Sensory neurons second-order

The cell bodies of second-order sensory neurons are found in the dorsal horn. These neurons receive input from afferent neurons (first-order sensory neurons) entering the CNS from the periphery of the body through the dorsal... 

Second-order sensory neuron that transmits impulses to higher levels of the CNS... 

As discussed, the first-order neuron is the afferent neuron that transmits impulses from a peripheral receptor toward the CNS. Its cell body is located in the dorsal root ganglion. This neuron synapses with the second-order neuron whose cell body is located in the dorsal horn of the spinal cord or in the medulla of the brainstem. The second-order neuron travels upward and synapses with the third-order neuron, whose cell body is located in the thalamus. Limited processing of sensory information takes place in the thalamus. Finally, the third-order neuron travels upward and terminates in the somatosensory cortex where more complex, cortical processing begins. 

The mechanism of action of these anesthetics involves the blockade of sodium channels in the membrane of the second-order sensory neuron. The binding site for these anesthetics is on a subunit of the sodium channel located near the internal surface of the cell membrane. Therefore, the agent must enter the neuron in order to block the sodium channel effectively. Without the influx of sodium, neurons cannot depolarize and generate an action potential, so the second-order sensory neuron cannot be stimulated by impulses elicited by pain receptors associated with the first-order sensory neuron. In other words, the pain signal is effectively interrupted at the level of the spinal cord and does not travel any higher in the CNS. In this way, the brain does not perceive pain. 

The second-order sensory neuron transmits impulses ultimately to the left side of the brain. This permits the awareness of pain, identification of its source, and, if necessary, postural adjustment. As discussed, impulses in this pathway do not play a role in the reflex per se. 

Centrally mediated hyperalgesia involves the hyperexcitability of second-order sensory neurons in the dorsal horn of the spinal cord. In the case of severe or persistent tissue injury, C fibers fire action potentials... 

Stimulation of a nociceptor in the periphery of the body elicits action potentials in the first-order neuron, which transmits the signal to the second-order neuron in the dorsal horn of the spinal cord. From the spinal cord, the signal is transmitted to several regions of the brain. The most prominent ascending nociceptive pathway is the spinothalamic tract. Axons of the second-order sensory neurons project to the contralateral (opposite) side of the spinal cord and ascend in the white matter, terminating in the thalamus (see Figure 8.1). The thalamus contributes to the basic sensation or awareness of pain only it cannot determine the source of the painful stimulus. 

Morphine may be administered orally, intravenously, or epidurally. An advantage of epidural administration is that it provides effective analgesia while minimizing the central depressant effects associated with systemic administration. The mechanism of action with the epidural route of administration involves opioid receptors on the cell bodies of first-order sensory neurons in the dorsal root ganglia as well as their axon terminals in the dorsal hom. Stimulation of these receptors inhibits release of substance P and interrupts transmission of the pain signal to the second-order sensory neuron. 

Olfactory perception translates abstract chemical features of odorants into meaningful neural information to elicit appropriate behavioral responses (Shepherd, 1994 Buck, 1996). Specialized bipolar olfactory sensory neurons (OSNs) are responsible for the initial events in odor recognition. These have ciliated dendrites exposed to the environment, and a single axon that extends into the brain and forms synapses with second order projection neurons (PNs) (Shepherd, 1994 Buck, 1996). In arthropods and mammals, the first olfactory synapse is organized into glomeruli, spherical structures in which afferent olfactory neuron axons synapse with projection neuron dendrites (Hildebrand and Shepherd, 1997). 

Hains, B. C., Saab, C. Y., Klein, J. P., Craner, M. J., and Waxman, S. G. (2004). Altered sodium channel expression in second-order spinal sensory neurons contributes to pain after peripheral nerve injury. J. Neurosci. 24, 4832-4839. 

Del Punta K, Puche A, Adams NC, Rodriguez I, Mombaerts P (2002b) A divergent pattern of sensory axonal projections is rendered convergent by second-order neurons in the accessory olfactory bulb. Neuron 35 1057-1066... 

At present, the functional architecture of gustatory receptor gene expression, in particular for sweet and bitter, is being unravelled however, the functional configuration of these receptors as monomer, dimers or oligomers is not clear, and neither are their downstream intracellular signalling cascades, the transmitter used by their host sensory neurons and the precise connectivity of these cells to second-order intemeurons. 

In the primitive nervous system, sensory cells evolved from general epithelial cells. Primitive nervous systems of modern echinoderms and lower deuterostomes are still composed of three cell types that include the primary sensory cells, the neurons that connect the sensory cells to distal targets, and a supporting cell that serves the special physiological needs of such a system (Lacalli, 2001). The basic structural plan of the retina is comparable to such a primitive nervous system. In the course of evolution, the photoreceptive system developed specialized photoreceptor cells (rods and cones), intra-retinal second-order neurons (bipolar cells), and tertiary output neurons (ganglion cells). This evolution perhaps took place in photopic conditions therefore early photoreceptor cells were more like cones. 

In pain pathways, central sensitization involves enhanced efficiency of excitatory synaptic transmission pathways within the CNS, notably in the dorsal horn of the spinal cord at the synapse between the sensory nociceptor fibres and second-order relay neurones (Ji et al. 2003). It has been suggested that central sensitization in the cough reflex loop might also underlie hypertussive states (Bonham et al. 2004, 2006a, b), but to date, there are few data confirming this. 

The axons of the sensory receptor cells conduct the nerve impulses to the central nervous system. In the thoracic ganglion some second-order neurons connecting via synapses with the axons of tarsal receptor cells have been identified (Dethier, 1976 Rook et al., 1980). The corresponding second-order neurons for the labellar sensory neurons are located in the suboesophagal ganglion (Dethier, 1976 van Mier et al., 1980 Stocker and Schorderet, 1981). 

The olfactory receptor cells in insects are usually found on the antennae (Schneider, 1964). These primary sensory ceUs receive the information from the odor signals and transfer it via their own axons to the second order neurons in the antennal lobe (Fig. 2.1). The receiving part of the receptor cell is located within the olfactory organ or sensilla, whose outer structure can be seen on the... 

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