Transcript Document

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 antiinflammatory 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
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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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