Ascending spinal cord pathways
Sensory (afferent) pathways originate from first-order neurons whose cell bodies lie in the sensory ganglia of spinal nerves (spinal ganglia). Their axons enter the spinal cord through the dorsal roots and divide into two portions:
- Medial group (thick, heavily myelinated fibers) heads into the dorsal funiculus. Here, each fiber branches in a T-shape. Ascending branches go to the dorsal horn and substantia gelatinosa, and some of them reach the medulla oblongata, forming the fasciculus gracilis (fasciculus gracilis) and fasciculus cuneatus (fasciculus cuneatus) tracts. Descending branches descend 6–7 segments, connecting with the gray matter of the dorsal columns (in the lumbar region they form a medial bundle, and in the sacral region they adjoin the fasciculus gracilis).
- Lateral group (thin fibers) goes to the posterolateral marginal nucleus (nucleus marginalis), then to the dorsal column. Next, the axons ascend in the lateral funiculus (partially crossing via the anterior white commissure to the opposite side).
| Pathway name | Course and localization | Function |
|---|---|---|
| Spinothalamic tracts (anterior and lateral) | 2nd neuron: proper nucleus of the dorsal horn. Decussation in the white commissure. Ascend to the thalamus as the spinal lemniscus | Lateral: pain and temperature. Anterior: tactile sensation |
| Posterior spinocerebellar tract | 2nd neuron: Clarke's column (nucleus dorsalis). Runs laterally into the dorsolateral part of the lateral funiculus, via the inferior cerebellar peduncle to the cerebellum | Proprioception (from muscles and tendons) |
| Anterior spinocerebellar tract | 2nd neuron: intermediate-medial nucleus. Partial decussation. Via the superior cerebellar peduncle to the cerebellar vermis | Proprioception |
| Spino-olivary tract | From cells of the dorsal horns, crosses over, goes to the inferior olivary nuclei | Information from cutaneous, muscle, and tendon receptors |
Also distinguished are the spinoreticular tract (uncrossed, projects to the brainstem reticular formation) and spinotectal tract (terminates in the tectum of the midbrain).
Ascending brainstem pathways
At the brainstem level, major pathways form that collect sensory information:
- Medial lemniscus (lemniscus medialis). It is a direct continuation of the gracile and cuneate fasciculi. The axons of their second-order neurons (in the matching nuclei of the medulla oblongata) form the internal arcuate fibers, decussate, and head toward the thalamus.
- Trigeminal lemniscus (lemniscus trigeminalis). Collects fibers from the sensory nuclei of cranial nerves V, VII, IX, and X, crosses to the opposite side, and also terminates in the thalamus.
- Auditory pathway. Begins at the spiral ganglion (cochlear ganglion). Second-order neurons lie in the dorsal and ventral cochlear nuclei. After decussation (in the trapezoid body), the fibers form the lateral lemniscus. The pathway passes through the medial geniculate body, auditory radiation, and reaches the cortex of the superior temporal gyrus.
- Radiations. Transmit signals from subcortical centers to the cortex. Optic radiation (radiatio optica) connects the lateral geniculate body with the occipital lobe. Thalamic radiations connect its nuclei with the cortex (anterior radiation with the frontal lobe, central radiation with the postcentral gyrus).
Descending pathways
Descending (efferent) pathways originate in the cerebral cortex, brainstem nuclei, or cerebellum. They terminate on the motor nuclei of the brainstem or in the ventral horns of the spinal cord, providing a motor response.
1. Pyramidal system Responsible for voluntary (conscious) movements. The pyramidal tract (tractus pyramidalis) starts in the precentral gyrus of the cortex, passes through the corona radiata, posterior limb of the internal capsule, cerebral peduncles, and pons. In the pyramids of the medulla oblongata, decussation occurs, after which the pathway divides into the lateral and anterior corticospinal tracts. A similar function for the head and neck muscles is performed by corticonuclear fibers (fibrae corticonucleares), running to the motor cranial nerve nuclei.
