16. The medial lemniscus is formed by the secondary-order sensory fibers arising from the contralateral gracile and cuneate nuclei. After the decussation of medial lemniscus, the fibers turn upwards to form the medial lemniscus and ascend through the medulla oblongata, pons and midbrain, terminate into the ventral posterolateral nucleus of thalamus. The medial lemniscus is the important tract for conducting the proprioceptive and fine touch sensations of the contralateral trunk and limbs.
17. There are four pairs of parasympathetic nuclei in brainstem. Accessory nucleus of oculomotor nerve is also called the Edinger-Westphal nucleus that is situated in the median plane and dorsomedial to the nucleus of oculomotor nerve. The nucleus gives off the parasympathetic preganglionic fibers that pass through the oculomotor nerve and are relayed in the ciliary ganglion; the postganglionic fibers innervate the ciliary muscle and the sphincter pupillae. Superior salivatory nucleus is in the reticular formation of lower pons and surrounds the caudal portion of nucleus of facial nerve. The nucleus sends the parasympathetic preganglionic fibers into the facial nerve to relay in the pterygopalatine ganglion and submandibular ganglion respectively. The postganglionic fibers regulate the secretion of the lacrimal gland, submandibular gland and sublingual gland mainly.
Inferior salivatory nucleus is situated in the reticular formation of the medulla, just below the superior salivatory nucleus. This nucleus sends the parasympathetic preganglionic fibers into the glossopharyngeal nerve to relay in the otic ganglion; the postganglionic fibers control the secretion of parotid gland. Dorsal nucleus of vagus nerve is located within the vagus triangle. It gives off the parasympathetic preganglionic fibers, which are along the branches of the vagus nerve into the parasympathetic ganglions in the visceral organs of neck, thorax and most of the abdomen to relay. The postganglionic fibers innervate the actions of those organs.
18. Motor nucleus of trigeminal nerve is located in the reticular formation of the pons; ventromedial to the pontine nucleus of the trigeminal nerve. The efferent fibers of the nucleus run ventrolaterally into the mandibular division of the trigeminal nerve to innervate the masticatory muscles and the tensor tympani of middle ear etc. Nucleus of facial nerve lies in the ventrolateral portion of the reticular formation of lower pons. The fibers from upper part of the nucleus only innervate the expression muscles above the eye; the fibers from lower part of the nucleus just innervate the expression muscles of the face below the eye.
Nucleus ambiguus is situated within the reticular formation of medulla oblongata. The nucleus receives the afferent fibers from the bilateral corticonuclear tracts and sends the efferent fibers that join the glossopharyngeal, vagus and accessory nerves respectively and supply the muscles of pharynx and larynx.
Accessory nucleus includes two parts: the cranial part is connected with lower part of the nucleus ambiguus, but the caudal part is called the accessory nucleus which extends downward as low as the level of the fifth cervical segment of spinal cord. The efferent fibers of it control the trapezius and sternocleidomastoid mainly.
19. The visceral sensory nucleus in the brainstem is solitary nucleus. This nucleus is composed of two parts. The minor upper part is considered the gustatory nucleus by some authors, which relays the specialized taste stimuli from the tongue and epiglottis by the facial, glossopharyngeal and vagus nerves. The major lower part relays the general sensory stimuli from the viscera by the glossopharyngeal and vagus nerves.
20. Nucleus of oculomotor nerve is situated in the ventral portion of the central gray matter at the level of the superior colliculi of midbrain. Its efferent fibers form the oculomotor nerve to innervate ipsilateral most of the extraocular muscles, except the lateral rectus and the superior obliquus, and the contralateral levator palpebrae superioris.
Nucleus of trochlear nerve lies in the ventral area of the central gray matter at the level of the inferior colliculi of the midbrain. The nucleus sends the efferent fibers to innervate the contralateral superior obliquus. Nucleus of abducent nerve occupies the inferior portion of the pons just within the facial colliculus. It's efferent fibers form the abducent nerve to innervate the lateral rectus of the ipsilateral eye. Some cells of the nucleus also control the medial rectus of the contralateral eye.
