Lab 6: Sensory Systems

Lab Summary

This laboratory session will focus on the visual, auditory, somatosensory and gustatory pathways. Now that many of the structures of these pathways are somewhat familiar from other laboratories, focus your attention on their functional roles and on diagnostic differences for lesions at different points along the pathways. 

SECTION 1: Visual System 

SECTION 2: Auditory System 

SECTION 3: Somatosensory System 

SECTION 4: Gustatory (Taste) System 

SECTION 5: Olfactory System

SECTION 6: Review

Visual System

On the serially cut horizontal and sagittal fixed brains, trace the optic nerves to the optic chiasm. At the chiasm axons carrying information from the nasal retinas (for the peripheral visual fields) cross to innervate the contralateral lateral geniculate nucleus (LGn). Review the schematic of the visual pathways in Figure 6.1A and identify the position of each visual structure in horizontal brain section Figure 6.1B and on midsagittal cut Figure 6.1C.

Do the crossed axons come from neurons in the nasal or temporal portion of the retina? What would be the sequence of visual loss from a pituitary tumor pressing on the center of the chiasm?

Using Figure 6.1A, review the following terminology:

Geniculate and extra-geniculate pathways

Follow the optic tracts around the base of the hypothalamus and the cerebral peduncles, to the termination of many of these axons in the lateral geniculate nucleus. Retinogeniculate axons carry information that will reach the primary visual cortex (striate cortex, area 17) and will be important for analyzing the form, motion and color of objects and space. Another group of retinal axons will form the extrageniculate visual pathway, bypassing  the LGn on their way to terminate in the superior colliculus (tectum) and pretectal area. These fibers enter the colliculus as the brachium of the superior colliculus. The extra-geniculate pathway is important for directing visual attention toward stimuli in the environment, the control of pupillary responses and the generation of saccadic eye movements. Some of the targets of the tectal and pretectal nuclei are the Edinger Westphal nucleus and the frontal eye fields in the frontal lobe cortex for the control of eye movements. This extra-geniculate pathway is relevant for visual attention and orientation, while the geniculate pathway sends massive projections to the visual cortex and will process the bulk of visual perception and interpretation of the environment.  In addition to these two channels, the optic chiasm is located underneath the hypothalamus, and some axons carrying retinal information innervate hypothalamic nuclei: : suprachiasmatic nucleus for maintenance of circadian rhythms and other hypothalamic nuclei for sympathetic control of pupillary dilation.

Consider the path of the fibers from the LGn to the cortex as they pass around the lateral ventricle as the geniculo-calcarine projection or optic radiation (Figure 6.2). Note a portion of the optic radiation containing axons related to the inferior retina (superior visual field) loop over and around the lateral ventricle into the temporal lobe on their way to the occipital cortex. These fibers are referred to as Meyer’s loop. What would be the visual field deficit as a result of a lesion or vascular damage to this region? See the Lab 6 quiz for the answer.

On the medial surface of the fixed half brain, locate the primary visual cortex (Figure 6.1).

The primary visual cortex is also known as striate cortex or Brodmann’s area 17. The majority of area 17 is located on the banks of the calcarine sulcus and extends into the adjacent cuneus and lingual gyri. A small portion of area 17 extends out onto the surface of the occipital pole. Areas 18 and 19 are adjacent surface regions of the occipital cortex that are secondary and tertiary visual areas.

Using Brodmann’s cortical map (Figure 6.3) locate these 2 cortical regions. Examine the occipital pole of the coronally and horizontally cut brains you looked at in the first lab. Identify the calcarine sulcus. On the cut surface note the lateral depth of the sulcus; the topographic map of the retina extends along the banks of this sulcus. Closely examine the cut surface of the occipital lobe and the calcarine sulcus. This area of primary visual cortex is characterized by a dense fiber stripe (even visible in fresh tissue) localized within a portion of layer 4 (4B) and is called the stria of Gennari (Figure 6.4). This dense band is formed by the dense f myelinated projections  from the LGn and transversely oriented cortico-cortical axons of area 17 passing through the middle of layer 4. It is this ‘striped’ appearance that gave rise to the term ‘striate cortex’.

