7. 7
3 broad functions:
1. It serves as a conduit for the ascending tracts and
descending tracts connecting the spinal cord to the
different parts of the higher centers in the forebrain
2. Inportant reflex centers present - respiration, cvs and
control of consiousness
3. Nuclei for CN 3 to 12
8. 8
MEDULLA OBLONGATA:
Connects Pons with Spinal cord.
Conical in shape.
The central canal in lower half continues as that of
spinal cord while the upper half expands as the cavity of
fourth ventricle.
The region between the anterior median sulcus and the
antero lateral sulcus is occupied (on either side of the
midline) by an elevation called the pyramid.
9. 9
The elevation is caused by a large bundle of fibres that descend
from the cerebral cortex to the spinal Cord (corticospinal fibres).
Some of these fibres cross from one side to the other in the lower
part of the medulla, obliterating the anterior median fissure. These
crossing fibres constitute the decussation of the pyramids.
Some other fibres emerge from the anterior median fissure, above
the decussation, and wind laterally over the surface of the medulla to
enter the cerebellum. These are the anterior external arcuate fibres.
OLIVES = INFERIOR OLIVARY NUCLEI
In the groove between the pyramid and olive emerge the rootlets of
hypoglossal nerve.
11. The posterior surface of the superior half of medulla forms the
lower part of the floor of the fourth ventricle
The posterior surface of the inferior half of the medulla is
continuous with posterior aspect of the spinal cord and has
POSTERIOR MEDIAN SULCUS
GRACILE TUBERCLE= GRACILE NUCLEUS(MEDIAL)
CUNEATE TUBERCLE=CUNEATE NUCLEUS(LATERAL)
14. 14
INTERNAL STRUCTURE OF
THE MEDULLA:
Considered at 4 levels
1. Level of decussation of Pyramids
2. Level of decussation of lemnisci
3. Level of the olives
4. Level just inferior to the Pons
17. The region just behind the pyramids is occupied by a prominent
bundle of fibres, the medial lemniscus, on either side of the midline.
The medial lemniscus is formed by fibres arising in the nucleus gracilis
and the nucleus cuneatus. These fibres cross the midline and turn
upwards in the lemniscus of the opposite side.
The crossing fibres of the two sides constitute the sensory decussation.
The region lateral to the medial lemniscus contains scattered neurons
mixed with nerve fibres. This region is the reticular formation.
More laterally there is a mass of white matter containing various tracts.
18. The pyramids, the medial lemniscus, the spinal nucleus and tract of the
trigeminal nerve, and the reticular formation are present in the same relative
position as at lower levels. The medial lemniscus is, however, much more
prominent and is somewhat expanded anteriorly.
Lateral to the spinal nucleus (and tract) of the trigeminal nerve we see a large
compact bundle of fibres. This is the inferior cerebellar peduncle which connects
the medulla to the cerebellum.
Posteriorly, the medulla forms the floor of the fourth ventricle. Here it is lined by
a layer of grey matter in which are located several important cranial nerve nuclei
The inferior olivary nucleus forms a prominent feature in the anterolateral part
of the medulla at this level. It is made up of a thin lamina of grey matter that is
folded on itself like a crumpled purse. The nucleus has a hilum that is directed
medially.
19. 19
Olivary nuclear complex:
Largest = Inferior olivary nucleus
Smaller dorsal and medial accessory olivary nuclei also are present
The cells of the inferior olivary nucleus send fibres medially across
the midline to enter the cerebellum through inferior cerebellar
peduncle.
Afferent fibres reach from spinal cord via spino-olivary tracts and
from the cerebellum and cerebral cortex
Function of olivary nuclei= voluntary muscle movement
20. 20
PONS:
The Pons is anterior to the cerebellum
and connects the medulla with the
midbrain.
1 inch
Anterior surface has basilar groove
which lodges the basilar artery.
Connected to cerebellum via Middle
cerebellar peduncle.
21. On either side of the lower part of the Pons there is a region called the cerebello-pontine
angle. This region lies near the lateral aperture of the fourth ventricle. The facial,
vestibulocochlear and Glossopharyngeal nerves, the nervus intermedius, and sometimes
the labyrinthine arteries lie in this region.
The pons is divisible into a ventral part and a dorsal part
The ventral (or basilar) part contains numerous transverse and vertical fibres. Amongst the
fibres are groups of cells that constitute the pontine nuclei.
When traced laterally the transverse fibres form the middle cerebellar peduncle.
The vertical fibres are of two types. Some of them descend from the cerebral cortex to end
in the pontine nuclei. Others are corticospinal fibres that descend through the pons into
the medulla where they form the pyramids.
