SOMATIC AFFERENT PATHWAYS

The designation somatic afferent is applied to those pathways of fiber tracts and intercalated nuclei that convey information from the wide array of receptors of various types that are scattered throughout the skin and the deeper somatic tissues.

The somatic afferent system is concerned with a variety of sensory modalities: touch, pressure, vibratory sensation, thermal sensation, pain, and the kinesthetic sensations relating to joint angulation and muscle tension. The primary neurons concerned with all these senses are located within the dorsal root ganglia of the spinal nerves (and corresponding ganglion of the tri-

Figure 8-40 A simplified conjectured diagram of the relay scheme of the limbic system. The fiber tracts indicated by dotted lines are bent laterally out of the plane of the drawing. 1, Olfactory bulb; 2, medial olfactory tract; 3, cingulum (in gyri supracal- losus and cinguli); 4, gyrus dentatus; 5, hippocampus; 6, fornix; 7, mamillary body; 8, hypophysis; 9, optic chiasm; 10, piriform lobe; 11, lateral olfactory tract; 12, mammillothalamic tract; 13, projection fibers entering the cingulum; 14, projection fibers to reticular formation.

geminal nerve where structures of the head are con­cerned), and their axons enter the central nervous system by way of the dorsal roots of the spinal nerves (and afferent root of the trigeminal nerve). The axons branch on entering the central nervous system. Some branches end on interneurons within the gray substance of the segment of entry or of an adjacent segment; these neurons in turn project on ventral horn cells of the same or neighboring segment, so completing the short neuron chain that provides the anatomical basis for local reflex responses.

Other branches of the primary axons connect directly, or through interneurons, with higher centers, thus pro­viding pathways that initiate more complex integrated responses. The ascending pathways concerned with most sensory modalities (including a fraction of those concerned with registering joint position) ultimately reach the somatosensory area of the cerebral cortex, providing the mechanism for conscious perception. None of these ascending pathways is entirely isolated from other parts of the brain; all are variously connected to other centers by collateral branches at different levels.

The Lemniscal System

There are two large ascending pathways that enter con­sciousness. One, termed here the lemniscal system though other names are used, is followed by impulses that provide for a high degree of spatial discrimination of touch, for accurate assessment of the intensity of pressure, for repetitive vibratory sensation, and for a part of joint proprioception. The initial link in this pathway is provided by the chief branches of the axons of the primary sensory neurons that enter the cord (Figure 8-6). These pass at once to the dorsal funiculus of the cord, where they adopt a very orderly arrange­ment (Figure 8-18); those that enter through sacral nerves occupy the most medial positions, while those that enter at more cranial levels assume progressively more lateral positions. A glial septum that appears within the dorsal funiculus at midthoracic level divides it into two parts: the medial division, which constitutes the gracile fasciculus, contains fibers from the hindlimb and caudal trunk; the lateral division, the cuneate fas­ciculus, contains fibers from the forelimb, the cranial part of the trunk, and neck. Both tracts end within like- named nuclei of the dorsal part of the medulla oblon­gata, where they raise slight surface elevations, the gracile and cuneate tubercles (Figure 8-23/13,14).

The somatotopic organization of this pathway is pre­served throughout its length, including the thalamic nucleus and the cortex. The cortical representation is of contralateral parts of the body and reflects the generos­ity of their sensory innervation, not their absolute sizes. There is also some segregation by modality.

Figure 8-41 The lemniscal (black) and extralemniscal (white) projections from the trunk and head to the telencephalon. d, Decussation; LCV lateral part; MCV medial part of the caudoventral thalamic nucleus.

Figure 8-42 A simplified scheme of the extralemniscal pro­jections ascending from the spinal cord to the telencephalon. The uninterrupted black and white lines represent the projec­tions within the lateral system; the interrupted black lines represent the bilateral and multisynaptic projections within the medial system.

The Extralemniscal System

The extralemniscal system conveys a second group of somatic afferent modalities, characterized by slower propagation and less precise localization of the origi­nating stimuli. The information conveyed relates to the cruder varieties of touch and pressure, to temperature, and to pain. The primary axons of this system end on neurons of the dorsal horns within a segment or two of entry. The information is processed via several interneu­rons before leaving the dorsal horn (Figure 8-6). The axons of the second-stage neuron then pass into the white substance of the cord and ascend to higher brain centers. The projection of pain signals from the spinal cord to the brain occurs via multiple ascending systems, which can be divided into medial and lateral groups by their projections.

The tracts of the medial group tend to project into the core of the neuraxis to the level of the limbic system. The group comprises the spinothalamic tract (see Figure

Figure 8-43 Three-dimensional representation of the extralemniscal projection in the dog. 1, Spinal nerve; 2, dorsal horn of spinal cord; 3, spinothalamic tract; 4, trigeminal nerve; 5, nucleus of the spinal tract of the trigeminal nerve; 6, medial part of the caudoventral thalamic nucleus; 7, somatosensory cortex.

8-42) that projects into the medial and intralaminar thalamic nuclei, the spinoreticular tract composed of fiber bundles located bilaterally within the ventral and ventrolateral zones of the spinal white substance and ending in the reticular formation of the brainstem as far rostrally as the diencephalon, and a loosely orga­nized group of propriospinal pathways that originates and ends in the spinal gray substance and forms a mul- tisynaptic ascending fiber system. The medial group, in contrast to the lateral, shows little variation among vertebrates.

The lateral group also comprises tracts projecting onto the medial MCV and thence to the neocortex: the spinothalamic tract, the spinocervicothalamic system, and the second-order dorsal column pathway (Figures 8-42 and 8-43).

