Neural pathwaysIn this article, we describe the neural pathways of the brain and the spinal cord tracts. Have you ever wondered how sensation from your fingertips reaches your brain and how conscious and reflexive actions return to your fingers? Does it happen instantly or in several smaller steps? Do all nervous transmissions follow the same route? If you think about our nervous system, with a few exceptions, almost all nerves reaching or exiting the brain pass through the spinal cord, so it must be a pretty special structure.
In fact, it is. Running up and down the spinal cord are neural pathways called tracts, these represent the highways of the central nervous system (CNS). The brain also contains these neural communicating pathways, providing connections between several of its structures.
This article will describe all of the above. We’ll take a look at the concept of a neural pathway and introduce the spinal cord tracts as well as the intracerebral interconnections.
- Meaning of pathways and tracts
- Spinal cord tracts
- Pathways in the brain
Meaning of pathways and tracts
Before diving into any specifics, let’s wrap our heads around some key terms and concepts. A neural pathway is a bundle of axons that connects two or more different neurons, facilitating communication between them. Tracts are pathways that are located in the brain and spinal cord (central nervous system). The nomenclature is quite varied, resulting in tracts being called ‘lemnisci’, ‘peduncles’, or ‘fasciculi’ depending on their course, location, and projection.
Tracts are formed by neurons synapsing onto one another, and these neurons can be classified as first-order, second-order and third-order neurons depending on their location and order within the tract. Furthermore, tracts are named according to their origin (first half of the term) and termination (remaining part). For example, the spinothalamic tract begins in the spinal cord and ends in the thalamus, while the corticospinal tract starts in the cerebral cortex and finishes in the spinal cord. Therefore, if you understand anatomical terminology you don’t need to memorize the names–not bad, right?!
Neural pathways are a hot topic among anatomy students, which is why we have created a custom quiz about them. Take this quiz and test your knowledge about the neural pathways!
Spinal cord tracts
The spinal cord consists of ascending and descending tracts. The ascending ones are sensory pathways that travel through the white matter of the spinal cord, carrying somatosensory information up to the brain. They allow you to feel sensations from the external environment (exteroceptive) such as pain, temperature, touch, as well as proprioceptive information originating from the body’s interior for example muscles and joints.
Each ascending pathway follows the same general structure with first-order, second-order and third-order neurons. First order neurons are afferent in nature, transmitting nerve impulses from peripheral receptors to second-order neurons located in the posterior gray horn of the spinal cord. Second-order neurons ascend up through the spinal cord and project onto third-order neurons in the subcortical structures of the brain, such as the thalamus. Finally, these third-order neurons then pick up the impulse and carry it to the cerebral cortex. There are ten ascending tracts: dorsal column (fasciculus gracilis, fasciculus cuneatus), spinothalamic (anterior, lateral), spinocerebellar (anterior, posterior, cuneo-), spinotectal, spinoreticular, and spino-olivary.
Dorsal column pathways
Let’s take a look at each pathway more closely. The gracilis and cuneate fasciculi, also known as the dorsal columns, are located side-by-side in the dorsal funiculus of the spinal cord and carry fine and discriminative touch as well as proprioceptive sensations. Together with the medial longitudinal fasciculus, these tracts form the so-called ‘dorsal column medial lemniscus pathway’. First-order neurons ascend ipsilaterally (on the same side) through the spinal cord and synapse in the gracilis and cuneate nuclei of the medulla oblongata. The second-order neurons decussate (cross the midline) and keep ascending as the medial lemniscus until the thalamus. After synapsing there, third-order neurons reach the primary somatosensory cortex where the sensations are mapped out and the source pinpointed.
If you want to learn more details about how proprioception and discriminative touch reach the brain, take a look below:
There are two spinothalamic tracts: anterior and lateral. The anterior spinothalamic tract transports course touch and pressure sensation and is located in the anterior funiculus. The pathways begin with peripheral first-order neurons reaching the posterior grey horn. Fibers from the posterior grey horn (second-order neurons) ascend within the ipsilateral anterior funiculus for seven segments of the spinal cord, decussate, then travel all the way to the thalamus. Finally, third-order neurons project from the thalamus onto the primary somatosensory cortex.
The lateral spinothalamic tract travels in the lateral funiculus and carries pain and temperature sensations. Similar to its anterior sibling, this tract also decussates but it happens much earlier, after about two to three ipsilateral spinal segments. While crossing the medulla, it forms the anterolateral tract (spinal lemniscus) together with the anterior spinothalamic and spinotectal tracts. The second-order neurons of the anterior spinothalamic tract also end up in the thalamus and the subsequent third-order neurons project onto the primary somatosensory cortex.
