Spinal cord
Spinal Cord
Features, internal structure, gray matter, white matter, spinal nerves, ascending and descending tracts, and applied physiology.
Notes
The spinal cord is a part of the central nervous system and is connected with the brain.
It extends from the foramen magnum to the lower border of the first lumbar vertebra in adults.
The spinal cord gives rise to spinal nerves and contains ascending and descending tracts connecting the spinal cord with the brain.
Major Areas Covered
- Features of spinal cord
- Internal structure of spinal cord
- Gray matter
- White matter
- Spinal nerve roots
- Spinal nerves
- Ascending tracts
- Descending tracts
- Effects of spinal cord lesions
- Spinal shock
- Hemisection of spinal cord
- Diseases of spinal cord
Features of Spinal Cord
Situation and Extent
Spinal cord is a part of central nervous system, other part being the brain.
It extends from the foramen magnum where it is continuous with medulla oblongata of brain, and lower limit is first lumbar vertebra in adults.
Coverings of Spinal Cord
Spinal cord is covered by membranous sheaths called meninges, which include:
- Dura mater
- Arachnoid mater
- Pia mater
Meninges are responsible for protection and nourishment of the nervous tissues.
Shape and Length
Spinal cord is cylindrical in shape.
It has a length of about 45 cm in males and 43 cm in females.
Enlargements of Spinal Cord
Spinal cord has two spindle-shaped swellings called cervical enlargement and lumbar enlargement.
Cervical enlargement supplies upper extremity and lumbar enlargement supplies lower extremity.
Conus Medullaris and Filum Terminale
Spinal cord becomes narrow rapidly below the lumbar enlargement and forms a cone-shaped termination called conus medullaris.
A slender non-neuron filament called filum terminale extends from conus medullaris downwards and is formed of sensory and motor roots.
Fissure and Sulci
A deep narrow cleft in the anterior median region is called anterior median fissure.
Depth of this fissure is about 3 mm.
Lateral to the anterior median fissure on either side, there is a slight depression called anterolateral sulcus.
It denotes the point of anterior nerve root exit.
A depression called posterior median sulcus is present on the posterior surface of spinal cord.
This sulcus is continuous with a thin glial septum called posterior median septum, which extends into the spinal cord for about 5 mm and reaches the gray matter.
On either side, lateral to posterior median sulcus, there is a posterior intermediate septum which continues with the posterior intermediate sulcus.
It extends about 3 mm into the spinal cord.
Segments of Spinal Cord
Spinal cord is made up of 31 segments, which are listed below:
| Segment | Number | Spinal Nerves |
|---|---|---|
| Cervical | 8 | Cervical spinal nerves |
| Thoracic | 12 | Thoracic spinal nerves |
| Lumbar | 5 | Lumbar spinal nerves |
| Sacral | 5 | Sacral spinal nerves |
| Coccygeal | 1 | Coccygeal spinal nerve |
Spinal Nerves
Spinal nerves are mixed nerves consisting of both motor and sensory fibers which carry afferent and efferent impulses.
Spinal Nerve Roots
Each spinal nerve is formed by a posterior (dorsal) root and an anterior (ventral) root.
Posterior or Dorsal Root
Posterior or dorsal root is formed by afferent (sensory) nerve fibers.
It contains the posterior root ganglion, which is formed by the soma of sensory neurons.
Anterior or Ventral Root
Anterior or ventral root is formed by efferent (motor) nerve fibers.
Both the nerve roots on either side leave the spinal cord and pass through corresponding intervertebral foramina.
Intervertebral Foramina
Cervical and thoracic roots are shorter, whereas lumbar and sacral roots are longer.
Longer roots are known to reach the corresponding intervertebral foramina.
This bundle of descending roots surrounding the filum terminale resembles the hair of a horse and is called cauda equina.
Internal Structure of Spinal Cord
Neural substance of spinal cord is divided into gray matter and white matter.
Gray Matter
Gray matter forms the central part of spinal cord.
White Matter
White matter surrounds the gray matter.
Gray Matter
Gray matter has a butterfly-shaped appearance in transverse section.
It resembles the letter H.
In the center of gray matter there is a small canal called central canal.
