MODULE 4
THE ANTEROLATERAL SYSTEM
- - The term anterolateral system refers to the group of sensory pathways which ascend through the spinal cord in the anterolateral part of the white matter. While these pathways are difficult to trace, because they do not stand out on slides, it is important (from a clinical point of view) to work out the details because this is the route by which information regarding pain and temperature is conveyed from the body to the brain. Blumenfeld describes the system on pages 268-9 and shows it in Figures 7.2 and 7.3. Fitzgerald discusses it on pages 186-7 and in Figures 15.10, 11, 12, 13. Haines treats it in the greates detail, at the start of his Chapter 18 (pages 281-290. We show it in our Figure 4-1. All authors agree that the anterolateral system actually contains at least two pathways. The first is called the spinothalamic pathway. Because it is the newest component of the system, and the most direct route to the thalamus, it is also known as the neospinothalamic or direct pathway. The second is known as the spinoreticular pathway - also called the paleospinothalamic or indirect path. Many authors now add a third pathway to the list. It consists of ascending fibers that terminate at the midbrain level in the superior colliculus and underlying periaqueductal gray matter. It is called the spinomesencephalic tract. All three components are shown in Blumenfeld's Figures 7.2. As we will see the anterolateral pathway differs from the dorsal column-medial lemniscus in several ways (compare Figure 3-2 with Figure 4-1).
- - Let's start by describing the features shared by all three parts of the anterolateral pathway as shown in Figure 4-1 and Figure 4-2.
----- 1) The axons of the first order neurons are relatively small in diameter ( A-delta or C fibers in Blumenfeld Table 7.2). They enter in the more lateral part of the dorsal root entry zone, and are represented schematically by the thin orange lines in Figure 3-4. See also Fitzgerald Figure 15.5.
----- 2) These axons may travel up or down the cord for several segments in the dorsolateral fasciculus of Lissauer, and then turn deeply to end on second order neurons in the dorsal horn. The manner of termination is complex, and somewhat different for each of the three tracts
----- 3) The second order neurons give rise to axons that cross the midline in the ventral white commissure and ascend contralaterally in the anterolateral fasciculus.
----- 4) The pathway is somatotopically organized at spinal levels, but this organization is less apparent as the pathway ascends through the brainstem.
----- 5) Axons of one tract, the spinothalamic one, ascend all the way to terminate in the thalamus. The other two terminate at brainstem levels.
- - Now let's describe each of the tracts in greater detail.
THE SPINORETICULAR PATHWAY
- - The sensory modality conveyed by this pathway is one of long-lasting, dull, aching pain or a burning sensation. It is the most difficult pain to endure and is sometimes described as "slow" because there is a brief but detectable interval between the stimulus and the onset of the pain. First order neurons have their cell bodies in dorsal root ganglia and send unmyelinated C fibers into the spinal cord, passing through the lateral part of the dorsal root entry zone and ultimately terminating in laminae II and III. Neurons in these laminae project by a multisynaptic route to lamina V. Neurons within lamina V, and perhaps deeper laminae as well, (considered second order neurons) send axons across the midline in the Anterior White Commissure to ascend in the anterolateral pathway as shown in Figure 4-2. The axons in this spinoreticular division of the anterolateral pathway do not ascend directly to the thalamus. Rather, they gradually "peel off", as the pathway passes through the brainstem, and terminate on neurons of the reticular formation. Cells of the reticular formation that have received a spinoreticular input relay this information via the central tegmental tract, that terminates within the intralaminar nuclei of the thalamus. Neurons in these nuclei (considered third order ones) project broadly to the cerebral cortex. All this is summarized in Figure 4-1 and you can use Figure 4-3 to trace the pathway on our slides
THE SPINOMESENCEPHALIC PATHWAY
- - Some of the fibers described above ascend as far as the midbrain, ending within the superior colliculus. This region is called the tectum, and this part of the pathway is also called the spinotectal tract. Fibers may also end in the periaqueductal gray region, just below the colliculus. The precise role of this pathway is uncertain. Some believe it is responsible for initiating eye movement in response to painful stimuli. Others (Blumenfeld) suggest it represents the afferent or input side of a system which participates in the modulation of pain.
THE SPINOTHALAMIC PATHWAY
- - One sensory modality conveyed by this pathway is one of sharp, brief, well-localized pain. First order neurons send is A delta fibers into the cord which also pass through the lateral part of the dorsal root entry zone. They may pass up or down the cord for several segments in Lissaur's Tract before turning deeply to terminate upon second order neurons in laminae I and V. Second order neurons give rise to axons which cross the midline in the Anterior White Commissure and ascend all the way to the thalamus. The main termination of this pathway is in the nucleus ventralis posterolateralis, but some fibers may end more medially, in the intralaminar nuclei - as shown in Figure 4-3. Use this figure to trace the pathway.
- - The spinothalamic pathway also conveys the sensations of warmth and cold through the brain. The first order neuron axons are C fibers (warmth) and A delta fibers (cold), as shown in Blumenfeld's Table 7-2. Second order neurons, found mainly in lamina 5, follow the expected route to the thalamus.