2. Extrapiramidal system Provides unconscious regulation of tone, balance, and movement coordination. Includes a number of tracts:
- Rubrospinal tract (tractus rubrospinalis);
- Tectospinal tract (tractus tectospinalis);
- Vestibulospinal tract (tractus vestibulospinalis, divided into lateral and medial);
- Pontine and medullary reticulospinal tracts;
- Olivospinal tract (tractus olivospinalis);
- Central tegmental tract (tractus tegmentalis centralis).
Briefly: answers to topical questions
What specific type of sensation is conducted by the fasciculus gracilis and fasciculus cuneatus (goll's and burdach's tracts)?
The fasciculus gracilis and fasciculus cuneatus (Goll's and Burdach's columns) conduct conscious proprioceptive and tactile sensation. The types of sensation conducted include:
- Conscious proprioceptive sensation — muscle-joint sense and posture;
- Vibration sense — perception of vibration;
- Touch and pressure — tactile sense;
- Discriminative sensation — ability to distinguish two touch points.
These pathways provide a conscious reaction to the listed types of stimuli.
In which specific sulcus or gyrus of the cortex does the optic radiation terminate?
The optic radiation (radiatio optica) terminates in the cortex of the occipital lobe of the brain, specifically along the banks of the calcarine sulcus (sulcus calcarinus). The cortical end of the visual analyzer (area 17) is located in this region. Anatomical landmarks of the visual cortex projection also include the cuneus (cuneus) and lingual gyrus (gyrus lingualis).
In which specific thalamic nuclei do the fibers of the medial lemniscus terminate?
The fibers of the medial lemniscus (lemniscus medialis) terminate in the ventrolateral nuclei of the thalamus (optic thalamus). The cell bodies of the third-order neurons of the sensory pathway, to which nerve excitation is transmitted, are located precisely in these nuclei. From here, the axons of the third-order neurons run through the posterior limb of the internal capsule to the cortex of the postcentral gyrus.
Which descending tracts run in the anterior funiculus of the spinal cord?
The following descending motor tracts run in the anterior funiculus of the spinal cord:
- Anterior corticospinal tract (tractus corticospinalis anterior) — formed by uncrossed fibers of the pyramidal tract that decussate in the white commissure of the spinal cord;
- Tectospinal tract (tractus tectospinalis) — a tectospinal pathway that directly adjoins the anterior fissure of the spinal cord and provides unconscious motor reactions.
Pay attention to the localization of 2nd-order neurons: for the spinothalamic pathways (pain, temperature, touch), they lie directly in the spinal cord (decussation occurs immediately), whereas for the fasciculus gracilis and cuneatus (gracile and cuneate fasciculi — proprioception), they ascend to the medulla oblongata, where the decussation of the medial lemniscus takes place.
Class and exam questions
Where are the cell bodies of the first neurons located for all sensory ascending pathways?
The cell bodies of the first-order neurons for all modalities of sensation always lie outside the CNS — in the spinal ganglia.
What is the difference between the lateral and anterior spinothalamic tracts?
The lateral spinothalamic tract conducts pain and temperature sensation, while the anterior tract is responsible for tactile sensation.
What is the medial lemniscus and how is it formed?
The medial lemniscus is a bundle of fibers in the brainstem. It is formed by the axons of second-order neurons of the gracile and cuneate nuclei after they decussate (internal arcuate fibers).
Where does the pyramidal tract decussate?
The decussation of the fibers of the pyramidal tract occurs in the region of the pyramids of the medulla oblongata.
What else to study on this topic
- Topography of neural pathways in a transverse section of the spinal cord.
- Detailed course of the extrapyramidal system fibers and their influence on muscle tone.
- Internal capsule: arrangement of ascending and descending tracts within its limbs.
- Formation of the lateral lemniscus and centers of the auditory analyzer.
- Role of the reticular formation and the spinoreticulospinal tract.
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