Afferent way.

Sanchez Alarcon Jiunnery Ximena.

Abstract.

Afferents way are chains of neurons that transmit impulses from the periphery, that is, from the receivers to the cortex and cerebellum.

Some tactile sensations are transmitted, proprioception, cold, heat, pain, besides olfaction, sight, taste and balance. They`re classified into two groups, somatic and visceral.

Somatic carry the proprioceptive and nociceptive information as well as the vibration, which is why their receptors are found in various organs, including the largest in the human body, the skin. Some of these receptors are those of Krause for fio, Meissner to light touch, pressure and Pacini for vibration, heat and Riffini for Merckel for pressure and texture of objects.

Special visceral afferents originate in specialized sensory organs of smell, taste, sight and balance; while the vessels are found under which detect pressure changes.

Introduction.

They are chain of neurons which transmit impulses from the periphery, that is, from the receivers to the cortex and cerebellum.

The stimuli that convey are:

Tactile sensation (pressure) Proprioception ( body position and movement) thermic sensation(heat and cold) Pain

They are classified into 2 groups:

Somatic afferent way: medular origin and troncular origin. Visceral afferent way: general and specially.

Somatic afferents.

They are responsible for bringing information to the NCS from somatic structures.The information is transported through some cranial and spinal nerves.Somatic afferent systems have two main components: a means for detection of painful stimuli (nociceptive) and thermical and a way to detect mechanical stimuli (touch surface deep touch or proprioception and vibration).They can be conscious or unconscious.

If the receptors are localized in the skin or the subcutaneous tissue and respond to external stimuli, are called exteroceptors.Those located in the visceral organsare called visceroceptoresor interoceptors.The proprioceptors are receptors located in skeletal muscles, tendons and joints.

The accuracy with which a stimulus can be identified varies from one region to another due to the difference in the number of receptors per area.

Afferent neurons are pseudomonopolares. The body is located in the ganglia of posterior root the spinal nerves or some ganglia of cranial nerves like trigeminal nerve ganglia. Central extension of neuronsgo from the ganglion to the neuraxis and the peripheral from skin, the skeletal muscles, tendons or joints to the node.

Visceral afferents.

General visceral afferents originating in the receptors of the vessels and viscera of the head and trunk, which mostly respond to stretch and chemical stimuli.The fibers of this division are within certain cranial nerves and allthe spinal.

Special visceralafferents originate in specialized sensory organs of olfaction and gustation.Fibers leading olfactory information exist only in the olfactory nerve and the ones leading taste information are leading a small group of cranial nerves.

ROUTE OF PAIN AND TEMPERATURE.

Pain is a complex sensation, in which a nociceptive stimulus leads to the perception of the site where it occurs and also triggers a series of events such as increased levels of attention, emotional reaction, autonomic response and a tendency to remember the event and their circumstances.

Correspondingly, multiple pathways carry nociceptive informationrostrally from the spinal cord.The sensation of pain has several modes of presentation; superficial pain that is triggered from stimuli in the skin and is easily located, while the deep pain that originates in skeletal muscles, tendons and joints are sparsely located.The termical sensation can be distinguished in two modalities: heat and cold. For cold receptors are activated temperature below 33 ° C and heat receptors increase their activity at a temperature above 33 ° C.

Most nociceptors (pain receptors) and thermoceptors (receptors for heat and cold) are represented by free nerve endings that are continuous with afferent C and Aδ type, which are poorly myelinated.The pain pathway and temperature have the same course.

The cell bodies of these fibers are located in the dorsal root ganglion and the central portion is continued through dorsal root to penetrate into the spinal cord where the fibers diverge and form ascending and descending branches.These fibers run through

the tract of Lissauer and send collaterals that terminate in laminae I (marginal zone) and II (jelly substance). Other fibers stablish synapses with neurons whose axons form the ascending tracts for pain and temperature, as:

Ventrolateral thalamic spinocerebellar tract (TETVL) fibers that intersect at rostral or upper segments are occupying inner portions of the tract, this provides a somatotopic organization in a form that lower portions of the body are represented laterally and the higher medially.The TETVL ascends to the brain stem, the dorsolateral to the olive and the medial lemniscus.

TETVL fibers terminate somatotopically organized in the nucleus ventral-posterior and lateral of the thalamus. Thalamic intralaminar nuclei also receive TETVL terminations.Thalamic thalamocortical projections are distributed in the somatosensory cortex and are arranged in a somatotopic.

Spino-reticular tract: TETVL fibers are accompanied by fibers terminating in the brainstem reticular formation and form the spino-reticular tract. This tract formed by crossed fibers and direct, is related to several reflexes and represents a connection to the intralaminar nuclei of the thalamus.

