LABORATORY STUDY OF THE SHEEP BRAIN

WEEK 2 Medial Structures

 

You should be able to work on the cranial nerves before Wednesday.  (Possible extra credit?  Know the role of each CN – for example optic nerve = incoming vision signals)

 

Cranial Nerves.  Human Brain 5.7 – KNOW THE LOCATION AND NAME *NOT JUST CNX or Cranial Nerve 3* FOR EACH CRANIAL NERVE – Color code the pairs of nerves, it will help!  Sheep Brain Handout will help you, there are good labeled images on the Online Sheep Brain Dissection Guide (http://www.uofs.edu/sheep/framerow.html)   – I will only ask you to name cranial nerves where it is absolutely clear which nerve it could be.

a.    The origins of the olfactory (CNI) and optic (CNII) nerves have been indicated on the week 1 sheet.

b.    The oculomotor nerve (CNIII) arises from the ventral surface of the cerebral peduncles close to its medial border.

c.    The trochlear nerve (CNIV) is found caudal and lateral to the oculomotor nerve and rostral and medial to the trigeminal nerve.

d.    The trigeminal nerve (CNV) is a very large flat nerve extending forward from the lateral border of the pons.   It arises by two roots; the larger sensory root is placed lateral to the smaller motor root.

e.    The abducens nerve (CNVI) arises from the trapezoid body about 1/8 inch from the ventral median fissure.

f.     The facial nerve (CNVII) arises from the trapezoid body lateral to the abducens.

g.    The vestibulocochlear nerve (CNVIII) is seen just caudal to the facial nerve. Having arched downward from the dorsolateral border of the medulla, it appears from under the flocculus of the cerebellum.

h.       The glossopharyngeal (CNIX) and vagus (CNX) nerves arise by a series of filaments from a groove along the lateral border of the medulla.  CNIX arises from this groove just caudal to the trapezoid body and just lateral to the olive.  CNX arises caudal to CNIX.

i.         The accessory nerve (CNXI) is on the same line with CNIX and X, but caudal to them; it is seen running forward along the side of the spinal cord and medulla, receiving filaments from both.

j.        The hypoglossal nerve (CNXII) arises as a series of roots from the ventral surface of the medulla, near the caudal end of the lateral border of the pyramid.

 

Medial sagittal section of brain.

 

  1. The corpus callosum, the great band of fibers passing across the midline between the two hemispheres, has been cut transversely in this section. It appears as an elongated mass of white matter that slopes in a rostroventral direction.  The caudal club-shaped end is the splenium. The body runs from the splenium to the genu.  The rostral extremity bends ventrally as the genu (knee) and ends in the caudally-turned rostrum (beak).  The corpus callosum should also be identified as the particular region such as “splenium of the corpus callosum” or “body of the corpus callosum” – using just “splenium”, “body”, or “corpus callosum” is not acceptable.

 

2.      The cingulate gyrus (gyrus cinguli) is the gyrus just dorsal to the corpus callosum, bounded dorsally by the cingulate sulcus (sulcus cinguli).

 

  1. The cortical area under the rostrum of the corpus callosum, appearing as a ventrocaudally-turned continuation of the gyrus cinguli, is a part of the rhinencephalon.   It is in close relation to the medial olfactory gyrus and anterior perforated substance.

 

  1. The fornix is the white sickle-shaped band that lies under the caudal end of the corpus callosum.   It arches in a rostroventral direction to end behind the optic chiasm.  The fibers of the fornix arise in the hippocampus, a cortical elevation in the floor of the inferior horn of the lateral ventricle, in the temporal lobe.  The fiber bundles from the two sides arch upward to join under the corpus callosum, some of the fibers crossing the midline as commissural fibers.   Rostral to the thalamus, the fibers again separate into two bundles, the anterior column of the fornix to terminate ventrally into mammillary bodies. 

 

  1. The septum pellucidum is the thin double-layered membrane which covers the triangular space between the genus of the corpus callosum and the anterior columns of the fornix.   It separates the two lateral ventricles toward their rostral extremities.

 

  1. The thalamus is the large rounded mass of gray matter around whose rostral aspect the anterior columns of the fornix arch.  The thalami are bilateral paired egg-shaped masses, joined together across the midline by the massa intermediate, which has been cut transversely in making this section.

