Somatosensory+System+II

=**Somatosensory System II**=


 * 13. Outline the peripheral organization of primary somatosensory neurons in spinal and cranial nerves, ganglia, and roots as they project the spinal or brainstem levels of the central nervous system.**

__Spinal Cord Entering Neurons__ As a neuron travels from the body wall to the spinal cord, it sequentially groups with other sensory neurons into a fascicle, peripheral nerve, plexus (for inputs above and below T2-T12), spinal nerve, dorsal root ganglion, and finally into the dorsal root. Alternatively, some distal axons travel in autonomic structures before reaching the spinal nerve. The total input to the spinal cord comes from 31 pairs of dorsal roots, and related dorsal root ganglia and spinal nerves.

All the structures distal to the dorsal root ganglia contain the distal axons of primary sensory neurons and are part of the peripheral nervous system. The dorsal root ganglia contain the cell bodies of primary sensory neurons and each dorsal root contains the central axons.

__Brainstem Entering Neurons__ Sensory neurons group together to form fascicles, cranial nerves, cranial nerve ganglia, cranial roots, and enter the CNS at brainstem levels. This input to the brainstem comes for 4 pairs of cranal roots from the trigeminal (CN V), facial (CN VII), glosspharyngeal (CN IX), and vagus (CN X) systems.

Trigeminal: Fascicles of axons enter the ophthalmic, maxillary, and mandibular nerves. The distal axons have cell bodies in the trigeminal ganglion and central axons in the trigeminal root which enter the brainstem at the lateral pons.

Facial: Fascicles of axons have cell bodies in the geniculate ganglion and central axons in the facial root which enters the brainstem at the junction of the pons and medulla.

Glossopharyngeal: Fascicles of axons have cell bodies in either the superior or inferior (petrosal) ganglion of CN IX and central axons enter the brainstem in the medulla.

Vagus: Fascicles of axons have cell bodies in the superior or inferior (jugular) ganglion of CN X and central axons enter the brainstem at the medulla.

All structures distal to the cranial nerve ganglia (i.e., cranial nerves) contain distal axons of primary neurons; the cranial ganglia contain the cell bodies, and the cranial roots contain the central axons.

__Exception__ Some cell bodies of sensory neurons innervating the facial muscles are located in the mesencephalic nucleus of the trigeminal complex in the brainstem. The distal axons of these neurons reach face muscles through peripheral trigeminal structures.


 * 14. Describe the receptive field innervation territories of cranial nerves to the face, peripheral nerves to the hand, and dorsal root dermatomes to the body.**

Each primary sensory neuron has a receptive field and any peripheral group of sensory neurons has a receptive field equal to the sum of the receptive fields of all sensory neurons in that group called an innervation territory. The receptive field of one dorsal root is called a dermatome.

Skin surfaces covering the face and external ear structures are within the innervation territories of CN V, CN VII, CN IX, and CN X. Spinal dermatomes have innervation territories covering the rest of the body surface, starting at the occiput of the head. Dermatomes territories on the occiput and neck begin with C2 and not C1 because C1 does not innervate skin territory on the body surface (instead innervating neck muscles).

Innervation territories of peripheral nerves and dorsal root dermatomes for the same part of the body do not always correspond because of regrouping of fibers when they cross a plexus. e.g., the median nerve territory overlaps 3 dermatomes due to regrouping of the median nerve axons at the brachial plexus.

Innervation of deeper body wall regions include:

CN V – oral and nasal cavities CN IX – oral and nasal cavities, pharynx, and carotid sinus CN X – Thoracic and abdominal viscera, trachea, and esophagus

Some inputs entering at spinal levels and traveling through autonomic structures also innervate viscera. These fibers are unmyelinated C-fibers (free nerve endings).


 * 15. Describe the major divisions of spinal cord grey and white matter.**

The spinal cord is divided into a central grey region (neurophil region with high density of cell bodies, dendrites, axon terminations, and synapse) and a surrounding white region (high density of fibers, particularly myelinated fibers making up tracts).

