Somatosensory+System+III

=**Somatosensory System III**=


 * 21. Describe simple concepts of integration at somatosensory synapses.**

Central axons of primary sensory neurons terminate in the (1) spinal grey, (2) dorsal column nuclei, (3) main trigeminal nucleus, (4) spinal trigeminal nuclei, and (5) solitary nucleus. These terminations make excitatory synapses on second order neurons in a divergent or convergent fashion and can participate in local and descending inhibition.

__Divergence__ Central terminations of primary sensory neurons can make synaptic connections with a large number of second order neurons.

__Convergence__ Dendrites and soma of a secondary neuron may have a large number of synaptic connections from many primary neurons.

__Local Inhibition__ Local lateral inhibition is one mechanism for constraining divergence of excitatory connections. The input fiber can have divergent endings on multiple postsynaptic cells, some of which have inhibitory synapses on postsynaptic cells which are weakly excited by the input fiber. This allows the excitatory divergence of the input signal to be reduced by the inhibition of a local neuron.

__Descending Inhibition__ Inhibition of synaptic activity can also occur through the synaptic connections of long axon neurons which have their cell bodies in higher order structures. Fibers from the somatosensory cortex descending the dorsal column nuclei to excite local inhibitory interneurons can affect the divergent or convergent function between primary and secondary neurons.


 * 22. Point out examples of transmitters and modulators used at initial somatosensory synapses.**

Neurotransmitters and neuromodulators are released by the central terminals of the primary sensory neurons. Transmitters include excitatory amino acids such as glutamate. Modulators include peptides such as substance P, vasoactive intestinal polypeptide, and calcitonin gene-related peptide.

Interneurons release inhibitory amino acids such as GABA.


 * 23. Describe the major sensory modality, nuclei where the synapses are made, tracts through which axons travel, and decussation level for the dorsal column-medial lemniscal, trigeminal lemniscal, anteriolateral, and trigeminothalamic systems.**


 * **Primary Modality** || **Origin** || ** Nuclei** ||**Decussation** || **Tract** || **Ventroposterior Nucleus** ||
 * Touch and Proprioception || Body || Dorsal Column Nuclei (Gracile or Cuneate) || Medulla ||Dorsal Column Medial Lemniscal System || VPL ||
 * Touch and Proprioception || Face || Main Trigeminal Nucleus || Pons ||Trigeminal Lemniscal System || VPM ||
 * Pain, Temperature, and Crude Touch || Body || Dorsal Horn || Spinal Cord || Anteriolateral System ``*`` || VPL ||
 * Pain, Temperature, and Crude Touch || Face || Spinal Trigeminal Nuclei || Medulla || Trigeminothalamic System || VPM ||

See Page 4-6 of Dr. Wall’s lecture notes for Somatosensory System III.

``*`` The Anteriolateral system is subdivided into the spinothalamic, spinal tectal, and spinoreticular tracts. The spinothalamic tract ascends to terminate in the VPL of the thalamus where third orderneurons project through the internal capsule and synapse on fourth order neurons in the primary somatosensory cortex. In spinotectal and spinoreticular tracts, the axons terminate on third order neurons in the periaqueductal grey and colliculi in the tectum and the reticular formation of the brainstem, respectively.


 * 24. Explain the basis for __modality__ organization in the ascending systems to primary somatosensory cortex.**

The modality basis for the organization in ascending systems to primary somatosensory cortex is based on the input modalities from primary sensory neurons. Primary sensory neurons follow the labeled line principle in which each individual nerve fiber is specialized by its receptor type, location of nerve ending, and threshold of activation to carry information about one modality.

However, although the ascending pathways are dominantly modality coded,there is also limited convergence and divergence across pathways. For example, touch and proprioception fibers that terminate in the dorsal horn can synapse with spinothalamic pain neurons. Nonetheless, modality dominance ins sufficient that lesions of a given pathway result the loss of the indicated modality.

There are different ascending systems from the face and body, and inputs from one side of the body project to the opposite side of the thalamus and cortex due to decussation of second order neurons at specific levels in each system.


