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KeywordsUnconditioned Stimulus Classical Conditioning Motion Sickness Pavlovian Conditioning Penile Erection
A complex system of degrading unwanted (e.g., misfolded) cellular proteins. Impairment in this system can lead to abnormal aggregation of intracellular proteins.
Transient worsening of neurological symptoms with elevation of body core temperature is known as Uhtoff's phenomenon. Worsening symptoms, such as visual blurring or leg heaviness can occur in a hot environment or with exercise. These symptoms, sometimes called pseudo-exacerbations, may result from transient nerve conduction block.
Circadian rhythms are biological rhythms that occur with a periodicity of approximately 24 h. Ultradian rhythms are biological rhythms that occur with substantially shorter periodicity, such that multiple cycles occur during each 24 h day. Examples of ultradian rhythms include rhythms of parathyroid and insulin secretion, heart rate, blood pressure, and episodes of REM sleep. While circadian rhythms have been demonstrated to be based in all organisms on self-sustaining cellular oscillations, the mechanistic bases for ultradian rhythms – including whether any of them are self-sustaining – have remained obscure.
A reflex is a stereotyped behavior that reliably occurs at a characteristic latency after a specific stimulus. An unconditioned reflex is a reflex that has not been modified by a well-defined conditioning experience, such as a classical or operant conditioning protocol.
Many (and perhaps all) unconditioned reflexes are in reality the products of interactions early in life between genetic endowment and activity-dependent central nervous system plasticity. Thus, the traditional distinction between conditioned and unconditioned reflexes is meaningful only when defined in terms of a specific experimental protocol.
Unconditioned Response (UR)
In classical conditioning, the unconditioned response (UR) is a response evoked by the unconditioned stimulus. The UR includes mostly reflexive autonomic response, such as salivary secretion and vasomotor responses, and muscle movements, such as flexion reflex and eyelid reflex.
Unconditioned Stimulus (US)
In classical conditioning, the unconditioned stimulus (US) is a stimulus presented following the conditioned stimulus. The US is chosen to elicit innate reflexive responses (unconditioned response), such as food in the salivary conditioning protocol and airpuff in the eyelid conditioning protocol.
Uncontrolled Manifold Hypothesis
A hypothesis that states that the central nervous system created a subspace (an uncontrolled manifold, UCM) in the space of independent elemental variables and organizes co-variation of elemental variables to restrict much of their variability to the UCM.
Uncorrelated Broadband Noises
Two noises with flat magnitude spectra whose fine structures are uncorrelated. Specifically, noises are said to be uncorrelated if one is given the amplitude (e.g., in volts) of one of the noise bursts at an instant in time, one cannot predict the amplitude of the other noise burst at that instant or any other instant in time. Perfectly correlated noises, by contrast, have identical magnitude and phase spectra.
Under- or Indeterminate System
A mathematical system is called under- or indeterminate if it has fewer system equations than unknowns. Indeterminate systems typically have an infinite number of possible solutions.
The property of a material body such that all its points are mutually materially isomorphic. In an evolving uniform body (undergoing growth or remodeling) the material isomorphisms are functions of time.
The surgical or chemical ablation of the sensory receptors of one inner ear.
Unity of Science
Unity of Science is the idea that the various empirical sciences form a coherent “unified” system by being connected by reductive relations, and that physics is the ultimate reduction base. An associated idea propounded in the Vienna Circle is that there is a physicalist language common to all sciences in which any observation can be expressed.
The unloading reflex, or unloading response, is the automatic and stereotypical dis-facilitation of a homonymous muscle after it has been rapidly shortened/ unloaded. The response is caused by the removal of the afferent feedback component of the reflex control loop.
A mechanically sensitive sensory ending which conducts slowly (∼1 m/s). An afferent C-fiber. In the skin, a subclass knows as “tactile C fibers” possess mechanical thresholds similar to those of myelinated cutaneous mechanoreceptors.
