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Ascending Tracts | Spinothalamic Tract
Ninja Nerd
Overview
This video explains the anterolateral system, also known as the spinothalamic tract, which is responsible for transmitting sensations of crude touch, pressure, pain, and temperature from the body to the brain. It details the different types of nerve fibers (A-delta and C fibers) involved, the receptors they originate from (nociceptors, thermoreceptors, mechanoreceptors), how these signals enter the spinal cord, and their subsequent pathways to various brain regions, including the thalamus, reticular formation, and cerebral cortex. The video also touches upon associated tracts within the anterolateral system and their roles in sensory processing and emotional responses to stimuli.
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Chapters
- The anterolateral system is also called the spinothalamic tract.
- It is divided into the anterior (ventral) and lateral spinothalamic tracts.
- Historically, the anterior tract was thought to carry crude touch and pressure, while the lateral tract carried pain and temperature.
- Current research suggests these divisions are not always distinct, with some overlap in function.
Understanding the basic divisions of the anterolateral system provides a framework for comprehending how different sensory information is processed and transmitted to the brain.
- Pain and temperature sensations are detected by nociceptors and thermoreceptors.
- A-delta fibers are lightly myelinated and transmit fast, sharp (pinprick) pain and cold sensations.
- C fibers are unmyelinated and transmit slow, dull, aching, or burning pain, and hot sensations.
- A-delta fibers are activated by mechanical stimuli and cold temperatures, while C fibers are primarily activated by chemical stimuli, heat, and mechanical stimuli.
Differentiating between A-delta and C fibers helps explain why we experience different qualities of pain and temperature sensations and how quickly we perceive them.
A-delta fibers are stimulated by the sharp pain of a pinprick, while C fibers are responsible for the burning sensation after touching a hot stove.
- Crude touch and superficial pressure are detected by receptors like Merkel's discs, peritrichial nerve endings, and free nerve endings.
- These receptors are associated with the anterior (ventral) spinothalamic tract.
- The specific pathways for crude touch and pressure are less detailed in this section compared to pain and temperature.
Knowing the receptors for touch and pressure clarifies how these less urgent sensations are also conveyed to the brain via the anterolateral system.
Feeling the light brush of clothing on your skin involves receptors for crude touch.
- Sensory fibers enter the spinal cord via the dorsal root and synapse in specific laminae (layers) of the dorsal horn.
- C fibers (slow pain) primarily synapse in Rex laminae II and III.
- A-delta fibers (fast pain) primarily synapse in Rex laminae I and V.
- Fibers carrying crude touch and pressure may synapse in laminae II, III, IV, or V.
- After synapsing, second-order neurons cross to the contralateral side of the spinal cord via the anterior white commissure to ascend.
The precise entry points and crossing over within the spinal cord are crucial for understanding how sensory information is routed to the opposite side of the brain for processing.
A pain signal from your left foot enters the spinal cord, synapses, crosses to the right side, and then ascends towards the brain.
- The Tract of Lissauer is a small bundle of nerve fibers within the spinal cord.
- It contains both ascending and descending fibers from the initial sensory neurons.
- These fibers can ascend or descend one to three spinal cord segments before synapsing.
- This tract is important for understanding the spread of pain and the effects of spinal cord lesions.
The Tract of Lissauer explains why a lesion at a specific spinal cord level can affect sensation in segments above and below it, and on the opposite side of the body.
A lesion at spinal segment T6 might affect pain sensation a few segments below T6 on the contralateral side due to the ascending and descending fibers in the Tract of Lissauer.
- The ventral spinothalamic tract (crude touch, pressure) ascends in the anterior white column.
- The lateral spinothalamic tract (pain, temperature) ascends in the lateral white column.
- The lateral spinothalamic tract is further divided into the paleo (C fibers, slow pain) and neo (A-delta fibers, fast pain) spinothalamic pathways.
- These pathways synapse in various nuclei of the thalamus, such as the ventral posterior lateral (VPL) and ventral posterior inferior (VPI) nuclei.
- Some fibers also project to the reticular formation, periaqueductal gray, hypothalamus, and other brainstem nuclei.
This outlines the major routes by which sensory information travels up the spinal cord to reach different processing centers in the brain.
Fast pain signals travel via the neo-spinothalamic pathway to the VPL nucleus of the thalamus.
- From the thalamus, sensory information radiates to the primary somatosensory cortex (S1) and secondary somatosensory cortex (S2) for conscious perception.
- Pain signals, particularly from C fibers, also project to emotional centers like the cingulate gyrus and anterior insular cortex, contributing to the affective (emotional) experience of pain.
- The reticular formation receives a large input from C fibers and plays a role in arousal and alertness.
- Connections to the hypothalamus mediate autonomic responses to pain (e.g., changes in heart rate, blood pressure).
This explains how sensory input is not only perceived consciously but also influences our emotional state, autonomic functions, and level of awareness.
The unpleasant emotional component of chronic pain is processed in areas like the cingulate gyrus, influenced by C fiber input.
- The anterolateral system also includes other tracts like the spinotectal, spinomesencephalic, spinohypothalamic, and spinoreticular tracts.
- The spinotectal tract sends information to the superior colliculus for eye and head movements towards stimuli.
- The spinomesencephalic tract connects to the periaqueductal gray (PAG) and amygdala, involved in pain modulation and emotional responses like fear.
- The spinohypothalamic tract influences autonomic responses via the hypothalamus.
- The spinoreticular tract projects heavily to the reticular formation, contributing to arousal and the alerting aspect of pain.
These associated tracts highlight the complex network involved in sensory processing, demonstrating how stimuli can trigger reflexes, emotional reactions, and autonomic changes.
When you step on something sharp, the spinotectal tract helps direct your eyes and head towards the source of the pain.
Key takeaways
- The anterolateral system is the primary pathway for pain, temperature, crude touch, and pressure.
- A-delta fibers transmit fast, sharp pain, while C fibers transmit slow, dull pain.
- Sensory information crosses to the opposite side of the spinal cord before ascending to the brain.
- Pain perception involves not only conscious awareness but also emotional and autonomic responses.
- The Tract of Lissauer plays a role in the spread of sensory information within the spinal cord.
- The reticular formation's role in arousal is significantly influenced by pain signals.
- Different pathways within the anterolateral system project to distinct brain areas, mediating different aspects of sensory experience.
Key terms
Anterolateral SystemSpinothalamic TractA-delta fibersC fibersNociceptorsThermoreceptorsDorsal hornAnterior white commissureTract of LissauerThalamusReticular formationCingulate gyrusAnterior insular cortexSomatosensory cortex
Test your understanding
- What is the functional difference between A-delta fibers and C fibers in transmitting pain signals?
- How does sensory information from the body cross to the contralateral side of the central nervous system?
- What are the primary roles of the spinotectal, spinohypothalamic, and spinoreticular tracts within the anterolateral system?
- Why is the input of C fibers to the reticular formation significant for our experience of pain?
- How do signals from the spinothalamic tract reach conscious perception in the cerebral cortex, and what other brain areas are involved in the emotional aspects of pain?