Grey matter is divided into dorsal, intermediate, and ventral horns, each of which can be divided up into 10 lamina with I being most dorsal superficial, IX being the most ventral superficial and X being deepest in the middle. The lamina are identified as:

I – marginal zone II – substantia gelatinosa III-IV – nucleus proprius VII – Clarke’s nucleus and intermediolateral nuclus IX – motor nuclei

White matter is divided into the dorsal (posterior), lateral, and ventral (anterior) colums. Between the dorsal and lateral columns, where the dorsal roots attach to the spinal cord is lissaeur’s tract which contains unmyelinated neurons. The spinal cord merges rostrally into the medulla of the brainstem. The spinal column white matter continues into the medulla as the fasciculus gracilis and fasciculus cuneatus.


 * 16. Describe the locations in the medulla of the dorsal column nuclei, solitary nucleus, and trigeminal nuclei.**

Upon entering the spinal cord or brainstem, the central axons of primary sensory neurons travel in tracts of white matter, projecting their axons termination zones in grey matter. Terminations of primary sensory neurons are always ipsilateral. There are 5 major central termination zones of primary sensory axons:

(1) Spinal grey (2) Dorsal column nuclei (gracile and cuneate nuclei) (3) Main trigeminal nucleus (4) Spinal trigeminal nuclei (5) Solitary nucleus

These structures are located in the medulla or pons and are bilateral. See Figures 14, 15 on page 152, 153 of the notes)


 * 17. Describe the major central termination zones of primary sensory neurons.**

//For inputs form body to spinal cord (dorsal roots C1 and below)//:

__Touch and Proprioception__: Large diameter A-alpha and A-beta touch and proprioception axons from the body enter the spinal cord via dorsal roots and project into the dorsal column.. Most of these axons turn and ascend up the dorsal column, traveling through the cuneate or gracile fascicle and terminating in the respective cuneate or gracile nucleus of the dorsal column nuclei in the medulla. Other branches leave the dorsal column and terminate in the deep layers of the spinal grey matter (layers III-VII) around the level they enter the spinal cord and mediate reflexive actions.

__Pain, Temperature, and Crude Touch__: Small diameter A-delta and C pain, temperature, and crude touch axons enter the spinal cord via dorsal roots and project through lissauer’s tract to terminate in the superficial layers (layers I-II) of the dorsal horn around the level they enter the spinal cord.

//For inputs from face, mouth, external ear, and viscera to brainstem (cranial roots V, VII, IX, and X)//:

__Touch and Proprioception__: Most large diameter touch and proprioception axons  fomr the face enter the brainstem via CN V, project into the trigeminal tract and terminate on the main trigeminal nucleus. Other axons branch in the trigeminal tract and terminate in the deeper layers (layer III-VI) of the spinal trigeminal nuclei.

__Pain, Temperature, and Crude Touch__: Small diameter pain, temperature, and crude tocuh axons from the face, mouth, and outer ear enter the brainstem via cranial roots of CN V, CN VII, CN IX (superior ganglion), or CN X (superior ganglion). They project through the trigeminal tract and terminate in the superficial layers (layer I-II) of the spinal trigeminal nuclei.

Small diameter pain, temperature, and crude touch axons form the oral and nasal cavities, and carotid sinus, and pain and crude touch axons form the thoracic and abdominal viscera enter the rostral medulla via cranial roots of CN IX and CN X, respectively. They travel through the solitary tract before terminating in the solitary nucleus.

The visceral inputs form cranial nerves are supplemented by visceral inputs from sensory neurons with distal axons in autonomic structures that enter the CNS at spinal elves and terminate in the superficial layers of the dorsal horn around the small diameter pain inputs form the body surface.


 * 18. Explain the __modality__ organization of central projections and terminations of primary sensory neurons.**

Primary sensory inputs from the surface of the body (e.g., spinal nerves) vs. surface of the face (e.g., CN V, CN VII, CN IX, CN X) enter the nervous system at different levels (spinal cord vs. brain stem) and travel through different tracts (dorsal column and lissauer’s tract vs. trigeminal tract), and terminate in different nuclei (dorsal column nuclei and spinal layers vs. main and spinal trigeminal nuclei).

For inputs from both body and face surfaces, tough and proprioception modality inputs have different tracts and termination zone than pain, temperature, and crude touch modality inputs.

