๐ฏ Objectives
The main objective of this lesson is to study:
- The Brain and the Peripheral systems ๐ง ๐
- Brain: Forebrain ๐, Mid brain ๐, Hind Brain ๐ functioning of each anatomical location in the CNS. Starting from the posterior located areas up to the anterior-most ๐
- Cerebral Cortex ๐ง : its layers ๐, Corpus Callosum ๐ค and the two hemispheres ๐
๐ Main Purpose
Students will come to relate the importance of location of various areas in the Diencephalon ๐ฏ and the Telencephalon ๐, their functions โ๏ธ and their relationship to other areas ๐. The students would also understand how control and modulation of behavior related to these neuroanatomical site takes place ๐ฎ.
๐ฏ 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 ๐ฅ and the Thalamus ๐ฏ
๐ฅ Hypothalamus
Hypothalamus: it lies at the base of the brain ๐ง , on both sides of the 3rd ventricle ๐ง. It is very small in size ๐ as compared to other brain areas but is very important in its role and function โ๏ธ. Hypothalamus is located immediately beneath the thalamus ๐ฝ and lies above the pituitary ๐.
Hypothalamus is part of the Limbic system ๐, and in all situations requiring a fight-flight ๐ฅ๐ or other survival responses โ ๏ธ. There are several important fiber bundles passing through the hypothalamus, including the MFB (Medial forebrain bundle) ๐ which carries major neurotransmitters ๐งช along with other fibers to the higher areas โฌ๏ธ.
โ๏ธ Superchiasmatic Nucleus (SCN)
Underneath the hypothalamus lies the superchiasmatic nucleus (SCN) โ๏ธ. The SCN plays an important role in the day night cycles ๐๐ i.e. diurnal cycles โฐ. Within this region we also see the mammillary bodies ๐ง which are important in memory ๐.
๐ฌ Hypothalamic-Pituitary Connection
The hypothalamus also secretes its own hormones ๐งช which act as releasing factors for hormonal secretions in the pituitary. The neurosecretory cells of the hypothalamic hormones are located near base of hypothalamus ๐ฝ, very close to the pituitary. Therefore hypothalamic-pituitary connection is important for the neural and endocrinal connections ๐. The brain ๐ง talks to the pituitary through the hypothalamus, and viceโversa. All the endocrinal glands communicate for fear ๐จ aggression ๐ก, temperature regulation ๐ก๏ธ, food ๐ฝ๏ธ and water ๐ง, and mating โค๏ธ.
๐ง Thirst and Feeding Regulation
The Anterior hypothalamus ๐ is important in thirst regulation ๐ง and there are important sensors which continuously monitor fluid (and isotonic saline levels) of neurons โ๏ธ. The Ventromedial (VMH) and Lateral Hypothalamus (LH) are involved in regulation of feeding ๐ฝ๏ธ. Research has shown that the VMH is the satiety center (have-enough-food-stop-signal) ๐, and the lateral hypothalamus is the center for initiation of feeding ๐ด. Hypothalamus important in all major survival and maintenance functions โ ๏ธ; it is related to the management functions of the body ๐ฎ and survival rather than intellectual functioning ๐ง .
๐ฏ Thalamus
Thalamus: this is the largest component of Diencephalon ๐ with an ovalish shape ๐ฅ; it has two lobes which are connected by the mass intermedia which go through the midline of the third ventricle ๐ง. The thalamus comprises of large clusters of nuclei ๐งฌ. The thalamus is a staging area ๐ญ, information received from sensory receptors systems and other lower areas and then it is projected to the relevant cortical areas ๐ง .
๐ก Sensory Relay Nuclei
Sensory relay nuclei: All afferent sensory input ๐ฅ is received classified ๐, sorted ๐ and then passed through the thalamus, these go from the receptor to the Thalamus which then sends the selected projections to the related cortices ๐ง . The Visual ๐๏ธ, auditory ๐ input goes through the lateral geniculate nuclei ๐ต and the medial geniculate nuclei ๐ข, and somatosensory sensory fibers go to the somatosensory cortices ๐ค. There is a reciprocal one to one relationship of these connections in the cortex ๐. If one fiber degenerates in the cortex, it would lead to atrophy of the relevant thalamic areas โ ๏ธ.
