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Chapter 4: Somatosensory Pathways

Patrick Dougherty, Ph.D., room of Anesthesiology and Pain Medicine, MD Anderson Cancer facility (content listed by Chieyeko Tsuchitani, Ph.D.) Reviewed and also revised 07 Oct 2020
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This chapter explains the general organization the somatosensory pathways and the anatomy of the somatosensory pathways involved in processing discriminative touch and also proprioceptive information, and also those associated with sharp pain and also cool/cold information. Discriminative touch and proprioceptive information enable for the recognition of objects by touch, carry out for a sense of our body image and is offered for preserving balance and posture. Sharp, pricking pain and cool/cold information allows for the detection and localization of potential tissue-damaging stimuli in a fashionable manner.

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4.1 basic Organization the Somatosensory Pathways

Sensory pathways consists the chain of neurons, from receptor organ to cerebral cortex, that are responsible for the late of sensations.

4.2 usual Anatomical attributes

Somatosensory stimuli activate a chain the neurons beginning with the peripheral first-order (1°) afferent and ending in the cerebral cortex (e.g., number 4.1).


Figure 4.1 typical anatomical features of spinal somatosensory pathways


Within each somatosensory pathway,

The 1° afferent is a pseudounipolar neuron that has actually its cabinet body situated in a peripheral (spinal or cranial) ganglion. It has a peripheral axon that creates or innervates somatosensory receptors and also a main process that synapses with 2° afferent neuron(s) in a spinal cord or mind stem nucleus. The 2° afferent may synapse v 3° afferent neurons in the spinal cord or might ascend the neuraxis come synapse through 3° afferent neurons in the thalamus. over there is a decussation (i.e., axons crossing the midline come the opposite side of the spinal cord or brain stem) in every somatosensory pathway listed below the level of the thalamus. every somatosensory pathways incorporate a thalamic nucleus. The thalamic neurons send their axons in the posterior limb of the internal capsule to finish in the cerebral cortex. many somatosensory pathways terminate in the parietal lobe the the cerebral cortex.

each somatosensory pathway is called after a major tract or cell core in the pathway.

In general, conscious perception that sensory stimuli needs the joining of neurons in the thalamus and also cerebral cortex. Because that example, electrical stimulation of a framework in pathways connecting muscle and also joint receptor to the cerebellum (e.g., electrical stimulation the the anterior spinocerebellar tract) will not develop a emotion of body movement, as these pathways carry out not include the thalamus or cortex. In contrast, electrical stimulation that a framework in the posterior column-medial lemniscal pathway (e.g., electric stimulation the the medial lemniscus) may an outcome in a emotion of body movement, as this pathway has the thalamus and also terminates in the cerebral cortex.

4.3 Peripheral Somatosensory Axons

The morphology of the peripheral somatosensory axon is related to the receptor it innervates or forms and also to the sensory info it dead (Figure 4.2).

The team I and II 1° afferent axons, which type the muscle/tendon receptors and carry human body proprioceptive information, have the largest diameter and the thickest myelin of all the somatosensory 1° afferent axons. The form C 1° afferent axons, which kind free nerve endings and carry dull pain, deep pain, rudely touch or warm/hot information, room the the smallest 1° afferent axons and also are unmyelinated. The kind Aδ1° afferent axons, which form free nerve endings and carry sharp pain or cool/cold information, space thinly myelinated and larger 보다 the kind C axons. The type Aβ 1° afferent axons, which form encapsulated end in skin and also joints or hair follicle end or Merkel disks in skin, are myelinated and have diameter much less than group I afferents and also greater 보다 the type Aδ 1° afferent axons.

Figure 4.2 The relationship in between axon diameter, myelin thickness and conduction velocity that somatosensory 1° afferent peripheral processes.


The morphology the the peripheral somatosensory axon is likewise related to the conduction velocity the the action potentials produced by the axon.

