The central nervous system (CNS) contains numerous nerve fibers that group together to form pathways between its various parts. These neural pathways represent the communicating highways of the CNS. They can be located solely within the brain, providing connections between several of its structures, or they can link the brain and the spinal cord together.
Neural pathways that connect the brain and the spinal cord are called the ascending and descending tracts. They are responsible for carrying sensory and motor messages to and from the periphery. For example; this is how sensation from your fingertips reaches your brain and how conscious and reflexive actions return to your fingers.
This article will describe the anatomy and function of our neural pathways. We’ll take a look at the concept of a neural pathway and introduce the spinal cord’s ascending and descending tracts as well as two important intracerebral interconnections.
|Definition||Neural pathways are groups of nerve fibers which carry information between the various parts of the CNS.
- What are neural pathways and tracts?
- Spinal cord tracts
- Pathways in the brain
What are neural pathways and tracts?
First, 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 neural pathways that are located in the brain and spinal cord (central nervous system). Each tract runs bilaterally; one on each side of the cerebral hemisphere or in a hemisection of the spinal cord. Some of the tracts decussate, or crossover, to descend or ascend on the contralateral side. The level of decussation varies in each tract. The nomenclature is quite diverse, resulting in pathways being called ‘lemnisci’, ‘peduncles’,‘fasciculi’, or ‘tracts’, 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!
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Spinal cord tracts
The spinal cord consists of ascending and descending tracts. The ascending tracts 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 from muscles and joints.
The sensory pathways start from receptors located in our skin, organs, muscles, etc. These specialized sensory organs register physical and chemical changes in our body’s external and internal environment and convert these changes into electrical impulses. This afferent information then travels from these receptors, via peripheral nerves, to the CNS, where they join with the relevant ascending tract.
Each ascending pathway follows the same general structure as first-order, second-order and third-order neurons. First-order neurons are afferent in nature. The sensory input from the receptors is sent through the peripheral nerve to the dorsal root ganglia. The body of the first-order neuron, within the ganglia, projects its axons to the posterior gray horn of the spinal cord. Here, it synapses with second-order neurons that ascend along the spinal cord and project onto third-order neurons which are found in the subcortical structures of the brain, such as the thalamus. These third-order neurons pick up the neural impulse and carry it on to the cerebral cortex.
There are ten ascending tracts: dorsal column (fasciculus gracilis, fasciculus cuneatus), spinothalamic (anterior, lateral), spinocerebellar (anterior, posterior, Cuneo-), spino-tectal, spino-reticular and spino-olivary.
Dorsal column pathways
Let’s now take a look at each pathway more closely. The gracilis and cuneate fasciculi, also known as the dorsal columns, are two ascending pathways located side-by-side in the dorsal funiculus of the spinal cord. They 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’ (DCML pathway).
First-order neurons ascend ipsilaterally (on the same side) through the spinal cord. They synapse in the gracilis and cuneate nuclei of the medulla oblongata, where the body of the second-order neuron lies. The axons of the second-order neuron immediately decussate (cross the midline) and ascend superiorly. At this point the dorsal column pathway is renamed as the medial lemniscus, and the fibers continue to ascend until the thalamus. After synapsing in the thalamus, third-order neurons pass through the posterior one-third of the posterior arm of the internal capsule and project to 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. It is located in the anterior funiculus of the spinal cord. The lateral spinothalamic tract carries pain and temperature sensations. It is found in the lateral funiculus of the spinal cord.
The anterior spinothalamic tract begins with peripheral first-order neurons located in the spinal ganglion. Axons of the first-order neurons reach the posterior gray horn of the spinal cord through the posterior root of the spinal nerve. Fibers from the dorsal grey horn (second-order neurons) ascend within the ipsilateral anterior funiculus for seven segments of the spinal cord, decussate, then travel on 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 of the spinal cord and carries the sensations of pain and temperature. Similar to its anterior sibling, first-order neurons located in the spinal ganglion send axons to the posterior gray horn, specifically in the Rexed laminae regions I, IV, V and VI, where they synapse with second-order neurons. These decussate across the anterior white commissure and ascend in the (now contralateral) lateral spinothalamic tract. While crossing the medulla, these fibers join with those from the anterior spinothalamic and spinotectal tracts to form the anterolateral tract (spinal lemniscus). The second-order neurons of the lateral spinothalamic tract synapse in the thalamus and the subsequent third-order neurons, together with the anterior spinothalamic tract, cross through the posterior third of the posterior arm of the internal capsule. These neurons then project onto the primary somatosensory cortex, where the information about external stimuli is decoded and analyzed.
Now that we understand the tracts involved in somatosensation, how are they integrated with movement? For example, how can our fingers follow the rim of a glass, or how can we walk in a coordinated fashion? These actions occur with the help of our spinocerebellar tracts.
