1. FOREBRAIN (TELENCEPHALON)
Joel Havemann (from A Life Shaken, 2002)
What seems astonishing is that a mere three-pound object, made of the same atoms that constitute everything else under the sun, is capable of directing virtually everything that humans have done: flying to the moon and hitting seventy home runs, writing Hamlet and building the Taj Mahal -- even unlocking the secrets of the brain itself.
Ivan Petrovich Pavlov ( ; in a lecture given in 1913 and published in Lectures on conditioned reflexes. Twenty-five year of objective study of the higher nervous activity [behavior] of animals, London: Martin Lawrence, 1928, p. 222.)
If we could look through the skull into the brain of a consciously thinking person, and if the place of optimal excitability were luminous, then we should see playing over the cerebral surface, a bright spot with fantastic, waving borders constantly fluctuating in size and form, surrounded by a darkness more or less deep, covering the rest of the hemisphere.
Georgia Bishop PhD
We’re now going to focus on the organization of the forebrain or telencephalon. This is what most people think about when you say “the brain”. It is interesting that what Pavlov wished for 100 years ago has now come to pass. With the advent of procedures such as functional MRI, or PET scans we can see excitability of different parts of the cerebral cortex as individuals carry out different thought or motor tasks. He would be quite pleased. Most of the material in this module is a review of material present previously in several different lectures or e-learning modules. There is some new material

2. OBJECTIVES Describe the Anatomical Organization of the Cerebral Cortex
1. Identify the lobes of the cerebral cortex and their borders including the frontal lobe, parietal lobe,
temporal lobe, occipital lobe, cingulate lobe, and insula.
2. Describe the general function of each cortical lobe
3. Identify specific sulci in the cerebral cortex including the central sulcus, pre-central sulcus, post-central
sulcus, lateral sulcus, calcarine sulcus, cingulate sulcus, and parieto-occipital suclus.
4. Identify specific gyri in the cerebral cortex including the precentral gyrus, postcentral gyrus, parahippocampal
gyrus, superior temporal gyrus, inferior frontal gyrus and cingulate gyrus.
5. Recognize what is meant by Brodmann’s Areas.
6. Recognize the motor and sensory homunculi in the primary motor and sensory cortex
7. Differentiate between interhemispheric and intrahemispheric fiber tracts that link different regions of
the cerebral cortex.
8. Differentiate between neocortex, archicortex and paleocortex, give examples of each.
9. Locate and describe the axons contained within the internal capsule
List the names of the major arterial branches arising from the internal carotid and vertebral arteries and the
regions of the CNS that each vessel supplies
These are the objectives for this block.

3. DIVISIONS of the FOREBRAIN
1. CORTICAL – Thin Outer Mantle ~1.5 – 4.5 Mm Thick
2. SUBCORTICAL – Collection Of Nuclei In Deep White Matter Below Cortex. Primarily
These Consist Primarily Of Nuclei Called The Basal Ganglia (More Later).
3. LIMBIC – Hippocampus, Amygdala
ASSOCIATED FIBER TRACTS:
Internal Capsule
Corpus Callosum
Anterior Commissure
Superior Longitudinal Fasciculus
CORTEX
SUBCORTICAL NUCLEI WM
The forebrain may be divided into 3 general regions including the outer mantle of grey matter called the cortex. Internal to the cortex is the white matter consisting of axons traveling to and from the cerebral cortex. The area *** enclosed by box is shown at higher magnification in the next image. Deep in the white matter, several areas of grey matter are evident. These collectively are called subcortical nuclei that are part of the basal ganglia which we have already described. Finally, not shown in this image are structures related to the limbic system including the hippocampus and amygdala, both of which are located in the temporal lobe which were described in another e-learning module. Finally, *** there are fiber tracts that connect different parts of the cerebral cortex. We will identify these on subsequent slides.