21. The most evident characteristics of this section are as follows: the central canal has been spread to form the lower part of fourth ventricle; and the olive with the inferior olivary nuclei are presented lateral to the pyramid very clearly. A median sulcus and a pair of sulcus limitans are appeared on the rhomboid fossa. The nucleus of hypoglossal nerve, dorsal nucleus of vagus nerve and vestibular nuclei are situated in the gray matter of the floor of fourth ventricle from medial to lateral. The portion ventrolateral to the vestibular nuclei is the inferior cerebellar peduncle. The pyramidal tracts are still located in the pyramids. Closely dorsal to the pyramidal tracts and by the midplane are the medial lemniscus, tectospinal tract and medial longitudinal fasciculus successively from ventral to dorsal. The extensive area between the inferior cerebellar peduncle and the inferior olivary nuclei is the reticular formation in which some cranial nuclei, such as nucleus of solitary tract, spinal nucleus and tract of trigeminal nerve and nucleus ambiguus are situated. The other long tracts are located in the lateral border of the medulla oblongata.
22. In the center of the section, the transverse fibers of dumb-bell shaped trapezoid body are presented. The basilar part is bulged ventrally and contains the transverse fibers, longitudinal bundles of fibers and numerous dispersed neurons called as the pontine nucleus. The transverse fibers arising from the pontine nucleus cross the midplane to form the contralateral middle cerebellar peduncle. The longitudinal fibers are belong to the pyramidal tracts and separated to several small bundles by transverse fibers of pons. The fourth ventricle is shrunk. The fibers of superior and middle cerebellar peduncles are located on the lateral wall of fourth ventricle. In the lateral portion of tegmentum, the motor nucleus and pontine nucleus of trigeminal nerve are situated at both sides of the fibers of trigeminal nerve which run ventrolaterally and emerge through the ventrolateral surface of pons. The other long tracts are still in the original places.
23. The central canal is called the cerebral aqueduct and surrounded by the periaqueductal gray matter. The eminences dorsolateral to the periaqueductal gray matter are inferior colliculi which contain the corresponding nuclei. The mesencephalic nucleus of trigeminal nerve appears at the lateral border of periaqueductal gray matter, while the nucleus of trochlear nerve is in the ventral portion of it and close to the midplane. The medial longitudinal fasciculus is ventral to the nucleus of trochlear nerve. The most ventral portion of the cerebral peduncle is the crus cerebri which is formed by the pyramidal tract in the middle 3/5, frontopontine tract in the medial 1/5 and parieto- occipito-temporo-pontine tracts in lateral 1/5. The portion between the substantial nigra and periaqueductal gray matter is the tegmentum, which is formed by the reticular formation mainly. The medial, trigeminal and spinal lemnisci are in the ventrolateral portion of the tegmentum.
24. There are three kinds of fibers in the facial nerve. The special visceral motor fibers come from the nucleus of facial nerve, the general visceral motor fibers origin from the superior salivatory nucleus, and the special visceral sensory fibers conducting the taste from anterior 2/3 of tongue enter the upper part of solitary nucleus.
25. If the medulla oblongata is lesion, it can damage to the nucleus ambiguus. The nucleus sends the efferent fibers that join the glossopharyngeal, vagus and accessory nerves respectively to innervate the movement of the muscles of soft palatine, pharynx and larynx. So, the cough, hoarseness and dysphagia may present if the lesion involved to the nucleus ambiguus.
26. The main function of the cerebellum is to maintain posture (equilibrium), to regulate
muscle tone, and to coordinate skilled voluntary and programmed movements.