 * 25. Explain major features of __somatosensory__ organization of the ascending systems to primary somatosensory cortex.**

Ascending systems are also somatotopically organized. There is an orderly progression of inputs from the lower body, to upper body, to face at both thalamic and cortical levels.

//As the primary sensory neurons ascend up the dorsal column nuclei (gracile and cuneate nucleui), the lower body, upper body, and face inputs are represented from medial to lateral. In the thalamic ventroposterior (VP) nucleus, the lower body, upper body, and face inputs switch and are represented from lateral to medial. Lastly, in primary somatosensory cortex, the lower body, upper body, and face inputs switch back and are represented from medial to lateral.//

Maps of the body and face are separated in the dorsal column nuclei and main trigeminal nucleus but fuse to a single map in the VP nucleus and re-reflected in the unified map of the body and face in the primary somatosensory cortex.

Detailed representations of each small body region are present at each level of the ascending neuraxis. For example, the cuneate nucleus, VPL nucleus of thalamus, and primary somatosensory cortex each contain zones where inputs from each finger are represented.

//Pathways for pain, temperature, and crude touch also contain some degree of somatotopic organization. For example, anteriolateral system second order fibers in the anterior-lateral columns are organized such that fibers transmitting signals from leg, trunk, arm, and neck are located in sequence from ventrolateral to dorsomedial at cervical spinal levels.//


 * 26. Describe levels of the ascending pathways which are regulated by descending systems.**

Signal transmission up ascending systems is regulated by descending projections form the somatosensory cortex and the brainstem.

__Somatosensory Cortical Regulation__ Pyramindal cells in layers V and VI of the somatosensory cortex send descending axons which terminate in subcortical structures at each level of the ascending pathways including the thalamus ventroposterior (VP) nucleus, main and spinal trigeminal nuclei, dorsal column nuclei, and spinal cord grey matter. These descending cortical projects typically act by synapsing on inhibitory interneurons that alter the convergence/divergence of ascending signals.

__Descending Brainstem Regulation__ Neurons in the periaqueductal grey in the midbrain and pons use enkephalin neurotransmitter to excite neurons in the raphe maqnus nucleus and lateral tegmental nucleus in the pons-medulla. The raphe and lateral tegmental neurons project to the dorsal horn, using serotonin and norepinephrine as neurotransmitters, respectively. Both excite enkephalin interneurons in the dorsal horn to inhibit synaptic transmission of pain signals by inhibiting presynaptic terminals of primary pain afferents or spinal cord neurons of the spinothalamic system. By preventing pain signals from reaching second order neurons, they diminish pain signals from reaching the cortex.


 * 27. Point out somatosensory functions beyond the production of body feelings.**

Besides ascending systems to cortex for generating body and face sensations, somatosensory inputs also contribute to reflexes, cerebellar functions, autonomic functions, and central arousal.

__Reflexes__ Proprioception inputs terminate in the deep layers of spinal grey matter and serve as inputs to spinal interneurons which synapse on motor neurons in spinal reflex pathways.

__Cerebellar Functions__ Proprioception inputs from lower limb terminate on Clarke’s nucleus (lamina VII) of the spinal grey and synapse on neurons that send axons up the spinal lateral column to the cerebellum through the dorsospinocerebellar tract. Upper limb proprioception inputs synapse on the cuneate nucleus neurons which send axons to the cerebellum by the ceneocerebellar tract. These pathways are important in maintaining posture and movement control functions of the cerebellum.

__Autonomic Functions__ CN IX and CN X inputs from body cavity and viscera terminate in the solitary nucleus which has connections to structures that regulate autonomic functions (e.g., parabrachial nucleus, hypthalaums, amygdale).

__Central Arousal__ Pain, temperature, and crude touch inputs activate neurons that project up the anteriolateral system. Some of these inputs activate spinoreticular tract axons which activate the reticular formation in the brainstem to regulate arousal of higher brain structures.


 * 28. Point out examples of referred pain and how it is explained.**

Pain that arises from problems in visceral or deep structures is commonly erroneously localized to skin or superficial body locations in a phenomenon called “referred pain.”

