Anatomy and Physiology of Children and Adolescents - M. R. Sapin 2007

Nervous System (General Layout of the Structure and Development of the Nervous System)
Central Nervous System
Neural Pathways of the Brain and Spinal Cord

Impulses generated by receptor stimulation are transmitted via neuronal processes to their Cell bodies. Through numerous synapses, Neurons interconnect to form pathways along which nerve impulses travel in one specific direction only—from sensory neurons through interneurons to effector neurons. This is due to the morphofunctional properties of synapses, which conduct excitation in a single direction only: from the presynaptic membrane to the postsynaptic membrane.

Along certain neuronal pathways, impulses travel centripetally from their site of origin in the Skin, mucous membranes, locomotor Organs, or vessel walls to the Spinal Cord or Brain. Along other pathways, impulses are conducted centrifugally from the brain to the periphery, reaching effector organs such as Muscles and glands. In the course of evolution and the Progressive development of the Central Nervous system, The Structure of reflex arcs has grown more complex. Complex reflex arcs have emerged, formed by neurons located in higher segments of the spinal cord, the Basal Ganglia of the brain, and the Cerebral Cortex. Neuronal processes extend upward from the spinal cord to various brain structures, forming tracts that interconnect nerve centers. These tracts constitute nerve pathways (tracts). A nerve tract is a collection of closely apposed nerve fibers that connect various centers of the BRAIN AND SPINAL cord, running within specific zones of their White matter and conducting specific nerve impulses.

Three groups of nerve tracts (nerve fibers) are distinguished in the spinal cord and brain: association, commissural, and projection fibers.

Association nerve fibers (short and long) interconnect groups of neurons (nerve centers) located within the same hemisphere of the brain. Short (intra-lobal) nerve fibers connect neighboring areas of Gray matter and typically reside within a single lobe of the brain. Long (inter-lobal) association bundles connect areas of gray matter situated at considerable distances from one another, usually in different lobes. These include: the superior longitudinal fasciculus, which connects the cortex of the frontal lobe with the parietal and occipital lobes; the inferior longitudinal fasciculus, which links the gray matter of the temporal lobe to the occipital lobe; and the uncinate fasciculus, which connects the cortex of the frontal pole with the anterior part of the temporal lobe.

In the spinal cord, association fibers interconnect neurons located in different spinal segments. These fibers form the own tracts of the spinal cord (intersegmental fasciculi), which lie close to the gray matter.

Commissural nerve fibers connect corresponding centers (gray matter) of the right and left cerebral hemispheres, forming the corpus callosum, the fornix commissure, and the anterior white commissure. The corpus callosum interconnects the newer, evolutionarily younger Regions of the cerebral cortex of the right and left hemispheres. Within each hemisphere, these fibers fan out to form the corona radiata.

The anterior commissure contains fibers that connect areas of the temporal lobe cortex of both hemispheres and belong to the rhinencephalon (the gray matter of the hippocampi and temporal lobes of both hemispheres).

Projection nerve fibers (nerve tracts) connect the spinal cord with the brain, the Brainstem nuclei with the basal ganglia and cerebral cortex (ascending tracts), as well as the brain with the spinal cord (descending tracts).

Ascending Projection Pathways are afferent, or sensory. Via these pathways, nerve impulses resulting from the effects of various environmental factors reach the cerebral cortex, including impulses originating from the Sense Organs, The Musculoskeletal System, Internal Organs, and Blood Vessels. Accordingly, ascending projection pathways are divided into three groups: exteroceptive, proprioceptive, and interoceptive.

1. Exteroceptive pathways transmit impulses from the skin (pain, Temperature, Touch, and pressure) and from the organs of special sense (visual, auditory, gustatory, and olfactory). The pathway for pain and temperature sensitivity (the lateral spinothalamic tract) consists of three neurons. The receptors of the first (sensory) neuron, which perceive these stimuli, are located in the skin and mucous membranes. The Cell body of the first neuron lies in the spinal sensory ganglion (dorsal ROOT ganglion). Its central process runs within the dorsal root into the posterior horn of the spinal cord and terminates in synapses on the Cells of the second neuron. The axons of the second neurons, whose cell bodies lie in the posterior horn, cross to the opposite side of the spinal cord via the anterior gray commissure and enter the lateral funiculus, forming the lateral spinothalamic tract. This tract ascends into the Medulla Oblongata, passes through the tegmentum of the Pons and the tegmentum of the Midbrain, and terminates in the thalamus. The axons of the thalamic cells (the third neuron) project to the internal granular layer (layer IV of the cortex) of the postcentral gyrus, where the cortical end of the general sensibility analyzer is located.

