TRACTS OF THE SPINALCORD
Definition of Tract – A collection of nerve fibres that connects two masses Grey matter within the central nervous system is called a tract. Some tracts are called fasciculi or lemnisci Types- Tracts may be ascending or descending. Nomenclature- They are usually named after the masses of grey matter connected by them.. Descending Tracts A The pyramidal or corticospinal tract descends from the cerebral cortex to the spinal cord. It consists of two parts: 1. The lateral corticospinal tract.which lies in the lateral funiculus; and 2. The anterior corticospinal tractwhich lies in the anterior funiculus B. Extrapyramidal tracts. These are: 1. The rubrospinal tract. 2. The medial and lateral reticulospinal tracts. 3. The olivospinal tract. 4. The vestibulospinal tract. 5. The tectospinal tract.
Fig- Descending Tracts of Spinal Cord
Scheme to show Descending tracts passing through Brain stem
Formula- ROT- C1-C5, MAOV- uncrossed descending tracts
Ascending Tracts 1. Lateral spinothalamic tract 2. Anterior spinothalamic tract 3. Fasciculus gracilis (medially) 4. Fasciculus cuneatus (laterally) 5. Dorsal or posterior spinocerebellar tracts. 6. Ventral or anterior spinocerebellar tract 7. Spino-olivary tract. 8. Spinotectal tract.
Fig -Ascending Tracts of Spinal Cord
Scheme to show Ascending tracts passing through Brain stem
Description of Tracts
1. Pyramidal Tract- Corticospinal Tract or Themotor corticospinal are mainly made up4). of axons neurons Origin lying in -the area of tracts the cerebral cortex (area Some of fibres also arise from the premotor area (area 6) and some from the somatosensory area (areas 3)
Course & Termination
From this srcin fibres pas to the posterior limb of internal capsule , then they pass through middle 2/3rd of crus cerebri of mid brain, then the fibres descend through the ventral part of pons to enter pyramid in the upper part of medulla. At lower end of medulla 80% of fibres cross to opposite side, called pyramidal decussation. The fibres that have crossed enter lateral column of white matter of spinal cord and descend as lateral corticospinal tract. Most of these fibres terminate by synapsing through the internuncial neurons at the anterior horn cells The 20% of fibres that do not cross enter anterior white column of spinal cord to form anterior (or ventral) corticospinal tract. The fibres of this tract also cross at appropriate levels to reach grey matter of the opposite half of spinal cord and synapse with intemuncial neurons similar to those of lateral corticospinal tract.
Thus the cerebral cortex through lateral and anterior corticospinal tracts controls anterior horn cells of opposite half of spinal cord See figure next page
Function(i) The cerebral cortex controls voluntary movements of opposite half of body through anterior hom cells. (ii) Influence of corticospinal tract is supposed to be facilitatory for flexors and inhibitory for extensors.
Extrapyramidal Tracts- These are Indirect pathways through which cerebral cortex influence spinal cord. Function of all these tracts is to control the voluntary movements of skeletal muscles of the body through anterior horn cells
1. Rubrospinal tract:
Origin & Situation - This tract is formed by the axons of red nucleus which lies in the upper part of mid brain. Course- The fibres cross to the opposite side in the tegmentum of midbrain; thus constituting the ventral tegmental decussation. The tract descend through the pons and medulla oblongata and enter the lateral white column of spinal cord. The fibres terminate by synapsing through internuncial neurons with anterior hom cells.
Function- The tract is believed to be facilitatory to flexors and inhibitory for extensors..
2: Reticulospinal tracts:
Origin & Course a) The medial reticulospinal tract is formed by the fibres from reticular formation in pons and descends to same side cervical segments only (no crossing over occurs). It lies in the anterior white column of spinal cord. b) The lateral reticulospinal tract srcinate from reticular formation in medulla and the fibres cross to the opposite side in the medulla and descend up to thoracic segments of spinal cord. It lies in the anterolateral white column of spinal cord.
TerminationBoth the tracts terminate by synapsing with the neurons anterior grey horn cells (interneurons)
FunctionReticulospinal tract- is facilitatory for trunk and limb muscles and inhibitory to neck muscles. Ie tract is concerned with postural adjustment of head, trunk and limbs.
3. Olivospinal tract: Its fibres srcinate from the inferior olivary nucleus in medulla oblongata, descend to spinal cord, lie in the antero-lateral column of white matter and synapse with the anterior horn cells. 4. Tectospinal Tract-
Origin and course- Fibres of this tract arise from neurons in the superior colliculus (midbrain). The fibres cross to the opposite side in the upper part of the tegmentum of the midbrain. The crossing fibres
form the dorsal tegmental decussation (Fig. 11.9). The tract descends through the pons and medulla into the anterior white column of the spinal cord. Termination- The fibres terminate by synapsing with ventral (anterior) horn cells in cervical segments of the cord, through internuncial neurons located in laminae VI to VIII of the spinal grey matter. 5. Vestibulospinal tract:
Origin & Course - The fibres arise from lateral vestibular nucleus lying at ponto-medullary junction. The fibres descend uncrossed to spinal cord. This tract is situated in the anterior white column of spinal cord. Termination- These fibres synapse with anterior horn cells. FunctionThe vestibulospinal tract is facilitatory to motor neurons supplying extensor muscles of neck, back and limbs; and is inhibitory to flexor muscles. Just reverse of rubrospinal.