The conduction velocity of an axon is identified by electrically stimulating the axon and recording the time (latency) that takes the electrically elicited action potential to reach a recording electrode (Figure 4.3). The street traveled indigenous the electric stimulating website to the recording site divided by the latency provides the conduction velocity that the axon.

As questioned in previously chapters, the larger and much more heavily myelinated the axon, the better its conduction velocity (Figure 4.3). Consequently, the 1° afferent axons carrying information compelled for fine motor control and also rapid reflex responses (i.e., those forming body proprioceptors) conduct activity potentials rapidly, whereas those delivering information about body and object temperature conduct activity potentials at a much slower rate.

The whole nerve potential or compound activity potential (CAP) is videotaped extracellularly native an electrically stimulated nerve and is the sum of the signals created by every of the individual action potentials the the axons forming the nerve. (Figure 4.3) The combined nerve (afferent and also efferent axons) compound activity potential has actually three prominent peaks that are called A, B and also C. The conduction velocity of one axon identify the axon"s donation to the compound activity potential peaks. Special, the much faster the axon conduction velocity, the shorter the latency the axon response and the higher the axon"s contribution to the much shorter latency peaks (e.g., to compare columns lid Peak and Conduction Velocity in Table I). The axons contributing to a offered compound activity potential height (e.g., peak A) are called according to the top name (e.g., kind A axon). As soon as the family member amplitudes that the peaks different from those created by "normal" nerves, the types of damaged axons can be assessed by determining which peaks room abnormal. Consequently, the compound activity potential is supplied clinically to detect nerve damage and to monitor the progress of the renewal of damaged nerves.


Figure 4.3 The entirety nerve potential (aka compound action potential or CAP) tape-recorded from a peripheral nerve in response to electric stimulation the the nerve. (A)The compound action potential is taped proximal come an electrical stimulus ceded to a peripheral nerve. (B) The fiber kind based ~ above the compound activity potential peaks. (C) The voltage adjust (compound activity potential) recorded proximal come the stimulating electrode is plotted together a duty of time (in msec) adhering to the electrical stimulus pulse. Also listed along the abscissa (at each arrow) is the axon diameter (in micrometers) that axons contributing come the peaks in the whole nerve potential.


Table i Somatosensory Receptors and their Peripheral Axons
Receptor Type Axon3 Group CAP Peak Conduction Velocity Axon Diameter Information Processed
Muscle Spindle: Annulospiral endings 1a 70-120 m/sec 1-20 μM Muscle length and also velocity

Muscle Spindle: Flower Spray endings

II 30-70 m/sec 6-12 μM Muscle length
Golgi Tendon Organ Ib 70-120 m/sec 12-20 μM Muscle tension
Joint: Pacinian II 30-70 m/sec 6-12 μM Joint movement
Joint: Ruffini II 30-70 m/sec 6-12 μM Joint angle
Joint: Golgi Tendon Organ II 30-70 m/sec 6-12 μM Joint torque
Meissner corpuscle II 30-70 m/sec 6-12 μM Touch, flitter or movement
Pacinian corpuscle II 30-70 m/sec 6-12 μM Vibration
Ruffini corpuscle II 30-70 m/sec 6-12 μM Skin stretch
Hair follicle II & III Aβ & Aδ 10-70 m/sec 2-12 μM Touch movement
Merkel complex II 30-70 m/sec 6-12 μM Fine touch
Free Nerve endings III 5-30 m/sec 1-6 μM Sharp pain or cool/cold
Free nerve endings IV C 0.5-2 m/sec M Dull or aching pain, or touch or warm

For historical reasons, the terminology based upon axon conduction velocity (Group I, II, III and also IV) is supplied for afferent and also efferent axons innervating muscles and tendons. And the terminology based on the compound action potential (Type A, B or C) is supplied for afferent axons innervating the skin, joints and viscera.

