Sensory system.

Sándor Katz M.D., Ph.D.

Sensory pathways - receptors

Sensory pathways (ascending tracts)-Fasciculus gracilis (Goll’s) and cuneatus (Burdach’s)

• Both tracts convey fibers for positionsense (conscious proprioception) andfine cutaneous sensation (touch,vibration, fine pressure sense, two-pointdiscrimination) =EPICRITICSENSATION.

• The fasciculus gracilis carries fibersonly from the lower limbs, while thefasciculus cuneatus carries fibers onlyfrom the upper limbs, therefore it is notpresented in the spinal cord below theT3-(6) level.

Sensory pathways (ascending tracts)-Fasciculus gracilis and cuneatus

• Receptors: Meissner’s and Paciniancorpuscles. Muscle spindles and tendon organsfor the conscious proprioception.

• 1st neuron: pseudounipolar neurons in thedorsal root ganglion. The fibers divide intolong ascending and short descending fibers.

• The descending fibers give off collateralbranches (in the cervical region: Schultze’s commatract, in the thoracic and lumbar regions: Flechsig’soval field, in the sacral region: Philippe-Gombault’strigone) that synapse with posterior or anteriorhorn’s neurons. (They are involved withintersegmental reflexes).

• Many of the long ascending fibers travelipsilaterally in the posterior funiculi to thegracile and cuneate nuclei (2nd neuron) in themedulla oblongata.

Sensory pathways (ascending tracts)-Fasciculus gracilis and cuneatus

• The axons (called the internal arcuate fibers)cross in the medulla oblongata and travel asa single bundle (medial lemniscus) to theVPL of thalamus (3rd neuron).

• (Many axons from the 2nd neurons in thenucleus cuneatus relay to the cerebellum,traveling through the inferior cerebellarpeduncle (called cuneocerebellar tract),carrying muscle joint sense.)

• The axons of the third neurons terminate inthe primary somatosensory cortex, located inthe postcentral gyrus.

Sensory pathways (ascending tracts)-Spinothalamic tracts

• The anterior spinothalamictract is the pathway for crudetouch and pressuresensation, while the lateralspinothalamic tract conveyspain, temperature, tickle,itch and sexual sensation=PROTOPATHIC SENSATION.

Sensory pathways (ascending tracts)-Lateral spinothalamic tract

• Receptors: Free nerve endings in the skin for pain andtemperature sensation.

• The pain impulses are transmitted to the spinal cord infast-conducting delta A-type fibers (initial sharp pain)and slow-conducting C-type fibers (prolonged burning,aching pain).

• The cell bodies of these nerve endings are located in thedorsal root ganglion (1st neuron).

• The central processes of first neurons pass through thedorsal root into the white matter of the spinal cord anddivide into descending and ascending branches(dorsolateral tract of Lissauer), then they terminate onsecondary sensory neurons in the posterior horn(including cells of the substantia gelatinosa) (2ndneuron).

• The axons of second neurons cross in the anterior whitecommissure in the corresponding spinal segment andascend in the anterolateral funiculus on the opposite(contralateral) side.

Sensory pathways (ascending tracts)-Lateral spinothalamic tract

• As the lateral spinothalamic tract ascends throughthe medulla oblongata, it is accompanied by theanterior spinothalamic tract and spinotectal tract.Together they form the spinal lemniscus.

• In the midbrain, the spinal lemniscus lies in thetegmentum lateral to the medial lemniscus.

• Many of the fibers of lateral spinothalamic tractterminate in the VPL of thalamus (3rd neuron).

• The axons of third neurons radiate to the primarysomatosensory cortex in the postcentral gyrus.

The contralateral half of thebody is represented asinverted, with the hand andmouth situated inferiorly andthe leg situated superiorly, andwith the foot and anogenitalregion on the medial surface ofthe hemisphere.

Gate Control Theory

Massage, acupuncture andlow frequency electricalstimulation can relieve pain.Although the precisemechanism for thesephenomena is not understood,the theory was proposed in1965 by Ronald Melzack andPatrick Wall.It was suggested that at theside where the pain fibersenter the CNS, inhibition couldoccur by means of connectorneurons excited by large,myelinated afferent fiberscarrying information of nonpainful touch and pressure.

