ANTEROLATERAL SYSTEM (ALS)
Hopefully you remember from the spinal cord module that the cells of origin of pain and temperature conveying axons in the spinal cord lie in the dorsal horn. Axons arising from these dorsal horn cells cross and ascend in the anterolateral portion of the white matter of the spinal cord (hence the name Anterolateral System; ALS). Thus the cells of origin of the ALS (or spinothalamic tract) lie in the contralateral dorsal horn. Axons in the ALS are destined for the ventral posterolateral (VPL) nucleus of the thalamus. Since the thalamus lies ROSTRAL to the midbrain, which is the most rostral part of the brain stem, the ALS is present in each of the 10 brain stem slides that you need to learn.
Once the pain and temperature information traveling in the ALS reaches the VPL nucleus of the thalamus (the thalamus is the GREAT GATEWAY TO THE CORTEX), it is relayed by a thalamic neuron to the somatosensory cortex (postcentral gyrus; areas 3, 1 and 2). Dont forget: pain and temperature information reaches cells in the dorsal horn via the central processes of dorsal root ganglion cells (delta and C fibers; neuron #1). Dorsal horn cells (neuron #2) project to the contralateral VPL via the ALS. Finally, cells in VPL (neuron #3) project to areas 3, 1 and 2 (somatosensory cortex) for perception of the pain and temperature.
Interruption of the ALS anywhere in the brain stem results in loss of pain and temperature from the contralateral side of the body (arms, trunk and legs), but NOT the head. We will deal with the head later!
The location of the ALS is difficult to see in fiber-stained sections, since most of these axons are either lightly myelinated or non-myelinated (i.e., slowly conducting). You must focus hard on remembering the location of the anterolateral fibers in each of the ten brain stem levels. I will include this system in many of the problem-solving questions in order to help you remember its location. YOU SHOULD NEVER FORGET THE ALS/SPINOTHALAMIC TRACT!
AN INTERESTING CLINICAL OBSERVATION
Clinical case reports often comment that lesions involving areas of the brain stem adjacent to the anterolateral system result in an IPSILATERAL HORNERS SYNDROME. The explanation for this finding is that descending fibers from the HYPOTHALAMUS (a major autonomic center) travel close to the ALS in the brain stem (you will not see these descending fibers in the ten brain stem levels). Their interruption means that the PREGANGLIONIC SYMPATHETIC neurons in the lateral cell column (T1-L2) of the spinal cord have lost an important drive. The most obvious clinical finding would be a CONSTRICTED PUPIL (MIOSIS) in the ipsilateral eye since the parasympathetic input is now in control. There would also be slight drooping (PTOSIS) of the ipsilateral upper eyelid due to the absence of sympathetic drive to the superior tarsal (smooth) muscle. There is also lack of sweating (anhidrosis) and vasodilatation (flushed face). Remember, sympathetics innervate sweat glands and constrict blood vessels of the face. All problems are IPSI to the brain stem lesion. For our problem solving excercises A HORNERS WILL BE PRESENT ANY TIME THE LESION INVOLVES THE ALS ABOVE T2! REMEMBER, THE PROBLEMS RELATED TO HORNERS WILL BE IPSI. WHILE THOSE RELATED TO THE ALS WILL BE CONTRA.