Dissector Answers - Neuroanatomy 1

Learning Objectives:

Upon completion of this session, the student will be able to:

  1. Identify the following surface features of the brain: cerebrum, cerebellum, brainstem; longitudinal fissure; lateral sulcus (including opercula) and central sulcus; and the pre- and post-central gyri; frontal, parietal, temporal, occipital and insular lobes.
  2. Identify the three meninges (dura, arachnoid, and pia) and the middle meningeal artery.
  3. Identify the three parts of the brainstem (midbrain, pons, and medulla oblongata), the 12 pairs of cranial nerves as they arise from the brain and brainstem, and the relative position of cranial nuclei.
  4. Identify each of the 12 pairs of cranial nerves in the three cranial fossae (anterior, middle and posterior), and the relative positions of their sensory ganglia (if appropriate).
  5. Identify the surface features of the cerebellum, including the lateral hemispheres connected by the vermis, the lobes (anterior, posterior, and flocculonodular) and lobules (10, but not their names), cerebellar cortex and folia, peduncles (superior, middle, inferior), and the tonsils.
  6. Identify the inner features of the cerebellum, including the superior and inferior medullary velum and 4th ventricle, and the cerebellar nuclei (dentate, globose, emboliform, and fastigial).

Learning Objectives and Explanations:

1. Identify the following surface features of the brain: cerebrum, cerebellum, brainstem; longitudinal fissure; lateral sulcus (including opercula) and central sulcus; and the pre- and post-central gyri; frontal, parietal, temporal, occipital and insular lobes. (NI19A, NI19B, NI19C, NI20, TG7-53A, TG7-53B)
The lobes of the cerebral hemispheres are named for the bones that overlie them: frontal bone, parietal bone, temporal bone, and occipital bone.

The frontal lobes lie on the anterior cranial fossa, above the orbits. The temporal lobes sit in the middle cranial fossae behind the orbits. The occipital lobes sit above the cerebellum, which occupies the posterior cranial fossa.

There are 5 lobes of each cerebral hemisphere, but only 4 are visible on the surface of the brain. They are the frontal lobe, Parietal lobe, occipital lobe, and temporal lobe.

The brain features many grooves or sulci separating multiple gyri or rounded ridge-like elevations. The major sulci and fissures include the midline longitudinal fissure separating the two cerebral hemispheres, the lateral sulcus that lies above the temporal lobe, the parieto-occipital sulcus and preoccipital notch (both of which can be difficult to locate) and the central sulcus, which separates the precentral gyrus from the postcentral gyrus.

The central sulcus separates the precentral gyrus, the primary motor cortex, and the postcentral gyrus, the primary sonatosensory cortex.

Down in the bottom of the lateral sulcus, overshadowed by parts of the frontal, parietal, and temporal lobes called opercula, is the insula, the hidden 5th lobe.

Some people consider the limbic system as a 6th lobe, with its cingulate gyrus lying above the corpus callosum, the big connection between the two cerebral hemispheres.

2. Identify the three meninges (dura, arachnoid, and pia) and the middle meningeal artery. (NI45A, NI59A, NI59B, TG7-46, TG7-47, TG7-48)
The brain has the same three coverings of meninges as the spinal cord. There is an outer dura mater, a thin arachnoid mater, and then the pia mater covering the surface of the brain just as it does the spinal cord. The middle meningeal artery, a branch of the maxillary artery, supplies most of the dura, and its location at the side of the head puts it at risk of rupture with a blow to the "temple", which would produce an epidural hematoma.

The dura mater of the brain forms curtain- and tent-like structures that serve two purposes. They contain spaces called dural venous sinuses that serve to drain the brain and skull. They also help to anchor the brain within the skull, reducing the risk of concussion. The cerebrospinal fluid is a water bath that also helps in reducing this risk, as do the arachnoid trabeculae, numerous fine threads that pass from arachnoid to pia, acting like Lilliputian strings to hold the brain in its water bath and prevent it from bumping into the inside of the skull.

The falx cerebri separates the cerebral hemispheres and nearly touches the corpus callosum, uniting them. It attaches anteriorly to the crista galli, a projection of the ethmoid bone, and posteriorly to the tentorium cerebelli. The tentorium cerebelli forms a tent over the cerebellum, holding it in the posterior cranial fossa. Its tentorial notch allows the midbrain to connect down to the pons. The small falx cerebelli lies between the cerebellar hemispheres.
3. Identify the three parts of the brainstem (midbrain, pons, and medulla oblongata), the 12 pairs of cranial nerves as they arise from the brain and brainstem, and the relative position of cranial nuclei. (NI34A, NI34B, NII64, NII65, TG7-54, TG7-55A, TG7-55B, TG7-76)
The three parts of the brainstem are the midbrain, pons, and medulla oblongata. The cerebellum sits behind the brainstem in the posterior cranial fossa.

