The Anatomy of the Posterior Commissure

Abstract

AIM: The connections of posterior commissure are defined. Its fibers mediate the consensual light reflex by interconnecting the pretectal nuclei. The fiber connections from the thalamic, pretectal, superior colliculus and the habenular nuclei are known, but they have not been shown anatomically. The present study is a fiber dissection study to define the anatomy of the posterior commissure. MATERIAL and METHODS: Twenty formalin-fixed sheep heads were used in the study. The specimens were fixed in 10% formalin solution for 3 weeks. The arachnoidal and vascular structures were removed by using a surgical microscope magnification (x6-x40) and brains were again fixed for 4 weeks at -20 degrees C. The fiber dissections were performed at Marmara University, Rhoton Laboratory. Also, a radiological tractographic study was carried on five healthy volunteers to see the posterior commissure cortical connections. RESULTS: In fifteen sheep brains, the dimensions of the posterior commissure were measured as 1.36 mm (range 0.5-2.5 mm) width, and as 4.6 mm (range 3-6 mm) length. In the dissection study, a frontotemporooccipital fascicle was observed to connect with the fibers of the posterior comnnissure. Diffusion tensor imaging scans showed the frontotemporooccipital tract to extend to posterior commissural region. CONCLUSION: To our knowledge, this is the first anatomical and tractographic study regarding the posterior commissure. However, further human cadaveric studies are necessary.

Full text

Orgnal Investgaton
Turk Neurosurg 2015, Vol: 25, No: 6, 837-843 837
Received: 21.07.2014 / Accepted: 16.12.2014
DOI: 10.5137/1019-5149.JTN.12122-14.2
e Anatomy of the Posterior Commissure
Posterior Komissür’ün Anatomisi
Nuriye Guzin OZDEMIR
İstanbul Training and Research Hospital, Neurosurgery Clinic, Istanbul, Turkey
Corresponding Author: Nuriye Guzin OZDEMIR / E-mail: guzozdemir@yahoo.com
ABSTRACT
AIM: The connectons of posteror commssure are defned. Its fbers medate the consensual lght reex by nterconnectng the pretectal
nucle. The fber connectons from the thalamc, pretectal, superor collculus and the habenular nucle are known, but they have not been
shown anatomcally. The present study s a fber dssecton study to defne the anatomy of the posteror commssure.
MATERIAL and METHODS: Twenty formaln-fxed sheep heads were used n the study. The specmens were fxed n 10% formaln soluton for
3 weeks. The arachnodal and vascular structures were removed by usng a surgcal mcroscope magnfcaton (x6-x40) and brans were agan
fxed for 4 weeks at -20°C. The fber dssectons were performed at Marmara Unversty, Rhoton Laboratory. Also, a radologcal tractographc
study was carred on fve healthy volunteers to see the posteror commssure cortcal connectons.
RESULTS: In ffteen sheep brans, the dmensons of the posteror commssure were measured as 1.36 mm (range 0.5-2.5 mm) wdth, and as 4.6
mm (range 3-6 mm) length. In the dssecton study, a frontotemporooccptal fasccle was observed to connect wth the fbers of the posteror
commssure. Duson tensor magng scans showed the frontotemporooccptal tract to extend to posteror commssural regon.
CONCLUSION: To our knowledge, ths s the frst anatomcal and tractographc study regardng the posteror commssure. However, further
human cadaverc studes are necessary.
KEYWORDS: Duson tensor magng, Fber dssecton, Posteror commssure
ÖZ
AMAÇ: Posteror komssür’ün ler, pretektal nükleusları bağlayarak konsensual ışık reeksne aracı olur. Talamk, pretektal, tektal, superor
kolkulus ler ve habenular çekrdeklern lernn posteror komssür’de çaprazlaştığı blnmektedr. Ancak anatomk olarak gösterlmemştr.
Posteror komssür anatomsn tanımlamak çn yapılan lf dseksyon çalışması sunulmuştur.
YÖNTEM ve GEREÇLER: Yrm koyun beynnde Marmara Ünverstes Rhoton Laboratuvarı’nda dseksyon yapıldı. Spesmenler %10 formaln
solüsyonunda 3 hafta bekletld. Cerrah mkroskop (x6-x40) altında araknod-vasküler yapılar çıkartılarak beynler -20°C’de bekletld. Beş
beyn, posteror fksasyon-dseksyon yetersz olduğu çn çalışmadan çıkartıldı. Beş beynde anteror, lateral, posteror ve medal dseksyon
yapılarak her aşamada fotoğraandı. On beynde posteror komssür ortaya konacak şeklde hpokampal komssür düzeynden dseksyon
yapıldı. Radyolojk çalışmada 5 gönüllü hastaya, dfüzyon tensor görüntüleme yapılarak posteror komssür’ün lf bağlantısı ncelend.
BULGULAR: On beş koyun beynnde posteror komssür’ün ortalama kalınlığı 1,36 mm (0,5-2,5 mm), uzunluğu 4,6 mm (3-6 mm) olarak bulundu.
Posteror komssür’ün her k tarafta superor ve nferor kolkulusları bağladığı; habenula, pretektal, tektal ve perakuaduktal bölgeden kısa
lerle bağlantı kurduğu görüldü. Bazı spesmenlerde hpokampal komssür lernn splenal bölgede bu lere eşlk ettğ zlend. Talamk
dseksyondan sonra gözlenen frontotemporooksptal faskülün posteror komssür’e uzandığı saptandı.
SONUÇ: Çalışmada dseksyonla posteror komssür’ün frontotemporooksptopontn bağlantısı gösterlmştr. Araştırma posteror komssür’ün
lf bağlantılarını ortaya koyacak nsan kadavra çalışmalarıyla desteklenmeldr.
ANAHTAR SÖZCÜKLER: Dfüzyon tensor görüntüleme, Lf dseksyonu, Posteror komssür
INTRODUCTION
Cerebral white matter is composed of myelinated bers
classied into three types of fasciculi; association, commissural
and projection bers. Association bers interconnect dierent
cortical regions of the same hemisphere, commissural bers
interconnect the two hemispheres across the median plane,
and projection bers connect the cortex with caudal parts of
the brain and the spinal cord (2, 7).
Commissural bers are corpus callosum, anterior commissure,
hippocampal commissure and the posterior commissure. The
posterior commissure is located in the inferior pineal lamina
and it is one of the commissural bers of the brain known to
be important in the pupillary light reex. Its bers acquire
their medullary sheath early. Various nuclei are associated
with the posterior commissure. The best known of them is
the interstitial nucleus of the posterior commissure, nucleus
of Darkschewitsch; another one is the interstitial nucleus of
Cajal. The rst one is located in the central grey substance
of the upper end of the cerebral aqueduct, in front of the
nucleus of the oculomotor nerve. The second one is near
the upper end of the oculomotor fasciculus (5, 6, 14, 22). The
posterior commissure is detected and dened early in the
human embryo but the connections of this structure except
for the nucleus of Darkschewitsch are unknown. Its bers
interconnect the pretectal nuclei and mediate the consensual

