Question

How can you consciously tell the difference between a touch on the right arm versus a touch on the left foot?

Answer 1

the pathways involved in proprioception are

1. The sensations like deep touch, pressure, tactile localisation (the ability to locate exactly the pro prioceptive part touched), tactile discrimination (the ability to localise two separate points on the skin that is touched), stereognosis (ability to recognise that is touched), stereognosis (ability to recognise shape of object held in hand) and sense of vibration are carried by fasciculus gracilis and fasciculus cuneatus.

2 Fasciculus gracilis (tract of Goll): It commences at the caudal limit of spinal cord and is composed mainly caud of the long ascending branches of the medial division of fibres of dorsal nerve roots. These are the first order neuron fibres from dorsal root ganglia. These run directly upwards (without relaying in the spinal grey matter) in the posterior column of white matter of spinal cord. As the tract ascends, it receives accession from each dorsal root. The fibres which enter in the coccygeal and lower sacral region are thrust medially by fibres which enter at higher levels.

Fasciculus gracilis which contains fibres derived Fasciculus gracilis which contains fibres derived from lower thoracic, lumbar, sacral and coccygeal segments of spinal cord occupies the medial part of posterior column of upper part of spinal cord and is separated from fasciculus cuneatus by postero separated from fasciculus cuneatus by postero- intermediate septum.

3 Fasciculus cuneatus (tract of Burdach): It commences in mid-thoracic region. It derives its fibres from upper thoracic and cervical segments.

Both fasciculi contain first neuron fibres from central process of dorsal root ganglia and end by synapsing with the neurons in nucleus gracilis and nucleus cuneatus, situated in the medulla oblongata from cuneatus, situated in the medulla oblongata from where second neuron fibres take origin.

Now the fasciculus gracilis and fasciculus cuneatus terminates to Relays in and nucleus gracilis and nucleus cuneatus respectively.

2nd order fibres form medial lemniscus which reaches posterolateral ventral nucleus of thalamus

From thalamus another relay occurs and the third order neurons ends in broadmans area 3, 1, 2 ( somesthatic area) of cerebrum.