Parahippocampal Gyrus

The parahippocampal gyrus is a cortical convolution located on the medial and inferior surface of the temporal lobe. It lies adjacent to the hippocampal formation and forms an important component of the limbic lobe.

The parahippocampal gyrus contains cortical regions that provide a major interface between the hippocampal formation and widespread cerebral association cortex. Through these connections, it participates in memory, contextual processing, recognition of scenes and environments, and spatial navigation.

Its anterior portion curves medially to form the uncus, while more posterior regions continue toward medial occipitotemporal and retrosplenial-related cortical networks. The entorhinal cortex, located primarily in the anterior parahippocampal region, is particularly important because it provides a major route by which cortical information enters and leaves the hippocampal formation.

Location

The parahippocampal gyrus lies along the medial aspect of the temporal lobe and extends onto its inferior surface.

It is positioned inferior and medial to the hippocampal formation and forms part of the cortical surface surrounding medial temporal structures.

Boundaries

The parahippocampal gyrus is bounded laterally by the collateral sulcus along much of its course.

Medially, it is related to the hippocampal sulcus, hippocampal formation, ambient cistern, and other medial temporal structures depending on the anteroposterior level examined.

Collateral Sulcus

The collateral sulcus separates much of the parahippocampal gyrus from more lateral occipitotemporal cortex.

It is an important landmark on the inferior surface of the temporal and occipital lobes and helps define the lateral boundary of the parahippocampal region.

Relationship to the Hippocampus

The hippocampus lies deep to the medial temporal cortex and is closely related to the parahippocampal gyrus.

The two structures are anatomically and functionally interconnected but are not synonymous. The parahippocampal gyrus is cortical tissue on the cerebral surface, while the hippocampus is a specialized allocortical structure associated with the floor of the temporal horn.

Relationship to the Temporal Horn

The hippocampal formation lies between portions of the parahippocampal region and the temporal horn of the lateral ventricle.

This relationship is important in coronal sections and MRI, where the parahippocampal gyrus can be identified inferior to the hippocampal formation.

Uncus

The uncus is the hook-shaped medial projection of the anterior parahippocampal gyrus.

It forms a prominent landmark on the medial temporal surface and is closely related to the amygdala, hippocampal structures, cerebral peduncle, oculomotor nerve, and medial edge of the tentorial opening.

Relationship to the Amygdala

The amygdala lies deep within the anteromedial temporal lobe, near the anterior parahippocampal and uncinate region.

The amygdala and parahippocampal cortex participate in interconnected limbic networks that combine emotional significance, context, memory, and sensory information.

Relationship to the Fusiform Gyrus

The fusiform gyrus, also called the occipitotemporal gyrus in some classifications, lies lateral to the parahippocampal region.

The collateral sulcus forms an important boundary between these cortical territories.

Relationship to the Cingulate Gyrus

The parahippocampal and cingulate gyri are major cortical components of the classical limbic lobe.

They are connected by the cingulum and participate in distributed networks involving memory, emotion, attention, and contextual processing.

Entorhinal Cortex

The entorhinal cortex occupies an important portion of the anterior medial temporal cortex associated with the parahippocampal gyrus.

It serves as a major gateway between widespread neocortical association regions and the hippocampal formation.

Entorhinal-Hippocampal Connections

Information from association cortex can reach the hippocampal formation through the entorhinal cortex.

One major route is the perforant pathway, which projects from entorhinal cortex toward the dentate gyrus and hippocampal fields.

Hippocampal output can return through the subiculum and entorhinal cortex toward distributed cortical regions.

Perirhinal Cortex

The perirhinal cortex lies within the medial temporal cortical region adjacent to the entorhinal and parahippocampal areas.

It participates prominently in processing information concerning objects and familiarity and provides input to medial temporal memory systems.

Posterior Parahippocampal Cortex

Posterior portions of the parahippocampal region are strongly involved in processing spatial layouts, environmental context, and visual scenes.

These areas interact with retrosplenial, hippocampal, occipital, and parietal networks involved in navigation and representation of places.

Hippocampal Formation

The hippocampal formation generally includes the hippocampus proper, dentate gyrus, and subicular complex.

The parahippocampal gyrus surrounds and communicates extensively with these structures, especially through entorhinal and subicular pathways.

Subiculum

The subiculum occupies a transitional position between the hippocampus proper and parahippocampal cortical regions.

It is an important output component of the hippocampal formation and projects both toward cortical regions and through white matter pathways associated with the fornix.

