OBJECTIVES FOR GROSS ANATOMY OF THE FOREBRAIN, BRAINSTEM, AND VENTRICULAR SYSTEM©

Blair H. Turner, Ph.D.

Gregory B. Stanton, Ph.D

Duke Tanaka, Jr., Ph.D.

The student will be able to:

FOREBRAIN

1. Identify the major divisions of the brain and list the principal nuclear and fiber components.

2. List the lobes of the brain and the general function of each.

3. Describe the components of the basal ganglia.

4. Identify the major commissures.

BRAINSTEM

1. Know the major structures in each division of the brainstem

2. Be able to identify these structures on gross brainstem models and in pictures or photographs of brainstem sections.

3. Know the difference between the cerebral and cerebellar peduncles. Tell what axons travel in each peduncle.

4. Describe the functional differences between the superior and inferior colliculi.

5. Define the term "tectum"

6. Describe the major components of the cerebellum.

7. Locate in cross sections the major nuclei and tracts of the medulla including the inferior olive, the vestibular nuclei, the dorsal column nuclei, the dorsal columns and medial lemniscus, the pyramidal tract.

VENTRICULAR SYSTEM

1. Recognize each of the ventricles and ventricular subdivisions on a endocast model of the ventricles and in slides, drawings or photographs of brain sections.

2. Know the circulation of cerebrospinal fluid.

3. Know the organization of the brain ventricles.

GROSS ANATOMY OF THE BRAIN AND VENTRICULAR SYSTEM

I. INTRODUCTION

In describing the gross anatomy of the brain, the following gross neuroanatomical terminology will be used.

A. Gray Matter - Definitions

A general term for concentrations of nerve cell bodies. A nucleus (such as the red nucleus or abducens nucleus) is a definable group of cell bodies in the central nervous system (CNS). The cerebral and cerebellar cortices are layered arrangements of cell bodies on the surface of the cerebrum and cerebellum. A ganglion is a collection of cell bodies in the peripheral nervous system (PNS).

B. White Matter - Definitions

A general term for collections of axons. A tract is a bundle of axons that originates and terminates within the CNS. Other examples of axonal groupings in the CNS are funiculus, fasciculus, lemniscus, peduncle, or column. Commissures (anterior, posterior, corpus callosum) are collections of axons in the CNS that interconnect symmetrical structures in both halves of the brain. Decussations are axons that cross the midline to terminate on the contralateral side. Nerves are axons passing between the CNS and PNS.

C. General Organization

The gross anatomy of the brain is organized by embryonic regions. Each region relates to neural structures that develop from the walls of the brain vesicles. There are five regions. The telencephalon, diencephalon and mesencephalon are related to brain vesicles of the same name. The metencephalon and myelencephalon are regions related to the rhombencephalic vesicle. Each vesicle develops into a ventricle in the mature brain. The major structures that develop in each region are listed below.(Fig. 11.3)

II. TELENCEPHALON

A. Cerebral Hemispheres

The two cerebral hemispheres are joined together by the corpus callosum. The hemispheres have lateral and medial surfaces and frontal, temporal and occipital poles. The surfaces are enfolded forming gyri (convexities) and sulci and fissures.

1. Lateral surface

The lateral surface of the cerebral hemispheres are divided into lobes by sulci, fissures and imaginary lines. The principal sulci on the lateral surface of the hemisphere are the central sulcus, which divides the frontal from the parietal lobes, and the lateral (Sylvian) fissure which divides the temporal lobe from the fronto-parietal lobes. Posterior parts of the parietal and temporal lobes are divided from the occipital lobe by an imaginary line drawn from the preoccipital notch on the basal surface of the hemisphere to the superior part of the parieto-occipital sulcus on the medial wall of the hemisphere. A primary functional area is localized in each lobe.

