At the age of 20, a man has around 176,000 km and a woman, about 149,000 km of myelinated axons in their brain. The brain performs an incredible number of tasks:It controls body temperature, blood pressure, heart rate and breathing. Your brain, spinal cord and peripheral nerves make up a complex, integrated information-processing and control system. The parietal lobe, which sits behind the frontal lobe, deals with the perception and integration of stimuli from the senses. The temporal lobe, which runs along the side of the brain under the frontal and parietal lobes, deals with the senses of smell, taste, and sound, and the formation and storage of memories. Several other essential parts of the brain lie deep inside the cerebral hemispheres in a network of structures called the limbic system.
The amygdala, an almond-shaped structure involved in processing and remembering strong emotions such as fear. The hippocampus, which is buried in the temporal lobe, is important for learning and short-term memory. The thalamus, located at the top of the brain stem, receives sensory and limbic information, processes it, and then sends it to the cerebral cortex.
The hypothalamus, a structure under the thalamus, monitors activities such as body temperature and food intake. Activation of the GVE components induces salivation from the submandibular and sublingual glands and secretion from the lacrimal and mucous glands of the nasal and oral cavities.
These effects are commonly due to an external blow to the face or overexposure to cold weather and affect peripheral branches of the facial nerve after the nerve exits the skull.
The effects of damage to the facial nerve upon sensory systems include primarily loss of taste sensation from the anterior two thirds of the tongue and loss of general sensation from the back of the ear and external auditory meatus. FIGURE 14-9 (A) Distribution of the sensory (general somatic afferent) and motor (special visceral efferent) components of the trigeminal nerve, including the sensory arrangement of the sensory divisions of the trigeminal nerve. FIGURE 14-9 (B) Organization and distribution of the central trigeminal pathways from the periphery to the cerebral cortex. As noted earlier, the central processes of each of the divisions of the trigeminal nerve enter the skull through different foramina. Both experimental and clinical studies have revealed that the fibers mediating pain and temperature sensation are distributed to the caudal aspect of the spinal nucleus, whereas conscious proprioception, pressure, and tactile sensation are distributed through the main sensory nucleus and possibly parts of the spinal nucleus. The receptors for some of the fibers contained within the mandibular branch are muscle spindles. From this discussion, it is reasonable to conclude that fibers of the trigeminal system share parallel relationships with several of the sensory pathways of the spinal cord. Concerning the sensory components, applying pain, tactile, or temperature stimuli to the area in question can test for damage to any of the sensory branches. As a bonus, site members have access to a banner-ad-free version of the site, with print-friendly pages.Click here to learn more. It is, nevertheless, one of the body's biggest organs, consisting of some 100 billion nerve cells that not only put together thoughts and highly coordinated physical actions but regulate our unconscious body processes, such as digestion and breathing. It accepts a flood of information about the world around you from your various senses (seeing, hearing, smelling, tasting, touching, etc). The billions of neurons in the two hemispheres are connected by thick bundles of nerve cell fibers called the cerebral hemispheres.
The limbic system links the brain stem with the higher reasoning elements of the cerebral cortex.
This part of the brain is thought to be the site where short-term memories are converted into long-term memories for storage in other brain areas. The motor nucleus of the facial nerve is found in the ventrolateral aspect of the tegmentum of the lower pons. Not shown is the distribution of fibers to the muscles of facial expression contained in the motor root for the special visceral efferent component of CN VII. Several other actions include reflex closing of the eyelids upon touching the cornea,and reflex contraction of the stapedius muscle following a loud noise.
The GVE component of the facial nerve arises from the superior salivatory nucleus located in the reticular formation of the lower pons (Fig.
Lesions of the intermediate nerve commonly produce disturbances in secretion of saliva and lacrimal secretion.
As indicated earlier, the facial nerve also contains a small GSA component, which conveys cutaneous sensation from the back of the ear and external auditory meatus.4 The cell bodies lie in the geniculate ganglion, and the central processes enter the CNS through the intermediate nerve. Corticobulbar fibers provide bilateral inputs to the dorsal half of the motor nucleus of the facial nerve. A lower motor neuron paralysis may result from damage to the facial nerve, its peripheral branches, or the facial nucleus. The trigeminal nerve is very large and can be easily seen upon its emergence at the level of the middle of the pons near the position of emergence of the middle cerebellar peduncle.
The cell bodies of the GSA component of the trigeminal nerve are located in the trigeminal (also called the Gasserian or semilunar) ganglion.


