Dorsal column–medial lemniscal (DcmL) pathway

The area of skin innervated by a dorsal root is a dermatome. These are number for the spinal cord segment served by the dorsal root. The Cutaneous low threshold (simply stimulated) mechanoreceptor main afferent axons relaying skin mechanoreceptor an proprioceptor signals enter the dorsal roots to synapse with the interneurons, DHCs (dorsal horn cells), in Rexed laminae III–VI. DHCs mediate or alter spinal reflexes. Each afferent sends a collateral up the dorsal columns to synapse with neurons in the dorsal column nuclei (DCN) in the medulla. The cuneate nucleuses receive input from C1–8 and T1–6, while the gracile nucleus gets its inputs from the T7–12, lumbar and sacral spinal segments. The Lateral inhibition in DCN shapes this input as shown in figure below:

The Axons of DCN neurons cross the midline to rise on the opposite side of the cord as the medial lemniscus, terminating in the ventroposterolateral (VPL) part of the ventrobasal thalamus as shown in figure.The VPL neurons give ascend to thalamo-cortical axons, that project to the primary somatosensory cortex SI (Brodmann’s regions1, 2, 3a, and 3b), located over the postcentral gyrus. The SI neurons in turn project to SII as shown in figure below.

                           Figure: The dorsal column–medial lemniscal system. All neurons exposed are excitatory.

Figure: Position of the primary (SI) and secondary (SII) somatosensory cortex of the left cerebral hemisphere, lateral aspect. The Numbers refer to the Brodmann regions.

The common properties of the DCML system are as follows:

  ? The huge strength of its synaptic connections.

  ? The properties of neurons are matched to those of the sensory receptors supplying them, therefore the dynamic features of stimuli are transmitted with high fidelity.

  ? The Somatotopic mapping preserves localization at each stage. The body maps are found in VPL, DCN, and each of the four areas in SI. The Cutaneous and proprioceptor input are partially individually streamed as shown in figure.

  Figure: The interconnections of the thalamus and somatosensory cortex sight across the line A–B

The Neurons in SI are organized into columns aligned at right angles to the brain surface. Each column gets input from merely a single type of receptor, and from a particular location. The adjacent positions are represented in adjacent columns in a somatotopic way. The Extensive neural connections exist within a column; that is the connections between columns are sparse.

Lesion studies illustrate that region 3b is significant for tactile discrimination; region 1 is concerned with the analysis of texture, and region 2 with stereognosis, the capability to perceive the 3-dimensional shape of an object by touch. Additionally to the cutaneous input, region 2 gets input from muscles and joints and has the reciprocal connections with motor cortex. These are not included in altering ongoing movements but might inform the motor system of the sensory consequences of moving.

The secondary somatosensory cortex (SII) obtains input from the thalamus and SI. Most of the neurons in SII have bilateral RFs, i.e., stimuli in corresponding areas on both sides of the body will remind a response. Inputs from the contralateral body surface increases as a direct consequence of the decussation of the medial lemniscus. The Inputs from the ipsilateral body surface enter SII from the contralateral side through the corpus callosum. By integrating information from mutual sides of the body, the SII is a first stage in forming entire body perceptual experience. It enables the tactile discrimination learned by using one hand to be easily performed with the other.

The SII is significant in controlling movement in the light of somatosensory input through its connections with the motor cortex. Additionally, SII has inputs to the limbic cortex enabling the tactile learning.