Turk Neurosurg 2015, Vol: 25, No: 6, 837-843838
Ozdemir NG e Anatomy of Posterior Commissure
pupillary light reex. Some bers are believed to be derived
from the posterior part of the thalamus and from the superior
colliculus and to continue directly to the medial longitudinal
fasciculus. Fibers from the thalamic, pretectal, tectal region,
bers from the superior colliculus and the habenular nuclei
are known to connect with the posterior commissure, but
they have not been shown anatomically (3-5,12).
Although recent reports about other commissural bers
of the brain, such as corpus callosum, anterior commissure
and hippocampal commissure are present in the literature,
the anatomy and connections of the posterior commissure
have not been studied except for an early report (2, 5,10, 21).
The aim of this study was to evaluate the connections of the
posterior commissure and especially to show the cortical ber
relationship anatomically and tractographically.
MATERIAL and METHODS
Twenty formalin-xed sheep heads were used in the
study. The dissection was performed using the operating
microscope at the Rhoton Anatomy Laboratory of Marmara
University, Faculty of Medicine. Craniectomy to evacuate
the brains for the rst 5 heads and craniotomy for the latter
15 heads was performed with the help of a craniotome. The
specimens were xed in 10% formalin solution for 3 weeks.
The arachnoidal and vascular structures were removed by
using the magnication of the surgical microscope (x6-x40)
and brains were again xed for 4 weeks at -20°C, after which
the dissection began. Five specimens were excluded from
the study since the xations was not sucient to dissect the
posterior structures. Gross anatomical dissection was done to
dene the posterior structures of the brain. In 5 brains, the
dissections were done as dorsal, ventral, lateral, and medially
using Klingler’s method. Digital photographs were taken at
each step (Figures 1AD). Dissection was made on all sides
to understand the anatomic relationship of the posterior
commissure and then, only posteriorly to see the posterior
commissure anatomy (Figures 2A-D). In ten specimens,
the posterior commissure together with the tectum and
tegmentum were dissected, removing the upper part of the
telencephalon.
As a radiological study, ve healthy subjects with no history of
neurologic pathology were recruited for the study. All patients
provided written consent for the study, which was approved
by the institutional review board at the Marmara Neurologi-
cal Sciences Institute. Five volunteer patients underwent trac-
tographic investigation to understand the upper and lower
ber tract pathway of the posterior commissure. A radiologi-
cal tractographic study was carried on ve healthy volunteers
to see the posterior commissure cortical connections. Brain
magnetic resonance imaging (MRI) was performed using a
whole-body scanner (1.5- T, Siemens Magnetom, Espree, Ger-
many) with an eight-channel head coil. Diusion tensor im-
aging (DTI) tractography was performed using a single-shot
multislice spin echo-echo planar sequence. Slice thickness
was 3 to 5 mm.
RESULTS
In 15 sheep brains, the dimensions of the posterior
commissure were measured as 1.36 mm (range 0.5-2.5 mm)
width and 4.6 mm (range 3-6 mm) length.
The posterior commissure was observed to join both superior
and inferior colliculi on both sides (Figures 3A, B; 4). Short
bers coming from the habenula, pretectal, and tectal regions
Figure 1: A-D) Gross anatomical dissection: Dorsal, ventral, lateral and medial dissection views respectively.
A B
C D

