Ascending Pathways: Neuroanatomy | Medical Science Optional Notes for UPSC PDF Download

Sensory Pathways/Ascending Tracts

The ascending tracts denote the neural pathways through which sensory information from the peripheral nerves is conveyed to the cerebral cortex. From a functional perspective, these ascending tracts can be categorized based on the type of information they transmit, whether conscious or unconscious:

• Conscious tracts consist of the dorsal column-medial lemniscal pathway and the anterolateral system.
• Unconscious tracts comprise the spinocerebellar tracts.

Dorsal column-Medial Lemniscal pathway

The dorsal column-medial lemniscal pathway (DCML) is responsible for transmitting sensory modalities such as fine touch (tactile sensation), vibration, and proprioception. The name of this pathway is derived from the two main structures it encompasses. In the spinal cord, information is conveyed through the dorsal (posterior) columns, while in the brainstem, it is transmitted via the medial lemniscus. This pathway involves three groups of neurons: first-order, second-order, and third-order neurons.

First Order Neurones

The first-order neurons convey sensory information related to touch, proprioception, or vibration from the peripheral nerves to the medulla oblongata. These neurons follow two distinct pathways:

• Signals originating from the upper limb (T6 and above) traverse the fasciculus cuneatus, which is the lateral part of the dorsal column. Subsequently, they synapse in the nucleus cuneatus located in the medulla oblongata.
• Signals originating from the lower limb (below T6) travel through the fasciculus gracilis, which is the medial part of the dorsal column. These signals then synapse in the nucleus gracilis situated in the medulla oblongata.

Second Order Neurones

The second-order neurons originate in either the cuneate nucleus or gracilis. These fibers receive information from the preceding neurons and convey it to the third-order neurons located in the thalamus. Within the medulla oblongata, these fibers decussate, crossing to the other side of the central nervous system (CNS). Subsequently, they travel through the contralateral medial lemniscus to reach the thalamus.

Concerning the third-order neurons, they are responsible for transmitting the sensory signals from the thalamus to the ipsilateral primary sensory cortex of the brain. These neurons ascend from the ventral posterolateral nucleus of the thalamus, traverse the internal capsule, and ultimately terminate at the sensory cortex.

The Anterolateral System

The anterolateral system comprises two distinct tracts: the anterior spinothalamic tract, responsible for transmitting sensory modalities such as crude touch and pressure, and the lateral spinothalamic tract, which conveys sensory modalities of pain and temperature. Similar to the DCML pathway, both tracts within the anterolateral system involve three groups of neurons.

First-Order Neurons:
First-order neurons originate from sensory receptors in the periphery. They enter the spinal cord, ascend 1-2 vertebral levels, and synapse at the tip of the dorsal horn in an area known as the substantia gelatinosa.

Second-Order Neurons:
Second-order neurons transmit sensory information from the substantia gelatinosa to the thalamus. After synapsing with the first-order neurons, these fibers decussate within the spinal cord and form two distinct tracts:

• Fibers related to crude touch and pressure enter the anterior spinothalamic tract.
• Fibers related to pain and temperature enter the lateral spinothalamic tract.

Despite their functional distinction, these tracts run alongside each other and can be considered as a single pathway. They ascend superiorly within the spinal cord, synapsing in the thalamus.

Third-Order Neurons:
Third-order neurons transmit sensory signals from the thalamus to the ipsilateral primary sensory cortex of the brain. Originating from the ventral posterolateral nucleus of the thalamus, they traverse the internal capsule and terminate at the sensory cortex.

Spinocerebellar Tracts

The DCML and anterolateral tracts serve as conduits for conscious sensations such as pain, touch, and temperature. In contrast, the tracts responsible for conveying unconscious proprioceptive information are collectively referred to as the spinocerebellar tracts. While we may not be consciously aware of these signals, they play a crucial role in helping our brain coordinate and refine motor movements. These tracts transmit information from the muscles to the cerebellum.

Within the spinocerebellar tracts, four distinct pathways exist:

1. Posterior Spinocerebellar Tract

   • Conveys proprioceptive information from the lower limbs to the ipsilateral cerebellum.

2. Cuneocerebellar Tract

   • Conveys proprioceptive information from the upper limbs to the ipsilateral cerebellum.

3. Anterior Spinocerebellar Tract

   • Conveys proprioceptive information from the lower limbs. The fibers decussate twice, terminating in the ipsilateral cerebellum.

4. Rostral Spinocerebellar Tract

   • Conveys proprioceptive information from the upper limbs to the ipsilateral cerebellum.

Sensory Pathway Lesions

Defects in the DCML Pathway A disruption in the DCML pathway results in the loss of proprioception and fine touch. However, due to a small number of tactile fibers traveling within the anterolateral system, patients can still perform tasks requiring tactile information processing. If the lesion occurs in the spinal cord, which is the most common scenario, the sensory loss is ipsilateral, as decussation takes place in the medulla oblongata. Vitamin B12 deficiency and tabes dorsalis (a complication of syphilis) are conditions associated with DCML lesions.

Injuries to the anterolateral system lead to an impairment of pain and temperature sensation. Unlike DCML lesions, this sensory loss is contralateral, as the spinothalamic tracts decussate within the spinal cord.

Brown-Sequard Syndrome, characterized by a hemisection (one-sided lesion) of the spinal cord, typically results from traumatic injury. It involves both the anterolateral system and the DCML pathway:

• DCML pathway: Ipsilateral loss of touch, vibration, and proprioception.
• Anterolateral system: Contralateral loss of pain and temperature sensation.
• It also affects the descending motor tracts, causing an ipsilateral hemiparesis.