Human Spinal Cord: Structure and Effects (With Diagram)

The human spinal cord is made up 31 segments. From each of these segments, a pair of spinal nerves takes origin.

Hence there are 31 pairs of spinal nerves. Of 31 pairs:

i. 8 belong to cervical segments

ii. 12 belong to thoracic segments

iii. 5 belong to lumbar segments

iv. 5 belong to sacral segments

v. 1 belongs to coccygeal segment.

In a transverse section of spinal cord, in the central part will be H-shaped gray matter and this will be surrounded by white matter. The gray matter and white matter on either side have continuity through commissures (Fig. 9.14).

In the gray matter, nerve cell bodies are present whereas the white matter is composed of compactly packed nerve fibers in the form of tracts. The tracts which carry impulses from peripheral parts of body towards higher parts of CNS through spinal cord are called ascending tracts. Likewise, tracts which carry information from higher parts of CNS to motor neurons present in spinal cord are called descending tract (Fig. 9.15).

Structural and functional integrity of both gray matter and white matter areas is essential for normal functioning of nervous system. The cell bodies in gray matter area and tracts in white matter are both present in a bilaterally symmetrical way in spinal cord.

Ascending Tracts:

i. Carry information from spinal cord to higher parts of CNS.

ii. They are essential for sensory perception as impulses should reach brain from peripheral parts of body.

iii. Most of these tracts end in sensory areas of cerebral cortex.

iv. Some of the important ascending tracts are dorsal column tracts, lateral spinothalamic tract, anterior/ventral spin thalamic tract.

Dorsal Column Tract or Posterior Column Tract or Tract of Goll and Burdach or Fascicules Gracilis and Cuneatus:

Sensations carried by these tracts are:

1. Fine touch

2. Tactile localization

3. Tactile discrimination (2 point discrimination)

4. Pressure sensation

5. Vibratory sensation

6. Proprioception or sense of position and joint movement and is also known as kinesthetic sensation.

7. Stereognosis.

The above sensations from peripheral parts of body are carried by posterior column tracts to cerebral cortex. Nerve fibers carrying these sensations are A beta fibers.

Receptors involved will be:

a. Merkel’s disk

b. Meissner’s corpuscle

c. Pacinian corpuscle

d. Ruffini’s end organ

e. Receptors in and around joints

Touch sensation has dual pathways. Fine touch sensation is carried by posterior column tracts and crude touch sensation is carried by anterior or ventral spinothalamic tract.

From the receptors, A beta fibers carry impulse. When posterior nerve fibers reach spinal cord, the nerve trunk of posterior spinal nerve separates into two divisions, namely medial and lateral divisions between dorsal root ganglion and spinal cord. The fibers going to contribute for formation of posterior column tracts enter spinal cord through medial division.

These fibers reach posterior funiculus of spinal cord and ascend up on same side of spinal cord. In the brainstem, at the level of medulla oblongata, these fibers synapse in two different nuclei, namely gracile and cuneate nuclei. First order neurons are posterior root ganglion cells.

The second order fibers take origin from the nuclei of gracile and cuneate and cross midline and to reach opposite side. These fibers that cross are known as internal arcuate fibers (80%). Internal arcuate fibers will be contributing for sensory decussation. Approximately, 20% of fibers which takes origin from gracile and cuneate nuclei, do not cross midline and these are known as external arcuate fibers.

They carry impulses to cerbellum of same side by passing through inferior cerebellar peduncle. The internal arcuate fibers as they ascend up further to form medial lemniscus after crossing. Fibers of medial lemniscus synapse in ventroposterolateral nucleus present in thalamus. Neurons extending from gracile and cuneate nuclei to thalamus are called as second order neurons.

Fibers which take origin from ventroposterolateral nuclei pass through posterior limb of internal capsule to end in sensory area (3, 1, 2) of cerebral cortex (primary somesthetic area) present in postcentral gyrus. These fibers which end in cerebral cortex are known as thalamic/sensory radiation fibers and constitute third order neurons (Fig. 9.16).

Sensory Homunculus:

In uncrossed tract, sacral segmental fibers will be medial most and cervical fibers will be lateral most. Knowledge of topographical arrangement afferent fibers in different ascending tracts is essential in certain spinal cord lesions especially when lesion is due to degeneration of tissue around spinal canal (syringomyelia).

Stereognosis:

It is ability of person to identify some familiar/known objects even with closed eyes. The impulses for this sensation will be carried by posterior column tracts.

The person is able to identify object based on:

i. Shape

ii. Size

iii. Texture of object

Fine touch, tactile localization, and tactile discrimination, etc. from face.

Fibers carrying sensations from receptor belong to 5th cranial nerve (carry sensory fibers from face). These fibers reach Gasserian ganglion. From ganglion, fibers enter pons to synapse in chief sensory nucleus. From chief sensory nucleus, 2nd order fibers take origin and cross the midline to reach the opposite side. These fibers ascend upward as trigeminal lemniscus. They synapse in ventroposteromedial nucleus of thalamus.

From ventroposteromedial nucleus, 3rd order fibers originate. They end in lateral most part of cerebral cortex area number 3,1, and 2 present in post-central gyrus where face is represented (Fig. 9.19).