Spino-mesencefalictract: TETVL fibers are also accompanied by fibers ending in the superior colliculus, in the periaqueductal gray and the midbrain raphe nuclei.The latter two are related to the pain inhibitory system, giving rise to descendants and polysynaptic pathways fibers terminating in posterior cords.

In the face, pain and temperature information is transmitted through the trigeminal nerve and its branches V1 (ophthalmic), V2 (jaw) and V3 (mandibular).

The bodies of the neurons are located in the trigeminal ganglion and central prolongations terminate in the spinal trigeminal nucleus in the medulla and upper spinal cord.

Nerve fibers that accompany the VII, IX and X cranial nerves also end in the spinal trigeminal nucleus.Spinal trigeminal nucleus, originate crossing fibers, and forming the lemnisco trigeminal mesencefalic ascend to reach the ventral-posterior nuclei and medial thalamus and the intralaminar nuclei of the thalamus.

From thalamic nucleus originate fibers constituting the thalamocortical projections. These projections contain nociceptive fibers terminate in the postcentralgyrus and the cingulate.

DEEP TOUCH OR DISCRIMINATIVE PATHWAY.

Touch is the experience of a gentle stimulus to the skin, but when the deep skin receptors are activated originates pressure sensation.And touch and pressure can be considered as a continuum according to the stimulus intensity.The path of deep touch is involved in the perception and appreciation of mechanical stimuli.Through deep touch we are able to discriminate the form (estereognosia), texture, motion and position sense of our body in space (proprioception).The vibration (pallesthesia) originates before oscillating stimuli as by placing a tuning fork on the surface of a joint.The purpose of the proprioceptors is mainly to give detailed information and continuing sense of limb position at rest (static) and during movement (kinetic).

In contrast to the pain receptors and temperature which are represented mainly by free tips, related receptors deep touch, vibration and proprioception, besides being in the free ends are encapsulated and are not as specialized.Receptors for pressure are represented by terminalscylinders, tactile corpuscles, discs contractile, lamellar bodies and receptors in hair follicles (peritraquialarborizations).Vibration is mediated Paccini corpuscles. The sense of position depends on several proprioceptors located at the joints, tendons and muscles (neuromuscular spindles and neurocinewy).

The bodies of the neurons associated with these receptors are located in the dorsal root ganglia, their prolongations central formed from Aα fibers, which continue through the posterior roots, penetrate the posterior cord, which are divided into upstream and downstream branches, both contain synapsing collateral with several groups of cells in the spinal cord.

Most ascending branches enter the ipsilateral spinal cord, forming the gracilis and cuneatus fasciculus. The gracilis fasciculus carries information from the lower limb and lower trunk, while the cuneatus fasciculus carries information from the upper trunk and ipsilateral upper extremity.

These tracts terminates in the spinal somatotopically organized as gracilis and cuneatus tubers.From them, originate fibers cross to form the contralateral medial lemniscus.

The fibers that cross form the decusation of medial lemnisco, located above decusation of pyramids.

The medial lemniscus is located initially in the midline, but as it rises is located more laterally, reaching Ventral Posterior Lateral nucleus of the thalamus.

Collateral fibers of the strands subsequent carrying proprioceptive information from the lower limb on the same side end in the neurons of Clarke's nucleus (T1-L1 /2).

From this core fibers projecting to the lateral cord of the same side and form the dorsal spinocerebellar tract enters the cerebellum through the inferior cerebellar peduncle and terminates in the ipsilateral cerebellar hemisphere and vermis.

Thalamic fibers projecting to the somatosensory cortex and do so in a topographically organized.

The fibers responsible for the sense of position in the face of the masticatory muscles and periodontal ligaments posseses a pseudomonopolar body within the neuraxis, specifically in the mesencephalic trigeminal nucleus, the central portion of these neurons send collaterals to the trigeminal motor nucleus, the core Pontic and spinal trigeminal nuclei to the reticular formation nuclei and cerebellum. This core fibers originate crossed and form trigeminal-thalamic tract, also called trigeminal lemnisco which addresses the ventral posterior nucleus of the thalamus.

Medial lemnisco fibers and the ones from trigemino-thalamic tract end organized in the poesterior ventral lateral and medial ventral porsteriornuclei respectively.Thalamus, projecting thalamocortical fibers, which rise for the rear arm of the internal capsule and end in the somatosensory cortex.

TOUCH SURFACE PATHWAY.

The receptors for light touch (not discriminative) are distributed in the skin, hair, joints and muscles.These receptors are represented by free endings, not related to nociceptive stimuli and respond to stimuli that are triggered by stretch, rub or squeeze the skin, but do not trigger pain.Light touch information to the trunk and extremities is carried along the path of pain and touch the deep, described above.

