Neuroscience Terms Starting With E
Neuroscience Glossary: E
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EEG
/ ee-ee-JEE / · Abbreviation: electroencephalography
EEG is the clinical and research technique of recording the brain's electrical activity through electrodes placed on the scalp, producing a continuous trace of voltage fluctuations used to diagnose neurological conditions and study cognitive function.
EEG requires no ionizing radiation, is non-invasive, and can be performed repeatedly, making it suitable for monitoring brain states in intensive care units, sleep laboratories, and operating rooms. Diagnosing epilepsy, classifying sleep stages, confirming brain death, and detecting encephalopathy all rely on characteristic EEG patterns. Temporal resolution is EEG’s greatest strength: it captures electrical events with millisecond precision, far finer than the seconds-scale resolution of functional MRI.
Advances in signal processing, including independent component analysis and source localization algorithms, have improved spatial resolution and expanded research applications to include brain-computer interfaces and real-time neurofeedback.
Hans Berger recorded the first human EEG in Jena, Germany, in 1924, identifying the alpha rhythm at roughly 10 Hz in a relaxed, eyes-closed subject; his results were initially dismissed by the scientific community and not widely accepted until Edgar Adrian replicated them in 1934.
EEG and MRI measure the same thing. EEG measures electrical activity in real time with millisecond precision but cannot resolve brain structure, whereas MRI reveals anatomy and, in its functional form, tracks blood-flow changes on a timescale of seconds.
High-density EEG systems using 256 scalp electrodes combined with source-modeling algorithms can localize epileptic foci to within one to two centimeters. That accuracy approaches what is needed to guide surgical planning in patients whose seizures resist medication, with the additional benefit of millisecond-level time resolution unavailable to fMRI or PET imaging.
Efferent Neuron
/ EF-er-ent NYOOR-on / · Latin efferens (carrying away) + Greek neuron
Efferent Neuron is a nerve cell that carries signals away from the central nervous system to peripheral effectors such as muscles and glands, translating neural commands into physical responses.
Efferent neurons include somatic motor neurons that innervate skeletal muscles and autonomic efferent neurons that regulate smooth muscle, cardiac muscle, and glands. In the somatic motor system, upper motor neurons in the motor cortex synapse onto lower motor neurons in the spinal cord, which then directly contact muscle fibers at the neuromuscular junction. Damage to lower motor neurons produces flaccid paralysis with muscle wasting, while damage to upper motor neurons produces spastic paralysis with exaggerated reflexes.
Not all efferent output is motor: descending efferent fibers from the brainstem also modulate sensory processing in the spinal cord and adjust the sensitivity of muscle spindles through gamma motor neurons.
A single lower motor neuron and all the muscle fibers it innervates constitute a motor unit. The force of voluntary contraction is graded by recruiting more motor units and by increasing their firing frequency.
Efferent neurons carry only motor commands to muscles. Efferent fibers also descend from the brain to regulate sensory receptor sensitivity, including the gamma motor neurons that set the mechanical threshold of muscle spindles.
Fun Facts About the Nervous System →The neuromuscular junction, where somatic efferent axons contact skeletal muscle, uses acetylcholine as its neurotransmitter and is one of the most thoroughly characterized synapses in the peripheral nervous system. Each human motor neuron can innervate anywhere from a handful of muscle fibers in the extraocular muscles to more than a thousand fibers in large limb muscles such as the gastrocnemius, reflecting the precision required for fine versus gross movement.
Electroencephalogram
/ ee-lek-troh-en-SEF-ah-loh-gram / · Greek elektron + enkephalos (brain) + gramma (record)
Electroencephalogram is a recording of the brain's electrical activity obtained by placing electrodes on the scalp, measuring the summed postsynaptic potentials of large populations of cortical neurons.
EEG detects oscillatory patterns of different frequencies associated with distinct brain states, including delta waves during deep sleep, alpha rhythms during relaxed wakefulness, and faster beta activity during alert thinking. Its millisecond temporal resolution makes it uniquely suited for studying the timing of cognitive processes, even though its spatial resolution is far coarser than that of functional MRI. Clinically, EEG is a primary diagnostic tool for epilepsy, sleep disorders, and encephalopathy, and it remains the only bedside method capable of detecting subclinical seizures in real time.