Now examine the sequential slides in the Coronal Visual Pathway and Horizontal Visual Pathway sections (Figures 6.5 & Figure 6.6). In each series of sections trace the visual pathway from 1) the optic tract as it wraps around the hypothalamus and midbrain, 2) to terminate in the lateral geniculate nucleus (LGn) of the thalamus, 3) exiting from the LGN laterally, as the optic radiation running around the lateral ventricle back toward the medial surface of the occipital cortex, 4) to terminate in the striate cortex which forms the banks of the calcarine sulcus.

Figure 6.5 Horizontal Visual Pathway Series

Figure 6.6 Coronal Visual Pathway series

This pathway, as well as the extrageniculate pathways are schematized in Figure 6.7.

Examine the cytology of the lateral geniculate nucleus (Figure 6.8) and relate this to the passage of information from the retina to the visual cortex. Two divisions can be seen. The inner, or most ventral, two layers contain large cells (1&2) and are called the magnocellular layers. Cells in the magnocellular layers (layers 1-2) receive retinal input from the M-type ganglion cells that have large receptive fields and are important for analyzing movement. The most dorsal or outer 4 layers of the LGn (layers 3-6) contain smaller cells, and are called the parvocellular layers. These layers receive retinal input from the P-type ganglion cells that are important for color, form and detail perception. The pattern of termination of retinal axons in the LGn is that crossed fibers arising from the contralateral nasal half of the retina terminate in layers 1, 4 and 6, while uncrossed temporal retina axons terminate in layers 2, 3 and 5. Thus, proceeding from lamina 1 to 6 the pattern is: contra (1), ipsi (2), ipsi (3), contra (4), ipsi (5), contra (6). Each LGn neuron receives direct synaptic input from retinal axons. Neurons of all 6 LGn laminae project to the primary visual cortex, area 17, where they terminate in layer 4.

Examine the myelin stained slide Figure 6.9 of the superior colliculus. As with the cortex and LGn, the superior colliculus is a layered structure. While it is unimportant for you to know specific layers, you should be aware that different modalities of input are processed in the different layers. For example, the most superficial layers (strata zonale, cinereum and opticum) are concerned with visual information processing. Retinal input arising from ganglion cells enters the superior colliculus via the stratum opticum, while input from areas of the occipital cortex enters through the stratum zonale. The superficial layers of the superior colliculus are primarily concerned with the detection of movement in visual space and with the control of head and eye movements and their primary projections are to the frontal eye fields, the pulvinar nucleus. and to the pretectal area. Lesions of the superior colliculus can result in deficits in spatial discrimination and in visual tracking. The deeper layers (strata lemnisci) of the colliculus receive a variety of inputs carrying, auditory and somatosensory information and project to intralaminar and posterior nuclear groups of the thalamus, the inferior colliculus, the inferior olive and areas of the brainstem reticular formation. Thus,the primary function of the deeper layers is to integrate sensory-motor information.

The calcarine fissure is easily seen medially, and the extent of the striate cortex can be traced. Do this using the Coronal Visual Pathway and Horizontal Visual Pathway sections in Figures 6.5 & 6.6.

Auditory System

The auditory system provides us with the ability to detect and interpret sounds within our environment, and to localize them in space. Air movement and pressure (sound) are transduced within the middle and inner ears into energy that is detected by the peripheral process of the bipolar sensory neurons of the spiral ganglion. These sensory neurons have a peripheral process that innervates the inner and outer hairs cells of the Organ of Corti and a central branch that forms the auditory component of the VIII cranial nerve. When these axons enter the brainstem, they send branches to the dorsal and ventral cochlear nuclei. Both of these nuclei can be identified on the brain stem slides in your collection. The dorsal cochlear nucleus is found at the level of the lateral recess, dorsal to the inferior cerebellar peduncle. The ventral cochlear nucleus is slightly more rostral, and is located ventrolateral to both the inferior cerebellar peduncle and the VIII nerve itself. The remaining pathway within the brainstem involves several nuclei and several opportunities for crossing before reaching the inferior colliculus in the midbrain. Auditory information then passes to the medial geniculate nucleus of the thalamus and then to Brodmann’s area 41, the primary auditory cortex located in the superior transverse temporal gyrus of the temporal lobe (see Brodmann’s cortical map, Figure 6.3).