Effects of Hemisection of Spinal Cord (when only one-half of spinal cord is sectioned) (Fig. 9.36):

Features of effects of hemisection of spinal cord have been tabulated in Table 9.3.

Dissociated anesthesia:

It is a condition in which in a particular part of body, certain sensations are retained, and certain sensations are lost.

Dissociated anesthesia is seen in:

a. Hemisection of spinal cord below the level of lesion

b. Syringomyelia—pain and temperature is lost bilaterally but other sensations are retained in the affected parts of the body.

c. Tabes dorsalis—postural sensations are lost but pain and temperature is retained in the affected parts of the body.

Effect of Complete Transection of Spinal Cord:

It may occur due to accidental injury (motor vehicle, sports accidents), gunshots wound, stab injury to spinal cord. In these types of injuries, spinal cord will be cut into two different parts. Signs and symptoms are seen below level of lesion in both halves of body.

Effect on Skeletal Muscles:

i. When spinal cord is transected completely (for example at the level of T6 that is mid-thoracic level) part of body supplied below spinal segment T6 is affected. There will be complete paralysis of skeletal muscles in lower parts of body.

So no voluntary movements are possible in both lower limbs. Person suffers from paraplegia. Paraplegia results when there is transaction of spinal cord occurs anywhere above lumbosacral plexus origin but below the level of brachial plexus.

ii. If lesion is at level of C6 or C7, upper and lower limbs get paralyzed and hence result in quadriplegia (paralysis of all four limbs).

Effect on Blood Pressure [BP]:

i. If lesion is at T6 or anywhere between T6 and sacral segments, fall of blood pressure limited. Fall of blood pressure is not much since sympathetic nerves function above the level of section will be normal.

ii. If lesion is above T1, there will be severe fall of blood pressure since all lateral horn cells are separated from vasomotor center which bring facilitatory impulses to lateral horn cells (sympathetic fibers take origin from LHC of thoracolumbar segments).

There will be complete loss of vasomotor tone.

Effect on Respiration:

i. If lesion is above C3, it leads to instantaneous death due to paralysis of all muscles of respiration.

ii. If lesion is below C5, diaphragmatic respiration continues, only intercostal muscles get paralyzed. (Intercostal muscles are supplied by nerve coming from T1 to T10 segments of spinal cord.)

Consequences of Complete Transaction of Spinal Cord:

1. Stage of spinal shock

2. Stage of recovery of reflex activity

3. Stage of reflex failure.

Spinal Shock:

i. Lasts for about 1-6 weeks in human beings.

ii. Duration of spinal shock is directly related to hierarchy/evolution of the animal. In frogs, spinal shock lasts for few minutes only (1-2 min).

Features observed during stage of spinal shock:

Signs and symptoms are seen below level of lesion in either side of body.

1. Loss of all sensations in affected parts of body as all ascending tracts are damaged.

2. Motor neurons which are required for motor activities in muscles are affected. Therefore, there will be loss of all voluntary movements in affected parts of body giving rise to flaccid type of paralysis.

3. Reticulospinal tract is damaged, so no excitatory influence to LHC in thoracolumbar segments. This brings about loss of vasoconstrictor influence on smooth muscles of blood vessels. Arterioles get dilated and blood pressure falls (especially diastolic blood pressure).

Extent of fall of blood pressure depends on level of transection. In addition to vasodilatation, venous return will also get decreased due to dilation of veins (loss of venomotor tone). Pooling of blood in venous system occurs. Decreased venous return affects cardiac output and contributes to fall in systolic blood pressure.

4. There will be complete loss of muscle tone due to no impulses from motor neurons to the muscles.

5. There will be loss of all types of reflexes (superficial, deep and visceral) even though reflex arc is intact. It is because motor neurons of spinal cord are under constant excitatory influence from higher parts of CNS.

When supraspinal facilitatory influences are suddenly withdrawn, motor neurons of spinal cord undergo functional depression. Reflexes will be lost completely resulting in a state of areflexia. Functional depression of motor neuron is known as diaschiasis.

6. Position of lower limbs will be determined by gravity and the part of body suffers from stagnant hypoxia and cyanosis occurs.

7. When lesion is above T6 segment, all visceral sensations are also lost. Reflexes, like micturition, defecation, etc., will be lost during this stage of spinal shock. But some amount of urine may dribble out at times. This occurs because of over flow incontinence.

8. Bedsores and ulcers: Fall of blood pressure and pressure contact of part of body with area of support, brings about compression of blood vessels. This leads to decreased blood flow to tissue. Tissues suffer from stagnant hypoxia and this result in atrophy of tissues resulting in bedsores, loss of secretions, etc.

When a patient is in a state of spinal shock, if proper care is not taken, he may not go to 2nd stage that is stage of recovery of reflex activity. Hence management of patient during stage of spinal shock is crucial.

Managing of patient during stage of spinal shock period:

1. When there is accumulation of urine in urinary bladder it may lead to infection. Therefore, bladder has to be catheterized to drain urine continuously. This prevents over distension of bladder and minimizes infection.

2. Administration of rectal enema to remove feces.

3. Bedsore development should be minimized not only to prevent tissue damage and bleeding, but also to prevent any infection. Infections of urinary tract and lungs are prevented by suitable anti­biotics.

4. Certain drugs, like amphetamines, which have ability to stimulate motor neurons of spinal cord may be given.