– pain and temperature (Lateral Spinothalamic), crude touch and pressure(ventral spinothalamic)
Trigeminal-thalamic
– discriminative tactile and proprioceptive sensations
Trigeminal-lemniscal
– pain and temperature for face
Sensory Receptors (mechanoreceptors) and associated Sensory Axons
Low-threshold (high-sensitivity) mechanoreceptors are innervated by A
a and Ab heavily myelinated fibers (large diameter axons)
Weak mechanical stimulation of the skin (light touch, pressure, vibration, skin tension) induces these to produce APs. The myelination ensures rapid central transmission of tactile information. Used by DCML pathway (see below).
High-threshold (low-sens.) mechanoreceptors are innervated by Ad and C lightly myelinated/unmyelinated fibers (small diameter axons)
Strong mechanical stimulation of the skin (pain, temperature, crude touch) induces these to produce APs. Used by Spinothalamic tract.
Dorsal column-medial lemniscus (DCML) pathway
Functions
:
mediates tactile discrimination, vibration, form recognition, and joint and muscle sensation.
mediates conscious proprioception.
receives input from Pacini’s and Meissner’s corpuscles, join receptors, muscle spindles, and Golgi tendon organs.
Components
:
(1) First-order neurons
– located in DRG at all levels
give rise to the fasciculus gracilis (from the lower extremity) or fasciculus cuneatus (from the upper extremity)
axons ascend in the dorsal columns and terminate in the gracile and cuneate nuclei of the medulla
the dorsal columns are somatotopically organized so that lower levels (gracilis tract) are more medial and higher levels (cuneate tract) are more lateral.
some axons of the DRG also collateral to Nucleus proprius (layer IV) of spinal gray matter
(2) Second-order neurons
– located in the gracile and cuneate nuclei of the caudal medulla
axons (called internal arcuate fibers) decussate and form a compact fiber bundle, the medial lemniscus
the medial lemniscus ascends through the contralateral brainstem to terminate in the ventral posterolateral (VPL) nucleus of the thalamus. **note the medialÛ lateral change in position
in medulla the lemniscus rotates so that the fibers from the cuneatus are dorsal and the fibers from the gracilis are ventral. In the pons the lemniscus rotates ninty degrees so that the fibers from the cuneatus go medially and the fibers from the gracilis go laterally.
(3) Third-order neurons
– located in the VPL nucleus of the thalamus
project via the posterior 1/3 of the posterior limb of the internal capsule to the postcentral gyrus, the somatosensory cortex (Brodman’s areas 3, 1, and 2)
Note: it is in the internal capsule that the tract is most vulnerable to cerebrovascular accident (CVA)
in the postcentral gyrus the axons mirror the position of their origin (i.e., axons that started medial in the spinal cord are now medial again but in the opposite side of the body.
As the medial lemniscus travels to the thalamus, it incorporates sensory information from CN V, VII, IX, X