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However, the number of taste buds varies widely; some humans have only ,…. He identified the taste buds and regarded them as terminations of nerves, described the minute structure of the brain, optic nerve, and fat reservoirs, and in was the first to see the red blood cells and to attribute the colour of blood to them.
They contain taste buds, which are sensitive to chemical constituents of food, and serous glands that secrete some of the fluid in saliva, a substance that moistens the oral cavity and helps lubricate food particles. The base, or upper rear portion, of the tongue has no papillae,…. More About Taste bud 6 references found in Britannica articles Assorted References major reference In human sensory reception: Taste gustatory sense association with flavour In flavour chemoreceptors In chemoreception: Taste discovery by Malpighi In Marcello Malpighi: Life tongue structure In tongue In human digestive system: Help us improve this article!
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Internet URLs are the best. Thank You for Your Contribution! There was a problem with your submission. Please try again later. Mapping and lesion studies on BSR were designed to determine the location of reward-relevant neurons as well as determine the signal pathways that are directly affected by brain stimulation. The site of intracranial self-stimulation leads to substantially different behavioral characteristics.
Sites along the length of the medial forebrain bundle MFB through the lateral and posterior hypothalamus , the ventral tegmental area VTA , and into the pons are associated with the strongest reward effects of stimulation. The lateral hypothalamus is a portion of the hypothalamus, and brain stimulation to this area at the level of the medial forebrain bundle produces the highest response rates and subsequently the highest reward potency in rodents.
Lesions in this region or along its boundary cause a loss of positive drive-reward behaviors as well as all other operant drive behaviors. The medial forebrain bundle MFB is the location of the most frequently investigated brain stimulation reward sites, and it is composed of a complex bundle of axons projecting from the basal olfactory regions and the septal nuclei.
The rewarding effect of MFB stimulation is mediated via the activation of the mesocorticolimbic dopamine system. The mesolimbic pathway connects the VTA to the nucleus accumbens. The nucleus accumbens is located in the ventral striatum and integrates information from cortical and limbic brain structures to mediate behaviors the reinforce reward. The VTA is the origin of dopaminergic cell bodies that comprise the mesocorticolimbic dopamine system.
BSR has been shown to result in the release of dopamine within the nucleus accumbens, which also occurs in response to natural rewards such as food or sex.
Electrophysiological data suggest stimulation of the MFB or VTA does not directly activate dopaminergic neurons in the mesolimbic reward pathway. These data suggest BSR is facilitated by initial excitation of descending, myelinated neurons which then activate the ascending, unmyelinated neurons of the VTA. Excitatory, cholinergic inputs to the VTA are thought to play a role in this indirect activation, but the neuroanatomical components of this circuit have yet to be fully characterized.
Subjects undergo stereotaxic surgery to permanently implant either a monopolar or bipolar electrode to the desired brain region. Electrodes are connected to a stimulating apparatus at the time of the experiment.
In experiments involving rats, subjects are trained to press a lever for stimulation, and the rate of lever-pressing is typically the dependent variable. The rewarding stimulus in BSR experiments is typically a train of short-duration pulses separated by interval pulses,  which can be manipulated experimentally using the independent variables of stimulation amplitude, frequency, and pulse duration.
The amplitude current of stimulation determines the population of neurons being activated by the implanted electrode. In certain approaches, this is adjusted for every subject due to minor variability in electrode placement, and therefore a slightly different population of affected neurons. This is called a discrete-trial current intensity procedure. Effective currents for BSR elicit responding above a certain rate 3 out of 4 trials, for example. The lowest current the animal responds sufficiently to is deemed the minimum effective current.
Within-subject study design is often implemented to help eliminate variability introduced by electrode placement. Between-subject study design requires rigorous histologic verification of electrode placement to ensure consistency between experimental groups. Subjects with imperfect electrode placement require a higher simulation amplitude to activate the reward circuitry and produce ICSS responding. Subjects with ideal anatomical placement will respond at lower stimulation amplitudes.
This corrective process is limited, however, since increasing the population of activated neurons can result in off-target activation of neighboring circuitry. This is often culminated in undesired motor side effects upon stimulation, due to the adjacency of the MFB to the internal capsule , a bundle of axons carrying descending motor information to the brainstem. Inadvertent stimulation of these axons can lead to motor output such as movement of the head or paw twitching.
At a constant minimum effective current, ICSS responding is recorded over a series of trials which vary in stimulation frequency. Each trial consists of a short priming phase of non-contingent stimulation, a response phase where responses are recorded and rewarded with stimulation, and a short time-out phase where responses are not recorded and no stimulation is delivered. This is repeated for a series of different ascending or descending frequencies, in.
While the amplitude of the stimulation influences which neurons are being stimulated, the frequency of stimulation determines the firing rate induced in that neuronal population. Generally, increasing stimulation frequency increases the firing rate in the target population. This is associated with higher ICSS response rates, eventually reaching a maximum level at the maximum firing rate, limited by the refractory properties of neurons.
The independent variables of stimulation train and pulse duration can also be varied to determine how each affects ICSS response rates. Longer train durations produce more vigorous responding up to a point, after which rate of responding varies inversely with train length. This is due to lever-pressing for additional stimulation before the previously earned train has finished. The reinforcement schedule can also be manipulated to determine how motivated an animal is to receive stimulation, reflected by how hard they are willing to work to earn it.
The number of required responses increases for each trial until the animal fails to reach the required number of responses. This is considered the "break-point" and is a good indication of motivation related to reward magnitude.
Stimulation intensity, pulse duration, or pulse frequency can be varied to determine dose-response functions ICSS responding using curve-shift analysis.
This approach generally resembles traditional pharmacological dose-response curve where the frequency of stimulation, rather than the dose of a drug, is examined. Lower stimulation frequencies fail to sustain ICSS responding at a probability above chance. Response rates increase rapidly over a dynamic range of stimulation frequencies as the frequency increases, until a maximum response rate is reached.
Curve-shift analysis is often used in pharmacological studies to compare baseline response rates to those following drug administration. The maximum response rate during baseline conditions is typically used to normalize data in a frequency-rate curve to a maximum control rate MCR.
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