๐ Dorsal Group of Fibers
The Dorsal group of fibers is multimodal ๐ (that is it receives and sends input to different sensory areas). This receives input from within the thalamic regions ๐ฏ, and project to the association cortex ๐ง . There is another group of fibers in the thalamus the Intrinsic or the non-specific ๐. If this area is stimulated it leads to wide spread electrical discharge in the cortex and other areas โก, evidence that this bundle is important in electrical activity of the cortex ๐ง . It is involved in sleep ๐ด awakening โฐ as well as attention ๐. It is also important in affective behavior ๐, memory ๐ (there is severe amnesia after lesions to midline thalamic area โ ๏ธ). Thus, the thalamus is important in every aspect of behavior ๐ฎ.
๐ Telencephalon: The End Brain
The End brain comprises mainly of the areas of the cerebral cortex ๐ง and some parts of the Corpus striatum ๐, the limbic system ๐. The two hemispheres of the cerebral cortex and the other parts are connected to each other by commissures (bands of fibers connecting the left โฌ ๏ธ and the right โก๏ธ hemispheres).
๐ค Corpus Callosum
Corpus Callosum is the major band of fibers joining the two hemispheres ๐ค it is wide, white and visible to the naked eye ๐๏ธ. The commissures are mainly for interaction between the two hemispheres and for crossing over of information ๐ (so that the two hemispheres can coordinate decisions ๐ฏ).
๐ง Cerebral Cortex
Cerebral Cortex: (cortex means outer covering ๐ณ: bark of the brain). This is made up of layers of grey matter ๐ฉถ which covers the white matter โช. The thickness of the cerebral cortex varies from 1.5 to 4.5 mm ๐; the average is about 2.00 mm. It is thickest in the primary motor cortex ๐ช about 4.5 mm thick and thinnest about 1.5 mm at the primary visual cortex ๐๏ธ. It is laid out in a straight line; the total surface of the cerebral cortex would be about 20 Sq foot ๐. How does this fit into the cranium ๐ฆด, it is squeezed into folds? These folds are irregular convolutions 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 cortex is these gyri ๐.
๐บ๏ธ Major Fissures
The two major fissures which are used as the dividing/identifying borders are:
- Central fissure or the Fissure of Rolando โฌ๏ธโฌ๏ธ separates the Frontal from the Parietal Lobe
- Temporal Fissure (or the Sylvian fissure) โฐ which separates the Frontal from the Temporal Lobe
The Central Fissure divides Cerebral Cortex into the anterior-posterior (frontal parietal) โฌ ๏ธโก๏ธ.
๐ Types of Cortices
There are several types of cortices the neocortex ๐ - the newly evolved areas of cortex, and the allocortex ๐ฆด - the older cortex
๐ฆด Allocortex (Paleocortex/Archicortex)
The allocortex or the paleocortex (/archicortex) is a three layered older cortical structure ๐ฆด subdivided into the apleo cortex and the archicortex. These two have very close ties with the limbic system ๐ and the olfactory system ๐.
- Paleocortex: (Have the primary olfactory cortex ๐ and other areas)
- Archicortex (consists of the hippocampal formation ๐) this is similar to the human cerebral cortex in terms of a) connections ๐ b) characteristics โจ, c) kinds of neurons found in these areas ๐งฌ.
๐ Mesocortex
Mesocortex: the middle cortex ๐ is found in the cingulate gyrus, para-hippocampal areas and also between the iso cortex and the allocortex
๐ Neocortex
Neocortex: the new cortex ๐, the iso cortex, consists of 6 layers ๐ which are more recent evolution ๐งฌ: organized in one-one: incoming in specific areas ๐ฅ. Outgoing in others ๐ค, association cortex takes care of higher order functions ๐ฏ
The higher the evolutionary scale ๐งฌ, the greater the neocortical development to the extent that in humans ๐จ where cortical development is maximal, 90% of the cortex is neocortex โญ. In the primitive or less developed animals the cortical surface is smooth ใฐ๏ธ, whereas in the more evolved and sophisticated animals the surface is rough and convoluted ๐. In rats ๐ the cortical surface is relatively smooth, in the squirrel monkeys ๐ต it is somewhat rougher and the cortex of the chimpanzees ๐ฆ and humans ๐จ increase in convolutions.
