General Organization Principles
The brain is structured on a similar principle to the spinal cord: gray matter represents collections of neuronal cell bodies, while white matter is formed by conduction pathways of myelinated fibers. However, in the brain, gray matter not only forms compact nuclei but also creates a continuous surface layer—the cortex, or mantle (pallium). The surface of the cortex is folded into numerous gyri, which is functionally crucial as it repeatedly multiplies the total surface area required to house the vast neural networks processing conscious sensation.
Ascending Pathway of the Arc: Conscious Sensation
For thermal, pain, or tactile irritation from the skin of the trunk and limbs to be perceived consciously, the signal follows a multi-step afferent pathway:
- Receptor Apparatus: Perception of the stimulus by cutaneous receptors.
- First Neuron (Sensory): The cell bodies of these neurons are located in the dorsal root ganglia (ganglion spinale). Their processes transmit impulses into the dorsal horns of the spinal cord.
- Second Neuron (Associative): Located in the proper sensory nucleus (nucleus proprius) of the dorsal horns. A key event occurs here: the axons of the second neurons cross to the opposite side of the spinal cord.
- Conduction Pathways: Following the decussation, the spinothalamic tracts are formed. The lateral tract runs in the lateral funiculus (carrying pain and temperature information), and the anterior tract runs in the anterior funiculus (responsible for touch). These tracts terminate in the thalamus (thalamus).
- Third Neuron (Second Associative): Represented by the nuclei of the thalamus. 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 sense of tension or dull aching pain in the muscles.
- Receptors include neuromuscular and neurotendinous spindles.
- The cell bodies of the first neurons also reside in the dorsal root ganglia; however, upon entering the spinal cord, their axons do not form synapses in its gray matter.
- The fibers ascend uninterrupted as part of the dorsal columns, forming the fasciculus gracilis (of Goll) and fasciculus cuneatus (of Burdach).
- The first synaptic relay occurs only at the medullary 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. Following information analysis and coordination with subcortical nuclei, a motor response is generated. The mechanism of voluntary movement control is executed via the pyramidal system.
The source of impulses is the giant pyramidal cells (Betz cells) located in the precentral gyrus of the cortex. Their axons form the pyramidal tracts descending through the brainstem. At the level of the medulla oblongata, the pathways split:
- Partial Decussation: Some fibers cross to the opposite side and descend in the lateral funiculus as the lateral corticospinal tract.
- Direct Fibers: Remain on their original side and descend in the anterior funiculus (anterior corticospinal tract), decussating segmentally within the spinal cord itself.
Both pathways terminate on large alpha motor neurons in the ventral horns of the spinal cord. From there, the signal travels to extrafusal fibers of skeletal muscles. If head muscles are involved in the movement, switching to the peripheral motor neuron occurs not in the spinal cord, but in the motor nuclei of cranial nerves at the level of the medulla oblongata or pons.
General Logic (Reflex Arc Cycle)
The closed cycle of signal propagation can be described by the following sequence: Stimulation of receptors $\rightarrow$ Spinal ganglia $\rightarrow$ Dorsal columns $\rightarrow$ Medullary nuclei $\rightarrow$ Sensory decussation (medial lemniscus) $\rightarrow$ Thalamus $\rightarrow$ Cerebral cortex (analysis) $\rightarrow$ Pyramidal tract $\rightarrow$ Partial decussation in the brainstem $\rightarrow$ Spinal funiculi $\rightarrow$ Ventral horns $\rightarrow$ Contraction of skeletal muscles.