Withdrawal reflex

Aplysia exhibits habituation. If the siphon is repeatedly touched lightly, the reflex withdrawal of gills and siphon gradually weakens. The snail has become habituated to the touch and its defensive reaction weakens as it no longer perceives a threat. 

Aplysia also exhibits sensitization. The paradigm is to give the snail a strong shock, either to the head or tail. This is perceived as noxious by the animal, which then reacts to a subsequent light touch on the siphon with an exaggerated reflex withdrawal. In both cases of learning, both short-term and long-term memory can be achieved. The latter requires repeated experiences interrupted with periods of rest. 

The cells comprising lamina V are more diverse than those of lamina IV and their dendrites extend vertically toward the superficial layers. Cell bodies in lamina V contribute to three projection pathways, the SCT, PSDC and STT. However, the STT cells appear to be predominant in this lamina. Lamina V plays an important role in nociception since it receives both A - and C-fibre inputs. Some cells in lamina V also respond to cutaneous low- and high-threshold mechanical stimuli and receive nociceptive inputs from the viscerae. Many of these neurons also project onto mononeurons and so act as interneurons in the polysynaptic withdrawal reflex to noxious stimuli. 

A reflex occurs when a particular stimulus always elicits a particular response. This response is automatic and involuntary in other words, it occurs without conscious effort. Therefore, reflexes are specific, predictable, and, furthermore, often purposeful. For example, the withdrawal reflex causes a body part to be pulled away from a painful stimulus so that tissue injury is avoided. Spinal reflexes require no input from the brain because they are elicited entirely at the level of the spinal cord. However, while the reflex is underway, nervous impulses are also transmitted to the brain for further processing. In fact, input from the brain may modulate a reflex or alter the response to a stimulus through conscious effort. 

A reflex is initiated by stimulation of a sensory receptor located at the peripheral ending of an afferent or first-order sensory neuron. This afferent neuron transmits impulses to the spinal cord. Within the gray matter of the spinal cord, the afferent neuron synapses with other neurons. As such, the spinal cord serves as an integrating center for the sensory input. The afferent neuron must ultimately synapse with an efferent or motor neuron. When the afferent neuron synapses directly with the motor neuron, it forms a monosynaptic reflex. An example of this type of reflex is the stretch reflex. When the afferent neuron synapses with an intemeuron that then synapses with the motor neuron, it forms a polysynaptic reflex, e.g., the withdrawal reflex. Most reflexes are polysynaptic. The motor neuron then exits the spinal cord to innervate an effector tissue, which carries out the reflex response. 

Figure 7.4 The withdrawal reflex coupled with the crossed-extensor reflex. A painful stimulus will elicit the withdrawal reflex, which causes flexor muscles to contract and move the affected body part away from the stimulus. At the same time, the crossed-extensor reflex causes extensor muscles in the opposite limb to contract. The straightening of the opposite limb provides support for the body.

An example of the mechanism of the withdrawal reflex is illustrated in Figure 7.4. When a painful stimulus activates a sensory receptor on the right foot, action potentials are transmitted along the afferent neuron to the spinal cord. By way of divergence, this neuron synapses with several other neurons within the gray matter of the spinal cord ... 

FIGURE 6-1 Path of excitation in a simplified spinal reflex that mediates withdrawal of the leg from a painful stimulus. In each of the three neurons and in the muscle cell, excitation starts with a localized slow potential and is propagated via an action potential (a.p.). Slow potentials are generator potential (g.p.) at the skin receptor the excitatory postsynaptic potentials (e.p.s.p.) in the interneuron and the motoneuron and end-plate potential (e.p.p.) at the neuromuscular junction. Each neuron makes additional connections to other pathways that are not shown. 

Recent data utilizing a polysynaptic withdrawal reflex suggest that, in intact rats, a disinhibition hypothesis may be necessary to explain some of the facilitatory effects of hallucinogens. Low to moderate doses of the hallucinogen 5-MeODMT administered into the lateral ventricle facilitated (i.e., decreased the latency of) the tail flick response to radiant stimulation of the tail (22). Furthermore, either spinal transection alone or systemic administration of 5-HT antagonists in intact... 

