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James Papez (1937)
The Papez Circuit was first proposed by James Papez in 1937 in a paper titled ‘’A proposed mechanism of emotion’’. He suggested that the circuit was essential for emotional experience and expression. Papez injected the rabies virus into the hippocampus of a cat, and monitored its advancement through the brain. From this, he identified a closed circuit within the brain which he believed essential for emotion. This circuit consisted of: the hippocampal formation (in particular the hippocampus), the mammillary bodies, anterior thalamic nucleus and the cingulate gyrus. At the time of publication Papez reported ‘no negative or contradictory evidence’; however he explicitly stated that he held no clinical evidence to hold this view. Instead his assumptions were based on theories proposed by Cannon, Bard and Dandy. Papez claimed his ideas sufficiently met the physiological requirements proposed by the aforementioned researchers. These theories suggested that emotional processes are located within specific structures including the diencephalon regions, and somewhere near the midline of the brain.

Maclean’s Limbic System (1952)
In 1952 Maclean expanded on Papez ideas to develop what is now known as the limbic system. The system is thought to include, but is not limited to, the hippocampus and associated structures, septal nuclei and limbic lobe. The limbic system is thought to play a role in emotion, motivation, behaviour, long-term memory, learning, and olfaction.

Current View
In light of Maclean’s findings the original Papez circuit has been updated to incorporate several other regions into the circuit. These include the prefrontal cortex, association cortex, amygdala and the hypothalamus. The circuit is now thought to be one of the major pathways of the limbic system, with current opinions suggesting the Papez circuit has little involvement in relation to emotion, and rather it is believed to be intrinsically involved in long-term memory.

Main Components:
The original Papez circuit consisted of;.

Hippocampal formation → via the fornix → mammillary bodies;

Mammillary bodies → via the mamillothalamic tract → anterior thalamic nuclei;

Anterior thalamic nuclei → projects to the cingulate gyrus;

cingulate gyrus → via the entorhinal cortex → hippocampal formation.

Hippocampal Formation:
The term hippocampal formation refers to the dentate gyrus, hippocampus (subiculum) and the parahippocampal gyrus. It is located within the medial temporal lobe of both cerebral hemispheres, and is thought to play a major role in memory, as well as attentional control and spatial navigation. Today’s consensus suggests there is little support for the structure’s direct role in emotion. The main projection of the hippocampal formation is to the mammillary bodies, via the fornix; whilst afferents are received from the cingulate gyrus projecting to the entorhinal cortex of the parahippocampal gyrus.

Mammillary Bodies:
The mammillary bodies are a pair of small globular bodies located at the end of the fornix, and are thought to be involved in the processing of recognition memory. They receive information from the hippocampus via the fornix and amygdala, and project to the anterior thalamic nuclei via the mammillothalamic tract.

Anterior Thalamic Nuclei:
The anterior thalamic nuclei are located at the rostral end of the dorsal thalamus. They are thought to play a role in modulating alertness, and in learning and memory. The anterior thalamic nuclei receive afferents from the mammillary bodies via the mamillothalamic tract and the subiculum via the fornix. They then project to the cingulate gyrus.

Cingulate Gyrus:
The cingulate gyrus is located directly above the corpus callosum. This structure is thought to be a fundamental part of both the limbic system and the Papez circuit, involved in learning and memory along with emotion formation and processing. The cingulate gyrus receives afferents from the thalamus (including the anterior thalamic nuclei) and projects to the entorhinal cortex via the cingulum, and onto the parahippocampal gyrus of the hippocampal formation; creating a great ‘loop’.

Fornix
The fornix is a c-shaped tract that connects the hippocampus to the mammillary bodies of the hypothalamus. The fornix extends from the hippocampus and arcs up to the anterior commissure where it splits into three parts; the precommissural fornix, the anterior commissure fornix and the postcommissural fornix (see fornix of the brain). The branch pivotal to the Papez circuit is the postcommissural fornix, which connects the hippocampus to the mammillary bodies. Damage to the fornix can cause impairments of temporal order memory for retrograde information. For example, patients with damage to the fornix are able to explain a holiday that occurred one year ago, and five years previously. However, they could not decide which event took place first or second.