Follow the auditory pathway using the auditory system schematic in Figure 6.10. Using the coronal auditory pathway slides (S7-18 to S7-28) in Figure 6.11 locate the auditory structures of the brainstem and thalamus indicated in TABLE 6.1. In following the auditory pathway through the brainstem, note the position of the superior olivary nucleus in the pons (not to be confounded with the inferior olive in the medulla), and the collection of fibers ventral to this nucleus, the trapezoid body, that consists of a portion of axons crossing to reach the contralateral  superior olive. The superior olive is the first point at which information from both cochlea can be received by a single neuron, and comparison of the timing and intensity of the signals allow this nucleus to detect sounds in the horizontal space.

Figure 6.11 Coronal auditory pathways slides

Axons from the cochlear nuclei and from the superior olives ascend toward the inferior colliculus as the lateral lemniscus. Throughout the brainstem, the lateral lemniscus maintains a position just dorsal to the spinothalamic tract. It becomes most distinct in the mesencephalon as it approaches the inferior colliculus. Locate this tract in the pons in slide S7-23 and note that this tract becomes less apparent in section S7-24 as the axons of this tract enter and synapse in the inferior colliculus. Neurons of the inferior colliculus then send their axons to the medial geniculate nucleus in the thalamus. As axons exit the inferior colliculus they form a superficial bundle of fibers called the brachium of the inferior colliculus. Try to locate the inferior colliculus, the brachium of the inferior colliculus and the medial geniculate nucleus on the brainstem dissections used in previous labs and in slide S7-26.

As you follow the auditory projections from the cochlear nuclei to the medial geniculate, be aware that there are crossing fibers and also ipsilateral projections at each level of the pathway. Thus, by the time this information reaches the medial geniculate nucleus, there is a substantial representation of auditory input from both ears within each nucleus. Axons of the MGn then ascend to the cerebral cortex and terminate in the ipsilateral auditory cortex. The primary auditory cortex (area 41) is located in the two transverse gyri that lie on the dorsal surface of the superior temporal convolution (Figure 6.12). The geniculate fibers pass through the sublenticular portion of the internal capsule, and most terminate in area 41, which is buried in the floor of the lateral sulcus. On the intact side of the fixed brain specimens, gently separate the banks of the lateral sulcus to see area 41.

The bilaterality of the auditory pathways is important for two reasons. First, bilateral input to several nuclei allows integration of information from both sides of the head for the localization of sounds. A sound from one side of the head will be louder and will be heard sooner by the ipsilateral ear than by the contralateral ear. Both intensity and phase differences are used to compute the location of a sound source. Secondly, peripheral lesions of the cochlea or the primary afferents of the cochlear nuclei will result in unilateral deafness. In contrast, central lesions may result in somewhat diminished sound perception, but not deafness because of the bilaterality of the ascending auditory pathways.

Somatosensory System

Review the ascending sensory pathways that you followed through the spinal cord and brainstem in labs 2-4. What is the organization of the posterior (dorsal) column / medial lemniscal and spinothalamic / anterolateral systems at each level of the brain and spinal cord and how are these sensory pathways spatially related to each other at each level of the neuraxis (Figure 6.13A-B)? Can you identify an area of the CNS where a lesion or tumor might result in a deficit in the perception of pin prick for the ipsilateral face and contralateral body? Be sure you can locate and understand the functional role of the dorsal columns, the tract of Lissauer, the dorsal horn, the anterolateral tract, the cuneate and gracile nuclei, the medial lemniscus and the spinothalamic tract in the brain stem, as well as the VPL and VPM nuclei in the thalamus. Be sure to locate the spinal tract of V, the main sensory nucleus of V and the mesencephalic nucleus. By now, many of these structures should be old friends (Figure 6.13C). What limb of the internal capsule carries the projection from VPL and VPM to the cortex? On the whole brain, locate the primary somatosensory area (SI) on the postcentral gyrus. The secondary somatosensory area (SII)lies on the upper bank of the lateral sulcus and extends into the parietal lobe. Recall their functional differences: SI is highly organized topographically and according to receptor type. SII is less organized, many cells receive input from large areas of the body, and many cells respond to a variety of stimuli, including pain. Locate the posterior parietal cortex, an important association area for the somatosensory system.

What right and left body and facial functional losses of somatic sensation would result from the following?

