spinal cord anatomy 2

Spinal Cord Anatomy

lecture 2

Abbas A. A. Shawka

Medical student

2nd grade

Subjects

• To list the major tracts in the spinal cord.

• To estimate the origin and destiny of these tracts.

• To relate the functions of these tracts to some of the clinical correlate concerning these tracts.

Cross section gross features• Spinal cord consist of central geay

mater ( H-shaped ) and peripheral white mater.

1. Anterior median fissure

2. Posterior median sulcus

3. Grey commissure

4. White commissure

5. Anterior horn

6. Lateral horn

7. Posterior horn

8. Central canal

9. Anterior white column

10. Lateral white column

11. Posterior white column

*The posterior white columns are

largest in the cervical region, and

the anterior grey horns largest in

the cervical and lumbar regions.

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posterior white column

Tract Type Function

Gracile tract (1) Ascendingtouch and some associated sensations.Cuneate tract (2) Ascending

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lateral white column

Tract Type Function

Lateral corticospinal tract (1)

descendingprincipal motor pathway and the most importantof all cord tracts

Anteriorlateral( spinothalamic) tract (2)

Ascending the pathway for pain (nociception) and thermal sensations

Anterior and posterior spinocerebellar tract (3)

Ascending Maintain unconscious proprioceptive

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Anterior white columnTract Type Function

medial longitudinal fasciculus 1- tectospinal tract 2- medial vestibulospinal

tracts

descending - The function of the tectospinal tract is to mediate reflex postural movements of the head in response to visual and auditory stimuli.

- head coordination and eye movement.

anterior spinothalamic tract ( shared with lateral )

Ascending conduct information about pressure and crude touch (prothopatic)

lateral vestibulospinaltract

descending maintain upright and balanced posture.

Anterior corticospinaltracts

Descendng Unimportant motor pathway

- Destination for afferent fibers - 1st , 2nd and 3rd order neurons • There are three possible destinations

for all incoming fibres:

1. the cortex of the opposite cerebral hemisphere, via thalamic relay, for conscious sensation;

2. the cerebellum, for muscular coordination;

3. the brainstem or spinal cord, for reflex actions.

The cell bodies of the first group of neurons (first neurons or first order neurons) lie outside the central nervous system, in the posterior root ganglia of spinal nerves or the equivalent ganglia of cranial nerves.

The cell bodies of the second neurons are in the spinal cord or brainstem

those of the third neurons are in the thalamus.

Ascending tracts

• The most important ascending tracts … ( WILL BE STUDIED )

1. Anteriolateral tract ( spino-thalamic tract ) : locate in anterior and lateral white column.

2. Cunate tract : locate in the posterior white column

3. Gracile tract : locate in the posterior white column

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Anteriolateral ( spino-thalamic tract )

• Concerned with pain (nociception) and temperature sensations, crude touch, and the sensations of itch, tickle and sexual orgasm.

• lying in front of the level of attachment of the denticulate ligament and extending into the anterior white column.

• Although in the lowest part of the cord the fibres from any one segment cross transversely, the crossing takes place more and more obliquely at higher levels so that in the cervical region the decussation may require the height of four or five segments before it is completed.

Pain fibers

Dorsolateral tract( of lessiure )

gelatinous substance

Anteriolateraltract

Run up or down for 1 or 2 segments

Crossing anterior to central canal


• The track is laminated, with fibressubserving impulses from the sacral segments lying most laterally, while those from cervical segments are the most deeply placed.

• The fibres for crude touch occupy the more medial part of the tract, and have crossed less obliquely than the pain and temperature fibres.

Pain fibers

Dorsolateral tract( of lessiure )

gelatinous substance

Anterolateral tract

Run up or down for 1 or 2 segments

Crossing anterior to central canal


ThalamusReticular formation of

brainstem

Spinal lemniscus system

Anterolateral tract

Thalamus

Spino-reticulo-thalmic system

rubbing an injured part to reduce the pain !!• The extent to which the anterolateral

tract cells are stimulated to pass on impulses from spinal cord to brain is subject to control and modification by both spinal and supraspinal influences.

• For example, fibres from mechanoreceptors, apart from passing up in the posterior white columns, give branches that synapse with anterolateral tract cells, and their effect is to inhibit transmission from the tract cells (‘closing the gate’ to nociceptive impulses).

• Other inhibitory influences come from supraspinal levels via the reticular formation, so that cerebral and limbic activities such as memory, past experience and emotion can depress or even completely suppress pain,

anterolateral cordotomy

• The knife is inserted in front of the line of attachment of the denticulate ligament, so avoiding damage to the corticospinal tract which lies behind this.

• Because of the obliquity of the crossing of fibres, the cut must be made several segments above the desired level of analgesia.

• Although cordotomy should theoretically result in permanent pain relief, it rarely lasts for more than a few months (2 years at the most), an example of the plasticity of the nervous system and the way new pathways can develop.

• The procedure is rarely performed now on account of advances in analgaesictherapy.