Tactile sensation originating in the face and neck, is derived from the same type of sensory receptors that are found in other body parts, however, due to the location unique structures in this region, some receptors are associated with special functions, such as the direction of movement or force dental bite.

A large quantity of encapsulated receptors, especially Merkel discs, are in the region around the mouth and lips.The information is transmitted by the three branches of the trigeminal nerve, whose bodies lie in the trigeminal ganglion.

The central portion of the fibers entering the brain stem forking and some end up in the principal trigeminal nucleus at the bridge and others in the spinal trigeminal nucleus.Originate from the core fibers which intersect and are part of lemnisco-trigeminal contralateral, also originate fibers which ascend byipsilateral dorsal trigémiotalámico fascicle. Both fascicles terminate in the VPM nucleus of the thalamus contralateral and ipsilateral respectively.Thalamic fibers are projected to the somatosensory cortex.

The lateral medial ventral and the back lateral ventralnuclei of the thalamus, projecting fibers with nociceptive information of not discriminative touch and touch deep, rising by the rear arm of the internal capsule and end in an organized manner in the primary somatosensory area (S1) in the postcentralgyrus.A smaller number of fibers reaches the secondary somatosensory secondary area (SII), on the upper lip of the lateral sulcus.

The SI area can be divided into four areas along the entire postcentralgyrus: Broadman areas 3a, 3b, 1 and 2.3b area, receives the majority of the fibers coming from the ventral posterior nucleus of the thalamus and associated with theprocessing oftactile information contains a complete map of skin receptors and different parts of the body are represented somattopically.

Further processing of the information takes place in areas 1 and 2 that receive input from area 3.

Area 1 is related to the texture of objects, while the area 2 with the discrimination of the size and shape.The area 3a is activated by the proprioceptive receptors and is associated with motor activity.

OLFACTORY SYSTEM.

Olfaction is odor perception resulting from the detection of odorous particles dispersed in the environment.For many mammals smell is the primary means to perceive information about the environment around them. Macrosmatic animals have a well-developed olfactory system that is trusted to recognize food, predators and prey, and locate potential mates.

The human is less dependent of smell because their olfactory system is less developed, but the man is able to recognize thousands of odors, many of them at low concentrations.Olfaction plays a significant role in safety, nutrition and quality of life.

The receptors responsible for the transduction of odorant molecules found in the olfactory mucosa, which has an area of 1 to 2 cm2 and is located at the top of the nasal cavity: On the bottom surface of the cribriform plate, at the level of nasal septum and medial wall of the superior turbinate.

Olfactory mucosa is made by a layer of mucus covering the olfactory epithelium.In the transition between the human respiratory epithelium and the olfactory is gradual.Olfactory epithelium contains three types of cells: receptor neurons, sustentacular cells (support), and basal cells.

The bodies of bipolar neurons for the olfactory receptors are localized to the basal portion of the olfactory epithelium.

Each neuron has an apical dendrite extension and no-myelinated axon which is directed at baseline.Apical dendrite extends to the surface epithelium where it

terminates in the form of vesicle olfactory from which emerge cilia whichnon-motile, protruding into the mucus layer.These cilia contain receptors for odorants particles.

The no-myelinated axons of receptor neurons cross the cribriform plate of the etmoides, and are grouped to form fascicles, which together form the olfactory nerve (first cranial nerve). These axons terminate in the olfactory bulb.

Olfactory receptor neurons have a continuous turnover and have an average lifespan of 30 to 60 days.They are replaced by receptor neurons originating from undifferentiated basal cells, namely the basal cells are primitive cells that give rise to receptor neurons.

Support cells are columnar and extend from the own lamina to the surface epithelium, which have microvilli which extend into the mucus layer. These cells provide mechanical support to receptor neurons and additional are secretory and its contents extend into the mucus layer that apparently plays a role in the binding of odorous particles.

A fourth type of cell is described in the human mucosa, microviliares cells, which have a projecting apical process within the mucus layer and an basal extension which is introduced into lamina propria.

Own Lamina has abundant Bowman glands which are found only in the olfactory mucosa. Serous secretion of these glands together with the support of the cells, forming the layer of mucus covering the olfactory mucosa.

The particles to be inhaled volatile odoriferous make contact with the mucus layer lining the olfactory epithelium.These particles interact with hydrosoluble proteins, odorant binding proteins.

Once through the mucus layer odorous particles bind to receptors of the cilia where transduction occurs.It is estimated that there are at least 1000 different types of odorant receptors that lead to the activation of a second messenger pathway through olfactory specific G protein which in turn activates adenylyl cyclase to produce cAMP.

