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UNIT 2 - Gross Mature Structures

Unit 2 - Learning Objectives

A. Prosencephalon
B. Diencephalon
C. Mesencephalon
D. Rhombencephalon
E. Myelon (Spinal Cord)
F. Cerebellum

A. Prosencephalon

1. Telencephalon

Encephalization and expansion in size and importance of the rostral portion of the brain occurs with the development of man to allow for increased discrimination of sensory and motor activity.  The prosencephalon expands in all directions and folds over upon itself as it develops.   As it balloons out, two major divisions occur whereby the outer telencephalon grows around the inner diencephalon.   Note that the division between the telencephalon and diencephalon is not as distinct as the other secondary brain vesicles since the latter is completely surrounded by the telencephalon.

External Configuration

As the telencephalon grows within the confines of the skull, characteristic grooves are formed on its surface.   The larger and longer of these grooves are referred to as fissures, while the smaller are called sulci.   These grooves are what divides the most external layer of the telencephalon, known as the cerebral cortex, into four functional lobes known as:   the frontal lobe, the parietal lobe, the temporal lobe, and the occipital lobe.   The longitudinal cerebral fissure divides the telencephalon into a right and left cerebral hemisphere.   The furrows, or folds between the grooves are called gyri.   The names of the major fissures, sulci, and gyri are important terms of reference which are frequently used when discussing the telencephalon.

Links to related animations:

12. Medial aspect Cerebral Cortex
13. Lateral & Inferior aspect of Cortex
14. Inferior aspect of Cortex
15. Lateral aspect of Cortex
16. All aspects of Cortex
49. Outter Cortex & Cerebellum

Internal Configuration

The telencephalon and C.N.S. is made up of two types of tissue, gray matter and white matter.   The gray matter is comprised largely of cell bodies and lies in the periphery of the cerebral cortex.   The white matter is primarily comprised of cell processes, or fibers, and is located in the center of the telencephalon. White matter can be subdivided into three groups on the basis of their connections:
1. Association fibers connect different parts of the same cerebral hemisphere. 
2. Commissural fibers connect the two cerebral hemispheres with each other.
3. Projectional (longitudinal) fibers connect the cerebral cortex to other subcortical structures such as the brain stem or spinal cord.
These same terms are commonly used in describing white matter throughout the C.N.S.

2. Ventricular System

The ventricular system and the central canal of the spinal cord are the remnants of the embryonic neural tube and in the adult nervous system they contain cerebrospinal fluid.   These hollow ventricles have important structures surrounding them that comprise their borders.   The two lateral ventricles are located within the telencephalon.  They are horn-like in appearance with one being found within each cerebral hemisphere.   These two lateral ventricles are connected to a centrally located third ventricle found in the diencephalon by the interventricular foramen, or foramen of munro.  The cerebral aqueduct is located in the mesencephalon and connects the third ventricle to the fourth ventricle, which is located in the metencephalon and the myelencephalon.   Each ventricle will be discussed in conjunction with the brain vesicle in which it anatomically belongs.

The lateral ventricles are arc-like structures which run from an anterior to a posterior position, radiating in a medial to lateral direction.   They are broken down into four components:  the anterior, or frontal horn, the body or central horn, the posterior or occipital horn, and the inferior or temporal horn. These ventricles are bordered by various structures in the brain. Those structures that surround the body or central horn are as follows. The roof being the corpus callosum, the floor being the thalamus, the lateral wall being the caudate nucleus, and the medial wall being the fornix. The medial and lateral walls of the lateral ventricles follow a path that is similar to the ventricle itself, but not identical (see diagram). These boundaries are considered to be the classical boundaries for the lateral ventricles, but do not constitute all of the boundaries for the remaining three components of the lateral ventricle.

Links to related animations:

4. Lateral Ventricle & Amygdala
8. Lateral Ventricle boundaries
20. Ventricles, Caudate & Cerebellum
35. Lateral Ventricle & Hypothalamus
40. Lateral Ventricle & Putamen
43. Ventricles
51. Lateral Ventricle & Amygdala
52. Ventricles

Fig. 1a –

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Fig. 1b –

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Click for Printable PDF Figure 1b.

