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The dorsal column sustains an immense amount of connections with the peripheral nervous system. It relays information related to many different types of touch, motor functionality, and more. The nerve signals that travel through the dorsal column also travel a bit differently than most others, in that they decussate at a different point in their transmission pathway, and the stages of transmission and points of synapsing are different from other spinal tracts.
In fact, because of the nerve connections that are regulated by the dorsal column, along with its wide variety of functions, the stimulation of the dorsal column is used as a form of treatment for chronic pain like complex regional pain syndrome (CRPS).
Just like all other sensory, motor, and cognitive pathways in the body, however, the dorsal column is continuously sending an incredible amount of neuronal information from the receptors in the skin, muscles, and more, straight to the brain, through the brainstem, and other structures such as the thalamus. Let’s look a bit closer at the structure and functionality of the dorsal column.
Nerve Connections of the Dorsal Column
The dorsal column, also known as the medial lemniscal pathway, is an ascending pathway of the spinal cord (meaning it is responsible only for sending information from receptors and elsewhere in the peripheral nervous system up toward the brain) and is located on the posterior portion of the spinal cord. It is responsible for transmitting sensory information related to
- Discriminative touch: picking up fine touch, and touch that occurs at more than one point of contact
- Stretching of skin and muscles
- Proprioception: awareness of the position of joints and muscles in space (anatomical spatial awareness)
The receptors that work with the medial lemniscal pathway are the…
Meisner’s corpuscles. These are in the dermal papillae of the skin (structures located between the epidermis and dermis layers of the skin) and take in information related to discriminative touch. (Discriminative touch is also known as “fine touch,” wherein receptors provide highly detailed information on what is being touched, and assisted by its position in the skin, it informs the dorsal column on very gentle, or “fine” touch, such as a spider web or strand of hair on the skin.)
Merkel’s discs. These are in the stratum basale (also known as the stratum germinativum) of the skin – which is the deepest of the five layers of the epidermis. Merkel’s discs are receptors of fine touch and superficial pressure.
Pacinian corpuscles. These receptors are just a bit deeper than the Meisner’s corpuscles and Merkel’s discs and can be found in more than one layer – in the dermis, hypodermis, and more. Their location enables them to respond to deep pressure.
Peritrichial nerve endings. These nerve fibers are found in the hair follicles and respond to the bending of the hair and are partially receptive to fine touch.
Ruffini corpuscles. These mechanoreceptors are also called bulbous corpuscles or Ruffini endings and respond to the stretching of the skin. They do this by detecting tension deep in the skin and fascia (the fibrous membrane that envelops muscles, internal organs, and many other bodily tissues).
Proprioception (specifically as it relates to skeletal muscles) that is regulated by the medial lemniscal pathway is transmitted by two distinct types of fibers (that, of course, divide even further into more types of fibers!):
Extrafusal fibers: fibers that surround the majority of muscle spindles. They are the main ones responsible for the contraction of the muscle.
Intrafusal fibers: fibers that function as proprioceptors, detecting the stretching of muscles (this detection is assisted by structures known as the Golgi tendon organs). Intrafusal fibers break further down into two sub-types of proprioceptive fibers (also called “1A” fibers)…
- Nuclear chain fibers: Detects the onset of the stretching – it is important to note that it is the “onset” specifically that is detected by these fibers! This is because they work in tandem with the second type of intrafusal fiber…
- Nuclear bag fibers: Detect the progressive stretch, or entire act of the stretch, of the muscle.
In the medial lemniscal pathway, there are three different types of nerve fibers: A, B, and C. The differences between the three lies in the degree of myelination of the axons, which influences the speed at which thy transmit signals. They are ordered from the least to the most myelinated: A being the least, C being the most. Of course, like everything in the human nervous system, we have to make things more complicated.
There are three sub-types of A fibers, each even more specialized for certain types of physical sensation: they are called alpha, beta, and delta.
- A-alpha fibers work along with muscle fibers and the Golgi tendon organs and, even though they’re the least myelinated of the three sub-types, they can still transmit action potentials of up to 120m per second! The A alpha fibers assist in detecting the stretching of the skin and muscles.
