Neuroscience Terms Starting With H
Neuroscience Glossary: H
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Hippocampus
/ hip-oh-KAM-pus / · Greek hippos (horse) + kampos (sea monster)
Hippocampus is a curved, seahorse-shaped structure in the medial temporal lobe that is critical for forming new declarative memories, supporting spatial navigation, and consolidating short-term memories into long-term memory.
The hippocampus receives convergent input from association cortices across the brain and relays processed information to long-term cortical storage sites during memory consolidation. It contains place cells that fire when an animal occupies specific locations, and grid cells in the adjacent entorhinal cortex collectively form a spatial mapping system. Among the first regions damaged in Alzheimer’s disease, the hippocampus explains why memory loss for recent events is usually the first symptom.
Patient Henry Molaison, known for decades as H.M., had both hippocampi removed in 1953 to treat epilepsy. He could not form new declarative memories afterward, showing that the hippocampus is needed for memory consolidation.
The hippocampus is not a general-purpose memory structure for all types of learning. It is specifically required for episodic and spatial memory, while procedural skills and conditioned fear responses depend on other structures such as the basal ganglia and amygdala.
London taxi drivers, who must memorize thousands of street routes to pass the Knowledge exam, have measurably larger posterior hippocampi than control subjects, with size correlating with years of taxi driving experience. Drivers with more than 40 years of experience show the greatest posterior hippocampal volume, suggesting that sustained spatial memory demands drive structural change in this region.
Hyperpolarization
/ hy-per-poh-lar-ih-ZAY-shun / · Greek hyper (over) + Latin polus (pole) + -ization
Hyperpolarization is a shift in a neuron's membrane potential to a more negative value than the resting potential, reducing the likelihood that the cell will generate an action potential.
When inhibitory neurotransmitters such as GABA open chloride channels, negatively charged chloride ions flow into the cell, driving the membrane potential further from the threshold for firing, typically around negative 55 millivolts in mammalian neurons. Potassium efflux through voltage-gated channels after an action potential produces a similar effect, creating the afterhyperpolarization that briefly silences the neuron and enforces a refractory period. This refractory period limits firing frequency and ensures that action potentials propagate in only one direction along the axon, because the membrane behind the advancing signal remains hyperpolarized and unresponsive.
Certain pacemaker neurons in the heart and brain exploit hyperpolarization rather than resist it: the hyperpolarization-activated cyclic nucleotide-gated (HCN) channels open specifically when the membrane becomes more negative, generating an inward current called Ih that drives the cell back toward threshold and produces rhythmic, spontaneous firing.
Fun Facts About the Nervous System →Hyperpolarization always silences neurons. Some neurons express HCN channels that open during hyperpolarization, generating a depolarizing current that returns the membrane toward threshold and can trigger rhythmic bursting.
In the spinal cord of a cat, Ia inhibitory interneurons hyperpolarize antagonist motor neurons during the stretch reflex, shifting their membrane potential roughly 5 to 10 millivolts below resting. This inhibition occurs within tens of milliseconds and ensures that the quadriceps and hamstrings do not contract simultaneously during limb extension.
Hypothalamus
/ hy-poh-THAL-ah-mus / · Greek hypo (below) + thalamos (inner chamber)
Hypothalamus is a small brain region below the thalamus that regulates homeostasis by controlling hunger, thirst, body temperature, circadian rhythms, and the pituitary gland's hormonal output.
Although weighing only about four grams, the hypothalamus controls core body functions through both neural and hormonal mechanisms, integrating signals from the body and higher brain regions to maintain internal stability. It contains distinct nuclei regulating feeding behavior, reproductive function, stress responses, water balance, and sleep-wake cycles. The hypothalamic-pituitary axis coordinates the body’s endocrine response to physiological and psychological challenges.
The suprachiasmatic nucleus in the hypothalamus is the master biological clock, synchronizing circadian rhythms across the body through neural signals and hormonal output, with individual neurons that continue to oscillate with a near-24-hour period even when isolated in a dish.
The hypothalamus does not simply switch hunger on and off. It integrates multiple hormonal signals, including leptin from fat tissue and ghrelin from the stomach, and weighs them against energy expenditure data before adjusting appetite and metabolism.
Lesions of the lateral hypothalamus in rats abolish eating entirely, while lesions of the ventromedial hypothalamus produce hyperphagia and obesity, demonstrating that distinct hypothalamic nuclei exert opposing controls on food intake. Animals with ventromedial lesions can reach more than twice their normal body weight within weeks of the injury.