The increase in cAMP ciliary allows opening ciliary membrane channels which facilitates flow of cations to receptor neurons and starts a gradual depolarization.

An olfactory neuron expresses only one type of odorant receptor subtypes and specific odorant receptors preferentially distributed within one of the four zones symmetrical bilateral olfactory epithelium.Such specificity allows olfactory information has an additional processing pattern in the olfactory bulb. Although olfactory neuron expresses only one type of olfactory receptor, such cell can respond to a wide range of odorants.

The olfactory bulb consists of five layers of neurons and nerve fiber layers including: the olfactory nerve, the glomerular, outer plexiform, mitral cells and granule cells.Fibers forming the olfactory nerve penetrate the olfactory bulb, and end exclusively on the olfactory glomeruli.

Each neuron axon olfactory synapses in one or two glomeruli. These axons terminations are arranged such that the receptor neurons expressing the same receptor subtype terminating at the glomeruli. This suggests that each glomerulus receives fibers of a single type of receptor.The glomeruli are the most prominent structures in the olfactory bulb and contain the endings of olfactory receptor neurons ramify and synapse with the dendrites of mitral cells and tufted cells.

These two neurons are functionally similar and form the efferent pathway of the olfactory bulb.As olfactory receptor neurons, the dendritic cell mitral and tufted synapse only in one glomerulus.

Glomeruli are adjacent neurons peri-glomerular wooded dendrites extending into the glomerulus and short axon distributed in close vicinity to five clusters. As can be seen significant neuronal convergence within the glomerulus, thousands of receptor neurons converge into a single glomerulus creating axodendriticas excitatory synapse (glutamate, carnosine) with the ends of the mitral cells, periglomerular and tufted.

The other set of connections within the glomerulus are dendodendritics reciprocal synapses between mitral cells, periglomerular and tufted. Inthese synapses involved in glutamate and GABA.The glomerulus also end nerve processes coming from the locus coeruleus (noradrenergic), and raphe nuclei (serotonergic).

Granulosa cells are the main olfactory bulb interneurons, axons have not and through dendrodendritics inhibitory synapses with mitral and tufted cells, modulate the activity of the latter.

TASTE SYSTEM.

The term spice is used by some authors as taste sense, as said the set of feelings that arise to stimulate the taste buds.We often assume that this is the same as the complex sensation we perceive when we eat or drink, but this integration in tasting food is the result of the combination of three different types of stimulation simultaneously: olfactory stimulation by fumes food, direct stimulation of the taste buds and free endings burn trigeminal sensory system.The latter detects the spicy, pungent, temperature and texture of food.

On the surface of the tongue are the taste buds, taste receptors, which are located in the taste buds, which by their morphological characteristics are called: fungiform, foliate and circumvallate.

The fungiform papillae are more abundant in the anterior part of the tongue, each containing between 3-5 taste buds.The foliate are located at the edge of the tongue, to the rear and are formed by folds of mucous and contain between 100 to 150 taste buds. Finally a series of 8-9 circumvallate papillae arranged in a V, defining the anterior two thirds of the tongue.Each contains 250 taste buds.

In summary it is considered that there are about 5000 taste buds on the tongue distributed. Circumvallate and foliate papillae relate von Ebner's glands, draining the product at the base of these buds and influence the microenvironment.In the soft

palate, pharynx and larynx top of the taste buds are also not located in taste buds but distributed in the epithelium.Apparently relate to mechanisms and swallowing reflex responses.

Each taste bud contains 40 to 60 for taste receptor cells that extend from the basal lamina to the surface epithelium.These are fusiform cells with microvilli which extend into a small hole, the taste pore.The pore forms a pocket that allows for contact between the microvilli and the external environment.

Afferent fibers penetrate the basement membrane and branching at the base of taste bud. Receptor cells respond to more than one type of flavor. Buds fibers, visceral afferent special branch and innervate multiple taste buds that may be located in different taste, and they also respond to more than one type of flavor. In the deep part of the taste bud, the receptor cells synapse with special visceral afferent fibers of the facial nerve, glossopharyngeal and vagus.

Recipient cells have a half life of one to two weeks before being replaced by orig cells that are originated for the differentiation of the basal cells, which migrate from the epithelial cells which surround the taste bud.

Facial nerve fibers innervating the taste buds of the fungiform and foliate papillae of the two thirds of the tongue and soft palate taste buds.Glossopharyngeal fibers innervate circumvallate papillae, most foliate taste buds and pharynx.Vagal fibers innervating the taste buds of the epiglottis and esophagus.Recipient cells are differentiated from the surrounding tongue epithelium and dependent interaction with visceral fibers to ensure its existence because denervation of a tonge area causes degeneration of the receivers.

