Sensory

Dorsal Columns

The dorsal columns pathway is responsible for fine touch and two-point discrimination, perception of vibration, and conscious proprioception (knowledge of where our joints, limbs, and bodies are in space). Because of its function, it is designed for speed of transmission and specificity of information. It accomplishes this through Meissner corpuscles (for fine touch), Pacinian corpuscles (for vibration), and muscle spindles and Golgi tendon organs (for proprioception). As this is a very fast-conducting pathway, it is not surprising that it uses highly myelinated fibers (Aα and Aβ).

The first-order neuron in this pathway, which is pseudo-unipolar, starts in the periphery with its specialized receptor. Its axons run centrally and then through the cell body (which is connected via a short process, as it is pseudo-unipolar in structure). From the cell body, the central process moves medially and into the dorsal column of the spinal cord. This all occurs ipsilaterally to the initial stimulus.

There are two separate components of the dorsal column in the spinal cord. The fasciculus gracilis contains fibers from dermatomal levels T7 and below. The fasciculus cuneatus contains the ascending fibers from dermatomal levels T6 and above. This pathway is designed for speed. It minimizes synapses and crossing of fibers over others. So when incoming fibers enter the spinal cord, they take up the most medial location they can in order to be out of the way. Thus, the fasciculus gracilis is located medially (on each half of the spinal cord) and the fasciculus gracilis is located laterally within the dorsal column portion of each side of the spinal cord. From here, fibers ascend in their respective components of the dorsal column. These are still the first-order neurons, as there has not been a synapse. They remain ipsilateral to the stimulus. Fibers ascend to the brainstem, where they will have their first synapse in the caudal medulla.

The fasciculus gracilis and fasciculus cuneatus have corresponding nuclei (nucleus gracilis for fibers from T7 and below, and nucleus cuneatus for fibers from T6 and above) in the caudal medulla. These nuclei contain the cell bodies of the second-order neurons of the pathway. These second-order neurons immediately decussate as the internal arcuate fibers. Once the fibers have decussated, they end up in a collection of axons called the medial lemniscus, which continues to ascend. Information is now contralateral to the location of the initial synapse.

Fibers in the medial lemniscus continue to ascend in the brainstem until they reach the ventroposterolateral (VPL) nucleus of the thalamus, which is responsible for sensation (all modalities) from the body. The third-order neuron, whose cell body lies in VPL, then projects to the somatosensory cortex for conscious perception of the stimulus. Not surprisingly, there is a clear organization to the layout of the cortex. Medial portions of the cortex are responsible for the leg, whereas lateral portions of the cortex are responsible for sensation of face. The arm is between them.

Spinothalamic Tract

The spinothalamic tract pathway is responsible for the perception of pain and temperature (both hot and cold). This is accomplished not through specialized receptors but rather through free nerve endings.

Whereas the dorsal columns pathway is designed for speed and specificity, this pathway is designed for sensitivity. This makes developmental and evolutionary sense. When a person steps on a nail, for example, it is much less important for them to know which exact millimeter of the foot has been affected than it is for them to know that there is a painful stimulus coming from the right foot and that they need to move it quickly. This is a slow-conducting pathway and uses lightly myelinated (Aδ) or unmyelinated (C) fibers.

The first-order neuron in this pathway, which is pseudo-unipolar, starts in the periphery (with its “receptor,” free nerve endings). Its axons run centrally past the cell body (located in the dorsal root ganglion) and into the spinal cord via the dorsal root. Once the first-order neuron enters the spinal cord, it ascends two segments in the dorsolateral fasciculus of Lissauer. Once the axons have ascended to their eventual level, they dive into the grey matter of the spinal cord where they will synapse on the second-order neuron of the pathway in the dorsal horn of the spinal cord. The second-order neuron then decussates in the anterior white commissure of the spinal cord before taking its place in the anterolateral portion of the spinal cord. This is why this pathway is also called the anterolateral system. These fibers are now contralateral to the initial stimulus. As these fibers layer into this tract, laterally located fibers are responsible for the lower extremities, and those responsible for sensation from the trunk and then the upper extremity fill in medially. These second-order fibers ascend in the anterolateral portion of the spinal cord. As they run superiorly into the brainstem, the tract becomes known as the spinal lemniscus. Just as with the dorsal columns pathway, these ascend to the VPL nucleus of the thalamus. The remainder of the pathway is identical to the dorsal columns. The third-order neuron of the pathway has its cell body in the VPL nucleus of the thalamus. It projects rostrally to the cortex for conscious perception of pain and temperature sensation.