๐ฏ Cortical Organization
The cortex is organized in a one-to-one manner:
- a) Incoming information goes to specific areas ๐ฅ (most of the cortex is receiving information)
- b) Outgoing information is sent out from motor cortex ๐ช and other relevant areas (visual information sent out by visual cortex ๐๏ธ) ๐ค
- c) Association cortex takes place of other important and higher order functions ๐
๐ง Lobes of the Cerebral Cortex
๐ Frontal Lobe
Frontal Lobe: lies rostrally to the Central fissure ๐, and caudally to the precentral fissure are the primary motor area ๐ช. The primary motor area is most important in movement of the body ๐. The motor homunculus ๐บ๏ธ is the dictionary of motor movements, where each motor movement and muscle are mapped. The body parts are represented in well-defined but a disproportionate manner. For example the tongue ๐ and the thumb ๐ gets greater representation as compared to the body torso and the extremities (depending on the evolutionary importance of the areas ๐งฌ) Electrical stimulation of specific areas in the primary motor cortex leads to movement in the contralateral area of the body โฌ ๏ธโก๏ธ, and lesions lead to contralateral paralysis โ ๏ธ (as in stroke ๐ฉบ). Rostral to the precentral sulcus is the premotor area ๐ฎ; this is involved in initiating of a movement and changes in the already ongoing movement ๐. Rostral to premotor is the Brodmann's area 8 which has the frontal eye fields ๐๏ธ, (for conjugate eye movements ๐). There are other important areas such as the Broca's area ๐ฃ๏ธ which is important in speech articulation and production ๐ฌ. If this area is lesioned it leads to aphasia ๐ซ.
๐ Parietal Lobe
Parietal: lies caudally (behind) the central sulcus ๐, and primary somatosensory cortex is located here ๐ค. The sensory homunculus ๐บ๏ธ is mapped in the same manner as the motor homunculus (not in proportion to the size of the body part, but in direct proportion to the needs ๐ฏ). Stimulation of the areas leads to sensations of tingling and numbness in the contralateral part โก. The parietal lobe is also involved in the behavioral interaction of individual with the space around him ๐. If lesioned this led to sensory neglect of contralateral space ๐ซ (e.g., the patient would shave contralateral half of face ๐, eat half the plate ๐ฝ๏ธ, and not respond to chairs and tables in contralateral half of damage). The parietal is also involved in object recognition ๐, and language comprehension ๐.
โฐ Temporal Lobe
Temporal: lies caudal to the lateral gyrus in the superior area and is primary auditory cortex ๐, Wernicke's area ๐ฌ important for speech comprehension ๐. The spoken language is comprehended here ๐ฃ๏ธ.
In the inferior temporal lobe ๐ฝ, the perception of visual form and color is located ๐จ (this is in the close vicinity of the occipital and the parietal lobes).
๐๏ธ Occipital Lobe
Occipital Lobe: The primary and secondary visual cortex ๐๏ธ visual processing is carried out here. This is laid out in very well-organized layers ๐, in Brodmann's areas 17, 18, 19. This is the striated cortex ๐, i.e., the layered cortex, where both the left โฌ ๏ธ and the right โก๏ธ eye images get represented in these layers. Hubel and Wiesel have identified ocular dominance columns ๐๏ธ๐๏ธ.
๐ Cytoarchitectonics
Cytoarchitectonics: The cellular architecture of the cerebral cortical layers ๐๏ธ. 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 ๐.