Cingulum

The cingulum is a major association fiber bundle connecting medial frontal, cingulate, parietal, and medial temporal regions.

Posteriorly and inferiorly, cingulum fibers extend into the parahippocampal region, providing an important anatomical link between cingulate and medial temporal limbic cortices.

Afferent Connections

The parahippocampal region receives information from widespread cortical association areas.

Important sources include:

• Temporal association cortex
• Parietal association cortex
• Occipital visual association cortex
• Prefrontal cortex
• Retrosplenial and posterior cingulate regions
• Perirhinal cortex
• Limbic structures

Efferent Connections

Parahippocampal regions project extensively toward the entorhinal cortex, hippocampal formation, retrosplenial cortex, cingulate cortex, and other association areas.

These reciprocal pathways allow information processed within hippocampal memory circuits to interact with distributed cortical representations.

Connections with the Hippocampus

Connections between the parahippocampal cortex and hippocampal formation are central to medial temporal memory organization.

The entorhinal cortex channels cortical information into hippocampal circuits, while hippocampal output returns through the subiculum and entorhinal region.

Connections with the Cingulate Cortex

The parahippocampal region communicates with the cingulate cortex through pathways including the cingulum and interconnected association networks.

These connections participate in memory, contextual processing, spatial orientation, and limbic integration.

Connections with the Retrosplenial Cortex

The retrosplenial cortex has strong functional and anatomical relationships with the parahippocampal and hippocampal systems.

Together, these regions participate in navigation, scene processing, spatial memory, and the use of environmental context.

Connections with Association Cortex

The parahippocampal region receives highly processed information from multiple sensory and association systems.

This organization allows the medial temporal memory system to integrate information about objects, locations, scenes, events, and their relationships.

Memory

The parahippocampal gyrus participates in declarative memory through its connections with the hippocampal formation and association cortex.

It is particularly important for providing contextual and spatial information that can be incorporated into memories.

Episodic Memory

Episodic memories contain information about events and the context in which they occurred.

The parahippocampal region contributes information about environmental context and spatial relationships, while hippocampal circuits help bind these elements into coherent episodic representations.

Contextual Processing

A major function of parahippocampal networks is processing context.

The meaning of an object or event often depends on its surroundings, location, and relationship to previous experiences. Parahippocampal-hippocampal interactions help represent these contextual relationships.

Scene Processing

Posterior parahippocampal cortical regions respond strongly during processing of visual scenes and environmental layouts.

This information is important for recognizing places and understanding the spatial organization of an environment.

Spatial Navigation

The parahippocampal gyrus participates in a broader navigation network that includes the hippocampus, entorhinal cortex, retrosplenial cortex, posterior cingulate cortex, parietal cortex, and thalamic-related systems.

These networks help represent locations, landmarks, routes, and relationships among environmental features.

Recognition Memory

Medial temporal cortical regions surrounding the hippocampus contribute to recognition memory.

Different regions contribute different types of information, with perirhinal networks particularly associated with objects and familiarity and parahippocampal networks strongly associated with contextual and spatial information.

Papez Circuit

The parahippocampal region participates in the cortical portion of the classical Papez circuit.

A simplified sequence is:

1. Hippocampal formation
2. Fornix
3. Mammillary bodies
4. Mammillothalamic tract
5. Anterior thalamic nuclei
6. Cingulate cortex
7. Cingulum
8. Parahippocampal and entorhinal regions
9. Hippocampal formation

Parahippocampal Gyrus and Limbic Lobe

The parahippocampal gyrus forms a major portion of the limbic lobe on the medial cerebral surface.

Its continuity with other medial cortical regions and its close relationship with the hippocampal formation make it a key anatomical component of limbic and memory networks.

Parahippocampal Gyrus Compared with Hippocampus

Feature	Parahippocampal Gyrus	Hippocampus
Location	Medial and inferior temporal cortical surface	Deep medial temporal lobe along temporal horn
Structure	Cortical gyrus	Specialized allocortical structure
Major association	Context, scenes and cortical interface	Memory binding and spatial-contextual representation
Important connection	Entorhinal cortex	Entorhinal cortex and fornix
Shared role	Medial temporal memory processing

Parahippocampal Gyrus Compared with Cingulate Gyrus

Feature	Parahippocampal Gyrus	Cingulate Gyrus
Location	Medial temporal lobe	Medial hemisphere around corpus callosum
Major association	Memory, context and spatial processing	Emotion, motivation, attention and memory
Connection	Linked through the cingulum and broader limbic networks
Limbic relationship	Major cortical components of the limbic lobe

Blood Supply

The parahippocampal gyrus receives arterial supply predominantly from branches of the posterior cerebral artery, particularly temporal and hippocampal-related branches.

