ANTEROLATERAL SYSTEM: PAIN and TEMPERATURE SENSATION
The dorsal horn is a layered structure that receives afferent inputs from the skin, viscera and other structures. It is concerned particularly with nociceptive transmission. Signals generated by injurious stimuli to the body are passed to second order neurones, but the strength of the message passed to the brain can be modulated in a number of ways. The second order neurones have axons that pass rostrally in the contralateral antero-lateral white matter, and communicate with the thalamus, per
iaqueductal grey matter and the brainstem reticular formation.
Dorsal Horn neurones receive inputs from afferents entering the spinal cord through the dorsal roots; some of these afferents are myelinated (and concerned with touch and proprioception) and others are unmyelinated (and concerned prodominantly with pain and temperature sensation).
These different fibre groups have different destinations within the dorsal horn, where the post-synaptic neurones are arranged in layers. The superficial layers are concerned with transmission of nociceptive signals and their modulation.
Most of the post-synaptic neurones project to the brain, but others are interneurones that have axons that do not leave the dorsal horn.
Dorsal Horn neurones belonging to the anterolateral system project to thalamus, either directly (the spino-thalamic tract), or indirectly (with synapses on neurones in the reticular formation or the midbrain peri-aqueductal grey matter that also project to the thalamus).
Spinothalamic tract: This pathway is concerned with pain and temperature. Small myelinated and unmyelinated (A-delta and C) axons of dorsal root ganglion cells concerned with nociception synapse in the dorsal horn.
The axons of these second-order neurones cross to the opposite side of the cord within a few segments, then pass rostrally and directly to the posterior group of nuclei in the thalamus, without synapsing en route.
The axons of third order neurones in the pathway project from the ventral posterior thalamus to areas of the cortex including the second somatosensory area SII.
Spinal pathways for Pain
The Spinothalamic tract is concerned with nociception (pain) and temperature sensation. Sometimes these axons are called the anterior and lateral spinothalamic tracts because, in the cord, theyare grouped into two bundles that proceed directly to the posterior thalamus.
The small myelinated and unmyelinated (A-delta and C) axons of dorsal root ganglion cells concerned with nociception synapse on second-order neurones in the superficial layers of the dorsal horn.
The axons of these second order neurone cross to the opposite side of the cord within a few segments, then pass rostrally in the antero-lateral columns directly to the posterior group of nuclei in the thalamus, without synapsing en route.
The thalamic nuclei concerned with nociception are in the posterior ventral thalamus, and they process the information carruied by the spinothalamic system and pass it on to the cortex..
The axons of the third order neurones in the pathway project from the ventral posterior thalamus to sensory areas of the cortex including the second somatosensory area SII.
Alternative Pain Pathways
Image source: ib.cnea.gov.ar
The images show the spino-reticular and spino-mesencephalic ascending systems that are also concerned with nociception. The information they carry is utilised in the brainstem and midbrain per-aqueductal grey matter, and is also passed onwards to the posterior thalamic nuclei. So in addition to being indirect pathways to the thalamus (i.e. they synapse in the brainstem), the site of termination are also involved in modulating nociceptive transmissionn in the dorsal horn.
Alternative Pain Pathways
The antero-lateral white matter of the cord contains the spino-thalamic tract and other nociceptive pathways arising from the contralateral dorsal horn. These axons project to the reticular formation and the midbrain as well as the thalamus, and together they form the Anterolateral System.
Spino-Reticular and Spino-Mesencephalic Tracts
The reticular formation of the brainstem is a hotchpotch of neurones that tend not to be grouped into well defined nuclei. They are the substance of the brainstem which surrounds the identifiable nuclei (such as the cranial nerve nuclei) and the fibre tracts that course through the pons and medulla. Some groups of neurones with indistinct boundaries are given names, such as the nucleus gigantocellularis reticularis and the midline raphe nuclei.
The left hand diagram shows the spino-reticular tract, whose axons terminate in the medulla and pons, where third order neurones project to the thalamus.
The Spino-Mesencephalic Tracts synapse in the grey matter surrounding the aqueduct of the midbrain.
The reticular formation and the periaqueductal grey matter both send descending axons back to the dorsal horn, and these are known to modulate the strength of the nociceptive signal transmitted to the thalamus.
Another pathway that carries nociceptive information rins in the dorsal columns of some animals, and is called the dorsal coumn post-synaptic pathway. This is not the classical view of the function of the dorsal columns, which are commonly viewed as being concerned with touch and vibration rather than pain. Nevertheless, when alternative pathways for pain are considered in patients who have had a cordotomy, the possibility remains that the dorsal column post-synaptic system, originating in the dorsal horn, may contribute to nociception.
Image source: www.slideshare.net
Cordotomy results in sectioning in the anterolateral system. A need is inserted into the spinalcord at C2, and currents are passed into the anterolateral system so as to damage the ascending axons; this is one invasive method of treating intractable chronic pain.
Transection of the anterolateral system in the spinal cord.
Neurosurgeons sometimes section the anterolateral system in an attempt to relieve chronic intractable pain by dividing the axons in the spinal cord.
Transection of the anterolateral system can be performed under direct vision in a neurosurgical operation, but is more usually performed using a needle introcuced through the skin into the spinal cord.
The objective is to section axons of the spinothalamic and associated anterolateral pathways.
This procedure can be successful, but breakthrough pain sensation can occur after a year or so, and one reason for this may be that there are alternative ascending pain pathways in the anterolateral system and other smaller pathways (such as the post-synaptic dorsal column pathway).