|

Sensory Neurones and Ascending Tracts

Last updated:

Overview

The ascending tracts are mainly sensory and carry impulses from peripheral nerves to the cerebral cortex. They can be divided into:

  • Conscious tracts:
    • Dorsal column-medial lemniscus pathway
    • Anterolateral system:
      • Anterior spinothalamic tract
      • Lateral spinothalamic tract
  • Unconscious tracts:
    • Spinocerebellar tracts:
      • Anterior spinocerebellar tract
      • Rostral spinocerebellar tract
      • Cuneocerebellar tract
      • Posterior spinocerebellar tract

Dorsal Column-Medial Lemniscus Pathway

Overview

The dorsal column-medial lemniscus pathway (DCML, dorsal column) is responsible for fine touch, proprioception, and vibration sense. The pathway consists of 3 groups of neurones:

  • First order neurones:
    • Carry sensory impulses from peripheral nerves to nuclei in the medulla:
    • Nucleus gracilis for the lower limb, nucleus cuneatus for the upper limb
  • Second order neurones:
    • Start in the nucleus gracilis or cuneatus in the medulla and decussate to reach the thalamus
  • Third order neurones:
    • Send impulses from the thalamus to the sensory cortex of the brain

In summary:

  • The dorsal column-medial lemniscus pathway (DCML, dorsal column) is responsible for fine touch, proprioception and vibration sense
  • Neurones conveying these senses decussate in the medulla

Clinical significance

If a lesion occurs in the DCML pathway, fine touch, vibration, and proprioception are lost:

  • If this occurs in the peripheral nerves or spinal cord, the signs and symptoms are ipsilateral (because this occurs before decussation which happens in the medulla)

Romberg’s test can be used to identify lesions in the DCML pathway. This is discussed more in Neurological Signs and Symptoms.

Vitamin B12 and folate deficiencies can lead to DCML pathway lesions.

Anterolateral System

Overview

The anterolateral system consists of two tracts:

  • Anterior spinothalamic tract: crude touch and pressure
  • Lateral spinothalamic tract: pain and temperature

Like the dorsal column-medial lemniscus pathway, these also have three groups of neurones:

  • First order neurones: from sensory receptors in the periphery:
    • They enter the spinal cord at a region 1-2 vertebral levels up called the substantia gelatinosa
  • Second order neurones: from the substantia gelatinosa to the thalamus:
    • These fibres decussate within the spinal cord and form the anterior and lateral spinothalamic tracts
  • Third order neurones:
    • Carry sensory impulses from the thalamus to the primary sensory cortex

In summary, the anterolateral system is comprised of:

  • Anterior spinothalamic tract: crude touch and pressure
  • Lateral spinothalamic tract: pain and temperature

Neurones conveying these senses decussate in the spinal cord

Clinical significance

If a lesion occurs in the anterolateral system, either crude touch and pressure and/or pain and temperature sensation is lost:

  • If the lesion is before the spinal cord, signs and symptoms are seen ipsilaterally (as decussation has not yet happened)
  • If a lesion occurs after entering the spinal cord, signs and symptoms are generally contralateral (as they are likely to be after the point of decussation)

An example is Brown-Séquard syndrome, which describes a Hemisection of the spinal cord due to trauma:

  • DCML pathway: ipsilateral loss of touch, vibration, and proprioception (as decussation happens in the medulla)
  • Anterolateral system: contralateral loss of pain and temperature sensation (as decussation happens in the spinal cord)
  • Corticospinal tracts (motor): ipsilateral hemiparesis

Spinocerebellar Tracts

Overview

The spinocerebellar tracts convey unconscious information such as proprioception from the muscles to the cerebellum. There are four main pathways:

  • Posterior spinocerebellar tract:
    • Carries proprioception from the lower limbs to the ipsilateral cerebellum
  • Anterior 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
  • Rostral spinocerebellar tract:
    • Carries proprioceptive information from the upper limbs to the ipsilateral cerebellum

Clinical significance

Lesions in the spinocerebellar tracts can lead to ipsilateral impairment of muscle coordination, however, in reality, this rarely happens in isolation. There are usually lesions in the descending motor tracts leading to weakness and/or paralysis which usually masks the loss of coordination.

Author