The four basic tastes, traditionally recognized, are sweet, salty, bitter and sour, but surely there are others. Although some parts of the tongue are more sensitive to certain tastes like the tip of the tongue to sweet, sides to acid andsalty, and back to the bitter, actually all parts of the tongue are sensitive to these four types of flavors. The receptor cells use multiple methods to convert the chemical stimulus in an electrical stimulus. Transduction processes include:

1. For the transduction process of sodium chloride does not involve a receptor molecule, simply sodium channels in the apical portion of the recipient cell, allow the entry of Na + ions, which depolarizes the membrane.

2. Tart flavors that contain abundant H + depolarize the recipient cell membrane either by a movement of protons through Na +channels or blocking conductance K + channels.

3. Sweet substances, however, bind to the G protein that activates cAMP is increased which leads to the closure of K + channels, which are normally open.

4. The bitter substances, which are found in many of the toxic substances, have a protective function and are avoided by humans and other animals.These substances interact with the G protein or block K + channels directly.

Taste information not only reaches levels of consciousness to the perception of flavor, as well as its role in autonomic responses and the acquisition of food placed as the sensory system is more closely associated with the hypothalamus and the limbic system.Taste information are conveyed by the pairs VII, IX and X through special visceral afferents.The chorda tympani brings nerve taste information from the two thirds of the tongue and joins the pair VII.

The greater superficial petrosal nerve also joins the VII couple and brings soft palate taste information.The bodies of the facial nerve neurons that carry taste information are located in the geniculate ganglion, and from there the central fibers are part of the intermediate rib, part of VII, penetrate the brainstem cerebello-pontine angle join the solitary tract and end in the caudal solitary nucleus.

Taste information from circumvallate papillae and the back of the foliate papillae is carried by the branches of IX pairlinguo-tonsillar torque and the epiglottis from the esophagus is transported by the superior laryngeal nerve fibers, branch pair X.The bodies of these neurons are located in lower nodes and the central portion of the fibers penetrate the brainstem by the retro-olivary groove and terminate in the core alone.

The solitary nucleus, primary visceral afferent nucleus of the brainstem, is divided from the functional point of view in the rostral (gustatory) and caudal (cardiorespiratory).

From rostral nuclei projectto neighboring nuclei, involved in activities such as swallowing reflex, salivation, chewing and coughing. Other fibers projecting to the ipsilateral thalamus through central tegmental tract and terminate in the medial portion of the ventral posterior medial nucleus of the thalamus.

This core projecting fibers ending in the insula, medial surface in front of the cover near the base of the central groove and the area 3b of the center post-convolution.This pathway, in contrast to other afferents, is exclusively ipsolateral.

The gustatory cortex is projected orbital information to the frontal lobe cortex, where the taste information is integrated with the olfactory and into the amygdaloid complex through which ends taste information in the hypothalamus and limbic system.

VISUAL SYSTEM.

The visual system is unique in terms of quality and quantity of information that allows us to elaborate an image about the world around us.Light stimuli are received by the photoreceptors in the retina and the initial processing is done in light stimulimidbrain, most of the fibers end in the lateral geniculate body, which in its turn sends information to the primary visual cortex in the occipital lobe. From there, visual information is distributed in partnership visual areas in the occipital, temporal and parietal.

The retina.

Is the innermost layer of the eyeball, which is constituted by the neural retina and the retinal pigmentary epithelium. For describing the layers and retinal cells we use the internal terms, which refers to structures located into the vitreous, and the external is used inreferense to the structures located to the pigmentary epithelium.

Pigmentary epithelium is composed of a layer of cuboidal cells attached closely with each other, blocking the flow of plasma or ions.This epithelium supplies the neural retina with nutrients in the form of glucose and essential ions, protects photoreceptors from potential damage by high levels of light and maintains the anatomy of photoreceptors via phagocytosis.

The neural retina contains photoreceptors that absorb photons and convert light energy into a nerve impulse.These pulses are processed in different layers of the neural retina.

The junction between thepigmentary epithelium and neuronalretine is unstable and this is what promotes retinal detachment where neuronal retinal pigment epithelial separates.

The neural retina is composed of seven layers, of which the two outermost are formed by the photoreceptors, the intermediate by interneurons, layer six containing the ganglion cells bodies, and seven layer, the innermost, consisting of ganglion neurons axons, which converge toward the optical disk to form the optic nerve.

