Somatosensory Pathways to the Brain
Dorsal spinothalamic tract. Pathway that carries fine-touch and pressure fibers.
Ventrolateral thalamus. Part of the thalamus that carries information about body senses to the somatosensory cortex.
Ventral spinothalamic tract. Pathway from the spinal cord to the thalamus that carries information about pain and temperature.
As the axons of somatosensory neurons enter the spinal cord, they divide, forming two pathways to the brain. The haptic-proprioceptive axons ascend the spinal cord ipsilat erally (on the same side of the body on which they enter), whereas nocioceptive fibers synapse with neurons whose axons cross to the contralateral side of the spinal cord be fore ascending to the brain. Figure 10-18 shows these two routes through the spinal! cord. The haptic-proprioceptive pathway is shown as a sold red line and the nocio-ceptive pathway as a dashed red line.
The haptic-proprioceptive axons are located in the dorsal portion of the spinal cordj and form the dorsal spinothalamic tract. These axons synapse in the dorsal-columnj nuclei located at the base of the brain. Axons of neurons in the dorsal-column nuclei then cross over to the other side of the brainstem and ascend through the brainstem as part of a pathway called the medial lemniscus.
These axons synapse in the ventrolateral thalamus. The neurons of the ventrolateral thalamus send most of their axons to the somatosensory cortex, but some jaxons go to the motor cortex. Thus, three neurons are required to carry haptic-proprioceptive information to the brain: dorsal-root ganglia neurons, dorsal-column nuclei neurons, and thalamic neurons.
The nocioceptive axons, as already stated, take a different route to the brain. They synapse with neurons in the dorsal part of the spinal cord's gray matter. These neurons, in turn, send their axons to the ventral part of the other side of the spinal cord, where they form the ventral spinothalamic tract. This tract joins the medial lemniscus in the brainstem to continue on to the ventrolateral thalamus.
Some of the thalamic neurons receiving input from jaxons of the ventral spinothalamic tract also send their jaxons to the somatosensory cortex. So, again, three neurons are required to convey nocioceptive information to the brain: dorsal-root neurons, spinal-cord gray-matter neurons, and ventrolateral thalamic neurons.
Notice that the haptic-proprioceptive and the nocioceptive pathways enter the spinal cord together, separate in the spinal cord, and join up again in the brainstem. Thus, two separate pathways in the spinal
Figure 10-18
Dual Somatosensory Pathways to the Brain As neurons from the dorsal-root ganglia enter the spinal cord, the somatosensory pathways to the brain diverge.
Figure 10-18
Dual Somatosensory Pathways to the Brain As neurons from the dorsal-root ganglia enter the spinal cord, the somatosensory pathways to the brain diverge.
Dorsal-root ganglion neurons respond to fine touch and pressure; joint, tendon, and muscle change; and pain and temperature.
The ventral spinothalamic tract receives input from pain and temperature neurons and then joins the pathway called the medial lemniscus.
Spinal cord
Dorsal-root ganglion neurons respond to fine touch and pressure; joint, tendon, and muscle change; and pain and temperature.
The ventral spinothalamic tract receives input from pain and temperature neurons and then joins the pathway called the medial lemniscus.
Spinal cord
Unilateral damage to spinal cord
Unilateral damage to spinal cord
Unilateral damage causes loss of fine-touch and pressure sensation on the same side of the body below the cut...
.and loss of pain and temperature sensation on the opposite side of the body below the cut.
Unilateral damage causes loss of fine-touch and pressure sensation on the same side of the body below the cut...
Figure 10-19
.and loss of pain and temperature sensation on the opposite side of the body below the cut.
Effects of Unilateral Injury Damage to only one side of the spinal cord has different effects on fine-touch and pressure sensations compared with those on pain and temperature sensations.
Figure 10-20
Monosynaptic Reflex cord'convey somatos^^
age in the spinal cord results in distinctive sensory losses to both sides of the body below the site of injury.
As is illustrated in Figure 10-19, there is loss of hapsis and proprioception on the side of the body on which the damage occurred and there is a loss of nocioception on the opposite side of the body. Unilateral damage to the dorsal roots or in the brainstem or the thalamus affects hapsis, proprioception, and nocioception equally, because these parts of the pathways for hapsis and proprioception and that for nocioception lie in close proximity.
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