The spinal cord is only second to the brain in its importance to neuronal communication in the human nervous system. Through it travel signals that relay information regarding several different types of sensations, motor functions, and even contributions to general consciousness. The direction of nerve signals is highly dependent on what type of signal is being transmitted. For example, a sensation related to heat would be picked up by sensory receptors in the skin and sent through the spinal cord upward toward the brain. Any necessary command on what to do about the heat sensed – move away, for instance – is sent down the spinal tracts and out to the appropriate muscles.
Types of Sensory Neurons in the Spinal Tract
In the spinal tract are two types of neurons: sensory neurons and motor neurons. Generally speaking, the sensory neurons travel up the spinal cord to transmit action impulses to the brain, thalamus, midbrain, or whatever its terminal point need be. There are two types of sensory neurons: somatic sensory neurons, which innervate the skin and skeletal muscles, and the visceral sensory neurons, which innervate the internal organs.
The path of a sensory motor neuron is as follows: mechanoreceptors located in one of the dermal layers or muscle spindles responds to a given stimulus and transmits an action impulse to the spinal nerve. The impulse then travels into the dorsal root, then into the dorsal gray horn, where it synapses onto an interneuron. This interneuron then decussates (crosses the midline to the opposite side of the spinal cord) and transmits the signal up toward the brain.
Several hundred (even thousands!) of interneurons are synapsed at once, meaning that synapsing onto motor neurons in the ventral gray horn is occurring simultaneously with the synapsing of sensory neurons. This signal travels through the ventral root, out of the spinal cord, and innervates the skeletal muscle cells.
So what if this sensory pathway was cut off before the brain? Do you need the brain to react to sensory information? No – not for all senses, anyway. However, if the brain was an essential part of a given sensory pathway in its original form, such damage would render you unable to interpret the sensation from its original point or receptor(s) (hand, leg, etc.). Without the brain, you would still be able to react to a sensation, though, only in the form of a reflex. Whether the signal travels all the way up to the brain or not has no impact on the path of the signal from the sensory neuron, to the interneuron, to the motor neuron, to the skeletal muscle. This pathway, absent from the brain, is called the “spinal reflex.”
A Brief Overview of the Motor Neurons
Note: The pathway of motor neuron action impulses is one that is descending, as commands are sent from the brain, down the spinal cord, to the muscles. Because of this, motor neuron functions will only be covered briefly here.
Now, onto the motor neurons, which can be split into somatic motor neurons and autonomic motor neurons. Somatic motor neurons control information that is sent to skeletal muscles for voluntary movement and reflexes. Autonomic motor neurons, on the other hand, send motor impulses to internal organs, controlling actions like respiration and the heartbeat.
Interestingly, with regard to voluntary motor control, it is possible that the sensory activation of the skin might not only cause a muscle on that respective side of the receptor to contract, but a stimulus might also cause a muscle on the other side of the body to contract!
Motor control of the internal organs is called an “autonomic reflex.” In an autonomic reflex, visceral sensory neurons send action impulses to the brain from the internal organs and, based on the gathered information, motor commands are sent back to the internal organs and smooth muscles.
The Spinothalamic Tract
The spinothalamic tract, also known as the anterolateral system, is organized into two main divisions: the anterior and lateral divisions. The anterior spinothalamic tract is believed to detect crude touch and pressure, whereas the lateral tract picks up pain and temperature. (The naming mechanism of the spinal nerve bundles is as follows: The first part of the name will tell you where it originates, the second part of the name will tell you the destination of the tract. For example, “spinothalamic” spino = spinal, thalamic = thalamus).
Receptors of the Spinothalamic Tract
The receptors that work along with the spinothalamic tract are called “nociceptors,” and these specifically detect tissue damage and extreme temperatures. There are two different types of nociceptors: A-delta, which detects fast pain or “pinprick” pain, and C fibers that pick up slow pain (lasting, enduring pain).
More specifically, A-delta fibers pick up fast pain and thermal sensations, as well, meaning that they respond to multiple types of stimuli:
- For example, during a mechanical stimulus, say you stub your toe: On impact, there are protein channels which are deformed when you strike your toe, thereby allowing sodium to enter the channel and activate the A-delta fiber.
- During a thermal stimulus, the permeability (ability to allow materials in and/or out of the cell wall) of the protein channel fluctuates. When temperatures are very cold, the channel becomes more permeable, allowing sodium into the channel and activating the A-delta fiber by triggering an action potential.
C fibers respond to one more type of stimulus than A-delta fibers: mechanical, temperature, and chemical stimuli. (C fibers are less myelinated than A fibers, meaning their action potentials travel just a bit slower than A fibers.)
