Conduction functions
The main task of the nerve tracts is to reliably connect peripheral receptors with higher divisions of the CNS. Two powerful streams of impulses pass through the white matter, ensuring bidirectional communication.
Afferent (ascending) stream This channel is directed upward and carries information from cutaneous, temperature, pain, proprioceptive, and visceral receptors. Key sensory highways include:
- Fasciculus gracilis and fasciculus cuneatus (gracile and cuneate fasciculi);
- Spinothalamic tract;
- Spinocerebellar tract.
Efferent (descending) stream This channel brings motor commands down from higher centers to motor neurons and interneurons of the lateral horns. Spinal motor centers are strictly controlled from above via:
- Pyramidal tract.
- Extrapyramidal tracts, among which several specialized tracts are distinguished:
- Reticulospinal tract — monosynaptically excites alpha and gamma motor neurons of trunk muscles and proximal limb segments.
- Rubrospinal and corticospinal tracts — critically important for fine motor skills of the fingers and toes.
- Tectospinal tract — triggers protective visual and auditory startle reflexes (superior colliculus reactions).
- Vestibulospinal tract — responsible for maintaining balance and proper posture formation.
Intrinsic functions and the Bell-Magendie law
Intrinsic (segmental) functions are carried out by the segmental apparatus of the gray matter. They include somatic and autonomic reflexes of varying degrees of complexity. The most important feature is that the central link of these reactions is closed directly within the spinal segments.
The functional organization of a segment obeys a strict rule of entry and exit, known as the Bell-Magendie law:
- Posterior roots are always sensory (conduct afferent impulses to the brain).
- Anterior roots are always motor (transmit efferent excitation to effector organs).
Experimental proof A classical experiment is performed on a spinal frog. If all posterior roots on the right side are severed, the limb completely loses sensation, but the animal is still able to move it. If, however, the anterior roots on the left are severed, the limb retains sensation, but movement becomes completely impossible (paralysis ensues). This proves that the full integration of the simplest reflex acts is possible in an autonomous mode within a single spinal segment.
Spinal shock
Spinal shock is a specific condition that occurs after a complete transection (severing) of the spinal cord. It is characterized by the total disappearance of all its functions below the level of injury.
The features of shock progression depend on the evolutionary position of the species. In frogs, reflex activity recovers extremely quickly — usually within 10–15 minutes. However, in humans, functional recovery of a severed spinal cord practically does not occur.
Mechanisms of occurrence (theories):
- Irritation theory (F. Golz): Initially, it was assumed that shock is the result of severe traumatic tissue irritation at the moment of transection.
- Theory of function loss (modern concept): C. Sherrington and H. Trendelenburg proved the first theory to be erroneous. They discovered that shock develops fully even during bloodless cold blockade, when mechanical trauma is absent. Moreover, repeated transection of the brain slightly below the level of the first injury does not cause a recurrent episode of shock.
Conclusion: Modern physiology states that spinal shock occurs exclusively as a result of the sudden absence of functional connections (afferentation) with the higher controlling centers of the brain, and not due to physical tissue trauma per se.
Briefly: answers to topical questions
Where does decussation of the spinothalamic tract fibers occur?
Decussation of the spinothalamic tract fibers occurs in the anterior white commissure of the spinal cord. In this white commissure, which represents white matter along the longitudinal fissure, the axons of the second neurons of the anterior and lateral spinothalamic tracts cross.
Where are the cell bodies of the first and second neurons of the gracile and cuneate tracts located?
The cell bodies of the first neurons of the gracile and cuneate tracts are located in the posterior root ganglia (spinal ganglia). The cell bodies of the second neurons are localized in the medulla oblongata, where the axons of the fasciculi terminate in the corresponding nuclei:
- Gracile nucleus (nucleus gracilis);
- Cuneate nucleus (nucleus cuneatus).
What successive stages does the development of spinal shock in humans consist of?
The development of spinal shock and subsequent changes in spinal cord injury include the following stages:
- Acute period (spinal shock stage): inhibition of all spinal cord functions; atonia; loss of all types of sensation below the level of the lesion; acute urinary retention is possible.
- Subacute stage: acute urinary retention is replaced by paradoxical ischuria — urine is excreted drop by drop or in small portions.
- Recovery stage (several weeks later, post-shock period): segmental reflex functions are restored; tendon reflexes gradually revive and muscle tone increases.
Which specific autonomic reflex centers are localized in the lateral horns of the spinal cord?
In the lateral horns (cornu laterale), the centers of the sympathetic (thoracic and upper lumbar segments) and parasympathetic (sacral segments) divisions are localized. They provide the following autonomic reflexes:
- Regulation of vascular tone;
- Sweating;
- Piloerection (goosebumps);
- Viscerovisceral reflexes (regulation of internal organ function);
- Defecation and urination;
- Erection and ejaculation.
To avoid confusing the roots, use the phrase: "Anterior — Motor, Posterior — Sensory". It is easy to remember by association: You feel with your back (posterior/dorsal), and you move forward (anterior/ventral).
Class and exam questions
What is the essence of the Bell–Magendie law?
The law states that the dorsal roots of the spinal cord perform only a sensory function (signal input), while the ventral roots perform exclusively a motor function (command output).
Which theory of spinal shock is considered modern?
The modern theory is the loss of function theory. It proves that shock occurs due to the loss of descending stimulatory influences from the brain, which is confirmed by cold blockade experiments.
What is the function of the tectospinal tract?
The tectospinal tract participates in the realization of visual and auditory startle reflexes, providing rapid protective reactions to unexpected stimuli (superior colliculus reactions).
What else to study on this topic
- Mechanisms of cutaneous and pain sensation conduction via the spinothalamic tract
- Differences in the functioning of alpha and gamma motor neurons of the spinal cord
- Features of autonomic reflexes at the level of spinal segments
- The role of proprioceptors in the formation of the gracile and cuneate tracts
- Clinical manifestations of pyramidal and extrapyramidal tract lesions
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