Mammillothalamic tract
The mammillothalamic tract (see mammillothalamic fasciculus) ascends from both the fibres of the fornix and cells within the mammillary bodies. The mammillothalamic tract then connects to the dorsal tegmental nuclei, ventral tegmental nuclei and the anterior thalamic nuclei (see thalamic nuclei). The hippocampal-anterior thalamic pathway is thought to be essential for episodic memory. Damage to the mamillothalamic tract is linked to amnesia and Korsakoff's syndrome .Gold and Squire investigated a patient called PN, who had suffered from damage to the mammillary nuclei, mamillothalamic tract and anterior thalamic nuclei. Following this damage PN showed normal recognition memory and priming, however he displayed severely impaired declarative memory (i.e. both anterograde amnesia and retrograde amnesia).

Updated Circuit
When Maclean discovered the limbic system, it was suggested that the original Papez circuit needed updating to include these other connections outside of the great loop. This suggestion was based on the fact that the original Papez circuit did not include regions now thought to be essential for emotion (e.g., amygdala), rather the regions of the original circuit were more cognitive based (e.g., memory). The current state of research implies that even with the new additions of areas essential for emotion, the Papez circuit is primarily involved in memory. . 	The updated circuit includes: Amygdala, hypothalamus, prefrontal cortex and association cortex.

Amygdala
The amygdalae are almond shaped nuclei located within the medial temporal lobes. The role of the amygdala is thought to be central for emotional reactions, emotional behaviour & motivation, and the processing of memories. The amygdala receives afferents from all senses (i.e., olfactory, auditory, visual and somatosensory), and sends efferent’s to the hypothalamus and hippocampal formation, more specifically the hippocampus and the entorhinal cortex. The original Papez circuit was missing this ‘emotional’ component. Although the updated Papez circuit is currently not considered to be intrinsically involved in emotion (unlike the limbic system), it is vital that this emotional centre is incorporated into the Papez circuit. This is because emotion can significantly affect the way memories are produced and stored. For example, you are more likely to remember an emotionally significant event.

Hypothalamus
The hypothalamus is located above the brain stem and below the thalamus, forming the ventral part of the diencephalon. It has many functions including, fatigue, circadian cycles, thirst, hunger and body temperature. The hypothalamus receives inputs from, and projects to, a vast amount of regions. In relation to the Papez circuit these include inputs from the anterior hypothalamic nucleus and the mammillary bodies, and projections to the amygdala, mamillothalamic tract and the fornix.

Prefrontal Cortex
The prefrontal cortex is located in front of the motor and premotor areas (see frontal lobe), forming the anterior part of the frontal lobe. The prefrontal cortex as a whole is implicated in complex cognitive behaviours known as executive functions; such as decision making, expression of personality, short term memory, and moderating social behaviour. . 	 The prefrontal cortex receives inputs from, and projects to, a vast amount of regions. In relation to the Papez circuit these include connections with the amygdala, cingulate gyrus, hippocampal formation and the mediodorsal nucleus of the thalamus Goldman-Rakic, Selemon and Schwarts (1984) emphasise a multi-channel communication link between the prefrontal cortex and the hippocampus. In particular they suggest that prefrontal projections to the parahippocampal gyrus and subiculum carrying highly specific information. These bi-directional projections also allow the prefrontal cortex to retrieve memories stored by the hippocampus

Association Cortex
The association cortex, (see association area), refers to any part of the cerebral cortex outside of the primary areas. It is responsible for the integration of sensory information and the production of behaviour. The association cortex receives inputs from, and projects to, a vast amount of regions. In relation to the Papez circuit these include inputs from the thalamus, and outputs which reach the hippocampus, thalamus and other association cortices. The association cortex therefore has a bidirectional link with components that form the Papez circuit. The association cortex can be split into three cortices; parietal, temporal and frontal (see association area). Research suggests the function of each of these cortices is as follows.

•	The parietal association cortex is involved in attending to complex stimuli.

•	The temporal association cortex is important for identifying the nature of stimuli (i.e., recognition memory), •	and the frontal association cortex plays a vital role in the planning of appropriate behaviour to the aforementioned stimuli.