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Introduction to the Nervous System 3 Consciousness is the awareness of sensation. It is a phenomenon associated with the excitation of cerebral cortical neurons*. Therefore, stimuli that are consciously appreciated must give rise to an excitatory state that reaches the cerebral cortex. These stimuli are relatively few in number: Olfactory, gustatory, pain, touch, pressure, temperature (electromagnetic radiation of the infrared spectrum), light (electromagnetic radiation of the visible spectrum), auditory, stimuli that act on the vestibular and ampullary receptors of the internal ear, and proprioceptive (joint and muscle sense) stimuli. It turns out that, with the possible exception of stimuli that excite vestibular and ampullary receptors, each of these modalities results in excitation of neurons in a specific area of the cerebral cortex, which is designated accordingly: Visual cortex, auditory cortex, somatosensory cortex, etc. *Some texts state that low levels of consciousness are present in the thalamus. Fig. 1. Canine brain, lateral view. Inset shows the brain with left hemisphere removed. cruciate sulcus cruciate sulcus somatosensory cortex motor cortex Fig. 2. Canine, left cerebral hemisphere, lateral view, showing sensory and motor areas of the cerebral cortex. From Lehrbuch der Anatomie der Haustiere, Band IV; Nickel, Schummer, Seiferle, G. Böhme, Ed., 1992; Verlag Paul Parey. cruciate sulcus Fig. 3. Canine, left cerebral hemisphere, medal view, showing sensory and motor areas of the cerebral cortex. From Lehrbuch der Anatomie der Haustiere, Band IV; Nickel, Stimuli act on receptors Schummer, Seiferle, G. Böhme,of the body. Afferent and interneurons provide the which theParey. excitatory state extends from receptors to the cerebral Ed.,path 1992;by Verlag Paul cortex. Interruption of any part of this pathway or loss of receptor function or of the specific cortical function results in loss of the sensation mediated. Consciousness and the reticular activating system. What happens when receptor input is lost owing to injury, disease, or other cause? Since all efferent output is due to input, if all input were erased, presumably all output would be lost. The animal would probably not die; for the heart would continue to beat and other functions of smooth muscle and gland would not be lost. These autonomic effectors would lose their neural regulation but would continue to function. The reticular formation. In contrast to the large aggregations of cell bodies within the cns, designated nuclei and grey matter, and the aggregation of axonal fibers, designated tracts, a dispersed network of neurons, observed under the light microscope as a meshwork of cell bodies and processes, is designated reticular formation. The reticular formation extends from the thalamus of the diencephalon to the caudal spinal cord. The length of the cord it is a lateral, weblike extension of the grey matter (dorsal to the lateral horn of the grey matter of the thoracic cord) in relation to the lateral funiculus. Within the brainstem, it appears as an irregular and diffuse feltwork, filling the spaces between the larger nuclei and tracts. In the thalamus of the diencephalon, it consists of nuclei in relation to the external and internal medullary laminae. The laminae are thin layers of medullated fibers that separate major thalamic nuclei. In the brainstem, smaller, ill-defined, areas constituting the respiratory, cardiac, micturition, and vomiting centers are present within the reticular formation. reticular formation Fig. 4. Spinal cord x-section. From Handbuch der Vergleichenden Anatomie der Haustiere by Wilhelm Ellenberger and Hermann Baum; revised 1943, by Otto Zietzschmann, Ernst reticular formation Fig. 5. Cross-section of the rostral pons at its junction with the mesencephalon. The right-half is myelin-stained, making the fiber tracts black; nuclear areas remain pale with this stain, providing contrast. The large grey area dorsally is made up of a felt-work of fibers and neuron cell bodies, the reticular formation. From Gray’s Anatomy, 18th US edition; 1920. The figure was taken from Wikipedia. The reticular activating system is an interneuronal system that maintains consciousness. I believe (key word; other than its description as an “arousal” system, there is little to describe its precise manner of function) that it functions in this way: Probably no neuron is excited by a single other neuron. Spatial and temporal summation of interneurons and efferent neurons generally results from the input provided by neighboring synaptic boutons of several or many neurons. This is undoubtedly true of cortical neurons (Note: As a neuron that begins and ends within the cns, cortical neurons satisfy our definition of inteneurons.) that, excited, yield the phenomenon of awareness, the consciousness of sensation. All afferent neurons are probably excitatory and, arriving at the CNS, give off collaterals that feed into the reticular formation. From the formation, there are generated excitatory pathways that excite and inhibitory pathways that inhibit succeeding neurons. A major excitatory pathway is designated the reticular activating system (RAS), which maintains sub-threshold excitation of cortical neurons and is responsible for consciousness. With the arrival of excitation provided by interneuronal pathways, which, as well-defined tracts, lead in more direct fashion from specific receptors, threshold is reached, the cortical neuron is excited, and the particular sensation is realized. A prominent part of the RAS is in the midbrain and midbrain lesions of animals may result in a comatose or semi-comatose state. Consider also how one normally prepares for sleep, which is a state of diminished consciousness. We do this by lowering receptor input: We turn off the light; we turn off the radio; we retire to the bed and thus diminish tactile stimulation. In this way, the input to the reticular formation and the activity of the RAS are substantially diminished. Consciousness is not erased, however, and a sufficiently strong stimulus (for example, the ring of the alarm) is consciously perceived. Note: From the pontomedullary reticular formation, important excitatory and inhibitory reticulospinal tracts pass to efferent neurons innervating striated muscle. Fig. 6. Diagram of proposed origin of the reticular activating system. Collateras of afferent neurons feed into the system, creating