- A-beta fibers work with all of the aforementioned mechanoreceptors to inform the nervous system of pressure, touch, vibration, and discriminative touch.
- A-delta fibers are crucial to the detection of quick, fleeting pains such as the prick of a pin.
Organization of the Dorsal Column
As you may recall, like all spinal tracts, all of these fibers, A through C, enter the spinal cord through the dorsal root ganglion, travel to the dorsal gray horn, and then into a fasciculus below the T6 vertebra called the fasciculus gracilis without synapsing. Did you catch the huge difference here? Most spinal tracts enter the spinal cord through the dorsal root ganglion and synapse right away in either the dorsal, ventral, or lateral horns. The medial lemniscal pathway does not!
(Note: Transmitting the information from below T6 means its gathering information from the lower limbs, and, generally, the lower half of the body.)
The receptors and muscle spindles, on the other hand, transmit signals from above T6 and send it through a separate fasciculus lateral to the fasciculus gracilis – the fasciculus cuneatus.
The nerves of the medial lemniscal pathway are organized in the following manner (laterally to medially, meaning from the outer region of the body, inward): S L TC, representing sacral, lumbar, thoracic, and cervical. What this does is ensures that action impulses from the lowest portions of the body are sent to the brain through the most medial route, and, as they ascend in origin, are sent via more lateral routes.
So, what is something else you notice that sets the dorsal column apart from most other spinal nerve tracts? Typically, spinal tracts take another step in the process of sending information to the brain: After synapsing in either the dorsal or ventral gray horns, spinal nerves will decussate, or cross over, to the opposite side of the spinal cord to then complete their journey to the brain, thalamus, cerebellum, or other nervous system structure. The medial lemniscal pathway, however, does not decussate at this point. Because of this, this pathway is referred to as “ipsilateral” – meaning “the same side.”
Pathway of Action Impulses Through the Dorsal Column
Speaking of the thalamus, the fasciculus gracilis and fasciculus cuneatus also send impulses from the spinal cord up to the thalamus for the sensations of touch, pressure, and proprioception (also known as kinesthesia). Kinesthesia (kinesis = movement, esthesia = sensation) means the sensation of movement – you can feel the movement and positioning of your body parts without having to look at them or anything else. Your body maintains individual awareness of movement because of the sensation transmitted to your brain. Remember that if the spinal cord was severed at some part, you would lose this awareness. You may still be able to move the body parts reflexively, but you would not be physically aware of their movement.
Going back to the fact that the medial lemniscal pathway doesn’t synapse where it enters the spinal cord: It instead extends vertically and synapses in the medulla oblongata, specifically, onto one of the two structures known as the nucleus cuneatus or the nucleus gracilis. Laterally to medially, these are organized like so: N.C. – N.G. – N.G. – N.C. This part of the synapse is known as the first-order neuron.
From there, the second-order neuron decussates and travels upward toward the thalamus. The nerves synapse onto the third neuron, which then sends the signal to the cerebral cortex. (This is somewhat similar to the spinothalamic tract.)
The point at which the tract decussates is called the internal arcuate fiber, and it is found in the dorsal part of the medulla oblongata. From here, the tract moves up through the pons, midbrain, and as it continues, becomes the left and right medial lemniscus tracts. The now distinct tracts continue up the brainstem, to eventually synapse in the thalamus on the ventroposterior lateral nucleus.
The pathway then moves up to the posterior third limb of a projection fiber known as the internal capsule, through the corona radiata, which then informs various parts of the cerebral cortex – various sensations of the body are also sent to highly specific parts of the parietal lobe, down to the point!
Whew, time to take a breath! That was a lot, wasn’t it? The human nervous system is a very complex thing. Keep in mind that this was only one pathway! Communication of pressure, fine touch, stretching, and more – not just from the skin, but muscles as well – are all transmitted through this singular pathway! There’s still much more to learn about the functions of your nervous system that keep you alive and functioning!
Ninja Nerd Science. (2018, January 9). Ascending tracts | Dorsal column: Medial lemniscus pathway [Video file]. Retrieved from https://www.bing.com/videos/search?q=dorsal+columns&view=detail&mid=645FA3B0B39D1455CE51645FA3B0B39D1455CE51&FORM=VIRE