Knowledge of the dissociation of pain fibers from other forms of somatosen-sory inputs has been applied clinically; for example, severing the sensory root fibers after they enter the spinal tract of CN V can alleviate intense forms of trigeminal neuralgia. These fibers mediate unconscious proprioceptive signals to the brain and terminate in the mesencephalic nucleus of CN V, which represents part of the first-order sensory neuron. For example, the pathways that mediate pain and temperature sensation from the head to the thalamus via the ventral trigeminothalamic tract can be likened to the lateral spi-nothalamic tract, the pathways that mediate conscious proprioception and tactile inputs to the thalamus via the dorsal trigeminothalamic tract can be likened to the medial lemniscus, and the pathway involving inputs to the mesencephalic nucleus and its projection to the cerebellum shares similarities with the posterior spinocerebel-lar tract.
The motor trigeminal nucleus is located just medial to the main sensory nucleus of CN V and is separated from it by root fibers of the trigeminal nerve (Fig. Because the trigeminal nerve contains both motor and sensory components, dysfunctions of either component may result from lesions of this nerve.
Failure to recognize any of these stimuli would indicate a sensory loss to the affected area. It continuously receives sensory information, and rapidly analyzes this data and then responds, controlling bodily actions and functions.
Even though it is the smallest of the three main players, its functions are crucial to survival.
It plays a key role in developing and carrying out instinctive behaviors and emotions and also is important in perceiving smells and linking them with memory, emotion, and instinctive behaviors. The cell bodies for these neurons lie in the geniculate ganglion, and the peripheral processes run in the lingual and chorda tympani nerves.
Once these fibers have entered the CNS, they then enter the spinal tract of the trigeminal nerve and synapse upon neurons of the spinal trigeminal nucleus. However, inputs to more ventral aspects of this nucleus, whose axons innervate facial muscles below the forehead, are from the contralateral cerebral cortex. The ganglion itself is located on a cleft of the petrous bone lateral to the cavernous sinus. Some trigeminal pain fibers associated with the posterior aspect of the face may even extend as far caudally as C2 of the spinal cord. For this reason, the unusual feature is that the mesencephalic nucleus, which appears to be similar in appearance to cells of the Gasserian ganglion, does not lie outside the CNS but, instead, is situated within it.
In this manner, the same kinds of sensory inputs that reach the brain from the body region can also reach the brain from the region of the head by virtue of specific ascending pathways within the trigeminal complex. If there is a paralysis or paresis affecting the muscles of mastication, they will become flaccid after showing spasticity. The left hemisphere appears to focus on details (such as recognizing a particular face in a crowd). The cerebellum has two hemispheres, which receive information from the eyes, ears, and muscles and joints about the body’s movements and position.
The brain stem controls the functions that happen automatically to keep us alive—our heart rate, blood pressure, and breathing. Because this is the nerve of the second branchial arch and innervates muscles derived from the mesenchyme of this arch, it is classified as SVE.
They initially ascend in a dorsomedial direction to the region of the floor of the fourth ventricle.
Pregan-glionic neurons exit the brain in the intermediate nerve, which emerges between the facial and auditory-vestibular nerves. Centrally, they pass into the brainstem through the intermediate nerve and terminate in the rostral half of the solitary nucleus. In this manner, cutaneous sensation originating from the facial nerve is transmitted to the cerebral cortex via the trigeminal system in a manner similar to that described earlier for GSA inputs from the vagus and glossopharyngeal nerves. If muscle tone is lost, the affected side of the face may take on the appearance of an empty, smooth, mask-like expression. The SVE component, which innervates skeletal muscle from the mesenchyme of the first branchial arch, is much smaller in size than the sensory branches but provides motor innervation of the muscles of mastication. There are three principal divisions of the sensory components of the trigeminal nerve: ophthalmic, maxillary, and mandibular (Fig. From both the main sensory and spinal nucleus, second-order neurons arise, and their axons are distributed to the VPM of the contralateral thalamus. Although the mesen-cephalic nucleus is functionally similar to the dorsal root ganglion, it represents an anomaly in that it is the only sensory structure whose first-order cell bodies lie within the CNS and not in the periphery.
Axons of the motor nucleus pass in a ventrolateral direction and exit the brain at the approximate level of entry of the sensory fibers. Moreover, if the patient is asked to open his or her mouth, and the pterygoid muscles are affected, then the jaw will deviate to the affected side.
Severe pain can result from irritation of the trigeminal nerve by such factors as inflammation, tumor, or vascular lesion. This will produce (in addition to some problems with eating) damage to both the lateral spinothalamic tract as well as to the spinal trigeminal tract and nucleus.
The right hemisphere focuses on broad background (such as understanding the relative position of objects in a space).
