From Neurone to Brain 9

Ascending spinal tracts take impulses from pain, thermal, tactile, muscle and joint receptors to the brain. 

The first order neurons enter the dorsal root making synapses that mediate spinal reflexes and intersegmental co-ordination. The main fibre remains on the inpsilateral side, terminating with the second neuron in the spinal grey matter or in the medulla oblongata of the brain stem. The second neuron's axon decussates to the opposite side ascending to the thalamus, terminating at the third neuron. This neuron has its cell body in the thalamus and the axon passes to the somatosensory cortex in the parietal lobe.

The two main tract systems that follow this process are the dorsal columns and the spinothalamic tract.

The dorsal columns are between the dorsal median sulcus and the dorsal horn. It has 2 tracts, the fasciculus gracilis (medial) and the fasciulus cuneatus (lateral). These carry impulses involved with proprioception and fine touch. The fibres in the fasciculus gracilis join at the sacral lumbar and lower thoracic levels so includes the lower limb. The fibres in the fasciculus cuneatus enter via the upper thoracic and cervical dorsal roots so includes the upper limb.

Both sets of fibres ascend without interruption to the medulla oblongata, terminating upon second order neurons with the cell bodies located in the nucleus gracilis and nucleus cuneatus. The second order neurons decussate in the medulla as internal arcuate fibres then ascend as medial lemniscus. This terminates in the ventral psoterior nucleus of the thalamus. This will then project to the somatosensory cortex in the postcentral gyrus of the parietal lobe.

The spinothalamic tract lies laterally and ventrally to the ventral horn of the spinal grey matter. It carries information on pain, thermal sensations and non-discrimintive touch and pressure. The second order neurons lie in the contralateral dorsal horn, receiving input from primary afferent fibres. Spinothalamic axons decussate to the opposite side passing through the ventral white commissure entering the contralateral spinothalamic tract. The touch and pressure axons ascend further than the pain and temperature axons. The spinothalamic axons go on to form the spinal lemniscus. The majority of the fibres terminate in the ventral posterior nucleus of the thalamus contacting the third order thalamocortical neurons projecting to the somatosensory cortex.

The spinoreticulothalamic system is the route that noiceptive sensory impulses…