Note the the faster conducting somatosensory 1° afferents (Group Ia) innervate skeleton muscle and also the slowest (C-fibers) form the receptor of the pain systems. When one can expect painful, tissue damaging stimuli to have actually priority over all various other somatosensory stimuli, the afferent information required to control the reaction come the ache stimuli room conveyed through the faster conducting muscle and joint afferents. Also afferents providing much more exact information about the place of a cutaneous stimulus, the Aβ axons, command at a faster rate than the Aδ and C axons transporting information about painful stimuli.

4.4 Somatotopic company

Somatosensory neurons are topographically (i.e., spatially) organized so that surrounding neurons represent neighboring regions of the human body or confront (Figure 4.4). This organization is kept by a precise point-to-point somatotopic sample of relations from the spinal cord and brain stem to the thalamus and also cortex. Consequently, within every somatosensory pathway there is a complete map (spatial representation) the the body or face in every of the somatosensory nuclei, tracts, and cortex. Additional information top top somatotopic organization is presented in thing 5 of section II.


Figure 4.4 The somatotopic depiction of the body and face in the major somatosensory cortex (i.e., the postcentral gyrus and posterior paracentral lobule).


4.5 Somatosensory Pathways

The sensory info processed by the somatosensory systems travels along various anatomical pathways depending on the info carried. Because that example, the posterior column-medial lemniscal pathway dead discriminative touch and proprioceptive info from the body, and also the main sensory trigeminal pathway dead this information from the face. Whereas, the spinothalamic pathways lug crude touch, pain and also temperature info from the body, and also the spinal trigeminal pathway tote this details from the face.

4.6 Medial Lemniscal Pathway: human body Discriminative Touch and also Proprioception

The posterior (dorsal) pillar - medial lemniscal pathway (i.e., the medial lemniscal pathway) carries and processes discriminative touch and proprioceptive details from the body (Figure 4.5). That is vital to store in mind the within the medial lemniscal pathway, the afferents delivering discriminative touch information are maintained separate native those transferring proprioceptive details up to the level the the cerebral cortex.


Figure 4.5 The medial lemniscal pathway. Press PLAY to see the course of the pathway. Click on the framework labels to see their locations in the sections. Click on the label "Cuneate fasciculus" or "Gracile fasciculus" to watch the somatotopic organization of the posterior funiculus and also the blood supply provided by the posterior spinal artery. Click the brand "Medial lemniscus" to check out its somatotopic organization and the blood supply noted by the paramedian branches the the basilar artery.


The peripheral axons the the 1° afferents room myelinated, large or tool diameter axons. Each axon travels via a posterior root, spinal nerve and peripheral nerve come skin, muscle or joint- wherein it forms or innervates a somatosensory receptor.

The 1° medial lemniscal afferent peripheral process that finish in the

skin, are Aβ axons the branch come innervate hair follicles or Merkel’s cell or type Meissner, Pacinian or Ruffini corpuscles. joints, room Aβ axons the branch to type encapsulated endings similar to the Ruffini and Pacinian corpuscles and also Golgi tendon organs. muscle, are team I and also II axons the branch to end in muscle spindles (Ia and also II axons) or Golgi tendon guts (Ib axons).

The 1° medial lemniscal afferent main axons

join a posterior root, beginning the spinal cord, and ascends come the mind stem in the posterior tower of the spinal cord (Figure 4.5). the coccygeal to mid-thoracic posterior roots (i.e., approximately T7) ascend the spinal cord in the ipsilateral gracile fasciculus. the the upper thoracic (level T6 and above) and cervical roots collection in the ipsilateral cuneate fasciculus. of the gracile and also cuneate fasciculi are collectively called the posterior funiculus or posterior column. ascends the spinal cord in the posterior funiculus approximately the medulla there is no synapsing or decussating (i.e., there is no crossing the midline to the contralateral fifty percent of the spinal cord).

In the medulla,

the 1° afferents in the gracile fasciculus synapse in the gracile cell core the 1° afferents in the cuneate fasciculus synapse in the cuneate nucleus. the axons of the gracile and also cuneate nuclei (2° afferents) pass anteriorly and also decussate to kind the medial lemniscus, contralateral to your cells the origin. over the level that the gracile and also cuneate nuclei, each half of the human body is represented contralaterally (e.g., left fifty percent of body in best medial lemniscus) within the medial lemniscal pathway.

