The descending spinal tracts have an incredibly important job that is exactly the opposite of the ascending tracts. What about their purposes are opposite, though? The descending tracts handle all information – which can still be sensory-related – traveling from the central nervous system to the peripheral nervous system.
The action impulses sent from the descending spinal tracts are typically motor plans specifically constructed in reaction to information gathered from a stimulus that was passed to the cerebellum or cerebrum through the ascending spinal tracts.
The ascending spinal tracts pick up this information via all kinds of sensory structures in the skin, eyes, muscles, and everywhere else on the body that, in some form, interacts with the external world.
Once this information has been processed by the central nervous system, it is determined whether your body needs to retreat from the stimulus, move further into the stimulus, or other forms of interaction (or no interaction at all). These reactions are made possible by all action impulses sent through the descending spinal tracts into the musculoskeletal system and other major physiological systems.
Networks of the Descending Spinal Tracts and How They Work
Just like all parts of the body, the descending spinal tracts are connected to other physiological systems through neuronal networks sending information to and from the brain. Neurons that are assigned to a particular muscle or muscle group tend to be located in the same cluster, called a “motor neuron pool.”
These motor neuron pools are loosely organized into groups according to where in the body their assigned muscle or muscle group is located: Medial pools primarily innervate the trunk (basically, any part of the body outside of the head and limbs), also referred to as “proximal” musculature, whereas the lateral groups mostly innervate the musculature of the limbs and digits, known as the “distal” musculature.
The motor neurons in the spinal cord are directly or influenced by axons from several different fiber tracts. When the neurons are influenced directly by only one axon, they are called “monosynaptic,” and when indirectly, by many axons, they are known as “polysynaptic.” These terms are representative of different pathways of neuron communication in reflexes:
A well-known example of a monosynaptic reflex is the patellar tendon reflex: You’ve more than likely had to go get a physical at the doctor’s office before, and during that physical, you may have had the classic reflex test, when the doctor taps your kneecap and your lower leg leaps forward.
What is being tested here is the health of your nervous system, illustrated by how well it can respond to a mechanical stimulus. Although there are no receptors on the patellar tendon itself, it is connected to receptors on the quadriceps muscle. These receptors are activated because the tendon is stretched when tapped by the doctor’s mallet.
The receptors then activate an afferent neuron that takes information to your spinal cord for processing. Here is the key of the monosynaptic reflex: In this type of reflex, there is only one synapse. The integration and processing of the neuronal information is only performed in the postsynaptic cell, the motor neuron, in this case.
Once all information is processed, an efferent neuron (a motor neuron) is activated and sends an action impulse to the effector. Finally, the effector causes the knee to extend, and BOOM! You have your patellar tendon reflex action!
The polysynaptic reflex, on the other hand, involves the taking in of information from multiple axons using more than one synapse – some of these synapses may even come from different pathways. A great example of a polysynaptic reflex would be the quick withdrawal of your hand when you touch something hot.
The pain receptors in your hand are activated when you touch a hot stove, or when hot water splashes on you, for example. An action potential is sent via afferent neurons to the spinal cord, and here is where the major differences between a monosynaptic reflex and a polysynaptic reflex take place: When the afferent neuron sends information to the spinal cord, it does not synapse directly onto a motor neuron.
Rather, it synapses onto an interneuron, which then diverges onto the efferent neuron (again, this would be the motor neuron, in this case) to create the movement of withdrawing your hand or limb from the heat source.
These are only two of many examples of how action potentials are transmitted from your spinal cord to your proximal and distal musculature. Keep in mind, though, that not all of the information being sent from the spinal cord comes in the form of a reflex. Every single movement of every muscle in your body can and will be passed through the brain when it is not a reflex.
The Descending Tracts of the Human Nervous System
In general, the descending tracts of the central nervous system are pathways by which motor signals are sent from the brain to the lower motor neurons. These lower motor neurons go on to innervate muscles to produce movement in skeletal, smooth, and/or cardiac muscles.
There are two categories into which the descending tracts of the nervous system can be organized: pyramidal and extrapyramidal. The pyramidal tracts are responsible for the voluntary control of the musculature of the body and face. The extrapyramidal tracts are responsible for the involuntary and automatic control of all musculature including muscle tone, balance, posture, and locomotion.
At the point of termination of the descending spinal tracts, the neurons synapse with a lower motor neuron, which makes all the neurons in the descending motor system classified as “upper motor neurons.” The cell bodies of the descending motor tracts’ neurons are found in either the cerebral cortex or in the brain stem, with their axons remaining in the central nervous system.
The Pyramidal Tracts
These tracts are referred to as “pyramidal” due to their nerve fibers that compose the triangular cross-section of the medulla oblongata called the “pyramid.” This is a white matter structure containing fibers of the corticospinal and corticobulbar tract – at the medullary pyramid, the fibers decussate before descending further into the nervous system.
All of these fibers, again, originate from the motor cortex. The group of fibers that exits the motor cortex are collectively referred to as the “corona radiata.” The nerves of the corona radiata then pass through the internal capsule, and then the medulla. The majority (about 80%) of the corticospinal tract decussates just after the pyramid, and descends from there. (Note: It is at this decussation and descension that the tract becomes the lateral corticospinal tract.)
The anterior corticospinal tract contains only about 20% of the nerve fibers of the corticospinal tract
The corticonuclear tract controls facial muscles. Facial nerves and trigeminal nerves.
