The Ascending Tracts of the Spinal Cord

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Nerve Cell Groups in the Posterior Gray Columns

• There are four nerve cell groups of the posterior gray column: two that extend throughout the length of the cord and two that are restricted to the thoracic and lumbar segments.

• The substantia gelatinosa group is situated at the apex of the posterior gray column throughout the length of the spinal cord. It is largely composed of Golgi type II neurons and receives afferent fibers concerned with pain, temperature, and touch from the posterior root.

• Furthermore, it receives input from descending fibers from supraspinal levels. It is believed that the inputs of the sensations of pain and temperature are modified by excitatory or inhibitory information from other sensory inputs and by information from the cerebral cortex.

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Nerve Cell Groups …• The nucleus proprius is a group of large nerve cells situated

anterior to the substantia gelatinosa throughout the spinal cord. This nucleus constitutes the main bulk of cells present in the posterior gray column and receives fibers from the posterior white column that are associated with the senses of position and movement (proprioception), two-point discrimination, and vibration.

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Nerve Cell Groups …• The nucleus dorsalis (Clarke's column) is a group of nerve

cells situated at the base of the posterior gray column and extending from T1-L3-L4.

• Clarke’s column is a major relay center for unconscious proprioception. Sensory information from muscle spindles and tendon organs is carried by axons of larger neurons in posterior root ganglia, which synapse onto neurons in the spinal cord including cells in Clarke’s column. From Clarke’s column, information continues rostrally until it reaches the cerebellar cortex. This relay pathway is generally known as the dorsal spinocerebellar tract.

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Nerve Cell Groups …• The visceral afferent nucleus is a group of nerve cells of

medium size situated lateral to the nucleus dorsalis; it extends from the T1-L3 segment of the spinal cord. It is believed to be associated with receiving visceral afferent information.

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Commissure and Central Canal

• In transverse sections of the spinal cord, the anterior and posterior gray columns on each side are connected by a transverse gray commissure; the gray matter resembles the letter H. In the center of the gray commissure is situated the central canal. The part of the gray commissure that is situated posterior to the central canal is often referred to as the posterior gray commissure; similarly, the part that lies anterior to the canal is called the anterior gray commissure.

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PAIN AND TEMPERATURE PATHWAYS

(Lateral Spinothalamic Tract)

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Pain• Pain is described as an unpleasant sensation associated with a

specific part of the body (Nociception). It is produced by processes that either damage, or are capable of damaging, the tissues. Such damaging stimuli are called noxious and are detected by specific sensory nerve fibers called nociceptors. These nociceptors are free nerve endings with cell bodies in the dorsal root ganglia and terminate in the superficial layers of the dorsal horn of the spinal cord

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The free nerve endings• The free nerve endings (nociceptors ) in peripheral and cranial

nerves are the specific receptors for pain. Nociceptors are sensitive to mechanical, thermal, or chemical stimuli.

• Injured tissue may release prostaglandins or other neuroactive molecules (such as histamine, serotonin, and bradykinin), which lower the threshold of peripheral nociceptors and thereby increase the sensibility to pain (hyperalgesia). Aspirin and other nonsteroidal anti-inflammatory drugs inhibit the action of prostaglandins and act to relieve pain (hypalgesia or analgesia).

• The thinly myelinated A-delta fibers convey discrete, sharp, short-lasting pain. The unmyelinated C fibers transmit chronic, burning pain.

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First order neuron• The A-delta & C fibers

of the first-order neuron terminate by synapsing with cells in the posterior gray column, including cells in the substantia gelatinosa. Substance P, a peptide, is thought to be the neurotransmitter at these synapses

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Posterolateral tract of Lissauer

• The axons entering the spinal cord from the posterior root ganglion proceed to the tip of the posterior gray column and divide into ascending and descending branches. These branches travel for a distance of one or two segments of the spinal cord and form the posterolateral tract of Lissauer.

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tract of Lissauer

Analgesia • The word analgesia comes from the Greek words of An

(negative) and Algesis (pain) hence not sensing pain. We can think of analgesia as the process in which the sensation of pain is attenuated or inhibited. This is most often accomplished by the use of pharmaceuticals such as opioids that inhibit the activation of the neuronal pathways that relay pain sensations from the periphery to the central nervous system.

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Pain control and modulation• Pain can be modulated by the release of opioid peptides. Neurons of

the periaqueductal gray in the midbrain have excitatory synaptic connections with serotonergic neurons in nucleus raphe magnus and with noradrenergic neurons in the lower brainstem reticular formation. The serotonergic neurons (1) have inhibitory synapses with nociceptive projection neurons and (2) excitatory synapses with endorphin-containing interneurons (E), which have inhibitory synapses with the nociceptive projection neurons (P). The noradrenergic (norepinephrine) neurons also have excitatory synapses with the endorphin-containing interneurons. These activities modulate the excitatory synaptic influences of the glutamate and substance P transmitters of the A-delta fibers with the nociceptive projection neurons

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Lateral Spinothalamic Tract second-order neurons

• The axons of the second-order neurons now cross obliquely to the opposite side in the anterior gray and white commissures within one spinal segment of the cord, ascending in the contralateral white column as the lateral spinothalamic tract

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