2. Medial surface

The medial surface is made up of the medial extensions of the frontal, parietal, temporal and occipital lobes. In addition, gyri of the limbic system form a central ring of cortex bordering the corpus callosum and rostral parts of the brainstem. The cingulate gyrus lies just above the corpus callosum. It is separated from the frontal lobe and anterior parts of the parietal lobe by the cingulate sulcus. The parahippocampal gyrus is located on the ventromedial surface of the temporal lobe. The fornix, a major tract of the limbic system, can be seen in a medial view of the cerebral hemisphere. The fornix originates from the hippocampus, limbic cortex buried within the temporal lobe. The limbic system is associated with emotional behaviors. Two other important sulci seen on the medial surface are the parieto-occipital sulcus, which forms the border between these lobes, and the calcarine sulcus which is a landmark for the primary visual cortex.

3. The lobes of the telencephalon

a. Frontal lobe

The precentral gyrus is the location of the primary motor cortex. It is somatotopically organized (body mapped) in a mediolateral direction (See, Fig. 11.28). Cortical areas controlling the foot muscles are represented on the medial wall of the hemisphere and cortical areas controlling face, jaw, tongue and laryngeal muscles (muscles important in production of speech) are located laterally near the lateral fissure. Anterior to the speech musculature representation in the primary motor cortex is the speech association cortex called Broca's speech area.

b. Parietal lobe

The postcentral gyrus is the primary cortical receiving area for somatosensory input from skin and muscle receptors of the body and head. Like the precentral gyrus, the postcentral gyrus is somatopically organized in the mediolateral direction, with cells in the medial portion of the gyrus responding to tactile stimulation of the foot and cells in the lateral part of the gyrus responding to tactile stimulation of the face, jaw, and tongue.

c. Occipital lobe

The boundary between the parietal and occipital lobes is not clear cut on the gross brain, but in general it is considered to be a hypothetical line drawn from the parieto-occipital fissure dorsally to the preoccipital notch ventrally. The neocortex of the occipital lobe which surrounds the calcarine sulcus is the receives visual input.

d. Temporal lobe

The primary cortical area for auditory input is located on the superior temporal gyrus buried within the lateral fissure. Note that the temporal lobe has middle and inferior gyri as well.

The cortex at the junction of the temporal and parietal lobes posterior is known as Wernicke's speech area. In most individuals, Broca's and Wernicke's are larger on the left side. This asymmetry probably underlies the hemisphere lateralization of the expressive functions of language. For this reason, the left hemisphere is often called the dominant hemisphere.

B. Commissures

The two cerebral commissures, the corpus callosum and the anterior commissure, are sectioned in a midsagittal cut between the two hemispheres. The corpus callosum interconnects most of the cerebral hemispheres but the anterior interconnects the anterior temporal lobes and olfactory structures.

C. Internal Capsule

The internal capsule is a large white matter channel of axons projecting to and from the cerebral cortex. It is divided into two parts, the anterior limb which contains fibers related to the frontal lobe and posterior limb which contains fibers from the caudal lobes. The anterior limb divides the caudate and putamen nuclei. The posterior limb passes between the thalamus and the globus pallidus. Cells in the thalamic nuclei contribute most of the axons in the internal capsule that terminate in the cerebral cortex.

D. Lateral Ventricles

The lateral ventricles are the C-shaped ventricular cavities in each hemisphere. Each lateral ventricle is divided into an anterior horn associated with the frontal lobe, a posterior horn deep to the occipital lobe and an inferior horn in the temporal lobe. The anterior horns are separated by a thin, non-neural membrane called the septum pellucidum.

III. DIENCEPHALON

A. Thalamus

The dorsal thalamus is an egg-shaped mass of nuclei that project topographically to the cerebral cortex. These nuclei relay visual, auditory, somatosensory, motor, and multimodal information. A thalamus on one side is bordered laterally by the internal capsule and medially by the third ventricle. A thin layer of ependyma separates the dorsal part of the thalamus from the lateral ventricle.

