The Ascending Tracts

Original Author: Oliver Jones
Last Updated: September 1, 2016
Revisions: 45

This article is about the ascending tracts – the pathways by which sensory information from the peripheral nerves is transmitted to the cerebral cortex. In some texts, ascending tracts are also known as somatosensory pathways or systems.

Functionally, the ascending tracts can be divided into the type of information they transmit; conscious or unconscious:

  • Conscious tracts – Comprised of the dorsal column-medial lemniscal pathway, and the anterolateral system.
  • Unconscious tracts – Comprises of the spinocerebellar tracts.

In this article, we shall look at the anatomy of these tracts in some detail, and consider the clinical implications.


The Dorsal Column-Medial Lemniscal Pathway

The dorsal column-medial lemniscal pathway (DCML) carries sensory modalities of fine touch (tactile sensation), vibration and proprioception.

Its name arises from the two major structures to comprise the DCML. In the spinal cord, sensation travels via the dorsal columns. In the brainstem, it is transmitted through the medial lemniscus.

There are three groups of neurones involved in this pathway:

Fig 1.0 - Overview of the DCML pathway

Fig 1.0 – Overview of the DCML pathway

First order neurones carry sensory information from touch or proprioceptive receptors to the medulla oblongata. There are two different pathways which the first order neurones take:

  • Signals from the upper limb travel in the fasciculus cuneatus (the lateral part of the dorsal column). They then synapse in the cuneate nucleus of the medulla oblongata.
  • Signals from the lower limb travel in the fasciculus gracilis (the medial part of the dorsal column). They then synapse in the gracile nucleus of the medulla oblongata

Second order neurones begin in the cuneate nucleus or gracilis. The fibres receive the information from the preceding neurones, and delivers it to the third order neurones in the thalamus. Within the medulla oblongata, these fibres decussate (cross to the other side of the CNS). They travel in the contralateral medial lemniscus to reach the thalamus.

Third order neurones take the sensory signals from the thalamus to the primary sensory cortex of the brain. They ascend from the ventral posterolateral nucleus of the thalamus, through the internal capsule, terminating at the sensory cortex.


The Anterolateral System

The anterolateral system consists of two separate tracts:

  • The anterior spinothalamic tract carries the sensory modalities of crude touch and pressure.
  • The lateral spinothalamic tract carries the sensory modalities of pain and temperature.

Much like the DCML pathway, both the tracts of the anterolateral system have three groups of neurones:

Fig 1.1 - The right anterolateral system.

Fig 1.1 – The right anterolateral system.

First order neurones arise from the sensory receptors in the periphery. They enter the spinal cord, ascend 1-2 levels, and terminate at the tip of the dorsal horn (an area known as the substantia gelatinosa).

Second order neurones carry the sensory information from the substantia gelatinosa to the thalamus. Arising from a synapse with the preceding neurones, the neurones decussate (cross to the other side of the CNS). Here, fibres split:

  • Crude touch and pressure fibres enter the anterior spinothalamic tract.
  • Pain and temperature fibres enter the lateral spinothalamic tract.

Although they are functionally distinct, these tracts run alongside each other, and they can be considered at a single pathway. The fibres travel in their respective pathways, synapsing in the thalamus.

Third order neurones take the sensory signals from the thalamus to the primary sensory cortex of the brain. They ascend from the ventral posterolateral nucleus of the thalamus, through the internal capsule, terminating at the sensory cortex.

Fig 1.2 - The location of the ascending tracts, within the spinal cord.

Fig 1.2 – The location of the ascending tracts, within the spinal cord.


The Spinocerebellar Tracts – Unconscious Sensation

Both the DCML and the anterolateral tracts transmit conscious sensations, such as pain, touch and temperature. The tracts that carry unconscious proprioceptive information are collectively known as the spinocerebellar tracts. Although we cannot physically acknowledge these signals, they help our brain co-ordinate and refine motor movements.

They transmit information from the muscles to the cerebellum.

Within the spinocerebellar tracts, there are four individual pathways:

  • Posterior spinocerebellar tract – Carries proprioceptive information from the lower limbs to the ipsilateral cerebellum.
  • Cuneocerebellar tract – Carries proprioceptive information from the upper limbs to the ipsilateral cerebellum.
  • Anterior spinocerebellar tract – Carries proprioceptive information from the lower limbs. The fibres decussate twice – and so terminate in the ipsilateral cerebellum.
  • Rostral spinocerebellar tract – Carries proprioceptive information from the upper limbs to the ipsilateral cerebellum.

Clinical Relevance: Injury to the Ascending Tracts

Injury to the DCML Pathway

A lesion of the DCML pathway causes a loss of proprioception and fine touch. However, a small number of tactile fibres travel with the anterolateral system, and so the patient is still able to perform tasks requiring tactile information processing.

If the lesion occurs in the spinal cord (which is more common), the sensory loss will be ipsilateral. DCML lesions can occur in Vitamin B12 deficiency and Tabes dorsalis (a neurological disorder seen in neurosyphilis).

Injury to the Anterolateral System

Damage to the anterior and lateral spinothalamic tracts will cause a loss of pain and temperature sensation. In contrast to DCML lesions, this sensory loss will be contralateral (the spinothalamic tracts decussate within the spinal cord).

Brown-Séquard syndrome refers to a hemisection (one sided lesion) of the spinal cord. This is most often due to traumatic injury, and involves both the anterolateral system and the DCML pathway:

  • DCML pathway – ipsilateral loss of tactile sensation and proprioception
  • Anterolateral system – contralateral loss of pain and temperature sensation.

It will also involve the descending motor tracts, causing ipsilateral hemiparesis.

Injury to the Spinocerebellar Tracts

Lesions of the spinocerebellar tracts present as an ipsilateral loss of muscle co-ordination. However, the spinocerebellar pathways are unlikely to be damaged in isolation.

Accompanying these types of lesions is Injury to the descending motor tracts. This will cause muscle weakness or paralysis, and usually masks the loss of muscle co-ordination.

 

Rate This Article

99

Average Rating:

Quiz

Question 1 / 4
Which of the following functions are not supported by the DCML?

Quiz

Question 2 / 4
Which tracts carry unconcious proprioceptive information?

Quiz

Question 3 / 4
Third order neurones of the DCML ascend from which structure to terminate in the sensory cortex?

Quiz

Question 4 / 4
Damage to the anterolateral system will present with which symptoms?

Results