Now that we understand the tracts involved in somatosensation, how are they integrated with movement? For example, how can our finger follow the rim of a glass or how can we walk in a coordinated fashion? With the help of our spinocerebellar tracts. The posterior and anterior spinocerebellar tracts sense proprioception from muscle spindles, Golgi tendon organs, and joint receptors. As a result, they are involved in movement coordination and posture maintenance.
The posterior spinocerebellar tract is specific for the lower limbs. The fibers originate from the posterior grey horns, travel posterolaterally through the white matter without decussating and project onto the cerebellar cortex by passing through the inferior cerebellar peduncle. The posterior spinocerebellar tract equivalent for the upper limb is the cuneocerebellar tract. It consists of external arcuate fibers that accompany the posterior spinocerebellar tract.
In contrast, the anterior spinocerebellar tract is more complex. The majority of fibers from the second-order neurons decussate, and then reach the contralateral cerebellum by passing through the superior cerebellar peduncle. The fibers then cross over again, ending up in the ipsilateral cerebellar cortex. Therefore, the anterior spinocerebellar tract decussates two times in total.
Since we’ve seen the major ascending tracts of the spinal cord, we can now move on to the last three minor ones. The spinotectal tract (also known as the spinomesencephalic tract) is responsible for spinovisual reflexes, allowing you to turn your gaze and head toward a sudden flash of light appearing in your field of vision, for instance. The fibers are also crossed and travel in the anterolateral white column. They ultimately project on the superior colliculus, part of the tectum of the midbrain (mesencephalon).
The spino-olivary tract also transmits cutaneous and proprioceptive information. This crossed tract ascends in the anterolateral white column, ending in the inferior olivary nuclei in the medulla oblongata. From here it decussates again and travels to the cerebellum via the inferior cerebellar peduncle. The ascending tracts of the spinal cord can get quite confusing, but they are favourite topics in neuroanatomy exams.
If you want to learn more or clarify any confusion, take a look below.
Now that we understand how information travels up through the spinal cord, let’s see how information travels in the opposite direction, by discussing the descending tracts of the spinal cord. These motor pathways travel through the white matter carrying information from the brain to peripheral effectors. They are involved in involuntary and voluntary motion, reflexes, regulation of muscle tone and visceral functions.
The general structure of descending tracts is similar to the ascending ones but in reverse. First-order neurons travel from the cerebral cortex or brainstem and synapse in the anterior gray horn of the spinal cord. Very short second-order neurons transmit the impulse to third-order neurons which are also located in the anterior grey horn. The latter efferent neurons subsequently travel all the way to smooth and skeletal muscles, innervating them. The descending tracts are named corticospinal, corticobulbar (or corticonuclear), reticulospinal, tectospinal, rubrospinal and vestibulospinal. The corticospinal and corticobulbar tracts form the pyramidal tract, which is under voluntary control. The remaining tracts are grouped together into the extrapyramidal system, which is under involuntary control.
The corticospinal tract is involved with the speed and agility of voluntary movements. Each one originates mainly from the primary motor cortex of the precentral gyrus (Brodmann area 4) and consists of only two neurons rather than three. The first-order or upper motor neurons (UMN) descend until the medulla oblongata, where ~90% of them decussate, forming the lateral corticospinal tracts. The remaining ones travel ipsilaterally as the anterior corticospinal tracts and decussate further down the spinal cord, beyond the medulla oblongata.
The descending fibers of anterior tracts travel through the anterior funiculus of the spinal cord, while those of the lateral tracts travel through the lateral funiculus. The fibers continue until the anterior grey horn where they synapse with the second-order or lower motor neurons (LMN). The latter project onto peripheral effector muscles, resulting in movement.
The corticospinal tract received its alternative name, pyramidal tract, because it forms the pyramid while passing through the medulla oblongata.
Reading about visual concepts like the corticospinal tracts can be slightly confusing. Take a look at the learning materials given below that simplify and present the subject in a visual way.
