ABSTRACT
The auditory pathway is somewhat more complex than the somatosensory pathways, first because it is bilateral and second because there are more synaptic stations (nuclei) along the way, with numerous connections across the midline� It also has a unique feature-a feedback pathway from the central nervous system (CNS) to cells in the receptor organ, the cochlea�
The specialized hair cells in the cochlea respond maximally to certain frequencies (pitch) in a tonotopic manner; tones of a certain pitch cause patches of hair cells to respond maximally, and the distribution of this response is continuous along the cochlea� The peripheral ganglion for these sensory fibers is the spiral ganglion� The central fibers from the ganglion project to the first brainstem nuclei, the dorsal and ventral cochlear nuclei, at the level of entry of the vestibulocochlear nerve (cranial nerve [CN] VIII)at the ponto-medullary junction (see Figure 1�8, Figure 6�11, and Appendix Figure A�8)�
Many of the fibers leaving the cochlear nuclei synapse in the superior olivary complex, with the majority crossing but some remaining ipsilateral� This nucleus is located in the lower pons (see Appendix Figure A�7)� Fibers crossing to and from the other side are found in a structure known as the trapezoid body, a compact bundle of fibers that crosses the midline in the lower pontine region (see Figure 6�11 and Appendix Figure A�7)� The main function of the superior olivary complex is sound localization; this is based on the fact that an incoming sound will not reach the two ears at the exact same moment�
Fibers from the superior olivary complex either ascend on the same side or cross (in the trapezoid body) and
ascend on the other side� They form a tract, the lateral lemniscus, that begins just above the level of these nuclei (see also Figure 6�11)� Some fibers from the cochlear nuclei that cross in the intermediate acoustic stria and others that cross in the dorsal acoustic stria (not shown here, see Figure 3�3) join the lateral lemniscus� The lateral lemniscus carries the auditory information upward through the pons (see Figure 6�2 and Appendix Figure A�6) to the inferior colliculus of the midbrain (see dorsal views in Figure 1�9 and Figure 3�3)� There are nuclei scattered along the way, and some fibers may terminate or relay in these nuclei; the lateral lemnisci are interconnected across the midline (not shown)�
Almost all the axons of the lateral lemniscus terminate in the inferior colliculus (see Figure 6�2)� The continuation of this pathway to the medial geniculate nucleus of the thalamus is also discussed in Figure 6�2�
In summary, audition is a complex pathway, with numerous opportunities for synapses� Even though named a “lemniscus,” it does not transmit information in the efficient manner seen with the medial lemniscus� Although the pathway is predominantly a crossed system, there is also a significant ipsilateral component� There are also numerous interconnections between the two sides�
The auditory system is shown at various levels of the brainstem� The cochlear nuclei comprise the first CNS synaptic relay for the auditory fibers from the peripheral spiral ganglion; these nuclei are found along the incoming CN VIII at the level of the upper medulla� The superior olivary complex, consisting of several nuclei, is located in the lower pontine level (see Appendix Figure A�7), along with the trapezoid body, containing the crossing auditory fibers� By the mid-pons, the lateral lemniscus can be recognized (see Appendix Figure A�6)� These fibers move toward the outer margin of the upper pons (see Appendix Figure A�5) and terminate in the inferior colliculus (see Appendix Figure A�4)�
This illustration shows the projection of the auditory system fibers from the level of the inferior colliculus, the lower midbrain, to the thalamus, and then to the cortex�
Auditory information is carried via the lateral lemniscus to the inferior colliculus (see Figure 6�1), after several synaptic relays� There is another synapse in this nucleus, making the auditory pathway overall somewhat different from and more complex than the medial lemniscal and different from the visual pathways (discussed in the next part of the chapter)� The inferior colliculi are connected to each other by a small commissure (not labeled)�
The auditory information is next projected to a specific relay nucleus of the thalamus, the medial geniculate (nucleus) body (MGB; see Figure 4�3)� The tract that connects the two, the brachium of the inferior colliculus, can be seen on the dorsal aspect of the midbrain (see Figure 1�9 [not labeled] and Figure 3�3); this is shown diagrammatically in the present figure� The medial geniculate nucleus is likely involved with some analysis and integration of the auditory information�
