig. 8.7)

The paramedian region of the medulla is supplied by the branches of vertebral artery. The vascular involvement (ischaemia) of this region produces following signs and symptoms:

– Contralateral hemiplegia/paralysis of arm and leg, due to damage of pyramid.

– Ipsilateral paralysis and atrophy of the half of the tongue, due to damage of hypoglossal nerve.

– Contralateral loss of position and vibration sense due to damage of medial lemniscus.

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