Anterior portions of the medial temporal region may also receive contributions from branches associated with the anterior choroidal and middle cerebral circulations. The precise vascular distribution varies.

Venous Drainage

Venous drainage from the parahippocampal region communicates with basal and deep cerebral venous systems.

Medial temporal veins can drain toward the basal vein and neighboring venous channels.

MRI Appearance

The parahippocampal gyrus is readily evaluated on high-resolution magnetic resonance imaging, particularly on coronal and sagittal images.

The collateral sulcus provides an important landmark for identifying its lateral boundary, while the hippocampal formation can be visualized superior and medial to portions of the gyrus.

Clinical Significance

Parahippocampal Lesions

Lesions involving the parahippocampal region can impair memory, contextual processing, recognition of environments, and spatial navigation.

The precise deficit depends on lesion location, laterality, extent, and involvement of the hippocampus, entorhinal cortex, amygdala, or neighboring association cortex.

Medial Temporal Lobe Damage

Extensive bilateral damage involving the parahippocampal region and hippocampal formation can produce severe impairment of new declarative memory formation.

This reflects disruption of both hippocampal processing and the cortical pathways that provide information to and from the hippocampus.

Temporal Lobe Epilepsy

The parahippocampal gyrus can participate in seizure networks associated with mesial temporal lobe epilepsy.

Seizure activity may involve the hippocampus, amygdala, entorhinal cortex, parahippocampal cortex, and interconnected temporal structures.

Uncinate Herniation

Because the uncus forms the medial projection of the anterior parahippocampal gyrus, displacement of the medial temporal lobe can produce uncal herniation through the tentorial opening.

This can compress adjacent structures, particularly the ipsilateral oculomotor nerve and cerebral peduncle, and may also compromise nearby cerebral vessels.

Oculomotor Nerve Compression

Compression of the oculomotor nerve during uncal herniation can produce pupillary dilation and abnormalities of extraocular movement.

The close relationship between the uncus and the nerve explains this important neurological sign.

Cerebral Peduncle Compression

Progressive medial temporal displacement can compress the cerebral peduncle against the tentorial edge.

This may disrupt descending motor pathways and produce significant motor deficits.

Posterior Cerebral Artery Compression

Medial temporal herniation can also affect the posterior cerebral artery or its branches.

Vascular compromise may produce ischemic injury involving occipital or medial temporal structures.

Neurodegenerative Disease

The entorhinal and parahippocampal regions can be affected early in neurodegenerative disorders associated with progressive memory impairment.

Pathology in these regions can disrupt communication between hippocampal circuits and widespread association cortex.

Topographical Disorientation

Damage involving posterior parahippocampal and interconnected retrosplenial networks can contribute to topographical disorientation, in which a person has difficulty recognizing or navigating familiar environments.

Surgical Anatomy

The parahippocampal gyrus is an important landmark in surgical approaches to the medial temporal lobe.

Its relationships with the hippocampus, amygdala, temporal horn, choroidal fissure, tentorial edge, cerebral peduncle, posterior cerebral circulation, and basal cisterns are particularly important during procedures involving mesial temporal structures.

Key Anatomical Features

Feature	Key Point
Location	Medial and inferior temporal lobe
Lateral boundary	Collateral sulcus
Anterior projection	Uncus
Deep relationship	Hippocampal formation
Major cortical gateway	Entorhinal cortex
Major white matter relationship	Cingulum
Major functions	Memory, context, scene processing and navigation
Limbic relationship	Major component of limbic lobe
Major arterial supply	Predominantly posterior cerebral artery branches
Important clinical relationship	Uncal herniation and medial temporal pathology

Anatomical Importance

The parahippocampal gyrus forms a critical cortical interface between the hippocampal formation and widespread association cortex. Information concerning objects, scenes, spatial relationships, and context converges within medial temporal cortical networks before interacting with hippocampal memory circuitry.

The entorhinal cortex provides a major gateway into and out of the hippocampal formation, while posterior parahippocampal regions interact with retrosplenial, parietal, and visual association systems involved in representing environments and spatial context.

Its anatomy also has major clinical importance because the anterior parahippocampal gyrus forms the uncus. The close relationship of the uncus to the tentorial opening, oculomotor nerve, cerebral peduncle, and posterior cerebral circulation explains the neurological consequences of medial temporal lobe herniation.