Fig. 1c –

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1.Superior frontal gyrus
2. Superior frontal sulcus
3. Central sulcus
4. Precentral gyrus
5. Postcentral gyrus
6. Supramarginal gyrus
7. Angular gyrus
8. Postcentral sulcus
9. Parieto-occipital sulcus
10. Superior parietal lobule
11. Intraparietal sulcus
12. Precentral sulcus
13. Middle frontal gyrus
14. Inferior frontal sulcus

15. Inferior frontal gyrus
16. Anterior ascending ramus of lateral sulcus
17. Transverse temporal gyrus
18. Anterior horizontal ramus of lateral sulcus
19. Superior temporal gyrus
20. Superior temporal sulcus
21. Middle temporal gyrus
22. Stem of lateral sulcus
23. Inferior temporal sulcus
24. Inferior temporal gyrus
25. Preoccipital notch
26. Posterior branch of lateral sulcus
27. Triangular part of inferior frontal gyrus
28. Opercular part of inferior frontal gyrus

Superolateral surface of right cerebral hemisphere, showing sulci and gyri, The Cerebral Hemispheres, The Human Brain: Chapter 5,
http://www.vh.org/adult/provider/anatomy/BrainAnatomy/Ch5Text/Section02.html

Fig. 2 –

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1.Medial frontal gyrus
2. Cingulate sulcus
3. Cingulate gyrus
4. Central sulcus
5. Paracentral lobule
6. Callosal sulcus
7. Isthmus of cingulate gyrus
8. Subparietal sulcus
9. Precuneus
10. Parieto-occipital sulcus
11. Cuneus
12. Calcarine sulcus or fissure
13. Rostrum of corpus callosum
14. Genu of corpus callosum

15. Trunk of corpus callos
16. Splenium of corpus callosum
17. Choroid plexus in interventricular foramen
18. Interthalamic adhesion
19. Habenular trigone
20. Hypothalamic sulcus
21. Pineal body
22. Anterior (rostral) commissure
23. Tectum of midbrain
24. Mamillary body
25. Medial longitudinal fasciculus
26. Choroid plexus of 4th ventricle

Medial surface of the brain, with blood vessels removed, The Cerebral Hemispheres, The Human Brain: Chapter 5,
http://www.vh.org/adult/provider/anatomy/BrainAnatomy/Ch5Text/Section02.html

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Fig. 3 –

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1. Medial frontal gyrus
2. Cingulate gyrus
3. Central sulcus
4. Paracentral lobule
5. Cingulate sulcus
6. Callosal sulcus
7. Subparietal sulcus
8. Precuneus
9. Parieto-occipital sulcus
10. Cuneus
11. Isthmus of cingulate gyrus
12. Lingual gyrus
13. Calcarine sulcus or fissure
14. Medial occipitotemporal gyrus
15. Collateral sulcus

16. Parahippocampal gyrus
17. Uncus of parahippocampal gyrus
18. Rhinal sulcus
19. Subcallosal area
20. Paraterminal gyrus
21. Indusium griseum
22. Rostrum of corpus callosum
23. Genu of corpus callosum
24. Trunk of corpus callosum
25. Splenium of corpus callosum
26. Fimbria of hippocampus
27. Cut surface of thalamus
28. Anterior (rostral) commissure
29. Interthalamic adhesion
30. Column of fornix
31. Septum pellucidum

Medial surface of right cerebral hemisphere, with blood vessels removed, The Cerebral Hemispheres, The Human Brain: Chapter 5, http://www.vh.org/adult/provider/anatomy/BrainAnatomy/Ch5Text/Section02.html

Fig. 4 –

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1.Olfactory bulb
2. Orbital sulci and gyri
3. Olfactory tract
4. Gyrus rectus
5. Olfactory trigone
6. Optic chiasma
7. Tuber cinereum with infundibulum
8. Mamillary body
9. Posterior (interpeduncular) perforated substance
10. Basis pedunculi
11. Substantia nigra

12. Superior cerebellar peduncle
13. Mesencephalic (cerebral) aqueduct
14. Pineal body
15. Splenium of corpus callosum
16. Rhinal sulcus
17. Parahippocampal gyrus
18. Medial occipitotemporal gyrus
19. Lateral occipitotemporal gyrus
20. Collateral sulcus
21. Occipitotemporal sulcus
22. Lingual gyrus

Inferior surface of the brain after transection through lower midbrain and removal of cerebellum, The Human Brain: Chapter 5,
http://www.vh.org/adult/provider/anatomy/BrainAnatomy/Ch5Text/Section02.html

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Fig. 5 –

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1. Frontal pole of left cerebral hemisphere
2. Olfactory bulb
3. Olfactory tract
4. Orbital gyri and sulci
5. Straight gyrus
6. Temporal pole of left cerebral hemisphere
7. Olfactory trigone
8. Optic nerve
9. Optic chiasma
10. Anterior (rostral) perforated substance
11. Optic tract
12. Tuber cinereum with infundibulum
13. Oculomotor nerve
14. Mamillary body
15. Uncus of parahippocampal gyrus

