Brain and spinal cord pathways: an overview

Conduction pathways are organized bundles of white matter axons that connect different levels of the nervous system and facilitate the transmission of signals between receptors, the spinal cord, brainstem, thalamus, cortex, and motor centers. It is convenient to think of them as "highways" passing through strictly defined areas of white matter and carrying specific types of information. [1]

Classical transmission logic is built on "links" of neurons. For somatosensory signals, the first neuron in the dorsal root ganglion, the second neuron in the spinal cord or brainstem, the third neuron in the thalamus, and then the thalamocortical projections to the primary somatosensory cortex are often distinguished. For motor commands, the upper motor neurons in the cortex transmit signals to the lower motor neurons in the brainstem or anterior horns of the spinal cord. [2]

A key rule of clinical neurology is the "level of decussation." Some systems decussate early (for example, the anterolateral pain and temperature system usually crosses to the opposite side at the level of the spinal cord), while others decussate late (the posterior funiculi cross in the medulla oblongata, forming the medial lemniscus). Therefore, the side of symptoms depends on the location of the lesion relative to the decussation. [3]

Another important principle is somatotopy, or the "body map" within the tracts. In the posterior funiculi and medial lemniscus, in the corticospinal tract, and in a number of brainstem fascicles, there are consistent patterns in the arrangement of fibers from different body segments. This helps interpret focal symptoms and neuroimaging results. [4]

Table 1. Basic classification of conduction pathways and their meaning

Group	Where do they take place?	What do they connect?	Example of clinical significance
Projective ascending	Spinal cord, brainstem, thalamus, internal capsule	Receptors and cortex	Loss of sensation due to damage to a specific tract
Projective descending	Cortex, internal capsule, brainstem, spinal cord	Cortex and motor neurons	Weakness and signs of upper motor neuron damage
Associative	White matter of the hemisphere	Shares of one side	Attention deficit, speech, visual recognition in case of damage to the bundles
Commissural	Interhemispheric connections	2 hemispheres	Interhemispheric integration disorders in corpus callosum lesions

Sources. [5]

Ascending somatosensory pathways: posterior funiculi and medial lemniscus

The posterior funiculi and medial lemniscus are the primary pathway for fine touch, vibration, and conscious proprioception from the trunk and extremities. In the spinal cord, primary fibers ascend along their own side as part of the posterior funiculi, with switching and crossing occurring in the lower portions of the medulla oblongata, after which the signal travels upward as part of the medial lemniscus to the thalamus. [6]

The thalamic stage is usually associated with the ventral posterolateral nucleus of the thalamus, after which thalamocortical fibers reach the postcentral gyrus. This pathway is important not only for "sensations" but also for the precision of movements: the brain requires information about joint position and muscle sensation to adjust the motor program. [7]

In practice, damage to the posterior cords produces a characteristic profile: impaired vibration, positional sensitivity, and discriminative tactile sensation, while pain and temperature may persist. This "splitting of modalities" is due to the fact that pain and temperature sensitivity operate through a different system. [8]

The clinical landmark for the side depends on the level: with a spinal cord lesion above the fiber entry, symptoms will most often be ipsilateral for the posterior columns, and with a lesion above the decussation of the medial lemniscus in the brainstem, they will be contralateral. This rule is useful when quickly linking a symptom to anatomy. [9]

Table 2. Posterior funiculi and medial lemniscus: key characteristics

Parameter	The essence
Modalities	Vibration, subtle touch, conscious proprioception
1 neuron	Spinal ganglion
2 neurons	Grammatical and cuneate nuclei of the medulla oblongata
Crossroads	In the medulla oblongata, the medial lemniscus is then formed
3 neurons	Thalamus with further projection to the postcentral gyrus
Typical deficiency	Loss of vibration and positional sensitivity with preserved pain and temperature

Sources. [10]

Anterolateral system: pain, temperature, rough touch and "fast crossing"

