Human Anatomy - Kotsan I. Ya. 2009

Ascending and descending pathways of the brain and spinal cord
Ascending projection pathways

Ascending (afferent, centripetal) pathways are sensory pathways that project the body's surface, Internal Organs, as well as Muscles, tendons, ligaments, and joints to the sensory and motor centers of the Cerebral Cortex. A characteristic feature of all sensory pathways is that the first (receptor) Neurons lie outside the Brain AND SPINAL cord in the periphery (within the respective spinal ganglia or sensory ganglia of the Cranial Nerves).

The first neurons of the sensory pathways are represented by pseudounipolar Cells. Their peripheral processes run as part of the Spinal Nerves to the periphery and terminate in specialized endings—receptors, which is why the first neuron is referred to as the receptor neuron. The central processes of these neurons extend toward the Spinal Cord, pass through its dorsal roots, and, upon entering the neural tissue, immediately segregate into bundles depending on the modality of sensation they conduct. Subsequently, the axons reach the corresponding intermediate nuclei of the spinal cord or Brainstem, where they synapse with second-order (II) neurons.

Afferent fibers ascend within the posterior or lateral funiculi of the spinal cord, occupying a precisely defined position. In the region of the Pons and Midbrain, the axons of second-order neurons ascend as part of the medial lemniscus, formed by these fibers, to the thalamic nuclei, where they terminate near third-order (III) neurons.

The axons of third-order neurons, whose Cell bodies are located in the thalamus, pass through the posterior limb of the internal capsule and extend to the respective cortical nuclear centers (integration centers), forming the corona radiata within the White matter OF the cerebral hemispheres together with the fibers of the motor pathway.

Thus, all afferent pathways conducting various sensory impulses are relayed in the thalamus, which serves as the integration site for afferent impulses.

S. B. Dzugayeva (1975) notes: "The information received by the thalamus is integrated, correlated, emotionally colored, qualitatively transformed, and distributed accordingly to its destinations within the neocortical formations. The thalamus does not simply mechanically receive and transmit afferent information to higher centers; rather, it processes it, imparts a distinct nuance, and dispatches it in a qualitatively altered state to the cortical fields for further analysis and synthesis."

Afferent (sensory) pathways ensure the fine-tuned regulation of corresponding motor responses.

Depending on the type of receptors that generate the Nerve Impulse, ascending Projection Pathways are divided into three groups: exteroceptive, interoceptive, and proprioceptive.

Exteroceptive pathways (from Latin exter, externus — external) perceive and conduct impulses from the external environment. These pathways may originate from contact exteroceptors located in the Skin, which perceive pain, Touch, pressure, and Temperature changes, or from distance receptors that perceive impulses generated in the receptors of the visual, olfactory, auditory, and vestibular systems. Exteroceptive pathways serve as an intermediate link between external receptors and the sensory analyzers located in the cerebral cortex.

Exteroceptive pathways that conduct impulses from cutaneous receptors are designated According to the Location (origin and termination) of the second-order neuron in the given afferent chain.

The pathway for pain and thermal sensation—the lateral spinothalamic tract (tractus spinothalamicus lateralis)—consists of a chain of three sequentially arranged neurons. The Cell body of the first (sensory-pseudounipolar) neuron lies in a spinal or cranial ganglion. Its dendrite extends from a receptor located in the skin or mucous membrane, perceiving pain, warmth, or cold. The central process of the first neuron reaches the posterior horns of the spinal cord as part of the dorsal roots of the spinal nerves. Within the posterior horn, in the proper nuclear region, lies the cell body of the second neuron, upon which the axon of the first neuron terminates in a synapse. The axon of the second neuron immediately crosses to the opposite side of the spinal cord via the anterior white commissure and ascends within the lateral funiculus. In the brainstem, the lateral spinothalamic tract passes dorsal to the olivary Nucleus in the Medulla Oblongata, through the posterior part of the pons, and through the tegmentum of the midbrain as part of the spinal lemniscus, finally reaching the thalamus. In the thalamus, the axon of the second neuron synapses with the third-order neuron. The axon of the third neuron passes through the posterior limb of the internal capsule and reaches the neurons of the cerebral cortex in the postcentral gyrus of the parietal lobe, where the cortical analyzer of general sensitivity is located.

The pathway for touch and pressure sensation—the anterior spinothalamic tract (tractus spinothalamicus anterior)—also consists of a chain of three sequentially arranged neurons. The cell body of the first sensory neuron is located in the spinal ganglion. The peripheral process of this pseudounipolar neuron extends to the skin and terminates in a specific receptor that perceives touch and pressure, while its central process runs via the dorsal ROOT to the posterior horns of the spinal cord, where it synapses with the second neuron in the gelatinous substance. The axon of the second neuron immediately crosses to the opposite half of the spinal cord and reaches the anterior funiculus. Within the anterior funiculus of the spinal cord, the fibers of the anterior spinothalamic tract ascend through all higher spinal segments, and then via the medulla oblongata, pons, and midbrain reach the thalamus. In the thalamus, the axon of the second neuron contacts the third. The axon of the third neuron reaches the postcentral gyrus of the parietal lobe after passing through the posterior limb of the internal capsule.

