Human Anatomy - H. I. Koliadenko 2009

Nervous System
Central Nervous System
Neural Pathways of the Brain and Spinal Cord

Based on their Functions, Neural Pathways are divided into Projection Pathways (sensory and motor), reflex pathways, and Association Pathways.

Motor projection pathways transmit impulses from the center (cortex) to the periphery. However, both projection and association pathways also incorporate a portion of the reflex pathways.

The Pathways of the Spinal Cord are divided into ascending pathways, which conduct impulses from sensory and interneurons from bottom to top in an ascending direction, and descending pathways, which carry impulses from higher nerve centers to motor Neurons located at various levels of the spinal cord.

The ascending pathways of the spinal cord include: the gracile and cuneate fasciculi, the spinothalamic tract, and the anterior and posterior spinocerebellar tracts (Fig. 168). The gracile and cuneate fasciculi are the pathways for deep proprioceptive Muscle and joint sensitivity of the limbs and trunk. The first (also known as Goll's fasciculus) lies on both sides of the postcentral sulcus, while the second (Burdach's fasciculus) runs adjacent to it, more laterally. Both of these pathways originate from muscle and tendon receptors. Their sensory fibers, as part of the Spinal Nerves, approach the spinal ganglion, which houses the body of the first neuron. From here, nerve fibers enter the tubercle of the cuneate Nucleus and the clava. The Cells of these structures mark the beginning of the second neuron, whose axons ascend to the Medulla Oblongata and further up to the thalamus, from whose lateral nucleus the third neuron originates. The axons of the third neuron in these pathways proceed to the Cerebral Cortex in the postcentral gyrus, which contains the core of the general sensitivity analyzer. The gracile fasciculus conducts impulses from the receptors of the lower limbs and the lower half of the body up to the V thoracic segment, whereas the cuneate fasciculus carries them from the upper limbs and the upper half of the body above the V thoracic segment.

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Fig. 168. Course of the sensory ascending pathway:

1 — thalamus; 2 — section of the Pons; 5, 8, 9 — central processes of the second neurons of the sensory pathway; 4 — section of the medulla oblongata; 5 — central processes of the first neurons of the sensory pathway; 6 — pseudounipolar Cells of the spinal ganglion; 7 — sensory cells of the posterior horn of the spinal cord; 10 — sensory cells of the thalamus; 11 — central processes of the third neurons of the sensory pathway (thalamocortical tract)

The lateral spinothalamic tract is located in the lateral funiculi of the spinal cord. This pathway conducts Temperature and pain impulses from Skin and mucous Membrane Receptors. The body of the first neuron of this pathway lies in the spinal ganglion. The central process, as part of the posterior ROOT, enters the posterior horn of the spinal cord, where the second neuron is located. Its dendrites cross to the opposite side through the anterior gray commissure. Ascending through the lateral funiculus of the spinal cord and the Brainstem, the axons of the second neuron pass through the medulla oblongata, pons, and cerebral peduncles, reaching the thalamus, which synapses with the bodies of the third neuron. The processes of the third neuron extend to the postcentral gyrus of the cerebral cortex, where the centers for pain and temperature sensitivity are located.

The first neuron of the anterior spinocerebellar tract (Gowers' tract) is also located in the spinal ganglion; its axons, as part of the posterior sensory roots, travel to the intermediate part of the Gray matter of the spinal cord. This is where the second neuron of this pathway begins, whose axons emerge from the gray matter of the spinal cord through the posterior horns, locate in the white lateral columns of the same side, and ascend. Some fibers continue on their own side, while others cross over to the opposite side via the anterior commissure. Further, the fibers of this tract proceed to the medulla oblongata, pons, and the tectum of the Midbrain, where the fibers that did not decussate in the spinal cord undergo cross-over. Afterward, all fibers enter the superior cerebellar peduncles and reach the anterior section of the cerebellar cortex.

The posterior spinocerebellar tract (Flechsig's tract) originates from the first neuron located in the spinal ganglion. Its axon, as part of the posterior root, enters the gray matter of the posterior horn of the spinal cord. The second neuron of this pathway begins at the Base of the posterior horn. The neurites of the second neuron penetrate the White matter OF the lateral Column on the same side and are situated at the periphery of the posterior section. Ascending, the fibers of the second neuron pass through the inferior cerebellar peduncles to the vermis and terminate in the anterior

and posterior sections of the vermis cortex, with some fibers Crossing Over to the opposite side.