16. Basis pedunculi
17. Basilar sulcus of pons
18. Trigeminal nerve
19. Abducens nerve
20. Pyramid of medulla oblongata
21. Facial nerve
22. Vestibulocochlear nerve
23. Glossopharyngeal nerve
24. Vagus nerve
25. Cranial roots of accessory nerve
26. Spinal roots of accessory nerve
27. Rootlets of hypoglossal nerve
28. Flocculus
29. Ventral rootlets of 1st cervical spinal nerve
30. Pyramidal decussation

Inferior surface of the brain with cranial nerve attachments, after removal of meninges and superficial blood vessels, The Human Brain: Chapter 5,
http://www.vh.org/adult/provider/anatomy/BrainAnatomy/Ch5Text/Section02.html

Longitudinal cerebral fissure

Fig. 6a –

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Inferior surface of the cerebrum, brain stem reoved. Posteroinferior view (xl.5).
Atlas of the Brain and Spinal Cord p. 113.

Click for Printable PDF Figure 6a.

A. Longitudinal cerebral fissure
B. Optic nerve (II)
C. Infundibulum
D. Midbrain (cut surface)
E. Cerebral aqueduct
F. Inferior colliculus
G. Splenium of corpus callosum
H. Isthmus of cingulate gyrus
I. Calcarine sulcus
J. Lingual gyrus
K. Parahippocampal gyrus
L. Uncus
M. Collateral sulcus
N. Medial occipitotemporal gyrus
O. Occipitotemporal gyrus
P. Lateral occipitotemporal
Q. Rhinal sulcus
R. Hippocampal sulcus

Can you locate:

Olfactory bulb and tract

Optic chiasma and tract

Mamillary bodies

Oculomotor nerve

Fig. 6b –

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Atlas of the Brain and Spinal Cord Figure 69, p 1.

Precentral sulcus

Lateral view of the brain, arachnoid and pia mater removed (x1.5)

Precentral gyrus
Central sulcus
Postcentral gyrus
Postcentral sulcus
Supramarginal gyrus
Angular gyrus
Occipital lobe
Lateral sulcus
Superior temporal gyrus
Superior temporal sulcus
Middle temporal gyrus
Inferior temporal sulcus
Inferior temporal gyrus

Can you locate:

Superior, middle, and inferior frontal gyri

Superior and inferior frontal sulci

Frontal, occipital, and temporal poles

Pons

Medulla

Cerebellum

Atlas of the Brain and Spinal Cord Figure 72, p 118.

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Fig. 7 –

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Cross-Sectional Anatomy of the Cerebrum (Horizontal Section): Cross-Sectional Anatomy of the Cerebrum,
http://www.smithsrisca.demon.co.uk/neuro-tracts.html

Fig. 8 –

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Cross-Sectional Anatomy of the Cerebrum (Coronal Section): Cross-Sectional Anatomy of the Cerebrum,
http://www.smithsrisca.demon.co.uk/neuro-tracts.htm

Fig. 9 –

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Limbic System, http://www.psyweb.com/Brain/brainl0.html

Click for Printable PDF Figure 9.

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Fig. 10 –

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The Ventricular System, ANATOMY BY LEVEL OF ORGANIZATION, ANATOMY AND FUNCTION, The Brain from top to bottom, http://www.thebrain.mcgill.ca/flash/i/i_01/i_01_cr/i_01_cr_ana/i_01_cr_ana.html

Fig. 12 –

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Basal Ganglia and Thalamus, The R-complex: the Reptilian Brain
http://www.psycheducation.org/emotion/R%20complex.htm

3. Basal Ganglia

There are four pairs of gray matter located centrally in the telencephalon that are collectively known as the basal ganglia. The first pair is the caudate nuclei.   They are similar in shape to the two lateral ventricles and are situated on the lateral side of them with one in each cerebral hemisphere. The pair of lens-shaped lentiform nuclei which are continuous with the heads (anterior portion) of the caudate nuclei constitute the second and third components of the basal ganglia.   Each lentiform nucleus is subdivided into a lateral putamen and a medial globus pallidus. The fourth pair of basal ganglia is the claustrum.   They are two very thin plates of gray matter and are located laterally to the lentiform nuclei. These four pairs of telencephalic basal ganglia are closely related to other subcortical structures which are also considered basal ganglia from a functional standpoint.   This classification will be discussed in further units.

4. Other Telencephalic Structures

There are two other structures that are not basal ganglia, but are still located in the telencephalon.   The first is the amygdala.   It is located within the temporal lobe as an enlargement fused to the tail of the caudate nucleus.   The second structure is the fornix.   It forms a bundle of fibers situated medial to, and paralleling the course of the lateral ventricle.

