Basic Neuroanatomy:Diencephalon, Hypothalmus, Telencephalon, Frontal Lobe

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Neurological Basis of Behavior (PSY - 610)

Lesson29

Basic Neuroanatomy

Objectives:

The main objective of this lesson is to study

The Brain and the Peripheral systems:

Brain: Forebrain, Mid brain, Hind Brain functioning of each anatomical location in the CNS.

Starting from the posterior located areas upto the anterior-most.

Cerebral Cortex: its layers, Corpus Callosum and the two hemispheres

Main purpose:

Students will come to relate the importance of location of various areas in the Diecephalon and the

Telencephalon, their functions and their relationship to other areas. The students would also understand

how control and modulation of behavior related to these neuroanatomical site takes place

Diencephalon:

This comprises of two major areas of the brain both equally important in their functions and their

involvement in a wide range of behaviors and connection, the hypothalamus and the Thalamus

Hypothalmus: it lies at the base of the brain, on both sides of the 3rd ventricle. It is very small in size a

compared to other brain areas but is very important in it s role and function. Hypothalamus is located

immediately beneath the thalamus and lies above the pituitary.

Hypothalamus is part of the Limbic system, and in all situations requiring a fight-flight or other

survival responses. There are several important fibre bundles passing through the hypothalamus,

including the-MFB Medial forebrain bundle which carries major neurotransmitters along with other

fibers to the higher areas.

Underneath the hypothalamus lies the superchiasmatic nucleus (SCN). The SCN plays an important

role in the day night cycles i.e. diurnal cycles. Within this region we also see the mammiliary bodies

which are important in memory.

The hypothalamus also secretes its own hormones which act as releasing factors for hormonal secretions

in the pituitary. The neurosecretory cells of the hypothalmic hormones are located near base of

hypothalamus, very close to the pituitary. Therefore hypothalamic-pituitary connection is important for

the neural and endocrinal connections. The brain talks to the pituitary through the hypothalamus, and

vice versa. All the endocrinal glands communicate for fear aggression, temperature regulation, food

and water, and mating. The Anterior hypothalamus is important in thrist regulation and there are

important sensors which continuously monitor fluid (and isotonic saline levels) of neurons. The

Ventromedial (VMH) and Lateral Hypohtalamus (LH) are involved in regulation of feeding. Research

has shown that the VMH is the satiety center (have- enough- food -stop -signal), and the lateral

hypothalamus is the center for initiation of feeding. Hypothalamus important in all major survival and

maintenance functions; it is related to the management functions of the body and survival rather than

intellectual functioning.

Thalamus: this is the largest component of Deincephalon with an ovalish shape; it has two lobes

which are connected by the mass intermedia which go through the midline of the third ventricle.

The thalamus comprises of large clusters of nuclei. The thalamus is a staging area, information

received from sensory receptors systems and other lower areas and then it is projected to the

relevant cortical areas. 1. Sensory relay nuclei: All afferent sensory input are received classified,

sorted and the passed through the thalamus, these go from the receptor to the Thalamus which then

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sends the selected projections to the related cortices. The Visual, auditory input goes through the

lateral geniculate nuclei and the medial geniculate nuclei, and somatosensory sensory fibers go to

the somatosensory cortices. There is a reciprocal one to one relationship of these connections in the

cortex. If one fiber degenerates in the cortex, it would lead to atrophy of the relevant thalamic areas.

The Dorsal group of fibers is multimodal (that is it receives and sends input to different sensory

areas). This receives input from within the thalamic regions, and project to the association cortex.

There is another group of fibers in the thalamus the Intrinsic or the non-specific. If this area is

stimulated it leads to wide spread electrical discharge in the cortex and other areas, evidence that

this bundle is important in electrical activity of the cortex. It is involved in sleep awakening as well

as attention. It is also important in affective behavior, memory (there is severe amnesia after lesions

to midline thalamic area). Thus the thalamus is important in every aspect of behavior

Telencephalon: The End brain comprises mainly of the areas of the cerebral cortex and some parts of

the Corpus striatum, the limbic system. The two hemispheres of the cerebral cortex and the other parts

are connected to each other by commissures (bands of fibers connecting the left and the right

hemispheres).

Corpus Callosum is the major band of fibers joining the two hemispheres it is wide, white and

visible to the naked eye. The commisures are mainly for interaction between the two hemispheres

and for crossing over of information (so that the two hemispheres acna coordinate decisions).

