Many years ago, Korbinian Brodmann attempted to correlate cellular differences in different areas of the Cerebral Cortex with functional localization.
His numbered maps are widely used today when referring to the Cortex, even though there may not be exact structural functional correlations.
Within the Cortex, cells appear to be arranged in columns that allow lateral spread of impulses. Input usually proceeds vertically and then spreads to the sides to affect additional numbers of cells.
Following the designation of Cortical areas as devised by Brodmann, we may locate particular types of functions in each Hemisphere.
Much evidence about function is provided by direct Brain stimulation during surgical procedures or by associating symptoms of pathology with lesions of the Brain.
Our Brains, like our faces, contain basic regions but also like our faces, they are not identical to one another.
Lying in the Precentral Gyrus is area 4, designated as the Primary Motor Area. It is an area conferring voluntary control over movement in humans.
It is Somatotopically oriented; that is, different Cortical regions project ultimately to specific muscles in a particular body area.
It may be noted that the body is represented in an upside down fashion, and that the areas where complex movements are required (ex. hands & face) have larger areas of representation; hence the disproportionality of the body as represented by the Homunculus. (View: Homunculus Image)
Supplementary Motor Area occupies areas 24 & 31 on the medial aspect of each Hemisphere. Body representation is more crude here, stimulation bringing contraction of larger groups of muscles.
The Axons from cells in these Motor Areas constitute 40 to 45% of the fibers giving voluntary control over muscular activity. Lastly, Efferent Fibers control muscles primarily on the opposite side of the body, since they cross in the BrainStem.
Area 6 is designated as the PreMotor Area and provides input to area 4. Stimulation in area 6 causes contraction of muscles only if area 4 is intact.
It is particularly concerned with movements of the Head, Neck, and Trunk. Some degree of learned motor activity may lie in area 6, because lesions here interfere with performance even though no voluntary paralysis results.
Area 8 is called the Frontal Eye Field, and if stimulated it causes Eye movements of a scanning nature.
The Motor Areas described form a part of what is called Electrically Excitable Cortex, because stimulation results in obvious movement. The remaining portion of the Frontal Lobes gives no obvious movement or sensation if stimulated.
These areas, 9 through 12, are designated as the Frontal Association Areas. In animals, damage in these areas causes Hyperactivity and Excessive Emotional Display, suggesting an Inhibitory Function of the region.
In humans, there is great diversity in symptoms displayed as a result of lesions in areas 9 through 12.
Changes are most often seen in Personality, Emotional Reactions, ability to accept life's Responsibilities, Moral, and Social Concepts.
Broca's Speech Area
Broca's Speech Area is found in areas 44 & 45 of the Frontal Lobe. The area lies predominately in the Left Cerebral Hemisphere, regardless of handedness, and lies in close association with the Motor Areas concerned with the Lips, Jaws, and Tongue, regions important in Vocalization.
Lesions in these areas produce Alterations in ability to vocalize may cause Speech Arrest, and grossly interfere with ability to express oneself, even in the absence of vocal paralysis.
The Parietal Lobes
The Somesthetic or general Sensory Areas include areas 3, 1, and 2 located in the Postcentral Gyrus. That the area contains some Motor Function is evidenced by the fact that stimulation gives generalized and Nonskilled Movements.
The area represents the termination of those pathways dealing with general Sensation, the Sensation of Touch, Pressure, Pain, Heat, Cold and Joint and Limb Position.
As in area 4, there is a Somatotopic representation, with the body upside down and areas of greater density of receptors receiving larger representation. The areas provide localization of a stimulus but do not apparently discern much about the quality of the stimulus.
Areas 5 & 7 are termed the Parietal Association Areas and do provide information concerning stimulus quality. Differences in intensity of the stimulus, textural differences, and Spatial Discrimination (such as of the shape of an object placed in the hand) are provided here.
Connections between areas 5 & 7 and the Visual and Auditory portions of the Brain also exist. These types of connections permit interrelationship of several Sensory inputs.
The Occipital Lobes (Visual Area)
Or area 17 occupies a large part of the Occipital lobe, especially along the borders of the Calcarine Fissure.
Input to the area is provided by fibers ultimately originating in the Retina. Both Eyes are represented in both Lobes, with Central Vision Posteriorly and Peripheral Vision Anteriorly.
Anterior to area 17, on the Lateral side of a Hemisphere and above and below it Medially, are areas 18 & 19, called the Visual Association Areas. Relating past to Present Visual Experience, Binocular Vision, and Depth Perception are some of the Vision related functions handled by these areas.
The Temporal Lobes
The Auditory Area includes regions 41 & 42. The Cochleas of both Ears are represented in one Temporal Lobe. Areas 22 & 21 are Association Areas and are probably also major regions of Memory Storage generally.
Other Functional Areas
The Taste Area corresponds to Brodmann's area 43. The area is located in association with the general Sensory Cortex serving the Tongue and Pharynx.
A Vestibular Area (anterior 40, lower 3, 1, & 2) has been placed in the Temporal Lobe Anterior to the Auditory Area. Most of the fibers of the Vestibular System project to the Cerebellum.p95
There is evidence to support the contention that all functions are not equally represented in both Hemispheres.
We have already spoken of the Speech Area as being a Left Sided Region. This creates what is called a Left Cerebral dominance for Language Function.
The Right Hemisphere appears to be Superior in NonLanguage Functions such as Spatial Perceptions and Creative Functons associated with Art and Music.
The Medullary Body
This portion of the DienCephalon consists of the Pineal Gland (Body) and the Habenular Nuclei.
The Pineal Gland produces Melatonin, a chemical unique to this gland. It has been suggested that there's an AntiGonadoTropic function for the gland.
Tumors that destroy the cells of the gland may cause early Puberty; tumors formed by the cells themselves cause delayed Puberty. Light exerts an influence on the gland via a Retinal-MidBrain-Pineal Pathway.
The Habenular Nuclei are part of an Olfactory-Visceral System by which Visceral function may be altered by Olfactory cues.
See the discussion of the Limbic System for additional reference to these Nuclei.
All parts of the HypoThalamus appear to be connected to one another. Output passes via nerves or chemicals to the Pituitary Gland, by nerves to the Limbic System and MidBrain, and from there to body's Viscera.
Temperature RegulationHuman body temperature is normally maintained at 98.6 (+,-) 1.5 F. A more or less constant body temperature reflects a balance between heat production and conservation and heat loss.
It also implies that there are control mechanisms that keep the temperature within these narrow limits.
Heat GainThe source of body heat is its chemical reactions. A basal level of heat production is ensured by those reactions necessary merely to sustain life, and above basal levels are supplied by increased muscular activity and by an increase in the production of several Hormones.
As external temperature decreases, there's a gradual and continual increase in muscle tone. If this is not sufficient to maintain body temperature, shivering will occur.
Shivering is a series of rapid muscular tremors that can increase heat production several fold in a few minutes. The primary source of Nerve impulses to muscles that undergo shivering is the HypoThalamus.
Accompanying this increase of muscle tone is Cutaneous Vasoconstriction that shunts the blood away from the upper layers of the skin to prevent loss of heat through body surfaces.
If there was any degree of sweating, this is also decreased. Again, the nervous pathways responsible for these changes are controlled by the HypoThalamus.p100
Nonmuscular increase of basal heat production is determined by increase in secretion of Thyroxin and Epinephine via HypoThalamic mechanisms.