Turk Neurosurg 2015, Vol: 25, No: 6, 837-843 839
Ozdemir NG e Anatomy of Posterior Commissure
and from periaqueductal grey matter nuclei were identied.
Some hippocampal commissural bers were seen to join
these bers at the splenial region but this was not shown
in all the specimens (Figures 2A-D). After removal of the
upper part of the telencephalon and posterior dissection, a
frontotemporoparietooccipital fascicle was observed. This
tract was followed to the tectal region and seen to end in the
superior collicular region. This tract was observed in all the
specimens. However, a tract towards the cerebellar region
could not be dissected (Figures 5A, B).
DTI scans showed the frontotemporooccipital tract to
extend to the posterior commissural region (Figures 7A, B).
As in anatomical dissection, an inferior tract towards the
cerebellum could not be observed (Figures 6A, B).
DISCUSSION
The commissures of the telencephalon are the anterior com-
missure, the hippocampal commissure, corpus callosum and
posterior commissure. Anterior and hippocampal commis-
sures are present in all vertebrates, whereas corpus callosum
is phylogenetically new and found in placental mammals
only. Corpus callosum is the most prominent commissure,
connecting most of the frontoparietal bers of both hemi-
spheres, limited anteriorly by the anterior commissure and
posteriorly by the hippocampal commissure (12).
In humans, the anterior commissure carries olfactory and
neocortical bers from the temporooccipital region, and the
hippocampal commissure carries bers from the hippocampal
formations together with neocortical bers (the splenium)
from the posteromedial aspect of the hemispheres (12).
The posterior commissure is not well dened in the literature.
The subcommissural organ, a specialized neuroepithelium
located at the dorsal midline underneath the posterior
Figure 2: A-D) Dissection showing the hippocampal commissural bers and removal of it to see the frontotemporoparietal connections
of the posterior commissure.
Figure 3: A, B) Sagittal sections of the sheep brain showing the posterior commissure under the pineal gland (red arrows).
A B
C D
A B