4. CEREBRAL CORTEX DIVISIONS OF THE CORTEX
The outer mantle, consisting of neurons that are distributed in 3-6 layers, is very thin measuring mm.
There are billion neurons, 50 billion glial cells, 100,000 km of axons, and 1014 synapses in the cerebral cortex.
Highly infolded. If laid out flat it would be ~ 2 square feet. About 1/3 of cortex is visible and 2/3 is located in sulci and fissures
DIVISIONS OF THE CORTEX
Isocortex (Homogeneous) Consisting Of:
Neocortex Which Represents % Of The Cortex In Humans.
All Of The Cortex That Is Seen On Outside Of Brain.
Most Recently Evolved Cortex - Plays A critical role in abilities and activities that reach highest level of development in humans
The cerebral cortex is the outermost part of the telencephalon. It is very thin, measuing only mm. This area contains all of the neurons that are essential for interpreting sensory information, generating voluntary motor activity, speaking, personality, making executive decisions, memory, etc. There are billion neurons and 50 billion glial cells in the cerebral cortex. The numbers are impressive; you do not need to know the number of neurons or glia in the cortex. Just appreciate that there are a lot of neurons and glia in the telencephalon. Given the large number of neurons, one might wonder how they all fit. It turns out nature solved that problem by highly infolding the cerebral cortex giving it the bumpy appearance. If it was laid out flat, it would measure 2 sq feet., about the size of a open newspaper. It turns out that only 1/3 of the cortex is visible, the rest is buried in the trenches or sulci. ***The cortex may be divided into ***neocortex which is the most recently evolved and is essential for allowing us to do those things only humans do. As we'll see, it consists of 6 layers. The allocortex*** is much older and is found in more primitive species. It basically consists of limbic and olfactory centers including the hippocampus and cingulate cortex. The allocortex only contains 3 layers
B. Allocortex (Gr. “Other”) – Consisting Of:
Archicortex (i.e., Hippocampus)
Paleocortex (Parahippocampal Gyrus of Temporal Lobe, Olfactory Cortex, Cingulate Cortex).

5. MAJOR SULCI OF LATERAL CORTEX
Post
Central
Sulcus
Central
Sulcus
Pre-central
Sulcus
Parieto-
Occipital
Sulcus *
Much of the material presented here is a review of material you have learned previously. This will serve as a refresher for the next module on cortical function. Let’s begin to identify the major lobes of the cerebral cortex. To define the lobes, we need to first identify some of the major depressions or sulci that form the borders of the different lobes. The most obvious depression is the lateral sulcus **** which seems to split the brain almost in half in the horizontal plane. Another prominent depression, is the central sulcus*** which seems to divide the brain in half in the transverse plane. The central sulcus does not quite reach the lateral sulcus and there is always a small bridge ***of cortex that separates them. The depression rostral to the central suclus*** is called the precentral sulcus, whereas the depression immediately caudal to the central sulcus is the**** postcentral sulcus. Finally, a very short, shallow sulcus near the caudal pole of the cerebral cortex*** is the parieto-occipital sulcus. We will see this better when we look at the medial view of the brain. There are many more depression, all of which are named. However, these are the ones you will be asked to learn.
Lateral
Sulcus

6. LATERAL CEREBRAL CORTEX - LOBES
BORDERS:
PARIETAL LOBE
FRONTAL LOBE
FRONTAL POLE OF BRAIN
CENTRAL SULCUS
LATERAL SULCUS
FRONTAL LOBE
PARIETAL LOBE
CENTRAL SULCUS
LINE BETWEEN BASE OF CORTEX AND
PARIETO-OCCIPITAL SULCUS
OCCIPITAL LOBE
LINE BETWEEN BASE OF CORTEX
AND PARIETO-OCCIPITAL SULCUS
OCCIPITAL
LOBE
TEMPORAL
LOBE
TEMPORAL LOBE
LATERAL SULCUS
NO CLEAR DEMARCATION BETWEEN
PARIETAL LOBE AND OCCIPITAL LOBE
POSTERIORLY
Having defined the sulci, we can now define the lobes visible on the lateral aspect of the cerebral cortex. The sulci from the previous slide have been added to the image. The most anterior part of the brain ***is the frontal lobe. It extends from the frontal pole of the brain to the central sulcus. The parietal lobe extends**** from the central sulcus to the parietal occipital sulcus. On the lateral aspect of the brain, there is no clear separation between the parietal lobe and the ***occipital lobe, the most caudal pole of the telencephalon. An imaginary line may be drawn from the dorsal point of this sulcus to the base of the hemisphere to better define the separation between the parietal lobe and the occipital lobe. The temporal lobe*** is the area below the lateral sulcus. There is no clear demarcation between the temporal lobe and the parietal and occipital lobes posteriorly. As we'll see, these areas interact to correlate visual, sensory, and auditory information to interpret the world.