The 2° medial lemniscal afferents

ascend the brain stem in the medial lemniscus come the diencephalon. End in the ventral posterolateral (VPL) cell nucleus of the thalamus. Moving cutaneous details terminate in the main point of the VPL. Carrying proprioceptive information terminate in the neighboring shell the the VPL.

The axons that the VPL 3° afferent neurons

travel in the posterior limb of the inner capsule. End in the postcentral gyrus and also posterior paracentral lobule of the parietal lobe.

The postcentral gyrus and also posterior paracentral lobule

are referred to as the primary somatosensory cortex. Space the major cortical receiving locations of the somatosensory system.

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 4.4).


Figure 4.6 Afferent neurons in the medial lemniscal pathway triggered by poignant the left foot through a wisp the cotton. Push PLAY come animate. The flash of irradiate at every synapse to represent the relax of neurotransmitter by the presynaptic axon terminal.


The action potentials ascend the spinal cord via the main process that the 1° afferent in the fasciculus gracilis that the posterior shaft until they with the medulla. In the medulla, the action potentials begin the release of neurotransmitter from the 1° afferent axon terminals onto 2° afferents within the gracile nucleus. The 2° afferent generates action potentials the are carried out by the axons, i beg your pardon decussate to kind the medial lemniscus. These action potentials are carried out by the 2° afferent axon contralateral to their website of origin and contralateral to the foot wherein the stimulus to be applied. The activity potentials ascend to the thalamus whereby they start the relax of neurotransmitter native the 2° afferent axon terminals. They release neurotransmitters ~ above the 3° afferents in the main point of the VPL the the thalamus. The action potentials created by the 3° VPL afferents are conducted by their axons, which take trip in the posterior body of the interior capsule, to the posterior paracentral lobule the the parietal cortex. These activity potentials begin the release of neurotransmitter from the 3° afferent axon terminals ~ above cortical neurons and initiate the higher-order processing of the economic stimulation information created by the Meissner corpuscle. The point-to-point relationships within the pathway administer the basis because that a somatotopic map that is supplied to find the area of call with the stimulus and for modality particular information used to recognize the stimulus as tactile and also from a Meissner corpuscle.

4.7 main Sensory Trigeminal Pathway: face Discriminative Touch and Proprioception

The main sensory trigeminal pathway carries and processes discriminative touch and also proprioceptive information from the challenge (Figure 4.7). Consequently, it is the cranial homologue the the medial lemniscal pathway.


Figure 4.7 The key sensory trigeminal pathway. Press PLAY to see the course of the pathway. Click on the framework labels to check out their locations in the sections.


The cranial 1° main sensory trigeminal afferent neurons

peripheral procedures are situated in the trigeminal (predominantly), facial, glossopharyngeal and also vagus nerves. kind mechanoreceptors in the skin, mucous membranes, muscles and joints the the face. The relationship between receptor kind formed and the axon Type/Group are comparable to those that the medial lemniscal 1° afferents. send their main axons come the mind stem. synapse in the key sensory trigeminal cell core (2° afferents).

The key sensory trigeminal 2° afferent axons

decussate immediately on leaving the key sensory trigeminal nucleus. join the contralateral ventral trigeminal lemniscus. over the level the the key sensory trigeminal cell core (i.e., the mid pons), dead information about the contralateral face (i.e., the ideal ventral trigeminal lemniscus dead information around the left next of the face).

The 2° key sensory trigeminal afferents in the ventral trigeminal lemniscus

ascend come the diencephalon. terminate in the ventral posteromedial (VPM) nucleus of the thalamus.

The axons the the 3° main sensory trigeminal afferents (VPM neurons)

take trip in the posterior body of the internal capsule. end in the postcentral gyrus the the parietal lobe.

The postcentral gyrus

is part of the major cortical receiving area the the somatosensory system.

