Ascending and descending spinal tracts

Transcript

Content Reviewers:

Yifan Xiao, MD

Ascending and descending spinal tracts are pathways that carry information up and down the spinal cord between brain and body.

The ascending tracts carry sensory information from the body, like pain, for example, up the spinal cord to the brain.

Descending tracts carry motor information, like instructions to move the arm, from the brain down the spinal cord to the body.

Both types of tracts are made up of neuronal axons that gather into long columns called funiculi, meaning long ropes, which are found inside the ventral, lateral and dorsal parts of the spinal cord.

Ascending tracts are sensory pathways that begin at the spinal cord and stretch all the way up to the cerebral cortex.

There are three types of ascending tracts, dorsal column-medial lemniscus system, spinothalamic (or anterolateral) system, and spinocerebellar system.

They are made up of four successively connected neurons.

First order neurons are found inside dorsal root ganglions from where they gather sensory input and send it to the second order neurons, usually found inside the spinal cord or brainstem.

They further transmit it to the third order neurons found inside the thalamus, and then the fourth order neurons in the cerebral cortex.

While ascending through the spinal cord these tracts cross over to the opposite side of the central nervous system, or CNS, meaning that the left side of the brain receives sensory input from the right side of the body and vice versa.

These crossings are called decussations and they happen at different levels of the CNS for each of these tracts.

Let’s start with the dorsal column-medial lemniscus system which is a sensory pathway that transmits delicate sensations like vibration, proprioception or sensation of the position of bodyparts, two-point discrimination, and touch.

Throughout the body there are receptors like mechanoreceptors found in the skin that sense touch, or proprioceptors, like the muscle spindles and Golgi tendon organs in the muscles and joints that sense body position.

When stimulated, they send sensory input through peripheral nerves that are made of the axons of first order neurons.

These nerves travel to the dorsal root ganglion where the first order cell bodies reside.

These ganglions send out short axons that enter the spinal cord and to the ipsilateral dorsal area, called the dorsal column.

Those axons that carry sensations from the lower parts of the body form the gracilis fascicle, which is the medial part of the dorsal column, while axons that carry sensations from the upper parts of the body make the cuneate fascicle, the lateral part of the dorsal column.

This organization where medial parts transmit lower body sensation and the lateral parts transmit sensation from higher body parts is called topographic organization. Okay, now look at how they ascend.

Both the gracilis fascicle and cuneate fascicle ascend through the ipsilateral spinal cord all the way up to the medulla where we can find the gracilis nucleus and cuneate nucleus.

Inside these two nuclei both fasciculi end by synapsing on the bodies of the second order neurons. Second order neuronal axons now decussate and form the pathway called medial lemniscus, which is Greek for ribbon, that ascends to the contralateral thalamus, more specifically to its ventral posterolateral nucleus.

Inside this nucleus is where medial lemniscus ends by synapsing on the third order neurons. Finally, the third order neuronal axons now ascend toward cerebrum, through the internal capsule to the primary somatosensory cortex which contains fourth order neurons, the final destination for the dorsal column.

Moving on! We have the spinothalamic system, also known as the anterolateral system, or ALS.

These are sensory pathways that transmit sensory information about crude touch from free nerve endings, pain from nociceptors, pressure from mechanoreceptors, and temperature from thermoreceptors from the skin and organs.

First order neurons travel from these receptors to the dorsal root ganglion just like with the dorsal column-medial lemniscus system, and then that information is sent through another bundle of axons that enters the ipsilateral spinal cord.

Here, they ascend one or two segments before synapsing directly or indirectly, through interneurons, on the second order neurons, whose cell bodies are found inside the posterior, or dorsal horn of the spinal cord.

Next, the second order neuronal axons decussate and form the anterior spinothalamic tract that transmits crude touch and pressure sensory inputs, and the lateral spinothalamic tract that transmits pain and temperature sensory input.

Both anterior and lateral spinothalamic tracts ascend through the contralateral spinal cord all the way up to the contralateral thalamus, again to its ventral posterolateral nucleus like the dorsal column system did, and end by synapsing on the third order neurons found inside this nucleus.

From here on it’s pretty much the same. Third order neuronal axons ascend toward the cerebrum through the internal capsule to the primary somatosensory cortex where they synapse on the fourth order neurons.

Okay. The last ascending tract, called spinocerebellar tract, is a sensory pathway that is in charge of sending sensory information that will help coordinate the muscles in the trunk and the limbs. These sensory input come from the proprioceptors.

But unlike dorsal column system, spinocerebellar tract transmits unconscious proprioceptive sensations about the position of your body parts like your stance, and how flexed are the joints like the knees and elbows.

These travel through first order neurons that form peripheral nerves and enter the dorsal root ganglion which send out axons that enter the spinal cord and synapse on the second order neurons found inside the ipsilateral gray matter.

Second order neuronal axons now make two spinocerebellar tracts. They either decussate right away and make the ventral spinocerebellar tract just lateral to the lateral spinothalamic tract, or they stay on the ipsilateral side and make the dorsal spinocerebellar tract.

Next, we can see the ventral spinocerebellar tract ascending to the superior cerebellar peduncle, passing through it and then decussating again before arriving at the ipsilateral cerebellar cortex where it synapses on neurons inside the cortex.

The dorsal spinocerebellar tract doesn’t decussate at all but ascends to the inferior cerebellar peduncle, passes through it and arrives at the ipsilateral cerebellar cortex as well, ending it’s journey by synapsing on the last neuron inside the cortex.

So compared to other sensory tracts, the spinocerebellar tract has three orders of neurons instead of four and transmits information between ipsilateral cerebellum and body.

Okay, now that we have finished ascending tracts let’s look into the descending tracts which are motor pathways that’s in charge of controlling muscles of the trunk and extremities.

Unlike sensory tracts that were made out of three to four successively connected neurons, motor tracts have only two; the upper and lower motor neurons. Upper motor neurons are found inside the cerebral cortex and deep nuclei of the brainstem, while lower motor neurons reside inside the anterior, or ventral, horns of the spinal cord.

From the ventral horns their axons leave the spinal cord as peripheral nerves that innervate the muscles in the body.

So first we have the direct motor pathways, also known as the pyramidal tracts. They send the motor input necessary for fine, conscious muscle movements like those needed to move the hand when drawing or writing.

When we look at the brain, in front of the somatosensory cortex there is the primary motor cortex that contains cells called pyramidal cells.

These cells are the upper motor neurons of the direct motor pathways and their axons make multiple tracts.

The anterior corticospinal tract, which controls the muscles of the trunk, descends through the internal capsule and cerebral peduncle to the spinal cord where it is positioned medial to the anterior spinothalamic tract.