The corticobulbar tract, otherwise known as the corticonuclear tract, influence the activity of the motor nuclei of the motor (oculomotor, trochlear, abducens, accessory, hypoglossal) and mixed cranial nerves (trigeminal, facial, glossopharyngeal, vagus). Through these motor nuclei and nerves, this tract controls the activity of muscles of the head, face, and neck. The corticobulbar tract connects the brain with the medulla oblongata, also referred to as the bulbus. This tract consists of only two neurons: UMNs travel from the primary motor cortex, frontal eye fields and somatosensory cortex all the way to the LMNs in the brainstem, which are represented by the cranial nerve nuclei. The corticobulbar tract is also part of the pyramidal tract.
While several tracts are involved in initiating motor functions, they must be regulated in order for them to be useful. The reticulospinal tract helps with these aspects by facilitating or inhibiting voluntary and reflex actions. To put it in context, it helps maintain your posture by inhibiting the flexors and augmenting impulses to extensors in order for you to stand upright.
These uncrossed fibers originate from the reticular formation spanning the brainstem and descend as the medial (pontine) and lateral (medullary) reticulospinal tracts through the anterior and lateral funiculi of the white matter, respectively. They synapse onto neurons in the anterior grey horns, which in turn regulate efferent neurons.
Thanks to your tectospinal tract, you are capable of moving your head swiftly towards the source of sudden visual stimuli. They originate from the superior colliculus of the midbrain, cross the midline, and descend through the brainstem by following the pathway of the medial longitudinal fasciculus (MLF). In the spinal cord they continue through the anterior funiculus and synapse in the anterior gray horn of the cervical segments of the spinal cord.
Have you ever thought how are you able to pick up an object and bring it closer to your body? By inhibiting the extensor muscles of the upper limb and activating the flexor ones, a function facilitated by the rubrospinal tract. The fibers originate from the red nucleus located in the midbrain tegmentum, cross the midline and descend through the contralateral lateral funiculus until the cervical region of the cord. They project on interneurons located in the anterior grey horns. Therefore, the rubrospinal tract is responsible for innervating the upper limbs only.
Another pathway involved in balance is the vestibulospinal tract. By receiving information from the semicircular canals of the inner ear, this tract activates the extensor muscles and inhibits the flexors. It originates from the vestibular nuclei of the brainstem and descends uncrossed through the anterior funiculus of the spinal cord, ending up in the anterior grey horn.
There are quite a lot of tracts to get your head around, right? Neuroanatomy is certainly not easy, but with constant reviewing and testing the information will be cemented into your brain. A good starting point would be the following study unit.
Pathways in the brain
Now that we’ve covered the neural pathways of the spinal cord, it’s time to take a look at the connections located in the second component of the CNS, the brain. There are many individual pathways within the brain but for the scope of this article we’ll look at the two main ones, called the limbic system and basal ganglia. Let’s examine them very briefly.
As the name suggests, the limbic system is located at the margin or border between the cerebral cortex and the hypothalamus. It is involved in emotions and behaviours, for instance, hunger, satiety, sexual arousal and even memory. As the limbic system is located at the interface between the cortex and subcortex, its anatomical components are derived from both areas:
- Cortical components: orbital frontal cortex, hippocampus, insula, cingulate gyrus, and parahippocampal gyrus
- Subcortical components: amygdala, olfactory bulb, hypothalamus, anterior thalamic nuclei, and septal nuclei.
In order for the limbic system to perform its function, it must behave as a unit. Therefore, all of the above components must constantly communicate with one another, an ability facilitated by the following neural pathways: alveus, fimbria, fornix, mammillothalamic tract and stria terminalis.
The basal ganglia or basal nuclei refers to a collection of grey matter masses situated deep in the white matter of the cerebral hemispheres. There are four main nuclei in total: caudate nucleus, lentiform nucleus (putamen and globus pallidus), amygdaloid nucleus (amygdala), and claustrum. The lentiform and caudate nucleus can be grouped together in a structure known as the corpus striatum.
In order to carry out their functions, the basal ganglia are interconnected by several pathways. Inputs are received by the caudate nucleus and the putamen from the cerebral cortex, thalamus, subthalamus and substantia nigra of the brainstem. These nuclei pass the information via direct and indirect pathways to the globus pallidus, which represents the main output nucleus. The globus pallidus integrates the signals and sends them back to the cortex via the thalamus and other structures, according to the direct or indirect pathways. Therefore, the basal ganglia essentially form a regulatory loop that modulates voluntary and involuntary movements.
Are you curious to find out more about the basal ganglia and clarify this complex neuroanatomical concept once and for all? We have several resources that simplify the subject and help you test your knowledge.