ASCENDING TRACTS (SENSORY PATHWAY)

Submitted under the guidance of : Dr. Sachin Shris, MBBS, MS

Submitted by : Nibedita Ayan

PATHWAY:• There is a continuous flow of information between the brain, spinal

cord, and peripheral nerves. • A group of neurons connecting two masses of grey matter is called a

tract.• Generally the pathways:

Consists of a chain of tracts, associated nuclei and varying number of relays (synapses)

Consist of two or three neurons Exhibit somatotopy (precise spatial relationships) Decussate Involve both the brain and spinal cord Are paired (bilaterally and symmetrically)

SOMATOSENSORY PATHWAY: When a tract relays sensory information from the receptors (peripheral nerve endings) to the brain , the tract is the Ascending (somatosensory) tract .• Monitor conditions both inside the body and in the external environment• Sensation-stimulated receptor passes information to the CNS via

afferent (sensory) fibers• Most sensory information is processed in the spinal cord , thalamus, or

brain stem. Only 1% reaches the cerebral cortex and our conscious awareness

• Processing in the spinal cord can produce a rapid motor response (stretch reflex)

• Processing within the brain stem may result in complex motor activities (positional changes in the eye, head, trunk)

ASCENDING TRACTS:• LATERAL SPINOTHALAMIC TRACT• ANTERIOR SPINOTHALAMIC TRACT• FASCICULUS GRACIALIS• FASCICULUS CUNEATUS• DORSAL (post.) SPINOCEREBELLAR TRACT• VENTRAL (ant.) SPINOCEREBELLAR TRACT• SPINO-OLIVARY TRACT• SPINOTECTAL TRACT

• Three major pathways carry sensory informationPosterior/Dorsal

column pathway (gracile & cuneate fasciculi)

Anterolateral pathway (spinothalamic)

Spinocerebellar pathway

All these three pathways involve a sequence of three neurons each.

>> First-order neuron • Delivers sensations to the CNS • The cell body is in the dorsal or cranial root ganglion

>> Second-order neuron • An interneuron with the cell body in the spinal cord or brain

>> Third-order neuron • Transmits information from the thalamus to the cerebral cortex

Based on the perception • For conscious perception:

Spinothalamic system• For unconscious perception:

SpinocerebellarSpino-olivarySpinotectalSpinoreticular

Dorsal Column• Contains two tracts, Fasciculus gracilis (FG) & fasciculus cuneatus (FC)

• Carry impulses concerned with proprioception and discriminative touch from ipsilateral side of body

• Contain the axons of primary afferent neurons that have entered cord through dorsal roots of spinal nerves

FG contains fibers received at sacral, lumbar and lower thoracic levels, FC contains fibers received at upper thoracic and cervical levels.

Fibers ascend without interruption where they terminate upon 2nd order neurons in nucleus gracilis and nucleus cuneatus

The axons of the 2nd order neurons decussate in the medulla as internal arcuate fibers and ascend through the brain stem as medial lemniscus.

The medial lemniscus terminates in the ventral posterior nucleus of the thalamus upon 3rd order neurons, which project to the somatosensory cortex (thalamocortical fibers)

Spino-thalamic Tracts

• Located lateral and ventral to the ventral horn

• Carry impulses concerned with pain and thermal sensations (lateral tract) and also non- discriminative touch and pressure (medial tract)

• Fibers of the two tracts are intermingled to some extent

• Fibers are highly somato-topically arranged, with those for the lower limb lying most superficially and those for the upper limb lying deeply

Information is sent to the primary sensory cortex on the opposite side of the body

Spinocerebellar Tracts• The spinocerebellar system

consists of a sequence of only two neurons

• Two tracts: Posterior & Anterior

• Located near the dorsolateral and ventrolateral surfaces of the cord

• Contain axons of the second order neurons

• Carry information derived from muscle spindles, Golgi tendon organs and tectile receptors to the cerebellum for the control of posture and coordination of movements

Spinotectal Tract• Ascends in the anterolateral

part in close association with spinothalamic system

• Primary afferents reach dorsal horn through dorsal roots and terminate on 2nd order neurons

• The cell bodies of 2nd order neuron lie in base of the dorsal horn

• Axons of 2nd order neuron cross to opposite side, and project to the superior colliculus in the midbrain

Spino-olivary Tract• Indirect spinocerebellar pathway (spino-olivo-

cerebellar)• Impulses from the spinal cord are relayed to

the cerebellum via inferior olivary nucleus• Conveys sensory information to the

cerebellum• Fibers arise at all level of the spinal cord

TRACT SENSATION FIRST ORDER

SECOND ORDER THIRD ORDER FINAL DESTINATION CROSS-OVER SITE

POSTERIOR COLUMNSFASCICULUS GRACIALIS Proprioception; inferior to T6 Dorsal root

gangliaNucleus Gracialis Posteriolateral ventral

nucleus of thalamusPrimary Sensory Cortex on opposite side (contralateral)

Uncrossed

FASCICULUS CUNEATUS Proprioception; superior to T6

Dorsal root ganglia

Nucleus Cuneatus Posteriolateral ventral nucleus of thalamus

Primary Sensory Cortex on opposite side (contralateral)

Uncrossed

SPINOTHALAMIC TRACTSLATERAL SPINOTHALAMIC Pain and temperature Dorsal root

gangliaSubstantia Gelatinosa

Posteriolateral ventral nucleus of thalamus

Primary sensory cortex ;contralaterally

Crosses in the same segment

ANTERIOR SPINOTHALAMIC Crude touch and pressure Dorsal root ganglia

Nucleus Proprius Posteriolateral ventral nucleus of thalamus

Primary sensory cortex ;contralaterally

Ascends 2-3 segments and then crosses

SPINOCEREBELLAR TRACTSPOSTERIOR SPINOCEREBELLAR Proprioception Dorsal root

gangliaClark’s Column NIL Cerebellar cortex ;

ipsilaterallyUncrossed

ANTERIOR SPINOCEREBELLAR Proprioception Dorsal root ganglia

Neurons of Posterior Horn

NIL Cerebellar cortex ; ipsilaterally

Crosses twice;-spinal cord-midbrain

NON SPECIFICSPINO-OLIVARY Proprioception - - - Olivary nuclei UncrossedSPINOTECTAL Afferent limb of reflex

movement of eyes and head- - - Tectum or superior colliculus Crossed

CLINICAL CO-RELATESTRACT CLIINICAL TEST RELATED DISORDERS

LATERAL SPINOTHALAMIC

Pain with pin prick Temperature test Brown Sequard’s

Syndrome (due to hemisection of spinal cord)

Syringomyelia

ANTERIOR SPINOTHALAMIC

Joint sense Vibration Tactile localization Romberg's test Stereognosis Crude touch Crude pressure

FASCICULUS GRACIALIS

FASCICULUS CUNEATUS

DORSAL SPINOCEREBELLAR Cerebellar tests (finger ,nose, heel-

knee tests)VENTRAL SPINOCEREBELLARSPINO-OLIVARY

Visual and proprioception testsSPINOTECTAL

th@nkU