23. 23
INTERNAL STRUCTURE OF PONS:
Divided into
Studied at two levels :
1. At level of facial colliculus(caudal part)
2. At level of trigeminal nuclei(cranial part)
26. 26
MIDBRAIN:
0.8 inch in length.
Connects Pons and cerebellum with the forebrain.
The midbrain is traversed by NARROW CHANNEL, THE CEREBRAL
AQUEDUCT, filled with CSF.
Posterior surface- 4 colliculi- Corpora Quadrigemina.
Superior colliculi- Visual.
Inferior colliculi- auditory.
In the midline below the Inferior colliculus- trochlear nerves emerge.
When the midbrain is viewed from the anterior aspect, we see two large bundles of
fibres, one on each side of the middle line. These are the crura of the midbrain.
27. The crura are separated by a deep fissure. Near the pons the fissure is narrow,
but broadens as the crura diverge to enter the corresponding cerebral
hemispheres. The parts of the crura just below the cerebrum form the posterior
boundary of a space called the interpeduncular fossa. The oculomotor nerve
emerges from the medial aspect of the crus of the same side.
Each colliculus is related laterally to a ridge called the brachium.
The superior brachium (also called the superior quadrigeminal brachium, or
brachium of superior colliculus) connects the superior colliculus to the lateral
geniculate body.
Similarly, the inferior brachium (also called the inferior quadrigeminal brachium
or brachium of inferior colliculus) connects the inferior colliculus to the medial
geniculate body.
Just below the colliculi, there is the uppermost part of a membrane, the superior
medullary velum, which stretches between the two superior cerebellar peduncles,
and helps to form the roof of the fourth ventricle.
28. For convenience of description, the midbrain may be divided as follows
A. The part lying behind a transverse line drawn through the cerebral aqueduct is called the
tectum. It consists of the superior and inferior colliculi of the two sides.
B. The part lying in front of the transverse line is made up of right and left halves called the
cerebral peduncles.
Each peduncle consists of three parts. From anterior to posterior side these are the
1. Crus cerebri (or basis pedunculi),
2. The substantia nigra.
3. The tegmentum.
30. The crus cerebri (or basis pedunculi) consists of fibres descending from the cerebral cortex.
Its medial one sixth is occupied by corticopontine fibres descending from the frontal lobe;
and the lateral one-sixth is occupied by similar fibres from the temporal, occipital and
parietal lobes. The intermediate two-thirds of the crus cerebri are occupied by corticospinal
and corticonuclear fibres. The fibres for the leg are most lateral and those for the head are
most medial.
The substantia nigra lies immediately behind and medial to the basis pedunculi. It appears
dark in unstained sections as neurons within it contain pigment (neuromelanin).
The substantia nigra is divisible into a dorsal part, the pars compacta; and a ventral part, the
pars reticularis.
The pars compacta contains dopaminergic and cholinergic neurons. Most of the neurons
in the pars reticularis are GABAergic. Superiorly, the pars reticularis becomes continuous
with the globus pallidus. The substantia nigra is closely connected, functionally, with the
corpus striatum.
32. 32
IMPORTANT POINTS:
Pretectal nucleus- light reflex
The oculomotor nucleus lies in relation to the ventral part of the central grey matter. The nuclei of
the two sides lie close together forming a single complex. The Edinger Westphal nucleus (which
supplies the sphincter pupillae and ciliaris muscle) forms part of the oculomotor complex. The
oculomotor complex is related ventrally to the medial longitudinal fasciculus.
Connections of the red nucleus
The red nucleus consists of a cranial parvocellular part and a magnocellular part. The magnocellular
part is prominent in lower species, but in man it is much reduced and is distinctly smaller than the
parvicellular part.
The red nucleus receives its main afferents from:
a) The cerebral cortex (directly, and as collaterals from the corticospinal tract); and
b) The cerebellum (dentate, emboliform and globose nuclei).
34. 34
The efferents of the nucleus are as follows:
The magnocellular part projects to the spinal cord through the rubrospinal tract;
and to motor cranial nerve nuclei (III, IV, V, VI, VII) through the rubrobulbar
tract.
Some fibres reach the reticular formation. The parvicellular part of the nucleus
gives origin to fibres that descend through the central tegmental fasciculus to
reach the inferior olivary nucleus. Some fibres reach the reticular formation.
The term reticular formation was originally used to designate areas of the central
nervous system which were not occupied by well defined nuclei or fibre bundles,
but consisted of a network of fibres within which scattered neurons were situated.
Such areas are to be found at all levels in the nervous system. In the spinal cord
there is an intermingling of grey and white matter on the lateral side of the neck
of the dorsal grey column. This area is sometimes referred to as the reticular
formation of the spinal cord. The reticular formation is, however, best defined in
the brainstem where it is now recognised as an area of considerable importance.