sub-threshold excitation of cerebral cortical neurons. reticular activating system (it extends from the thalamus through the brainstem and the length of the cord) Fig. 7. Diagram of a cerebral cortical neuron covered (this neuron is not “covered” but the cell bodies and dendrites of most interneurons would be entirely covered by boutons of excitatory and inhibitory neurons). The proposed mechanism is that with sub-threshold excitation maintained by the RAS, the arrival of excitation generated by specific receptors results in threshold excitation of specific cortical neurons and awareness of the stimulus. Memory, learning, and thinking are cortical functions. Awareness of what is seen, heard, touched, and otherwise experienced is a necessary prerequisite of memory, learning, and thinking, which of course determine much of behavior. Memory, learning, and thinking are all cortical, interneuronal, functions and have their effect mediated by interneuronal pathways that are ultimately expressed in excitation and inhibition of efferent neurons. Mechanisms of attention by which the effect of some receptors is accentuated and that of others suppressed probably (key word) are accomplished in part by efferent innervation to (and, presumably, inhibition of) receptors. The hair cells of the internal ear have been shown to receive efferent innervation, and such efferent innervation of receptors is likely present elsewhere. To the writer’s knowledge, the function of these efferents to receptors is not established. The motor cortex is the area of the cerebral cortex at which initiation of voluntary motor activity takes place. In all the species that we study, and in humans, the motor cortex is located immediately anterior to the somatosensory cortex. Voluntary, deliberate, motor activity is the result of processes of memory, learning, and thought. It is distinguished from involuntary, reflex, activity that, with the exception of olfactory reflexes, does not traverse the cerebral cortex. All stimuli of which the animal is aware give rise to impulses, the excitatory state, that pass to specific areas of the cerebral cortex. Conditioned on the stimuli, interneuronal processes of memory and learning integrate this input within the cortex, resulting in the initiation of deliberate motor activity in the motor cortex and the initiation of a response by skeletal muscle and autonomic effectors. Voluntary motor activity is modulated by subcortical nuclei of the cerebral hemisphere and the subthalamic nuclei of the diencephalon. Skeletal motor activity is coordinated by the cerebellum. Motor fibers descending from the cortex end on efferent neurons in the brainstem and spinal cord that supply skeletal muscle. These same motor fibers give off collaterals that synapse in pontine nuclei, aggregations of cell bodies between the fibers of the basis ponti. Axons of the cells of the pontine nuclei as pontocerebellar fibers collectively form the middle cerebellar peduncles and end in the cerebellar cortex. Descending motor fibers of the red nucleus of the midbrain give off collaterals to the inferior olivary nucleus of the medulla, which projects with olivocerebellar fibers to the cerebellar cortex. In this way, prospective voluntary movements are fed into the cerebellum. The cerebellum coordinates skeletal motor activity and determines the resting tone of muscle. It functions in the maintainance of normal posture and equilibrium (equi libra = equal balance). Every particle of the body mass is subject to the force of gravity. The force acting on each particle could be represented by a vector, the length of which is proportional to the force acting on the particle and the direction of which extends toward the force acting. These vectors can be added to yield a single vector, whose length is proportional to the total force acting on the body and which is directed toward the center of the earth from a point on the body, the body’s center of gravity. Owing to the displacement of its mass, every movement of the body necessarily alters the center of gravity and in the moving animal the center of gravity is continually changing. If there were no accommodating change in the tension of its muscles, the animal would be unable to maintain its posture and its equilibrium with respect to the supporting surface, the ground or floor. Every movement results in changes in the output to its muscles that determine its posture. This output, and the output that at the same time provides a smooth, coordinated, movement, is integrated in, and effected by, the cerebellum. The cerebellum receives input, impulses, from the neuromuscular and neurotendinous spindle receptors that give it information on the length of muscles and the rate of change in length of muscles and the tension of tendons; it receives input from all joint receptors. It also receives input from the vestibular and ampullary receptors of the middle ear, which are sensitive to the position of the head in space and to linear and angular acceleration. It has also been shown that there is input to the cerebellum from visual and auditory receptors. And it receives input by way of descending corticopontine, corticomedullary, corticospinal, and rubrospinal fibers. From this input, there follows an integration that establishes the necessary adjustments in output to efferent neurons supplying skeletal muscle to assure the maintenance of posture and the coordination of movement. All output of the cerebellar cortex is effected by the Purkinje cells of the cerebellar cortex, which act chiefly on deep cerebellar nuclei to bring about these adjustments. Consciousness is not a function of the cerebellum. Removal or destruction of the cerebellum leaves consciousness unaffected. Maintenance of posture, of equilibrium and coordination of muscle activity are functions of the cerebellum. In Nervous System 2 was a list of those actions and activities that are relatively easily examined. In boldface below are those most useful in examining cerebellar, muscle and joint receptors, and vestibular and ampullary internal ear function. Consciousness and behavior; Standing attitude (posture) and ambulation (how the animal moves); Animal’s action or lack of it in response to pain, tactile, visual and auditory stimuli; Facial expression, eye movement and resting position of the eyes; Appearance of pupil; Appearance of tongue, ability to swallow; Postural, limb, and anal reflexes; Atrophy of muscle, integrity of muscle reflexes.