Links to related animations:

1. Caudate & Putamen
2. Caudate & Putamen structures
3. Inner Telencephalic & Diencephalic
4. Lateral Ventricle & Amygdala
5. Caudate & Amygdala
6. Fornix, Hippocampus & Anterior Commissure
7. Caudate, Putamen & Anterior Commissure
8. Lateral Ventricle boundaries
9. Telencephalic & Diencephalic structures
11. Amygdala
17. Inner Tele & Diencephalic structures
19. Inner Prosencephalon & Brain Stem
23. Fornix & Hippocampus
24. Fornix & Hippocampus
26. Inner Cortical structures
28. Inner Prosencephalon
30. Inner Cortical structures
31. Posterior inner structures Cortex & Brain Stem
32. Inner Cortical structures
33. Fornix & Hippocampus
34. Inner Prosencephalon
36. Inner Prosencephalon
39. Lentiform Nucleus
41. Inner Prosencenphalon
42. Inner Prosencenphalon
48. Inner Prosencephalon
50. Caudate & Lentiform Nucleus
53. Inner Prosencephalon

Fig. 13 –

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Fig. 14 –

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Click for Printable PDF Figures 13 & 14.

Fig. 15 –

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Fig. 16 –

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Click for Printable PDF Figures 15 & 16.

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B.  Diencephalon

1. External Configuration

Exposed only from its ventral aspect, the diencephalon is surrounded dorsally and laterally by the cerebral hemispheres. External structures located within the vesicle are the optic nerve, the optic tract, the optic chiasm, the hypophysis (the pituitary), and the mamillary bodies,

2. Internal Configuration

The diencephalon is split into four major subdivisions.   They are the epithalamus, the thalamus proper, the subthalamus, and the hypothalamus.

a. Epithalamus

The epithalamus is the dorsal portion of the diencephalon and consists of a narrow band on the roof of the diencephalon which includes the posterior commissure, the pineal body, and a group of nuclei called the habenular nuclei.

b. Thalamus

The two ovoid bodies of the thalamus surround most of the third ventricle.   They can connect across the midline of the brain (through the third ventricle) by the massa intermedia.

c. Hypothalamus

The hypothalamus is located beneath and in front of the thalamus, and includes the mamillary bodies and other groups of nuclei. It consists of two columns and a base.

d. Subthalamus

The subthalamus consists of motor centers lying on either side of the junction of the hypothalamus and thalamus at a level below the thalamus.

Fig. 17 –

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1. Pulvinar of thalamus
2. Posterior median sulcus
3. Pineal body
4. Superior colliculus
5. Medial geniculate body
6. Brachium of inferior colliculus
7. Inferior colliculus
8. Lemniscal trigone
9. Frenulum of superior medullary velum
10. Trochlear nerve

11. Basis pedunculi
12. Superior medullary velum
13. Superior cerebellar peduncle
14. Median sulcus of rhomboid fossa
15. Middle cerebellar peduncle
16. Facial colliculus
17. Vestibular area
18. Striae medullares of 4th ventricle
19. Inferior cerebellar peduncle
20. Hypoglossal trigone
21. Vagal trigone

4-4: Brainstem: dorsal view The Human Brain: Chapter 5:,http://www.vh.org/adult/provider/anatomy/BrainAnatomy/Ch5Text/Section02.html

Links to related animations:

3. Inner Telencephalic & Diencephalic
9. Telencephalic & Diencephalic structures
10. Thalamus
17. Inner Tele & Diencephalic structures

Fig. 18 –

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Midsagittal section of the brain stem. CHAPTER 9 - THE DIENCEPHALON, http://www.hallym.ac.kr/~de1610/nana/chp-9n.htm

Links to related animations:

18. Posterior Diencephalic & Brain Stem
19. Inner Prosencephalon & Brain Stem
22. Optic Tract & Interior Capsule
30. Inner Cortical structures
31. Posterior inner structures Cortex & Brain Stem
32. Inner Cortical structures
34. Inner Prosencephalon
36. Inner Prosencephalon
41. Inner Prosencenphalon
42. Inner Prosencenphalon
48. Inner Prosencephalon
53. Inner Prosencephalon

Fig. 19 –

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Midsagittal view of the human brain.
(A) Major features apparent after bisecting the brain in this plane.
(B) The lobes of the brain seen from its medial surface.
(C) An enlarged view of the diencephalon and brainstem in this view.
The Midline Sagittal Surface of the Brain, The Organization of the Nervous System, Neuroscience, http://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=neurosci.figgrp.76

Fig. 20 –

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Click for Printable PDF Figure 20.

3. Major White Fibrous Structures

a. Corpus Callosum

The corpus callosum is a broad band of commissural fibers that connects the two cerebral hemispheres.   It is located on the floor of the longitudinal fissure and just superior to the lateral ventricles in the telencephalon.

