Hippocampus Anatomy, Function & Pathology | Neuroscience Tutorial

The brain uses specialized neural cells to create cognitive maps of space: place cells in the hippocampus fire at specific locations (place fields) to indicate position, grid cells in the entorhinal cortex use hexagonal firing patterns to encode distance and direction, border cells detect environmental boundaries, and head direction cells maintain orientation independent of location; these cells work together to enable spatial navigation and environmental awareness.

The brain's navigation circuit involves a hierarchical organization: Thalamus contains head direction cells providing compass-like orientation; Entorhinal cortex serves as an association area receiving all sensory inputs (visual, auditory, vestibular); Hippocampus contains place cells in CA3 and CA1 substructures. This circuit projects back to entorhinal cortex in a reentrant loop. Entorhinal cortex and hippocampus are central areas with cells tuned to spatial variables, forming the core of the mammalian navigation system.

The hippocampus is a cortical structure named for its seahorse-like shape, containing dentate gyrus (granule cells), CA3 (pyramidal cells), and CA1 (pyramidal cells). Information flows through entorhinal cortex → dentate gyrus (mossy fibers) → CA3 (Schaffer collaterals) → CA1 → subiculum → entorhinal cortex. The dorsal hippocampus is essential for spatial learning and memory. Place cells fire specifically at particular locations, transforming into target cells with rewards. Grid cells fire in repeating hexagonal patterns across space with different spacing, enabling efficient spatial coding similar to positional notation. Combined, these cells allow precise spatial localization using relatively few neurons.

Place cells are pyramidal neurons located in the hippocampus that fire when an animal is at a single specific location within an environment. Unlike grid cells which have multiple firing fields arranged in a hexagonal pattern, each place cell has only one preferred firing field. The firing of different place cells converges to provide information about the animal's exact location within the environment. This system works together with grid cells to form a complete spatial navigation circuit.

The brain uses specialized neurons called grid cells in the medial entorhinal cortex to create an internal coordinate system for spatial navigation. These cells fire in a periodic hexagonal pattern across space, allowing the brain to track position through self-motion cues like speed and direction. Combined with other cell types including place cells (which fire at specific locations), border cells (which respond to environmental boundaries), and head direction cells, the entorhinal-hippocampal system dynamically monitors spatial location and forms the foundation for episodic memories. This neural circuitry explains how we navigate our environment and why damage to these structures, as demonstrated by patient H.M., results in both memory loss and impaired spatial navigation.

Adult neurogenesis is considered structural plasticity because new neurons integrate into existing neural networks. New neurons in the dentate gyrus are densely packed, meaning a single new neuron can form multiple connections and significantly alter existing circuits. The laboratory developed a contextual discrimination learning model where mice learn to find platforms indicated by different cues in different mazes. Gamma radiation (5 Gy) reduces neurogenesis and impairs contextual discrimination learning several weeks after treatment, suggesting new neurons are specifically involved in contextual learning rather than basic spatial memory.

Adult neurogenesis in the hippocampus involves the continuous production of new neurons from progenitor cells in the dentate gyrus, which integrate into existing neural circuits over several weeks; these adult-born neurons play crucial roles in learning and memory, as evidenced by impaired spatial learning when neurogenesis is disrupted, and they exhibit unique properties such as high synaptic efficacy during early development and responsiveness to electrical activity in the surrounding network that influences their maturation and integration into hippocampal circuits.

The dentate gyrus of the hippocampus is the primary site for adult neurogenesis and serves as the gateway for memory formation. New neurons here are more excitable and plastic than mature neurons, making them the most efficient learning neurons in the adult brain. Alzheimer's disease damages this region, making neurogenesis a powerful strategy for cognitive reserve. The three main factors activating neurogenesis are aerobic exercise, enriched environments, and moderate caloric restriction. A fourth factor, learning new complex skills in a game context, has been identified as equally powerful.

Adult neurogenesis, the birth of new neurons after birth, occurs primarily in the dentate gyrus of the hippocampus throughout the entire human lifespan, with approximately one-third of hippocampal neurons being renewed over a 100-year life, though this rate varies significantly between individuals (12-63%). This continuous neurogenesis, demonstrated using carbon-14 dating from nuclear bomb tests, plays a critical role in learning and memory by providing both plasticity and stability to hippocampal function, with newly generated neurons having an average lifespan of about 7 years.

Alzheimer's disease is characterized by amyloid-beta plaque deposition and cholinergic neuron loss. Treatment: (1) Donepezil (acetylcholinesterase inhibitor) - drug of choice; (2) Anti-amyloid antibodies (aducanumab, lecanemab, donanemab) - new treatments. Absence seizures present with blank staring and 3 Hz spike-wave EEG. Treatment: ethosuximide (calcium channel blocker) is first-line; valproate is alternative. Vigabatine is for infantile spasms. The question describes a patient with progressive cognitive decline and cortical atrophy.

Two main drug classes treat Alzheimer's symptoms: memantine (NMDA receptor antagonist, 1989) and cholinesterase inhibitors (donepezil, rivastigmine, galantamine). Memantine reduces glutamate overstimulation; cholinesterase inhibitors preserve acetylcholine levels. Both slow cognitive decline but do not halt neuronal death, representing symptomatic rather than curative approaches.

Neurodegenerative diseases of the central nervous system, including Parkinson's disease, Alzheimer's disease, and amyotrophic lateral sclerosis (ALS), currently have no curative treatments and rely on symptomatic management; Parkinson's disease is treated with levodopa-carbidopa combination therapy to replace dopamine, dopamine receptor agonists (pramipexol, ropinirole), MAO-B inhibitors (selegiline, rasagiline), and COMT inhibitors (entacapone); Alzheimer's disease is managed with cholinesterase inhibitors (donepezil, galantamine, rivastigmine) and NMDA receptor antagonist memantina; and ALS is treated with riluzole to reduce glutamate excitotoxicity and baclofen for spasticity relief.

This section covers neurodegenerative disease treatment: (1) Alzheimer's disease - memantine (NMDA receptor blocker) for moderate-severe cases; monoclonal antibodies: aducanumab, lecanemab, donanemab (all target amyloid-beta); (2) Parkinson's disease - dopamine agonists (bromocriptine, pramipexole, ropinirole) cause peripheral vascular disease and cardiac valve fibrosis; amantadine (antiviral with NMDA blocking) causes lichenoid eruptions; (3) ALS - riluzole (NMDA blocker, antioxidant), tofersen (anti-sense oligonucleotide); (4) Restless legs syndrome - non-ergot dopamine agonists (pramipexole, ropinirole), gabapentin enacarbil.

Channelopathies represent the largest genetic cause of developmental and epileptic encephalopathies, with over 900 genes now implicated. Four primary sodium channel genes (SCN1A, SCN2A, SCN3A, SCN8A) produce distinct clinical phenotypes based on variant type (gain-of-function vs loss-of-function). SCN1A gain-of-function causes severe neonatal-onset DEE with arthrogriposis and favorable sodium channel blocker response, while loss-of-function causes Dravet syndrome with alternating hemispheric seizures and contraindicated sodium channel blocker use. SCN2A variants present differently by age: gain-of-function causes early infantile epileptic encephalopathy with suppression-burst patterns, while loss-of-function presents later with West syndrome features. Potassium channelopathies (KCNQ2, KCNT1, KCNA2) and calcium channelopathies (CACNA1A) add further complexity. Treatment responsiveness directly correlates with variant type, making accurate classification essential for precision therapy. Understanding these molecular mechanisms enables targeted treatment approaches including sodium channel blockers, potassium channel openers, and emerging gene-based therapies.