CHAPTER 13. THE LIMBIC SYSTEM AND HYPOTHALAMUS

OBJECTIVE:
    BE ABLE TO IDENTIFY IN GROSS SPECIMENS AND/OR BRAINSTEM SECTIONS AND WHEN POSSIBLE IN THE MRIs THE PARTS OF THE LIMBIC LOBE, THE COMPONENTS OF THE HIPPOCAMPUS AND THE PAPEZ CIRCUIT, THE AMYGDALA AND ITS MAIN CONNECTIONS, AND THE HYPOTHALAMUS AND ITS SUBDIVISIONS.

Limbic Lobe

Bordering the corpus callosum and rostral brainstem is the large arcuate convolution called the limbic lobe. Its components can be readily identified in brain specimens. Identify the cingulate gyrus (Pl. 5) above the corpus callosum, and the parahippocampal gyrus (Pl. 7), the most medial gyrus on the ventral surface of the temporal lobe.

Hippocampus

Identify the hippocampus (Pls. 15, 27, 28, 29, 33, 34, MRIs 4, 5, 6, 7, 11, 12, 13, 18) which extends from the parahippocampal gyrus to the floor of the temporal horn of the lateral ventricle.

Directly continuous with the entorhinal part of the parahippocampal gyrus is the subiculum a transitional zone of cortical tissue consisting of six layers where it meets the parahippocampal gyrus and three layers where it meets the hippocampus proper. The hippocampus proper folds on itself and is continuous with the dentate gyrus, both of which form the oldest parts of the cerebral cortex, the archipallium.
Papez Circuit

The hippocampus is the initial center in a reverberating pathway called Papez circuit.

Efferent fibers chiefly from the subiculum gather on the ventricular surface of the hippocampus proper as the alveus. The alveus becomes the fimbria of the fornix. Beneath the splenium of the corpus callosum, the fimbria leaves the hippocampus and becomes the crus of the fornix. The crus arches anteriorly and upon reaching the septum pellucidum becomes the body of the fornix which is suspended in the free margin of the septum pellucidum.

Anteriorly, the body of the fornix curves ventrally, enters the hypothalamus and proceeds posteriorly (Sls. 45, 44, 43, 42, 41, 40, 39) to the mamillary body (Sls. 37, 38).

Follow the mamillothalamic tract (Sls. 37, 38, 39, 40, 41, 42, 43, 44) from the mamillary body to the anterior thalamic nucleus (Pls. 25, 26, 31; Sls. 43, 44, 45, 46). Thalamocortical fibers from the anterior nucleus enter the anterior limb of the internal capsule and arch around the caudate nucleus to reach the cingulate gyrus. The cingulate gyrus projects to widespread areas of the frontal, parietal, and temporal lobes via the cingulum, an association bundle deep to the gyrus.

Through the cingulum impulses reach the parahippocampal gyrus whose entorhinal area (Sl. 50) located near the subiculum contains the input neurons to the hippocampus, thus completing the circuit.

The impulses in the reverberating circuit of Papez are thought to be responsible for the consolidation of memory and learning. Bilateral lesions of this circuit or of the entorhinal area result in anterograde amnesia.

Amygdala

Identify the uncus on the medial part of the parahippocampal gyrus (Pls. 6, 7, 8, 11 MRIs 13, 17). Deep to the uncus is the amygdala (Pls. 11, 24, 25, 26, MRIs 2,3, 18). It is located anterior to the hippocampus and is continuous with the tail of the caudate nucleus.

Input to the phylogenetically newer and larger lateral part of the amygdaloid nucleus arises mainly from sensory association areas in the cerebral cortex and reaches the lateral part via the entorhinal area (Sl. 50). Input to the phylogenetically older and smaller medial part, which consists partially of the uncus itself, comes directly from the olfactory pathway (see chapter 15).