27. The cerebellum is divided into three main functional areas. The archicerebellum consists of the flocculonodular lobe. It is also known as the vestibulocerebellum because of connecting with the vestibular nuclei and vestibular nerves. The paleocerebellum is composed of the vermis (medial zone) and intermedial zone of the hemispheres, receives the afferents from the spinal cord, also called the spinocerebellum. The neocerebellum consists of the lateral zone, connects with the cerebral cortex, also termed as the cerebrocerebellum.
28. Typical signs of the cerebellum injury: (1) ataxia, incoordination of voluntary movements; (2) nystagmus; (3) intention tremor; (4) hypomyotonia.
29. Vestibulocerebellum mainly receives the axonal terminals from the ipsilateral vestibular ganglion and vestibular nuclei. These mossy fibres enter in flocculonodular lobe via the inferior cerebellar peduncle, and conduct the messages of topognosis of head to the cerebellum. The efferents of vestibulocerebellum relay at the ipsilateral vestibular nuclei via the inferior cerebellar peduncle, then modulate the activity of motor neurons of spinal medial nuclear group and cranial nuclei innervating the extraocular muscles through the vestibulospinal tract and medial longitudinal bundle respectively. These pathways regulate muscle movement of trunk, playing an important role in the maintenance of body equilibrium.
30. Its afferents mainly consists of the spinocerebellar tract. The spinocerebellar tract transmits various movement information of body to the anterior lobe, medial and intermedial areas of posterior lobe via the inferior and superior cerebellar peduncles. The spinocerebellum also receives the visual, auditory, vestibular sensations and the input from sensory and motor cortex of cerebrum (relayed via pons). The efferents originated from spinocerebellum first relay at the fastigial and interposed nuclei, then depart from the cerebellum. The efferents of vermis project to ipsilateral vestibular nuclei and reticular formation via the inferior cerebllar peduncle after relayed at the fastigial nucleus. By innervating ipsilateral spinal medial motor neurons, it modulates the muscular tension and motor coordination of trunk and proximal limb muscles through the vestibulospinal and reticulospinal tracts. The efferents of intermedial cortex of posterior lobe relay at the interposed nuclei, project to contralateral red nucleus and ventrolateral nuclei of thalamus respectively via decussation of superior cerebellar peduncle. The relayed fibres of the ventrolateral nuclei of thalamus project to contralateral cerebral cortex. The red nuclei and cerebral cortex respectively send out the rubrospinal and corticospinal tracts to ipsilateral spinal lateral motor neurons, to modulate the muscular tension and motor coordination of extremities.
31. The cerebrocerebellum receives massive inputs of contralateral extensive cerebral cortex, including sensory, motor and association cortex. The fibres originated from extensive cerebral cortex terminate in ipsilateral pontine nuclei, then project to contralateral cerebrocerebellum via the middle cerebellar peduncle. The efferents of cerebrocerebellum, after relayed in the dentate nucleus, end in contralateral red nucleus and ventrolateral nuclei of thalamus via decussation of superior cerebellar peduncle. Further, they project to the motor areas of cerebral cortex. The motor centre of cerebral cortex sends out corticospinal tract to contralateral spinal lateral motor neurons via decussation of pyramid. Through this cerebellum-cerebrum feedback circuit, the cerebr0cerebellum affects the modulation of initiation, planning and coordination of limbs fineness movement, including determination of movement strength, direction and amplitude. The lesion of neocerebellum may usually involve the paleocerebellum, with symptoms of lower muscular tension, decrease of tendon reflex, incoordination and intention tremor. For example, inability of finger-nose test, dysdiadochokinesia, etc.
32. Non-specific projecting nuclei include the midline nuclei, reticular nuclei and intralaminar nuclei. Specific relaying nuclei include the ventral anterior nucleus, ventral intermediate nucleus and ventral posterior nucleus. Internuncial nuclei include the medial nuclear group, anterior nuclear group and dorsal layer of the lateral nuclear group.
