Spinocortical reflex arcs

General Principles of Organization

The brain is structured on a principle similar to the spinal cord: gray matter represents clusters of neuronal cell bodies, and white matter is formed by projection pathways of myelinated fibers. However, in the brain, gray matter not only forms compact nuclei but also creates a continuous covering—the cortex, or pallium (pallium). The surface of the cortex is folded into numerous convolutions, which is of paramount functional significance, as it repeatedly increases the total surface area required to accommodate the vast number of neural networks processing conscious sensation.

Ascending limb of the arc: conscious sensitivity

For thermal, pain, or tactile irritation from the skin of the trunk and limbs to be perceived consciously, the signal passes through a multi-step afferent pathway:

1. Receptor apparatus: perception of the stimulus by skin receptors.
2. First neuron (sensory): the cell bodies of these neurons are located in the spinal ganglia (ganglion spinale). Their processes transmit the impulse into the dorsal horns of the spinal cord.
3. Second neuron (associative): located in the proper nuclei (nucleus proprius) of the dorsal horns. A key event occurs here: the axons of the second neurons decussate to the opposite side of the spinal cord.
4. Pathways: after decussation, the spinothalamic tracts are formed. The lateral tract runs in the lateral funiculi (carrying pain and temperature information), and the anterior tract runs in the anterior funiculi (responsible for touch). The tracts terminate in the thalami (thalamus).
5. Third neuron (second associative): represented by the thalamic nuclei. From here, axons project to the cortex, predominantly to the postcentral gyrus (gyrus postcentralis).

Features of conscious proprioception

The pathway for transmitting information from the motor apparatus itself (proprioception) is more complex. Subjectively, this information is perceived as a feeling of tension or a pulling pain in the muscles.

• The receptors are neuromuscular and neurotendinous spindles.
• The cell bodies of the first neurons also lie in the spinal ganglia; however, upon entering the spinal cord, their axons do not form synapses in its gray matter.
• The fibers ascend in transit as part of the dorsal columns, forming the fasciculus gracilis (of Goll) and fasciculus cuneatus (of Burdach).
• The first relay occurs only at the bulbar level—in the corresponding nuclei of the medulla oblongata.

Cortical analysis and descending (efferent) pathway

In the cerebral cortex, the signal enters an extensive neural network rather than a simple linear circuit. After information analysis and coordination with subcortical nuclei, a motor response is generated. The mechanism of conscious movement control is implemented via the pyramidal system.

The source of impulses is giant pyramidal cells (Betz cells) located in the precentral gyrus of the cortex. Their axons form the pyramidal tracts passing through the brainstem. At the level of the medulla oblongata, the pathways divide:

• Partial decussation: some fibers cross to the opposite side and descend in the lateral funiculi as the lateral corticospinal tract.
• Direct fibers: remain on their own side and run in the anterior funiculi (anterior corticospinal tract), decussating segmentally within the spinal cord itself.

Both pathways terminate on large alpha motor neurons of the ventral horns of the spinal cord. From there, the signal goes to the extrafusal fibers of skeletal muscles. If head muscles are involved in the movement, the relay to the peripheral motor neuron occurs not in the spinal cord, but in the motor nuclei of the cranial nerves at the level of the medulla oblongata or pons.

General logic (reflex arc cycle)

The closed loop of signal transmission can be described by the following sequence: Receptor stimulation $\rightarrow$ Spinal ganglia $\rightarrow$ Dorsal columns $\rightarrow$ Nuclei of the medulla oblongata $\rightarrow$ Sensory decussation (medial lemniscus) $\rightarrow$ Thalamus $\rightarrow$ Cerebral cortex (analysis) $\rightarrow$ Pyramidal tract $\rightarrow$ Partial decussation in the brainstem $\rightarrow$ Spinal cord funiculi $\rightarrow$ Ventral horns $\rightarrow$ Skeletal muscle contraction.

Briefly: answers to topical questions

Class and exam questions

What else to study on this topic

• Cytoarchitectonics of the cerebral cortex and the structure of the pallium.
• Histological structure of spinal ganglia (ganglion spinale).
• Morphology of giant Betz cells and large alpha motor neurons.
• Microscopic structure of neuromuscular and neurotendinous spindles.
• Topography and cellular composition of the thalamic nuclei.

Other topics in the section «Nervous system: brain»

Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.