The major output of the amygdala occurs via the ventral amygdaloid path that courses anteromedially beneath the lentiform nucleus through the region of the anterior perforated substance (Pl. 25; to the hypothalamus and the medial dorsal nucleus of the thalamus (Sls. 39, 40, 41, 42). Thalamocortical projections of the medial dorsal nucleus travel via the anterior limb of the internal capsule to all parts of the prefrontal cortex and the anterior part of the cingulate gyrus.

Another tract associated with the amygdala is the terminal stria. This is located in the groove between the caudate nucleus and thalamus (Sl. 39) and carries reciprocal connections between the amygdaloid nucleus and the hypothalamus and accumbens nucleus.

Accumbens Nucleus

Identify the nucleus accumbens (Pls. 22, 23; MRIs 1, 2, Sl. 45) which receives projections from the amygdala and prefrontal cortex.

The accumbens projects to the ventral pallidum, an extensive area beneath the lateral part of the anterior commissure The ventral pallidum projects to the medial dorsal thalamic nucleus, which in turn projects to the prefrontal cortex.

Septal Nuclei

Identify the septal nuclei in the anterior part of the septum pellucidum above the anterior commissure (Pl. 24). The septal nuclei project to the midbrain reticular formation.

The Hypothalamus

Identify the hypothalamus which borders the ventral part of the 3rd ventricle and is divided into three regions from anterior to posterior: chiasmatic or anterior, tuberal or intermediate, and mamillary or posterior (Pl. 11). Each region is divided into medial and lateral areas that are more or less separated by a vertical plane through the fornix (Sls. 39, 40, 41, 42, 43, 44). The lateral hypothalamic area is comprised chiefly of scattered neurons and the longitudinally-running fibers of the medial forebrain bundle which interconnects the septal nuclei, hypothalamus, and midbrain reticular formation. The medial hypothalamic area consists of a number of nuclei in each region. The individual nuclei, other than the mamillary, cannot be localized precisely in myelin-stained sections.

The chiasmatic level is associated with parasympathetic activity and includes the supraoptic, paraventricular, anterior, and preoptic nuclei (Sl 43).

The supraoptic and paraventricular nuclei regulate urine production, uterine contraction, and milk ejection by controlling the release of vasopressin (ADH) and oxytocin from their axonal terminals on capillaries within the neurohypophysis. The anterior and preoptic nuclei are associated with parasympathetic activity and it is here that the heat-loss center is located. Increased temperature of the circulating blood stimulates the neurons of this area and elicits sweating and cutaneous vasodilation.

At the tuberal level are mainly the dorsomedial, ventromedial, tuberal, and arcuate nuclei (Sls. 40, 41).

The tuberal and acrcuate nuclei strongly influence the adenohypophysis by secreting releasing and inhibiting factors into the hypophysial portal system, a vascular channel between the infundibulum and the adenohypophysis.

In the mamillary region (Sls. 37, 38) are the mamillary nuclei and the posterior hypothalamic nucleus.

The posterior hypothalamic nucleus is associated with sympathetic phenomena. It is also sensitive to decreased body temperature and when stimulated induces cutaneous vasoconstriction, piloerection, and shivering in order to conserve and produce heat.
Neural input to the hypothalamus arises chiefly from the limbic system, and has been identified.

The neural output of the hypothalamus occurs via three main projections: the mamillothalamic tract, the mamillotegmental tract, and the periventricular system. The mamillothalamic and mamillotegmental tracts emerge from the mamillary body in the mamillary princeps (Sl. 38)which passes dorsolaterally for a short distance before dividing into the two tracts. The mamillothalamic tract has been followed rostrally to the anterior nucleus. The mamillotegmental tract passes caudally and terminates in the midbrain tegmentum. The periventricular system consists of axons that descend into the periaqueductal region of the midbrain and ascend into the thalamus. Although some direct connections occur between visceral and somatic motor centers and other nuclei in the brainstem and spinal cord, most descending hypothalamic connections are made via the reticular formation. Ascending hypothalamic projections pass to the medial dorsal thalamic nucleus Pls. 26, 28, and Sls. 37, 38, 39, 40, 41, 42) which is reciprocally connected with the hypothalamus and with the prefrontal cortex.