Cerebral Cortex: (cortex means outer covering: bark of the brain). This is made up of layers of

grey matter which covers the white matter. The thickness of the cerebral cortex varies from 1.5 to

4.5 mm; the average is about 2.00 mm. It is thickest in the primary motor cortex about 4.5 mm thick

and thinnest about 1.5 mm at the primary visual cortex. It is laid out in a straight line; the total

surface of the cerebral cortex would be about 20 sq.foot. How does this fit into the cranium, it is

sqeezed into folds. These folds are irregular convulations and grooves called sulci (for smaller size)

and fissures (for large sized). The area lying between two fissures is called gyrus and there is 2/3rds

of the cerebral collosum is these gyri. The two major fissures which are used as the

dividing/identifying borders are the Central fissure or the Fissure of Rolando separates the Frontal

from the Parietal Lobe and Temporal Fissure (or the Sylvian fissure) which separates the Frontal

from the Temporal Lobe. The Central Fissure divides Cerebral Cortex into the anterior-posterior

(frontal parietal).

There are several types of cortices the neo cortex- the newly evolved areas of cortex, and the allocortex

- the older cortex

The allocortex or the paleocortex (/archicortex) is a three layered older cortical structure subdividied

into the apleo cortex and the archicortex. These two have very close ties with the kinbic system and

the olfactory system.

Paleocortex: (Have the primary olfactory cortex and other areas)

Archicortex (consists of the hippocampal formation) this is similar to the human cerebral cortex in

terms of a) connections b) characteristics, c) kinds of neurons found in these areas.

Mesocortex: the middle cortex is found in the cingulate gyrus, parahippocampal areas and also between

the iso cortex and the allocortex

Neocortex: the new cortex, the iso cortex, consists of 6 layers which are more recent evolution:

organized in one-one: incoming in specific areas. Outgoing in others, association cortex takes care of

higher order functions

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Neurological Basis of Behavior (PSY - 610)

The higher the evolutionary scale, the greater the neocortical development to the extent that in humans

where cortical development is maximal, 90% of the cortex is neocortex. In the primitive or less

developed animals the cortical surface is smooth, whereas in the more evolved and sophisticated

animals the surface is rough and convulated. In rats the cortical surface is relatively smooth, in the

squirrel monkeys it is somewhat rougher and the cortex of the chimpanzees and humans increase in

convulations. In humans, rhesus monkeys or chimpanzees have a very large disproportionate, rough

surface of the cerebral cortex.

The cortex is organized in a one-to-one manner a) incoming information goes to specific areas ( most of

the cortex is receiving information) b) Outgoing information is sent out from motor cortex and other

relevant areas ( visual information sent out by visual cortex) c) Association cortex takes place of other

important and higher order functions.

Lobes of the cerebral Cortex:

Frontal Lobe: lies rostrally to the Central fissure, and caudally to the precentral fissure are the primary

motor area. The primary motor area is most important in movement of the body. The motor homunculus

is the dictionary of motor movements, where each motor movement and muscle is mapped. The body

parts are represented in well defined but a disproportionate manner. For, example the tongue and the

thumb gets greater representation as compared to the body torso and the extremeties ( depending on the

evolutionary importance of the areas) Electrical stimulation of specific areas in the primary motor cortex

leads to movement in the contralateral area of the body, and lesions lead to contralateral paralysis ( as

in stroke). Rostral to the precentral sulcus is the premotor area; this is involved in initiating of a

movement and changes in the already ongoing movement. Rostral to premotor is the Bordmanns area 8

which is has the frontal eye fields, (for conjugate eye movements). There are other important areas such

as the Brocas area which is important in speech articulation and production. If this area is lesioned it

leads to aphasia.

Parietal:lies caudally ( behind) the central sulcus, and primary somatosensory cortex is located here.

The sensory homunculus is mapped in the same manner as the motor humonculus (not in proportion to

the size of the body part, but in direct proportion to the needs). Stimulation of the areas leads to

sensations of tingling and numbness in the contralateral part. The parietal lobe is also involved in the

behavioral interaction of individual with the space around him. If lesioned these lead to sensory neglect

of contralteral space (e.g. the patient would shave contralteral half of face, eat half the plate, and not

respond to chairs and tables in contralateral half of damage). The parietal is also involved in object

recognition, and language comprehension.

Temporal: lies caudal to the lateral gyrus in the superior area and is primary auditory cortex,

Wernicke's area important for speech comprehension. The spoken language is comprehended here.

In the inferior temporal lobe, the perception of visual form and color is located (this is in the close

vicinity of the occipital and the parietal lobes.

