Conduction Functions
The primary task of the ascending and descending pathways is to reliably connect peripheral receptors with higher centers of the central nervous system. Two major streams of impulses travel through the white matter to ensure bidirectional communication.
Afferent (Ascending) Pathway This channel directs information upward from cutaneous, thermal, pain, proprioceptive, and visceral receptors. Key sensory pathways include:
- Fasciculus gracilis and fasciculus cuneatus (gracile and cuneate tracts);
- Spinothalamic tract;
- Spinocerebellar tract.
Efferent (Descending) Pathway This channel delivers motor commands from higher centers to lower motor neurons and interneurons in the lateral horns. Spinal motor centers are tightly controlled from above via:
- Corticospinal (pyramidal) tract.
- Extrapyramidal pathways, among which several specialized tracts are distinguished:
- Reticulospinal tract — monosynaptically excites alpha and gamma motor neurons of axial muscles and proximal limb segments.
- Rubrospinal and corticospinal tracts — critical for fine motor skills of the fingers and toes.
- Tectospinal tract — triggers protective visual and auditory startle reflexes (collicular reactions).
- Vestibulospinal tract — responsible for balance maintenance and proper postural control.
Segmental Functions and the Bell-Magendie Law
Segmental (own) functions are executed by the segmental apparatus of the gray matter. They include somatic and autonomic reflexes of varying complexity. A key feature is that the central processing of these reactions occurs directly within the spinal segments.
Functional organization of a spinal segment obeys a strict input-output rule known as the Bell-Magendie law:
- Dorsal roots are always sensory (conveying afferent impulses into the central nervous system).
- Ventral roots are always motor (transmitting efferent excitation to target organs).
Experimental Proof This is classically demonstrated using a spinal frog model. If all dorsal roots on the right side are transected, the limb completely loses sensation, but the animal can still move it. Conversely, if the ventral roots on the left side are transected, the limb retains sensation, but movement becomes completely impossible (paralysis ensues). This proves that integration of basic reflex acts can occur autonomously within a single spinal segment.
Spinal Shock
Spinal shock is a specific condition that occurs following a complete transection of the spinal cord. It is characterized by the total abolition of all spinal cord functions below the level of the lesion.
The clinical presentation of shock depends on the species' evolutionary level. In frogs, reflex activity recovers extremely quickly—usually within 10–15 minutes. However, in humans, functional recovery of a severed spinal cord virtually does not occur.
Mechanisms of Origin (Theories):
- Irritation Theory (F. Goltz): Initially hypothesized that shock results from severe traumatic tissue irritation at the moment of transection.
- Loss of Function Theory (Modern view): C. Sherrington and G. Trendelenburg disproved the first theory. They discovered that spinal shock fully develops even during a bloodless cold blockade, in the absence of mechanical trauma. Furthermore, re-transecting the cord slightly below the level of the first injury does not trigger another episode of shock.
Conclusion: Modern physiology establishes that spinal shock occurs solely as a result of the sudden loss of functional connections (afferentation) from higher controlling centers in the brain, rather than from physical tissue trauma per se.