In digital image processing, an unsharp mask is a simple method used to sharpen an image, effecting contrast enhancement. It does not increase image resolution, but rather improves small-scale acutance. A slightly blurred version of the original image is created and then subtracted from the original image, creating a highpass filtered difference image called the unsharp mask that emphasizes regions of transition (edges). This mask is then used to transform the original image to emphasize the contrast along these edges.
The scale of the contrast enhancement in this operation is determined by the blur distance. The more blurred the image that is initially subtracted from the original, the lower the corner frequency of the resulting unsharp mask, and the larger the scale of the contrast enhancement. The effects of larger-scale contrast enhancement are broader and less restricted to the sharp edges of the original image.
Unsupervised learning is a form of learning in computational models such as connectionist (artificial neural network) models. In contrast to supervised learning, unsupervised learning algorithms work without providing explicit feedback on the error of the net with respect to its input (i.e., no teaching signal). Learning develops by using internal or statistical structure of data set, so that the responses (output) will be fully characterized statistical properties of inputs. Often, the aim of unsupervised learning algorithms is to cluster the input according to similarity. While this is biologically more plausible than providing an external teaching signal, problematic issues in this context are how many clusters to form, and when to stop training. Often, weights are adjusted until some internal constraint is fulfilled. It has been proposed that unsupervised learning occurs in cortex-based learning.
Upper Motor Neuron Syndrome
Oldfashioned term meaning a syndrome resulting from lesion of the so-called upper motor neurons, i.e., neurons originating at higher levels of the central nervous system and synapsing on the motor neurons proper (lower motor neurons) in brainstem and spinal cord. Such lesions result in spasticity, enhanced Tendon reflexes (jerks) and Babinski Sign.
Upper Respiratory Tract Infections (URTIs)
The common cold is probably the most frequently occurring illness in humans worldwide. More than 200 different viruses cause colds, and rhinoviruses and coronaviruses are the culprits 25–60% of the time. Rhinoviruses often attack during the fall and spring seasons, while the coronavirus is common during the winter. The average adult has two or three respiratory infections each year. Epidemiological data suggest that endurance athletes are at increased risk for upper respiratory tract infection during periods of heavy training and the 1- to 2-wk period following race events.
The last common ancestor of all bilaterian phyla before the split of the protostome and deuterostome lineages.
Urethelium is the membrane lining the inner surface of the bladder.
Urethrogenital reflexes refer to a rat model that mimics the nerve and muscle responses associated with human sexual climax.
Urodeles are one of three orders of living amphibians (order Caudata). They are commonly referred to as salamanders and newts depending on the taxonomic group within the order. Urodeles are the most generalized of the amphibians, and in their basic body appearance most similar to known fossil amphibians. A number of urodele species diverge from the hypothesized amphibian ancestral state in interesting ways that have implications for evolutionary changes in the brain, including (in the same or different species) secondary reductions in brain size, large increases in genomic DNA content and subsequent increase in cell size, and the retention of juvenile characteristics in adults (“neoteny”). Others, such as the plethodontid salamanders, have highly specialized visual systems and tongue motor systems used for visually guided prey capture that is very similar to that used in the anurans.
The neural control of the lower urinary tract and reproductive organs (i.e., the urogenital organs) is dependent on autonomic and somatic nerves arising in the lumbosacral spinal cord. Many of the functions of these organs are regulated involuntarily by reflex mechanisms: the urogenital reflexes. Some reflexes are organized in the spinal cord, e.g., penile erection induced by tactile stimulation of the penis; whereas others require coordination between the brain and spinal cord, e.g., penile erections induced by visual erotic stimuli. Some reflexes are complex, requiring coordination between autonomic and somatic nerves, e.g., seminal emission-ejaculation; whereas others involve interactions between different organs, e.g., inhibition of micturition by stimulation of afferent nerves to the penis or vagina.
One of two otolith organs that sense gravity and linear acceleration such as from initiation of movement in a straight line. The utricle is oriented horizontally in the head, and registers accelerations acting in the horizontal plane.