Neuroanatomy of the Nervous System
World Small Animal Veterinary Association World Congress Proceedings, 2005
Humberto Morales Castro, MVZ Esp.
Hospital Veterinario Peques
Membrillo 121 Nueva Santa María

Important part of the anatomical diagnosis it is supported in the neurological exam of the patient one, thus also, the diagnostic differential and the definitive diagnosis of any neurological affection can be better evaluated if we take into account the anatomy of the nervous system and we achieve to correlate its functions. Afterwards we will try to see to the nervous system since the anatomic-functional point of view and with some anatomic-pathologic relation examples.

Basically the nervous system functions in form of a reflex arch, with two main ways or systems called system afferent and system efferent. These mechanisms include so much the autonomous nervous system as the nervous system volunteer (sensitive and motor).

The system afferent sensitive is that that contributes information of temperature, touch, proprioceptive positioning, and pain. It forms part of the nervous system peripheral consisting of a neuron unipolar that grasp information of the body through its receptors and carry it to the spinal marrow and to the cerebral stem (to the central nervous system). To medullar level, these neurons before entering to the marrow present its body neuronal to level of the dorsal roots and enter way the dorsal shafts to the spinal cord.

In the gray substance of the spinal cord, these neurons can do synapses directly with the body neuronal of a neuron multipolar whose function is that of producing an motor effect upon leaving its axon of the marrow way the shafts and ventral roots, peripheral nerve and to do synapses in the muscle effectors in which reflected arch is called. For which to this neuron it is called neuron effectors or lower motor neuron (LMN). However, the neuron sensitive can also do synapses with neurons intermediate or internuncials, which inside its functions is that of doing synapses with the body neuronal of a neuron multipolar that will ascend via the ascending tracts to the high centers of the cerebellum, brain and cerebral stem. Here the information is processed and later, in the high motor centers (located mainly in the cerebral bark) arises a neuron whose axon way descends the tracts descendents until doing synapses with the neuron effectors or LMN, functioning like initiator of the voluntary movement besides serving like modulator inhibitory or calming effect upon the LMN. To this neuron by having its body neuronal in the high centers is called it upper motor neuron (UMN).

The spinal reflex are obtained stimulating neuro receptors located in the fascias of the tendons or ligaments of the members, so much thoracic as pelvic, as well as of coetaneous and to be developed is necessary that patents be found the components of the reflected arch; receptor, way afferent, segment medullar, way efferent, organ defector. Any wound the some of its components will produce that the effect be seen diminished or absent and to this it is called sign of neuron motor lowers. By another side, if the damage is located for ahead of the segment medullar where enters the neuron sensitive (each reflection enters in a segment medullar specific), including in the high cerebral centers, the reflection will be performed but freed of the action modulator inhibitory of the UMN so that the effect will be of normal to increased in which sign is called of upper motor neuron. For example, for the patellar reflex, is stimulated upon striking the ligament of the rotula, the receptor neuron that perceives the stimulus through its receptor, as well as the effectors that stimulates to the quadriceps femoral enter and come out of the spinal marrow through the spaces inter vertebrates L4-L5 and L5-L6, so any wound that involve said segment medullar or the remainder of the reflected arch will produce decrease or the extension of the articulation of the knee (sign of lower motor neuron). By another side, if the spinal cord is affected by ahead of L4 or ascending ways or descendents located in the brain, the extension of the articulation of the knee will be seen of normal to increased (sign of upper motor neuron).

All the spinal reflections of the member thoracic arise of the medullar segment C6-T2, and all the spinal reflections of the pelvic legs as well as the anal and the micturition arise of the segment medullar L4-S3.

Before continuing with this chapter, is necessary to remember some terms necessary for the comprehension of the neurologic theme.

 Nuclei: neuronal bodies inside the central nervous system

 Ganglion: neuronal bodies out of the central nervous system

 Tract: joint of axons inside the central nervous system

 Nerve: joint of axons out of the central nervous system

To evaluate the brain and the cerebral functions we will divide this in three portions; brain, cerebellum, and cerebral stem.

The brain or forebrain is composed by the telencephalon and diencephalon, in this area found the cerebral cortex. Its have four lobes (frontal, parietal, temporal, and occipital) conforming the cerebral hemispheres besides the thalamus and the hypothalamus. The cerebellum is a great dorsal extension of the metencephalon. The brain stem is composed of the mesencephalon, the metencephalon, and of the myelencephalon.

Still and when the diencephalon is technically the most rostral aspect of the brain stem, this have functional and dysfunctional but similar to the brain that the remainder of the brain stem (since the medium brain to the medulla oblongata), so that we can say that the "forebrain" includes the telencephalon and the diencephalon.

Skilled responses and motor functions volunteers are processed in the frontal lobule. The frontal cortex is the terminal point of the system reticular activator (SRA) maintaining the conscious, alert and awake individual of its environment, besides participating in functions of gait and posture. Its main tract is the corticospinal, thus, the lobule frontal is mainly effector. The parietal lobe process the sensorial information such as touch, pain, and the proprioception. The thalamus may process more of the sensory information in the animals than in humans, the animals do not seem to depend on the parietal lobe for processing many sensations. The occipital lobe is necessary for an adequate vision and process of the visual information. The temporal lobe processes auditory information and aids in the localization of sound. Animals do not appear to depend on the temporal lobe for the complete ability to hear, since the information additive can also be processed in the cerebral stem. The temporal lobe is also responsible for some complex behavior. Parts of the temporal and frontal lobe cortex are included in the limbic system.

The limbic system is the responsible for many of the emotions and for the innate survival behavior such as protective, maternal, aggressiveness and sexual reactions. Parts of the hypothalamus are also included in the limbic system.

