SENSORY OR ASCENDING TRACTS Dr. Jamila El-Medany Dr. Essam Eldin Salama

OBJECTIVES By the end of the lecture, the student will be able to: Define the meaning of a tract. Distinguish between the different types of tracts. Locate the position of each tract. Describe the sensory pathway. Identify the different sensory spinal tracts and their functions. Identify the course of each of these tracts.

Grey matter of the spinal cord is completely surrounded by white matter, which consists of Ascending and Descending Nerve Fibers. The White matter is divided into Dorsal, Lateral & Ventral Columns or Funiculi.

WHITE MATTER TRACTS Definition: Bundles or fasciculi of fibers that occupy more or less definite positions in the white matter. They have the same Origin, Termination and carry the same Function. They serve to join the brain to the spinal cord. They are classified into: 1- Short Tracts (intersegmental). 2-Long Tracts: Ascending (sensory or afferent). Descending (motor or efferent). WHITE MATTER TRACTS

Intersegmental (propriospinal) tracts Definition: Fibers that interconnect adjacent or distant segments of the spinal cord. Position: They lie close to the grey matter

Ascending Tracts; Carry impulses from pain, thermal, tactile, muscle and joint receptors to the brain. Some of this information eventually reaches a conscious level (the cerebral cortex), while some is destined for subconscious centers (e.g. the cerebellum).

Ascending Or Sensory Tracts 1- Dorsal column tracts: (Gracile & Cuneate) Function: Transmit Proprioceptive (deep) sensations (sense of movement, position, vibration). Fine touch sensations (tactile localization, tactile discrimination & stereognosis). These senses reach a Conscious level (cerebral cortex). 2- Spinothalmic tracts: Function: Transmit impulses concerned with specific sensory modalities: pain, temperature, touch, that reach a Conscious level (cerebral cortex). 3- Spinocerebellar tracts: Function: Transmit impulses from tactile and stretch receptors to Subconscious centers (cerebellum)-muscle tone and coordinations. Ascending Or Sensory Tracts

Pathways that carry information to a conscious level share certain common characteristics. There is a sequence of three neurones between the peripheral receptor and the cerebral cortex.

The first-order neurone or primary afferent neurone) enters the spinal cord through the dorsal root of a spinal nerve and its cell body lies in the dorsal root ganglion. The main fiber remains on the ipsilateral side of the cord and terminates in synaptic contact with the second neurone either in the spinal grey matter or in the medulla oblongata of the brain stem.

The second order neurone has its cell body in the cord or medulla oblongata. Its axon crosses over (decussates) to the opposite side of the CNS and ascends to the thalamus, where it terminates upon the third neurone. The third-order neurone has its cell body in the thalamus. Its axon passes to the somatosensory cortex of the parietal lobe of the cerebral hemisphere.

Two main tract systems in the spinal cord fit into this pattern: Dorsal (posterior) columns and Spinothalamic tracts. Prof. Makarem

1. Dorsal column Tracts Function: Carry proprioceptive (deep) & fine touch sensations to sensory area of the cerebral cortex. The dorsal column contains the axons of the neurones that have entered the cord through the dorsal roots of spinal nerves. Fibers of Fasciculus Gracilis enter via the sacral, lumbar and lower thoracic levels; (lower limbs). Fibers of Fasciculus Cuneatus enter via the upper thoracic and cervical levels; (upper limbs). Fibers ascend to the Medulla oblongata where they terminate upon second-order neurones, (Nucleus Gracilis and nucleus Cuneatus in the medulla). Prof. Makarem

Axons of second-order neurones decussate in the medulla as the internal arcuate fibers (sensory decussation). Then it ascend in (opposite side) through brain stem as the Medial Lemniscus. The medial lemniscus terminates in the ventral posterior (VP) nucleus of the thalamus. Third-order neurone, (thalamocortical, or sensory radiations- run in the internal capsule) project to the somatosensory cortex. Prof. Makarem

II- Spinothalamic tracts Spinothalamic tracts lie lateral and ventral to the ventral horn of the spinal grey matter. They carry pain and thermal (temperature) sensations and also non-discriminative (crude) touch and pressure. The Lateral spinothalamic tracts carry pain and temperature sensations. The ventral spinothalamic tracts carry crude touch and pressure. But fibers carrying these modalities are probably intermingled, at least to some extent. Prof. Makarem

A-Lateral Spinothalamic Tract (pain & temperature) Function: Carries pain & Temperature to thalamus and sensory area of the cerebral cortex. Neurones: 3 Neurones Neurone I: Small cells in the dorsal root ganglia. Neurone II: Cells of substantia gelatinosa of Rolandi in the posterior horn. Neurone III: Cells of (VP) nucleus of the thalamus. The spinothalamic tract contains second-order neurones, the cell bodies of which lie in the contralateral dorsal horn.

After leaving the parent cell bodies, spinothalamic axons decussate to the opposite side of the cord by passing through the ventral white commissure, which lies ventral to the central canal of the cord, and, thus, enter the contralateral spinothalamic tract. Lesion: Syringomyelia, (widening of the central canal) leads to Loss of pain & temperature below the level of the lesion.

B- Ventral Spinothalamic (Crude touch & Pressure) Function: Carries crude touch & pressure to thalamus and sensory cortex. Neurones: 3 Neurones Neurone I: Medium sized cells in the dorsal root ganglia. Neurone II: Cells of main sensory nucleus or (nucleus proprius). Neurone III: Cells of VP nucleus of thalamus. Effect of lesion: Loss of crude touch sensation below the level of the lesion.

Spinocerebellar tracts Ascending pathways that carry impulses to a subconscious level are represented by the spinocerebellar tracts. Fibres of spinocerebellar tracts form dorsal and ventral tracts that are located at the dorsolateral & ventrolateral surfaces of the cord, respectively. Both tracts carry information derived from muscle spindles, Golgi tendon organs and tactile receptors to the cerebellum for the control of posture & coordination of movement. Prof. Makarem

Neurone I:Large cells of dorsal root ganglia. The spinocerebellar system consists of a sequence of only two neurones. Neurone I:Large cells of dorsal root ganglia. Neurone II: cells of the nucleus dorsalis (Clark's nucleus. Both spinocerebellar tracts contain second-order neurones. The tract neurones terminate directly in the cerebellar cortex. Prof. Makarem

Dorsal (Direct) Spinocerebellar tract Fibers of the dorsal spinocerebellar tract originate from the cells of Clarke's column at the base of the posterior horn. The axons ascend ipsilaterally to enter the cerebellum through the inferior cerebellar peduncle.

Ventral (Indirect) Spinocerebellar Tract Fibers of the ventral spinocerebellar tract decussate, ascend on the contralateral side of the cord and enter the cerebellum via the Superior cerebellar peduncle. Some axons then recross within the cerebellar white matter.

Spino-Olivary Tracts Project to accessory olivary nuclei and cerebellum. Contribute to coordination & associated primarily with balance.

Spinotectal Tracts Project to superior colliculi of midbrain. Involved in reflexive turning of the head and eyes toward a point of cutaneous stimulation.

Spinoreticular Tracts Involved in arousing consciousness in the reticular activating system through cutaneous stimulation.

Thank you