Pain control in the CNS

The Analgesia System

Stimulation of certain areas of thebrainstem can reduce or blocksensations of pain.These areas include the periventriculararea of the diencephalon, theperiaqueductal gray matter of themidbrain, and midline nuclei of thebrainstem. It is believed that fibers of thereticulospinal tract pass down to thespinal cord and synapse on cellsconcerned with pain sensation in theposterior gray column. The analgesicsystem can suppress both sharp prickingpain and burning pain sensations.(Related neurotransmitters areenkephalins, endorphins and serotonin,inhibiting the release of substance P inthe posterior gray column.)

Sensory pathways (ascending tracts)-Anterior spinothalamic tract

• Receptors: Impulses from tactile corpusculesand from receptors about the hair follicles.

• 1st neuron: pseudounipolar neurons in thedorsal root ganglion.

• The central processes of first neurons passthrough the dorsal root into the white matterand divide into descending and ascendingbranches (dorsolateral tract of Lissauer), thenthey terminate on secondary sensory neurons inthe posterior horn (including cells of thesubstantia gelatinosa) -2nd neuron.

Sensory pathways (ascending tracts)-Anterior spinothalamic tract

• The axons of second neurons cross in theanterior white commissure within severalspinal segments and ascend in theanterolateral funiculus on the opposite(contralateral) side.

• (As the anterior spinothalamic tract ascendsthrough the medulla oblongata, it is accompanied bythe lateral spinothalamic tract and spinotectaltract. Together they form the spinal lemniscus.)

• The axons terminate in the VPL of the thalamus(3rd neuron).

• The axons of third neurons radiate to theprimary somatosensory cortex in thepostcentral gyrus.

Sensory pathways (ascending tracts)-Spinocerebellar tracts

Receptors: musclespindles, tendon organsand joint receptors.

Sensory pathways (ascending tracts)-Posterior spinocerebellar tract

• Muscle spindles and tendon receptorsconvey proprioceptive informationfrom lower body part topseudounipolar neurons in the dorsalroot ganglion (1st neuron).

• The axons terminate at the dorsalnucleus (Clark’s) of gray matter (2ndneuron), which spans spinal cordsegments C8 to L3.

• The axons of the second neuronsascend ipsilaterally to the cerebellum,entering through the inferior cerebellarpeduncle.

• The axons terminate with mossy fibersin the stratum granulosum of cerebellarcortex.

Sensory pathways (ascending tracts)-Anterior spinocerebellar tract

• Muscle spindles and tendon receptors conveyproprioceptive information from lower bodypart to pseudounipolar neurons in the dorsal rootganglion (1st neuron).

• The axons terminate at the dorsal gray horn (2ndneuron), which spans spinal cord segments L4 toS3.

• The majority of the axons of second neuronscrosses in the anterior commissure and ascendscontralaterally to the cerebellum, enteringthrough the superior cerebellar peduncle.

• The axons terminate with mossy fibers in thestratum granulosum of cerebellar cortex.

• (It is believed that those fibers that crossed over tothe opposite side in the spinal cord cross backwithin the cerebellum.)

Spinotectal tract

It provides afferentinformation for spinovisualreflexes and brings aboutmovements of the eyes andhead toward the source of thestimulation.

• 1st neuron: in the dorsal rootganglia

• 2nd neuron: unknown• crossing in the median plane• termination: in the superior

colliculi

Other ascending tracts

Spinoreticular tract

It provides afferentinformation for the reticularformation, which plays animportant role in influencinglevels of consciousness.

• 1st neuron: in the dorsal rootganglia

• 2nd neuron: unknown• termination: neurons of the

reticular formation in themedulla oblongata, pons andmidbrain

Spino-olivary tract

It provides afferentinformation for thecerebellum from cutaneousand proprioceptive organs.

• 1st neuron: in the dorsal rootganglia

• 2nd neuron: unknown• crossing in the median plane• 3rd neuron: in the inferior

olivary nuclei in the medullaoblongata

• crossing in the midline• termination: cerebellum via

inferior cerebellar peduncle

Other ascending tracts

Visceral sensory tracts

It provides afferentinformation for the cerebralcortex about visceral pain,including ischemia, chemicaldamage and spasm ofsmooth muscle..

• 1st neuron: in the dorsal rootganglia

• 2nd neuron: posterior orlateral gray column

• the axons of 2nd neurons arebelieved to join thespinothalamic tracts

• 3rd neuron: VPL nucleus ofthe thalamus

• termination: postcentralgyrus

Ascending and descending tracts - overview

Thank you for your attention.References:Snell: Clinical Neuroanatomy, 7th ed.McGraw-Hill Company’s picturesNature ReviewsPearson Education