Occipital Lobe: The primary and secondary visual cortex visual processing is carried out here. This is

laid out in very well organized layers, in Brodmanns areas 17, 18, 19. This is the striated cortex, i.e. the

layered cortex, where both the left and the right eye images get represented in these layers. Hubel and

weasel have identified coular dominance columns.

Cytoarchitectonics: The cellular architecture of the cerbal cortical layers. There are six layers in which

cortex can be divided (not on an all or none basis), but it is mainly in terms of the organization of cell

layers.

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Neurological Basis of Behavior (PSY - 610)

Table: Layers of cerebral cortex

Lay

Cytoarchitecture,

network

Area

Order

name

migration

Molecular layer

Fibers going in a Primary area for Oldest (cellular)

network

fewer synapses

interneurons and glial

cells

granular Dense packing of small Dendrites of the 5th wave of

External

layer

and medium pyramidal pyramidal

neuronal

cells and interneurons project to layer 5 development

from other layers.

and

other

extensions

deeper

Pyramidal cells which Dendrites send 4th wave of the

III

External Pyramidal

to neuronal

( medium and large)

increase in size as the extensions

layers 1, axons migration

cell layers deepens

extend to other

deeper layers in

the same and

contralateral

hemisphere

densely 3rd wave of

granular Pyramidal cells are Most

Internal

densely packed. There is packed, project neuronal

layer

and stellate and granular to deeper layers, migration

(

pyramidal

cells terminating in this thalamocortical

granular)

fibers end here

layer

Large and medium sized Lowest density 2nd wave of

Internal Pyramidal

pyramidal stellate cells. as cells sends neuronal

Betz cells ( apical out projections migration

to other areas.

dendrite)

Multiform layer

Varying shapes and

1st wave of

sizes short axons and

neuronal

dendrites

migration

The first layer contains cells with horizontal fibres and horizontal cells of Cajal

Granule cells- Short branching axons and amny dendritic branches

Pyramidal cells shaped like a pyramid, send axons to layers below cortex. They also have long

Apical dendrite which extend to other layers ( and even down to the spinal cord) therefore a bigger

cell body is need to energize the cell to send messages out for longer extensions and carry messages

effectively.

Cerebral Cortex: Two Lobes

There are two independent lobes connected with the commissures. Many researchers have worked on

the question do we have two brains or one. Are there two independently functioning brains or do they

coordinate as one. Research by Gazzaniga, Milner, Sperry and others has shown that the two

hemispheres are specialized for different functions, Speech in left hemisphere (first identified by Broca)

and spatial functioning in right hemisphere. Milner carried out the WADA TEST on patients, in this test

one side of the brain is anesthetized with slow infusion of sodium amytal through the carotid artey

(major artery of the brain). It was reported by Milner that all right handed persons have left hemisphere

speech dominance (92%), as their speech stopped with the anesthetization of the left hemisphere.

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Neurological Basis of Behavior (PSY - 610)

There are specific disorder known as Aphasias, which are language and speech disorders with left

hemisphere damage

The Apraxias are movement disorders which occur when patient is required to perform on a verbal

command and fails to do so, even though spontaneously it can be performed.

(Additional references, Graham 735, Gazzaniga, Pinel, Kolb and Wishaw)

In order to explain the cerebral differences, there are several theories of cerebral asymmetry: Levy

and Sperry state that there are two basic modes of thinking: the analytical (LH)/synthetic and the

(RH) more gestalts more organized therefore the neural circuitry is differently wired.

Both hemispheres are equipotential upto two years states Lenenberg, that each hemisphere can take

on any role of specialization. However, Kinsberg states that each child is born with specialized

functions of the two hemispheres (the planum temporale, in the temporal lobe is large in the left side

in the fetus)

The research is ongoing and continues to this day to identify the specializations and roles of the two

Hemispheres.

The complexity of the cerebral cortex and that of the mysteries of how each neuron adds up to the

behaviours we exhibit are an interesting ongoing journey.

References:

1. Kalat J.W (1998) Biological Psychology Brooks/ Cole Publishing

2. Carlson N.R. (2005) Foundations of Physiological Psychology Allyn and Bacon, Boston

3. Pinel, John P.J. (2003) Biopsychology (5th edition) Allyn and Bacon Singapore

4 Bloom F, Nelson and Lazerson (2001), Behavioral Neuroscience: Brain, Mind and Behaviors (3rd

edition) Worth Publishers New York

5. Bridgeman, B (1988) The Biology of Behaviour and Mind. John Wiley and Sons New York

6. Brown,T.S. and Wallace.(1980) P.M Physiological Psychology

Academic Press New York

Note: References 2, 3, 4, 7 more closely followed in addition to the references cited in text.

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