Lab 13 - Systems II - Visual, Auditory, Chemical senses (gustation and olfaction), visceral, and limbic systems. Vision: process of discovering from images what is in the world and where it is. This is an informative processing task. The optic nerve contains about a million fibers compared to the auditory nerve's 30,000 fibers. This should indicate that this is a very important system. Visual pathway starts with the retina in the eye. The retina differs from other somatosensory receptors like those in the skin or ear because it is not a peripheral organ - it is part of the CNS, derived from neuroectoderm. Some consider the retina to be a small brain. There are two kinds of photoreceptors- rods and cones. These are found in the deepest layer of the retina, so light has to get through the more superficial layers to hit the receptors. There are more rods (which mediate night vision, being more sensitive to dim light) than there are cones (which are more sensitive to bright light and which detect form and color and mediate day vision). The fovea is the area of greatest resolution - here the superficial neural layers have shifted away to permit the photoreceptors to get the image with minimal distortion. Rods and cones synapse with biopolar cells which then in turn connect with ganglion cells. There are retinal interneurons which modulate the flow of information from rods and cones to ganglion cells - these are called horizontal cells and amacrine cells. the horizontal cells are involved with lateral interactions b/w receptor and biopolar cells. Amacrine cells mediate lateral interactions between bipolar and ganglion cells. The retinal ganglion cells send their axons into the optic disc/optic papilla, which is the origin of the optic nerve. The axons of the ganglion cells in the nasal hemiretinae then cross via the optic chiasm to the contralateral optic tract. fibers from the temporal hemiretinae do not cross, and remain in the ipsilateral optic tract. The proportion of crossed and uncrossed fibers varies among mammalian spp. each optic tract terminates in a) dorsal lateral geniculate nucleus, b) pretectal nuclei, and c) the superior colliculus. The DLG is a thalamic nucleus which has 6 laminae. Each lamina recieves either crossed or uncrossed fibers - crossed fibers from teh contralateral eye go to lamina 1, 4, and 6; uncrossed fibers from the ipsilateral eye go to laminae 2, 3, and 5. Most DLG cells then send axons to the cerebral cortex - generally the striate cortex - area 17. From retina to DLG to cortex, the visual system has a high degree of topographic localization - retinotopia. Realize that the striate cortex is not the end of the visual system. It projects into adjacent areas 18 and 19 and then these areas project to association areas in the parietal and temporal lobes as part of a serial visual information processing system. The optic tract holds fibers that terminate in a retinotopic manner in the superior colliculus and pretectum of the midbrain to initiate brainstem mediated visual reflexes (see lab 8). It also turns out that the superior colliculus and the pretectum are part of a parallel visual information processing pathway system - they connect with the pulvinar and lateral posterior nucleus (the association nuclei of the thalamus), which then project to a number of visually related association areas in the occipital, posterior parietal, and temporal areas of the cortex. So the pretectum and superior colliculus are more than midbrain reflex centers! Brief recap: light stimulates rods and cones--->synapse with bipolar cells-->synapse with retinal ganglion cells---> axons exit through optic disk as optic nerves-->axons of ganglion cells from ipsilateral temporal hemiretinae and contralateral nasal hemiretinae cross at optic chiasm--> fibers travel through optic tracts (crossed and uncrossed fibers) to DLG (still monocular synapses)-->axons from DLG go to striate cortex (binocular convergence occurs here) Note that also fibers from optic tracts go to the PTN (pretectal nuclei), and from there to the OLM (Oculomotor nuclei), which then projects out to the ciliary ganglion for pupillary control. Auditory system: Sound waves mechanically deform the tympanic membrane, causing movement of the bones of the middle ear, resulting in vibration of the basilar membrane containing hair cells and nerve endings - this is happening in the organ of corti, in the cochlea, in the inner ear. The vibration of the basilar membrane moves hair cells and activates associated nerve endings to produce nerve impulses, which are conducted to the auditory area (areas 41, 42) of the cerebral cortex via CN VIII and the central auditory pathways, where they are perceived as sound. Note that hair cells of the cochlea are tuned mechanically and electrically to enable them to transduce particular frequencies of sound into nerve impulses in auditory nerve fibers. Because of integrated activity of the central auditory pathways we can detect sounds of different pitch, loudness, and origin in space. We can selectively attend to sounds of particular interest - selective attention via feedback circuitry. Auditory pathway in a nutshell: primary afferent (bipolar ganglion cells) are in the spiral ganglion in the cochlea. The peripheral processes are closely associated with the cochlear hair cells. The central processes of the bipolar cells aggregate into the cochlear nerve (CN VIII) which then enters the brainstem at the pontomedullary border lateral to the facial nerve root. Primary afferent fibers synapse on cells in the dorsal and ventral cochlear nuclei - which are seen lateral to the inferior cerebellar peduncle (ICP). Secondary afferents can remain ipsilateral to the side of origin; the central audiotry projection from each ear is therefore bilateral, with a heavier contralateral projection in the CNS. Auditory fibers may take several routes across the midline - dorsally through the tegmentum or ventrally to form the trapezoid body. The trapezoid body also contains cell clusters called the nuclei of the trapezoid body, and fibers may synapse here. The lateral lemniscus forms lateral to the superior olives, and is the fiber tract to the inferior colliculus. Most lateral lemniscal fibers (coming from superior olive)(whch is where the projections from the cochlear nucleus go) lead directly to the inferior colliculus, where they synapse on cells that become the brachium of the inferior colliculus (BIC) (seen on