Neuroscience Terms Starting With D
Neuroscience Glossary: D
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Dendrite
/ DEN-dryt / · Greek dendron (tree)
Dendrite is a branching extension of a neuron that receives synaptic input from other neurons and conducts graded electrical signals toward the cell body for integration.
A single neuron can have thousands of dendritic branches covered with tiny protrusions called dendritic spines, each forming a synaptic contact with an axon terminal from another neuron. The size, branching complexity, and spine density of dendrites determine how many inputs a neuron can receive and how it integrates those signals. Dendritic spines are dynamic structures whose number and shape change with learning and memory formation through a process called synaptic plasticity.
Cerebellar Purkinje cells carry some of the most elaborate dendritic arbors in the nervous system, with a single cell receiving input from more than 100,000 parallel fiber synapses across a flat, fan-shaped tree.
Pyramidal neurons in the human cerebral cortex can carry more than 10,000 dendritic spines each, giving a single cortical cell integrative complexity that neuroscientists sometimes compare to a small neural network in its own right.
Dendrites passively receive signals and relay them unchanged to the cell body. Dendrites contain voltage-gated ion channels that can actively amplify, filter, or transform incoming signals before they reach the soma, meaning the dendrite itself performs computation.
When mice (Mus musculus) learn a new forelimb motor skill, dendritic spines on motor cortex pyramidal neurons form and stabilize within hours of training. The density of these new spines, typically a 10 to 20 percent increase over baseline, correlates with how accurately the animal performs the skill two days later, directly linking structural change to behavioral improvement.
Fun Facts About the Nervous System →Depolarization
/ dee-poh-lar-ih-ZAY-shun / · Latin de- (reversal) + polus (pole) + -ization
Depolarization is the reduction of a neuron's membrane potential from its negative resting value toward zero or positive values, driven by the net influx of positive ions that diminishes the electrical charge difference across the cell membrane.
At rest, the inside of a neuron sits at about negative 70 millivolts relative to the outside. When a depolarizing stimulus opens ligand-gated or mechanically gated channels, positive sodium ions flow inward, making the interior less negative. Reaching a threshold of roughly negative 55 millivolts triggers voltage-gated sodium channels to open in a regenerative cascade that produces a full action potential.
Below threshold, depolarization remains local and graded, decaying with distance unless it summates with other inputs.
Neurons can summate hundreds of subthreshold depolarizing inputs spatially across their dendrites or temporally over milliseconds, integrating them into a single decision about whether to fire an action potential at the axon hillock.
Depolarization and an action potential are the same event. Depolarization refers to any decrease in membrane potential toward zero, whereas an action potential is a specific, all-or-nothing event that occurs only when depolarization crosses the firing threshold.
Local anesthetics such as lidocaine block depolarization by physically occluding voltage-gated sodium channels in sensory neurons, preventing the action potentials that signal pain. A dentist's injection of about 1.8 milliliters of lidocaine solution typically silences a tooth's sensory nerve within two to three minutes, an effect lasting roughly 60 minutes before the channel block dissipates.
How To Become An Anesthesiologist? →Dopamine
/ DOH-pah-meen / · DOPA (precursor) + amine (chemical group)
Dopamine is a catecholamine neurotransmitter synthesized mainly in the substantia nigra and ventral tegmental area that drives reward-based learning, motivation, voluntary motor control, and working memory through four major projection pathways.
Dopaminergic neurons project through four main pathways: the mesolimbic pathway mediates reward and motivation; the mesocortical pathway supports cognition and working memory; the nigrostriatal pathway controls voluntary movement; and the tuberoinfundibular pathway regulates prolactin release from the pituitary. Rather than signaling pleasure directly, dopamine encodes prediction errors, firing strongly when an outcome is better than expected and falling silent when an expected reward is omitted. Parkinson’s disease results from the progressive loss of nigrostriatal dopamine neurons, reducing dopamine in the striatum by 60 to 80 percent before motor symptoms become clinically apparent.
Antipsychotic drugs treat schizophrenia partly by blocking D2 dopamine receptors, which implicates excess dopaminergic signaling in the positive symptoms of that disorder.
Wolfram Schultz's recordings from monkey midbrain neurons in the 1990s showed that dopamine cells shift their firing from the time of an unexpected reward to the cue that predicts it, providing the first direct neural evidence for a prediction-error teaching signal.
Dopamine is the brain's pleasure chemical. Dopamine mediates wanting and the motivation to pursue rewards rather than the subjective feeling of pleasure itself; the hedonic experience of enjoyment depends more heavily on opioid and endocannabinoid signaling in the nucleus accumbens.
Rats (Rattus norvegicus) with dopamine-depleted nucleus accumbens still display normal pleasure responses, such as positive facial reactions to sweet tastes, but will not perform learned behaviors to obtain those tastes. The dissociation, established in studies of 50 to 100 rats per condition, shows that dopamine drives the motivation to seek rewards rather than the enjoyment of receiving them.