The Somatosensory Tract

The spinothalamic tract is a sensory pathway originating in the spinal cord. It transmits information to the thalamus about pain, temperature, itch, and crude touch. The pathway decussates at the level of the spinal cord.

Somatosensory organization is divided into the dorsal column–medial lemniscus tract (the touch/proprioception/vibration sensory pathway) and the anterolateral system, or ALS (the pain/temperature sensory pathway). Both sensory pathways use three different neurons to get information from sensory receptors at the periphery to the cerebral cortex. 

These neurons are designated primary, secondary, and tertiary sensory neurons. In both pathways, primary sensory neuron cell bodies are found in the dorsal root ganglia, and their central axons project into the spinal cord.

Function

The types of sensory information transmitted via the spinothalamic tract are described as affective sensation. This means that the sensation is accompanied by a compulsion to act. For instance, an itch is accompanied by a need to scratch, and a painful stimulus makes us want to withdraw from the pain.

There are two subsystems:

1. Direct (for direct, conscious appreciation of pain).
2. Indirect (for affective and arousal impact of pain). 

Indirect projections are further divided into:

• Spino-reticulo-thalamo-cortical (part of the ascending reticular arousal system, also known as ARAS).
• Spino-mesencephalic-limbic (for affective impact of pain).

The Corticospinal Tract

The corticospinal tract conducts impulses from the brain to the spinal cord. It contains mostly motor axons. The corticospinal tract is made up of two separate tracts in the spinal cord: the lateral corticospinal tract and the anterior corticospinal tract. 

The corticospinal tract also contains the Betz cell (the largest pyramidal cells) that are not found in any other region of the body. An understanding of these tracts leads to an understanding of why one side of the body is controlled by the opposite side of the brain.

The corticospinal tract is concerned specifically with discrete, voluntary, skilled movements, such as the precise movement of fingers and toes. The brain sends impulses to the spinal cord that relay the message. 

This is imperative in understanding that the left hemisphere of the brain controls the RIGHT side of the body, while the right hemisphere of the brain controls the LEFT side of the body. The signals cross in the medulla oblongata, and this process is also known as decussation.

Spinal cord tracts

Spinal cord tracts are identified.

Spinal cord tracts are identified in this drawing. They are the pyramidal tracts, extrapyramidal tracts, spinocerebellar tracts, the lateral spinothalamic tract, and the anterior spinothalamic tract.

Function

The primary purpose of the corticospinal tract is to maintain voluntary motor control of the body and limbs. However, connections to the somatosensory cortex suggest that the pyramidal tracts are also responsible for modulating sensory information from the body. 

Some of these connections cross the midline; therefore, each side of the brain is responsible for controlling muscles for the limbs on opposite sides of the body. However, control of trunk muscles is on the same side of the body.

After a patient's pyramidal tracts are injured, the patient is paralyzed on the corresponding side of the body. Fortunately, they can re-learn some crude, basic motions, but not fine movements. This implies that the connections to these tracts are crucial for fine movement, and only partial recovery is possible if they are damaged.