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The DienCephalon is situated between the BrainStem and the Cerebrum. Its main components are:
• Thalamus
• SubThalamus
• EpiThalamus
• HypoThalamus

Thalamus
1 - The Thalamus is by far the largest portion and constitutes approximately 4/5 's of the DienCephalon's weight.

The Thalamus consists of a Cluster of Nuclei and is shaped somewhat like a yo-yo, with two large, Lateral portions connected in the center by a small stalk called the Intermediate Mass.

The space surrounding the Intermediate Mass and separating the two large portions of the Thalamus is the Third Ventricle of the Brain.

Most Sensory Input projects to the Thalamus where Afferent Neurons synapse with Thalamic Neurons, which send projections from the Thalamus to the Cerebral Cortex (Parietal Lobe).

Auditory Impulses synapse in the Medial Geniculate, of the Thalamus.

Visual Impulses synapse in the Lateral Geniculate Body, and most other Sensory Impulses synapse in the Ventral Posterior Nuclei.

The Thalamus also
• Influences Mood
• Body Movements associated with
• Strong Emotions
  • Fear
  • Rage

The Cerebrum is the largest portion of the Brain, weighing approximately 1200g in females and 1400g in males. Brain size is related to body size; large Brains are associated with large bodies and not with intelligence.

The Cerebrum is what most people think of when the term Brain is mentioned. The Cerebrum is divided into left and right Hemispheres by a Longitudinal Fissure.

The most conspicuous features on the surface of each Hemisphere are numerous folds called Gyri, which greatly increase the surface area of the Cortex, and intervening grooves called Sulci.

The general pattern of the Gyri is similar in all normal human Brains, but some variations exists between individuals, even between the two Hemispheres of the same Cerebrum.

Cerebral Lobes

Each Cerebral Hemispheres is divided into Lobes, which are named for the Skull Bones overlaying each one. The Frontal Lobe is important in Voluntary Motor Function, Motivation, Mood, and Aggression.

The Frontal and Parietal Lobes - are seperated by a prominent Sulcus called the Central Sulcus. (View: Image)

The Parietal Lobe - is the major center for the Reception and Evaluation of most Sensory Information.

Situated at the top of the Brain it processes information about Touch, Taste, Blood pH, Pressure, Pain, and Temperature (excluding Smell, Hearing, and Vision). (View: Image)

The Occipital Lobe - functions in the reception and integration of Visual Input and is not distinctly separate from the other Lobes.

The Temporal Lobe - receives and evaluates Olfactory and Auditory Input and plays an important role in Memory.

Its Anterior and Inferior portions are referred to as the Psychic Cortex and they are associated with Brain functions such as Abstract Thought and Judgement.

The Temporal Lobe is separated from the rest of the Cerebrum by a Lateral Fissure, and deep within that Fissure is the Insula, often referred to as a Fifth Ventricle.

The Gray Matter on the outer surface of the Cerebrum is the Cortex, and clusters of Gray Matter deep inside the Brain are Nuclei. The White Matter of the Brain between the Cortex and Nuclei is the Cerebral Medulla.

Not to be confused with the Medulla Oblongata; "Medulla" is a general term meaning the center of a structure or Bone Marrow.

The Cerebral Medulla consists of Nerve Tracts that connect the Cerebral Cortex to other areas of Cortex or other parts of the CNS.

Three Tracts Categories

• Association Fibers - connect different areas of the Cerebral Cortex within the same Hemisphere

• Commissural Fibers - connect one Cerebral Hemisphere to the other Hemisphere (Corpus Callosum)

• Projection Fibers - which are between the Cerebrum and other parts of the Brain and Spinal Cord

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Somesthetic Cortex

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Sensory Pathways project to specific regions of the Cerebral Cortex, called Primary Sensory Areas, where those Sensations are perceived.

Most of the PostCentral Gyrus (located Posterior to the Central Sulcus) is called the Primary Somesthetic Area, or General Sensory Area.