Fig. 20b –

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b. Internal Capsule

The internal capsule, located in the telencephalon, is a broad band of projectional fibers which interconnect cortical and subcortical structures. It is located medially to the lentiform nucleus and is subdivided into an anterior limb, a genu, and a posterior limb, giving it a V-shaped appearance. The internal capsule also has a fan-shaped portion, called the corona radiata, which extends above the level of the caudate nucleus, carrying information to the higher telencephalic structures.

Fig. 21 –

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Click for Printable PDF Figures 20b & 21.

c. External and Extreme Capsule

The external and extreme capsules consist of two thin layers of white fibrous matter that enclose the claustrum and are believed to be composed primarily of association fibers.   They are located in the telencephalon just medially to the insular cortex.

4. Superior Longitudinal Fasciculus

The superior longitudinal fasciculus runs the entire length of the cerebral cortex and is found in the telencephalon superior to the Sylvian fissure.   It consists of association fibers.

5. Superior and Inferior Occipitofrontal Fasciculi

These fasciculi are found in the telencephalon and consist of association fibers.   The superior occipitofrontal fasciculus is located superior and lateral to the caudate nucleus.   The inferior occipitofrontal fasciculus is found in the temporal lobe lateral to the lateral ventricle.

6. Cingulum

The cingulum is located in the telencephalon lateral to the cingulate gyrus, which is found in the frontal and parietal lobes.   It consists of association fibers.

7. Anterior and Posterior Commissures

The two bands of fibers consist of commissural fibers.   The anterior commissure is found in the telencephalon and sits at the top of the anterior columns of the hypothalamus.   The posterior commissure is found in the diencephalon constituting part of the epithalmic area. 

8.Ventricular System

The slit-like third ventricle is located in the midsagittal plane of the diencephalon.   The cerebral aqueduct extends from the third ventricle through the midbrain to the fourth ventricle, found in the metencephalon and the myelencephalon.

The boundaries of the third ventricle are formed by the following structures:

Anterior Wall  -  Lamina Terminalis (neural tissues which extend from the optic chiasma to the anterior commissure)

Lateral Wall  -    Hypothalamus (approximately anterior 25%)

Thalamus (approximately posterior 75%)

Posterior Wall  - Pineal Body (stalk)
Habenular Nuclei

Posterior Commissure

Roof is formed by the choroid plexus which is attached to the Striae
Medullaris Thalmi (a band of neural fibers located on the superior medial aspect of
the thalamus)

Floor Hypothalamus

Pineal Body Stalk (posterior portion) 

Fig. 21a Frontal Section of Prosencephalon –

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Fig. 21b –

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Click for Printable PDF Figures 21a & 21b.

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C.  Mesencephalon

The mesencephalon is the smallest of all primary brain vesicles and is similar in structure to the remaining vesicles whose discussion follows.   Each of these remaining vesicles contains three major areas.   They are, from dorsal to ventral:   the dorsal tectum, the tegmentum, and the basilar portion.   The ventricular system found in these brain vesicles separates the tectum from the tegmentum.

1. External Configuration

The dorsal portion of the mesencephalon contains four rounded prominences called the corpora quadrigeminae.   These structures (two superior and two inferior colliculi) are concerned with the visual and auditory systems respectively.
The cerebral peduncles, two large columns, which appear to be connecting the hindbrain to the forebrain, are apparent from the lateral and ventral views of the mesencephalon.

Fig. 22 –

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Scheme showing the course of the fibers of the lemniscus; medial lemniscus in blue, lateral in red. Henry Gray (1821–1865).  Anatomy of the Human Body.  1918.

Fig. 23 –

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Hind- and mid-brains; postero-lateral view. Henry Gray (1821–1865). Anatomy of the Human Body. 1918.

2. Internal Configuration

a. Tectum

The tectum of the midbrain consists of the paired colliculi identified previously.

b. Ventricular System

The cerebral aqueduct originates from the third ventricle and extends through the mesencephalon.   This portion of the ventricular system separates the dorsal tectum from the ventral cerebral peduncles which internally forms the tegmentum and basilar portion.

c. Tegmentum

The tegmentum of the mesencephalon consists of the dorsal portion of the cerebral peduncles.   The red nucleus and the substantia nigra, which are a group of cells related to the motor system, are both located in the tegmentum.   Also found in this area are several fibrous tracts which are afferent in nature, and a complex mass of cells and fibers which are part of the reticular formation.

d. Basilar Portion

The most ventral portion of the mesencephalon is known as the basis pedunculi.   It contains long projectional efferent fibers which are concerned with motor control, and are thus efferent in nature.

