The Horizontal Neurologic Levels
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This is Chapter 4 from RC’s best-selling book:
“Basic Principles of Chiropractic Neuroscience”
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Chapter 4: The Horizontal Neurologic Levels
and Related Clinical Concerns
This chapter describes the basic functional anatomy and clinical considerations of the horizontal aspects of the supratentorial, posterior fossa, spinal, and peripheral levels of the nervous system.
The reader should keep in mind that the various aspects of the nervous system as described in this manual (eg, longitudinal and horizontal systems) are only reference guides that are visualizations of a patient’s nervous system in the upright position. They can be likened to the lines of longitude and latitude on a globe of the earth.
Such systems do not exist physically, but they do serve as excellent mental grid-like tools (viewpoints) during localization and areas in which and from which relationships can be described. For example, although the longitudinal systems take a general vertical course within the spinal column there are numerous alterations and they actually become horizontal when decussating. While the horizontal levels are spatially placed in and extend from the CNS in a general segmental manner, they soon take a widely diffuse course as they project toward their destinations. Thus, references to longitudinal and horizontal levels are just general viewpoints.
It is helpful for study purposes to isolate the body into certain systems, as described above, organize systems into organs, organs into tissues, tissues into cells, and cells into their components. However, we should keep in mind that, physically and functionally, there is only one integrated body and it is more than the sum of its parts. And even the body cannot be thought of as truly separate from its external environment. Although we may do this for study purposes, it is a limited viewpoint.
The human body is the most intricate and efficient electric instrument in the universe. In a state of perfect health, every organ, tissue, and cell functions in perfect unison, one with another. The primary regulator of such activity lies within the CNS. Its higher component, the brain, is much more than a conglomerate of relay stations. It is a highly evolved data gatherer, data processor (selector and organizer), labeler (identifier), transformer, and transmitter of energy.
We do not actually “see” with our eyes, “hear” with our ears, “taste” with our tongue, “smell” with our nose, or “feel” with our fingertips. Our specialized receptor organs are only peripheral sensors for visual, sound, liquid, gas, and pressure stimuli by which impulses are sent centrally so that we can be aware of our external environment.
It is primarily within the brain where all types of nerve impulse frequencies are transformed from input frequencies and amplitudes to selected (consciously or unconsciously) output frequencies and amplitudes. It is within the brain where sensations are transformed into musculoskeletal dynamics and where the stimuli from light, sound, heat, touch, thought, and other action potentials are each in their turn transformed into other frequencies, each influencing body function (specifically and holistically) in either a beneficial or destructive manner.
From a basic perspective, it has been previously described that a nerve is a medium in which energy is expressed in various frequencies and amplitudes and directed by various external, internal, and coordinated cause-and-effect processes. The sum of frequencies and amplitudes of a nerve can be stated in terms of nerve “tone.” There are ranges of normal tone, and if these ranges are consistently subnormal or abnormal, the nerve can be said to be acting in an unhealthy manner for the situation at hand; ie, inhibited (blocked, suppressed, raised-threshold) or irritated (overactive, hypersensitive, lowered-threshold).
When nerve tone reflects itself upon an effector organ (eg, muscle, gland, vessel), a subservient body part is said to be expressing a certain tone. Such descriptors as spasticity and flaccidness in reference to muscles, parasympathicotonia and sympathicotonia in reference to the autonomic nervous system, or anxiety and depression in reference to psychologic states are just a few selected labels to describe particular states of overall voluntary muscle, autonomic, and psychic tone. Thus, in this context, the sum tone of the individual can be considered to represent the general health status of the individual.
In capitalized letters on the title page of what is generally believed to be his first book, Text-Book of the Science, Art and Philosophy of Chiropractic, D. D. Palmer, the father of the science of chiropractic, stated, “FOUNDED ON TONE.” On page 7, he continues, “Life is the expression of tone. In that sentence is the basic principle of chiropractic.” He went on to state that “Tone is the normal degree of nerve tension. Tone is expressed in functions by normal elasticity, activity, strength and excitability of the various organs, as observed in a state of health. Consequently, the cause of disease is any variation of tone ….”
THE SUPRATENTORIAL LEVEL
The neural structures located above the tentorium cerebelli are those of the supratentorial level, all which are derivatives of the primitive telencephalon and diencephalon. The major systems at this level are the cerebral hemispheres, the diencephalon, the telencephalon, the limbic system, and central portions of the optic and olfactory nerves.