Afferent Fibers carrying general Sensory Input such as Pain, Pressure, and Temperature Synapse to the Thalamus, and Thalamic Neurons relay the information to the Somesthetic Cortex.

The Somesthetic Cortex - is organized topographically relative to the general plan of the body. Sensory impulses conducting input from the feet project to the most Superior portion of the Somesthetic Cortex, and sensory impulses from the face project to the most Inferior portion of the Somesthetic Cortex.

The pattern of the Somesthetic Cortex in each Hemisphere is arranged in the form of a half Homunculus (a little human) representing the opposite side of the body, with the feet directed Superiorly and the head directed Inferiorly.  (View Image)

In addition, the size of various regions of the Somesthetic Cortex is relative to the number of Sensory Receptors in the associated regions of the body.

Many Sensory Receptors are in the face, but far fewer are in a comparably sized area of the legs; therefore a greater area of the Somesthetic Cortex contains Sensory Neurons associated with the Face (the Homunculus has a disproportionaly large face).

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Primary Sensory Areas

The Primary Sensory Areas of the Cerebral Cortex must be intact for Conscious Perception, Localization and Identification of a stimulus.

The conscious perceptions of cutaneous sensations, although integrated within the Cerebrum, Are Perceived as though they were on the Surface of the Body.   (View: Image)

This is called Projection and indicates that the Brain refers a sensation to the superficial site where the stimulus interacts with the Sensory Receptors.

Cortical Areas immediately adjacent to the Primary Sensory Centers, Association Areas, are involved in the process of recognition.

The Somesthetic Association is Posterior to the Somethetic Area, and the Visual Association Area is Anterior to the Primary Visual Area (Visual Cortex).

Afferent action potentials originating in the Retina of the Eye reach the Visual Cortex where the image is "seen".

Action Potentials then pass from the Visual Cortex to the Visual Association Area where the present visual information is compared to past visual experience.

Based on this comparison the Visual Association Area "decides" whether or not the visual input is recognized and passes judgement concerning the significance of the input.

You pay less attention to a person you have never seen before than to someone you know.

The Visual Association Area, like other Association Areas of the Cortex, has reciprocal connections with other parts of the Cortex and input from those parts influences decisions.

These connections include input from the Frontal Lobe where Emotional Value is placed on the Visual Input.

Because of these numerous connections it is quite unlikely that Visual information can pass beyond the Visual Association Area without having several judgements made.

Concerning the input and may be one of the reasons why two people may witness exactly the same event and if questioned immediately afterwards present somewhat different versions of what happened.

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Primary Motor Area

The PreCentral Gyrus (Anterior to the Central Sulcus) also called the Primary Motor Area.

Efferent action potentials initiated in this region Control many Voluntary Movements, especially the Fine Motor movements of the Hands.

Cortical Neurons that control Skeletal Muscles are called Upper Motor Neurons and are not confined to just the PreCentral Gyrus; in fact, only 30% of them are located there.

Another 30% are in the PreMotor Area, and the rest are in the Somethetic Cortex.

The Cortical functions of the PreCentral Gyrus are arranged Topographically according to the general plan of the body - similar to that of the PostCentral Gyrus.

Nerve cell bodies providing Motor Function to the Feet are in the most Superior and Medial portions, whereas those for the Face are in the Inferior Region.

Muscle groups that have numerous Motor Units and therefore Greater Innervation are represented by a relatively larger area of the Motor Cortex.

For example, Muscles of the Hands and Mouth are represented by a larger area of the Motor Cortex than the muscles of the Thights and Legs.

The PreMotor Area is the Staging Area where Motor Functions are organized before they are initiated in the Motor Cortex.

For example, if a person decides to take a step, the Neurons of the PreMotor Area are stimulated first, and the determination is made there as to Which muscles must Contract, in What Order, and to What Degree.

Impulses are then passed to the Upper Motor Neurons (CortocoSpinal Tract) in the Motor Cortex, which actually initiate each planned movements.