7. GYRI
PRECENTRAL – PRIMARY MOTOR CORTEX.
Gyri immediately anterior are supplementary motor areas
PRECENTRAL
POSTCENTRAL
INFERIOR FRONTAL
Including Broca’s area for motor control of language production (on left)
INFERIOR
FRONTAL
ORBITAL PART
Most anterior portion
ORBITAL
WERNICKE’S
AREA
POSTCENTRAL- PRIMARY SENSORY CORTEX
BROCA’S
AREA
AUDITORY
AREA
More posterior gyri are sensory association areas – spatial orientation and directing attention.
SUPERIOR
TEMPORAL
OCCIPITAL
SUPERIOR TEMPORAL – Posterior part related to interpretation of language – Wernicke’s area
Let’s add one more level of refinement on the cortex by identifying specific bumps or gyri. These gyri extend down into the sulci. We will be identifying the visible portion on the lateral aspect of the cerebral cortex. Starting in the frontal lobe, a prominent gyrus *** called the precentral gyrus is located between the central and precentral sulci. This is the primary motor cortex where voluntary movements are initiated. The area immediately anterior to this gyrus are supplementary motor areas that are involved in planning movements. The most rostral *** part of the frontal lobe is called the prefrontal cortex. It can be divided into inferior frontal which contains ***an area called Broca’s area that is essential for generating speech. The most rostral ple of the frontal lobe is the ***orbital cortex and is involved in decision making, memory, personality, etc. Between the central *** and postcentral sulcus is the postcentral gyrus. This is primary sensory cortex. ***More posterior gyri (shown in green) are association areas and are involved in spatial orientation, and sterognosis. The next area to identify is the area immediately inferior to the lateral sulcus***. This is the superior temporal gyrus which is involved in processing auditory information***. The posterior portion of this gyrus *** is called Wernicke’s area and it is essential for interpreting speech. Finally most caudally are lateral occipital gyri which we will not name. These are visual association areas.
LATERAL OCCIPITAL GYRI – Visual association areas

8. INSULAR CORTEX
One other area cortex needs to be identified on the lateral surface. This part of the cortex is not visible unless the temporal and parietal lobes are separated. This deeply buried cortex is called the Insular cortex. In transverse or*** horizontal sections it can be recognized as the deep cortex covered by the temporal and parietal lobes. It is indicated by the green arrow in this image.
BURIED DEEP IN THE LATERAL SULCUS. COVERED BY GYRI FROM THE TEMPORAL, PARIETAL AND FRONTAL LOBES.
CONTAINS GUSTATORY, AUTONOMIC, PAIN, VESTIBULAR AREAS.

9. MAJOR SULCI OF MEDIAL CORTEX
Cingulate
Sulcus
Parieto-Occipital
Sulcus
Calcarine
Sulcus
Let's now look at the medial surface of the cortex. A few sulci will serve as landmarks. These include the cingulate*** sulcus which defines cortical tissue immediately above the corpus callosum. Caudally, we now see a better defined*** parieto-occipital sulcus that separates the parietal and occipital lobes. In the occipital lobe *** a large sulcus, called the calcarine sulcus divides the lobe into dorsal and ventral halves.