33. Specific relaying nuclei include the ventral anterior nucleus, ventral intermediate nucleus and ventral posterior nucleus. The ventral anterior nucleus and ventral lateral nucleus receive axonal terminals of corpus striatum, dentate nucleus and substantia nigra and project into the cerebral motor cortex to regulate body movement. The ventral posteromedial nucleus receives the trigeminal lemniscus and taste fibres of the solitary nucleus; the ventral posterolateral nucleus receives the fibres of the medial lemniscus and spinal lemniscus. The ventral posterior nucleus contains a well-ordered topographic representation of the body, which includes from lateral to medial, the sacral, lumbar, thoracic, cervical and cranial segments. The ventral posteromedial nucleus, which relays sensory information of the head and face, composes the central thalamic radiations and further projects into the superior part of postcentral gyrus.
34. The medial forebrain bundle is a loose grouping fiber pathway which runs mostly longitudinally through the hypothalamus, connecting forebrain autonomic and limbic structures with the hypothalamus and brainstem, receiving and giving small tracts throughout its course. It contains many different kinds of fibres: descending afferents from the septal area and orbitofrontal cortex; ascending afferents from the brainstem; efferents from the hypothalamus.
The fornix is the largest afferents of the hypothalamus. It originates from the hippocampus and ends in the mamillary nucleus, preoptic region, lateral zone and posterior hypothalamic nucleus. The amygdalohypothalamic fiber, which forms terminal stria, originates from the amygdaloid complex and terminates in the medial preoptic nucleus, anterior hypothalamic nucleus and supraoptic nucleus.
36. The language areas include the motor speech area, the writing area, the auditory speech area and the auditory speech area.
37. The caudate nucleus and putamen develop together and contain similar cells and, collectively, are termed the striatum. Functionally, the striatum and their interconnections and neurotransmitters form the extrapyramidal system, which includes midbrain nuclei such as the substantia nigra, and the subthalamic nucleus.
38. The cortex of the cerebrum is considered to comprise two types: archicortex and neocortex. The archicortex is found predominantly in the limbic system cortex and contains three layers, while the neocortex is more commonly found in most of the cerebral hemisphere and contains six layers. The paleocortex forms the transition between the archicortex and neocortex.
39. The limbic lobe includes the subcallosal gyrus, cingulate gyrus, parahippocampal gyrus, hippocampal formation, insula and temporal pole. The limbic system includes the limbic lobe, the amygdaloid body, septal nucleus, the anterior nuclear group of thalamus and hypothalamus. Its basic functions include feeding behavior, "fight-or-flight" responses, aggression,' and the expressions of emotion and of the autonomic, behavioral, and endocrine aspects of the sexual response.
41. The lateral ventricle may be divided into a central part, which occupies the parietal lobe, and from which anterior, posterior, and inferior horns extend into the frontal, occipital, and temporal lobes, respectively.
42. The internal capsule is a broad band of myelinated fibers that separates the lentiform nucleus from the medial caudate nucleus and thalamus. It has an anterior limb, genu, a posterior limb.
The anterior limb contains the anterior thalamic radiation, thalamocortical and corticothalamic tracts, the frontopontine tracts. The genu is usually regarded as containing corticonuclear tract mainly from precentral gyms and terminating in the largely contralateral motor nuclei of cranial nerves.
The posterior limb contains major ascending and descending pathways. It has the thalamolentiform, retrolentiform and sublentiform parts. The thalamolentiform part includes the corticospinal tract, the frontopontine and corticorubral tracts, the central thalamic radiation. The retrolentiform part includes the occipitopontine, posterior thalamic radiation and also the optic 'radiation. The sublentiform part contains the temporopontine tract and acoustic radiation.
43. The basal forebrain comprises several cellular groups of different sizes that are situated in the rostral and ventral aspect of the telencephalon. The nuclei usually considered together here are the paraolfactory cortex, diagonal band of Broca, nuclei of Meynert, septal nuclei, amygdaloid body, and the preoptic region of the hypothalamus.