Ascending Tracts- see in fig below 1 st, 2nd, 3rd order Sensory Neuron in a Sensory Pathway
A. First Sensory Neuron- the first neuron fibres start in the dorsal root ganglia (i) The perpheral process of these neurons in dorsal root ganglia run in peripheral nerves. (ii) The central process of these neurons in the dorsal root ganglia enter spinal cord through dorsal nerve root and end by synapsing with dorsal grey matter of spinal cord or brain stem B. Second Sensory neuron- The second neuron lies in spinal dorsal grey matter or grey matter in the brain stem.
C. Third Sensory neuron- The third neuron for sensory pathway lies in the Thalmus. From the thalamus third neuron fibres are projected to the somato sensory areas in the cerebral cortex 1. Spinothalmic TractA. lateral Spinothalmic tract B. Anterior Spinothalmic tract
A. Lateral spinothalamic tract: This tract carries the sensation of pain and temperature. The first neuron fibres start in the dorsal root ganglia. These relay by synapsing with neurons lying in the grey matter of lamina II and III. Pain fibres relay in Lamina II (substantia gelatinosa). The second neuron .fibres cross immediately to opposite side close to the central canal and ascend as tract in the lateral white column of spinal cord. 2. Anterior spinothalamic tract: This tract carries the fibres for crude touch and pressure. First neuron fibres are in the dorsal root ganglia. These relay in the grey matter of posterior horn or nucleus proprius (Laminae III-IV). The second neuron fibres ascend for 1-2 segments and cross to opposite side in the white commissure and ascend as a tract in the anterior white column of spinal cord.
The anterior and lateral spinothalamic tracts carry sensations from the opposite half of body. These lie in continuity with each other in the anterolateral white column of spinal cord showing somatotopic lamination. The sensations of pressure, touch, temperature and pain are lying medial to lateral. Pressure ensations are medial most near the anterior median fissure. Cervical segments are facing medially and sacral segments face laterally.
Functions- Both Lateral and Anterior Spinothalmic tracts Carry sensations like touch, pain, and temperature. 3. Tracts of Goll & Burdach-
A. Fasciculus gracilis (tract of Goll): st
first order neuron fibres arises from dorsal 1 Order Neuron - The from ganglia starting upwards the caudal most portion of spinal cord. root
Course of fibres in spinal cord - These run directly upwards (without relaying in the spinal grey matter) in the posterior column of white matter of spinal cord. As the tract ascends, it receives fibres from each dorsal root. The fibres ascending from lower spinal segments (coccygeal & sacral) are thrust medially by fibres which enter at higher levels. Fasciculus fibres coccygeal to lowercolumn thoracic segments spinal cord.gracilis It occupies the from medial part of posterior of upper part of of spinal cord. B. Fasciculus cuneatus (tract of Burdach):
1st Order Neuron- The first order neuron fibres arises from dorsal root ganglia starting upwards from thoracic region. It contains fibres from upper thoracic and cervical segments.
Both fasciculi ( Tract of Goll and Burdach) contain first neuron fibres from central process of dorsal root ganglia. Both these fascicule occupy posterior column of are cal;led Posterior Column Tracts. The fibres from both these facsiculi end by synapsing with the neurons in nucleus gracilis and nucleus cuneatus, situated in the medulla oblongata of the same side without crossing from where second neuron fibres take srcin.
2nd Order Neuron- Of both the tracts (Goll & Burdach) are Located in nucleus gracilis (for tract of Goll) and nucleus cuneatus (for Tract of Buirdach), in the medulla oblongata. Their axons cross midline forming sensory decussation. After crossing midline the fibres move upwards forming a bundle called medial Medial Leminscus mid brain and reach. The . leminscus ascends upwards through pons, Thalmus
3rd Order neuron- It is located in thalamus and sends axons to internal capsule, corona radiate and finally to somatosensory area of cerebral cortex. Functions of Tracts of Goll & Burdach1. Deep touch and pressure 2. Ability to localize the part touched (tactile localization) 3. (tactile Ability to recognize 2 points trounced as separate from each other discrimination) 4. Ability to recognize shape of object held in hands (Stereognosis) 5. Proprioceptive impulses – for sense of joint positions 6. Sense of vibration
5. Dorsal or posterior spinocerebellar tract:
1st order Neuron- It begins about the level of 2nd or 3rd lumbar segment of spinal cord. The first neuron fibres are the central processes of dorsal root ganglia. These relay in the dorsal nucleus (thoracic or Clark's column) which lies on the medial side of the base of posterior grey column in segments.