Pain and temperature from the trunk and extremities are conducted primarily through the anterolateral system, which includes the spinothalamic tract and a number of parallel pathways associated with wakefulness and defensive behavior. Educational schemes often distinguish a lateral component for pain and temperature and an anterior component for gross touch and pressure, but in reality these fibers run adjacent and functionally overlap. [11]

An important clinical detail is the decussation. After entering the spinal cord, pain and temperature fibers can ascend 1-2 segments, then switch in the posterior horn and cross to the opposite side through the anterior gray commissure. Therefore, with spinal cord hemisection, a loss of pain and temperature on the opposite side below the level of the lesion is often observed. [12]

In the brainstem, the anterolateral system passes through the tegmentum and maintains somatotopic patterns, then switches in the thalamus and proceeds to the cortex. With brainstem lesions, this results in "crossed" syndromes: for example, with lateral lesions of the medulla oblongata, sensory disturbances of the trunk and face may be combined with dysphagia due to the involvement of adjacent nuclei and pathways. [13]

The anterolateral system is also associated with the classic "dissociated" sensory loss seen in central spinal cord lesions, such as syringomyelia. A cavity in the central regions can damage the intersecting pain and temperature fibers in the anterior gray commissure, leaving the posterior funiculi relatively intact. [14]

Table 3. Anterolateral system: how to recognize it clinically

Sign	What usually suffers	Why does this happen?
Early contralaterality in spinal lesions	Pain and temperature on the opposite side	Crossing of the spinal cord through the anterior gray commissure
Segmental loss zone	Pain and temperature at the level of several segments	The fibers can rise 1-2 segments before crossing
Dissociated loss in central lesions	Pain and fever with preserved vibration	The crossing fibers are damaged, not the posterior funiculi
Brainstem syndromes	Combinations of sensory and cranial symptoms	The proximity of the anterolateral system to the nuclei of the brainstem

Sources. [15]

Descending motor systems: pyramidal and extrapyramidal tracts

The main pathway for voluntary movement is the corticospinal tract, often called the pyramidal tract. It begins primarily in the motor cortex, passes through the internal capsule and brainstem, forms the pyramids of the medulla oblongata, and then most of the fibers decussate at the pyramidal decussation and run in the lateral funiculus as the lateral corticospinal tract. [16]

A smaller portion of the fibers do not cross in the medulla oblongata and continue as the anterior corticospinal tract, exerting greater influence on the trunk and proximal muscles. Clinically, this explains why some lesions produce more pronounced impairments in fine hand movements than in gross axial stabilization. [17]

The corticonuclear tract provides voluntary control over the motor nuclei of the cranial nerves and the muscles of the face, tongue, and pharynx. Many nuclei are characterized by bilateral cortical innervation, so lesions above the brainstem do not always result in "complete loss," but there are important exceptions, such as the inferior facial nucleus.

The extrapyramidal descending tracts (reticulospinal, vestibulospinal, rubrospinal, and others) form a system for adjusting posture, muscle tone, and automated components of movement. These tracts are particularly important for balance and antigravity reactions, and the vestibulospinal tracts are directly involved in maintaining posture and stabilizing movement. [19]

Table 4. Pyramidal system and main descending tracts

Path	The main role	Where is the key intersection?	Typical deficit with lesions above the decussation
Lateral corticospinal	Precise voluntary movements, especially distal parts	Decussation of the pyramids in the medulla oblongata	Contralateral weakness and upper motor neuron signs
Anterior corticospinal	Proximal muscles and trunk	Some of the fibers cross segmentally	Usually less pronounced clinical effect
Cork nuclear	Voluntary movements of the muscles of the head and neck through the nuclei of the cranial nerves	Mainly at the level of the trunk to the cores	Depends on the bilateral innervation of a particular nucleus
Vestibulospinal tracts	Posture, balance, extensor tone	Mostly without a full "pyramidal" intersection	Postural instability, gait disturbances

Sources. [20]

Associative and commissural pathways: "connections within and between hemispheres"

