Papez circuit

The Papez circuit /peɪpz/,^{[unreliable source?]} or medial limbic circuit, is a neural circuit for the control of emotional expression. In 1937, James Papez proposed that the circuit connecting the hypothalamus to the limbic lobe was the basis for emotional experiences. Paul D. MacLean reconceptualized Papez's proposal and coined the term limbic system. MacLean redefined the circuit as the "visceral brain" which consisted of the limbic lobe and its major connections in the forebrain – hypothalamus, amygdala, and septum. Over time, the concept of a forebrain circuit for the control of emotional expression has been modified to include the prefrontal cortex.

The Papez circuit involves various structures of the brain. It begins and ends with the hippocampus (or the hippocampal formation). Fiber dissection indicates that the average size of the circuit is 350 millimeters. The Papez circuit goes through the following neural pathways:

A photograph of the inferior medial view of the brain when dissected clearly shows the layout of the Papez circuit. Due to the location of the structures in the circuit, the resulting shape is a limbus. This is what drove MacLean to call the circuit the limbic system when he later modified the circuit.

Various studies indicate that the Papez circuit is greatly influenced by the cerebellum and that perhaps the hippocampus is not the starting point of the circuit. Anatomically, this would make sense since the cerebellum is connected to the circuit with many fine fibers and fiber bundles. Chemical lesions on the cerebellum seem to have an inhibitory effect on the circuit. Animal behavioral studies show that electrical stimulation of the anterior cerebellum can cause arousal, predatory attack, and feeding responses, all of which are thought to be expressions of emotion. Overall, these studies provide evidence that the cerebellum may also be included in an emotional system of the brain.

In recent years, multiple additional limbic fiber connectivity has been revealed using Diffusion MRI-weighted imaging MRI techniques. The equivalent fiber connectivity of all these pathways has been documented by dissection studies in primates. The fiber tracts include the amygdalofugal tract, amygdalothalamic tract, stria terminalis, dorsal thalamo-hypothalamic tract, cerebellohypothalamic tracts, and the parieto-occipito-hypothalamic tract.

Based on Papez's experiment with aggression in rats and other studies, it was initially believed that the circuit was involved with emotion. The circuit connects the hypothalamus and the cortex and acts as the emotional system of the brain. "The cingulate cortex projects to the hippocampus, and the hippocampus projects to the hypothalamus by way of the bundle of axons called the fornix. Hypothalamic effects reach the cortex via a relay in the anterior thalamic nuclei."

However, there has not been additional proof of the circuit's role in emotion over the years. Now, the Amygdala is thought to play a key role in emotion, a structure that was not a part of the Papez circuit until 1952 when MacLean included it in the modified version of the circuit, the limbic system.

In the past few years, many experiments have been done that involve structures of the Papez circuit. Moreover, many cases where structures of the Papez circuit have been damaged show changes that indicate its real function.