The diencephalon is the part more rostral of the cerebral stem and includes to the hypothalamus and to the thalamus.

The hypothalamus modulates the autonomic nervous system. Many of the neurons motor high sympathetic and parasympathetic are originated in this place. Appetite, thirst, regulation of the temperature, balance electrolytic, sleeps and behavior responses are some hypothalamic functions. The gland pituitary is found united by a stem of the surface ventral of the hypothalamus. The gland pituitary controls many of the functions endocrines of the body. The thalamus is a complex of many nuclei with intricate functions. The nuclei ventral, abundant and medial constitute the real system nociceptive and proprioceptive of the cerebral cortex. Many of the process of sensory information occurs in the thalamus of the animals, more than in the lobule parietal. Part of the system reticular ascending (RAS) is projected since the medium brain through the thalamus and diffusing to the cerebral bark, and is the responsible for maintaining alert to the cerebral bark and to maintain awake to the animal. Another part of the system reticular ascending projects through the subthalamus the cerebral cortex. Still and when there is many structures located here, the diencephalon can have large wounds with relatively few clinical signs.

The cerebellum is situated over the fourth one ventricle in the union of the bridge and the marrow oblong. The cerebellum modulate and coordinates all the motor activity of the head, neck, trunk and extremities. The cerebellum also controls the muscular tone in the animals. The cerebellum is part of the system vestibular and maintains the equilibrium in the animal. These functions elaborates them not by direct action upon the motoneurons but by indirect influence through the tracts descendents, mainly acting like a comparator of the movements desired and the movements carried out.

The olfactory nerve or nerve cranial I (NC I) is a sensitive nerve, responsible for the olfaction and is located rostral to the hypothalamus. The optic nerve (NC II) is also a sensitive nerve and together with the optic chiasm are necessary for the vision.

The mesencephalon or medium brain contains great part of the system reticular ascending, passes through the tegmentum (lower half) of the midbrain through the thalamus and subthalamus to the cortex. When the animal does not receive or processes these impulses, tends to be slept. Wounds in the cerebral cortex, diencephalon and medium brain can produce state of comma. The mesencephalon or midbrain contains the NC III and IV. The nerve oculomotor common (NC III) contains a motor effector component, innerves the ocular muscles medial rectus, dorsal rectus, ventral rectus, inferior oblique and the muscle upper eyelid, as well as the pupil constrict through a parasympathetic way. The trochlear nerve or pathetic (NC IV) effector, innerves the muscle superior oblique dorsal of the eye. Processes pathologic in the abundant portion of the medium brain are capable to produce ataxia or paralysis.

The metencephalon contains the pons and the cerebellum. The nerve trigeminal (NC V) is a mixed nerve, its portion afferent is sensitive for the head (touch, temperature, proprioception and pain) and contributes motor effector activity of the masticators muscles. Some vital centers associates with the respiration are also located in the pons.

The cerebellum is situated over the fourth one ventricle in the union of the pons and the medulla oblongata. The cerebellum coordinates all the motor activity of the head, neck, trunk and extremities. The cerebellum also controls the muscular tone in the animals. The flocculonodular lobe of the cerebelo is part of the vestibular system and maintains the equilibrium in the animal.

In the myelencephalon or medulla oblongata is located the NC VI to the XII. The nerve abducent (NC VI) nerve motor effectors innervate to the lateral rectus and retractor muscles of the ocular globe. The facial nerve (NC VII) is mixed, its motor portion produces movement of the lips, ears and close the eyelids. The facial nerve, presents sympathetic fibers that innerves the nasopalatines tear glands and the mandibular and sublingual salivary glands, besides, its sensitive portion obtains information of the flavor of the two third cranial portions of the tongue. The nerve vestibule cochlear (NC VIII) sensitive specialized, in its portion vestibular maintains the equilibrium of the animal, while the portion cochlear is necessary to hear. The nerve glossopharyngeal (NC IX) presents effectors functions and innerves muscles of the larynx and the pharynx (phonation together with the vague one), as well as sympathetic fibers for the glands salivates zygomatic and parotid. The nerve glossopharyngeal also contributes sensation of the caudal portion of the tongue and the pharynx, as well as the sense of the flavor of the part caudal of the tongue. The vagus nerve (NC X) innerves the thoracic and abdominal glands, cardiac muscle and smooth muscle to the colon. Besides innerves the skeletal muscle of the larynx, pharynx (phonation), and the soft palate. Besides, the vagus nerve transmits information sensorial since the pharynx, larynx, thoracic and abdominal viscera. The spinal accessory (NC XI) effector nerve, innerves the trapezius, brachiocephalic, and sternocleidomastoid muscles. The hypoglossal (NC XII) nerve effector innerves the skeletal muscle of the tongue, give its mobility. Finally, vital centers for the control of the respiration, blood pressure, and cardiac rhythm are located through the central nucleus of the medulla oblongata.


 

References

1.  Cheryl L. Chrysman. Problems in Small Animal Neurology. Ed. Read & Febiger 1991

2.  Michael D. Lorenz; Joe N. Korngay. Handbook of Veterinary Neurology 4a Ed. Saunders 2004

3.  Curtis W. Dewey. Canine and Feline Neurology to practical guide. Ed. Iowa State Press 2003

4.  Fernando Pelllegrino; Adriana Suraniti; Luis Garibaldi. Neurología for the Practical Clinic. Ed. INTER-MEDICAL 2003

5.  Vicente Aige Gil. Neurología Veterinaria in the Dog and the Cat. Ed. Pulso 1998

Speaker Information
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Humberto Morales Castro, MVZ Esp.
Mexico


MAIN : Neurology : Basic Neuroanatomy
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