Association pathways connect cortical regions within a single hemisphere. Short arcuate fibers connect adjacent gyri, while long fasciculi connect distant lobes and facilitate the integration of speech, attention, visual recognition, and complex cognitive functions. Recent reviews emphasize that even "classical" fasciculi exhibit internal segmentation and functional differences. [21]

The superior longitudinal fasciculus is one of the main "corridors" between the frontal and parietotemporal regions, and its variants are associated with language and attention networks. Diffusion magnetic resonance imaging approaches have clarified the architecture of the fasciculus and shown that its "parts" can have different functional correlates. [22]

The inferior longitudinal fasciculus connects the occipital and temporal regions and is considered an important component of visual recognition and semantic processing. Damage to these pathways can lead to "higher" impairments, where primary vision is preserved but object or face recognition is impaired, especially when the corresponding cortical areas and connections are involved. [23]

Commissural pathways connect the two hemispheres. The most important of these is the corpus callosum, which facilitates the interhemispheric exchange of sensory, motor, and cognitive information. Modern research is clarifying the internal organization of the corpus callosum and how its different regions participate in interhemispheric integration. [24]

Table 5. Examples of association and commissural pathways and their role

Bundle	Type	What areas does it connect?	What is most often associated with the function?
Superior longitudinal fasciculus	Associative	Frontal and parietotemporal cortex	Language, attention, working control networks
Inferior longitudinal fasciculus	Associative	Occipital and temporal cortex	Visual recognition and semantic processing
Hooked bundle	Associative	Frontal and anterior temporal cortex	Semantics, behavior regulation, emotional connections
Corpus callosum	Commissural	2 hemispheres	Interhemispheric integration of sensory and motor programs

Sources. [25]

How conduction pathways manifest in the clinic and how they are assessed today

Classical "conductor-based" diagnosis relies on comparing three components: the level of injury, the modality (pain, temperature, vibration, strength), and the side of the deficit. For example, in Brown-Sequard syndrome (hemisection of the spinal cord), ipsilateral weakness and loss of vibration and proprioception are expected, with simultaneous contralateral loss of pain and temperature. [26]

Vascular syndromes of the spinal cord well illustrate the "anatomy by function" principle. In anterior spinal artery syndrome, the descending motor tracts and anterolateral system are usually affected, while the posterior cords may be preserved, resulting in a combination of severe weakness with loss of pain and temperature, but relatively preserved vibration. [27]

Brainstem syndromes show that cranial nerve nuclei run alongside the long conductors. Lateral medullary syndrome may cause dysphagia and hoarseness due to involvement of the nucleus ambiguus, along with sensory disturbances and other signs of damage to the lateral structures of the medulla oblongata. [28]

Neuroimaging complements clinical assessment. Magnetic resonance imaging remains the primary method for detecting lesions, and diffusion tractography helps assess the location and integrity of the major white tracts in neurosurgical planning. Meta-analyses have shown a reduced risk of postoperative neurological deficits when tractography is included in planning, although the accuracy of the method depends on the algorithm and data quality. [29]

Table 6. Quick clinical “key” for conduction pathways

Where is the hearth?	What suffers more?	The side of the deficit relative to the focus	A typical example
Half of the spinal cord	Force and vibration plus pain and temperature	Strength and vibration are more often ipsilateral, pain and temperature are contralateral	Brown-Séquard syndrome
The anterior 2 thirds of the spinal cord	Movement and pain and temperature	Depends on the level, often bilateral	Anterior spinal artery syndrome
Central parts of the spinal cord	Pain and temperature segmentally	Often bilateral at the level of the lesion	Syringomyelia with dissociated loss
Lateral part of the medulla oblongata	Swallowing, sensation, other brainstem functions	Combined, often "cross-over"	Lateral medullary syndrome
Medial part of the medulla oblongata	Pyramidal tract and medial lemniscus plus tongue	Contralateral weakness and sensitivity, ipsilateral tongue signs	Medial medullary syndrome

Sources. [30]