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. There are two main spinocerebellar tracts that carry information from the lower extremities; the dorsal (posterior) spinocerebellar and the ventral (anterior) spinocerebellar tracts. Whilst the cuneocerebellar and rostral spinocerebellar tracts carry information from the upper extremities.
The dorsal or posterior spinocerebellar tract (a.k.a. Flechsig's fasciculus) is specific for the lower limbs. The fibers originate from the posterior grey horn, travel posterolaterally through the white matter without decussating and project onto the cerebellar cortex by passing through the inferior cerebellar peduncle. Functionally, the dorsal spinocerebellar tract conveys sensory information from the muscle spindles, Golgi tendon organs, as well as from touch and pressure receptors of the lower extremities.
The ventral (anterior) spinocerebellar tract (a.k.a. Gowers fasciculus) also carries sensory information from the lower limb. However, while the dorsal spinocerebellar tract conveys information about the muscle tone of synergistic muscles, strength and speed of movement from the lower extremities, the ventral spinocerebellar tract appears to relay information regarding their status (posture) during their movement. The organization of the ventral spinocerebellar tract is more complicated than the dorsal, due to its numerous polysynaptic inputs and large receptive fields. The first order neuron is localized in the spinal ganglion. Its axon reaches the dorsal horn through the dorsal root and synapses with the second-order neurons. Their fibers immediately cross at the same level of the spinal cord through anterior commissural fibers and ascend contralaterally along the anterolateral funiculus. The majority of fibers from the second-order neurons reach the contralateral cerebellum by passing through the superior cerebellar peduncle and medullary velum. The fibers then cross over again, ending up in the ipsilateral cerebellar cortex. Therefore, the anterior spinocerebellar tract decussates twice, before synapsing in the vermal and paravermal regions of the cerebellum called the spinocerebellum.
The cuneocerebellar and rostral spinocerebellar tracts are the upper extremity homologs of the dorsal and the ventral spinocerebellar tracts, respectively. They carry proprioceptive information from the upper limbs and neck. Note that the "cuneo-" derives from the accessory cuneate nucleus, not the cuneate nucleus. These two nuclei are related in space, but not in function.
Now that we’ve seen the major ascending tracts of the spinal cord, we can move on to the last three minor ones. The spinotectal tract, spinoreticular tract, and the spino-olivary tract.
The spinotectal tract (also known as the spinomesencephalic tract) is responsible for spinovisual reflexes, allowing you to turn your head and gaze toward a visual stimulus (e.g., a sudden flash of light). The fibers cross the spinal cord to travel in the anterolateral white column. They ultimately project on the superior colliculus, part of the tectum of the midbrain (mesencephalon).
The spinoreticular tract is involved in influencing levels of consciousness and provides a pathway from the muscles, joints and skin to the reticular formation of the brainstem.
The axons of the first-order neurons are localized within the spinal ganglion. They enter the spinal cord from the posterior root ganglion and synapse with second-order neurons in the posterior horn of the gray matter. The axons from these neurons ascend the spinal cord in the lateral white column, mixing with the lateral spinothalamic tract. Most of the fibers are uncrossed and synapse with neurons of the reticular formation in the medulla oblongata, pons and midbrain.
The spino-olivary tract (a.k.a. Helweg’s fasciculus) also transmits cutaneous and proprioceptive information to the cerebellum. Similar to other ascending pathways, the first-order neurons are located in the spinal ganglion. They synapse with second-order neurons in the posterior gray column. The axons of the second-order neurons cross the midline as they enter the spinal cord and ascend within the contralateral ventral funiculus to reach the accessory olivary nucleus. After synapsing with third-order neurons in the inferior olivary nuclei in the medulla oblongata, the axons cross the midline again and enter the cerebellum through the inferior cerebellar peduncle.
The inferior olivary nucleus is a source of climbing fibers to Purkinje cells in the cerebellar cortex. Thus, the spino-olivary tract may play a role in the control of movements of the body and limbs.
If you want to learn more about the spinal cord, take a look at these study units.
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 of the spinal cord carrying information from the brain to peripheral effectors, the skeletal muscles. The descending tracts are involved in voluntary motion, involuntary motion, reflexes and regulation of muscle tone.
The general structure of descending tracts is similar to the ascending tracts 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, called interneurons, transmit the impulse to third-order neurons which are also located in the anterior grey horn at the same spinal cord level.
Because the second-order neurons are insignificant, we use only a two-order system for the descending (motor) tracts. This way, the first neuron in the pathway (the upper motor neuron) arises in the cerebral cortex or brainstem, descends along the spinal cord and synapses in the anterior gray horn. The second neuron in the pathway (lower motor neuron) leaves the spinal cord through the ventral (anterior) root. In the cervical, brachial and lumbosacral regions the ventral roots combine to form the so-called nerve plexuses. Peripheral nerves emerge from the distal aspect of these plexus, or in the case of the thoracic region directly from the ventral roots. These efferent neurons subsequently travel all the way to a specific skeletal muscle or muscle group (myotome), 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. The tract 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 un-decussated neurons travel ipsilaterally as the anterior corticospinal tracts. These decussate further down the spinal cord, below the level of the medulla oblongata.
