ANTEROLATERAL SYSTEM

ASCENDING TRACTS
(Sensory physiology)

Before going in to the details of ANTEROLATERAL group of ascending tracts, we should have a clear concept of ASCENDING TRACTS.... ..........

What the ASCENDING TRACTS are?

The answer to this question is that the spinal cord is basically a downward elongated part of central nervous system. Among many different functions which the spinal cord performs, in order to co-ordinate the different body functions, ONE VERY IMPORTANT FUNCTION is that “it acts as a conduit” for the transfer of information between the higher centers of the CNS, i.e. brain, and different internal and external receptor systems, i.e. PNS.

Other functions include,

• It acts as a relay center for many types of sensory information.
• It acts as a reflex center for eliciting the local reflexes.
• It integrates and performs its role in motor control for some repetitive motions, such as walking. Higher signals only make a correction, when necessary.
• Some other autonomic functions. Mainly sympathetic, and also parasympathetic to some extent.

The above mentioned functions along with others are mainly a function of CENTRAL GRAY MATTER of the spinal cord, in the neuronal cell bodies. But the spinal cord’s “conduit” function, which is our main topic of discussion now, is carried out through WHITE MATTER of the spinal cord.

So, the antero lateral pathway (also called anterolateral system) can be defined as,

'' the route of passage of primitive type of sensory information through the spinal cord white matter.''

click on the image to enlarge.

antero-lateral system

Overview of the transfer of sensory information through the antero-lateral system.

Receptor (e.g. pain) → peripheral process of sensory nerve root → cell body of the neuron is in dorsal root ganglion i.e. FIRST ORDER NEURON → central process enter the spinal cord via post root → the central process synapse with 2^nd order neuron in substantia gelatinosa of the spinal cord. → these second order neurons send processes  which cross to the opposite side → the same process (very long axon) ascend in the anterolateral white matter of the spinal cord on the contralateral side → in the brain stem, this long axon is part of spinal lemniscus˚ → ventro-anterior and vntroposteromedial nuclei of thalamus (2^nd order neuron synapse with 3^rd order neuron here.) → 3^rd order axons pass through the INTERNAL CAPSULE and then fibers eventually end in the somatosensory cortex of the cerebrum.

˚spinal lemniscus consist of spinotectal tract, anterior spinothalamic tract and lateral spinothalamic tract.