The Longitudinal Neurologic Systems
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This is Chapter 3 from RC’s best-selling book:
“Basic Principles of Chiropractic Neuroscience”
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Chapter 3: The Longitudinal Neurologic Systems
This chapter succinctly describes the basic structure and function of the six major longitudinal systems; viz, the sensory, motor, visceral, vascular, consciousness, and cerebrospinal fluid systems.
As we begin this chapter, it might be well for the reader to subjectively grasp the significance of the motor and sensory systems as far as possible. One exercise in this is to imagine that you had become unconscious and someone has placed you in a remote dark empty cellar, far beyond any source of environmental sound. The first thing you realize is that you are a total sensory and motor paralytic from the neck caudad. You are unable to move even a fingertip because your motor system is not functioning. Because there is no feeling, you do not know whether you are recumbent or tied in a chair. Your vision is normal, but there is no light. Your hearing is normal, but there is no sound. Your taste buds are functional, but there is nothing to eat or drink. Your olfactory organs are functional, but there are no detectable odors. There is little left except thought and memory.
After a time in this predicament, thoughts undoubtedly arise such as, “I wish I had really looked at the beauty of the world when I had a chance. I wish I had listened to the music of the masters and even the birds in my backyard when I had a chance. I gulped down so many delicious meals. I had a beautiful garden, but I rarely took time to appreciate its design and fragrance. I even failed to take time to appreciate the texture of my own clothes. I was in such a hurry to go nowhere that was more important. I missed so much.”
The human nervous system is a marvel in organizing and adapting to internal and external environmental changes:
(1) The receptors and afferent neurons of the visceral and somatic input systems are necessary to detect internal and external environmental changes.
(2) The visceral efferent neurons and the muscles of the motor output system must be stimulated if action is to be taken.
(3) The integrative system serves as intermediary stations via a complex arrangement of interneurons whose synapses control impulse strength and signal direction from the sensory system to the motor system.
The visceral (autonomic, vegetative) system is organized in a manner that is similar to the somatic system, but there are some important differences. Both visceral and somatic systems have longitudinal afferent sensory and efferent motor components. These fibers interact at each horizontal level of the axial nervous system to provide the various reflex mechanisms necessary to maintain homeostasis. See Table 3.1.
Table 3.1. Levels of the Autonomic Nervous System
and Major Structures
The visceral system is far more independent from consciousness than is the somatic system. Three mechanisms especially provide this independence:
(1) All somatic reflexes are mediated in the CNS; visceral reflexes can also occur in the periphery.
(2) Many viscera are directly regulated by circulating hormones, but their respective glands are under neural control.
(3) Visceral receptors do not require external stimuli; eg, glands function without external control or conscious awareness.
Disorders within a longitudinal system may occur at any one or more of the horizontal levels; viz, the supratentorial, posterior fossa, spinal, and peripheral levels. When dysfunction occurs, symptoms usually arise as pain, pressure, tingling, numbness, weakness, paralysis, incoordination, nausea, dizziness, or altered states of consciousness.
THE SENSORY SYSTEM
The CNS derives information to determine its functions in voluntary and involuntary behavior by way of data from the body’s internal and external media. The following review of the sensory system is designed to underscore how such environmental information is detected and translated by receptors, transmitted to the CNS, integrated, and perceived by conscious and subconscious faculties.
Daube/Sandok classify afferent (sensory) impulses according to their origin and specificity. See Table 3.2.
Table 3.2. Classification of Sensory Impulses
Basic Types of Sensory Perception
- Superficial sensation includes such afferent signals as touch, pain, warmth, cold, two-point discrimination, weight differentiation (barognosis), and graphesthesia. This group would also include the special sensations of vision, smell, hearing, and taste.
- Deep sensation includes the afferent signals of deep pressure, joint position and movement, equilibrium, deep pain, and vibration (pallesthesia). This group would also include the unconscious perception of such physiologic sensory input as muscle length and tension, arterial blood pressure, central venous pressure, lung inflation, cranial blood temperature, blood and CSF pH, blood oxygen and carbon dioxide levels, and plasma osmotic pressure.
- Combined sensations include
(a) stereognosis, the ability to recognize familiar objects placed on the hand; and
(b) topognosis, the ability to localize and perceive cutaneous stimuli.
Classic literature often uses Head’s two groups in classifying cutaneous sensibility.
- Protopathic. These essentially comprise the crude, poorly localized sensations perceived at the thalamic level such as simple touch, pain, and severe changes in temperature. Following nerve injury, protopathic sensations return rapidly (7—10 weeks) during regeneration.
- Epicritic. These include sensations requiring cerebral participation for fine discrimination such as light touch, mild—moderate temperature changes, two-point discrimination, barognosis, and graphesthesia. Epicritic sensations return slowly (12 years) during regeneration or do not return at all.
Sensory Neuron Levels
The function of the transducer-like receptor organs is to convert chemical, mechanical, photic, and thermal stimuli into neuronal potentials that can be transmitted to and within the CNS by specific pathways so that they can be perceived, interpreted, and integrated. This process is usually (but not always) conducted, processed, and used over three orders of neurons: primary, secondary, and tertiary.
The dendrites of peripheral sensory neurons receive impulses from sensory receptors, and their unipolar cell bodies lie outside the CNS in a spinal root ganglion. There are no synapses in a posterior (dorsal) spinal ganglion.
Each ganglion cell has a single nerve process (dendraxon) that divides so that the distal branch enervates the receptor and the proximal branch enters the CNS by a posterior root of the spinal cord or brain stem and then through the posterolateral sulcus at the posterior root entry zone. The proximal axons of a primary neuron may synapse immediately with secondary neurons in the CNS or ascend within a tract (usually a white matter fasciculus) for a considerable distance before they synapse. See Table 3.3.
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