Distance pathways conducting impulses originating from the receptors of the visual, olfactory, auditory, and vestibular systems will be discussed below in the chapter "Sense Organs".

Interoceptive pathways (from Latin internus — internal) conduct impulses from human internal organs, which house baroreceptors, chemoreceptors, and mechanoreceptors. There are close reciprocal connections between the CEREBRAL CORTEX AND the internal organs. A continuous stream of impulses flows from the cerebral cortex to the internal organs, regulating their activity, while simultaneously a feedback stream of impulses travels from the visceral receptors to the cortex, signaling the functional state of the internal organs and the processes occurring within them.

Each of the internal organs belonging to the digestive, respiratory, and urogenital systems, as well as the walls of Blood Vessels, contains a vast number of diverse sensory nerve endings—receptors termed interoceptors because they perceive stimuli originating from the internal organs.

The term "interoceptors" was first proposed in 1906 by C. Sherrington to designate receptors of the digestive tract specialized in detecting chemical stimuli.

Alongside interoceptors that do not normally elicit distinct conscious sensations, there are interoceptors that periodically signal vital physiological events within the Organism (thirst, hunger sensations, urges for urination and defecation, sensations related to the reproductive sphere, etc.).

The Diversity of receptor shapes is dictated by their functional Specificity. Thus, chemoreceptors perceive chemical stimuli caused by various substances differing in chemical composition (food, urine, blood, Lymph, etc.). Mechanical stimuli are perceived by mechanoreceptors. The degree of mechanical stretch of the vascular wall is detected by baroreceptors, which, In addition to blood vessels, are also located in the walls of the gastrointestinal tract, Gallbladder, and Urinary Bladder. Thermoreceptors (Krause end-bulbs for cold, Ruffini corpuscles for warmth) perceive temperature changes in organs.

The Pathways of the visceral analyzer run either as part of the V, VII, IX, and X cranial nerves or through all the dorsal roots of the spinal cord.

When the pathway runs within cranial nerves, the first neuron (sensory, afferent) is located peripherally in one of the ganglia (trigeminal/semilunar, geniculate, superior, or inferior) of the four cranial nerves (trigeminal, intermediate, glossopharyngeal, and vagus). It is represented by pseudounipolar cells whose process divides in a T-like fashion into central and peripheral branches. The peripheral processes—dendrites—run within the respective nerves to the internal organs, where they terminate in receptors. The central processes—axons—travel as part of one of the cranial nerves (V, VII, IX, X) to the brainstem, where they synapse with second-order neurons (sensory nuclei of the respective nerves).

The axons of the second-order neurons cross to the opposite side and ascend as part of the medial lemniscus to the thalamus, synapsing there with the third-order neuron.

The axons of the third-order neurons pass through the posterior third of the posterior limb of the internal capsule to the cortical core of the visceral analyzer, located in the lower portion of the postcentral and precentral gyri and in the limbic region of the cortex.

Part of the afferent pathways from the internal organs runs within the dorsal roots of the spinal cord. In this case, the cell bodies of the first (pseudounipolar) neurons are located in the spinal ganglia. Their peripheral processes—dendrites—travel as part of the greater and lesser splanchnic nerves and pelvic nerves, serving as conductors of visceral sensations, while their central processes enter the spinal cord and ascend within the lateral and posterior funiculi. Fibers running in the posterior funiculi of the spinal cord reach the thalamus, where they synapse with the next neuron, whose axon reaches the cortical visceral analyzer. Fibers running in the lateral funiculi terminate in the nuclei of the brainstem, Cerebellum, and thalamus.

Nerve fibers belonging to the interoceptive analyzer are thin. They conduct impulses at a relatively low velocity. Along the afferent fibers of the splanchnic nerves, impulses propagate at speeds ranging from 6 to 36 m/s. In the lateral and posterior funiculi of the spinal cord, fibers conducting impulses at speeds of 20–55 m/s have been identified.

The afferent innervation of internal organs is non-segmental in character. Internal organs and blood vessels feature a multitude of sensory innervation pathways, the majority of which consist of fibers originating from the closest spinal cord segments. These are the primary innervation pathways. Conversely, fibers of supplementary (accessory) visceral innervation pathways are located extra-segmentally and originate from distant spinal cord segments.

This structural pattern of afferent pathways ensures that information from internal organs is transmitted not only to the adjacent segments of the spinal cord containing the effector centers of that organ, but also to distant segments housing the centers of other organs. All of this enables the coordination of visceral activity at the spinal cord level. In the event of a complete spinal cord transection, these collateral pathways serve as the sole routes for afferent impulses from internal organs. However, their compensatory capacity is limited.

Proprioceptive ascending pathways (from Latin proprius, meaning \'one\'s own\') conduct nerve impulses from receptors located in the organs of The Musculoskeletal System—muscles, tendons, ligaments, joint capsules, and fascia (originating from the body itself). Proprioceptors respond to the degree of stretch of the soft Tissues of the musculoskeletal system and Muscle tone, signaling Changes in the relative positions of various parts of The Human Body (trunk, limbs, neck), while proprioceptive pathways transmit these signals to the cerebral cortex and cerebellum.

Depending on their destination—whether the nerve impulse is conducted to the cerebral cortex or the cerebellum—proprioceptive pathways are classified into cortical and cerebellar tracts.