the dorsolateral surface of the midbrain as a raised ridge connecting the inferior colliculus to the MGN of the thalamus). MGN cells get the auditory fibers from the inferior colliculus via the BIC, then they send axons into the internal capsule as the auditory radiation, to terminate in primary auditory cortex (areas 41 and 42) in the superior temporal gyrus. These areas are tonotopically organzied. Input from one ear projects bilaterally to both primary auditory cortices, but the contralateral projection predominates. Brief recap: sound-->basilar membrane vibration-->hair cell movement-->stimulation of bipolar ganglion cell of spiral ganglion-->through CN VIII-->synapse at cochlear nuclei-->ipsilateral projection to synapse at superior olive or contralateral projection through trapezoid body to synapse at contralateral superior olive-->through lateral lemniscus to synapse at inferior colliculus --> out through BIC to synapse at MGN --> out through internal capsule to auditory cortex. Chemical senses: gustation and olfaction Taste and smell inform us about the external world and also access the limbic system to influence states of emotion or evoke memories. These senses are phylogenetically primitive. Gustatory system: the receptor cells are modified epithelial cells which are organized into a complex sensory organ called the "taste bud". These are embedded in the epithelium of the tongue and pharynx. The taste receptor cells are constantly turning over about every 10 days. There is a taste pore connecting the external environment to the receptor. Microvilli (the actual transduction parts of the receptor) extend from the surface of the receptor through the taste pore to contact the saliva. One afferent nerve fiber innervates many receptor cells in each taste bud and also in several taste buds - this is divergence. Taste buds in different areas of oral cavity are innervated by afferent nerve fibers that travel into the cranial brainstem (caudal pons, and medulla) by different cranial nerves. CN VII (Facial n) innervates the anterior 2/3 of the tongue - here, the taste buds are in the form of fungiform papillae, and respond best to sweet and salty stimuli. Foliate papillae of the postero-lateral edge of the tongue are also innervated by this nerve, and respond best to sour. These are located anterior to the taste buds of the posterior 1/3 which respond best to bitter. CN IX (glossopharyngeal n) innervates posterior 1/3 - here, taste buds in circumvalate papillae, localized to the posterior 1/3 of the tongue, respond best to bitter stimuli CN X (vagus) innervates taste buds in the epiglottis and esophagus CN VII innervates taste buds on the palate Pathway: primarily uncrossed First synapse is in the taste bud - receptor cells synapse in terminals of the taste afferents which access the medulla via CN VII, IX, and X. In the medulla these gustatory fibers enter the solitary tract and terminate on the nucleus of the soliatary tract (NS), which is an important visceral projection nucleus. Rostral NS neurons project ipsilaterally to terminate in parvocellular medial thalamic nucleus ventralis posteromedialis (aka VPMpc). VMPpc neurons project via the internal capsule to the ventral most part of the coronal gyrus (taste area I) which is ventral and rostral to the somesthetic representation of the tongue, mainly in the lateral bank of the presylvian sulcus. SO gustatory information is represented in NS, VPMpc, ventral coronal gyrus, and presylvian sulcus. Gustatory information can also enter the hypothalamus and limbic system, important for affective qualities of taste perception. this access is via a relay of NS fibers in the parabrachial nucleus (pontine taste area). The detailed connectivity of this system needn't be learned. Olfactory system: Sense of smell is carried by some 50-100 million olfactory receptor cells that lie deep within a dorsal recess of the nasal cavity, and which is characterized by a yellowish brown patch of specialized epithelium called (surprisingly...) "olfactory epithelium". Within this epithelium is three cell types - the receptor cell (a bipolar neuron), a supporting cell, and basal cells. The peripheral process of the receptor cell extends to the surface of the nasal mucosa and ends in a ciliated knob. The cilia join with other cilia to form a dense mat in the mucous film on the mucosal surface. The cilia interact with the odorant molecules which penetrate the mucosal film. The axon (central unmyelinated) process of the receptor joins the processes of 10-100 other receptors to form a bundle of axons. these bundles pass as olfactory nerves through the cribiform plate, and then contact the olfactory bulb. The first synapse is in the telencephalon - this is unique. Olfactory receptors themselves are also unique in that they are neurons, and they regenerate from basal cells about every 60 days. Within the olfactory bulb, multiple receptor axons converge and synapse on various cells - mainly mitral and tufted cells, which are in special synaptic regions called "olfactory glomeruli". There is chemotopic organization of the olfactory bulb, in that stimulation of receptors with different odorants stimulates metabolic activity in different areas of the bulb. The mitral and tufted cells then project axons via the olfactory tract, which later divides into the medial and lateral olfactory striae, to secondary olfactory areas - the olfactory cortex. olfactory cortex has five parts which are all part of the paleocortex: - anterior olfactory nucleus, which interconnects the two olfactory bulbs via the anterior commissure - olfactory tubercle - prepiriform cortex (not illustrated) - amygdala (central nucleus subdivision) - entorhinal area which in turn projects to hippocampus Olfactory processing doesn't end in paleocortex, however. It involves the thalamus (nucleus medialis dorsalis [MD], an "association nucleus"), the orbitofrontal area of the neocortex, and the limbic system via the amygdala and hippocampus. The amygdala connects the olfactory cortex to the hypothalamus and midbrain tegmentum. This limbic pathway system is thought to mediate the affective component associated with olfactory stimuli, whereas the MD-neocortex system is involved in conscious perception of smell. Visceral system: review information from labs 5, 6, 9 on PNS, spinal cord, and limbic system. Limbic system: understand basic components, connectivity, and general function by doing the assigned reading.