Anatomy of the descending spinal cord pathways

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Introduction0:00–0:39

The spinal cord is like a highway that enables two-way communication between the brain and the rest of the body. It contains neural pathways, called spinal cord pathways or tracts, which either ascend or descend depending on the information they are carrying.
Both types of tracts are made up of neuronal axons that gather into long columns which are found inside the ventral, lateral and dorsal aspects of the spinal cord.
The descending tracts are the motor pathways that tell various muscles in the body to contract, like when lifting a heavy weight.Now, let’s start by looking at the anatomy of the spinal cord.

Spinal cord gross anatomy overview0:39–2:14

Anteriorly, there is a deep midline depression called the ventral median fissure and, posteriorly, there is a more shallow midline depression called the dorsal median sulcus.
Each half also has a ventrolateral sulcus, where ventral rootlets leave the spinal cord; and a dorsolateral sulcus, where dorsal rootlets enter the spinal cord.
The ventral and dorsal rootlets fuse to form the ventral and dorsal roots, respectively Ventral rootlets and roots carry motor fibers that travel from the spinal cord to different organs and muscles, while their dorsal counterparts - with a sensory ganglion, called the dorsal root ganglion, attached to each dorsal root - carry sensory fibers from organs and receptors throughout the body to the spinal cord.
Now, on a transverse section, the spinal cord has an area of gray matter shaped like a capital “H” in the middle. The gray matter is subdivided into the gray commissure, which is the strip connecting the two halves of the spinal cord that surrounds the central canal; and the peripheral regions known as horns.
There are two ventral, and two dorsal horns. Dorsal horns contain neuronal cell bodies that process information received from sensory fibers, entering the spinal cord from the dorsal roots and dorsal rootlets.
On the other hand, the ventral horns contain cell bodies of motor neurons, with motor fibers,exiting through the ventral rootlets and ventral roots.
Alright, now let’s look at the descending somatic motor, pathways, which control the activity of skeletal muscles.. These pathways originate from either the cerebral cortex or the brainstem and have various termination points, depending on the pathway.

Descending Somatic Motor Pathways2:14–2:59

Each pathway contains an upper motor neuron and a lower motor neuron. The upper motor neurons are found inside the cerebral cortex and various nuclei of the brainstem, while lower motor neurons are found inside the brainstem or the ventral horn of the spinal cord.
From the ventral horns, axons from lower motor neurons leave the spinal cord through the ventral roots and travel within peripheral nerves to innervate the skeletal muscles of the body.First up, there’s the corticospinal tract, which is a motor pathway that runs from the cerebral cortex to the spinal cord - hence the name “cortico-spinal”.

Corticospinal tract2:59–5:22

This tract is responsible for the voluntary movements of the limbs and trunk.The cell bodies of the first-order neurons of this pathway lie in the primary motor cortex.
The axons then descend via the corona radiata, through the internal capsule, the cerebral peduncle of the midbrain, and the ventral aspect of the pons to reach the ventral aspect of the medulla.
At the medulla these fibres bundle together and create swellings known as the pyramids, and the majority of the corticospinal fibers decussate - or cross over to the opposite side - at the level of the caudal medulla, in what’s known as the pyramidal decussation.
At this level, the corticospinal tract divides into the lateral corticospinal tract, and the anterior corticospinal tract.
The decussated fibers form the lateral corticospinal tract, while the uncrossed fibers continue as the anterior corticospinal tract.
Both the lateral and the anterior corticospinal tracts descend through the spinal cord. The lateral corticospinal tract continues through the contralateral lateral column of the spinal cord, and synapse in the ventral horn on the lower motor neurons.
Axons of the lower motor neurons leave the spinal cord through the ventral root to innervate muscles of the limbs, especially distal musculature that control fine movements.
On a cross section of the spinal cord, the lateral corticospinal tract is organized based on the region of the spinal cord it is supplying; from lateral to medial, it is organized into sacral, lumbar, thoracic and cervical fibers.
So, axons that control motor innervation of the upper limbs, terminate in the cervical region, and are therefore located more medially, whereas axons that control lower limb movements, terminate in the lumbar region and are more lateral.The anterior corticospinal tract descends uncrossed within the anterior column, with its fibers decussating only after they reach the desired spinal cord level, where they synapse with lower motor neurons in the ventral horn.
These axons also leave the spinal cord through the ventral root to supply muscles of the trunk.Now let’s talk about the reticulospinal tract, which originates from the reticular formation located in the pons and the medulla.

Reticulospinal tract5:22–5:57

Fibers that originate from the pons form the pontine reticulospinal tract, which descends to the spinal cord, mostly uncrossed.
Fibers originating in the medulla form the medullary reticulospinal tract, which terminates in the spinal cord with a mix of crossed and uncrossed fibers.
The fibers of the reticulospinal tract can either excite or inhibit motor neurons to influence voluntary movements, posture and reflexes.
Next, is the tectospinal tract, also known as the colliculospinal tract, which originates in the superior colliculi of the dorsal midbrain.

Tectospinal tract5:57–6:32

These fibers decussate and descend down the brainstem, enter the spinal cord and synapse on interneurons in the ventral horns of the cervical region.
Since the superior colliculi receive both visual and auditory information, the function of the tectospinal tract is to mediate postural reflexes of the head and neck in response to visual and auditory stimuli.
Now, the rubrospinal tract originates from the red nucleus located in the midbrain. Axons of this tract cross the midline after leaving the red nucleus, and then descend into the lateral column of the spinal cord’s white matter and synapse with interneurons in the ventral horn.

Rubrospinal tract6:32–7:03

Like the tectospinal tract, the rubrospinal tract also terminates in the cervical region and regulates flexor and extensor muscle activity in the upper limb, helping to produce smoother and more coordinated movements.Finally, there is the vestibulospinal tract, which originates from the vestibular nuclei located in the pons and medulla, just beneath the floor of the fourth ventricle.

Vestibulospinal tract7:03–7:41

Its fibers do not decussate, and descend into the spinal cord, terminating and synapsing on interneurons in the ventral horn of the spinal cord.
Now, the vestibular nuclei primarily receive sensory information about the position and motion of the head. Therefore, the vestibulospinal tract functions to carry motor commands that facilitate extensor muscles of the trunk and extremities which helps to maintain balance.
Alright, now just before we wrap up, can you remember the 5 descending spinal tracts? Alright, as a quick recap.

Quiz7:41–7:54

Descending spinal cord pathways consists of upper motor neurons, which form tracts that transmit motor commands from the brain to the spinal cord, and lower motor neurons that directly innervate the muscles of the body.

Review7:54–8:49

The tracts that originate from the cortex are the corticospinal tracts, which helps control the voluntary movement of our muscles.
The corticospinal tract is divided into the anterior corticospinal tract which mainly helps control the muscles of the trunk, and the lateral corticospinal tract which helps control muscles of the limbs.
The tracts that originate in the brainstem include the rubrospinal tract which originates in the red nucleus in the midbrain; the reticulospinal tract which originates in the reticular formation in the pons and medulla, the tectospinal tract originating from the dorsal midbrain, and the vestibulospinal tract, which originates from the vestibular nuclei