For primary sensory inputs from combined body and face surfaces, 4 modality systems can be distinguished:

(1) Body touch-proprioception system involving the dorsal column and related dorsal column nuclei (cuneate or gracile nuclei) (2) Body pain-temperature-crude touch system involving lissauer’s tract and superfical dorsal horn. (3) Face touch-proprioception system involving the trigeminal tract and the main trigeminal nucleus. (4) Face pain-temperature-crude touch system involving the trigeminal tract and spinal trigeminal nucleus.

This central modality organization has important clinical implications because central lesions may impact structures for some but not all modalities of input. For example, a lesion in the dorsal horn may affect the body pain-temperature-crude touch system but leave the other modalities unaffected.


 * 19. Explain the __somatotropic__ organization of central projections and terminations of primary sensory neurons.**

When inputs originating from adjacent locations of the body or face project to adjacent locations in the central nervous system, the projection system is said to be somatotopically organized, even thought it differs in size and/or shape than its origin locations. In addition to being organized by modality, central tracts and termination zones of primary sensory neurons are also somatotopically organized.

Inputs are from the ipsilateral half of the body. Central axons of primary sensory neurons in the spinal white and terminations of these axons in the grey of the dorsal horn and dorsal column nuclei originate entirely from receptors in the ispilateral half the body.

Inputs from the lower body and limb enter the spinal cord at lumbosacral regions, inputs from the upper body and limb enter the spinal cord at cervical levels, while intervening trunk inputs enter in between. Similarly, axon terminations of inputs from the lower body and limb map ont lumbar segments of spinal grey matter, whereas terminations from the upper body and limb map onto cervical segments of spinal grey matter, with intervening trunk mapping in between. Thus, fibers from the lower to upper body are somatotropically grouped in a dermatome pattern from caudal to rostral in dorsal root entry zones and spinal grey.

In fibers in the dorsal columns and terminations in the dorsal column nuclei, the mapping of lower-to-upper body inputs is along a medial-to-lateral axis rather than caudal-to-rostal. Fibers from the lower body and lower limb are most dense in the medial dorsal column while fibers from the upper body and upper limb are most dense in the lateral dorsal column.

As the cervical spinal cord merges into the lower medulla, the medial group of dorsal column fibers becomes the fasciculus gracilis and terminates in the nucleus gracilis, the corresponding medial nucleus of the dorsal column nuclei. The terminations in the nucleus gracilis forma pat of the foot and lower body. Similarly, the more lateral fibers from the upper body become grouped as the fasciculus caneatus and terminate on the nucleus cuneatus, the corresponding lateral nucleus of the dorsal column nuclei. The terminations in the nucleus cuneatus form a map of the hand and upper body.

Within both the nucleus gracilis and nucleus cuneatus, there is also a finer grain somatotopy. e.g., in the nucleus cuneatus, the terminations from the little finger distribute medially while the terminations from the thumb distribute laterally, with terminations from the intervening fingers in between.

Central somatotropic organization has important clinical implications because central lesion can impact on representations or tracts that process inputs from specific parts of the body.


 * 20. Explain how abnormal insertion of receptor-ion channel complexes into membranes of primary sensory neurons contributes to sensory changes after injury.**

Receptor-ion channel proteins of primary sensory neurons are normally inserted only in the membranes on the distal endings of their respective neurons. Therefore, sensory fibers normally are only activated and receptor potentials are normally only generated by these receptor-ion channels in the distal endings. These receptor-ion channel proteins are manufactured in the cell body and transported down the axon by axoplasmic transport and inserted into the membrane.

Following injury of the distal axon, the axon membrane distal to the injury often degenerates, leaving the surviving axon ending somewhere proximal to their normal ending site. Injured primary neurons can often develop an ability to discharge action potentials even though their distal receptors are lost due in response to a range of stimuli (e.g. mechanical displacement, temperature changes, local ischemia, circulating catecholamines/neuroactive agents, etc.).

These discharges are caused by abnormal insertion of receptor-ion channel proteins into the membranes of the axon stumps and cell bodies. Thus, receptor potentials normally generated only at the distal endings are now generated from ectopic membrane locations. These ectopic potentials are capable of inducing action potentials which can conduct centrally and contribute to unusual post-injury sensations such as chronic pain and phantom sensations.