It begins and ends in the hippocampus. The fornix carries its output to the mammillary bodies, the mammillothalamic tract runs on to the anterior nucleus of the thalamus, which projects to the cingulate gyrus; the cingulum leads back to the entorhinal cortex, which feeds the hippocampus. [1][2][3]
A circuit has no shape of its own in this atlas; lit: the parts it runs through, in order.
The 3D brain needs JavaScript. The steps below still say where the Papez circuit runs, in order.
The brain opens cut down the midline and seen from the right, so the inner face of the left half shows, where most of the loop lies. Step through lights one step at a time, about 1.2 s each, round the loop from the hippocampus, and back to step 1; tap a step below to light it alone, or Next step to go round the loop one step at a time. Drag to turn, pinch or scroll to zoom. Step through is off because your device asks for reduced motion; tap each step instead.
Each step lights the whole part: the thalamus is lit entire, though the circuit uses only its anterior nucleus, and the entorhinal cortex has no border of its own in the atlas, so its step re-lights the parahippocampal gyrus that holds it. The mammillothalamic tract and the cingulum have no shape here either. [4][2][5]
The loop, in 8 steps
The circuit starts in the hippocampal formation, at the subiculum. Its output fibres gather along the hippocampus as the fimbria, which runs on as the fornix. [1][6]Where is the hippocampus?
The fornix, a C-shaped bundle and the main output tract of the hippocampus, arches forward under the corpus callosum. Near the anterior commissure its back (postcommissural) fibres turn down through the hypothalamus to the mammillary bodies. [6]Where is the fornix?
The mammillary bodies are a pair of small round nuclei on the underside of the brain, at the ends of the fornix’s front arches; each holds a medial and a lateral mammillary nucleus, and they are often counted as the back of the hypothalamus. [7]Where is the mammillary bodies?
From the mammillary bodies the mammillothalamic tract (the bundle of Vicq d’Azyr), a heavily myelinated bundle, climbs to the anterior nuclei of the thalamus. The atlas has no shape for it, so this step lights nothing new: the mammillary bodies it leaves from stay lit. [8]
The anterior nuclei (anteromedial, anterodorsal and anteroventral) sit at the front end of the thalamus. Not a separate shape in this atlas; lit: the part that holds it, the thalamus. Besides the mammillothalamic tract they take fibres straight from the fornix, and they send theirs on to the cingulate gyrus. [2]Where is the thalamus?
The cingulate gyrus arches over the corpus callosum on the inner face of each hemisphere. Its white-matter core is the cingulum, a C-shaped bundle that follows it from beneath the front of the corpus callosum round the back and down into the temporal lobe. [3]Where is the cingulate gyrus?
The cingulum ends in the parahippocampal gyrus and uncus of the temporal lobe: the gyrus that surrounds the hippocampus on the underside of the brain. [3][9]Where is the parahippocampal gyrus?
The entorhinal cortex is the front part of the parahippocampal gyrus; the atlas draws no border for it, so this step lights nothing new. From its layers II and III the perforant path runs into the hippocampal formation (the dentate gyrus, the CA fields and the subiculum), which closes the loop at step 1. [5][10]Where is the entorhinal cortex?
James Papez proposed the circuit in 1937 as the basis of emotional experience. Paul D. MacLean later reworked the idea and coined the term limbic system for the limbic lobe and its connections with the hypothalamus, amygdala and septum. [1]
Its link to memory is now better supported than its role in emotion: the hippocampus and the anterior thalamus fire in step in the theta rhythm, and the circuit is tied above all to episodic and spatial memory. The amygdala, never part of Papez’s loop, is now given the key role in emotion. [1][2]
Traced by fibre dissection, the whole loop averages about 350 millimetres in length. [1]
The mammillary bodies’ projection to the anterior thalamus through the mammillothalamic tract supports recollective and spatial memory. [7][8]
Facts describe anatomy and normal function only. The 3D model is the adult male BodyParts3D brain (credits and licence); it has no shape for tracts, so a pathway is shown by the parts it runs through. A circuit is lit the same way, one link at a time, and its last step leads back to its first.