Be able to locate all of the cranial nerves on the brain. The first two, olfactory and optic, are outgrowths of the forebrain, while cranial nerves III through XII arise from the brainstem.

4 cranial nerves arise from forebrain or midbrain (2 from each), 4 from the pons, and 4 from the medulla.

The somatic motor nuclei of cranial nerves 3, 4, 6, and 12 lie beside the midline in the brainstem. The motor nuclei of cranial nerves 5, 7, and nucleus ambiguus for 9 and 10 are lateral, and that of cranial nerve 11 is lateral within the cervical spinal cord. The parasympathetic motor nuclei are intermediate in location. The parasympathetic nervous system is also called the craniosacral outflow because it comes from cranial nerves or pelvic splanchnic nerves. Cranial nerves 3, 7, 9, and 10 carry presynaptic parasympathetic fibers. Fibers from Edinger-Westphal nuclei join the oculomotor nerve to innervate smooth muscles of the eyeball. Fibers from superior salivatory nucleus join facial nerve to reach the lacrimal, submandibular, and sublingual glands. Fibers from the inferior salivatory nucleus join glossopharyngeal nerve to reach the parotid gland, and of course the dorsal motor nucleus of vagus innervates glands and smooth muscle of the gut, respiratory tract, and cardiac muscle of the heart.

Cranial nerves 1, 2, and 8 are purely sensory, of course. The sensory nucleus of trigeminal is, due to its extensive sensory distribution on the face.
4. Identify each of the 12 pairs of cranial nerves in the three cranial fossae (anterior, middle and posterior), and the relative positions of their sensory ganglia (if appropriate). (N11, N104, N114, TG7-07, TG7-51, TG7-52)
Nerve Cranial Exit
I Olfactory Cribriform plate
II Optic Optic canal
III Oculomotor Superior orbital fissure
IV Trochlear Superior orbital fissure
V Trigeminal Superior orbital fissure (V1); f. rotundum (V2); f. ovale (V3)
VI Abducens Superior orbital fissure
VII Facial Enters internal auditory meatus, travels through petrous temporal bone, leaves via stylomastoid f.
VIII Vestibulocochlear Enters internal auditory meatus, remains within petrous temporal bone
IX Glossopharyngeal Jugular f.
X Vagus Jugular f.
XI Accessory Jugular f.
XII Hypoglossal Hypoglossal canal

5. Identify the surface features of the cerebellum, including the lateral hemispheres connected by the vermis, the lobes (anterior, posterior, and flocculonodular) and lobules (10, but not their names), cerebellar cortex and folia, peduncles (superior, middle, inferior), and the tonsils. (NI35A, NI35B, TG7-52)
The cerebellum lies behind the brainstem, connected to it by the superior, middle, and inferior cerebellar peduncles.

The cerebellar hemispheres, or lateral hemispheres, are united by the vermis, meaning worm because of its appearance. The vermis has a fairly complex pattern of folding. The lateral hemispheres are divided by the primary fissure into a smaller anterior lobe and a larger posterior lobe, sometimes called middle lobe, because there is a small third lobe called the flocculonodular lobe under the cerebellum. The cerebellum features multiple parallel mini-gyri called folia, meaning leaves, and resembling the leaves of a book. The folia increase the surface area of the cerebellar cortex in the same way as the gyri increase cerebral cortex surface area.

The tonsil projects inferiorly from the posterior lobe. Increased intracranial pressure can cause the tonsils to herniate down through foramen magnum.

The small flocculonodular lobe is located on the inferior surface of the cerebellum anterior to the posterior lobe. It is the oldest part of the cerebellum, and functionally, it is part of the vestibulocerebellum. The other two functional parts of the cerebellum are the spinocerebellum, which is the three medial nuclei and the vermis, and the cerebrocerebellum, the lateral hemispheres and the dentate nuclei.
6. Identify the inner features of the cerebellum, including the superior and inferior medullary velum and 4th ventricle, and the cerebellar nuclei (dentate, globose, emboliform, and fastigial). (NI35C)
The cerebellum has 4 cerebellar nuclei. From lateral to medial they are the dentate, which is the largest cerebellar nucleus, with the emboliform nucleus medial to it, and the small globose nuclei between it and the fastigial nucleus beside the midline. Together, the emboliform and globose nuclei are referred to as the interposed nuclei, due to their position between the other two cerebellar nuclei.

