Brainstem Flashcards
Identify the features on the diagram and give an general overviewe of the functions of the brainstem
The brainstem consists of the medulla oblongata, pons and the midbrain and occupies the posterior cranial fossa of the skull
Connects the spinal cord with the
cerebral hemispheres and cerebellum and has long tracts of fibres present at all levels.
In a transverse section the brainstem can be divided into four areas from posterior to anterior. The tectum (roof over the
ventricular system), the ventricular system itself, the tegmentum (core of brainstem) and the basal
portion (situated most anteriorly).
The brainstem has three broad functions;
It serves as a conduit for the ascending and descending tracts of the spinal cord connecting them to
higher centres within the brain
It contains important reflex centres
It contains the important nuclei of the cranial nerves III through to XII
The superior & inferior colliculi, superior & inferior medullary vela, comprise the tectum that roofs the cerebral
aqueduct & 4th ventricle.
The tegmentum is the largest most central portion of the brainstem and houses the reticular formation, cranial nerve nuclei , ascending pathways from the spinal cord and some descending pathways.
The majority of the descending pathways travel through the basal portion of the brainstem.
Identify the major external features on the brainstem
Describe the anterior surface of the medulla
The medulla oblongata is approximately 3cm long, conical shape with it widening gradually in a superior
direction.
It is the most caudal part of the brainstem and connects with the pons superiorly with the spinal
cord inferiorly.
The junction of the medulla and the spinal cord is at the origin of the anterior and posterior roots of cervical spinal nerve 1 which corresponds approximately to the level of the foramen magnum.
It rests on the midline part of the occipital bone and its dorsal surface is covered by the cerebellum.
The rostral (superior) limit of the medulla is marked ventrally (anteriorly) by a prominent sulcus.
On the anterior surface of the medulla is the anterior median fissure, which is continuous with the anterior median
fissure of the spinal cord.
On each side of the anterior median fissure is a swelling called the pyramid (corticospinal tracts). The pyramids get thinner inferiorly and where the majority of the descending tracts decussate forming the decussation of the pyramids
(the decussating fibres obscure the anterior median fissure at this level).
The anterior external arcurate fibres emerge from the anterior median fissure above the decussation of the pyramids and pass laterally over the surface of the medulla to enter the cerebellum.
Posterolateral to the pyramids are the olives which are oval swellings produced by the underlying inferior olivary nuclei.
In the groove between the pyramids and olives (ventrolateral sulcus) emerge the rootlets of the hypoglossal nerve (CN XII)
The inferior cerebellar peduncles consist of white matter which connects the medulla to the cerebellum lie
posterior and lateral to the olives.
In the groove between the olives and the inferior cerebellar peduncle emerge the roots of the glossopharyngeal (CN IX), vagus (X) and cranial roots of the accessory (CN XI)
nerves.
Describe the major features of the posterior aspect of the medulla
The superior limit on the dorsal aspect of the medulla is an imaginary transverse line that joins the caudal margins of the cerebellar peduncles.
The posterior half of the medulla oblongata forms the
lower floor of the 4th ventriclewhich is often removed during dissection referred to as theopen medulla oblongata.
The inferior half of the posterior surface (closed
part) iscontinuous with the posterior aspect of the spinal cordand possesses aposterior median sulcus.
Immediately lateral to the posterior median sulcus are the gracile tubercles elongated swellings which continue from the spinal cord produced by the underlying gracile nucleus
Lateral to the gracile tubercle
is a similar swelling produced by the cuneate nucleus, the cuneate tubercle.
In the middle of the floor of the 4th ventricles, within the posterior aspect of the medulla, are delicate strands of nerve fibres which emerge from the median sulcus and run laterally to enter the inferior cerebellar peduncles known as the
striae medullares
The apex of the V-shaped boundary of the 4th ventricle is known as the obex
The floor of the 4th ventricle is in both the medulla oblongata (inferior portion) and the pons (superior portion) and is diamond shaped when the roof is removed so is referred to as the rhomboid fossa.
The inferior part of the rhomboid fossa (in the medulla oblongata) is marked by two triangles; the vagal trione
(ala cinerea)which marks the sites of thedorsal motor nucleus of the vagus nerve and the hypoglossal
trione which indicates the position of the underlying hypoglossal nucleus.
Outline the levels of the medulla oblongate
The medulla oblongata consists of both white and
grey matter and its internal structure can be
considered at four levels;
- (1) level of decussation of pyramids/ spinomedullary junction
- (2) level of decussation of leminci/ middle of medulla
- (3) level of the olives/ upper medulla
- (4) pontomedullary junction (not noted on diagram)
Describe the anatomy of the spinomedullary junction/ decussation of pyramids
MOTOR
A transverse section through the inferior half of the medulla oblongata passes through the decussation of
the pyramids, the great motor decussation
In the superior part of the medulla the corticospinal tract
fibres form the pyramid, inferiorly about 80% of its fibres decussate and descend contralateral as the lateral corticospinal tract; 10% travel ipsilaterally as the anterior (ventral) corticospinal tract which decussate lower down the spinal cord and 10% will remain ipsilateral
and travel with the lateral corticospinal tract.
SENSORY
The fasciculus gracilis and the fasciculus cuneatus ascend superior and posteriorly to the central grey
matterin thedorsal white column, with the spinal tract andnucleus of the trigeminal nerve
alongside.
The nucleus gracilis and the nucleus cuneatus appear as posterior extensions of the central grey matter.
The substantia gelatinosa of the spinal cord becomes continuous with the inferior end of the
nucleus of the spinal tract of the trigeminal nerve.
CEREBELLUM AND RETICULAR FORMATION
The dorsal spinocerebellar tract is nearing its point of departure into the inferior cerebellar peduncle.
Describe the anatomy of the middle of the medulla oblongata at the level of the decussation of the Lemnisci (sensory decussation)
SENSORY
The dorsal columns of the spinal cord ascend to the mid medulla before turning ventrally (anteriorly).
The gracile fasciculus synapses in the gracile nucleus, and the cuneate fasciculus in the cuneate nucleus. These
fibres give rise to the internal arcurate fibres which travel anteriorly and laterally around the central grey
matter, then curve medially toward the midline, toward the decussation of the lemnisci, the great
sensory decussation.
This decussation takes place anterior to the central grey matter and posterior to the decussation of the pyramids.
The fibres ascend to the thalamus as the medial lemniscus.
The nucleus of the spinal tract of the trigeminal nerve lies lateral to the internal arcuate fibres, the spinal
tract of the trigeminal nervelieslateral to the nucleus.
MOTOR
The hypoglossal nerve emerges at the lateral edge of the pyramid
Lateral to the hypoglossal nucleus is
the dorsal nucleus of the vagus nerve. Here also the cranial accessory nerve emerges from the nucleus
ambiguouswhich will be incorporated into thevagus nerve below the jugular foramen.
The dorsal longitudinal fasciculus contains autonomic fibres descending from the hypothalamus to the spinal cord.
Other motor pathways which can be seen at this level include the vestibulospinal tract.
CEREBELLUM AND RETICULAR FORMATION
At this level also the inferior and accessory olivary nuclei can be observed they send projections to the
contralateral cerebellar cortex.
The paramedian and lateral reticular formation along with the posterior spinocerebellar and the beginning of the inferior cerebellar peduncle can still be seen at this level.