The face is stood for in the lower half of the postcentral gyrus (See number 4.4).

Figure 4.8 illustrates the course of activity potentials created in an answer to poignant the left cheek with a wisp the cotton. A Merkel receptor in the left cheek is stimulated, and also its 1° afferent generates action potentials the are conducted by the 1° afferent abdominal axon, previous its pseudounipolar soma, into the brain stem.


Figure 4.8 Afferent neurons in the main sensory trigeminal pathway set off by touching the left cheek v a wisp of cotton. Push to animate. The speed of irradiate at every synapse represents the release of neurotransmitter through the presynaptic axon terminal.


The 1° afferent central process conducts the action potentials right into the pons whereby they start the relax neurotransmitter indigenous the 1° afferent axon terminals. The neurotransmitter is exit onto 2° afferents within the main sensory trigeminal nucleus. The 2° afferent generates activity potentials the are conducted along that axon, which decussates in the pons to join the ventral trigeminal lemniscus. These action potentials are conducted by the 2° afferent axon contralateral come their website of origin and contralateral to the site where the stimulus to be applied. The action potentials ascend to the thalamus wherein they initiate the release of neurotransmitter from the 2° afferent axon terminals. They relax neurotransmitters onto the 3° afferents in the core of the VPM the the thalamus. The action potentials generated by the 3° VPM afferents are carried out by their axons, which take trip in the posterior body of the interior capsule, come the postcentral gyrus of the parietal cortex. These activity potentials start the release of neurotransmitter indigenous the 3° afferent axon terminals onto cortical neurons and initiate the higher-order handling of the stimulus information generated by the Merkel cell. The point-to-point relations within the pathway carry out the basis for a somatotopic map that is supplied to find the area of contact with the stimulus and also for modality particular information provided to identify the stimulus together tactile and also from a Merkel cell.

There is a young proprioceptive component for the jaw in cranial nerve V that has actually 1° afferent cabinet bodies situated in the mesencephalic trigeminal nucleus. The peripheral axons of these afferents travel in the mandibular branch the the trigeminal nerve and also end in the jaw muscles and also joint. The main processes of many of this afferents end in the trigeminal engine nucleus the controls the muscle of the jaw. Couple of synapse in the main sensory trigeminal nucleus.

4.8 Neospinothalamic Pathway: body - spicy Prickling Pain and Cool/Cold

The neospinothalamic pathway carries and processes sharp, pricking pain and also dropping temperature (cool/cold) information from the body (Figure 4.9). The pain information lugged by the neospinothalamic pathway is fine localized and also the sensations room the short lasting “fast” or “first” pains elicited by tissue-damaging cutaneous stimuli. The neospinothalamic pathway is additionally characterized by somatotopic representation, which enables for precise localization the the pains stimulus.

Recall that there space multiple spinal pathways handling pain info (see Somatosensory solution Table I). Most of the ascending afferents the the spinal ache pathways travel through the neospinothalamic afferents in a fiber tract dubbed the "spinothalamic tract" or "anterolateral spinothalamic tract". Elements of these other pain pathways will certainly be mentioned below to assist you understand exactly how pain sensations may remain after damages to the neospinothalamic pathway. The ache pathways will be spanned in greater detail in later on chapters.


Figure 4.9 The neospinothalamic pathway.

Press play to view the course of the pathway. Click on the structure labels to check out their areas in the sections. Click on the brand "Spinothalamic tract" to view the somatotopic company of the street fibers and the blood supply noted by the anterior spinal artery. Click on the label "Medial lemniscus" to see the blood supply noted by the posterior worse cerebellar artery.


The 1° neospinothalamic afferents

have kind Aδ peripheral axons that form free nerve end in skin, muscles and also joints. have central processes that enter the spinal cord. synapse in the posterior marginal nucleus (2° afferents) the the posterior horn.

The 2° neospinothalamic afferent axons

decussate in the spinal cord anterior white commissure. type the lateral component of the spinothalamic street in the lateral funiculus.