Now, let’s discuss the two subdivisions of the pyramidal tracts: the corticospinal tract and the corticobulbar tract.
The Corticospinal Tract
The corticospinal tract innervates the musculature of the body apart from the head and neck and receives input from the primary motor cortex, premotor cortex, supplementary motor area, and somatosensory area.
The Corticobulbar Tract
The corticobulbar tract innervates the musculature of the head and neck and originates in the lateral region of the primary motor cortex. This tract receives input from the same structures as the corticospinal tracts and terminates on the motor nuclei of the cranial nerves. Here, the corticobulbar tract synapses with lower motor neurons, which then carry motor signals to the muscles of the face and neck.
(Note: Recall the naming convention of nerve tracts, that the beginning and end of the name will tell you where the tract begins and ends. In the case of corticobulbar, cortico refers to the cortex from which the nerves emerge, and bulbar is often used in anatomy and physiology as a reference to the medulla oblongata. So these nerve fibers terminate in the medulla before sending motor plans to the muscles of the face and neck.)
The Extrapyramidal Tracts
These tracts do not pass through the pyramid of the medulla and are involved in the maintenance of posture (meaning that they are also in control of the musculature of the trunk) as well as skilled voluntary movement. The extrapyramidal tracts arise from the red nucleus, vestibular nucleus, reticular formation and tectum.
The extrapyramidal tract also has subdivisions to further distinguish its unique responsibilities and functions within the nervous system. There are four subdivisions of the extrapyramidal tracts in total:
The Vestibulospinal Tract
The vestibulospinal tract controls balance and posture by innervating the “anti-gravity” muscles (the flexors of the arm, and extensors of the leg) through impulses sent through the lower motor neurons. The fibers of this tract work closely with the cerebellum, which, occasionally, can result in the cerebellum indirectly controlling the nervous activity of the spinal cord.
It arises from the vestibular nuclei (superior, inferior, lateral, and medial) and receives input from the inner ear (e.g., vestibule). There are two components of the vestibulospinal tract: lateral and medial.
The lateral vestibulospinal tract is responsible for the control of muscle tone in the postural muscles, inhibition of flexor motor neurons (ipsilaterally), and facilitates flexor activity on the opposite side of the body (opposite from itself).
The medial vestibulospinal tract, however, innervates the cervical and upper thoracic spine. The main function of this tract is to stabilize the head with regard to body position.
The medial vestibulospinal tract also controls reflexes of the head, especially those that require rapid adjustment of the head in response to an aural or visual stimulus for the purpose of maintaining a horizontal gaze. The same “righting” mechanism for the eyes is controlled through this tract as well.
The Reticulospinal Tract
The reticulospinal tract is also composed of two distinct sub-tracts, the medial reticulospinal tract and the lateral reticulospinal tract. The medial reticulospinal tract originates in the pons and is responsible for the facilitation of voluntary movements and also plays a part in increasing muscle tone.
The lateral reticulospinal tract arises from the midbrain as well, but from the medulla. This tract works in opposition of the medial reticulospinal tract by inhibiting voluntary movements and reducing muscle tone.
The Rubrospinal Tract
The nerve fibers of this tract have contralateral innervation (meaning they innervate muscles of the opposing side of the body) because they decussate as they emerge from the red nucleus in the midbrain and then descend into the spinal cord. Scientists are not quite sure of what their function is at the moment, but it is thought to play a role in fine motor control of the hands.
The Tectospinal Tract
The tectospinal tract receives input from the superior colliculus, a structure in the midbrain that gathers sensory information from the optic nerves. The nerves of this tract terminate at the cervical levels of the spinal cord, which allows it to solely coordinate head movements in response to visual stimuli.
The Central Role of the Descending Tracts Now that you have a solid foundation of the functionality and purpose of the descending tracts, you can begin to journey deeper into the How – and maybe even the Why – of the journey of sensory stimuli and action potentials through the nervous system!
The core purpose of these tracts, as you saw, is to control the musculature of the body – this is not restricted to voluntary movements of skeletal muscles, rather, all-encompassing: skeletal, smooth, and cardiac muscles are controlled by these tracts.
Getting a detailed look into the individual tracts will help to further your knowledge on exactly how the human nervous system allows us to interact with the world around us. So explore further and jump into more information on each of the descending spinal tracts!
- Academic Algonquin. (2013, August 16). Monosynaptic reflexes [Video file]. Retrieved from https://www.youtube.com/watch?v=t4N9qSKzlzw
- Academic Algonquin. (2013, August 16). Polysynaptic reflexes [Video file]. Retrieved from https://www.youtube.com/watch?v=tP4cCMMSfX0
- Jones, O. (2018, January 2). The descending tracts. Retrieved November 29, 2019, from https://teachmeanatomy.info/neuroanatomy/pathways/descending-tracts-motor/
- Kishore, K. (2017, February 22). What is the “trunk” of the body? Where is it located? | Socratic. Retrieved from https://socratic.org/questions/what-is-the-trunk-of-the-body-where-is-it-located
- Med Vids Made Simple. (2018, May 21). Descending tracts | Pyramidal and extra-pyramidal tracts | Med vids made simple [Video file]. Retrieved from https://www.youtube.com/watch?v=TMz9p7ODzFc
- Sampson, K. (n.d.). Vestibulospinal tract. Retrieved December 17, 2019, from https://www.physio-pedia.com/Vestibulospinal_Tract