The (neo)spinothalamic tract constitutes the classi­cal pain tract of primates, including humans. It is entirely crossed and ascends on the ventrolateral aspect of the ventral horn toward the MCV.

The spinocervicothalamic system is well developed in subprimate mammals, particularly carnivores. Sec­ond-order axons ascend ipsilaterally as the spinocervi- cal tract, which occupies the dorsolateral quadrant of the white substance and ends in the lateral cervical nucleus, located at the junction of the spinal cord and brainstem. The axons that emanate from this nucleus cross the midline and follow the medial lemniscus to end in the MCV, where they overlap the projection site of the (neo)spinothalamic tract.

The third system has been found in cats. It is com­posed of second-order axons that, surprisingly, ascend through the dorsal columns; in addition to being non­nociceptive, these are mainly composed of primary afferents. The postsynaptic, pain-conveying axons end in ipsilateral dorsal column nuclei. The third-order axons that cross the midline also run to the MCV. Sec­ond-order trigeminal axons arise from the caudal part of the descending trigeminal nucleus. Axons either join the lateral system and ascend to the MCV or join the medial system into the thalamic reticular formation. The third-stage axons project on an area of the somato­sensory cortex rostral to the area allocated to the lem­niscal system.

Models have been proposed to explain the respective roles of the lateral and medial pain-signaling systems in the generation of pain sensation and behavior. It has been proposed that the lateral and medial systems con­tribute differentially to the psychological dimensions of pain experience: one suggestion is that the lateral system conveys information regarding the sensory-discrimina­tive dimensions of pain, whereas the medial is mainly involved in the motivational-affective dimension via the reticular formation, medial thalamus, and limbic system.

Other Ascending Pathways

Ascending pathways transmit information—from muscle and tendon receptors—of which there is no con­scious appreciation. The pathways commence in the usual way with primary axons that terminate on dorsal horn cells within the initial and adjacent segments. The axons of the second-stage neurons associate in dorsal and ventral spinocerebellar tracts (Figure 8-18Z5,d), which follow separate routes to their projections on the cerebellar cortex. The dorsal tract takes a direct ipsilat­eral pathway that enters the cerebellum through the caudal peduncle; the information it conveys is obtained from stimulation of muscle spindles. In contrast, the ventral spinocerebellar tract is mainly concerned with transmitting information provided by tendon receptors. The fibers of this tract decussate within the cord close to their origins; they then ascend to midbrain level before they turn back to enter the cerebellum through the rostral peduncle. A second decussation within the cerebellar medulla restores the fibers to the side of the origin of the stimulus before they terminate within the cerebellar cortex. These two tracts are concerned only with information from the trunk and hindlimb; the equivalent representation of the forelimb follows a dif­ferent pathway that will not be described.

A further diffuse ascending pathway is provided within the reticular formation, the subject of the follow­ing section. It provides a means for integrating informa­tion conveyed by the pathways previously described with information from other afferent systems, somatic and visceral, general and special.

The Reticular Formation

The reticular formation extends from the spinal cord throughout the brainstem as a diffuse arrangement of neurons interspersed with fiber tracts. In the evolution­ary sense it is an old system.

Despite the impression of diffusion and lack of orga­nization it initially creates, closer analysis permits the recognition of numerous nuclear aggregations of varying size and architectonic character; some are suf­ficiently distinctive for their homologues to be recogniz­able in different species.

The reticular formation is connected to all projection systems within the central nervous system, whether afferent or efferent, and has reciprocal connections with the major integration centers within the brain. Thus, among its many ascending, descending, and transverse connections, there are such tracts as reticulocerebellar and cerebelloreticular, reticulothalamocortical and cor- ticoreticular tracts. The inescapable inference is that the reticular formation plays an important role in modulat­ing the activities of these integration centers.

The reticular formation occupies a large part of the brainstem; it is spread within the core, and when it reaches the thalamus, it contributes some of the nuclear groups of this complex structure. It also extends into the cervical part of the spinal cord.

The formation may be divided into parts distin­guished by their morphology. The medial part, the periventricular gray, is arranged mainly in relation to the ventricular system of the brain. It has proved impossible to analyze in detail but appears to provide multisynaptic pathways composed of an indeterminable number of neurons with short and much-branched processes.

The second component exhibits a more obvious orga­nization with more readily identifiable nuclei and tracts. It is restricted to the brainstem, extending from the floor of the medulla oblongata through the midbrain to the “reticular” nuclei of the thalamus. The reticular nuclei of the thalamus receive an input from lower parts of the formation and project diffusely on the entire neopal­lium. The spinoreticulothalamic tract, an important component of the system, may provide an alternative or complementary route to the spinothalamic system. The spinoreticulothalamic tract commences with the projection of primary afferent neurons on neurons within the dorsal horn. It contains axons that project for long distances and conduct more rapidly than those found in the spinothalamic tract.

One extensive ascending pathway that ultimately projects beyond the thalamus to the cortex is known as the ascending reticular activating system. It receives an input through collateral branches from all sensory systems, whether exteroceptive or enteroceptive (Figure 8-44). Its activation arouses the animal, making it more conscious of its circumstances and surroundings; dimi­nution of activity induces lethargy or sleep. The reticu­lar activating system has been regarded as the seat of consciousness, but most neurologists would assert that “there is no room or place where consciousness dwells.”

The reticular system also plays an essential role in motor control by means of a descending pathway that extends from the telencephalon to ultimate destinations on lower motor neurons of the brainstem and cord (p. 301).