10. GYRI – MEDIAL CEREBRAL CORTEX
CINGULATE GYRUS – Related to limbic system
PARAHIPPOCAMPAL GYRUS - Overlies hippocampus. Is continuous with cingulate gyrus at posterior end of corpus callosum.
Cingulate (Limbic)
Lobe (Gyrus)
Cuneus
UNCUS – Anterior End Parahippocampal Gyrus; Overlies Amygdala
Uncus
Parahippocampal
Lingual
The lobe located immediately below the cingulate sulcus is the cingulate or limbic lobe. This is part of the allo or old cortex.
The cingulate lobe***is also called the cingulate gyrus. As is evident here, it is related to the frontal lobe, the parietal lobe and the temporal lobe. Functionally, it is part of the limbic system. The portion in the temporal lobe *** is called the parahippocampal gyrus. Para means “next to” and as we'll see later, it is next to a limbic structure called the hippocampus. At the rostral end of the parahippocampal gyrus there is a swelling called the uncus***. This is related to the amygdala, another part of the limbic system. This bump is clinically important as you'll hear later. In the occipital lobe, 2 gyri may be identified on either side of the calcarine sulcus – the ***cuneus and the lingual gyri. These are visual association areas. So where is primary visual cortex? Actually *** it is buried deep in the calcarine sulcus and is not visible on the gross brain.
CUNEUS, LINGUAL – Visual association cortex
NOTE: Primary visual cortex is located on walls calcarine sulcus

11. Histology
CORTEX IS MADE UP OF PYRAMIDAL (RED ARROWS) AND NON-PYRAMIDAL CELLS (ORANGE ARROW)
PYRAMIDAL CELL IS PRIMARY OUTPUT NEURON OF THE CORTEX
Let's do just a little histology. There are many different types of neurons in the cerebral cortex. They can be subdivided into pyramidal cells (indicated by the red arrows) and non-pyramidal cells indicated by the orange arrows. We'll focus on the pyramidal cells as these are the neurons that send signals out of the cortex to other areas of the CNS such as the brainstem or spinal cord or to other areas of the cerebral cortex which would be considered inter or intrahemispheric projections. More on this later.

12. LAMINATION IN NEOCORTEX AND ALLOCORTEX
III – Intracortical
Connections
IV – Thalamic Input
V – Cortical Output:
Corticospinal
Corticostriate
Corticopontine
Corticobulbar
VI– Corticothalamic
The cerebral cortex is a layered structure. In the neocortex there are 6 layers labeled I – VI with layer I being closest to the outside of the brain. In the allocortex, there are only 3 layers. You have already discussed this in the presentation on the limbic system. In the neocortex, we will focus on 4 layers – III, IV, V and VI. Let’s start with layer IV*** This is the primary area where afferent axons from the thalamus terminate. These are afferents that carry specific information related to sensory input as well as motor input from the cerebellum and basal ganglia. The axons of pyramidal cells in Layer III *** interconnect different regions of the cerebral cortex both ipsilaterally and contralaterally forming association projections (ipsilateral connections) or commissural projections (contralateral connections). The axons of neurons in layer V *** leave the cerebral cortex and project to other regions of CNS such as the spinal cord (corticospinal axons), the brainstem (corticobulbar, corticopontine axons) or the striatum (corticostriate axons). Finally, neurons in layer VI *** project back to the thalamus to regulate thalamic output.
THERE ARE 6 LAYERS IN THE NEOCORTEX AND 3 LAYERS IN THE CORTEX RELATED TO THE LIMBIC SYSTEM