Layers two to seven are bounded by a pair of membranes limiting consisting glial processes, external limiting membrane is located between layers 1 and 2, and between the internal limiting membrane and the vitreous nerve fibers. There are five types of neurons in the retina, photoreceptors, the bipolar, the horizontal, amacrine cells and the ganglion cells.

The rods and cones are the photic receptors and both have a similar design. An outer segment is in contact with the retinal pigment epithelium and has hundreds of membranous discs, the cilium which is a narrowing that connects outer segment the inner segment containing mitochondria and innermost is the nucleus and ultimately

end up forming photoreceptors synaptic expansion called spherule in the cones and pedicle on the rods.

The rods are named for the cylindrical shape and the cones by the triangular shape of the outer segment. Lamellar disks are in the distal portion of the outer segment, are detached and phagocytosed by the retinal pigment epithelium, such that the outer segment is continuously renewed.

Rhodopsin in rod absorbs photons and initiates a series of changes aimed to hyperpolarization of the membrane, it is the conopsinain the cones. We describe three types of cones, each responds to different wavelengths of light, large or red cones that respond to long waves, medium or green cones respond to the small and medium-wave and blue cones respond to short wave light.

Because opsin genes for the large and medium cones are located on the X chromosome for Color blindness is more common in men.

Macula, located in the posterior pole of the eye, is where the layers of the retina and are thinned in the center of the macula cones are responsible only for color vision, it allows the greatest amount of light reaching these fotoreceptors optimally. In contrast, the rods are more sensitive to low levels of illumination, predominate in the peripheral retina.

The fotorecptores synapse with horizontal and bipolar neurons.In turn bipolar neurons synapse with amacrine and ganglion neurons at the level of the inner plexiform layer.

Horizontal neurons synapse with distal and proximal photoreceptors.Photoreceptors express glutamate and horizontal neurons GABA, allowing inhibition of photoreceptor proximal to horizontal neurons and define the receptive field.

Located between the photoreceptor and ganglion cells are bipolar neurons, which are the visual direct path.

Amacrine neurons are small, they are not distinguished the axon and dendrites are very wooded.A group containing GABA, while others contain glycine or acetylcholine.These cells modify the activity of bipolar neurons both proximal and distal to them.

The output neurons of the retina are the ganglion. Their axons converge on the optic disc and form the optic nerve.The optic nerve has no photoreceptors, this portion is called the blind spot in the visual field. There are two types of magnocellular ganglion neurons in the lateral geniculate body and parvocellular, so called because synapsewith small neurons in the lateral geniculate body.

The magnocellular predominate in the periphery of the retina, receive information primarily from the rods and are sensitive to moving stimuli.The parvocellular cones receive information and respond to color stimuli and static.

Projections of the retina.

Ganglion neurons send axons to the suprachiasmatic nuclei in the anterior hypothalamus, the pretectal nuclei in the midbrain, but most of the fibers are directed to the lateral geniculate bodies.

Within the retina, these fibers are unmyelinated, but through the sclera are coated with myelin.The optic nerve extends from the back of the eye to the optic chiasm, where the fibers coming from the nasal retina (corresponds to temporal fields) intersect and remain in the contralateral optic tract, while the fibers that come from the temporal retina (corresponding tonasal field) remain on the same side and continue in the ipsilateral optic tract. The courses optic tract cross lateral to the cerebral peduncle and terminate in the lateral geniculate bodies.

The lateral geniculate body contains six layers of cells separated by thin sheets of fibers myelinated.In the ventral end fiber optic tract while at the dorsal and lateral side the fibers are formed by the optical radiation.Layers 1 and 2 have magnocellular neurons, and the layers 3 to 6 parvocellular, division was correlated with node fibers.

The axons of the ganglion neurons originating from the temporal retina, ipsilateral rooming during his tour in the chiasm and optic tract, and terminate in layers 2, 3 and 5.Those from the nasal retina, intersect in the chiasm, optic tract contralateral taken and end in layers 1, 4 and 6 of contralateral lateral geniculate body.

Lateral geniculate bodies of projecting fibers to the primary visual cortex ipsilateral, the "optical radiation".The primary visual cortex is located in the upper and lower lips of the calcarine sulcus, for this reason this group of fibers are also called geniculocalcarinas.The optical radiation is divided into two installments, one that ends in the upper lip and one on the lower lip of the calcarine sulcus.The fibers terminate in the upper lip, originate from the medial portion of the lateral geniculate body and head straight to the occipital cortex, carrying information from the upper retina and therefore visual field contralateral inferior temporal and nasal side thereof.The ending in the lower lip, originating from the medial portion of the lateral geniculate body, are directed forward, forming the Meyer loop and then to the occipital cortex, carry information from the lower retina and therefore the contralateral superior temporal field and ipsilateral nasal.