• Damage to the left dorsal radicular arteries at T11
• An occlusion of the left posterior inferior cerebellar artery
• Occlusion of the thalamic branches of the left posterior cerebral artery
• An occlusion of the left middle cerebral artery

Review the vestibular system and the visceral afferent systems, including the afferents and efferents of the nucleus solitarius that we studied in labs 2-4.

Gustatory System

The gustatory system is specialized for detecting molecules that are collectively termed tastants. There are 5 basic tastes that are evoked by different chemicals: sweet (e.g. carbohydrates), salty (sodium), sour (acids), bitter (alkaloids) and umami (“meaty”, MSG).

The tongue has 3 regions where taste transduction occurs and each contains different papillae-containing taste buds. The epiglottis also has several taste buds (Figure 6.14).

On the anterior tongue are the small fungiform papillae, on the lateral tongue the foliate papillae and the posterior tongue the (circum-)vallate papillae (FIG1 A). There are no taste buds on the ventral tongue. The taste buds are located inside the papillae’s trough (fungiform, i.e. mushroom-shaped) or folds (vallate, foliate, Figure 6.14B).

Taste buds are onion-shaped groups of cells containing adult or developing taste-transducing cells and supporting cells Figure 6.15 . The small filiform papillae are not taste sensitive, but transduce touch, texture and thermal changes on the tongue. Taste cells are epithelial cells (not neurons) that continuously turnover with a lifespan of about 2 weeks. Tastants enter the taste bud’s pore and either activate receptors or pass into the taste cell. Taste cells then transduce these events by releasing serotonin into their cleft with primary gustatory neurons.

Central Gustatory Pathway

The cell bodies of the primary afferents of the gustatory system are located in the geniculate and inferior (petrosal and nodose ganglia). The peripheral processes of the sensory neurons of the geniculate ganglia (CN VII, facial) innervate taste buds on the anterior two-thirds of the tongue and soft palate, neurons of the petrosal inferior ganglion (CN IX, glossopharyngeal) innervate taste buds of the posterior third of the tongue  and peripheral processes of the sensory neurons of the nodose inferior ganglion (CN X, vagal) innervate taste buds of the epiglottis and esophagus (Figure 6.16). Hence, the fungiform papillae are innervated by the facial cranial nerve (via the chorda tympani nerve, named as it runs close to the tympanic membrane of the middle ear), the circumvallate by the glossopharyngeal (CN IX), the foliate by the facial and glossopharyngeal (CN IX and X),  and the epiglossal papillae by the vagal nerve (CN X, Figure 6.16). 

Figure 6.16 Cranial nerve innervation of the tongue for taste.

The central processes of the cells in the geniculate (facial nerve), petrosal (glossopharyngeal nerve) and nodose (vagus nerve) ganglia enter the medulla and travel in the solitary tract to terminate on second order neurons in the rostral portion of the solitary nucleus (Figure 6.17). Axons of the second order gustatory neurons of the solitary nucleus travel in the central tegmental tract to the ventral posteromedial nucleus (VPM) of the thalamus. Axons of VPM neurons then travel in the posterior limb of the internal capsule to the cortical taste area in the inferior margin of the postcentral gyrus and anterior insula. These projections do not cross midline. Axons from the nucleus of the solitary tract also project to the hypothalamus for control of salivation and appetite and to the amygdala for  learned taste aversions (Figure 6.19).

The organization of the taste system input among the other cranial nerves and nuclei is indicated in Figure 6.19. The taste cranial nerves (VII, XI and X) are considered mixed GSA and GVA. Only the anterior half of the solitary nucleus receives gustatory input.

Trace the gustatory pathway using Figure 6.20 and Figure 6.21.  Use the fixed whole brain specimens and the horizontal and coronal sectioned fixed brain sections to identify the postcentral and insular cortical areas that are involved in taste perception.

In contrast to common belief, there is no “taste map” on the tongue: all basic tastes can be sensed at all main 3 regions, but minor regional sensitivity differences do exist. More generally, there is no clear chemotopy at any stage along the taste system, which has a ‘fuzzy’ functional organization.

A total loss of taste is called ageusia and a partial loss hypogeusia and can be caused by neurological damage (e.g. head trauma or Bell’s palsy), endocrine issues, drugs side-effects and aging.  Changes in taste quality are called dysgeusia and commonly result from drugs side-effects, xerostomia (dry mouth syndrome) or can accompany burning mouth syndrome (prominent in postmenopausal women). 