13. THICKNESS OF CORTICAL LAYERS
The thickness of the cortical layers is not consistent across different lobes of the cerebral cortex. The thickness is reflective of the function of a particular lobe. For example, primary sensory cortex receives a large projection from the thalamus carrying sensory information on touch, pressure, pain, temperature, and proprioception from muscles and joints. This information terminates primarily in layer IV. Thus this layer*** is comparatively larger in sensory cortex. Motor cortex does receive input from the thalamus but not to the same level as primary sensory cortex so layer IV in this area is comparatively smaller. In contrast, it's primary role is to project to nuclei in the brainstem or to neurons in the spinal cord to initiate or regulate movement. Therefore, ***layer V is much larger in this part of the cortex compared to the sensory cortex. In Association cortex, we again see differences in the sizes of the layers. These regions of the cortex communicate with other areas of the cerebral cortex so layer III may be slightly larger. They also receive substantial input from nuclei in the thalamus that don't relay specific sensory or motor information such as the pulvinar. These differences in the sizes of the cortical layers was what was used to Dr. Brodman in developing his nomenclature.
ACROSS THE CORTEX, LAYERS ARE NOT EQUAL IN SIZE. THIS LEADS TO CYTOLOGICAL DIFFERENCES.

14. Korbinian Brodmann KORBINIAN BRODMANN 1868-1918
USED DIFFERENCES IN CYTOLOGICAL ORGANIZATION OF THE CORTEX TO DEFINE AREAS – BRODMANN AREAS
Korbinian Brodmann was a neurologist who lived in the late 1800s, early 1900s. Under the microscope, he measured the thickness of cortical layers in different regions of the cerebral cortex and found many differences. This image is from his original study.

15. BRODMANN’S CYTOARCHITECTURAL MAP
From these data, he defined up to 52 different cytologically distinct regions of the cerebral cortex. Many of these numbers are still used as you have seen in previous TLMs.

16. Study this LOBE BRODMANN NUMBER LOCATION Frontal 4
Precentral gyrus (Primary Motor area) 6
Premotor,Supplementary Motor
44,45
Inferior frontal gyrus (Broca’s area for language)
Parietal
3,1,2
Postcentral gyrus (Primary somatosensory area)
5,7
Somatosensory association area
Occipital 17
Banks of calcarine sulcus (Primary visual area)
18,19
Surrounding 17 (Visual Association areas)
Temporal 41
Primary auditory area 42
Auditory association area 22
Superior temporal gyrus (Wernicke’s area for language on left)
This table lists the ones that you are most likely to encounter in your studies and when you begin your neurology or neurosurgery rotations. They may also show up on your board examinations. Some of these numbers have been used in previous TLMs.

17. SOMATOTOPIC MAP - HOMUNCULUS
SOMATOSENSORY CORTEX
MOTOR CORTEX
The somatosensory and motor cortex are very organized with respect to the body representation. This is referred to as the Homunculus. As you can see, specific parts of the body are represented in distinct areas. Also note, there is not a uniform area for all regions. In both the somatosensory and motor cortex, the face has a disproportionate representation as does the hand and especially the digits. This clearly reflects the sensitivity of these areas and the fine motor control. In comparison, the trunk has a very small representation. You should know the relative distribution of different regions of the body on the cortex. Basically, the body is draped over the primary motor and primary somatosensory cortices with the lower limb hanging over the top and into the cleft between the two hemispheres and the head resting on the temporal lobe.

18. MOTOR HOMUNCULUS
If our bodies actually looked like the brain perceived the various regions this is what we would look like.*** Scary.

19. FIBER TRACTS – CORTICO-CORTICAL
CORPUS CALLOSUM – INTERHEMISPHERIC
Links related areas of frontal, parietal , caudal temporal, and occipital lobes
ANTERIOR COMMISSURE – INTERHEMISPHERIC
Links related areas of inferior temporal lobe
Corpus Callosum
Fornix
There are just a few fiber tracts we want to identify at this time. Primarily, we will define two cortico-cortical pathways. One type, links the right and left hemispheres. These are called interhemispheric. Generally, they link related areas. The major fiber bundle that mediates this linkage is the corpus callosum. This fiber tract links the frontal, parietal, caudal temporal, and occipital lobes. The rostral and inferior parts of the temporal lobe have their own fiber tract to link them called the anterior commissure. This image ***of a transverse section through the cortex illustrates the axons crossing the midline in the corpus callosum and in the anterior commissure.
Anterior
Commissure

20. INTRAHEMISPHERIC TRACT
Intrahemishperic tracts link cortical areas on the same side.
SUPERIOR LONGITUDINAL FASCICULUS –INTRAHEMISHERIC
Links Broca’s and Wernicke’s language areas
In addition to interhemispheric tracts, there are also pathways that link cortical areas on the same side. These are called intrahemispheric. An example of this is the superior longitudinal fasciculus (or arcuate fasiculus) that links parts of the language pathway namely Broca's area for generating speech and Wernicke's area for interpretation of language. In this diffusion tensor image, the superior longitudinal fasciculus are the yellow fibers labeled A.