Note the the fibers in the lateral spinothalamic tract space contralateral to your cells of origin and contralateral come the body area lock represent.

The crossed 2° neospinothalamic afferent axons

ascend the spinal cord and mind stem as component of the spinothalamic tract. take trip with various other pain (archispinothalamic) afferents that leave the spinothalamic tract and terminate in the brain stem as: spinoreticular yarn that finish in the reticular development of the brain stem. spinomesencephalic yarn that end near the periaqueductal gray that the midbrain. take trip with other pain (paleospinothalamic) afferents to the diencephalon wherein the neospinothalamic afferents end in the ventral posterolateral (VPL) nucleus of the thalamus. the paleospinothalamic afferents terminate in the intralaminar nuclei the the thalamus.

The spinothalamic afferent axons from the thalamus

travel in the posterior body of the inner capsule. VPL (i.e., the 3° neospinothalamics) finish in the postcentral gyrus and also posterior paracentral lobule that the parietal lobe. Intralaminar nuclei (i.e., paleospinothalamics) finish in the insula and also rostral cingulate gyrus.

The postcentral gyrus and also posterior paracentral lobule are

the neospinothalamic pathway discontinuation sites. the main cortical receiving areas for sharp, cut pain information. not the to exclude, cortical receiving area because that pain information.

The insula and rostral cingulate gyrus

are the archispinothalamic and also paleospinothalamic pathways" termination sites. obtain dull and also deep pain information. room responsible for poorly localized, much longer lasting pain sensations and add the emotionally (i.e., unpleasant) attributes to these sensations.

Figure 4.10 Afferent neurons in the neospinothalamic pathway caused by a pin prick to the left foot.

press PLAY to animate. The flash of irradiate at each synapse to represent the relax of neurotransmitter through the presynaptic axon terminal.


figure 4.10 illustrates the course of action potentials created in response to a pin prick right into the left foot. Totally free nerve endings in the left foot are engendered by the pen prick. Action potentials space generated and also conducted by the 1° afferent Aδ axon, past the pseudounipolar soma, and also into the spinal cord (Figure 4.10).

The action potentials enter the spinal cord via the central process of the 1° afferents to initiate the release neurotransmitter indigenous the 1° afferent axon terminals ~ above 2° afferents within the posterior marginal nucleus. The 2° afferent generates action potentials the are performed by that is axon, i beg your pardon decussates in the anterior white commissure the the spinal cord. The crossed 2° neospinothalamic afferent axons form the lateral component of the spinothalamic tract. The activity potentials carried out by the crossed 2° afferent axon are contralateral come their website of origin and contralateral come the foot whereby the stimulus to be applied. The action potentials ascend come the thalamus whereby they begin the relax of neurotransmitter from the 2° afferent axon terminals. They relax neurotransmitters ~ above the 3° afferents in the VPL the the thalamus. The activity potentials created by the 3° VPL afferents are conducted by your axons, which travel in the posterior body of the interior capsule, come the posterior paracentral lobule that the parietal cortex. These action potentials begin the release of neurotransmitter native the 3° afferent axon terminals ~ above cortical neurons and initiate the higher-order processing of the stimulus information created by the free nerve ending. The point-to-point relationships within the pathway provide the basis for a somatotopic map the is provided to situate the area of contact with the stimulus and also for modality certain information offered to identify the stimulus as a sharp pinprick.

4.9 Spinal Trigeminal Pathway: challenge Pain, Temperature and Crude Touch

The spinal trigeminal pathway carries and also processes crude oil touch, pain and temperature details from the confront (Figure 4.11) Consequently, the is the cranial homologue that the spinothalamic pathways i.e., homologous to all the spinothalamic pathways, the archi-, paleo- and neo-spinothalamic pathways. Together in the spinothalamic pathways, the afferents transporting crude touch info are kept separate native those moving temperature information and from others carrying pain information. Likewise the trigeminal afferents delivering sharp, cutting pain details are segregated native those transferring dull, burning pain and also deep aching pain information.