Fig. 24a –

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Gross structures on the midbrain, Mesencephalon, Wikipaedia the Free Encyclopedia, http://en.wikipedia.org/wiki/Midbrain

Fig. 24b –

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Click for Printable PDF Figures 24b & 29.

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D. Rhombencephalon

As a result of cellular differentiation within the hindbrain, two major subdivisions occur within this primary vesicle.   The rostral metencephalon becomes clearly divided from the caudal myelencephalon forming two distinct secondary vesicles.

1. Metencephalon

a. External Configuration

The cerebellum is located on the dorsal aspect of the metencephalon, and the pons in its ventral aspect.   The metencephalon laterally consists of three cerebellar peduncles, the superior, middle, and inferior.

Fig. 26 –

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Upper part of medulla spinalis and hind- and mid-brains; posterior aspect, exposed in situ. Henry Gray (1821–1865).  Anatomy of the Human Body.  1918.

Fig. 27 –

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Hind- and mid-brains; postero-lateral view, Henry Gray (1821–1865).  Anatomy of the Human Body. 1918.

b. Internal Configuration

i. Ventricular System

The tent shaped fourth ventricle is found both in the metencephalon and myelencephalon.   It consists of a diamond shaped floor called the rhomboid fossa, which is bisected by the eighth cranial nerve thus dividing the floor into a rostral and caudal portion.   The rostral portion being located in the metencephalon and the caudal portion in the myelencephalon.   Extending from the rhomboid fossa into the cerebellum are two lateral walls, rostral roof, and caudal roof.   The lateral walls consist of the middle cerebellar peduncle, which originates from the pons of the metencephalon, and the inferior cerebellar peduncle coming from the myelencephalon.   The rostral roof consists of the superior cerebellar peduncle, which originates from mesencephalon and the superior medullary velum which originates from the pia mater of the mesencephalon.   The caudal roof consists of the inferior medullary velum, which originates from the pia mater of the myelencephalon.

ii. Tectum

The cerebellum occupies the tectum of the metencephalon.

iii. Tegmentum

The tegmentum of the metencephalon, which is the dorsal section of the pons, is comprised of cranial nerve nuclei of the 5-6-7-8 cranium nerves, fibrous afferent pathways, cells of the reticular formation, and the origin of the middle cerebellar peduncles.

iv. Basilar Portion

The basilar portion of the metencephalon, its most ventral portion, is known as the pons proper and contains efferent pathways.

Fig. 28 –

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Coronal section of the pons, at its upper part. Henry Gray (1825–1861).  Anatomy of the Human Body.  1918.

2. Myelencephalon (Medulla Oblongata)

The myelencephalon is split into two sections.   They are the open medulla and the closed medulla. The open medulla is the rostral portion of the myelencephalon which houses the remaining part of the fourth ventricle. The closed medulla is the caudal portion of the myelencephalon which houses the central canal.

a. External Configuration

The dorsal portion of the myelencephalon is rostrally formed by the cerebellum, which includes the caudal portion of the rhomboid fossa or floor of the fourth ventricle, and by four protuberances known as tubercles at the level of the closed medulla.   These four tubercles are referred to as the tuberculi gracilis (medial) or clavae, and the tuberculi cuneatus (lateral). These four prominences are the nuclei of ascending sensory pathways.   Below these tuberculi, the dorsal median sulcus, two dorsal intermedial sulci, and two dorsal lateral sulci mark the brainstem and spinal cord.

On the ventral aspect of the myelencephalon three ventral sulci can be identified. They are the ventral median fissure, and two ventral lateral sulci. ; On either side of the ventral median fissure descending motor fibers form protuberances that are referred to as the pyramids. These fibers cross in the midline of the lower myelencephalon forming the pyramidal decussation. Lateral to the pyramids at the level of the open medulla, the inferior olivary nuclei form swellings that are known as the olives.

Fig. 29 –

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Click for Printable PDF Figures 24b & 29.

b. Internal Configuration

i. Ventricular System

As explained previously, the myelencephalon is comprised rostrally of the caudal portion of the fourth ventricle.   From this fourth ventricle extends the tube-like central canal, which becomes the ventricular system in the caudal or closed medulla portion of the myelencephalon.

ii. Tectum

At the level of the open medulla, the tectum is comprised of the cerebellum and the caudal half of the rhomboid fossa, or floor of the fourth ventricle.   At the level of the closed medulla it consists of the nuclei gracilis and the nuclei cuneatus.

3. Tegmentum

At the level of the open medulla the tegmentum is made up of afferent pathways, cells of the reticular formation, the inferior olivary nuclei, and the origin of the inferior cerebellar peduncle.   The inferior olivary nuclei are not present at the level of the closed medulla leaving only afferent pathways and the cells of the reticular formation to be located in that area.