๐ Layers of Cerebral Cortex
| Layer | Cytoarchitecture Name | Network Area | Order of Migration |
|---|---|---|---|
| I | Molecular layer ๐ | Fibers going in a network fewer interneurons and glial cells | Oldest (cellular) ๐ฆด |
| II | External granular layer ๐ต | Dense packing of small and medium pyramidal cells and interneurons from other layers | 5th wave of neuronal development |
| III | External Pyramidal (Medium and large) ๐บ | Pyramidal cells which increase in size as the cell layers deepen. Dendrites send extensions to layers 1, axons extend to other deeper layers in the same and contralateral hemisphere | 4th wave of neuronal migration |
| IV | Internal granular layer (Pyramidal and granular) ๐ถ | Pyramidal cells are densely packed. There are stellate and granular cells terminating in this layer. Most densely packed, project to deeper layers, thalamocortical fibers end here | 3rd wave of neuronal migration |
| V | Internal Pyramidal ๐ป | Large and medium sized pyramidal stellate cells. Betz cells (apical dendrite). Lowest density as cells sends out projections to other areas | 2nd wave of neuronal migration |
| VI | Multiform layer ๐ | Varying shapes and sizes short axons and dendrites | 1st wave of neuronal migration |
๐งฌ Cell Types
- The first layer contains cells with horizontal fibers ใฐ๏ธ and horizontal cells of Canal
- Granule cells ๐ต- short branching axons and many dendritic branches
- Pyramidal cells ๐บ 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 ๐ฆด) therefore a bigger cell body is needed to energize the cell to send messages out for longer extensions and carry messages effectively โก.
๐ Cerebral Cortex: Two Lobes
There are two independent lobes connected with the commissures ๐ค. Many researchers have worked on the question do we have two brains or one ๐ง ๐ง ? Are there two independently functioning brains or do they coordinate as one ๐. Research by Gazzaniga, Milner, Sperry and others has shown that the two hemispheres are specialized for different functions ๐ฏ, Speech in left hemisphere ๐ฃ๏ธโฌ ๏ธ (first identified by Broca ๐จโ๐ฌ) and spatial functioning in right hemisphere ๐บ๏ธโก๏ธ.
๐งช WADA Test
Milner carried out the WADA TEST on patients ๐งช, in this test one side of the brain is anesthetized ๐ with slow infusion of sodium amytal through the carotid artery (major artery of the brain ๐ด). It was reported by Milner that all right-handed persons ๐คโก๏ธ have left hemisphere speech dominance (92%) ๐ฃ๏ธโฌ ๏ธ, as their speech stopped with the anesthetization of the left hemisphere.
โ ๏ธ Brain Disorders
There is specific disorder known as Aphasias ๐ซ๐ฌ, which are language and speech disorders with left hemisphere damage โฌ ๏ธ.
The Apraxias are movement disorders ๐ถโ which occur when patient is required to perform on a verbal command and fails to do so, even though spontaneously it can be performed.
๐ง Theories of Cerebral Asymmetry
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) โฌ ๏ธ/synthetic and the (RH) โก๏ธ more gestalt more organized therefore the neural circuitry is differently wired ๐.
- Lenenberg states that both hemispheres are equipotential up to two years ๐ถ, that each hemisphere can take on any role of specialization ๐.
- Kinsbourne states that each child is born with specialized functions of the two hemispheres ๐งฌ (the planum temporal, in the temporal lobe is large in the left side in the fetus ๐ถโฌ ๏ธ)
The research is ongoing and continues to this day to identify the specializations and roles of the two Hemispheres ๐ฌ.
The complexity of the cerebral cortex ๐ง and that of the mysteries of how each neuron adds up to the behaviors we exhibit are an interesting ongoing journey ๐.
๐ References
- Kalat, J.W. (1998). Biological Psychology. Brooks/ Cole Publishing
- Carlson, N. R. (2005). Foundations of physiological psychology. Pearson Education New Zealand.
- Pinel, J. P. (2003). Biopsychology. (5th ed). Allyn & Bacon Singapore.
- Bloom, F., Nelson., & Lazerson. (2001), Behavioral Neuroscience: Brain, Mind and Behaviors. (3rd ed). Worth Publishers New York
- Bridgeman, B. (1988). The Biology of Behavior and Mind. John Wiley & Sons, New York
- Brown, T.S. & Wallace, P.S. (1980). Physiological Psychology. Academic Press, New York