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Physiology Of The Human Body

The Brain

Parts Of The Brain | Structure | Functions | Cerebrum
Basal Ganglia | MidBrain | BrainStem | DienCephalon
EpiThalamus | Thalamus | HypoThalamus | Pons
Medulla Oblongata | Vital Centers | Reticular Formation


The Central Nervous System

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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.

This chapter 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.   (View Image)

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.   (View Image)

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

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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.

Frontal Lobes

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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

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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.

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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

The term Medullary Body refers to all of the Myelinated Fibers (White Matter) of the Cerebrum. The Basal Ganglia are large masses of Gray Matter buried within the Medullary Body.

    Three types of Fibers compose the Medullary Body

1 - Commissural Fibers
Form the only means of connecting the two Cerebral Hemispheres to permit transfer of information between them. The Corpus Callosum is the major bundle of Commissural Fibers, estimated to contain 300 million fibers. The Anterior, Posterior, and Habenular Commissures are much smaller bundles.

2 - Projection Fibers
Are bundles of fibers that enter the Cerebrum from outside or originate within the Cerebrum and leave it. Ex. are the incoming Auditory and Visual projection fibers and the outgoing Motor pathways to Skeletal muscle.

3 - Association Fibers
Connect different parts of the same Hemisphere. These types of fibers enable one type of information to affect an entirely different part of the Cerebrum such as in Motor responses to Visual or Thermal stimuli.

The Basal Ganglia

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Include the Cadate Nucleus, the Putamen, the Globus Pallidus, the Claustrum, the Subthalamic Nucleus, and the Substantia Nigra.

The major connections that the Ganglia make are with Cerebral Cortex and Thalamus. (View Image)

An internal circuit is formed from Cerebrum to Ganglia to Thalamus and back to Cerebral Motor Areas. the Ganglia are thus postulated to influence the Motor Cortex.

The best clues about Basal Ganglia function are given by individuals who suffer lesions in particular portions of the system.

In general, it would seem that the Ganglia are concerned with suppression of certain types of Motor function that would destroy the purposeful nature of Motor Activity.

Chorea
is a rapid, semipurposeful movement, usually restricted to the distal extremity and facial muscles. Lesions within the Caudate or Putamen result in Chorea.

Sydenham's Chorea
develops secondarily to Rheumatic Fever. Huntington's Chorea is a degenerative hereditary disease that results in loss of Neurons, particularly in the Caudate Nucleus.

Ballism
is an involuntary and violent failing movement involving proximal muscles of the appendages. The SubThalamic Nucleus is most often involved in this disorder.

Athetosis
refers to slow, twisting movements of the extremities. Lesions in the Putamen are most often associated with this disorder.

Parkinson's Disease is the most common disorder associated with the Ganglia. The body muscles show rigidity, there is Involuntary Tremor and postural fixation of the neck and trunk, and equilibrial reflexes are impaired.

An association has been demonstrated with the Substantia Nigra and a low level of NorEpinephrine (norE) transmitter.

Administration of a precursor of norE called L-dopa (Levo-3,4-Dihydroxphenylalanine) can relieve many of the symptoms of the disease but doesn't arrest progression of the disease.

The DienCephalon

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EpiThalamus

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.

Thalamus

The paired Thalami constitute about 80% of the DienCephalon. In about 70% of all Brains that have been examined, the two Thalami are connected across the Midline by an Intermediate Mass.

The Thalamus consists of groupings of Neurons called Nuclei and appears to function mainly as a relay station to the Cerebrum for all varieties of Sensory Input Except Olfaction. It may also give Crude Awareness of Pain.

Thalamic Nuclei Classification Scheme:

The Reticular Nucleus

Receives Fibers from the Cerebrum, sends Fibers to other Thalamic Nuclei, and seems to Modify Neuronal activity in the Thalamus.

The Midline Nuclei
Receives impulses from the Viscera and Taste Buds and sends Fibers to the HypoThalamus for Visceral responses. There are no direct connections to the Cerebrum.