4. Basilar Portion

The basilar portion of the myelencephalon is occupied by the efferent pyramidal fibers and is called the pyramids.

Links to related animations:

18. Posterior Diencephalic & Brain Stem
19. Inner Prosencephalon & Brain Stem
21. Posterior Brain Stem
27. Cerebellum, Brain Stem & Spinal Cord
29. Brain Stem & Cranial Nerves
31. Posterior inner structures Cortex & Brain Stem

Fig. 30 –

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Click for Printable PDF Figures 30 & 40b.

Fig. 31 –

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Fig. 32 –

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Diagram of the dorsal aspect of the brainstem , Ocular Motor Nuclei & Nerves, http://www.city.ac.uk/optometry/Biolabs/OMotor_Nuclei&Nerves/OM%20Nerves.htm

Fig. 33 –

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diagram of the ventral aspect of the brainstem, Ocular Motor Nuclei & Nerves, http://www.city.ac.uk/optometry/Biolabs/OMotor_Nuclei&Nerves/OM%20Nerves.htm

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E. Myelon (Spinal Cord)

The spinal cord, an elongated and slightly flattened cylinder, is continuous with the medulla oblongata.   It originates just caudal to the pyramidal decussation and extends to between the twelfth thoracic vertebrae and the third lumbar vertebrae.

1. External Configuration

Sulci and a single large ventral medial fissure that are continuous from the medulla oblongata mark the external surface of the spinal cord.   The spinal cord varies in width and enlargements are noted at the cervical and lumbar levels.   At its base, the spinal cord narrows to a cone-shaped conus medullaris and terminates with the filum terminale (a non-neural thread).

Fig. 34 and Fig. 35 –

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Left Fig. 34 - Basic anatomy of the spinal cord, Spinal cord injury resource center, http://www.spinalinjury.net/html/_anatomy_of_the_spinal_cord_co.html

Right Fig. 35 - Cauda equina and filum terminale seen from behind. The dura mater has been opened and spread out, and the arachnoid has been removed. Henry Gray (1825–1861). Anatomy of the Human Body. 1918.

Fig. 36a and Fig. 36b –

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Left Fig 36a –Sagittal section of vertebral canal to show the lower end of the medulla spinalis and the filum terminale. Li, Lv. First and fifth lumbar vertebræ. Sii. Second sacral vertebra. 1. Dura mater. 2. Lower part of tube of dura mater. 3. Lower extremity of medulla spinalis. 4. Intradural, and 5, Extradural portions of filum terminale. 6. Attachment of filum terminale to first segment of coccyx. (Testut.)
Henry Gray (1825–1861).  Anatomy of the Human Body. 1918.

Fig. 37 –

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Diagrams of the medulla spinalis. Henry Gray (1825–1861). Anatomy of the Human Body.  1918

There are a series of dorsal (sensory) and ventral (motor) rootlets that are continuous along the length of the spinal cord. These rootlets collect laterally into dorsal and ventral roots, and finally into 31 pairs of spinal nerves.   Although the spinal cord is basically continuous and unsegmented, spinal nerves give it a segmented appearance and that portion of the cord associated with a pair of spinal nerves is referred to as a spinal segment.

Fig. 38 –

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Fig. 39 –

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2. Internal Configuration

The central canal is continuous with the ventricular system of the medulla and runs the entire length of the spinal cord. The cord is comprised of an innermost layer of gray matter (cell bodies) and an outermost layer of white matter (cell processes).

The white matter of each half of the spinal cord is divided into three funiculi (ventral, dorsal, and lateral) by the more centrally located gray matter and the entering dorsal and emerging ventral roots. Each funiculus is comprised of fiberous tracts known as fasciculi. The propriospinal fasciculi carry information between various levels of the spinal cord and contain both ascending and descending fibers. The fasciculi that run between the brain and spinal cord are basically named from place of origin to place of destination (e.g. spinothalmic, corticospinal); these tracts carry either ascending or descending information but not both types, as with the propriospinal fasciculi. A bundle of transverse fibers, the anterior white commissure, is comprised of fibers interconnecting the two halves of the cord and is located just anterior to the central canal and gray commissure.

The gray matter of the spinal cord is grouped into two left and two right longitudinal (comma shaped) bars connected by a transverse gray commissure.   Each bar can be regarded as having a dorsal (sensory) and ventral (motor) column, which are commonly called “horns” when viewed in a cross section.   In the thoracic through second lumbar segments, a small lateral protrusion called the lateral or intermedial lateral column can be seen.   This intermediolateral column plays a role in the autonomic nervous system.

Variations existing between spinal cord levels are illustrated below. The volume of white matter is seen to increase as one ascends the neuroaxis and the volume of gray matter is greatest at the cervical and lumbosacral levels (areas innervating the extremities).

