Human Anatomy and Physiology (with Age-Related Features of the Child's Body) - M.R. Sapin, V.I. Sivoglazov 2002

Nervous System
Brain
Conducting pathways of the brain and spinal cord

In The Nervous System, Nerve Cells establish contacts (synapses) with neighboring nerve cells, forming neuronal chains along which nerve impulses travel only in specific directions. From receptor (sensory) Neurons, impulses travel through interneurons to effector neurons. Within synapses, impulses are conducted in one direction only—from the presynaptic membrane to the postsynaptic membrane.

Along some neuronal chains, impulses propagate centripetally—from their site of origin in the Skin, mucous membranes, locomotor Organs, Blood Vessels, Tissues, and organs to the Spinal Cord or Brain.

Along other neuronal chains, impulses are conducted centrifugally—from the brain to the periphery, to effector organs: Muscles, glands, and tissues. Neuronal processes (nerve fibers) carrying impulses from the spinal cord to the brain, or in the opposite direction—from the brain to the spinal cord—are bundled together to form pathways. Pathways are tracts of closely packed nerve fibers running through specific Zones of the White matter OF the BRAIN AND SPINAL cord, connecting various nerve centers and conducting identical nerve impulses.

In the spinal cord and brain, three groups of nerve fibers (pathways) are distinguished: association, commissural, and projection fibers.

Association nerve fibers (short and long pathways) connect nerve centers located within the same hemisphere of the brain. Short (intralobar) fibers connect adjacent areas of Gray matter and are typically located within a single lobe of the brain or adjacent segments of the spinal cord. Long (interlobar) association bundles connect areas of gray matter located at a considerable distance from each other, usually in different lobes of the brain or Regions of the spinal cord. Long Association Pathways include the superior longitudinal fasciculus, which connects the frontal lobe cortex with the parietal and occipital lobes; the inferior longitudinal fasciculus, linking the gray matter of the temporal lobe with the occipital lobe; and the uncinate fasciculus, connecting the cortex of the frontal pole with the anterior part of the temporal lobe.

In the spinal cord, association fibers form the proper fasciculi of the spinal cord (intersegmental bundles), which are located adjacent to the gray matter.

Commissural nerve fibers (pathways) connect identical nerve centers of the right and left cerebral hemispheres. Commissural pathways pass through the corpus callosum, the commissure of the fornix, and the anterior commissure. The corpus callosum connects the newer, phylogenetically younger regions of the Cerebral Cortex of the right and left hemispheres, where the fibers fan out to form the radiation of the corpus callosum.

The anterior commissure contains fibers that connect areas of the temporal lobe cortex of both hemispheres belonging to the olfactory brain.

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

Ascending Projection Pathways (afferent, sensory) conduct nerve impulses to the cerebral cortex that arise from the effects of various environmental factors on the body, including impulses originating from the Sensory Organs, Musculoskeletal System, Internal Organs, and blood vessels. Accordingly, ascending projection pathways are divided into three groups: exteroceptive, proprioceptive, and interoceptive.

Exteroceptive pathways carry pain, Temperature, and tactile impulses from the skin and sensory organs (Vision, Hearing, taste, smell). The pathway for pain and temperature sensitivity (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, while its Cell body lies in the spinal ganglion. The central process of the sensory neuron enters the posterior horn of the spinal cord as part of the posterior ROOT 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 white 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 (III neuron) project to the internal granular layer of the cortex (IV layer) of the postcentral gyrus, which contains the cortical end of the general sensory analyzer.

The pathway for Touch and pressure (anterior spinothalamic tract) carries impulses from Skin Receptors to the cortical cells of the postcentral gyrus. The course of the fibers of the first neuron in this pathway is similar to the previous one. Most axons of the second neuron also cross via the anterior white commissure to the opposite side of the spinal cord into the anterior funiculus and ascend within it to the thalamus, and then to the postcentral gyrus. Some fibers of the second neuron travel within the ipsilateral posterior funiculus of the spinal cord along with the axons of the proprioceptive pathway projecting to the cortex.

Proprioceptive pathways conduct impulses from the organs of The Musculoskeletal System (muscles, tendons, joint capsules, ligaments). This pathway carries information to the cortex of the postcentral gyrus regarding body position, range of motion, Muscle tone, and tendon tension. Proprioceptive sensitivity allows a person to assess THE POSITION OF their body parts in space, analyze their own complex movements, and perform targeted corrections. The Cell bodies of the first neuron of this pathway also lie in the spinal ganglion. Their axons, as part of the posterior roots of the Spinal Nerves, bypass the posterior horn and enter the posterior funiculus, where they form the gracile and cuneate fasciculi. These nerve fibers ascend into the medulla oblongata to the gracile and cuneate nuclei. The axons of the second neurons emerging from these nuclei cross to the opposite side, forming the medial lemniscus, pass through the tegmentum of the pons and the tegmentum of the midbrain, and terminate in the thalamus, synapsing on the cell bodies of the third neurons. The axons of the thalamic neurons project to the cortex of the postcentral gyrus, to the neurons of cortical layer IV. Upon exiting the gracile and cuneate nuclei, some fibers of the second neurons pass through the inferior cerebellar peduncle to the ipsilateral cortex of the vermis. Another portion of the fibers crosses to the opposite side and also passes through the inferior cerebellar peduncle to the contralateral cortex of the vermis. These fibers carry proprioceptive impulses to the Cerebellum to adjust subconscious Movements of the musculoskeletal system. There are also proprioceptive anterior and posterior spinocerebellar tracts, which carry information to the cerebellum about the state of the musculoskeletal system and the motor centers of the spinal cord.