Giddiness, tension, anxiety, jitteriness, restlessness, emotional lability, excessive dreaming, insomnia, nightmares, headaches, tremor, withdrawal and depression, bursts of slow waves of elevated voltage in EEC, especially on over-ventilation, drowsiness, difficult concentration, slowness on recall, confusion, slurred speech, ataxia, generalized weakness, coma, with absence of reflexes, Cheyne-Stokes respirations, convulsions, depression of respiratory and circulatory centers, with dyspnea, cyanosis, and fall in blood pressure. 

Whereas pain is used to refer to the total subjective phenomena associated with injury and suffering, nociception more specifically refers to the aspect of sensory transmission. Nociception is the more correct term for the experimental study of pain, because it is objectively measured through a behavior in animals and humans alike (e.g., withdrawal reflexes, verbal ratings). The actual subjective experiences are private and known to each subject alone. However, for the sake of convenience, the word pain will be used here to address both aspects. Similarly, the word analgesia, meaning the relief from pain, will be used in place of the more objective term antinociception. 

To summarize there are two learning paradigms in the context of a reflex action controlled by a single ganglion in an experimental animal in which electrical measurements of nerve action are relatively easy. This experimental system permitted Kandel to map the neural circuit responsible for the gill-withdrawal reflex. The neural... 

Temazepam (Restoril) [C-IV] [Sedative/Hypnotic/ Benzodiazepine] Uses Insomnia, anxiety, depression, panic attacks Action Benzodiaz ine Dose 15-30 mg PO hs PRN X in elderly Caution [X, /-] Potentiates CNS dqjressive effects of opioids, barbs, EtOH, antihistamines, MAOIs, TCAs Contra NAG Disp Caps SE Confusion, dizziness, drowsiness, hangover Interactions T Effects W/ cimetidine, disulfiram, kava kava, valerian T CNS depression W/ anticonvulsants, CNS depressants, EtOH t effects OF haloperidol, phenytoin X effects W/ aminophylline, dyphylline, OCPs, oxtriphylline, rifampin, theophylline, tobacco X effects OF levodopa EMS Use caution w/ other benzodiazepines, antihistamines, opioids and verapamil, can T CNS depression concurrent EtOH can T CNS depression abruptly D/C after >10 d use may cause withdrawal OD May cause profound CNS depression, confusion, bradycardia, hypotension, and altered reflexes flumazenil can be used as antidote, activated charcoal may be effective... 

Digitalis has mild vasoconstrictor action increasing the peripheral resistance. But in CHF patients peripheral resistance decreases due to withdrawal of reflex sympathetic overactivity. Venous tone improves in normal as well as CHF patients. It has no significant effect on coronary circulation. 

Effects of autonomic blockade on the response to phenylephrine (Phe) in a human subject. Left The cardiovascular effect of the selective K-agonist phenylephrine when given as an intravenous bolus to a subject with intact autonomic baroreflex function. Note that the increase in blood pressure (BP) is associated with a baroreflex-mediated compensatory decrease in heart rate (HR). Right The response in the same subject after autonomic reflexes were abolished by the ganglionic blocker trimethaphan. Note that resting blood pressure is decreased and heart rate is increased by trimethaphan because of sympathetic and parasympathetic withdrawal. In the absence of baroreflex buffering, approximately a tenfold lower dose of phenylephrine is required to produce a similar increase in blood pressure. Note also the lack of compensatory decrease in heart rate. 

Figure 15.1 Some research disciplines and methods of relevance for organic research ordered according to two methodological dimensions the complexity of the research world (spanning from causal over adaptive to self-reflexive entities) and the degree of involvement of the researcher (adapted from Alroe and Kristensen 2002). The triangular form shows that simple research worlds allow the researcher to intervene and withdraw from the research world in closely connected processes, whereas the involved and detached stances are widely different in relation to complex research worlds.

Wiesenfeld-Hallin, Z. and Duranti, R. D-Arg1, D-Try7,9, Leu11-substance P (spantide) does not antagonise substance P-induced hyperexcitability of the nociceptive flexion withdrawal reflex in the rat, Acta Physiol. Scand. 1987, 129, 55-59. 

Short-term memory is not affected by inhibitors of protein synthesis, but alteration of synaptic proteins and membranes may be induced by covalent modification of existing macromolecules.1016 One way in which this happens has been described for Aplysia. As the snail learns a simple gill-withdrawal reflex, the duration of the action potentials in sensory neurons is increased, and there is a greater release of transmitters. This change comes about because stimulation of the sensory neuron causes simultaneous activation of... 

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