Figure 4.11 The spinal trigeminal pathway.

Press play to check out the food of the pathway. Click on the structure labels to check out their areas in the sections. Click the label "Spinothalamic tract" to view the vascular it is provided to the spinal trigeminal tract and also nucleus, i beg your pardon is listed by the posterior worse cerebellar artery.


The 1° spinal trigeminal afferents

have actually Aδ and C peripheral axons that kind free nerve end in the dura and face. on beginning the brain stem, kind the spinal trigeminal tract.

The spinal trigeminal tract

extends native mid pontine levels (the level of entry of trigeminal nerve) under to C1 the the spinal cord. is composed of spinal trigeminal 1° afferent axons (predominantly of the trigeminal nerve). 1° afferents synapse in the spinal trigeminal nucleus (2° spinal trigeminal afferents).

The 2° spinal trigeminal afferent axons

decussate and kind the ventral trigeminal lemniscus contralateral to their cells that origin. ascend in the ventral trigeminal lemniscus together crossed 2° spinal trigeminal afferents. travel through afferents the leave the ventral trigeminal lemniscus as trigeminoreticular fibers, which end in the mind stem reticular formation. space joined through the overcome 2° main sensory trigeminal afferents in ~ mid-pons. travel v afferents that leave the ventral trigeminal lemniscus together trigeminomesencephalic fibers, i m sorry terminate near the midbrain periaqueductal gray. end in the VPM and in the intralaminar nuclei of the thalamus.

Multiple thalamic nuclei process information in this pathway

the VPM processes sharp pricking pain. the intralaminar nuclei processes various other poorly localized emotion of dull, burn pain, deep, aching pain, temperature and crude touch.

The 3° spinal trigeminal afferent axons indigenous the thalamus:

travel in the posterior body of the interior capsule. finish in multiple areas of the cerebral cortex.

The spinal trigeminal pathway terminates in multiple cortical areas:

the intralaminar nuclei axons terminate in the cingulate gyrus and insula that the cerebral cortex, which carry out for poorly localized sensations of dull and aching pain, temperature and crude touch.

Figure 4.12 Afferent neurons in the spinal trigeminal pathway caused by a pin prick applied to the left cheek. Push to animate. The flash of light at every synapse represents the release of neurotransmitter through the presynaptic axon terminal.


figure 4.12 illustrates the food of action potential produced in an answer to a pin prick come the left cheek. Cost-free nerve end in the left cheek are engendered by the pen prick. Activity potentials room generated and conducted by the 1° afferent Aδ axon, previous the pseudounipolar soma, and also into the brain stem

The 1° afferent central process bypasses the key sensory trigeminal nucleus and also descends the mind stem in the spinal trigeminal tract. The action potentials are conducted in this descending street to the spinal trigeminal nucleus, wherein they start the release neurotransmitter from the 1° afferent axon terminals. The neurotransmitter is exit onto 2° afferents in ~ the spinal trigeminal nucleus. The 2° afferent generates activity potentials that are performed along that axon, i beg your pardon decussates to kind the ventral trigeminal lemniscus. These activity potentials are performed by the 2° afferent axon contralateral to their site of origin and also contralateral come the cheek whereby the stimulus was applied. The activity potentials ascend come the thalamus where they begin the release of neurotransmitter indigenous the 2° afferent axon terminals. They relax neurotransmitters ~ above the 3° afferents in the VPM. The action potentials produced by the 3° VPM afferents are performed by their axons, which travel in the posterior body of the interior capsule, come the postcentral gyrus of the parietal cortex. These activity potentials begin the relax of neurotransmitter from the 3° afferent axon terminals top top cortical neurons and also initiate the higher-order handling of the stimulus information produced by the cost-free nerve ending. The point-to-point relations within the pathway carry out the basis because that a somatotopic map the is used to find the area of contact with the stimulus and for modality particular information supplied to recognize the stimulus together a sharp pinprick.