The occipital cortex has 6 layers, layer IV is characterized by beingmore wide, containing a band of myelinated fibers, the stria of Gennari, which corresponds to geniculocalcarinas fibers.Layer VI is also prominent and fibers originating there that go to the lateral geniculate body.The neurons in the occipital cortex are organized in columns that extend perpendicularly.The macula is represented in the occipital region near the occipital pole.

Alterations.

The visual deficit resulting from damage to any portion of the visual pathway, is named according to certain convections.Because images on the retina are inverted and reverts, the temporal retinal damage produce nasal field deficit and superior retinal lesions produced deficits in the lower visual field.

Hemianopsia refers to the average loss of visual field.

The quadrantanopia is the loss of one quarter of the visual field.

The eponymous term denotes a condition in which the visual field loss is similar for both eyes, is said to the same side.

VESTIBULAR SYSTEM.

The vestibular system is very sophisticated because it is part of a multimodal system that coordinates information received from various sensory systems. Vestibular system provides information to the central nervous system required to determine the position of the head, as well as the speed and direction of the movements to which it is subjected.

This information is integrated to the central level, where coupled with visual and proprioceptive, allow schemes establish the position and dynamics of body movement.This complex process is therefore dependent on, the visual environment and the control of eye position, on one hand, and, secondly, information derived from somatosensory and vestibular systems.

Bony labyrinth exists located in the middle of the pyramid petrosa further containing the cochlea and the semicircular canals lobby.

The membranous labyrinth lies within the labyrinth bony shell and leave a space between the membranous endosteum and the perilymphatic space called. Semicircular canals is three membranous tubes having a sectional diameter of 0.4 mm. Each forming about 2/3 of a circle with a diameter of approximately 6.5 mm.

Are arranged so as to form a coordinate system. By drawing a line through the upper pole of the internal auditory meatus and another passing through the inferior orbital rim, you get Frankfur plane, used to locate the position of the semicircular canals. According to this, the horizontal semicircular canal or earlier is 30 degrees below this line, with your head up. The other two channels are located in a horizontal position, nearly orthogonal to each other, forming respective angles of 45 degrees.

The upper channel is angled as anterolateral on the roof of the utricle, and the rear channel is angled downward, laterally behind the utricle. The two vertical channels terminate in a common orifice at the back side of the utricle.

Since the planes of the channels are not aligned perfectly, the natural movements of the head orthogonal stimulate two and often three channels.

Lobby bone cavity is the central part of the maze sandwiched between the internal auditory canal and the tympanic cavity. It is egg-shaped, higher than long. Is oradado through the round window, the oval window and the holes of the semicircular canals, such as ampullary ampullary. In its inner part there are two impressions or pits one utricular and other semiovoide or hemispheric or saccular.

The vestibular aqueduct is a narrow channel that connects the lobby with endocranium. Contains the endolymphatic canal satellite and vein. It originates below

the edge of the common channel hole and ends in the temporary cerebellar phase midway between the sigmoid sinus and internal auditory canal, 10 mm behind the internal auditory canal, and corresponds to the endolymphatic sac.

The cochlear aqueduct is narrower than the vestibular aqueduct. He contacted the perilymphatic spaces of the cochlea with the subarachnoid spaces of the cerebellar fossa. Originates front of the round window working down through petrosa mass parallel to the inner ear canal, and ends in cerebellar fossa in the lower edge of the temporal bone 5 mm. below the inner auditory canal, through a hole at the fossa pyramidal.

Add it, the vestibular system comprises otolith organs, utricle and saccule, responsive to linear accelerations, and semicircular canals, previous, later and lateral, are arranged orthogonally to each other and responsive to angular accelerations.In each of these structures are sensory cells which are hair mechanoreceptors, and are located in specific areas such as the tops of the blisters in the case of the semicircular canals and maculae of the case otolith organs.

The apical portion of the outer hair cells emanate from 50 and 100stereo cilia and one true cilium which is located eccentrically, called kinocilio, which has the characteristic arrangement 9 +2 of its structural proteins, whereas the stereocilia have an actin core similar to the microvilli.

Hair cell is in contact with a liquid called apical endolymph meanwhile, while other liquid bathes perilymph its basal end.The different chemical composition of both liquids causes the cell located outside the apical membrane a high concentration of K + ions and Na + ions low, while in its basement membrane concentrations are reversed.

The location of kinocilio and position of the stereocilia morphologically define functional polarization of hair cells, as these play an important role in the transduction process mechano-electric.

Representation of a vestibular organ of a mammal.