The basal forebrain is comprised of a neurochemically heterogeneous population of neurons, including cholinergic, r-Aminobutyric Acid (GABA ergic) and peptidergic neurons that project to the cerebral cortex, hippocampus, thalamus, posterior hypothalamus and brain stem. This highly complex brain region has been implicated in attention, motivation, and memory as well as in a number of neuropsychiatric disorders such as Alzheimer's disease, Parkinson's disease, and schizophrenia.
44. The hippocampus is a primitive cortical structure that has been "folded in" and "rolled up" so that it is submerged deep into the parahippocampal gyrus. It extends the length of the floor of the inferior horn of the lateral ventricle and becomes continuous with the fornix below the splenium of corpus callosum. The hippocampus has been divided into CA1, CA2, CA3 and CA4 areas.
The hippocampus receives input from many parts of the neocortex, especially the temporal neocortex. The fomix is an arched white fiber tract extending from the hippocampal formation to the mamillary bodies of the hypothalamus and septal area. The hippocampal efferent axons travel in the fornix and synapse on neurons in the mamillary bodies. These neurons project axons, within the mamillothalamic tract, to the anterior thalamus. The anterior thalamus projects, in turn, to the cingulate gyms, which contains a bundle of myelinated fibers, the cingulum, that curves around the corpus callosum to reach the parahippocampal gyms. Thus, the following circuit is formed parahippocampal gyrus→ hippocampus→ fornix→ mamillary bodies→ anterior nuclear group of thalamus→cingulate gyrus→ parahippocampal gyrus. This circuit, called the Papez circuit, ties together the cerebral cortex and the hypothalamus.
45. The neocortex consists of up to six well-defined layers of ceils. The organization of these layers is referred to as cytoarchitecture. The outermost molecular layer contains non-specific afferent fibers that come from within the cortex or from the thalamus. The external granular layer is a rather dense layer composed of small cells. The external pyramidal layer contains pyramidal cells, frequently in row formation. The internal granular layer is usually a thin layer with cells similar to those in the external granular layer. These cells receive specific afferent fibers from the thalamus. The internal pyramidal layer contains, in most areas, pyramidal cells that are fewer in number but larger in size than those in the external pyramidal layer. These cells project to distal structures. The multiform layer consists of irregular fusiform cells whose axons enter the adjacent white matter.
46. The white matter of the cerebral hemisphere is deep to the cerebral cortex and consists of myelinated nerve fibers of many sizes. These fibers form connections within the brain. The white matter contains the commissural fibers, the association fibers, and the projection fibers.
The association fibers connect the various portions of a cerebral hemisphere and permit the cortex to function as a coordinated whole. They include the arcuate fibers, the uncinate fasciculus, the cingulum, the superior longitudinal fasciculus, the interior longitudinal faseiculus.
The commissural fibers interconnect the two cerebral hemispheres which include the corpus callosum, the anterior commissure and the commissure of the fornix. The projection fibers connect the cerebral cortex with lower portions of the brain or the spinal cord. They are composed of afferent and efferent nerve fibers, which form a compact band known as the internal capsule. The internal capsule is a broad band of myelinated fibers that separates the lentiform nucleus from the medial caudate nucleus and thalamus. It has an anterior limb, genu, a posterior limb. In horizontal section, it presents a V-shaped appearance, with the genu (apex) pointing medially.
(Shao Xujian, Hu Haitao, Li Yueying, Luo Xuegang, Peng Yingji, He Hongwen)
Multiple Choice Questions
1. Which of the following nerves may be injured alter fracture of the medial epicondyle of the humerus?
A. median nerve
B. ulnar nerve
C. radial nerve
D. musculocutaneous nerve
E. axillary nerve
2. Which of the following nerves may be injured after fracture of the fibular neck?