Topics and Speakers John H. Martin, PhD
| ||||||
Synopsis
In this lecture, Dr John H. Martin compares the dorsal column-medial lemniscal (DCML) and anterolateral systems (ALS), the pathways that mediate somatic sensations, both in anatomy and function. These systems differ in subtle but clinically paramount ways. These two pathways follow the subsequent route—receptor, periphery, spinal nerves, thalamus, and finally cortex—in that order. Attention is also paid to the characteristics of receptors, dorsal root ganglion neurons, and briefly, to the emotional content of pain.
The DCML mediates our mechanical sensations such as touch, sense of limb position, and vibration; whereas the ALS mediates our "protective sensations," that is, our feelings of pain and temperature. Both systems are three-neuron circuits to the cortex but differ in many ways. For the DCML, the first neuron is the mechanoreceptor neuron, whose cell body lies in the dorsal root ganglia and its axon projects into the central nervous system (CNS). Its principal branch extends up through the spinal cord where it branches with dorsal column nuclei (neurons in the medulla). The second neuron in this system has its axon crossing the midline and ascending into the medial lemniscus. The third neuron is in the ventral posterior nucleus (VPN) and its axon projects to the primary sensory cortex.
For the ALS, the first neuron is a nociceptor instead of a mechanocereptor neuron. Even though its cell body also lies in the dorsal root ganglia, its axon extends outward into the spinal cord, instead of the CNS. This axon synapses with neurons in the dorsal horn instead with neurons in the medulla. The secondary neuron crosses over the midline and projects to the brain in one of three important tracts: the spinoreticular, the mesencephalic, or the spinothalamic tracts. It is either one of these three tracts that is considered the "third neuron" in the pathway. The spinothalamic tract branches into several locations in the thalamus, like the VPN, and is believed to be involved in the localization of pain. In addition, there is another branch that extends to the medial dorsal nucleus, which selectively projects to the cingulate cortex (part of the limbic system) and is believed to be the source of the emotional aspect of pain.
Dr Martin then goes into the differences of the receptors of both systems. Both systems have the cell bodies of their primary neurons residing in the dorsal root ganglia. Neurons that reside here differ into two main ways: in their receptive fields and their adaptation rate. The receptive field is defined as "a spatial area within which a stimulus will activate the neuron." Hence, for a mechanoreceptor it would be a mechanical stimulation and for a thermoceptor, it would be a change in temperature. The adaptation rate is how the neuron responds to constant enduring stimulation. There are slowly adapting receptors and rapidly adapting receptors with the latter tending to be the majority in numbers. For instance, a rapidly adapting receptor (like a Pacinian corpuscle) would fire briskly at the onset of a stimulus but fire less over time as the stimulus continues.
Once the receptors fire as a consequence of stimuli (in this case from the skin), we find ourselves in the peripheral nerves. Anatomists have found a direct correlation between axon diameter and myelin sheath with information conductor velocity; hence, the larger the axon and the more myelin, the faster the impulse will travel. Interestingly, the faster the information travels, the more precise the representation of pain will be.
The peripheral nerves then communicate with the spinal nerves. Most sensory axons enter the spinal cord through the dorsal root into its gray matter or white matter. Depending on the size of the axon, there is a different termination pattern. Usually the larger diameter axons terminate within the deeper laminae of the spinal cord whereas the smaller diameter axons terminate within the superficial laminae. (The relevance of the dermatome and somatotopic organization are discussed in relation to axon branch termination.) The target of the pathways in the spinal cord—either DCML or ALS—is the ventral posterior lateral thalamus. From here, the projections reach the cortex.
The cortex has six principal layers but more than fifty different functional regions. Korbinian Brodmann was the pioneer in identifying these areas and assigning them a function. There are different projections of the somatic sensory neurons into the brain, but it is well understood that the projection into the posterior parietal cortex is important to our sense of spatial awareness. Other projections, such as into the medial dorsal which in turn ends at the anterior cingulate cortex is associated with emotional pain, since the latter is part of the limbic system. However, these projections and their association with pain are only now being elucidated.
Of course, there are still mysteries to unravel with these two somatosensory systems. Why do northern Europeans have higher thresholds for pain than Mediterraneans? Or why do some people experience pain without tissue trauma? Questions like these are the driving force behind current research into the somatic sensory system.