The descending fibers of the 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 (skeletal) muscles, resulting in movement.
The corticospinal tract received its alternative name, pyramidal tract, because it forms a 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, influences the activity of the motor nuclei of both motor (oculomotor, trochlear, abducens, accessory, hypoglossal) and mixed (trigeminal, facial, glossopharyngeal, vagus) cranial nerves. Through these cranial 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. Like the corticospinal tract, this tract also consists of only two neurons; UMNs travel from the primary motor cortex, frontal eye fields and somatosensory cortex all the way to LMNs located in the brainstem. The LMNs are represented by the cranial nerve nuclei. The corticobulbar tract is also part of the pyramidal tract.
While several tracts, such as corticospinal, corticonuclear, are involved in motor functions, they must be regulated in order to be useful. This is the role of the extrapyramidal system.
The reticulospinal tract, which is part of this involuntary system, helps with motor regulation by facilitating or inhibiting voluntary and reflex actions. To put it in context, this tract helps maintain your posture by inhibiting the flexors and augmenting impulses to extensors in order for you to stand upright.
The uncrossed fibers of the reticulospinal tract originate from the reticular formation spanning the brainstem. They descend as the medial (pontine) and lateral (medullary) reticulospinal tracts through the anterior and lateral funiculi of the spinal cord white matter, respectively. These fibers synapse onto neurons in the anterior grey horns, in the anteromedial portion of laminae VII and VIII, where they influence motor neurons supplying paravertebral and limb extensor musculature.
In addition to its role of facilitating or inhibiting voluntary and reflex actions, the reticulospinal tract is also involved in breathing, it mediates the pressors and depressors of the circulatory system and, in conjunction with the lateral vestibulospinal tract, helps in maintaining balance and making postural adjustments. Muscle tone, balance maintenance and postural changes form a necessary background upon which voluntary movement is executed, which explains why these pathways have numerous synapses with the lower motor neurons.
Thanks to the tectospinal tract, you are capable of moving your head swiftly towards the source of a sudden auditory or visual stimuli. Fibers of the tectospinal tract originate in the superior colliculus, which receives information from the retina and cortical visual association areas. These fibers then project to the contralateral (decussating dorsal to the mesencephalic duct) and ipsilateral portion of the first cervical neuromeres of the spinal cord and to the cranial nerves responsible for eye movement (CN III, IV and VI), located in the brainstem. The tectospinal tract then continues to descend in the ventral funiculus of the spinal cord until it reaches the neurons within cervical laminae VI-VIII where the fibers synapse with lower motor neurons of the neck muscles.
The tectospinal tract is responsible for controlling the movement of the head in response to auditory and visual stimuli. Therefore, it has been assumed this tract is responsible for head position and movement depending on visual input received by the superior colliculus.
The rubrospinal tract originates from the red nucleus located in the midbrain tegmentum. Its axons cross the midline and descend through the pons and medulla oblongata to enter the lateral funiculus of the spinal cord. The fibers terminate by synapsing with internuncial neurons in the anterior gray column at the level of laminae V, VI and VII, where it influences the lower motor neurons of the upper limbs.
The rubrospinal tract is considered to be responsible for the mediation of fine involuntary movement, along with other extrapyramidal tracts, including the vestibulospinal, tectospinal, and reticulospinal tracts. In other words, it coordinates the flexion/extension of muscle groups in order to execute large amplitude movements.
In humans, the rubrospinal tract is very small and its clinical importance is uncertain. It may participate in taking over motor functions after pyramidal (corticospinal) tract injury.
Another pathway involved in balance is the vestibulospinal tract. By receiving information from the semicircular canals of the inner ear, this tract activates our body’s extensor muscles and inhibits the flexors, correcting our physical position in space and thus correcting our balance. The tract originates from the vestibular nuclei (CN VIII) of the brainstem and descends uncrossed through the anterior funiculus of the spinal cord, ending up in the anterior grey horn. At this level, the fibers synapse with interneurons and lower motor neurons responsible for antigravity muscle tone in response to the head being tilted to one side. Additionally, the activity of these neurons is indirectly influenced by the cerebellum and the labyrinthine system.
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.
Do you struggle with remembering all the ascending or descending tracts? Try to improve your memory by better note-taking.
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; the limbic system and basal ganglia. Let’s examine them very briefly.
The limbic system is located at the margin between the cerebral cortex and the hypothalamus. It is involved in emotions and behaviors, 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 continuously communicate with one another, an ability facilitated by the following neural structures: 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: striatum, globus pallidus, subthalamic nucleus and substantia nigra.
To carry out their functions, the basal nuclei are connected 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.