From the medial geniculate nucleus the auditory pathway continues to the cortex� This projection, which courses beneath the lenticular (lentiform) nucleus of the basal ganglia, is called the sublenticular pathway, the inferior limb of the internal capsule, or simply the auditory radiation� The cortical areas involved with receiving this information are the transverse gyri of Heschl, situated on the superior temporal gyrus, within the lateral fissure� The location of these gyri is shown in the inset as the primary auditory areas (also seen in a photographic view in Figure 6�3)�
More exact auditory analysis occurs in the cortex� Further elaboration of auditory information is carried out in the adjacent cortical areas� On the dominant side for language, these cortical areas are adjacent to Wernicke’s language area (see Figure 4�5)�
Sound frequency, known as tonotopic organization, is maintained all along the auditory pathway, starting in the cochlea� This can be depicted as a musical scale with high and low notes� The auditory system localizes the direction of a sound in the superior olivary complex (discussed with
Figure 6�1); this is done by analyzing the difference in the timing that sounds reach each ear and by the difference in sound intensity reaching each ear� The loudness of a sound would be represented physiologically by the number of receptors stimulated and by the frequency of impulses, as in other sensory modalities�
This view of the brain includes the midbrain level and the thalamus, with the lentiform nucleus lateral to it� The lateral ventricle is open (cut through its body) and the thalamus is seen to form the floor of the ventricle; the body of the caudate nucleus lies above the thalamus and on the lateral aspect of the ventricle�
The auditory fibers leave the inferior colliculus and course via the brachium of the inferior colliculus to the medial geniculate nucleus of the thalamus� From here the auditory radiation courses below the lentiform nucleus to the auditory gyri on the superior surface of the temporal lobe within the lateral fissure� The gyri are shown in small “figurine” (above the main part of the illustration) and in Figure 6�3�
This diagram also includes the lateral geniculate body (nucleus), which subserves the visual system and its projection, the optic radiation (to be discussed with Figure 6�4 and Figure 6�6)�
The temporal lobe structures are also shown, including the inferior horn of the lateral ventricle, the tail of the caudate nucleus, the hippocampal formation, and adjoining structures relevant to the limbic system (see Figure 9�5A in Section 4)�
The auditory pathway has a feedback system, from the higher levels to lower levels (e�g�, from the inferior colliculus to the superior olivary complex)� The final link in this feedback is somewhat unique in the mammalian CNS because it influences the cells in the receptor organ itself� This pathway, known as the olivo-cochlear bundle, has its cells of origin in the vicinity of the superior olivary complex� It has both a crossed and an uncrossed component� Its axons reach the hair cells of the cochlea by traveling in CN VIII� This system changes the responsiveness of the peripheral hair cells�
This photographic view of the left cerebral hemisphere is shown from the lateral perspective (see Figure 1�3 and Figure 4�5)� The lateral fissure has been opened, and this exposes two gyri that are oriented transversely, the auditory gyri� These gyri are the areas of the cortex that receive the incoming auditory sensory information first� They are also known as the transverse gyri of Heschl (also shown in Figure 6�2)�
The lateral fissure forms a complete separation between this part of the temporal lobe and the frontal and parietal lobes above� Looked at descriptively, the auditory gyri occupy the superior aspect of the temporal lobe, within the lateral fissure�
Cortical representation of sensory systems reflects the particular sensation (modality)� The auditory gyri are organized according to pitch, thus giving rise to the term tonotopic localization� This is similar to the representation of the somatosensory system on the postcentral gyrus (somatotopic localization; the sensory “homunculus”)�
Further opening of the lateral fissure reveals some cortical tissue that is normally completely hidden from view� This area is the insula or insular cortex (see Figure 1�4)� The insula typically has five short gyri, and these are seen in the depth of the lateral fissure� It is important not to confuse the auditory gyri and insula� The position of the insula in the depth of the lateral fissure is also shown in a dissection of white matter bundles (see Figure 2�3) and in the coronal slice of the brain (see Figure 2�9A)�
The lateral fissure has within it a large number of blood vessels, branches of the middle cerebral artery, which have been removed (see Figure 8�4)� These branches emerge and then become distributed to the cortical tissue of the dorsolateral surface, including the frontal,
temporal, parietal, and occipital cortex� Other small branches to the internal capsule and basal ganglia are given off within the lateral fissure (discussed with Figure 8�6)�
The loss or decrease of hearing on one side may result from problems in the external ear (due to infection or excess ear wax) or middle ear (due to fluid, or infection, or pathology of the ossicles) that interfere with the transmission of the sound waves�
Diminished hearing, particularly for the higher frequencies, is common with advancing age and is often accompanied by a “ringing” noise, called tinnitus�
A tumor of cranial nerve (CN) VIII (within the internal auditory canal), called a schwannoma, also called an acoustic neuroma, is not rare and causes hearing loss on the side of the lesion� Because of its location, as the tumor grows it begins to compress the adjacent nerves (including CN VII)� Eventually, if left unattended, additional symptoms result from further compression of the brainstem and an increase in intracranial pressure� Modern imaging techniques allow early detection of this tumor� Surgical removal, however, still requires considerable skill so as not to damage CN VIII itself (which would produce a loss of hearing) or CN VII (which would produce a paralysis of facial muscles) and adjacent neural structures; focused radiotherapy has also been used to destroy the tumour�
Because the auditory system has a bilateral pathway to the cortex, a lesion of the pathway or cortex on one side does not lead to a total loss of hearing (deafness) on the same side or the opposite ear� Nonetheless, the pathway still has a strong crossed aspect; the auditory spectrum for speech is directed to the dominant hemisphere�
Note to the Learner: The auditory system is further described in the Integrated Nervous System with a clinical case involving a tumor of CN VIII; animation of the pathway is found on the Web site for the Integrated text�
The visual image exists in the visual field, the outside world� This image is projected onto the retina and is known as the retinal field� The visual fields are also divided into temporal (lateral) and nasal (medial) portions� The temporal visual field of one eye is projected onto the nasal part of the retina of the ipsilateral eye and onto the temporal part of the retina of the contralateral eye� The primary purpose of the visual apparatus (e�g�, muscles) is to align the visual image on corresponding points of the retina of both eyes� Because of the lens of the eye, the upper visual field projects to the lower retina (and the converse for the lower visual field); therefore, the image on the retina is “upside down�”
Visual processing begins in the retina with the photoreceptors, the highly specialized receptor cells, the rods and cones� The central portion of the visual field projects onto the macular area of the retina, composed of only cones, which is the area required for discriminative vision (e�g�, reading) and color vision� Rods are found in the peripheral areas of the retina and are used for peripheral vision and seeing under conditions of low level illumination� These receptors synapse with the bipolar neurons located in the retina, the first actual neurons in this system (functionally equivalent to dorsal root ganglion neurons)� These connect with the ganglion cells (still in the retina), whose axons leave the retina at the optic disc to form the optic nerve (cranial nerve [CN] II)� The optic nerve is in fact a tract of the central nervous system (CNS) because its myelin is formed by oligodendrocytes (the glial cell that forms and maintains CNS myelin)�
After exiting the eyeball, the optic nerves course through the orbit and exit through the optic foramen to enter the interior of the skull� In the area above the pituitary gland, the nerves undergo a partial crossing (decussation) of fibers in a structure called the optic chiasm�
Note to the Learner: There is no synapse in the optic chiasm�
The fibers from the nasal retina on one side cross the midline and join with those from the temporal retina from the other eye (which do not cross) to form the optic tract� Thus, the image of the visual world that started in
different parts of the retina of the two eyes is now brought back together in the optic tract (described in detail later)� The result of this rearrangement is to bring together the visual information from the visual field of one side from both eyes to the opposite side of the brain�
Using a specific example, the visual object on the left side, the visual field, projects to the nasal retina for the left eye and the temporal retina for the right eye�
