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HANDBOOK OF I'HYSIOLOGY 



NEUROPHYSIOLOGV I 



FIG. 12. Diagrammatic representation of intricate pathways 

 through which the primary olfactory cortex may be brought 

 into relation with certain 'rhinencephalic' structures and cer- 

 tain regions of the diencephalon and midbrain. The stria 

 terminalis (S.T.^ arises in the amygdala and terminates in 

 part in the hypothalamus (HTP.'). The amygdala probably 

 also has more direct hypothalamic connections and is also 

 connected with the septum (SEPT.) through the diagonal 

 band of Broca (D.B.B.}. The fornix bundles may convey ac- 

 tivity in both directions between the hippocampus and the 

 septum, the anterior thalamus (.4) and hypothalamus, which 

 terminates partly in the mammillary body (-V/). The mam- 

 millary body establishes anterior thalamic connections through 

 the mammillothalamic tract. The hippocampus (HIPP.) is 

 reciprocally connected with the entorhinal area (E.NT.AREA). 

 The tegmental nuclei {T.N.') and the periaqueductal grey 

 matter of the midbrain (M.B.) may receive fibers from the 

 entorhinal area through the stria medullaris (S.M.) and 

 through the periventricular fiber systems. The tegmental 

 nuclei may also be reciprocally connected with the mammil- 

 lary body through the mammillotegmental tracts and the 

 mammillary peduncle. 



Higher Order Olfactory Connections 



It is apparent from anatomical and physiological 

 studies that the primary olfactory cortex is only in- 

 directly connected with many of the cortical and 

 subcortical regions included in classical accounts of 

 the rhinencephalon. This is supported by the long 

 latencies of responses recorded in the more remote 

 regions. In the cat under pentobarbital anesthesia, 

 whereas responses appear in the ventral part of the 

 head of the caudate nucleus after 3.5 to 12.0 msec, 

 and in the hippocampal gyrus, subiculum and an- 

 terior end of the hippocampus after 7.0 msec, slower 

 responses are seen in the ventral parts of the hippo- 

 campus only after 10 to 33 msec, and in the caudal 

 and dorsal regions of the hippocampus after 1 7 to 

 38 msec. Responses appear in the stria medullaris 

 after 17 msec, and in the inammillothalamic tract 

 after 25 to 34 msec (22). 



Certain studies have emphasized the possible role 

 of such regions as the septum, the hippocampal 

 formation and the adjoining pyriform cortex in 

 mechanisms of alerting and emotional arousal. Al- 

 thotigh responses from olfactory stimulation can be 

 recorded in these regions, they are also accessible to 

 other sensory stimuli including those from tactile, 

 \isual and auditory modalities (46, 50, 57, 65). 

 Maclean ct al. (65) have recorded in the rabbit 

 regular rhythmic discharges at 13 to 20 waves per 

 sec. in the anterior pyriform cortex and in the hippo- 

 campal formation during respiration of smoke-filled 

 air. Similar responses were evoked by gustatory and 

 painful stimulation. They concluded that the hippo- 

 campus responds to olfactory stimuli in a less pre- 

 dictable manner and probably after a longer latency 

 than the pyriform area. They did not seek responses 

 in the posterior pyriform area (entorhinal area of 

 the hippocampal gyrus). 



Although a great body of evidence confirms the 

 role of the anterior pyriform or prepyriform region 

 as the major primary olfactory cortical area, path- 

 ways from it to adjoining rhinencephalic structures, 

 such as the hippocampus, may well be circuitous 

 (fig. 12). Thus neuroanatomical studies have indi- 

 cated that the amygdaloid nuclei project largely via 

 the stria terminalis i^undles to the hypothalamus 

 (3, 42, 44). The hypothalamus in turn establishes 

 connections with midline and intralaminar thalamic 

 nuclei (30, 70). Here further relays may pass via the 

 fornix to the hippocampal formation (49, 50), and 

 ultimately such activity may reach the midbrain 

 tegmentum via the entorhinal area and the stria 

 medullaris (2). Stimulation of the amygdala (48) has 

 indicated widespread subcortical projections from 

 both corticomedial nuclei (forming part of the 

 primary olfactory corte.x) and from basolateral nuclei. 

 Short latency responses (presumably monosynaptic) 

 appear in o\crlapping primary projection fields 

 covering the basomedial part of the telencephalon 

 and adjoining rostral pole of the diencephalon. From 

 here responses pass by short multisynaptic relays 

 through secondary projection fields which include a 

 central core of grey matter stretching from the hypo- 

 thalamus to the midbrain tegmentum. Short latency 

 responses from the corticomedial group extend 

 caudally into the hypothalamus further than those 

 following basolateral stimulation, but no tegmental 

 responses follow stimulation of the corticomedial 

 nuclei. This study confirms the role of the stria tenni- 

 nalis as an important efferent pathway from the 

 amygdala to the hypothalamus. 