Gray Matter Divisions
- Anterior gray horn
- Posterior gray horn
- Lateral gray horn
Gray Commissure
Gray matter on either side of the spinal cord is connected by gray commissure.
White Commissure
White matter on either side is connected by white commissure.
White Matter Divisions
White matter is divided into three columns:
- Anterior white column
- Lateral white column
- Posterior white column
Gray Matter
Gray matter is formed by the cell bodies of neurons, dendrites and neuroglia.
Neurons in Gray Matter
Neurons in gray matter are classified according to the length of their axons into:
- Golgi type I neurons
- Golgi type II neurons
Nuclei in Anterior Gray Horn
Anterior gray horn contains neurons involved in motor function.
Motor neurons are of three types:
- Alpha motor neurons
- Gamma motor neurons
- Renshaw cells
Nuclei in Lateral Gray Horn
Lateral gray horn has an intermediolateral nucleus.
Neurons of this nucleus give rise to sympathetic preganglionic fibers which leave the spinal cord through anterior roots.
Intermediolateral nucleus is present from T1 to L2 spinal cord segments.
Nuclei in Posterior Gray Horn
Posterior gray horn contains nuclei of sensory neurons which receive impulses from various receptors.
- Marginal nucleus
- Substantia gelatinosa of Rolando
- Chief sensory nucleus
- Clarke’s nucleus
Laminae of Gray Matter
| Region | Laminae | Nucleus / Neurons |
|---|---|---|
| Posterior gray horn | Lamina I | Marginal nucleus |
| Posterior gray horn | Lamina II | Substantia gelatinosa of Rolando |
| Posterior gray horn | Laminae III, IV and V | Chief sensory nucleus |
| Posterior gray horn | Lamina VII | Dorsal nucleus of Clarke |
| Lateral gray horn | Lamina VII | Intermediolateral nucleus |
| Anterior gray horn | Lamina VIII | Motor neurons |
| Anterior gray horn | Lamina IX | Alpha motor neurons, gamma motor neurons and Renshaw cells |
| Around central canal | Lamina X | Neuroglia |
White Matter
White matter surrounds the gray matter.
It is formed by bundles of nerve fibers.
White matter is divided into three columns:
- Anterior white column
- Lateral white column
- Posterior white column
Tracts in Spinal Cord
Tracts are collections of nerve fibers passing through the spinal cord.
Spinal tracts are divided into:
- Short tracts
- Long tracts
Short Tracts
Short tracts connect different parts of the spinal cord.
They are of two types:
- Association or intrinsic tracts which connect adjacent segments of spinal cord on the same side.
- Intersegmental tracts which connect opposite halves of the spinal cord.
Long Tracts
Long tracts connect spinal cord with other parts of the central nervous system.
They are divided into:
- Ascending tracts
- Descending tracts
Ascending Tracts
Ascending tracts of spinal cord carry impulses of sensory function to brain.
Ascending tracts are formed by two or three groups of neurons:
- First order neurons
- Second order neurons
- Third order neurons
First Order Neurons
First order neurons receive sensory impulses from receptors and send them to sensory neurons.
Their cell bodies are situated in the posterior root ganglion.
Second Order Neurons
Second order neurons are sensory neurons present in the posterior gray horn.
Fibers from second order neurons form ascending tracts of spinal cord and terminate in different parts of the brain.
Third Order Neurons
Third order neurons are present in subcortical areas.
Their fibers carry sensory impulses from subcortical areas to cerebral cortex.