Note to the Learner: Making a sketch diagram of the visual system using colored pens or pencils is a simple and effective way of understanding the visual pathway�
Most of the visual fibers in the optic tract terminate in the lateral geniculate nucleus (LGN or LGB), a specific relay nucleus of the thalamus (see Figure 4�3)� The lateral geniculate is a layered nucleus (this is a unique arrangement for a thalamic nucleus-see Figure 6�6); the fibers from each eye synapse in specified layers� Note that the image in the lateral geniculate nucleus remains “upside down�”
After processing, a new pathway begins that projects to the primary visual cortex, area 17 (see Figure 6�5 and Figure 6�6)� The projection, called the optic radiation, consists of two portions with some of the fibers projecting directly posteriorly deep in the parietal lobe, whereas others sweep forward alongside the inferior horn of the lateral ventricle in the temporal lobe, called Meyer’s loop; both then project to the visual cortex of the occipital lobe�
The final destination for the visual fibers is the cortex along the calcarine fissure of the occipital lobe, located on the medial surface of the brain; this is the primary visual area, V1 or calcarine cortex, also known as area 17 (described in Figure 6�6)� Note again that the image of the objects in the visual cortex is still “upside down�”
Cortical areas adjacent to the calcarine cortex, areas 18 and 19, further process the visual information; additional visual regions in the inferior aspect of the brain deal with specific aspects of vision, such as the recognition of faces (see Figure 1�5)� Other areas in the parietal lobe process visuo-spatial information (discussed with Figure 6�13)�
Note that the visual pathway-from cornea to the calcarine cortex-extends through the whole brain (excluding the frontal lobe), hence its importance in the assessment of nervous system integrity�
The visual pathway is easily testable, even at the bedside� Students should be able to draw the visual field defect in both eyes that would follow a lesion of the optic nerve, at the optic chiasm, and in the optic tract�
This is an unusual but instructive image-it is a reconstruction, from several T1 magnetic resonance images, of the visual pathway� Because this pathway, from orbit to cortex, does not exist on one plane, several “cuts” were used to piece together the whole pathway including the optic radiation from lateral geniculate to calcarine cortex�
This illustration, using a T1 magnetic resonance image of the medial aspect of the brain, shows the approximate path taken by the visual pathway, from the orbit to thalamus and the visual radiation to the occipital cortex�
This radiological image is a reconstruction, using T1 magnetic resonance images of the brain in several planes (see the upper illustration), of the visual pathway as if it exists in a single plane�
The optic nerves leave the eyeball, course through the orbit, and enter the skull, where the optic chiasm is located, and there is a partial decussation consisting of the fibers from the nasal retina (the lateral or temporal visual fields)� The optic tract continues to the lateral geniculate nucleus of the thalamus�
The optic radiation has been drawn onto the illustration on one side, showing Meyer’s loop swinging anteriorly into the temporal horn, whereas the other projection goes directly posteriorly�
Both end in the calcarine cortex (see Figure 6�6)�
Some lesions of the optic radiation are difficult to understand:
• Loss of the fibers that project from the lower retinal field, those that sweep forward into the
temporal lobe (Meyer’s loop, see Figure 6�4 and also Figure 6�6), results in a loss of vision in the upper visual field of both eyes on the side opposite the lesion, specifically the upper quadrant of both eyes�
• Loss of those fibers coming from the upper retinal field that project directly posteriorly, passing deep within the parietal lobe, results in the loss of the lower visual field of both eyes on the side opposite the lesion, specifically the lower quadrant of both eyes�
From the developmental perspective, the retina is an “extension” of the brain, and the optic nerve is in fact a central nervous system tract� In its path through the orbit, the optic nerve is ensheathed by the meninges of the brain, dura, arachnoid, and pia, with a typical subarachnoid space containing cerebrospinal fluid� Any increase of intracranial pressure may be reflected via this space onto the optic nerve and cause its compression, as well as compromising the blood vessels supplying the retina (the arteries and veins) running within the nerve�
The end result of this process will cause “blurring” of the optic disc, called papilledema, as seen with an ophthalmoscope (also discussed with the introduction to Section 3)� This is an ominous clinical sign!
