Spinothalamic tracts are the ascending tracts present in the spinal cord. An ascending tract is a bundle of nerve fibers that carries signals from the peripheral body parts to the higher centers of the brain. As the name indicates, spinothalamic tracts originate in the spinal cord and terminate in the thalamus, which is a part of the forebrain.
Spinothalamic tracts include an anterior tract and a lateral tract. These tracts are essential for carrying the sensations of pain, temperature, light touch and pressure from the peripheral body parts, such as limbs and the trunk. Any disease or pathology that results in damage to the spinothalamic tract results in loss of perception of the above-mentioned sensations.
In this article, we will discuss different aspects of spinothalamic tracts such as location, structure, functions and different diseases that involve these tracts. After reading this article, you will have a complete understanding of the spinothalamic tracts. So, keep reading!
In this section, we will discuss the anatomical aspects of spinothalamic tracts such as location, pathways, final destination, and termination, etc.
As mentioned in the introduction, spinothalamic tracts include an anterior tract and a lateral tract. The anterior spinothalamic tract is present in the anterior part of the spinal cord in front of the anterior motor neurons.
The lateral spinothalamic tract is present on the lateral side of the spinal cord. We already know that the spinal cord is divided into two symmetrical halves. A pair of these tracts i.e. an anterior spinothalamic tract and a lateral spinothalamic tract is present in each half of the spinal cord.
Whenever we discuss the pathway of an ascending tract, we discuss it under three headings of anatomical organization; first-order neurons, second-order neurons and third-order neurons. We will these organizations with respect to the anterior and lateral spinothalamic tract.
First Order Neurons
By definition, first-order neurons are those neurons whose cell bodies are present in the dorsal root ganglion. The peripheral process of this neuron connects with the receptor, while the central process enters the spinal cord and terminates by synapsing with the second-order neurons.
In the case of the Anterior Spinothalamic tract, the peripheral process of first-order neurons ends as free nerve endings that perceive light touch and pressure. The central process enters the spinal cord and terminates at the neurons in substantia gelatinosa, the second-order neurons.
In the case of the Lateral Spinothalamic tract, the peripheral process ends as free nerve endings which perceive heat and temperature. The central process of the first-order neurons terminates at the neurons in substantia gelatinosa.
The second-order neurons of both the anterior and the lateral spinothalamic tracts are located in the substantia gelatinosa, a collection of neurons in the posterior gray column of the spinal cord.
The axons of second-order neurons cross to the opposite side of the spinal cord. On the contralateral side of the spinal cord, half of these fibers arrange anteriorly and ascend as Anterior Spinothalamic tract. The other half of axons join laterally and ascend as Lateral Spinothalamic tract.
The crossing of axons in the spinal cord means that the spinothalamic tracts ascending in the left half of the spinal cord are actually carrying sensations from the right half of the body.
The anterior and lateral spinothalamic tracts continue to ascend in the spinal cord till they reach medulla oblongata, a part of hindbrain. In medulla oblongata, the two tracts join the spinotectal tract to form Spinal Lemniscus.
The spinal lemniscus continues to ascend through the rest of the brain stem. It passes through pons and finally terminates in thalamus by synapsing with the third-order neurons.
Third Order Neurons
The third-order neurons of both the anterior tract and the lateral spinothalamic tract are located in the ventral posterolateral nucleus of the thalamus. They receive anterior and lateral spinothalamic tracts in the form of spinal lemniscus.
The axons of third-order neurons pass through the internal capsule to reach the cerebral cortex.
The ultimate destination of the anterior as well as the lateral spinothalamic tract is the sensory cortex. The axons of third-order neurons terminate in the primary somatosensory area of the cerebral cortex.
This area is resent in the posterior central gyrus, located just behind the fissure present on the lateral side of the brain. You can easily differentiate this area by looking at the brain. Just trace a big gap on the lateral side of the brain. The area present just behind this gap or fissure is the posterior central gyrus.
In this section, our main focus will be on understanding the functions of the anterior and lateral spinothalamic tracts.
Anterior Spinothalamic Tract
The two functions associated with the anterior spinothalamic tract are the perception and relaying the sensations of light touch and pressure.
As the name indicates, it refers to the touch sensation perceived when a very thin or light object is touched with the body. When your friend touches the skin of your hand or arm with a hair, the tip of a pencil or a feather, the touch sensation perceived at that time is called light touch. You become aware of this sensation because anterior spinothalamic tract carries the nerve signals to your brain.
It is the sensation perceived when some force is applied against the body. For example, when your friend presses the back of his pen against your body, the sensation you feel at that time is pressure sensation. You perceive this sensation because the anterior spinothalamic tract carries signals to the sensory cortex.
Lateral Spinothalamic Tract
The lateral spinothalamic tract carries the sensations of pain and temperature from the peripheral body parts to the sensory area of the brain.
The pain you feel when your skin is pricked by the needle of an injection is due to lateral spinothalamic tract. Not only this, the pain during fatigue, body aches and other such conditions are also carried by the lateral spinothalamic tract. This tract is essential for the perception of pain.
The sensation of high temperature while sitting near a fire or while touching a hot object is carried to the upper centers of the brain by the lateral spinothalamic tract. In the absence of this tract, the brain will never become aware of the high temperature.
Under this heading, we will discuss how different diseases can involve the spinothalamic tracts and what are their clinical manifestations.
It is a condition caused by syphilis. The syphilis infection causes the selective destruction of nerve fibers at the point where they enter the posterior root of the spinal cord. The destruction occurs mostly in the lower thoracic and lumbosacral segments of the spinal cord. This lesion also involves the first-order neurons of the spinothalamic tracts.
The clinical symptoms include stabbing pain in the lower limbs, paresthesia, numbness and loss of sensations in some parts of the skin of the trunk and lower limbs.
Brown Sequard Syndrome
It is the hemisection of the spinal cord in which one half of the spinal cord loses its function. It may be due to fracture dislocation, bullet wound or stab wounds, etc. Due to involvement of the spinothalamic tracts, the brain fails to perceive sensations of light touch, pressure, pain, and temperature from the opposite half of the body below the point of lesion.
Complete Cord Transection
It is a pathological condition in which the entire spinal cord loses all its functions below the point of lesion. It can also be caused by fracture dislocation, stab or bullet wound, an expanding tumor. The spinothalamic tract’s involvement results in loss of sensations of light touch, pressure, temperature, and pain from both sides of the body.
Spinothalamic tracts are the ascending tracts present in each half of the spinal cord.
These include an anterior spinothalamic tract and a lateral spinothalamic tract. They are respectively present on the anterior and lateral aspects of the grey column of the spinal cord.
The first-order neurons of both these tracts are present in the dorsal root ganglia.
The second-order neurons are present in substantia gelatinosa, located in the posterior grey column of the spinal cord.
The axons of second-order neurons cross to opposite sides and ascend as anterior and lateral spinothalamic tracts.
In medulla oblongata, the two tracts join spinotectal tract to form spinal lemniscus which terminates in the thalamus.
The third-order neurons in the ventral posterolateral nucleus of thalamus send axons to the postcentral gyrus, the primary sensory area.
The anterior spinothalamic tract is associated with the sensations of
- Light touch
The lateral spinothalamic tract is associated with the sensations of
Different diseases and lesions that can involve the spinothalamic tracts include;
- Tabes Dorsalis
- Brown Sequard Syndrome
- Complete Cord Transection