35. The reticular formation extends throughout the length of
the brainstem. In the medulla it occupies the region dorsal
to the inferior olivary nucleus. In the Pons it lies in the
dorsal part, while in the midbrain it lies in the tegmentum.
36. 36
Ascending reticular
activating system:
It has been seen that many ascending tracts passing through the
brainstem are intimately related to the reticular formation. Many of
the fibres in these tracts give off collaterals to it. These come from the
spinothalamic tracts, from secondary trigeminal pathways and from
auditory pathways. These collaterals terminate predominantly in the
lateral part of the reticular formation. Fibres arising here project to the
intralaminar and reticular nuclei of the thalamus. These nuclei in turn
project to widespread areas of the cerebral cortex. These pathways
form part of the ascending reticular activating system (ARAS) which is
believed to be responsible for maintaining a state of alertness
37. 37
Summary of Functions of the
Reticular Formation:
A. Somatomotor control:
Through its direct connections with the spinal cord; and
indirectly through the corpus striatum, the cerebral cortex and
the cerebellum, the reticular formation has an influence on fine
control of movements including those involved in postural
adjustments, locomotion, skilled use of the hands, speech etc.
38. 38
B. Somatosensory control:
The reticular formation influences conduction through Somatosensory pathways.
Similar effects may also be exerted on visual and auditory pathways.
C. Visceral control:
Physiological studies have shown that stimulation of certain areas in the reticular
formation of the medulla has great influence on respiratory and cardiovascular function.
The region influencing respiratory activity corresponds approximately to the giganto-
cellular nucleus and parvocellular nucleus. Stimulation of the gigantocellular nucleus and
the upper part of the ventral reticular nucleus causes depression of vasomotor activity
while stimulation of other areas has a pressor effect. These effects are through connections
between the reticular formation and autonomic centres in the brainstem and spinal cord,
but the pathways concerned are not well defined.
D. Neuroendocrine control:
Through its connections with the hypothalamus, the reticular formation influences
activity of the adenohypophysis and of the neurohypophysis.
39. 39
CONTROL OF EYE MOVEMENTS:
When the gaze is shifted from one object to another the eyes undergo sharp
movements called saccades. The controlling centres determine the velocity and extent
of a saccade. A different kind of control is required when the eyes follow a moving
object (pursuit movement).
A. The areas of cerebral cortex involved in programming and co-ordination of all eye
movements are:
(a) the frontal eye field (area 8);
(b) the supplemental eye field located on the medial surface of the hemisphere (area 6).
(c) the prefrontal cortex (anterior to the frontal eye field); and
(d) the posterior eye field (in the parietal lobe).
40. B. The areas of cerebral cortex listed above project to the superior
colliculus (some of them passing via the basal nuclei). The superior
colliculus also receives fibres from the retinae, from pretectal nuclei,
and from the cerebellum.
C. The superior colliculus projects to two important centres which are
as follows.
(1)The vertical gaze centre lies in the midbrain. It is located in the
rostral interstitial nucleus (a collection of neurons within the medial
longitudinal fasciculus), and in the interstitial nucleus of Cajal. This
centre controls vertical eye movements.
41. 41
(2) The horizontal gaze centre lies in the pons, in the paramedian
reticular formation. The centre controls horizontal eye movements.
In addition to the afferents from the superior colliculus, these centres
also receive:
(a) some direct cortical fibres; and
(b) fibres from the cerebellum.
Efferents from the vertical and horizontal gaze centres project to the
oculomotor, trochlear and abducent nuclei.
42. 42
D. The cranial nerve nuclei supplying muscles of the eyeball receive the
following afferents.
(a) Corticonuclear fibres.
(b) Fibres from the vertical and horizontal gaze centres described above.
(c) Direct fibres from the superior colliculus and from pretectal nuclei.
(d) Fibres from vestibular nuclei. These travel through the medial
longitudinal fasciculus. They provide information about movements of
the head and neck (from the vestibular apparatus).
Through the vestibular nuclei, the oculomotor trochlear and abducent
nuclei are brought under cerebellar control.
43. 43
E. The final control of vertical gaze is by fibres passing from
the oculomotor and trochlear nuclei (to reach muscles
producing upward and downward movements of the
eyeballs). The final control of horizontal gaze is mainly by
the abducent nerve (lateral rectus), but is also influenced by
the oculomotor nerve (medial rectus). The horizontal gaze
centre is also involved incontrol of eye movements when the
gaze follows a moving object. These are called ‘pursuit
movements’. The control of these movements is complex