Interoceptive pathways conduct impulses from internal organs and blood vessels. Receptors located within them (mechano-, baro-, chemo- receptors) perceive information regarding the state of Homeostasis, the intensity of metabolic processes, The chemical composition of tissue fluid and blood, vascular pressure, etc.

Descending pathways carry impulses from the CEREBRAL CORTEX AND subcortical centers to the brainstem nuclei and the motor nuclei of the anterior horns of the spinal cord. Descending pathways are divided into two groups: the pyramidal (or main motor) pathway, and the extrapyramidal pathways. Pyramidal pathways carry impulses from the cerebral cortex to the skeletal Muscles of the HEAD, neck, trunk, and limbs. Extrapyramidal pathways carry impulses from subcortical centers and various cortical areas to the motor nuclei of Cranial and Spinal Nerves, and then to muscles, as well as to other nerve centers of the brainstem and spinal cord.

The main motor, or pyramidal, pathway is a system of nerve fibers along which voluntary motor impulses travel from the giant pyramidal neurons (Betz cells) located in the cortex of the precentral gyrus (layer V) to the motor nuclei of the Cranial Nerves and the anterior horns of the spinal cord, and from there to the skeletal muscles. Depending on the direction and Location OF THE fibers, the pyramidal pathway is divided into three parts: the corticonuclear tract, which goes to the cranial nerve nuclei, and the lateral and anterior corticospinal tracts, which go to the nuclei of the anterior horns of the spinal cord (Fig. 95).

The corticonuclear tract is a bundle of axons from the giant pyramidal cells of the precentral gyrus that passes through the genu of the internal capsule and the Base of the cerebral peduncle. In the midbrain, pons, and medulla oblongata, the fibers of the corticonuclear tract cross to the opposite side to the motor nuclei of the cranial nerves, where they terminate in synapses on their neurons. The axons of the motor nuclei neurons exit the brain as part of the corresponding cranial nerves and project to the skeletal muscles of the Head and Neck.

The lateral and anterior corticospinal tracts also originate from the giant pyramidal neurons of the precentral gyrus. The fibers of these tracts pass through the anterior part of the posterior limb of the internal capsule, then through the base of the cerebral peduncle and the pons, and enter the medulla oblongata, where they form its pyramids. At the boundary between the medulla oblongata and the spinal cord, a portion of the corticospinal tract fibers decussates to the opposite side, continues into the lateral funiculus of the spinal cord (forming the lateral corticospinal tract), and gradually terminates in the anterior horns of the spinal cord, synapsing on the motor cells of the anterior horns. This pathway is called the lateral corticospinal tract. Other fibers of the corticospinal tract, which do not cross to the opposite side at the boundary between the medulla and the spinal cord, descend within the anterior funiculus of the spinal cord. This bundle of fibers forms the anterior corticospinal tract. Its fibers cross segmentally to the opposite side via the white commissure and terminate in synapses on the motor neurons of the contralateral anterior horns of the spinal cord. The axons of the motor cells of the anterior horns exit the spinal cord as part of the anterior roots and innervate skeletal muscles.

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Fig. 95. Diagram of the 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.

Arrows indicate the direction of nerve impulses

Extrapyramidal pathways are phylogenetically older than the pyramidal ones; they have numerous connections with both the brainstem and the cerebral cortex, which controls and regulates the extrapyramidal system. Extrapyramidal pathways originate in various regions of the cerebral cortex and the brainstem, and terminate on the cells of the motor nuclei of the brainstem and the anterior horns of the spinal cord. The Influence of the cerebral cortex on the extrapyramidal system and extrapyramidal pathways is mediated through the cerebellum, red nuclei, reticular formation (which is connected to the thalamus and corpus striatum), and vestibular nuclei. One of the Functions of the red Nucleus is to maintain the muscle tone necessary for involuntary postural balance. From the red nuclei, nerve impulses travel to the motor nuclei of the anterior horns of the spinal cord via the rubrospinal tract.

The vestibulospinal tract, which connects the vestibular nuclei with the anterior horns of the spinal cord, plays an important role in coordinating human body movements when balance is disturbed. This pathway is connected to the cerebellum and, via the medial longitudinal fasciculus, to the motor nuclei of cranial nerves III, IV, VI, and other pairs. This interconnection ensures that the position of the eyeballs is maintained during head and neck movements. The axons of the first neurons of the vestibulospinal tract descend within the anterior funiculus of the spinal cord and terminate in synapses on the motor cells of the anterior horns of the spinal cord. Neurons of the reticular formation provide a connection between the vestibulospinal tract and the basal nuclei of the cerebral hemispheres.

The cerebral cortex regulates the functions of the cerebellum, which is involved in motor coordination, via the pons along the corticopontocerebellar pathway.

Thus, the PATHWAYS OF THE brain and spinal cord establish connections between afferent and efferent (effector) centers, completing complex reflex arcs in The Human Body.

Some reflex arcs are completed in phylogenetically older nuclei located in the brainstem, which mediate functions characterized by a certain automatism, occurring without conscious involvement, though under the control of the cerebral hemispheres. Other reflex arcs are completed with the involvement of the cerebral cortex, the higher Divisions of the Central Nervous System, and enable voluntary actions of organs and Organ Systems. Pathways functionally integrate the body into a unified whole, ensuring the coordination of its activities.