Major Ascending Tracts
| Tract | Origin | Course | Termination | Function |
|---|---|---|---|---|
| Anterior spinothalamic | Chief sensory nucleus | Crossing in spinal cord and ascending through spinothalamic tract | Ventral posterolateral nucleus of thalamus | Crude touch sensation |
| Lateral spinothalamic | Substantia gelatinosa of Rolando | Crossing in spinal cord and ascending through spinothalamic tract | Ventral posterolateral nucleus of thalamus | Pain and temperature sensations |
| Ventral spinocerebellar | Marginal nucleus | Crossing in spinal cord | Anterior lobe of cerebellum | Subconscious kinesthetic sensation |
| Dorsal spinocerebellar | Clarke nucleus | Uncrossed fibers | Anterior lobe of cerebellum | Subconscious kinesthetic sensation |
| Spinotectal | Chief sensory nucleus | Crossing in spinal cord | Superior colliculus | Spinovisual reflex |
| Fasciculus dorsolateralis | Posterior nerve root ganglion | Component of lateral white column | Substantia gelatinosa | Pain sensation and temperature sensation |
| Spinoreticular | Intermediomedial nucleus | Crossed and uncrossed fibers | Reticular formation of brainstem | Consciousness and awareness |
| Spino-olivary | Non-specific | Uncrossed fibers | Olivary nucleus | Proprioception |
| Spinovestibular | Non-specific | Crossed and uncrossed fibers | Lateral vestibular nucleus | Proprioception |
| Fasciculus gracilis | Posterior root ganglia | Uncrossed fibers | Nucleus gracilis in medulla | Tactile sensation, tactile localization, tactile discrimination, vibration sensation, conscious kinesthetic sensation and stereognosis |
| Fasciculus cuneatus | Posterior root ganglia | Uncrossed fibers | Nucleus cuneatus in medulla | Tactile sensation, tactile localization, tactile discrimination, vibration sensation, conscious kinesthetic sensation and stereognosis |
| Comma tract of Schultze | Posterior root ganglia | Short descending branches of fasciculus gracilis and fasciculus cuneatus | Cervical and thoracic segments of spinal cord | Establishment of intersegmental communications and formation of short reflex arc |
Dorsal Spinocerebellar Tract
Dorsal spinocerebellar tract is otherwise called Flechsig’s tract.
Origin
Fibers arise from Clarke’s nucleus in the posterior gray matter.
Clarke’s nucleus is present in the upper lumbar segments.
Course
The tract is formed by uncrossed fibers.
Axons from Clarke’s nucleus run to the lateral column of the same side and reach the medulla oblongata.
From there, fibers reach the cerebellum through the inferior cerebellar peduncle.
Termination
Fibers terminate in the cortex of anterior lobe of cerebellum.
Function
Along with the ventral spinocerebellar tract, dorsal spinocerebellar tract carries impulses of subconscious kinesthetic sensation (non-sensory impulses).
Effect of Lesion
Unilateral lesion leads to loss of subconscious kinesthetic sensation on the same side.
Ventral Spinocerebellar Tract
Ventral spinocerebellar tract is formed by fibers of second order neurons of the gray matter.
Origin
Fibers arise from marginal nucleus.
The marginal nucleus is present in lower lumbar and sacral segments.
Course
Fibers cross in the spinal cord and ascend in the lateral white column of the opposite side.
They reach the superior cerebellar peduncle.
Some fibers cross again and terminate in the cerebellum.
Termination
Fibers terminate in the anterior lobe of cerebellum.
Function
Ventral spinocerebellar tract carries impulses of subconscious kinesthetic sensation.
Effect of Lesion
Bilateral lesion causes loss of subconscious kinesthetic sensation on both sides.
Spinotectal Tract
Spinotectal tract is considered a component of spinothalamic tract.
It is constituted by fibers of second order neurons.
Fibers cross in the spinal cord and ascend in the lateral white column.
After taking their fibers to opposite lateral column, they pass through the brainstem and terminate in the superior colliculus.
This tract is concerned with spinovisual reflex.
Fasciculus Dorsolateralis
Fasciculus dorsolateralis is otherwise called tract of Lissauer.
It is a component of the fibers of the posterior root ganglion.
It is situated in the lateral white column.
It consists of fibers of the first order neurons.
It is concerned with transmission of pain and temperature sensations.
Spinoreticular Tract
Spinoreticular tract is formed by fibers of second order neurons situated in the posterior gray matter.
Fibers ascend in the lateral white column and terminate in the reticular formation of brainstem.
Some fibers cross and some remain uncrossed.
Fibers from this tract are concerned with consciousness and awareness.
Descending Tracts
Descending tracts of spinal cord are formed by motor fibers and carry impulses from brain to spinal cord.
Descending tracts carry motor impulses to brain and spinal cord.
Pyramidal Tracts
Pyramidal tracts are concerned with voluntary movements.
Pyramidal tracts are the most important tracts involved in voluntary motor control.