The Motivation & the Foresight to Plan and Initiate Movements occur in the next most Anterior portion of the Brain, the PreFrontal Area, an area of Association Cortex that is well developed only in primates, especially humans.

It is involved in motivation and regulation of emotional behavior and mood. The large size of this area in humans may account for our relatively well-developed forethought and motivation and for our emotional complexity.

The PreMotor Area must be intact for a person to carry out Complex Skilled or Learned Movements, especially ones related to manual dexterity.

Impairment in the performance of learned movements, called Apraxia can result from a lesion in the PreMotor Area, it is characterized by Hesitancy in Performing these movements.

In relation to its involvement in motivation, the Prefrontal Area is the Functional Center for Aggression.

In the past, one method used to eliminate uncontrollable aggression in mental patients was to surgically remove or destroy the Prefrontal regions of the Brain (Prefrontal or Frontal Lobotomy).

This operation was sucessful in eliminating aggression, but it also eliminated the motivation to do much else and dramatically altered the Personality.

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Speech

In the vast majority of people the Speech Area is in the Left Cortex. Two major Cortical areas are involved in Speech:

• Broca's Area (Motor Speech Area) in the Inferior portion of the Frontal Lobe, initiates the complex series of movements, necessary for any Speech to take place.

• Wernicke's Area (Sensory Speech Area) a portion of the Parietal Lobe, necessary for Understanding and Formulating Coherent Speech.

To repeat a word that one hears requires the functional integrity of the following pathway. Action Potentials from the Ear reach the Primary Auditory Area (Auditory Cortex) where the word is heard; the word is recognized in the Auditory Association Cortex and is comprehended in portions of Wernicke's Area.

Then Action Potentials representing the word are conducted through Association Fibers that connect Wernicke's and Broca's Areas (The Arcuate Fasciculus is the primary Nerve Pathway connecting Wernicke's and Broca's Areas).

In Broca's Area the word is formulated as it will be repeated; impulses then go to the PreMotor Cortex where the movements are programmed and finally to the Motor Cortex where the proper movements are triggered.

Speaking a written word is somewhat similar. The information enters the Visual Cortex, passes to the Visual Association Cortex where it is recognized and continues to Wernicke's Area where it is understood and formulated as it will be spoken.

From Wernicke's Area it follows the same route for repeaating audibly received words.

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Speech Disorders

• Aphasia
  Absent or defective Speech or Language Comprehension, results from a lesion in the Language Area of the Cortex. The several types of Aphasia depend on the site of the lesion.

• Receptive Aphasia
  With defective Auditory and Visual Comprehension of Language and Defective Naming of objects and Repetition of spoken sentences, is caused by a lesion in Wernicke's Area.

• Jargon Aphasia
  A person may speak fluently but unintelligibly, usually from a lesion in the Nerve Tract (Arcuate Fasciculus), between Wernicke's and Broca's Areas.

• Conduction Aphasia
  Is when a person has Poor Repetition but relatively Good Comprehension, it can result from a lesion in the tracts between Wernicke's and Broca's Areas.

• Anomic Aphasia
  Caused by the isolation of Wernicke's Area from the Parietal or Temporal Association Areas, it is characterized by Fluent but Circular Speech resulting from poor word finding ability.

• Expressive Aphasia (Broca's Aphasia)
  Caused by a lesion in Broca's Area, normally characterized by Hesitant and Distorted Speech.

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The Basal Ganglia

The Basal Ganglia are a group of Functionally Related Nuclei located bilaterally in the Inferior Cerebrum, DienCephalon and MidBrain.

The SubThalamic Nucleus is located in the DienCephalon, and the Substantia Nigra is located in the MidBrain.  (View Interior: Image)

The Nuclei in the Cerebrum are collectively called the Corpus Striatum (Stripped Body) and include the Caudate Nucleus (Tail) and Lentiform Nucleus (Lens Shaped).

They are the Largest Nuclei of the Brain and occupy a large portion of the Cerebrum.   (View Larger: Image)

The Basal Ganglia play an important role in planning and coordinating motor movements and posture. Complex Neural connections link the Basal Ganglia with the Cerebral Cortex.

