Brain stem

The structure belonging to the trunk is a rhomboid fossa (fossa rhomboidea). Its lower part belongs to the medulla oblongata, the upper - to the bridge. The rhomboid fossa is the bottom of the fourth cerebral ventricle (ventriculus quartus) and contains on its surface components such as medullary (brain) strips (striae medullares), triangles of the hyoid and vagus nerves, and vestibulo-cochlear field (area acustica). The median groove (sulcus medianus posterior) divides a longitudinally rhomboid fossa.

The same principle of localization of afferents and efferents as in the spinal cord is preserved in the brain stem. The role of sensory and motor roots is taken by the cranial nerves. In addition to the initial four components, the so-called so-called appear in the trunk. “Special” afferents and efferents serving derivatives of gill arches. Special afferents (SSA) are represented by the VIII pair - n. vestibulocochlearis serving specific inner ear receptors. Special visceral afferents (SVA) in the brain stem are represented by fibers from taste buds (VII, IX and X nerves, and the core of the single pathway common to them).

Special visceral efferents (SVE) are represented by nerves that innervate the muscles phylogenetically derived from the muscles of the gill arches of primary aquatic organisms. For humans, these are: masticatory muscles (innervation of the V nerve), facial muscles (VII), muscles of the larynx and pharynx (IX, X), as well as sternocleidomastoid and trapezius muscles of the neck (XI nerve).

In the lower part of the trunk (medulla oblongata), the dorso-ventral orientation of the components remains, as in the spinal cord (GSA, GVA, GVE, GSE), upstream the orientation changes to the lateral-medial and ceases to be linear. The GSA is superseded by the SSA ventrally, and the SVE component is also superseded by the ventral.

Thanks to the studies of such scientists as R. Magnus and I.F. Klein, it was established that the medulla oblongata have a complex system of reflex centers providing a certain position in the body due to static and static-kinetic reflexes. These reflexes, in fact, are the mechanisms of redistribution of muscle tone in such a way that a pose that is comfortable for the animal (postural-tonic reflexes) or return to that of an uncomfortable one (rectifier reflexes) is maintained, and balance is maintained during acceleration (statokinetic reflexes) . These reflexes are carried out with the participation of such stem formations as the reticular formation, the red nucleus, and the vestibular nuclei.

A reticular formation is a formation going from the spinal cord to the thalamus in the rostral (to the cortex) direction. In addition to participating in the processing of sensory information, the reticular formation has an activating effect on the cerebral cortex, thus controlling the activity of the spinal cord. With the help of this mechanism, the tonus of skeletal muscles, sexual and autonomic functions of a person are controlled. For the first time, the mechanism of the effect of the reticular formation on muscle tone was established by R. Granit: he showed that the reticular formation is able to change the activity of γ- motor neurons , as a result of which their axons (γ-efferents) cause contraction of muscle spindles, and, as consequence, increased afferent impulse from muscle receptors. These impulses, entering the spinal cord, cause the excitation of α-motor neurons, which is the cause of muscle tone.

It has been established that two clusters of neurons take part in fulfilling this function of the reticular formation: neurons of the reticular formation of the bridge and neurons of the reticular formation of the medulla oblongata . The behavior of neurons of the reticular formation of the medulla oblongata is similar to the behavior of neurons of the reticular formation of the bridge: they cause activation of the α-motor neurons of the flexor muscles and, therefore, inhibit the activity of the α-motor neurons of the extensor muscles. Neurons of the reticular formation of the bridge act exactly the opposite, excite the α-motor neurons of the extensor muscles and inhibit the activity of the α-motor neurons of the flexor muscles. The reticular formation is connected with the cerebellum (part of the information goes to the medulla neurons (from the nuclei of the cork-shaped and spherical cerebellum), and from the tent to the bridge neurons) and to the cerebral cortex, from which it receives information. This suggests that the reticular formation is a reservoir of non-specific sensory flow, possibly involved in the regulation of muscle activity. Although so far the need for a reticular formation, duplicating the functions of neurons of the vestibular nuclei and the red nucleus, remains unclear.

Vestibular nuclei (from lat. Vestibulum - vestibule) is an organ that fixes a change in body position in space and located in the inner ear . Excitation of the vestibular nuclei occurs under the action of an adequate stimulus acting on the vestibular apparatus. Starting from the Deuterium nucleus - one of the main nuclei - as well as from the superior and medial nuclei, the vestibulospinal pathway affects the alpha motor neurons of the spinal cord: the vestibular nucleus neurons excite extensor α-motor neurons, and primarily on the axial muscles (vertebral column muscles) and simultaneously inhibit the α-motor neurons of the flexors by the mechanism of reciprocal innervation. During experimental transection of the vestibulospinal tract, a predominance of tone in the flexor muscles is observed.

Also, from the vestibular nuclei of the medulla oblongata, there is a path to the so-called medial bundle directed towards the spinal cord. This bundle performs an important function: it unites all the nuclei of the nerves involved in the regulation of muscle activity in the eyeball. Signals coming from the vestibular nuclei fall on the longitudinal medial bundle, due to which, when the vestibular apparatus is activated, the phenomenon of nystagmus occurs.

Thus, with irritation of the vestibular apparatus, a redistribution of muscle tone and a change in the activity of the muscles of the eyeball occur, as a result of which the animal is able to maintain balance and direct its gaze in the right direction.

The red core is located in the midbrain. Neurons of this nucleus receive information from the cerebral cortex and cerebellum, that is, all information about the position of the body in space, about the state of the muscular system, skin. The effect on the alpha motor neurons of the spinal cord is carried out using the rubrospinal tract. The rubrospinal tract begins from the cells of the red nucleus located in the cortex of the legs of the brain. Activation of the red nucleus neurons causes an exciting postsynaptic potential in the flexor muscle motor neurons, and inhibitory postsynaptic potentials in the extensor motor neurons. In this regard, the rubrospinal tract is similar to the corticospinal tract .

1. Big Medical Encyclopedia / Ch. ed. B.V. Petrovsky. - 3rd ed .. - Soviet Encyclopedia, 1982. - T. 13. - S. 351.
2. Agadzhanyan N.A., Tel L.Z., Tsirkin V.I., Chesnokova S.A. Human physiology. - SPb. : Sotis, 1998 .-- S. 54-57.
3. Antonen E.G. Motor pathways // Spinal cord (anatomical, physiological and neurological aspects).