- Let’s use the example of the game “hot hands,” where you and a friend line up your hands and take turns slapping each other to see who has the fastest evasion reflex (yes, this is a real game!): When you’re too slow and your hand gets slapped, there are certain chemicals released in an inflammatory response to the mechanical stimuli including metabolic acids/protons, potassium, bradykinin, and histamines. Once these are released, they initiate an action potential in the C fibers to relay pain sensory information.
A few more bonus receptors that work alongside the ascending spinal tracts to pass on information regarding mechanical, thermal, and chemical stimuli – specifically from crude touch and pressure – are Merkel’s discs, peritrichial nerve endings, and free nerve endings.
The spinothalamic tract conducts impulses from every level of the spinal cord up to the thalamus, conveying information specifically regarding pain and temperature. The sensory neurons, or nociceptors, are also referred to as “first-order neurons” because they are the first neuron in a sequence of three: sensory neurons enter the gray matter of the spinal cord, where they synapse. Then the impulse goes on to enter the white matter, ascending into the ventral posterolateral nucleus of the thalamus, and finally, to the cerebral cortex.
Action Impulse Pathway of Sensory Information
At the point of the first synapse event, different receptors target distinct points of the gray matter of the spinal cord (a part of the “butterfly” or “H” shape in the center of the spinal cord) called the dorsal horn. On this dorsal horn are layers called the Rexed laminae, which are distinguished by the type of sensory information they receive.
- When the C fibers are activated by their receptors are activated by the response to a mechanical, thermal, or chemical stimulus by their peripheral process of the receptors, the action potential is first passed through a pseudounipolar neuron (located within the dorsal root ganglion), then the central process, and synapses onto Rexed laminae II and III. From these Rexed Laminae, it continues to synapse as it decussates and travels through the anterior white commissure, and vertically upward through the remainder of the spinal cord.
- A-delta fibers follow a route identical to that of the C fibers (as far as the receptors’ peripheral process, pseudounipolar neuron, and the central process), except that they synapse onto Rexed laminae I and V. These also pass through the anterior commissure, into the anterolateral tract, then vertically up to the brain.
- Lastly, the Merkel’s discs, peritrichial nerve endings, and free nerve endings all travel through the Rexed laminae III, IV, and V. However, as they progress past the anterior commissure, these nerve tracts travel further into the anterior white column of the spinal cord before continuing vertically toward the brain.
The tracts that form the route through which these impulses travel vertically up toward the brain are called the Tract of Lissauer.
The A-delta fibers are believed to be a part of what is called the neospinothalamic pathway, whereas the C fibers are incorporated into the paleospinothalamic pathway (pay attention to the names here: neospinothalamic, neo meaning new, and paleospinothalamic, paleo meaning “older” or “primitive.” These names are reflective of scientists’ knowledge of these tracts). All of these tracts come together in what is called the anterolateral system.
The Anterolateral System
As the tracts of the anterolateral system travel upward and group together, they are each destined to terminate in specific points of the thalamus, depending on the type of sensory information they are transmitting. (Note: the paleospinothalamic tract is made up of C fibers, whereas the neospinothalamic tract consists of A fibers.)
The different points to which these tracts travel are called the ventroposterior lateral nucleus and the ventroposterior inferior nucleus. Another part of the thalamus that these nerve tracts travel to is called the intramedullary lamina – specifically, the intralaminar nuclei. Major intralaminar nuclei are called the centromedian nucleus and the parafasciculus nucleus. From these structures, the anterolateral system continues on to the cerebral cortex, informing the emotional aspects of pain. Other tracts can travel to the somatosensory cortex, the cingulate gyrus, and the anterior insular cortex – all of these play a part in allowing the anterolateral system to partially regulate the emotions associated with pain.
Apart from these two nuclei, a majority of the C fiber nerve tracts of the anterolateral system terminate in a structure called the reticular formation. Nerve tracts informed by the A-delta fibers, however, give off very few nerve tracts to the reticular formation. The A-delta fibers inform similar structures to the C fibers, in addition to the corona radiata and specific points of the cerebral cortex including the primary and secondary sensory cortexes to allow the brain to interpret the sensation of pain.
- Fink. (2013, January 17). The spinal cord & spinal tracts; part 1 by Professor Fink [Video file]. Retrieved from https://www.bing.com/videos/search?q=youtube+the+spinal+cord&view=detail&mid=7D51F1D2918F665B2CC97D51F1D2918F665B2CC9&FORM=VIRE
- Fink. (2013, January 17). The spinal cord & spinal tracts; part 2 by Professor Fink [Video file]. Retrieved from https://www.bing.com/videos/search?q=youtube+the+spinal+cord&&view=detail&mid=D0413958C915777BBCEBD0413958C915777BBCEB&&FORM=VDRVRV
- Ninja Nerd Science. (2018, January 8). YouTube [Video file]. Retrieved from https://www.youtube.com/watch?v=Xy2J_zzRjDI
- Cross section of Spinal Cord
- The Spinothalamic Tract