The structures of the diencephalon, all of which are located between the cerebral hemispheres and the midbrain, surround the 3rd ventricle. The major structures of the diencephalon are the thalamus, hypothalamus, and epithalamus.
The Thalamic Body
The thalamus of each hemisphere is the largest structure in the diencephalon. Its prominent medial portion is called the pulvinar; its lateral oval swelling is called the lateral geniculate body. The centroposterior aspect of the thalamus is the main body of the thalamus. Its nuclei, arranged in anterior and posterior tiers adjacent to the lateral wall of the 3rd ventricle, serve as relay stations for the several ascending and descending tracts that were described in Chapter 3.
The Anterior Thalamus
That portion of the thalamus which lies between the thalamic body and midbrain is the anterior thalamus. It contains nuclei and passing tracts associated with the control circuits of the basal ganglia and the motor system. The subthalamic nucleus is the major nucleus of the anterior thalamus, and a lesion here produces hemiballismus.
The hypothalamus is considered by many to be the central homeostatic core of the autonomic nervous system. It serves as an integrater and regulator of consciousness, defensive emotions (eg, fear, rage), appetitive behavior (eg, hunger, thirst, sex drive), water metabolism, sleep, body temperature, endocrine and cardiovascular functions, and blood osmolality.
The hypothalamus is below and forward to the anterior thalamus and separated from the thalamus by the hypothalamic sulcus. Its major structures are the hypothalamic nuclei in the walls of the 3rd ventricle, tuber cinereum, and mamillary bodies. The hypothalamic nuclei and several tracts are part of the limbic system. Connections are made with all areas of the brain stem and cerebral hemispheres. Although the pituitary gland is not considered part of the hypothalamus, its stalk (infundibulum) attaches to the tuber cinereum. Some authorities consider the optic chiasm a part of the hypothalamus.
The epithalamus lies behind the thalamic body, above the anterior thalamus in the posterior wall of the 3rd ventricle, forming part of the roof of the diencephalon. It contains the posterior commissure and pineal gland. The latter secretes melatonin and endorphin containing hormones. The pineal gland begins to calcify after the age of 20 years. It then can serve as a central landmark, during roentgenography, that may be shifted by a space-occupying lesion.
The telencephalon is derived from lateral evagination of the rostral part of the embryonic neural tube. The major structures of the telencephalon are the basal ganglia, the subcortical white matter, and the cerebral cortex. These structures fill the entire cranial vault above the tentorium cerebelli and are separated by the falx cerebri.
The Basal Ganglia
The basal ganglia, which surround the ventricles, are three large masses of gray matter (caudate nucleus, putamen, and globus pallidus) located deep within the cerebral hemispheres, anterolateral to the thalamus. The synaptic complexes of the basal ganglia have important function as a control circuit in regulating muscle tone and motor behavior.
It is within the basal ganglia that many lesions of the extrapyramidal tract occur. Parkinsonism and involuntary movements (eg, athetosis, chorea) are typical examples. If the internal capsule becomes involved (eg, hemorrhage), contralateral spastic hemiplegia results.
The Subcortical White Matter
The large mass of white matter beneath the cerebral cortex that surrounds the basal ganglia, contains an array of neuroglia and dense unsynapsing myelinated tracts of afferent (corticipetal) and efferent (corticifugal) projectional neurons that interplay with cortical neurons and such subcortical structures as the basal ganglia, thalamus, hypothalamus, red nucleus, brain-stem reticular formation, cerebellum, and spinal cord (eg, direct activation pathway). This white matter also contains long and short association neurons, which connect ipsilateral hemispheric areas. Short association fibers (eg, subcortical) connect adjacent gyri, while the long fibers connect widely separated areas (eg, the cingulum, fornix, and fasciculi uncinate, arcuate, superior longitudinal, inferior longitudinal, and occipitofrontal).
Numerous transverse commissural neurons are also found in the subcortical white matter. They connect homologous areas of the two hemispheres. The largest commissural mass is the broad corpus callosum, which forms the roof of the lateral and 3rd ventricles.
The Cerebral Cortex
The outer neural surface of the five lobes of the brain consists of a relatively thin mantle of gray matter surrounding the subcortical white matter. It has millions of pyramidal and stellate (granule) cell bodies, organized into approximately six laminated layers, and associated vertically and horizontally running axons, dendrites, and blood vessels.
Specific functions are shown in Table 4.1.
As Guyton points out, we know less about the mechanisms of the cerebral cortex than we do almost all other parts of the brain, though it is by far the largest portion of the nervous system.
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