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Ozdemir NG e Anatomy of Posterior Commissure
commissure, releases the subcommissural organ spondin,
a large glycoprotein belonging to the thrombospondin
superfamily that shares molecular domains with axonal
path-nding molecules. The subcommissural organ is
thought to be involved in the development of the posterior
commissure (3,12). In our anatomical study, we observed the
posterior commissure but we could not distinctly identify the
subcommissural organ.
The posterior commissure is located in the posterior third
ventricle, composed of the roof, oor, posterior wall and
both lateral walls. The posterior wall within the third ventricle
extends from the suprapineal recess above to the cerebral
aqueduct below and consists of the suprapineal recess, the
habenular commissure, the pineal body and its recess, the
posterior commissure and the cerebral aqueduct. The only
structure in the posterior wall in the quadrigeminal cistern
is a pineal body and is concealed by the splenium above,
thalamus laterally and the quadrigeminal plate and the
vermis below (20). In this study, the posterior commissure was
Figure 4: Sagittal section showing the posterior commissure (red
arrow).
Figure 5: A) Pineal gland is seen in close-up view with the ber tract extending to the frontotemporal region on the left side (red and
blue arrows respectively). B) Pineal gland and after removal of the pineal gland, the posterior commissure (red arrow) and the ber
tract is seen after a horizontal dissection.
Figure 6: A, B) Pineal gland and after removal of the pineal gland; the frontotemporooccipitopontine tract connecting with the
posterior commissure and superior colliculus in close-up views (white and red arrow respectively).
A B
A B

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Ozdemir NG e Anatomy of Posterior Commissure
reference to the pupilloconstrictory pathway via the posterior
commissure was also reported (14). All these studies
were related to the light reex function of the posterior
commissure which was already known. We also observed the
posterior commissure to connect the right and left pretectal
regions in accordance with the literature reports and classical
anatomical knowledge.
Besides being a connective area between the tectal areas, the
posterior commissure may have a relationship with the optic
pathways. We know that visual pathways originate from the
temporal half of the ipsilateral retina as nerve bers of the
optic tract. The optic tract begins at the posterolateral corner
of the optic chiasma and passes posterolaterally between
the anterior perforated substance and tuber cinereum. Their
bers lie superior to medial aspect of the crus cerebri. The
optic tract enters the lateral geniculate body and divides into
two roots, the medial of which passes to the medial geniculate
body. The optic radiation starts at the lateral geniculate
body. The minority of optic tract bers bypass the lateral
geniculate nucleus entirely, traveling to the midbrain. These
bers mediate the pupillary light reex in the pretectal area
as described. The posterior commissure connects the left and
right pretectal area and contains bers from cell bodies in the
pretectal and neighboring regions, including the dendrite of
Müller cell M1. However, the studies showed that interaction
between neurons in the left and right pretectal regions is not
crucial for the responses (11, 19).
Two patterns of visual pathways were found. The optic ra-
diation ran more commonly deep in the whole superior and
middle temporal gyri and superior temporal sulcus. The op-
found to be located in the posterior wall of the third ventricle
by dissecting 15 brains of sheep and there was no variation.
The posterior commissure connects the bers of the pretectal
area. There are myelinated and non-myelinated bers
connecting the pineal gland and the pretectal area. Recently,
a new paired tract has been found distinct from the posterior
commissure. These tracts form a structural component of the
wall of the pineal recess. These bers are myelinated, invade
the pineal gland, and can be traced a short distance into the
lateral pre-tectal area (15). Also, a quantitative analysis of
the myelinated axons of commissural bers in the rat brain
demonstrated a heterogeneous distribution of myelinated
axons in the posterior and habenular commissures (13).
Functionally, the posterior commissure serves as a pathway
for impulses related to eye-movements, and there is a close
association between the posterior commissure and the
medial longitudinal bundle. However, a relationship was
observed between the development of the cerebellum
and the posterior commissure that suggests that the more
important functions of the commissure might be associated
with posture and the integration of body movement rather
than with movement of the eyes and visual inuences (5, 15,
17).
Experimental studies are present showing the association of
the periaqueductal neurons with the posterior commissure
and functional and anatomical ber analysis of the posterior
commissure has been performed to study the pupillary reex
(6).
A study showing the posterior commissural connections
of area pretectalis and neighboring structures with special
Figure 7: A, B) DTI showing the frontotemporal ber connections of the posterior commissure.
A B