 

  1. The third ventricle, the cavity of the diencephalon, is a narrow vertical cleft forming a ring around the intermediate mass.  Its walls are constituted as follows:
    1. The lateral walls are formed mostly by the thalami.
    2. The floor and ventral part of the lateral walls are formed by the hypothalamus, whose ventrally-seen structures have already been identified.
    3. The rostral wall is formed by:

                                                               i.      the lamina terminalis, a thin plate joining the two hemispheres. It is the original rostral boundary of the telencephalon.  In this section it extends from the optic chiasm upward to the anterior commissure, a fiber bundle connecting olfactory regions of the two hemispheres, which appears here in cross section as a round white spot.

                                                             ii.      a small caudally-running portion of the rostrum of the corpus callosum.

                                                            iii.      a small part of the fornix just above the anterior commissure. Just caudal to this part of the fornix are located, on either side, the interventricular formina (sing. foramen), the opening by which the third ventricle communicates with the lateral ventricles of hemispheres.

    1. The roof or epithalamus is formed by:

                                                               i.      rostrally, a very thin ependymal layer which is thrust downward into the ventricle by a fold of richly vascular pia mater.  Thus is formed the choroid plexus of the third ventricle, which extends rostrally and laterally through the interventricular foramina to form the choroid plexus of the lateral ventricles. 

                                                             ii.      caudally, the pineal body or epiphysis, a small pinecone-shaped body whose caudal surface rests upon the midbrain.

                                                            iii.      The habenular trigone, a small triangular area just rostral to the pineal body.  It contains a nucleus concerned with olfactory reflexes.

                                                           iv.      the posterior commissure appears as a round white spot in the roof just caudal to the pineal body.  Some of its fibers connect the two superior colliculi; the connections of the rest are not certain.  The posterior commissure is dorsal to the junction of the third ventricle and the cerebral aqueduct (aqueduct of Sylvius), the cavity of the mesencephalon.

  1. The mesencephalon or midbrain consists of the following structures:
    1. Dorsal to the cerebral aqueduct, a thick roof plate forms two pairs of rounded eminences, the superior and inferior colliculi.  The superior colliculi, which are visual reflex centers, are much larger and more caudal than the inferior colliculi, which are reflex auditory centers.  The pineal gland is a singular midline structure that lies rostrally in between the two superior colliculi.
    2. Ventral to the cerebral aqueduct are the thick cerebral peduncles connecting the brainstem to the cerebral hemispheres.  The dorsal part, under the aqueduct, is the tegmentum, a reticular formation of mingled fibers and gray masses which extends caudally into the rhombencephalon.  The ventral part of the peduncle is the basis pedunculi, composed of longitudinal fiber tracts connecting the forebrain with lower levels.
  2. The rhombencephalon.
    1. View the cerebellum in sagittal section through the vermis.  Notice how the folia of gray cortex dip inward between the tree-like branchings of the medullary white matter.  Locate the fissure prima, the prepyramidal sulcus, and the nodule.  The nodule is the lobule of the vermis located in the most ventrocaudal position, just caudal to a recess of the fourth ventricle that vertically divides the ventral part of the vermis.
    2. The anterior medullary velum is the thin sheet forming the roof of the pontile (rostral) part of the fourth ventricle.
    3. The roof of the caudal part of the fourth ventricle is very thin and is invaginated by the vascular pia mater to form the choroid plexus of the fourth ventricle.  This has been removed in most of the sagittal sections.
    4. By tipping the cerebellum very slightly away from the brain stem, the rhomboid form of the fourth ventricle will be apparent.
    5. At its rostral end, the fourth ventricle connects with the cerebral aqueduct.
    6. At its caudal end, the fourth ventricle connects with the central canal of the closed part of the medulla, and this canal is in turn continuous with the central canal of the spinal cord.  The caudal portion of the fourth ventricle is termed the obex.

 

 

 

 

 

Any of the above bold and underlined structures are fair game in the human Neuroanatomy book but it particular pay attention to the following plates:

 

CRANIAL NERVES 5.7

VENTRICLES:  5.4

COMMISSURES:  5.6

THALAMUS:  6.4

HYPOTHALAMUS:  6.5

OVERALL MIDLINE STURCTURES:  10.4   10.5   

 

 

NOTES ON TAKING THE LAB PRACTICALS:

 

Remember that the lab practicals are CUMULATIVE.

 

On the lab practical you must be as specific as possible.  If a pin is in the hippocampal gyrus and the sheet does not say “lobe” then you must answer hippocampal gyrus and not temporal lobe.  If the pin is in the vermis, you will not get credit for answering cerebellum.

 

Ventricles, Gyri, Sulci, Fissures, and Nerves will be so marked on your lab practical sheet.

 

To help prepare for the lab practical, you should quiz your lab partner and have them quiz you on more than 1 brain (they don’t all look alike!).