Dorsal Column-Medial Lemniscal System

   The neuronal pathway which carry, 

• sense of vibration,  

• highly localized touch sensations,

• touch sensations which require fine gradations of intensity, 

• position sense 

through central nervous system is called, Dorsal Column-Medial Lemniscal System
 


     It includes one of the ascending tracts,

• dorsal column, present in the white matter of the spinal cord and 

• medial lemnisci in the brain through the medulla, pons, mid-brain and to the thalamus. where 3rd order neurons are present.

fig.1.1

Route:

                 Highly localized sensation is well developed in the fingers of human and allows us to feel fine textures and determine what an unknown object in our hands is without looking at it (stereognosis).

First Order Neuron: 
             Present in the dorsal root ganglion,

• peripheral process (as dendrite) from receptor - dorsal root ganglion
• central process (as axon) from dorsal root ganglion - gracile  nucleus & cuneate  nucleus in the medulla.
• pseudo-unipolar neuron.
  

• it has long axon which, unlike the antero-lateral system, don't cross the mid-line in the spinal cord
  
  

                This fine sensation is detected by Meirssner`s corpuscles that lie in the dermis of the skin close to the epidermis. When these structures are stimulated by slight pressure, an action potential is started. Alternatively, proprioceptive muscle spindles and other skin surface touch receptors such as merkel cells, Ruffini endings, Pacinian corpuscles, and hair follicle receptors (Peritrichial endings) may involve the first neuron in this ascending pathway of the spinal cord.

               The action potential travels up an axon (the cell body of the neuron will be in a dorsal root ganglion). (The neurons are classified as pseudo-unipolar, so they are regarded as having just one long process, which includes both a peripheral branch dendrite and central branch axon.) So the sensation travels from the skin, along the axon, past the neuronal cell body, and into the dorsal column of the spinal cord.

The axons continue inside the spinal cord, running up the posterior (dorsal) white column as,

• fasciculus gracilis  formed by axons from lower part of body, medially present.
• fasciculus cuneatus. axons from lower part of body above T6, lateral to mentioned above.

              At the medulla oblongata, these axons synapse with neurons in the gracile nucleus and cuneate nucleus.

Second Order Neuron:
             Present in the  gracile nucleus & cuneate nucleus,

• cross the mid-line in medulla as internal arcuate fibers and after crossing, it becomes medial lemniscus.
• they present sensory decussation in the mudulla.
• The medial lemniscus rotates 90 degrees at the pons.
• 
  

                Here, fibers from lower body lie lateral to those from upper. 
 At the medulla, the medial lemniscus is orientated perpendicular to the way the fibres travelled in the posterior columns. For example, in the columns, lower limb is medial, upper limb is more lateral. At the medial leminiscus, axons from the leg are more ventral, arm fibres more dorsal. Fibres from the trigeminal nerve (supplying the head) come in dorsal to the arm fibres, and travel up the lemniscus too.

 The secondary axons from neurons giving sensation to the head, stay at around the same place, while the leg axons move outwards.

                The axons travel up the rest of the brainstem, and synapse at the thalamus (at the ventral posterolateral nucleus for sensation from the neck, trunk, and extremities, and at the ventral posteromedial nucleus for sensation from the head).

Third Order Neuron:

                Neurons beginning in the thalamus travel up the posterior limb of the internal capsule, and again head and leg swap relative positions. The axons synapse in the primary sensory cortex, with and upper body more lateral while lower body sensation most medial (e.g., the paracentral lobule).
                NOTE:  To make your concept more clear, listen to the Lectures by dr Najeeb, the greatest teacher, ever.

                                         Dr Najeeb's Lecture on ascending tracts click here.