Ascending and descending spinal tracts
Definitions & Key takeaways
Ascending and descending spinal tracts are neural pathways within the spinal cord, which carry information up and down the spinal cord connecting the brain to the rest of the body.
The ascending tracts include the dorsal column-medial lemniscus system, the spinothalamic system, and the spinocerebellar system. These tracts carry sensory information from the periphery, such as pain, touch, and temperature to the brain.
The descending tracts include lateral and anterior corticospinal tracts, the vestibulospinal, the rubrospinal, and the reticulospinal tracts. These tracts carry motor information, such as a motor command to move the arm, from the brain down the spinal cord to the appropriate part of the body.
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.
Once it gets to the desired spinal segment it, decussates and synapses with lower motor neurons in the ventral horn. Lower motor neurons axons leave the spinal cord through the ventral root and peripheral nerves and reach the muscles of the trunk.
Lateral corticospinal tract also descends through the internal capsule and cerebral peduncle but once it gets to the medulla it decussates at the place called decussation of pyramids.
Now it continues through the contralateral spinal cord where it descends medial to the posterior spinocerebellar tract until it reaches the ventral horn where it synapses with lower motor neurons.
Lower motor neuron axons leave the spinal cord through the ventral root and peripheral nerves and reach the muscles of the extremities.
The corticobulbar tract also starts in the motor cortex where the upper motor neurons are located. The axons of these upper motor neurons form the corticobulbar tract which travels lateral to the corticospinal tract to reach the brainstem.
These axons will depart the tract and synapse directly with the contralateral lower motor neurons for Cranial Nerves V, VII, XI, and XII at their corresponding levels of the Pons and medulla.
Some of the upper motor neurons branch into two fibers which synapse with both the ipsilateral and contralateral motor nuclei.
These include cranial nerves V, which controls the muscles for chewing, XI, which controls the muscles of the neck, and the part of VII that innervates the muscles in the upper half of the face.
This means the muscles innervated by these nerves receive motor signals from the motor cortex from both hemispheres of the brain.
The upper motor neurons of cranial nerve VII that control the lower half of the face, and cranial nerve XII, which control tongue movement, crossover in the brainstem without branching and only synapse with the contralateral nuclei.
So the muscles innervated by these cranial nerves only receive motor information from the contralateral cerebral cortex.
Okay now, indirect motor pathways, also known as the extrapyramidal tracts, are a group of descending tracts that have their upper motor neurons originate from the deep nuclei in the brain stem instead of the motor cortex.
These tracts innervate larger muscles that play a role in maintaining balance, body posture and coarse movements. So first, we have the vestibulospinal tract which transmits motor input for the extensor muscles of the trunk and extremities helping us maintain balance.
For example, if you went outside for a walk and tripped over a rock with your right leg, vestibulospinal tract would send inputs to the extensor muscles of the left leg, extending it and prevent you from falling.
Inside the pons and the medulla there are four vestibular nuclei, the superior, medial, lateral, and inferior nuclei that gather information about body balance.
Inside the lateral vestibular nuclei are the upper motor neurons and their axons make the vestibulospinal tract. It descends to the ipsilateral spinal cord and synapses with short interneurons that activate the lower motor neurons in the ventral horn.
Their axons leave the spinal cord through the ventral root, go through the peripheral nerves and reach the extensor muscles that they innervate.
Next, is the reticulospinal tract which transmits motor input for the extensor muscles just like the vestibulospinal tract and together with it, helps maintain balance.
Throughout the brainstem from the medulla to the midbrain there is a network of neurons called the reticular formation, where “reticulum” means a net like structure.
This is made out of complexly connected neurons and it is not very well differentiated from the rest of the brainstem..It is in charge of many different processes like sleep, alertness and cardiovascular control, but it also has motor control.
Inside this reticular formation are the upper motor neurons whose axons make two reticulospinal tracts. The pontine, or medial, reticulospinal tract descends to the ipsilateral spinal cord where It continues without decussating, just like vestibulospinal tract.
There, it indirectly activates the lower motor neurons in the ventral horn whose axons travel to the extensor muscles. The other one is the medullary, or lateral, reticulospinal tract, which gives off an ipsilateral and contralateral branch.
Both branches travel through the ventromedial spinal cord where they either directly or indirectly inhibit lower motor neurons in the ventral horn that innervate the extensor muscles.
The next one is the tectospinal tract that transmits motor impulses for the neck muscles, enabling you to move your head so that your eyes can follow a moving object, like when you follow the waiter while he walks past you with someone else’s pizza.
On the dorsal side of the midbrain there is a pair of structures called superior colliculi, latin for the upper hill, which receives sensory input from the eyes.
It also holds the upper motor neurons whose axons make the tectospinal tract. On its way to the spinal cord, tectospinal tract decussates behind the cerebral aqueduct and continues to the contralateral spinal cord.
It descends to the cervical region where it ends by synapsing on the lower motor neurons in the ventral horn. Their axons go through the ventral root and peripheral nerves to the muscles of the neck that they innervate.
Finally, we have the rubrospinal tract that transmits motor impulses mainly for the flexor muscles of the extremities, helping produce smoother and more coordinated conscious movements.
In the rostral part of the midbrain there is a structure called the red nucleus, or nucleus ruber, that receives input from both cerebral and cerebellar cortex.
It holds the upper motor neurons whose axons make the rubrospinal tract. It decussates immediately after leaving the red nucleus and descends to the contralateral spinal cord in the neck region, where it synapses with interneurons that activate the lower motor neurons in the ventral horn.
These lower motor neurons send their axons out of the spinal cord to innervate the flexor muscles. Alright, as a quick recap.
Ascending and descending spinal tracts transmit information up and down the spinal cord between the brain and the body. There are three ascending tracts; the dorsal column-medial lemniscus system that transmits sensations like vibration, conscious proprioception, and fine touch, the spinothalamic (or anterolateral) system that transmits sensations like crude touch, pressure, pain and temperature, and the spinocerebellar tract that transmits sensations about the position of your body parts like your stance, and how flexed your knees, elbows, and wrists joints are.
As for descending tracts there are two main groups, direct motor pathways that transmit information necessary for the voluntary and fine movement, while indirect motor pathways mainly regulate balance,
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