Physiology of Human Brain...

[PHARM]

The Central Nervous System

The CNS consists of the Brain and the Spinal Cord. The Brain is an extremely complex organ that can be said to give us Appreciation of Sensory Input; serves as the originator and coordinator of motor activity; and acts as the Repository for Experience, Intelligence, Moral and Social Behavior.

We will look at the Brain as a number of seperate parts that work together to control and moderate body activity. A summary of major Motor and Sensory Pathways will be made after the individual components are considered.

Parts Of The Brain

The largest portion of the Brain is the Cerebrum, accounting for more than four fifths of the total Brain weight. The Cerebellum is the next largest part, and the BrainStem, consisting of the Medulla, Pons, and MidBrain is the smallest.

The DienCephalon is a more or less seperate functional region that is usually described with the Cerebrum.




The Cerebrum

The Cerebrum is in the form of two halves (left and right), or Hemispheres, each of which bears many folds, or Convolutions.

Upfolds are termed Gyri, and shallow downfolds are called Sulci. Deep infoldings, called Fissures are found in several areas of each Hemisphere and subdivide the Hemisphere into Lobes.




The outer surface of the Cerebrum is covered with a 2.5 to 4.0 mm thick layer of Neurons containing Gray Matter, termed the Cerebral Cortex.

Most of the rest of the Hemisphere consists of Myelinated Fiber Tracts. The White Matter or Medullary Body.
The Cortex, because it contains the greatest number of Neurons, acts as the region giving Appreciation of Sensations, serves as the source of Motor activity, and contains areas responsible for Moral and Social Values.

Structure

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.

Functions

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.

The MidBrain

The MidBrain is the uppermost 1.5 cm or so of the BrainStem. Its anterior portion consists mainly of two large bundles of fibers called the Cerebral Peduncles.

The Peduncles carry Motor Projection Fibers from the Cerebrum to the Spinal Cord and to the Cerebellum.

Just posterior to the Peduncles lies the Substantia Nigra, mentioned previously in connection with the Basal Ganglia.

The posterior portions of the MidBrain contain the Red Nucleus, a group of Neuron Cell Bodies that give rise to a Motor Pathway (RubroSpinal Tract) conveying impulses concerned with Muscle Tone to the Skeltal muscles, and the Colliculi.

The BrainStem

The Superior Colliculi receive impulses from the Occipital (Visual) Cortex of the Cerebrum for reflex movements of the Eyes, such as when following objects that are moving across the visual field. The Inferior Colliculi are part of the Auditory Pathway to the Cerebrum.
Some fibers pass to the Superior Colliculus, producing Eye movements in response to sound, such as when turning the eyes toward the source of a sound.

The Cell Bodies giving rise to the Third (Oculomotor) & Fourth (Trochlear) Cranial nerves lie in the MidBrain.

Lesions in the MidBrain typically affect Auditory and Visual reflexes and, if the Peduncles are involved, produce deficits in Voluntary Movement.

The Medulla Oblongata

Is the inferior 3 cm or so of the BrainStem. It is continuous through the Foramen Magnum of the Skull with the Spinal Cord.

Prominent in the Anterior portion of the Pons are the CorticoSpinal fibers that mostly cross over in the Medulla and continue down the cord as the CorticoSpinal Tracts involved in Voluntary Movement.

Several obvious Nuclei (Gracile, Cuneate) are present in the Posterior part of the Medulla. They are areas for Synapses of Ascending Pathways carrying Sensory Information. The Nuclei of Cranial Nerves 9 (Glossopharyngeal), 10 (Vagus), 11 (Accessory), and 12 (Hypoglossal) lie in the Medulla.

Vital Centers

Some portions of the Cranial Nerve Nuclei form the so called Vital Centers of the Medulla. These centers include the following:

Cardiac Centers

CardioAcceleratory and CardioInhibitory centers are basically Reflex Centers, receiving impulses that rise in receptors in several body areas and sending impulses to the Heart to regulate its rate of beat according to the need for Oxygen and activity levels.

Respiratory Centers

Inspiratory, Expiratory, and Apneustic Centers and Vagal Nuclei form part of the system responsible for Intake of Air into the Lungs and further Expiratory Activity.

VasoMotor Centers

VasoDilator and VasoConstrictor Centers deal with the Diameter of Muscular Blood Vessels and thereby control the Blood Pressure.

All of these Vital Centers are intergrated in their activity so that they complement one another to achieve a desired end result.

During exercise, for example, Heart rate increases, VasoConstriction increases Blood Pressure, and Breathing is stimulated to increases Oxygen intake and Carbon Dioxide elimination.

Also within the Medulla, and usually including portions of the Medullary Cranial Nerve Nuclei, are the Non-Vital Centers for Intergration of the Acts of:
.Swallowing
.Vomiting
.Sneezing
.Coughing
Lesions in the Medulla are especially serious if they involve the Vital Centers. Before the advent of Polio Virus vaccines, damage to the Inspiratory Center by the Virus resulted in Respiratory Paralysis and required the patient to be placed in an iron lung in order to breathe.

Hypotension (Low Blood Pressure) can result from failure of the VasoConstrictor Center.