The pathway for touch and pressure (the anterior spinothalamic tract) transmits impulses from Skin Receptors to the cortical cells of the postcentral gyrus. The course of the first-neuron fibers of this tract is analogous to that of the lateral spinothalamic tract. Most axons of the second neuron of the anterior spinothalamic tract also cross to the opposite side of the spinal cord via the anterior gray commissure, enter the anterior funiculus, and ascend within it toward the thalamus. A portion of the second-neuron fibers runs within the posterior funiculus of the spinal cord alongside the axons of the proprioceptive pathway destined for the cortex.

2. Proprioceptive pathways conduct impulses from muscles, tendons, joint capsules, and ligaments. These pathways convey information regarding body position, range of movement, Muscle tone, and the degree of tendon tension. The cortical proprioceptive pathway carries impulses of muscle and joint sense to the postcentral gyrus cortex. Proprioception enables a person to assess THE POSITION OF body parts in space, analyze complex voluntary movements, and perform targeted adjustments to them. The cell bodies of the first neuron in this pathway also lie in the spinal ganglion. The axons of these neurons travel within the dorsal root and, bypassing the posterior horn, enter the posterior funiculus, where they form the fasciculus gracilis and fasciculus cuneatus. These bundles ascend into the medulla oblongata to The Nucleus gracilis and nucleus cuneatus. The axons of the second neurons, emerging from these nuclei, cross to the opposite side to form the medial lemniscus. They then pass through the tegmentums of the pons and midbrain and terminate in the thalamus by synapsing on the cell bodies of third neurons. The axons of the latter project to the postcentral gyrus cortex, terminating on neurons of cortical layer IV. Another portion of the second-neuron fibers, upon emerging from the gracile and cuneate nuclei, travels via the inferior cerebellar peduncle to the cortex of the cerebellar vermis on the same side. A third group of fibers crosses to the midline and, likewise via the inferior cerebellar peduncle, reaches the cerebellar vermis cortex on the opposite side. These fibers transmit proprioceptive impulses to the Cerebellum to help coordinate subconscious Movements of the musculoskeletal system. In addition to those described, there are anterior and posterior spinocerebellar pathways, which convey information regarding the state of the musculoskeletal system and spinal motor centers to the cerebellum.

3. Interoceptive pathways conduct impulses from internal organs and blood vessels. Receptors located within them (mechanoreceptors, baroreceptors, and chemoreceptors) monitor Homeostasis (such as metabolic rates, the Chemical composition of Blood and tissue fluid, and vascular pressure).

Descending nerve pathways conduct impulses from the CEREBRAL CORTEX AND subcortical centers to brainstem nuclei and the motor nuclei of the anterior horns of the spinal cord. These pathways are divided into two groups: pyramidal and extrapyramidal. The former serve as the primary motor pathways. Via the appropriate motor nuclei of the brain and spinal cord, they transmit impulses from the cerebral cortex to the skeletal Muscles of the HEAD, neck, trunk, and limbs. Extrapyramidal pathways convey impulses from subcortical centers and various cortical areas to the motor nuclei of the Cranial and Spinal Nerves, subsequently reaching the muscles, as well as to other motor centers in the brainstem and spinal cord.

The principal motor, or pyramidal, pathway is a system of nerve fibers through which voluntary motor impulses from giant pyramidal neurons (Betz pyramidal cells) located in the precentral gyrus cortex (layer V) are directed to the cranial nerve motor nuclei and the anterior horns of the spinal cord, and thence to the skeletal muscles. Depending on the direction and trajectory of its fibers, the pyramidal pathway is divided into three parts: the corticonuclear tract, which runs to the cranial nerve nuclei; and the lateral and anterior corticospinal (pyramidal) tracts, which run to the motor nuclei of the spinal anterior horns (Fig. 105).