Changes in taste perception (i.e. gustatory) should be differentiated from olfactory changes (i.e. smell). Taste (gustation) should also not be confused with flavor, which forms as a sensory integration of smell, taste and somatosensory food properties.

Figure 6.21 Gustatory pathway

Olfactory System

Contrary to popular belief, the olfactory sense, i.e. sense of smell, is as important and capable as other human senses. Further, although humans rely on olfaction much less than many other species with a more developed sense of smell (e.g.: dogs, pigs), humans are less capable than some species like dogs or pigs, similarly capable at detecting and discriminating odors as many other species, including monkeys.

Two modes of smell exist. Odorants can be either sniffed (inhaled) from outside the nose (orthonasal smell) or exhaled from inside the oral cavity from food odorants during ingestion (retronasal smell). These two modes involve opposing flow directions of odorants across the olfactory epithelium in the nasal cavity in opposite directions (Figure 6.22), which mean different sensory and perceptual properties.

The nasal cavity has a complex shape with folded narrow cavities called conchae or turbinates which are covered with epithelium and mucus. Only part of this epithelium is sensitive to odorants. The olfactory epithelium contains the olfactory receptor neurons which transduce the binding of odorants into a neural response.

The cell bodies of olfactory receptor neurons are located in the epithelium and project dendrites inside the mucus in which the receptor covered cilia reside (Figure 6.22, left). Humans express about 400 different functional odor receptors, about half that of rodents and dogs, of about a 1000 total genes (i.e. 600 pseudo genes). The olfactory receptor gene family is the largest of any gene family in the entire mammalian genome. Each receptor has a molecular receptive range, that varies between the receptor being sensitive to only a very specific set (1-2 different molecules) to quite a wide range (hundreds different molecules). 

Olfactory sensory neurons project their axons through the porous dorsal bone overlaying the olfactory cavity called the cribriform plate Figure 6.24. Their synapses terminate in the glomeruli of the olfactory bulb (Figure 6.23, right). Each bulb contains about 5000 glomeruli, roundish neuropils of about 150 micron in diameter, which form the outer shell of the olfactory bulb, and provide the first neural relay between the first order sensory neurons and second order output mitral and tufted neurons.

Each olfactory sensory neuron expresses only 1 olfactory receptor gene. About 5000-20,000 receptor neurons expressing the same receptor converge onto 10 glomeruli in humans in the olfactory bulb.

Just like the epithelial cells of the taste system, the olfactory sensory neurons in the olfactory epithelium  also turn over, being replenished continuously. Also, in most mammals, interneurons in the olfactory bulbs are replenished by young neurons via the rostral migratory stream originating in a neurogenic niche in the lateral walls of the rostral lateral ventricle. However, this neurogenic niche and pathway, although still present in newborns, dissapears by age 2 in humans.

On the fixed whole brains, locate these regions of the olfactory system (Figure 6.26):

Olfactory bulb – receives input from olfactory receptor neurons located in the olfactory mucosa via the olfactory nerve. Mitral and tufted cells of the olfactory bulb then send axons via the olfactory tract to the olfactory cortex.

Olfactory tract – contains axons from bulb that terminate in the anterior olfactory nucleus, prepyriform cortex, amygdaloid complex, and entorhinal cortex. Olfactory cortical areas are unique among sensory areas in that they receive direct sensory input from secondary sensory neurons (Mitral and Tufted neurons) without an associated relay nucleus in the thalamus.

Primary Olfactory cortex – includes the piriform cortex and the periamygdaloid cortex.

Piriform lobe – includes the lateral olfactory gyrus and most of the parahippocampal gyrus and the uncus. This area receives input from the lateral olfactory tract.

Anterior commissure – contains two sets of axons, l) axons from anterior olfactory nucleus to the contralateral bulb and contralateral anterior olfactory nucleus, and 2) axons connecting areas of the two temporal lobes.

Entorhinal area – small posterior part of lateral olfactory gyrus plus parahippocampal gyrus.

Think about the multiple paths that olfactory information takes in the brain. Figure 6.27 illustrates the multiple central targets of olfactory related information. In Laboratory 7 you will see that many of the targets of olfactory information are related to the Limbic system that is important in the regulation of emotion and memory.

Review