21. INTERNAL CAPSULE AND CORONA RADIATA
ALL AFFERENTS AND EFFERENTS RELATED TO THE CORTEX TRAVERSE THE INTERNAL CAPSULE .
INTERNAL CAPSULE:
BORDERS:
Lateral: Putamen, Globus Pallidus
Medial: Caudate Nucleus and Thalamus A
INTERNAL CAPSULE:
DESCENDING FIBERS INCLUDE:
Corticobulbar Axons Corticopontine
Corticospinal
Corticostriatal G P
ASCENDING FIBERS INCLUDE:
Thalamocortical Axons
Tracts Terminating In Thalamus R
The last major fiber tract to define is the internal capsule. We have looked at this tract before. It is the pathway used by all afferent and efferent axons related to the cerebral cortex. The internal capsule ***separates the caudate and putamen rostrally. This is called the anterior limb of the internal capsule. Caudally, the internal capsule separates the thalamus from the basal ganglia. This is called the posterior limb of the internal capsule. Within the internal capsule*** you will find axons descending from the cortex to the brainstem or spinal cord or to the subcortical nuclei such as those to the basal ganglia (called corticstriatal). Also present *** are ascending axons from the thalamus to the cortex. Tracts that terminate in the thalamus will also be present in this large fiber bundle. The portion of the internal capsule between the anterior and posterior limbs is called the genu, as there is a slight bend here. Axons that course posterior to the globus pallidus are referred to as retrolenticular.

22. Blood Supply to the Cortex
Finally, Let's review the blood supply to the cerebral cortex. Remember, the cortex has a very dense vascular supply from several different arteries.

23. BLOOD SUPPLY TO THE CEREBRAL CORTEX
2 VESSELS SUPPLY MEDIAL ASPECT OF CORTEX:
ANTERIOR CEREBRAL (BRANCH OF INTERNAL CAROTID ARTERY)
POSTERIOR CEREBRAL (BRANCH OF BASILAR ARTERY)
INTERNAL
CAROTID A.
ANTERIOR
CEREBRAL A.
POSTERIOR
FRONTAL
LOBE
PARIETAL
OCCIPITAL
CINGULATE
CORTEX
BASILAR A. CC
For review, the medial side of the cerebral cortex receives it’s blood supply from 2 major branches. These are the Posterior Cerebral which is the terminal branch of the basilar artery and the Anterior Cerebral which is a branch of the internal carotid artery. The posterior cerebral supplies the occipital lobe and medial portions of the temporal lobe. The anterior cerebral artery supplies the frontal and parietal lobes. There is an area of overlap at the border of the parietal and occipital lobes. Note also that both arteries extend over the dorsal surface of the brain to reach the lateral side.

24. BLOOD SUPPLY TO THE CEREBRAL CORTEX
Major Blood Vessel To Lateral Cortex Is Middle Cerebral A. (Branch Of Internal Carotid A.
Also See Terminal Branches Of Anterior and Posterior Cerebral Arteries
ANTERIOR CEREBRAL A
MIDDLE
CEREBRAL A
POSTERIOR
The lateral side of the cerebral cortex receives it’s blood supply from the middle cerebral artery, a branch of the internal carotid. Also, the branches of the anterior and posterior cerebral arteries that wrap around from the medial side supply portions of the lateral cortex.

25. Organization of the Forebrain Quiz

26. Forebrain

27. Forebrain - Cross Section

28. Thank you for completing this module
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