Fig. 40a –

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Spinal Cord Cross Section (Model), Nervous System & Special Senses,
www.bio.psu.edu/faculty/strauss/anatomy/nerv/nervous.htm

Fig. 40b –

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Click for Printable PDF Figures 30 & 40b.

Figure 41a –

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Spinal cord levels, Spinal Cord Anatomy, http://academic.uofs.edu/student/mcnallye2/scia.htm

Figure 41b –

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Click for Printable PDF Figure 41b.

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3. Laminae Classification

The most widely adopted model for classification of the various cell regions within the gray matter of the spinal cord has been determined by Rexed.   Based on extensive studies of the cytoarchitectural features of neurons throughout the length of the spinal cord, nine separate laminae have been distinguished.   The laminae seen within a “typical” segment and their proposed functions are presented below.

Laminae I-IV Primary receiving area for exteroreceptive fibers, also called the Substantia Gelatinosa

Laminae V-VI Primary receiving area for proprioceptive fibers and also receive extensive connections from higher centers, also called Clark’s Column or Nucleus Dorsalis.

Laminae VII Similar to structure and function as the reticular formation; deals with reflex reactions of the spinal cord, also called the Internuncial Pool or Nucleus Proprius.

Laminae VIII Connects laminae VII with laminae IX.   It has no functional significance.

Laminae IX Contains a mass of cell interconnections as well as the origin of gamma (medial) motor neurons.

Laminae IX; Consists primarily of alpha motor neurons (lateral).

Figure 42 –

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Figure 9.15  Schematic drawing of the general organization of the spinal cord gray matter and white matter.

Click for Printable PDF Figure 42.

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F. Cerebellum

1. External Configuration

The external structure of the cerebellum is similar to that of the cerebrum.   The cortex (gray matter) of the cerebellum is located outside of a centrally located mass of white matter called the corpus medullaris.   The cortex has a surface of fissures, sulci, and gyri that greatly increase its area.   The gyri of the cerebellum are known as folia because of their appearance.

The cerebellum is connected to the brainstem by three cerebellar peduncles, or pillars:   the brachium conjunctivum or superior cerebellar peduncle, the brachium pontis, or middle cerebellar peduncle, and the restiform body, or inferior cerebellar peduncle.

Links to related animations:

27. Cerebellum, Brain Stem & Spinal Cord
29. Brain Stem & Cranial Nerves

2. Internal Configuration

The corpus medularis is comprised of both afferent and efferent white fibers.   Located within this white matter are four pair of nuclei:   the fastigial nuclei, the dentate nuclei, the emboliform nuclei, and the globus nuclei.   All these nuclei are involved in the motor system of the cerebellum.

3. Cerebellar Regions

Anatomically, the cerebellum consists of two flocculi, a superior and inferior vermis, and two cerebellar hemispheres.   These structures are divided into three lobes:   the flocculonodular lobe, the anterior lobe, and the posterior lobe.   The two vermil areas are subdivided as follows:   1. The superior vermis which connects the right and left hemispheres of the cerebellum, consists of the lingula, central lobule, culmen, folium, and declive.   2. The nodulus, two tonsils, uvula, pyramis, and the tuber form the inferior vermis.

a. Flocculonodular Lobe

The flocculonodular lobe is comprised of two flocculi and one nodule.

b. Anterior Lobe

The anterior lobe is comprised of the cerebellar cortex located anterior to the primary fissure, including the superior vermis in that area.

c. Posterior Lobe

The posterior lobe is comprised of the cerebellar cortex located posterior to the primary fissure including the superior vermis in that area and the inferior vermis except for the nodulus.

Links to related animations:

20. Ventricles, Caudate & Cerebellum
27. Cerebellum, Brain Stem & Spinal Cord
49. Outter Cortex & Cerebellum

Figure 43 –

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Median sagittal section of brain (http://cns.georgetown.edu/medneuro/lab1/).

Figure 44 –

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Fig. 1. Midsagittal section through the brainstem and cerebellum, http://www.hallym.ac.kr/~de1610/nana/chp-8n.htm#I

Click for Printable PDF Figure 44.

Figure 45 –

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Upper surface of the cerebellum (Henry Gray (1825–1861).Anatomy of the Human Body. 1918.

Figure 46 –

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Under surface of the cerebellum (Henry Gray (1825–1861) Anatomy of the Human Body. 1918.

Figure 47 –

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Nomenclature for specific regions of the cerebellum.  The lingula, not seen in these drawings, is a small flattened portion of the superior vermis beneath the central lobule and adherent to the superior medullary velum (Fig. 10-2)

Click for Printable PDF Figure 47.

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