The corticonuclear tract is a bundle of axons originating from the giant pyramidal cells of the precentral gyrus. This tract passes through the genu of the internal capsule and the basis pedunculi of the midbrain. The fibers of the corticonuclear tract cross to the contralateral side toward the cranial nerve motor nuclei, where they terminate in synapses on their neurons. The axons of the motor neurons belonging to these nuclei emerge from the brain as Components of the respective Cranial Nerves and course toward the skeletal muscles of the Head and Neck.

The lateral and anterior corticospinal (pyramidal) tracts originate from the giant pyramidal neurons of the precentral gyrus. The fibers of this pathway converge toward the internal capsule, pass through the anterior part of its posterior limb, continue through the basis pedunculi of the midbrain and the basilar pons, and enter the medulla oblongata, forming its pyramids. At the bulbospinal junction, a portion of the corticospinal fibers crosses to the contralateral side, continues into the lateral funiculus of the spinal cord (lateral corticospinal tract), and progressively terminates in the anterior horns of the spinal cord via synapses on motor neurons. The corticospinal fibers that do not cross at the bulbospinal junction descend within the anterior funiculus of the spinal cord, forming the anterior corticospinal tract. These fibers cross segmentally to the opposite side via the white commissure of the spinal cord and terminate in synapses on the motor neurons of the contralateral anterior horn. The axons of the anterior horn cells emerge from the spinal cord within the ventral roots to innervate the skeletal muscles. Thus, all pyramidal pathways are crossed pathways.

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Fig. 105. Pyramidal pathways:

1 — precentral gyrus; 2 — thalamus; 3 — corticonuclear tract; 4 — transverse section of the midbrain; 5 — transverse section of the pons; 6 — transverse section of the medulla oblongata; 7 — decussation of the pyramids; 8 — lateral corticospinal tract; 9 — transverse section of the spinal cord; 10 — anterior corticospinal tract. The arrows indicate the direction of Nerve Impulse propagation

Extrapyramidal pathways are phylogenetically older than pyramidal pathways. They maintain extensive connections with cells and nuclei in the brainstem and with the cerebral cortex, which exerts control and regulation over the extrapyramidal system. Consequently, the cerebral cortex can be considered the ultimate origin of extrapyramidal pathways, while their termination sites include the brainstem nuclei and the spinal anterior horns. The Influence of the cerebral cortex is mediated through several structures: the cerebellum, red nuclei, and reticular formation (which is linked to the thalamus and corpus striatum via vestibular nuclei). One of the Functions of the red nucleus is to maintain the muscle tone required for the involuntary maintenance of bodily equilibrium. From the red nucleus, nerve impulses are relayed to the motor nuclei of the spinal anterior horns (rubrospinal tract).

An important role in coordinating human movement and maintaining balance is played by the vestibulospinal tract, which connects the vestibular nuclei with the anterior horns of the spinal cord. The first neuron of this pathway originates in the nuclei of cranial nerve VIII. These nuclei are connected to the cerebellum and, via the medial longitudinal fasciculus, to the motor nuclei of cranial nerves III, IV, and VI. This ensures stable gaze and eye position during movements of the head and neck. The axons of the second neurons of the vestibulospinal tract descend within the anterior funiculus of the spinal cord and terminate in synapses on the motor neurons of the spinal anterior horns. Neurons of the reticular formation provide functional linkage between the vestibulospinal tract and the basal ganglia.

The cerebral cortex regulates cerebellar functions—which are involved in motor coordination—via the corticopontocerebellar pathway through the pons.

Thus, the nerve tracts of the brain and spinal cord establish connections between afferent and efferent (effector) centers, closing complex reflex arcs within The Human Body. Certain pathways transmit nerve impulses to the nuclei of phylogenetically older brain regions located in the brainstem, which govern automatic functions operating independently of conscious awareness, albeit under the regulatory control of the cerebral hemispheres. Other pathways operate with the active participation of the cerebral cortex (the highest centers of the central nervous system) to mediate voluntary actions of organs and Organ Systems. Functionally, these Neural Pathways integrate the Organism into a unified whole, ensuring coordinated action.