The major effect of the Basal Ganglia is to inhibit unwanted muscular activity and disorders of the Basal Ganglia result in exaggerated, uncontrolled movements.
(Also See: Basal Ganglia)

The Limbic System

Portions of the Cerebrum and DienCephalon are grouped together under the title *Limbic System*. The term Limbic is not precise and is used differently by various authors.

The term Limbus means Border, and the term Limbic refers to the Medial Portion, or Border of the Temporal Lobe.

Structurally the Limbic System consists of:
• The Cingulate (to surround) Gyrus, located along the Inner Surface of the Longitudinal Fissure just above the Corpus Callosum, and the Hippocampus;
• Various Nuclei such as Anterior Nuclei of the Thalamus and the Habenular Nuclei in the EpiThalamus;
• Parts of the Basal Ganglia;
• The HypoThalamus, especially the Mamillary Bodies;
• The Olfactory Cortex;
• Tracts connecting the various Cortical and Ganglia (the Fornix is one such Tract).

The Limbic System influences Emotions, the Visceral Responses to those Emotions, Motivation, Mood, and sensations of Pain and Pleasure.

One of the major sources of Sensory Input into the Limbic System is the Olfactory Nerves. The Smell of Food stimulates the Hunger Center in the HypoThalamus. (View: Image)

In animals such as dogs and cats, Olfactory detection of Pheromones molecules released into the air by one animal that attract another animal of the same species, usually of the opposite sex are important in reproduction.

Lesions in the Limbic System can result in voracious appetite, increased (often perverse) sexual activity, and docility (including loss of normal fear and anger responses).

Since the HyppoCamus is part of the Temporal Lobe, damage to that portion of the Limbic System can also result in a loss of memory.

The Hyppocamus and the adjacent Cortex are very important in the transition of information from short to long term memory, and the cells undergoing Calcium induced shape changes associated with long term memory are localized in that region of the Brain.   (Also See: Limbic System)

Cerebellum

The term Cerebellum means Little Brain. It communicates with other regions of the CNS through three large nerve tracts, the Superior, Middle, and Inferior Cerebellar Peduncles.

The Cerebellum is organized like the Cerebrum, with Gray Matter both on the inside as Nuclei and on the outside as Cortex. The Cerebellar Cortex has Gyri and Sulci, but the Gyri are much smaller than those of the Cerebrum.

The Cerebellum consists of three portions: the FlocculoNodular Lobe. a narrow Central Vermis, and two large Lateral Hemispheres.

The FlocculoNodular Lobe is the simplest portion of the Cerebellum and is involved in Balance.

The Anterior portion of the Vermis is involved in Gross Motor Coordination, and the Posterior Vermis and Lateral Hemispheres are involved in Fine Motor Coordination, producing smooth, flowing movements.

A major function of the Cerebellum is that of a comparator.

Impulses from the Motor Cortex descend into the Spinal Cord to initiate Voluntary Movements, and at the same time impulses are sent from the Motor Cortex to the Cerebellum, giving the Cerebellar Neurons information representing the intended movement.

Simultaneously, impulses from the Proprioceptive Neurons (providing information about the position of the body or body parts) that innervate the Joints and Tendons of the structure being moved, reach the Cerebellar Cortex.

These impulses give the Cerebellar Neurons information from the Periphey about the actual movements. The Cerebellum compares impulses from the Motor Cortex with those from the moving structures (it Compares the Intended movement with the Actual movement),

If a difference is detected, the Cerebellum sends impulses to the Motor Cortex and the Spinal Cord to correct the discrepancy. The result is smooth and coordianted movements.

With training a person can develop highly skilled and rapid movements that are accomplished more rapidly than can be accounted for by the comparator function of the Cerebellum.

In these cases the Cerebellum can "learn" highly specialized motor functions through specific, repeated comparator activities.

(Also See: Cerebellum)