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Ozdemir NG e Anatomy of Posterior Commissure
interstitio-spinalis tracts in a downward direction towards
the medial part of the medial longitudinal bundle, forming
the most cephalic portion of the medial longitudinal bundle.
This anatomy may explain how the posterior commissure
may have a cerebellar role, rather than a functional role in
the light reex in some animals. It has been suggested that
there are bers passing from the commissural nuclei to the
ipsilateral globus pallidus, and also to the rubral area (5, 14).
Knowing that there is a pathway between the cerebellum and
the rubral area via the cerebellar peduncles, the commissural
bers might have a connection to the cerebellar area. We
could not show the ber relationship between the posterior
commissural bers and the cerebellum, and the vestibular
tracts crossing at the posterior commissure. In sheep, the
cerebellum has a major role in balance and coordination and
it is reasonable not to be able to demonstrate the vestibular
bers crossing at the posterior commissure. This is also true
for the human brain.
The importance of anterior and posterior commissure and
individual variations and tractographic anatomical study of
the insular region have been reported in the literature (1, 18),
but there is no tractographic study related to the posterior
commissure. We used DTI for identication of the bers of
the posterior commissure in the human brain, but the patient
number was limited and a distinct tract could not be shown
in all the patients.
In this anatomical and radiological study, we showed the
tract connection of the posterior commissure with the
thalomocortex anatomically and tractographically with DTI as
a preliminary work, but we could not demonstrate the inferior
relationship.
CONCLUSION
The posterior commissure connects the superior colliculi
which is involved in the bilateral pupillary light reex. We
observed a frontotemporooccipitopontine tract that may
play a role visual information. To our knowledge, this is
the rst anatomical study showing the bers with the ber
dissection technique using the Klingler’s method and
tractographic study regarding the posterior commissure.
However, there are limitations of this study, as the study was
done on sheep brains and there are anatomical dierences.
Further human cadaveric and anatomical studies, and also
further tractographic studies with a larger patient group,
are necessary to understand the ber connections of the
posterior commissure.
ACKNOWLEDGEMENT
I wish to include special thanks to the radiology technician
Serdar Erbil at the Marmara University Radiology Department
for giving his time and eort for the tractography of the
volunteer patients.
tic radiation was closely surrounded in all cases by an inferior
longitudinal fascicle (ILF) and a parietooccipitotemporopon-
tine fascicle. The anatomy of the inferior longitudinal fascicu-
lus and its function are not totally understood. It connects the
occipital lobe with the anterior temporal lobe and is reported
to subserve the language semantics in parallel with the infe-
rior occipital fasciculus. It joins the posterior occipitotemporal
regions to the temporal pole and is relayed by the uncinate
fasciculus connecting the anterior temporal pole to the fron-
tobasal areas (11, 16, 19).
The inferior longitudinal bundle was originally thought
to consist of long tracts connecting the visual areas of the
occipital lobe with the anterior lobe, possibly playing a
role in visual memory. We also observed in our study that
the frontotemporooccipitopontine bers connect with the
posterior commissure. However, the anatomical dierences
between sheep and human brain stop us from declaring
that this is the parietooccipitotemporopontine fascicle
accompanying the inferior longitudinal fascicle together
which surround the optic tract, since animal studies have
shown that animals do not have an inferior longitudinal
fascicle. The fascicle we identied may be the tract playing
a role in visual formation, and this information from the
opposite hemisphere is thought to be carried by the posterior
commissure (8, 9, 11,16).
The occipitopontine projection originates in cortical areas
that respond to visual stimulation. The occipitopontine tract
is reported to be associated with eye pursuit movements
and may also be associated with visual perception (16).
Each occipital hemisphere receives information from the
opposite half of the visual world. The visual information is
transferred to a visual word form system located in the inferior
occipitotemporal white matter. Information from the left side
of the visual eld is received by the right visual cortex and is
transferred to the word form system in the left hemisphere.
This transfer occurs through the posterior commissure
and splenium of the corpus callosum and is disrupted in
alexia without agraphia (9). We could not show the inferior
longitudinal projection system, since it is absent in animals.
In this anatomical study, a distinct fascicle was shown
anatomically from the frontotemporooccipitopontine region
to the superior and inferior collicular area (tectal region) and
crossing at the posterior commissure in all the specimens
after removal of the upper part of the telencephalon and
posterior dissection.
Another function of the posterior commissure is related
to the vestibular pathway. Vestibular tracts as vestibular
mesencephalic, and the lateral vestibular tegmental cross in
the posterior commissure before connecting with the nucleus
of the posterior commissure, and the interstitial nucleus. The
eerent connection is the commissuro-medullaris, and the

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