Questions and Answers:

3. Note dural attachment to the calvaria and the base of the skull. Is there any difference? (N98A, N101, N102, TG7-46, TG7-47, TG7-48, TG7-49)
The spinal dura consists of one layer and is a tube with lateral extensions covering nerve rootlets. The cranial dura, on the other hand, splits to form two layers.
  1. An external periosteal layer is the periosteum covering the internal surface of the calvaria.
  2. An internal meningeal layer, a strong fibrous membrane that is continuous at the foramen magnum with the spinal dura mater covering the spinal cord.
The periosteal layer adheres to the internal surface of the skull, and its attachment is tenacious along the suture lines and in the cranial base. The external periosteal layer is continuous at the cranial foramina with the periosteum on the external surface of the calvaria; it is NOT continuous with the dura mater of the spinal cord. In most areas, the meningeal layer is intimately fused with the periosteal layer and cannot be separated from it. The fused external and internal layers of dura over the calvaria can be easily stripped from the cranial bones. A blow to the head can detach the periosteal layer from the calvaria without fracturing the cranial bones. In the cranial base the two dural layers are firmly attached and difficult to separate from the bones. Consequently, a fracture of the cranial base tears the dura and results in leakage of CSF.
4. Examine falx cerebri, falx cerebelli, tentorium cerebelli, and diaphragma sellae. Are these infoldings periosteal or meningeal? Define attachments and relationship of each and the compartmentalization of the cranial cavity produced by these infoldings. (N103, N104A, N104B, TG7-47, TG7-48, TG7-49A, TG7-49B)
The internal meningeal layer of dura draws away from the external periosteal layer of dura to form dural infoldings, which separate the regions of the brain from each other and form dural venous sinuses. These separations in the dural layers form the dural venous sinuses (compartments). See the objective question above for attachments and relationships of these dural infoldings.
6. What is the tentorial notch? (N104A, N104B, TG7-47)
The tentorial notch is the opening in the tentorium cerebelli for the brainstem (specifically the midbrain).
7. Observe meningeal arteries in all cranial fossae. Which is the largest? How is it held within the dura? Relation to greater wing of sphenoid? (Significance?) (N99B, N104A, N104B, TG7-51)
The largest of the meningeal arteries is the middle meningeal artery. It is a branch of the maxillary artery. It goes through the foramen spinosum and supplies most of the dura mater except for the floors of the anterior and posterior cranial fossae. It runs forward for a short distance in a groove on the greater wing of the sphenoid bone, lying between bone and dura, and then divides into anterior and posterior branches. Significance: The dura is sensitive to pain, especially where it is related to the dural venous sinuses and meningeal arteries. Consequently, piercing the dura where the meningeal arteries enter the base of the skull or near the vertex causes pain and is a major source of headaches. In addition, the rupture of the middle meningeal artery by fracture of the greater wing of the sphenoid bone causes an epidural hematoma.
8. What is the innervation of dura? (N104A, N104B, TG7-51)
The dura is innervated by all three divisions of the trigeminal nerves, the vagus nerves, and the hypoglossal nerves.

A. Anterior and posterior ethmoidal branches of the ophthalmic division of the trigeminal nerve in the anterior cranial fossa.

B. Meningeal branches of the maxillary and mandibular divisions of the trigeminal nerve in the middle cranial fossa.

C. Meningeal branches of the vagus and hypoglossal nerves in the posterior cranial fossa.

    9. What cranial nerves exit through the jugular foramen? (N11, N104A, N104B, TG7-07, TG7-51)

    Cranial nerves IX, X, and XI.

    17. Examine the pia mater on the brain. How does it differ from the arachnoid mater in covering the brain? (N98A, N101, N102, TG7-46, TG7-49)
    The pia mater on the brain is a delicate, intimate, areolar investment of brain and spinal cord that enmeshes the blood vessels on their surfaces. It is a vascular membrane. On the other hand, the arachnoid is a delicate transparent membrane composed of a blend of collagenous and elastic fibers and squamous mesenchymal epithelial cells. It is NOT vascular and is NOT attached directly to the surface of the brain or spinal cord. Arachnoid trabeculae are thin strands that conect the arachnoid to the pia mater. (WB 322-23)