1. The SPINOTHALAMIC tracts convey action potentials generated by pain, temperature or gross touch from receprtors to the brain.



1. FIRST ORDER neurons begin at the receptors.



1. Receptors have their cell bodies in the dorsal root ganglia of the cord.



2. They synapse in posterior horn gray of the spinal cord .



2. SECOND ORDER neurons have cell bodies in the posterior horn gray.



1. Their axons decussate through the anterior white commissure of the cord and cross to the anterior half of lateral column.



2. They then turn rostrally.



3. They ascend to thalamus. In the brainstem and thalamus, the axons aggregate with the axons from the posterior column-lemniscal system to form the Medial Lemniscus.



3. THIRD ORDER neurons cell bodies lie in the ventral nuclear group of the thalamus and radiate to somesthetic cortex.



1. They terminate in the postcentral gyrus



2. With association fibers in the paracentral gyrus



3. Of the parietal lobe.



4. Lesions affecting the second or third order neurons of the spinothalamic tract will produce unilateral paresthesia on the contralateral side.

1. TRIGEMINOTHALAMIC tracts convey somesthetic impulses from the facial dermatomes to the brain.



1. These somesthetic impulses include fine touch as well as gross touch, but the routes of the two types of sonesthesis are not the same.



2. GROSS TOUCH FROM FACE:



1. FIRST ORDER NEURONS have cell bodies in the semilunar ganglion, and axons course from the periphery to the mid-pons level of the brainstem.



2. These axons form the descending tract of V.



1. This tract travels caudally through the lower lateral pons and lateral medulla to C-1 thru C-4.



2. They synapse at Nucleus of Descending Tract of V ain segments C-1 through C-4.



3. SECOND ORDER NEURONS have cell bodies in the nucleus of the descending tract of V and send axons across the midline.



1. These axons then turn and travel rostrally...

1. Forming the ventral ascending tract of V.



2. They pass through lateral medulla, pons and midbrain forming the trigeminal lemniscus.



3. Axons end in the thalamus



1. THIRD ORDER NUURONS have their sell bodies in the ventral nuclear group of the thalamus. lateral course.



1. The axons take a lateral course from the thalamus to the...



2. Posterior limb of internal capsule.



3. They terminate in the postcentral gyrus of theparietal lobe.





1. FINE TOUCH FROM FACE:



1. FIRST ORDER NEURONS have their distal processes in distribution of V to the facial dermatomes.



1. The cell bodies are in the semilunar ganglion.



2. Axons synapse in Chief Sensory Nucleus of V at the mid pons level.

1. SECOND ORDER NEURONS have cell bodies in Chief Sensory Nucleus of V.



1. They send axons rostrally in Dorsal Secondary Ascending Tracts of V.



2. Some of these axons cross, some do not.



3. They all synapse in the posterior portion of ventral nuclear group of thalamus.



2. THIRD ORDER NEURONS have their cell bodies in the ventral nuclear group of the thalamus.



1. They send axons to the posterior limb of the internal capsule.



2. These axons terminate in the facial area of somesthetic cortex.



1. Unilateral lesions affecting the second and third order neurons of the trigeminothalamic tracts may produce contralateral gross touch symptoms, but the bilateral representation of the fine touch tracts may mask these symptoms and make them clinically undetectable.







1. The POSTERIOR COLUMN LEMNISCAL SYSTEM conveys fine touch impulses from the periphery of the body, especially the extremities, to the brain.



1. FIRST ORDER neurons at the periphery have specialized nerve endings to be sensitive to fine touch. Their cell bodies are located in the in posterior root ganglia of the cord.



1. AXons course into the posterior columns of cord and turn in a rostral direction, traveling up the cord all the way to the brainstem..



2. At caudal medulla, they synapse at nucleus gracilis (lower ext) and nucleus cuneatus (upper extremity).

1. Second order neurons have their cell bodies in the Nuclei Cuneatus and Gracilis near the Spinobulbar Junction.



1. Their axons cross midline to form the "Sensory Decussation."



2. The collective second order axons join the spinothalamic tracts to form the Medial Lemniscus.



3. This bundle of axons courses through brainstem and into the thalamus.



2. Third Order neurons have their cell bodies in ventral nuclear group of dorsal thalamus.



1. Axons pass into the posterior limb of the internal capsule.



2. They terminate in the postcentral Gyrus.(Somesthetic Cortex of Parietal Lobe)