The descending pathways (Fig. 169) consist of the anterior and lateral corticospinal (or pyramidal) tracts, the rubrospinal, tectospinal, and vestibulospinal tracts.

The anterior and lateral corticospinal (pyramidal) tracts originate from Betz cells in the cerebral cortex of the anterior central gyrus. The axons of the first neuron pass through the internal capsule, then run as part of the cerebral peduncles of the midbrain, and enter the pons and medulla oblongata. In the region of the medulla oblongata, they form the pyramids. At the border between the medulla and the spinal cord, a portion of the fibers of this tract decussates and crosses to the opposite side. The crossed portion of the fibers enters the lateral columns of the spinal cord and terminates on the cells of its anterior horns. From there, the second neuron travels as part of the spinal nerve to the Muscles. This pathway is known as the lateral pyramidal tract. The uncrossed portion of its fibers is directed into the anterior columns of the spinal cord, forming the anterior pyramidal tract, whose fibers terminate on the motor cells of the anterior horns (the second neuron). As they descend the spinal cord, the Pyramidal Tracts gradually taper because their fibers terminate segmentally on the motor cells of the anterior horns. The pyramidal tracts conduct voluntary motor impulses from the cerebral cortex to the Muscles of the Trunk and limbs.

The rubrospinal tract (Monakow's tract). The first neuron of this pathway lies in the red Nucleus of the midbrain. Its fibers decussate after emerging from The Nucleus, then descend into the lateral columns of the spinal cord and terminate in the cells of the anterior horns of the spinal cord. The second neuron of this pathway runs from the anterior horns to the muscles. This pathway transmits impulses from the cerebellar cortex and the nuclei of the corpus striatum—which are connected with the red nucleus—to the spinal cord. It regulates muscle tone, coordinates movements, and controls body posture in space.

The tectospinal tract originates from the tectum of the midbrain and terminates on the cells of the anterior horns of the spinal cord. This pathway transmits stimuli that arise during protective body movements associated with the auditory and visual analyzers.

The vestibulospinal tract connects the vestibular apparatus of the Organ of Balance with the cells of the anterior horns of the spinal cord. This pathway conducts impulses that ensure body balance.

The extrapyramidal System of the Brain is the phylogenetically oldest system in terms of brain evolution. The nuclei of the extrapyramidal system include: the corpus striatum, the substantia nigra of the cerebral peduncles, the inferior olivary nucleus, the red nucleus, and the dentate nucleus of the Cerebellum. The extrapyramidal system performs the function of automatically regulating the contraction and relaxation of individual muscle groups, which is necessary for executing complex movements associated with the Functions of the pyramidal tracts.

Fig. 169. Course of the motor descending pathways:

1 — pyramidal tract; 2 — pyramidal decussation; 3 — lateral pyramidal tract; 4 — anterior pyramidal tract; 5 — segmental decussation of the fibers of the anterior pyramidal tract in the spinal cord; 6 — ventral (motor) roots of the spinal nerve; 7 — transverse fibers of the pons; 8 — corticopontine tracts

Fig. 170. Main cerebellar pathways and the rubrospinal tract:

1 — cerebral hemispheres; 2 — thalamus; 3 — red nucleus; 4 — tectum of the midbrain; 5 — dentate nucleus; 6 — cerebellum; 7 — transverse fiber of the pons; 8 — rubrospinal tract; 9 — neurocytes of the gracile and cuneate nuclei; 10 — bulbocerebellar tract; 11 — posterior spinocerebellar tract; 12 — anterior spinocerebellar tract; 13 — medulla oblongata; 14 — pons; 15 — corticopontine tracts; 16 — cerebral peduncles

Stimuli enter the extrapyramidal system from the cells of the thalami and the Hypothalamus, which constitute the receptor part of the extrapyramidal system. From there, impulses first travel to the corpus striatum, then to the red nucleus, dentate nucleus, and other nuclei, and finally to the skeletal muscles, predominantly via the rubrospinal tract (Fig. 170). It is known that the extrapyramidal system is closely connected with the Autonomic (vegetative) Nervous system and exerts an influence upon it.