2nd Order Neuron- It is present in dorsal nucleus (thoracic or Clark's column). This relay from 2nd neuron form dorsal spinocerebellar tract. This uncrossed tract ascends in the lateral column of white matter of spinal cord. It ascends to the level of medulla oblongata where its fibres pass through inferior cerebellar peduncle to reach thecerebellum 6. Ventral or anterior spinocerebellar tract: st
1 order Neurondorsal root ganglia.The first neuron fibres are the central processes of 2nd Order Neuron – they are derived from the large cells of posterior grey column (laminae V, VI) in the lumbar and sacral segments. The second neuron fibres are from opposite side mainly, very- few from the ipsilateral side. These ascend in the lateral white column of spinal cord anterior to the fibres of dorsal spinocerebellar tract to pass through the medulla oblongata and pons to finally reach cerebellum. Functions - Both spinocerebellar tracts control the coordination and movements of muscles posture of the body. both upper The ventral tract conveyscontrolling muscle and joint informationfrom and lower limbs, while the dorsal tract receives informationfrom trunk and lower limbs.
Clinical1. Syringomyelia:
It is the condition in whichinthere is dilatation cord usually beginning cervical region. of central canal of spinal Decussating fibres for pain and temperature in the grey commissureare damaged. This leads to bilateral loss of pain and temperature in the affected region. But touch and proprioception is present. Thus patient has dissociation of sensory loss. Usually the patient gets severe infections in the fingers due to uncared cuts and bruises.
Poliomyelitis: It is a viral disease which involves anterior hom cells leading to flaccid paralysis of the affected segments. It is lower motor neuron, paralysis. Following is the comparison between upper motor neuron and lower motor neuron paralysis:
2. Brownsequared Syndrome (Hemisection of Spinal Cord) ie cutting spinal cord laterally from half side horizontally- Effects at the level, above the level, below the level- The hemisection of the cord results in affect the corticospinal tract , one or both dorsal columns tracts and the spinothalamic tract. So the patient will present with three lesions:
The corticospinal lesion injury produces ipsilateral (same side) spastic paralysis - hemiparesis. The injury to fasciculus gracilis or fasciculus cuneatus results in ipsilateral (same side) loss of vibration and proprioception (joint position sense). The injury of the spinothalamic tact leads to pain and
temperature sensation being lost from the contralateral (opposite) side beginning one or two segments below the lesion. 3. Describe the 6 important ascending (sensory) tracts under following headings in tabulated form: a. Name b. Situation. c. srcin d. side. e. formed by which neuron fibre f. termination g. function
Q. Describe the five important decussations in b rainstem under the following headings in a tabulated forma. situation b. source c. fate of fibres after decussations d. termination e. function 1. Sensory Decussationa. situation - Medulla Oblongata b. source - The fibres of fasciculus gracilis and fasciculus cuneatus of right and left side cross the midline and form the sensory decussation. c. fate of fibres after decussations- After crossing midline the fibres move upwards forming a bundle called Medial Leminscus. d. termination- The medial leminscus ascends upwards through pons, mid brain and reach Thalmus e. function- Functions of Facsiculus gracilis and fasciculus cuneatus – following sensations carrieda. Deep touch and pressure b. Ability to localize the part touched (tactile localization) c. Ability to recognize 2 points trounced as separate from each other (tactile discrimination) d. Ability to recognize shape of object held in hands (Stereognosis) e. Proprioceptive impulses – for sense of joint positions f. Sense of vibration
2.Pyrimidal Decussationa. situation – lower end of medulla b. source – The corticospinal tracts are mainly formed by axons of neurons lying in the motor area of the cerebral cortex (area 4). Some fibres also arise from the premotor area (area 6) and some from the somatosensory area (areas 3). These fibres first enter posterior limb of internal capsule and then descend through brain stem (mid brain, pons and medulla). 80% of
fibres of corticospinal tract cross to opposite side at lower end of medulla forming pyramidal decussation. c. fate of fibres after decussations - The fibres that have crossed enter lateral column of white matter of spinal cord and descend as lateral corticospinal tract. d. termination- Most of these fibres terminate by synapsing through the internuncial neurons at the anterior horn cells e. functions(i) The cerebral cortex controls voluntary movements of opposite half of body through anterior hom cells. (ii) Influence of corticospinal tract is supposed to be facilitatory for flexors and inhibitory for extensors. 3. Dorsal Tegmental Decussationa. situation – upper part of Tegmentum (mid brain) b. source - fibres arise from superior colliculus and cross to opposite side to form dorsal tegmental decussation c. fate of fibres after decussations – the fibres after crossing descend as Tectospinal Tract through the pons and medulla into the anterior white column of the spinal cord. d. termination - The fibres terminate by synapsing with ventral (anterior) horn cells in cervical segments of the cord, through internuncial neurons located in laminae VI to VIII of the spinal grey matter. e. function- control the voluntary movements of skeletal muscles of the body through anterior horn cells. 4. Ventral Tegmental Decussationa. situation – upper part of Tegmentum (mid brain) b. source - fibres arise from red nucleus and these fibres cross to the opposite side in the tegmentum of midbrain; constituting theventral tegmental decussation.
c. fate of fibres after decussations – the fibres after crossing descend as Rubrospinal Tract through the pons and medulla into the lateral white column of the spinal cord. d. termination - The fibres terminate by synapsing through internuncial neurons with anterior horn cells. e. function- The tract is believed to be facilitatory to flexors and inhibitory for extensors..