B. Hypothalamus

The hypothalamus is a small region containing nuclei associated with limbic and vegetative functions such as appetite, thirst, temperature regulation, sex, and aggression. The hypothalamus is located ventral and rostral to the thalamus. The caudal boundary is marked by the mammillary bodies. The rostral nerve and tract is a derivative of the diencephalon. The stalk of the pituitary, the infundibulum, arises from the ventral surface of the hypothalamus and angles rostralward. Hypothalamic neurons exert much of their influence by neuronal and vascular signals to the pituitary.

C. IIIrd Ventricle

The third ventricle forms a narrow slit between the thalami and hypothalami. It communicates with the lateral ventricles by the interventricular foramen (of Monro). It forms two recesses above the optic tract and infundibulum.

IV. MESENCEPHALON (MIDBRAIN)

A. Tectum

The tectum forms the roof of the cerebral aqueduct, the ventricular cavity of the midbrain. It consists of the superior and inferior colliculi which function in reflexive movements of the head and eyes to sudden visual and auditory stimuli, respectively.

B. Mesencephalic Tegmentum

The region ventral to the cerebral aqueduct is the tegmentum. It contains cranial nerve nuclei III, IV, and two important nuclear complexes of the motor system, the red nucleus and the substantia nigra. In the core of the midbrain tegmentum is the reticular formation.

C. Cerebral Peduncles

The cerebral peduncles (Basis Pedunculi) are located in the most ventral part of the midbrain. They contain axons whose cell bodies are located in the precentral gyrus.

V. METENCEPHALON

A. Pons

The pons is composed of the pontine tegmentum and pontine nuclei. The tegmentum is a continuation of the midbrain tegmentum. It contains both ascending and descending tracts, several cranial nerve nuclei including those of the V, VII, and VIII nerves, and the reticular formation. The pontine nuclei are located ventral to the tegmentum. They are clumps of cells. Longitudinally oriented bundles of axons in the pons are continuous with axons in the cerebral peduncles. Many of these longitudinal bundles will collect to form the pyramids of the medulla.

B. Cerebellum

The cerebellum consists of a cortex and cerebellar nuclei embedded within the subcortical white matter. Axons going to a from the cerebellum pass by way of the cerebellar peduncles. The inferior cerebellar pedunclei carries input from the spinal cord and vestibular system and output to the brainstem. The middle cerebellar peduncle carries input from the pontine nuclei and the superior cerebellar pedunclei carries mostly output from the cerebellar nuclei to the midbrain and thalamus.

VI. MYELENCEPHALON

A. Medulla Oblongata

The myelencephalon is synonymous with the medulla oblongata. The medulla is continuous with the pontine tegumentum and spinal cord. The medulla can be divided by its relation to the fourth ventricle and central canal. The rostral, "open" medulla is related to the fourth ventricle. Cranial nerve nuclei (V, VIII, IX, X, XII) lie ventral to the ventricular floor. The "closed" medulla surrounds the central canal which continues into the spinal cord. Some of the same cranial nerve nuclei which appear at rostral levels are found in dorsal parts of this medullary level. In addition, two somatosensory tracts (gracilis and cuneatus) and related nuclei form raised tubercles on the dorsal surface. An important structure on the ventral surface of the medulla is the pyramids, located just lateral to the midline. A pyramid is a collection of axons which originates from cortical cell bodies. These axons descend into the internal capusle, cerebral peduncle and pons before they form the pyramids. At the spino-medullary junction the pyramidal axons decussate to the contralateral side enroute to all levels of the ventral horn of the spinal cord. The XII cranial nerve exits between the inferior olive and the pyramid on each side.

 

GENERAL ORGANIZATION OF THE CENTRAL NERVOUS SYSTEM

ORGANIZATION OF THE CEREBRAL HEMISPHERES

MEDIAL AND LATERAL VIEWS OF THE BRAINSTEM

DORSAL AND VENTRAL VIEWS OF THE BRAINSTEM

VENTRICULAR SYSTEM

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