Polysynaptic response

Stimulation of the peripheral nerve trunk of intact animals leads to generation of muscle action potentials of three types. According to the duration of latent periods, they fall into the following order M-response (the result of the direct stimulation of a-motor neuron axons), Fl-response (the monosynaptic response), and polysynaptic responses with the variable latent period from 8-12 up to about 40 ms. In test animals of the III group, the changes of temporal parameters refer mainly to the latent period and duration of M-response (Table 7.4). Polysynaptic responses occur at all intensities of excitation and have a more pronounced character than in intact rats. A marked level and more distinct differentiation of the peaks of the complex action potential were noted. 

Barbiturates act throughout the CNS nonanesthetic doses preferentially suppress polysynaptic responses. Facilitation is diminished, and inhibition usually is enhanced. The site of inhibition is either postsynaptic, as at cortical and cerebellar pyramidal cells and in the cuneate nucleus, substantia nigra, and thalamic relay neurons, or presyruq tic, as in the spinal cord. Enhancement of inhibition occurs primarily at synapses where neurotransmission is mediated by GABA acting at GABA receptors. 

Figure 7.3 Components of a reflex arc. As illustrated by the components of the reflex arc, reflexes may be processed entirely at the level of the spinal cord with no need for input from the brain. A monosynaptic reflex has a single synapse between afferent and efferent neurons a polysynaptic reflex has two or more synapses between these neurons. In this case, intemeurons lie between the sensory and motor neurons. The more intemeurons involved, the more complex the response is.

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. 

An interneuron together with a sensory afferent and motor efferent form a polysynaptic reflex (Figure 2.2) this comprises the initial stage of information input (sensory afferent), the processing/computing an appropriate response (interneurons) and the execution of a behavioural response (motor efferent). The simplest reflexes in the nervous system are monosynaptic reflexes, such as the familiar tendon (knee) jerk, these do not involve an interneuron. The sensory afferent activated by the mechano-receptor (the tap of the patellar hammer) forms a synapse with the motor efferent in the spinal cord, which then causes the skeletal muscle to contract and the crossed leg to jerk forward. With a synaptic delay of 1 millisecond (ms), the time between input and output increases with the number of synapses introduced into the circuit. As an... 

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... 

However, activation of the central nervous system (CNS) does not equate with EEG arousals or awakenings. CNS activation implies that integrative neurons were activated and sent information to descending pathways, the nucleus tractus solitarius, and sympathetic controlling cells. An ANS modulation is always associated with an efferent response. CNS activation may lead to an arousal, an awakening, or an important ANS activity change, but activation may be limited to a polysynaptic reflex response with ANS change and no EEG arousal (6). 

FIGURE 7.7. Action potentials of the rat gastrocnemius muscle under stimulation of N. tibialis effect of direct stimulation of the muscle (M-response, M), mono- (H-response, H) and polysynaptic (Ps) responses of control rats (a, h), rats of group III (c, d), II (e, f), and I (g, h). Ciphers within ftames indicate value of the stimulus, x-axis shows time of registration, msec y-axis shows amplitude of action potential, mV. 

The affective motivational component is mediated by slowly conducting c-fibers, and polysynaptic transmission to the limbic cortex [10,12]. It is responsible for continued pain perception, suffering, pain-related behaviors, hyperalgesia, reflex spasm (splinting behavior). It is also responsible for immobilization, and protection of the injury site. 

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