Once the cerebellum processes that information, it sends instructions to the body through the rest of the brain and spinal cord.


It also relays information between the brain and the spinal cord, which then sends out messages to the muscles, skin, and other organs. The GVE components serve as preganglionic parasympathetic neurons that are associated with a number of different processes, such as salivation, lacrima-tion, and secretion of mucous membranes within the nasal cavity. They then pass laterally over the abducens nucleus (CN VI) and descend in a ventrolateral trajectory, exiting the brainstem at the level of the caudal border of the pons (Fig. Taste information is transmitted from the solitary nucleus to the nucleus VPM of the thalamus and from the VPM to the taste-receiving regions of the lateral parts of the postcentral gyrus. Individuals can typically wrinkle their foreheads and close their eyes with unilateral lesions of the cerebral cortex. The angle of the mouth on the affected side may also droop, and, when attempting to display their teeth, patients cannot bring the angle of the mouth laterally. Fibers that arise from the spinal nucleus are distributed to the contralateral VPM via the ventral trigemi-nothalamic tract. Similar to neurons of the dorsal root ganglion, there is a second limb of the axon emanating from the mesencephalic nucleus, which transmits signals away from it to the motor nucleus of CN V.
This is called trigeminal neuralgia (sometimes called tic douloureux) and is often localized to a portion of one side of the face (usually associated with a specific branch of the trigeminal nerve that is subject to such irritation). Accordingly, there will be a loss of pain and temperature sensation on the ipsilateral side of the face and contralat-eral side of the body. The cerebellum’s work allows us to move smoothly, maintain our balance, and turn around without even thinking about it.
The SVA component is associated with the transmission of signals from the tongue to the brain, and the GSA component conveys somesthetic inputs to the brain from the region of the back of the ear and external auditory meatus. One branch joins the chorda tympani and then the lingual nerve and, ultimately, synapses in the submandibular ganglion. On the other hand, as a result of supranuclear lesions, patients will not be able to raise the corners of their mouths or move their lips contralateral to the lesions.
Concerning the peripheral distribution of the ophthalmic division, nerve fibers supply the forehead, cornea, upper part of the eyelid, dorsal surface of the nose, and mucous membranes of the nasal and frontal sinuses.
This provides the basis for a monosynaptic reflex that is sometimes referred to as the jaw-jerk reflex. The main functions of these muscles are to produce chewing and biting responses (see earlier description of jaw-jerk reflex). Neurologic pain may also occur in association with the viral disease, herpes zoster (shingles), affecting mainly the ophthalmic division of the trigeminal nerve, called ophthalmic zoster. It also is involved with motor learning and remembering how to do things like drive a car or write your name. Upon exiting the brain, fibers of the facial nerve enter the internal acoustic meatus and petrous portion of the temporal bone.
Post-ganglionic parasympathetic fibers arise from the sub-mandibular ganglion and supply the submandibular and sublingual glands. Thus, this distinction is characteristic of an upper motor neuron lesion with respect to the facial nerve and clearly different from a lower motor neuron lesion of this cranial nerve (described in the following section). Those fibers that pass ipsilaterally do so in the dorsal trigeminothalamic tract, whereas fibers passing contralaterally do so in the ventral trigeminothalamic tract. This is a stretch reflex because it occurs after stimulation of muscle spindles in the masseter muscle of the lower jaw. The fibers then continue along and through the facial canal and, ultimately, exit the skull through the sty-lomastoid foramen. Other preganglionic fibers in the intermediate nerve join the major petrosal nerve and terminate in the pterygopalatine ganglion.
Moreover, lesions of the facial nerve can also produce hyperacusis, which is an increase in sensitivity to sounds on the side of the lesion, because of a paralysis of the stapedius muscle. Collectively, it would certainly appear that the ventral trigeminothalamic tract is far more significant in transmitting sensory information from the trigeminal system to the thalamus.
Afferent impulses (1a fibers) cause a discharge of neurons in the motor nucleus, which is then followed by a jaw-closing response.
Therefore, motor deficits following a unilateral upper motor neuron lesion are generally not observed. From this ganglion, post-ganglionic parasympathetic fibers arise and supply the lac-rimal, nasal, and palatine glands.
Third-order sensory fibers arising from the VPM then project to the ipsilateral face region of the postcentral gyrus (Fig. Other fibers from the mesencephalic nucleus project to the cerebellum, thus providing the cerebellum with information concerning the status of muscles of the lower jaw. The third division, called the mandibular division, supplies the lower jaw, lower teeth, chin, parts of the posterior cheek, temple, external ear, anterior two thirds of the tongue, and floor of the mouth.



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