Aa: previous blisterAsd: previous semicircular canalBp: basilar papilla C: cochleaCc: common cross L: lagenaLa: lateral blister Lm: lagena maculaLsd: lateral semicircular canalMn: neglecta maculaPa: later blisterPsd: later semicircular canalS: sacculeSm: saccule macula U: utricleUm: utricle macula

Described two types of hair cells according to the manner in which contact axons which innervate:

a) Type I cells that are shaped narrow neck bottle and make contact with a terminal afferent which completely surrounds the cell.

b) Type II cells, which are cylindrical and receive afferent terminals as a button.This latter type of cells is considered phylogenetically oldest, and is forming the crests of fish and amphibians.Type I cells appear in reptiles and are phylogenetically most recent.

Hair cells have a number of ion channels in the lateral base side, which have been studied by voltage-clamp techniques and current thathave revealed differences in terms of expression of ion channels and their functional role.The main ion channels expressed by vestibular hair cells are K + channels and Ca2 +, however also been described nonspecific channels.Na + channels are usually not present on mature vestibular hair cells, but may be present in auditory hair cells.Importantly, the various ionic channels are not equally distributed among the hair cells. Is important to say that different ionic channels are not equally distribuited among the hair cells.These differences could be related with its sensitivity to the frequency, gain and dynamic properties, and may show specificity depending on the species with which they work.

Transduction mechanoelectric.

In the vestibular system, transduction is the conversion of mechanical energy contained in the external stimulus, such as movement of the head, in electrical changes in the hair cell.

Transduction in the hair cells occurs after a few µsec after beam deflection ciliary positive, suggesting that mechanical stimulus transmission is performed by directly communicated to ion channel.

The transduction channel is located near the distal end of the stereocilia and is considered a cation channel nonspecific, but because the K + is the positive ion most abundant in space endolymphatic the opening of these channels generate an incoming stream of K + which could depolarizate the cytoplasmic membrane.

The stereocilia are connected between themby proteins called tip links (tip-links).The deflection of bunch follicle voltage changes these connections and provides the energy necessary for opening the channel, which accordingly is in phase with the displacement of the stereocilia.

The movement of the cilia kinocilio direction causes depolarization in these cells, whereas a movement in the opposite direction leads to a hyperpolarization.Thus, the inclination of the cilia causes changes in the membrane potential and the release of neurotransmitter by the hair cell, finally determining a change in the discharge frequency of the afferent fibers which synapse with hair cells in its basal portion.

Afferent neurotransmission.

Ciliated cells make synaptic contact with afferent neurons, which are bipolar type are myelinated, have their soma and Scarpa ganglion synapse with the vestibular nuclei in the brain stem and cerebellar neurons directly.

The hair cellssynapse with afferent cells whichare bipolar, are myelinizated, have their soma in the scarpa ganglia and stablish synapse with vestibular nuclei at brainstem level or directly with cerebellum neurons, possibility of synapse transmission between cells and sensory afferent neurons is of a chemical nature, is based on electrophysiological and morphological evidence. It is also known that this transmission is performed by mechanisms similar to those of other well-known chemical synapses.

For example, we have demonstrated the presence of a synaptic delay and occurrence of spontaneous miniature excitatory postsynaptic potentials in goldfish saccule.

Afferent synapses in the vestibular system of various species, has been registred a basal electrical activity which is due to the spontaneous release of the neurotransmitter by the hair cells, which may mean an excitatory amino acid glutamate type, although other endogenous amino acids, such as aspartate and homocisteato, able to activate glutamate receptors, which are of two types: NMDA and non-NMDA (kainate, AMPA and metabotropic).

Conclusion.

Afferent way perform various functions, some of them, those related to sensory organs, for it have different receptors, signaling pathways and nuclei through which pass to reach the cortex, where they will be analyzed to transmit a response, which occurs before the stimulus given.

Add it, are involved with various nuclei of the cranial nerves, whose function innervation of muscles, organs and body regions.

The afferent way are relevant because they give us an overview of the things around us, including hazards or simply we eat, and discrimination whether can be harmful based on their color, shape, smell and taste, which helps survival.

References.

Fluur E, Mellstrom A. (1971); The otolith organs and their influence on oculomotor movements.; Exp Neurol; 30:139.

Osuna S., Rubiano A.(2002); Neuroanatomía funcional; 46-54.

Matamala; Grandes Vías Aferentes; Universidad de la Frontera.

García D., Castro F. (2011); ¿Cómo se conecta la olfacción? Mecanismos celulares y moleculares que dirigen el desarrollo de las conexiones sinápticas desde la nariz a la corteza.; Rev. Neurol.; 52 (8): 477-488.