Inputs from viscera that enter the spinal cord through autonomic structures have synapses on spinothalamic neurons that receive convergent pain inputs from skin or superficial body regions. Activation of visceral inputs activates the same ascending neurons that pain inputs from skin or superficial body locations that terminate in about the same spinal segment. Visceral problems are normally infrequent compared to pain from skin or superficial body so when visceral problems do arise, information is misinterpreted to originate from the skin and superficial body.

Examples of referred pain are pain from the heart referred to the chest and left arm and pain from the kidney or ureter referred to the lower abdomen.


 * 29. Describe how local anesthetic blockades work.**

Local anesthetics interfere with channel proteins conformation changes that cause channel opening. This blocks Na+ currents and action potential conduction, preventing signal from reaching the central nervous system. The blockade is reversible, lasting a few hours until the drug enters circulation and is excreted.

Local anesthetics can be applied topically to an area of skin in the form of sprays or ointments to block conduction in distal endings of axons in the skin. Local anesthetic at the dorsal root levels can produce even larger blocks.


 * 30. Predict sensory changes caused by somatosensory lesions.**

__Peripheral Nerve Section__ Peripheral nerve section will result in the loss of all modalities in the receptive field of the peripheral nerve. e.g., if the median nerve was transected at the level of the wrist, there will be a loss of touch and proprioception as well as pain, temperature and crude touch within the receptive field of the median nerve after the wrist. This is because the primary sensory nerves of all the modalities travel centrally together.

__Complete Spinal Section__ A complete spinal section will result in the interruption of both dorsal columns resulting in the loss of touch and proprioception and pain, temperature, and crude touch at all levels below the level of the lesion. Touch and proprioception signals cannot ascend through the dorsal columns because both are interrupted. Similarly, pain, temperature, and crude touch signals cannot ascend because both anteriolateral tracts are severed.

__Brown-Sequard Syndrome Spinal Hemisection__ In Brown-Sequard syndrome, there is a hemisection of the spinal column at a specific level. Touch and proprioception on the //same// side as the lesion are lost below the lesion because the dorsal column on that side is severed by the lesion, preventing signals from ascending. However, pain, temperature, and crude touch modality on the //opposite// side of the lesion are lost below the lesion, because these fibers decussate at the spinal cord levels; thus, the lesion interrupts the anteriolateral column carrying pain, temperature, and crude touch on the //opposite// side. A few levels of pain, temperature, and crude touch can be lost on the //same// side as the lesion if the lesion blocks lissauer’s tract on the same side as the pain, temperature and crude touch primary sensory axons enter the spinal grey. Some pain, temperature, and crude touch modality may be spared a few levels below the lesion on the //opposite// side of the lesion because some of those fibers may ascend a few vertebral levels higher before decussating to the opposite side anteriolateral tract, essentially going around the lesion.

__Syringomyelia__ In Syringomyelia, a lesion form in the central part of the spinal cord, resulting in loss of pain, temperature, and crude touch on both sides in a limited vertebral level range reflecting the extent of the lesion. This is caused by the disruption of decussating axons at the ventral commissure of the spinal cord at the levels of the lesion. Thus, pain, temperature, and crude touch signals are interrupted at the levels of the lesion. However, signals at other levels are spared because the decussation of axons still occurs at levels outside of extent of the lesion, allowing signals to ascend the anteriolateral tract.

__Incomplete Anteriolateral Tract Lesion__ In an incomplete lesion of the anteriolateral tracts, pain, temperature, and crude touch modality will be lost at levels below the lesion because signals traveling up the anteriolateral tract are interrupted and are unable to reach central structures. However, sacral sparing may occur because of the somatotopic organization of the anteriolateral tract places lower limb, upper limb, neck structures in a ventrolateral to dorsomedial relationship; if the lesion of the anteriolateral tracts is incomplete, it may spare the most ventrolateral region of the tract, allowing signals in that region to ascend all the way to central structures. That is, because the sacral region of the anteriolateral tract is somatotopically organized so far ventrolaterally, the incomplete lesion misses it, allowing those pain, temperature, and crude touch fibers to ascend the tract to central structures and, consequently, producing the sacral sparing.