Hans Berger recorded the first human EEG in 1924, identifying the alpha rhythm and establishing that scalp electrodes could capture meaningful brain signals.
Epileptic seizures produce characteristic EEG patterns, including spike-and-wave discharges, that can be detected even between clinical episodes. Identifying these interictal patterns guides both the classification of epilepsy type and the selection of appropriate antiseizure medication.
EEG does not record the activity of individual neurons. It detects the summed synchronous postsynaptic potentials of tens of thousands of neurons simultaneously, meaning EEG patterns reflect coordinated population activity rather than single-cell firing.
Neurofeedback training using real-time EEG allows patients to learn voluntary control of their own brain wave patterns. Clinical applications in attention deficit disorder, epilepsy, and anxiety have been supported by growing evidence, with some protocols training patients to increase sensorimotor rhythm power at 12 to 15 Hz to reduce seizure frequency.
Endorphin
/ en-DOR-fin / · Contraction of endogenous morphine
Endorphin is a neuropeptide produced by neurons in the brain and by the pituitary gland that binds to opioid receptors in the central nervous system, reducing pain perception and producing feelings of well-being.
The term endorphin encompasses several distinct peptides, including beta-endorphin, enkephalins, and dynorphins, each with different receptor affinities and distributions across the brain and spinal cord. Beta-endorphin, the most potent of these, binds mu-opioid receptors with an affinity roughly 18 to 33 times greater than morphine. Endorphins are released during exercise, excitement, and acute pain, and they modulate nociceptive signals at the level of the dorsal horn of the spinal cord as well as in brainstem structures such as the periaqueductal gray.
Their receptors are the same molecular targets exploited by opioid analgesic drugs, which is why morphine and related compounds produce such powerful pain relief.
The runner's high experienced by long-distance runners results from a combination of endorphin release and endocannabinoid activity. Brain imaging studies using positron emission tomography have confirmed increased opioid receptor binding in limbic regions, including the prefrontal cortex and anterior cingulate, during and after prolonged running.
Endorphins released into the bloodstream during exercise travel to the brain to produce their mood-lifting effects. Endorphin molecules do not cross the blood-brain barrier efficiently; the central effects of endorphins arise from neurons within the brain that synthesize and release them locally onto their targets.
Fun Facts About the Nervous System →The analgesic power of the placebo effect is partially mediated by endorphin release. Blocking opioid receptors with naloxone can reduce placebo analgesia within minutes in controlled trials, showing that expectation recruits endogenous opioid circuits rather than acting only as imagination.
Excitatory Synapse
/ ek-SY-tah-tor-ee SIN-aps / · Latin excitare (to arouse) + Greek synapsis (joining)
Excitatory Synapse is a synaptic connection at which neurotransmitter release increases the probability that the postsynaptic neuron will fire an action potential by depolarizing its membrane potential.
The most common excitatory neurotransmitter in the brain is glutamate, which acts on AMPA, NMDA, and kainate receptors to open cation channels that allow sodium influx and depolarize the postsynaptic membrane. A single excitatory postsynaptic potential is rarely sufficient to trigger an action potential; spatial or temporal summation of multiple EPSPs is usually required. The balance between excitatory and inhibitory synaptic input determines whether and when a neuron fires, and disruption of this balance underlies conditions ranging from epilepsy to schizophrenia.
Each pyramidal neuron in the human cortex receives an estimated 5,000 to 30,000 excitatory synaptic contacts, reflecting the massive convergence of information that precedes any single firing decision.
NMDA receptors at excitatory synapses require both glutamate binding and membrane depolarization to open, making them coincidence detectors. This property is the molecular basis of Hebbian learning and long-term potentiation, the synaptic strengthening mechanism most closely linked to memory formation.
Excitatory synapses are not always beneficial for neural function. Excessive glutamate release during stroke or traumatic brain injury causes excitotoxicity, a toxic overactivation of neurons that floods them with calcium and leads to cell death.
Cell Death →During spatial navigation in rats, hippocampal place cells fire action potentials when the animal enters a specific location in its environment. This location-specific firing depends on the summation of excitatory synaptic inputs from entorhinal cortex grid cells, and disrupting AMPA receptor transmission in the hippocampus abolishes place field stability within minutes.