Somatosensory neurons are topographically i. This organization is preserved by a precise point-to-point somatotopic pattern of connections from the spinal cord and brain stem to the thalamus and cortex. Consequently, within each somatosensory pathway there is a complete map spatial representation of the body or face in each of the somatosensory nuclei, tracts, and cortex. Additional information on somatotopic organization is presented in Chapter 5 of Section II.
The sensory information processed by the somatosensory systems travels along different anatomical pathways depending on the information carried. For example, the posterior column-medial lemniscal pathway carries discriminative touch and proprioceptive information from the body, and the main sensory trigeminal pathway carries this information from the face.
Whereas, the spinothalamic pathways carry crude touch, pain and temperature information from the body, and the spinal trigeminal pathway carries this information from the face. The posterior dorsal column - medial lemniscal pathway i. It is important to keep in mind that within the medial lemniscal pathway, the afferents carrying discriminative touch information are kept separate from those carrying proprioceptive information up to the level of the cerebral cortex.
Press PLAY to view the course of the pathway. Click on the structure labels to view their locations in the sections. Click on the label "Cuneate fasciculus" or "Gracile fasciculus" to view the somatotopic organization of the posterior funiculus and the blood supply provided by the posterior spinal artery. Click on the label "Medial lemniscus" to view its somatotopic organization and the blood supply provided by the paramedian branches of the basilar artery. Each axon travels via a posterior root, spinal nerve and peripheral nerve to skin, muscle or joint- where it forms or innervates a somatosensory receptor.
The lower part of the body foot and leg are represented in the posterior paracentral lobule, whereas the upper body chest, arm, and hand are represented in the upper postcentral gyrus See Figure 4. Press PLAY to animate. The flash of light at each synapse represents the release of neurotransmitter by the presynaptic axon terminal. The point-to-point connections within the pathway provide the basis for a somatotopic map that is used to locate the area of contact with the stimulus and for modality specific information used to identify the stimulus as tactile and from a Meissner corpuscle.
The main sensory trigeminal pathway carries and processes discriminative touch and proprioceptive information from the face Figure 4. Consequently, it is the cranial homologue of the medial lemniscal pathway. The face is represented in the lower half of the postcentral gyrus See Figure 4. Press to animate. The point-to-point connections within the pathway provide the basis for a somatotopic map that is used to locate the area of contact with the stimulus and for modality specific information used to identify the stimulus as tactile and from a Merkel cell.
The peripheral axons of these afferents travel in the mandibular branch of the trigeminal nerve and end in the jaw muscles and joint. The central processes of most of these afferents end in the trigeminal motor nucleus that controls the muscles of the jaw. Few synapse in the main sensory trigeminal nucleus.
The neospinothalamic pathway is also characterized by somatotopic representation, which allows for accurate localization of the painful stimulus. Recall that there are multiple spinal pathways processing pain information see Somatosensory Systems Table I.
Most of the ascending afferents of the spinal pain pathways travel with the neospinothalamic afferents in a fiber tract called the "spinothalamic tract" or "anterolateral spinothalamic tract". Elements of these other pain pathways will be mentioned below to help you understand how pain sensations may remain after damage to the neospinothalamic pathway.
The trigeminal is the largest cranial nerve and has three major branches:. It provides sensory information from the orbit and the forehead. It provides sensory information from the upper lip, cheeks and nose and the gums and teeth of the upper jaw. The sensory component provides sensory information from lips, gums and teeth of the lower jaw and the anterior portion of the tongue.
The motor component innervates the muscles of mastication. The nerve originates in the pons but emerges at the border between the pons and medulla. It passes through the superior orbital fissure. The motor neurons originate from the pons.
The somatic motor neurons innervate the muscles of facial expression and visceral motor neurons innervate the lacrimal and nasal mucous glands and the submandibular and sublingual salivary glands. The sensory neurons receive taste information from taste receptors in the anterior two-thirds of the tongue.
The facial nerve passes through the internal acoustic meatus and the stylomastoid foramen. The nerve has two branches:. Both branches leave the inner ear and reach the brain through the internal acoustic meatus. The ultimate destination of the afferent neurons are vestibular and cochlear nuclei in the pons and medulla. These structures are nestled near the center of the brain, and they developed together in our evolutionary ancestors.
Dalton said smell was designed to immediately send an emotional signal: good or bad, approach or retreat, eat or avoid. A third factor in this memory-triggering phenomenon, Dalton said, lies outside the brain, and has to do with language. Whenever the sticky-sweet scent hits my nostrils, I am transported back to the midnight masses of my youth: the drama of the Christmas story read from the pulpit, the glow of candles lighting the church, and the giddy anticipation of opening presents the next morning.
Retailers capitalize on the contextual connection between smell and place to draw people into their stores. ScentAir, a marketing firm that designs signature scents for businesses in more than countries, said many of its retail clients switch to holiday aromas this time of year.
The scent incorporates notes of pine needles, cinnamon, berries and orange. With it, and other holiday scents, Kindfuller said retailers aim to create new memories of shopping to draw people back to brick-and-mortar stores in an age of online retail. One client, a major clothing retailer, started using a woodsy pine scent in early November to remind people to start their Christmas shopping. Scent memories are fragile, though.
Over-exposure can reduce the emotional punch, which is why not many people have powerful holiday memories associated with common smells, like coffee.
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