The Specific Thalamic Nuclei

Medial Geniculate Body
This nucleus is part of the Auditory Pathway, receiving fibers from the Inferior Colliculus and the Organ of Corti. It sends fibers to areas 41 & 42 of the Cerebrum.

Lateral Geniculate Body
This Nucleus lies on the Visual Pathway, receiving impulses from the Optic Tract and sending impulses to the Visual Area 17 of the Cerebrum.

Ventral Posterior Nucleus
This Nucleus receives fibers from the Spinal Cord, carrying Pain, Tactile, and Thermal Sensations, and via the Trigeminal (Fifth Cranial) Nerve from the Head for the same sensations. Fibers are sent to areas 3, 1, and 2 of the Cerebrum.

Ventral Lateral & Ventral Anterior Nuclei
Both Nuclei receive fibers from several of the Basal Ganglia and the Cerebellum and send fibers to the Motor and Premotor Areas of the Cerebrum. These are clearly Motor Nuclei.

NonSpecific Thalamic Nuclei

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Describes Nuclei that send Fibers to the Association Areas of the Cerebrum. They receive Sensory Information and send it to Cerebral areas leading to Expression of Emotion, Memory Storage, Moods, and Feelings.

The Thalamic Syndrome
Is the result of lesions within the Sensory Portions of the Thalamus. There is initially little or no awareness of a sensation until a critical point is reached, whereupon the sensation is suddenly appreciated, usually in a disagreeable manner. There may be spontaneous and often excruciating Pain.

The HypoThalamus

    Weighs about 4g and is considered to be composed of a variety of Nuclei organized into three major areas:
  • The SupraOptic Area above the Optic Chiasm
  • The Tuberal Area above the Pituitary Gland
  • The Mammillary Area above the Mammillary Body

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.

The major functions of the HypoThalamus are concerned with regulation of those Homeostatic Functions related to Organism Survival.  ( Also See: HypoThalamus View: HypoThalamus Image)

Temperature Regulation

Human 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 Gain

The 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.

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Nonmuscular increase of basal heat production is determined by increase in secretion of Thyroxin and Epinephine via HypoThalamic mechanisms.

The MidBrain

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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.
(Also See: MidBrain)

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 Pons

Is about 2.5 cm in length is easily recognized by a large mass of fibers forming a conspicous bulge in its Anterior Aspect.

The bulge is termed the Basal Pons and consists of fibers descending to the Spinal Cord (CorticoSpinal Fibers) and fibers passing from it synapses in the Pons to the Cerebellum (PontoCerebellar Fibers).

The latter pathway notifies the Cerebellum of what the Cerebrum intends during Voluntary activity and is part of a system that the Cerebellum employs to coordinate and refine muscular activity.

The Posterior portion of the Pons, called the Tegmentum, contains large bundles of Sensory Fibers ascending to the Thalamus and the Nuclei of Cranial Nerves 5 (Trigeminal), 6 (Abducent), 7 (Facial), and 8 (VestibuloCochlear).

A Respiratory Center known as the Pneumotaxic Center lies within the Pons. It is part of the control mechanism that permits outflow of air (Expiration) from the Lungs.

The Medulla Oblongata

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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.

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.

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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.

The Reticular Formation

Located in the BrainStem and seperate from the Nuclei already mentioned are a columnlike series of Nuclei that constitute the Reticular Formation. The rather complicated names of these Nuclei need not trouble us here. The function of the Formation is important.

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The Nuclei of the Reticular Formation receive input from most of the Sensory Systems of the body and also from Cerebral Motor Regions.

Stimulation of The Formation causes, fibers that pass to the Thalamus and then to the Cerebral Cortex, Activation and Alertness of the Cortex in general. If one is asleep, there is immediate awakening.

The upper part of the Formation plus the pathways to Thalamus and Cortex are called The Reticular Activating System, which aids in maintaining the conscious state.

Drugs that make us sleepy and others that keep us awake appear to have a primary effect on this system.

Coma, a state of unconsciousness from which even the strongest stimuli cannot arouse the patient, seems to require Reticular Formation involvement, particularly of the ThalamoCortical Pathways.

(Also See: The Reticular Activating System)

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