Sensory and motor pathways.

Sándor Katz M.D., Ph.D.

Sensory pathways - receptors

Sensory pathways - receptors

Meissner’s corpuscle

Pacinian corpuscle

Sensory pathways (ascending tracts)-Fasciculus gracilis and cuneatus

• Both tracts convey fibers for positionsense (conscious proprioception)and fine cutaneous sensation(touch, vibration, fine pressure sense,two-point discrimination)=EPICRITIC SENSATION.

• The fasciculus gracilis carries fibersonly from the lower limbs, while thefasciculus cuneatus carries fibersonly from the upper limbs, thereforeit is not presented in the spinal cordbelow the T3 level.

Sensory pathways(ascending tracts)-Fasciculus gracilis and cuneatus

• Muscle-, tendon receptors and Vater-Pacini corpuscles for consciousproprioception. Receptors about the hairfollicles and additional receptorsmediate the fine touch sensation of theskin.

• 1st neuron: pseudounipolar neurons inthe dorsal root ganglion.

• The axons ascend ipsilaterally in theposterior funiculi to the gracile andcuneate nuclei (2nd neuron) in themedulla oblongata.

• The axons cross in the medial lemniscusto the VPL of thalamus (3rd neuron).

• The axons of the third neurons terminatein the primary somatosensory cortex,located in the postcentral gyrus.

Sensory pathways(ascending tracts)-Spinothalamic tracts

• The anteriorspinothalamic tract isthe pathway for crudetouch and pressuresensation, while thelateral spinothalamictract conveys pain,temperature, tickle, itchand sexual sensation=PROTOPATHICSENSATION.

Sensory pathways(ascending tracts)-Lateral spinothalamic tract

• Free nerve endings in the skin function asreceptors for pain and temperature sensation.

• The cell bodies of these nerve endings arelocated in the dorsal root ganglion (1st neuron).

• The central processes of first neurons passthrough the dorsal root into the white matter ofthe spinal cord (dorsolateral tract of Lissauer),then they terminate on funiculate neurons in thegray matter (2nd neuron).

• The axons of second neurons cross in the anteriorcommissure in the corresponding spinal segmentand ascend in the anterolateral funiculus on theopposite (contralateral)side.

• They terminate in the VPL of thalamus (3rdneuron).

• The axons of third neurons radiate to theprimary somatosensory cortex in the postcentralgyrus.

Sensory pathways(ascending tracts)-Anterior spinothalamic tract

• Impulses from tactile corpuscules and fromreceptors about the hair follicles.

• The cell bodies of these axons are located inthe dorsal root ganglion (1st neuron).

• The axons pass through the dorsal root andenter the gray matter where terminate onfuniculate neurons (2nd neuron).

• The axons of second neurons cross in theanterior commissure and ascend in theopposite anterior funiculus.

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

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

Sensory pathways(ascending tracts)-Spinocerebellar tracts

Sensory pathways(ascending tracts)-Posterior spinocerebellar tract

• Muscle spindles and tendonreceptors convey proprioceptiveinformation from lower body partto pseudounipolar neurons in thedorsal root ganglion (1st neuron).

• The axons terminate at the dorsalnucleus (Clark’s) of gray matter(2nd neuron), which spans spinalcord segments C8 to L3.

• The axons of the second neuronsascend ipsilaterally to thecerebellum, entering through theinferior cerebellar peduncle.

• The axons terminate with mossyfibers in the stratum granulosum ofcerebellar cortex.

Sensory pathways(ascending tracts)-Anterior spinocerebellar tract

• Muscle spindles and tendonreceptors convey proprioceptiveinformation from lower body partto pseudounipolar neurons in thedorsal root ganglion (1st neuron).

• The axons terminate at the dorsalgray horn (2nd neuron), whichspans spinal cord segments L4 toS3.

• The axons of the second neuronscross in the anterior commissureand ascend contralaterally to thecerebellum, entering through thesuperior cerebellar peduncle.

• The axons terminate with mossyfibers in the stratum granulosum ofcerebellar cortex.

Motor pathways(descending tracts)-Pyramidal tracts

• The most important pathway forvoluntary motor function.

• Some of its axons, thecorticonuclear fibers terminate atthe cranial nerve nuclei.

• Others, the corticospinal fibersterminate on the motor neurons inanterior horn of gray matter.

• A third group, the corticoreticularfibers are distributed to nuclei ofreticular formation.

Motor pathways(descending tracts)-Pyramidal (corticospinal) tracts

• The tracts originate in the motorcortex at the pyramidal cells (1stneuron).

• The corticospinal fibers pass throughthe internal capsule, continuing intothe brainstem and spinal cord.

• The fibers descend to the pyramidaldecussation in the medulla oblongata,where approximately 80% of themcross to the opposite side. The fiberscontinue into the spinal cord wherethey form the lateral corticospinaltract, which has somatotopicorganization: the fibers for the sacralcord are the most lateral, while thefibers for the cervical cord are themost medial.

Motor pathways(descending tracts)-Pyramidal (corticospinal) tracts

• The remaining 20% of corticospinalfibers continue to descend withoutcrossing, forming the anteriorcorticospinal tract, which bordersthe ventral median fissure.

• The anterior corticospinal tract isparticularly well developed in thecervical cord, but is not present in thelower thoracic, lumbar or sacralcords.

• The axons of the pyramidal cellsterminate via intercalated cells onalpha and gamma motor neurons,Renshaw cells and interneurons (2ndneuron).

• Most fibers of ant. corticospinal tractcross at the segmental level toterminate on the same motor neurons(3rd neuron) as the lateralcorticospinal tract.

Motor pathways(descending tracts)-Extrapyramidal tracts

• Unlike the pyramidal tract, whichcontrols conscious, voluntary motoractivities, the extrapyramidalmotor system (cerebellum, basalganglia and motor nuclei of thebrainstem) is necessary forautonomic and learned motorprocesses (e.g., walking, running,cycling).

Some important tracts:

• Tectospinal tract: coordinates headand eye movements.

• Rubrospinal tract: mediation ofvoluntary movements.

Ascending and descending tracts-overview

Thank you for your attention.References:McGraw-Hill Company’s picturesNature ReviewsPearson Education