Cuneate and gracile nucleus • The posterior white column is wholly

occupied by ascending fibres of the gracile and cuneate tracts.

• They are concerned with light (discriminative) touch, vibration sense, proprioception (muscle joint or kinaesthetic sense, the conscious appreciation of body position and movement), and the sense of fullness of the bladder and rectum.

• Their cell bodies lie in the posterior root ganglia of spinal nerves, and the fibres enter the cord from the medial side of the posterior nerve roots.

• Some branches from the incoming posterior root fibres are short and take part in segmental reflex arcs, but it is the long ascending fibres that form the tracts.

Cuneate and gracile nucleus • The fibres from the lowest parts of

the body lie nearest the midline, and incoming fibres are added progressively laterally ( laminated ) .

• In this way fibres from the perineum, lower limb and lower trunk form the gracile tract, and those from the upper trunk and upper limb form the more lateral cuneate tract

• The two tracts end in the lower part of the medulla by synapsing with the cells of the gracile and cuneate nuclei respectively.

• The axons from these (second order) nuclei immediately decussate to form the medial lemniscus which runs through the brainstem to the thalamus. After relay there, the axons of the third group of neurons pass via the internal capsule to cerebral cortex.

Cuneate & Gracile

Below T6 Above T6

Gracile cuneate

Spinocerebellar tract • Convey unconscious proprioceptive

information from cord to cerebellum.

• Some fibres from posterior root ganglion cells enter the cord through the posterior roots and synapse with the cells of the thoracic nucleus (Clarke’s column) at the base of the posterior horn in all the thoracic and first two lumbar segments of the cord. The axons of these cells move to the lateral edge of their own side of the cord as the posterior spinocerebellar tract, which runs up into the medulla to reach the cerebellum by the inferior peduncle

Posterior root

Thoracic nuclei

Lateral edge

Spinocerebellar tract

In base of posterior horn in all thoracic and first 2 lumba

Do NOT croos to the other side


• Other incoming fibres synapse with other posterior horn cells in lumbar and sacral segments; these give rise to fibres which mostly cross to the opposite side to form the anterior spinocerebellar tract, at the margin of the cord behind the anterior nerve roots.

• It takes an unexpectedly long route to the cerebellum, running through the brainstem to the midbrain and doubling back into the superior cerebellar peduncle of the opposite side.

• 2 CROSSING = the SAME SIDE !! = “ double crosser “ !!


• From cervical nerves, impulses destined for the cerebellum do not travel by these spinocerebellar tracts (since the thoracic nucleus does not extend above T1 level); they

• reach the accessory cuneate nucleus in the medulla by the

• cuneate tract, and thence by the cuneocerebellar tract

• enter the inferior peduncle.

Cervical roots

Cuneate tract

Cuneaocerebellartract

Inferior peduncle

Through cuneate nucleus

Corticospinal tract • The lateral corticospinal tract is

formed from the motor decussation in the lower medulla

• lies in the lateral white column at a level behind the attachment of the denticulate ligament. Almost all (98%) of the fibres end by synapsing with interneurons which in turn project to motor neurons of the anterior horn.

• (The 2% of fibres that synapse directly with motor neurons are those originating from the giant Betz cells.)

• About 55% of corticospinal fibres end on cervical and first thoracic anterior horn cells.

• Because of the medullary decussation, the cerebral cortex of one side controls the muscles of the contralateral side.

Corticospinal tract • The relatively unimportant uncrossed

fibres in the medulla continue as the anterior corticospinal tract, adjacent to the anterior median fissure, but it goes no lower than the upper thoracic part of the cord. The fibres eventually cross to the opposite anterior horn.

*Lateral corticospinal fibers synapse on ipsilateral anterior horn cells; anterior corticospinal fibers synapse on contralateral anterior horn cells

Reticulospinal tract• The lateral reticulospinal tract

arises from the medullary part of the reticular formation and the fibres run down in the lateral white column, largely intermingled with corticospinal fibres.

• Its influence on anterior horn cells is facilitatory.

• The medial reticulospinal tract comes from cells in the pontine reticular formation and descends in the anterior white column, to have an inhibitory action on motor neurons.

Reticulospinal tract• The lateral vestibulospinal tract

arises from the lateral vestibular nucleus of the medulla and runs down the cord approximately through the anterior nerve root region.

• A much smaller medial vestibulospinal tract descends from the medial vestibular nucleus to cervical segments of the cord.

• The reticulo- and vestibulospinalfibres synapse with interneurons which in turn project to the motor neurons.

• The lateral vestibulospinal tractprimarily affects trunk and limb girdle musculature, and is of great importance for posture and balance.

• the medial tract coordinates movement of the head.

Hypothalamospinal tract

• The hypothalamospinal tract provides a link between the hypothalamus and the thoracolumbar (sympathetic) and sacral (parasympathetic) lateral horn cells. The fibres lie adjacent to the lateral horn.

Thank you

spinal cord anatomy 2

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