4.10 break up Remarks

Clinically, it is necessary to remember what info is lugged by a particular pathway and the level that the pathway in ~ which decussation occurs (Figure 4.13).


Figure 4.13 The pathways involved with processing discriminative touch and also proprioception indigenous the body and also face and the pathways involved with processing sharp pain and temperature native the body and face.


For example, if the posterior funiculus to be sectioned, sparing the rest of the spinal cord, discriminative touch and also proprioception would be influenced but not pain, temperature, or crude touch. Also, the ns of discriminative touch and proprioception would certainly be ipsilesional (e.g., ~ above the appropriate side the the human body with section of the best posterior column) due to the fact that the 1° afferent axons in the posterior columns carry out not decussate. Consequently, damages to the posterior obelisk in the spinal cord would be suspected if a patient presented with a loss of discriminative touch and proprioception in the right leg and also foot, with no adjust in pains or temperature feeling in the human body or face. This area of ns resulted since the ascending medial lemniscal 1° afferent axons native coccygeal to lower thoracic levels were reduced off indigenous the mind stem, and also the information they brought could not be sent out on to the thalamus and also cortex. In contrast, a stroke affecting the posterior paracentral lobule would develop sensory deficits in discriminative touch, proprioception and also sharp pricking ache contralateral come the website of stroke. But such a stroke would influence other pain, temperature and crude touch sensations much less than a large spinal cord lesion due to the fact that these somatic sensations are stood for in diffuse locations of the cortex.

Table II The Nerve Roots and also Ganglia associated with the Somatic and Visceral Afferent Pathways
Nerve Root Ganglia Somatic Innervation Visceral Innervation
Spinal Cord: Sacral posterior root: S5 come S1 buttocks, earlier of leg and foot, genitals lower pelvic region, e.g., Rectum
Spinal Cord: Lumbar posterior root: L5 to L1 lower back, hip, pelvic area, side and front that leg and foot leg and pelvic region, e.g., bladder
Spinal Cord: Thoracic posterior root: T12 come T1 trunk (abdomen, back, and chest), part of arm

lower roots: lower abdomen (e.g., kidney, colon, appendix)

center roots: top abdomen (e.g., stomach, liver, bitterly bladder)

upper roots: Chest (e.g., diaphragm, esophagus, lung, heart)

Spinal Cord: Cervical posterior root: C8 to C2 shoulder, arm, hand, fingers, neck and ago of head minor to blood vessels and sweat glands of upper body and also extremities
Cranial Nerve: Vagus Nerve

jugular (superior)

nodose (inferior)

back the ear, exterior auditory canal and dura

none

throat, thoracic and ab viscera

Cranial Nerve: Glossopharyngeal

superior (jugular)

petrosal (inferior)

back that ear (minor), ear drum, middle ear

ear drum, center ear, Eustachian tube, tonsil, pharynx, soft palate and also posterior tongue

none

carotid body and also sinus

Cranial Nerve: Facial geniculate skin the ear minor - Parotid gland
Cranial Nerve: Trigeminal semilunar face, eye, oral and also nasal cavities, and also meninges none

Test her Knowledge

Make the ideal match between the 1° somatosensory axon type and the sensations lugged by the axon.


A. Vibration

B. Static muscle stretch

C. Dull, burning pain

D. Isotonic muscle contraction

E. Spicy "fast" pain

F. Dynamic muscle stretch


A. Vibration This is an not correct match.

B. Revolution muscle stretch

C. Dull, burning pain

D. Isotonic muscle contraction

E. Sharp "fast" pain

F. Dynamic muscle stretch


A. Vibration

B. Revolution muscle big This is an incorrect match.

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C. Dull, burn pain

D. Isotonic muscle contraction

E. Spicy "fast" pain

F. Dynamic muscle stretch


A. Vibration

B. Static muscle stretch

C. Dull, burning pain This is the exactly match!

The C fibers lug information around dull and also deep pain and also warm/hot native somatic structures. They perform not carry sharp "fast" pain information.