Major Pyramidal Tracts
- Anterior corticospinal tract
- Lateral corticospinal tract
Anterior Corticospinal Tract
Origin
Fibers originate from the primary motor area, premotor area and supplementary motor areas.
Course
Fibers descend through the internal capsule and brainstem.
About 20% of fibers do not cross to the opposite side but descend through the anterior white column of spinal cord.
These fibers cross to the opposite side at the level of the spinal cord and terminate in the anterior gray matter.
Termination
Fibers terminate in motor neurons of the anterior gray matter.
Function
Pyramidal tracts are concerned with skilled voluntary movements of the body.
Lateral Corticospinal Tract
The lateral corticospinal tract contains fibers that cross at the pyramidal decussation.
It is the major descending motor tract.
Extrapyramidal Tracts
Descending tracts other than pyramidal tracts are called extrapyramidal tracts.
These tracts are concerned with involuntary movements.
| Tract | Origin | Course | Function |
|---|---|---|---|
| Medial longitudinal fasciculus | Brainstem | Uncrossed fibers; extends up to upper cervical segments | Coordination of reflex movements and integration of movements of eyes and neck |
| Anterior vestibulospinal tract | Medial vestibular nucleus | Uncrossed fibers; extends up to upper thoracic segments | Maintenance of position of head and body during acceleration |
| Lateral vestibulospinal tract | Lateral vestibular nucleus | Mostly uncrossed; extends to all segments | Maintenance of position of head and body during acceleration |
| Reticulospinal tract | Reticular formation of pons and medulla | Mostly uncrossed | Coordination of voluntary and reflex movements, control of muscle tone and control of diameter of blood vessels |
| Tectospinal tract | Superior colliculus | Crossed fibers; extends up to lower cervical segments | Control of movement of head in response to visual and auditory impulses |
| Rubrospinal tract | Red nucleus | Crossed fibers; extends up to thoracic segments | Facilitatory influence on flexor muscle tone |
| Olivospinal tract | Inferior olivary nucleus | Mostly uncrossed; extends to neck region | Control of movements due to proprioception |
Medial Longitudinal Fasciculus
Medial longitudinal fasciculus descends through the posterior part of anterior white column of spinal cord.
It is formed by fibers from brainstem.
Most fibers descend on the same side.
It extends up to upper cervical segments.
It helps in coordination of neck movements and integration of eye and neck movements.
Anterior Vestibulospinal Tract
Anterior vestibulospinal tract is situated in the anterior white column of spinal cord.
Fibers arise from the medial vestibular nucleus of medulla oblongata.
Fibers descend through the anterior white column.
Most fibers are uncrossed and terminate in anterior motor neurons.
The tract helps in adjustment of position of head and body during acceleration.
Lateral Vestibulospinal Tract
Lateral vestibulospinal tract is situated in the lateral white column.
Fibers arise from the lateral vestibular nucleus.
Fibers descend throughout the spinal cord.
They terminate in anterior motor neurons directly or through interneurons.
It helps in adjustment of position of head and body during acceleration.
Reticulospinal Tract
Reticulospinal tract is situated in the anterior white column of spinal cord.
Fibers arise from reticular formation of pons and medulla.
Pontine reticular fibers are uncrossed and medullary reticular fibers are mostly crossed.
It is concerned with control of movement, muscle tone, respiration and diameter of blood vessels.
Pontine and medullary fibers have opposite effects on these functions.
Rubrospinal Tract
Rubrospinal tract is situated in the lateral white column.
Fibers arise from the large cells of the red nucleus in the midbrain.
Fibers cross the midline and descend into the spinal cord through the lateral white column.
Fibers terminate in anterior motor neurons.
It exerts a facilitatory influence on flexor muscle tone.
Tectospinal Tract
Tectospinal tract is situated in the anterior white column.
Fibers arise from the superior colliculus.
Fibers cross the midline and descend through the anterior white column.
Fibers terminate in anterior motor neurons.
It is responsible for movement of the head in response to visual and auditory stimuli.
Olivospinal Tract
Olivospinal tract is situated in the lateral white column.
Fibers arise from the inferior olivary nucleus.
Fibers are mostly uncrossed.
Fibers terminate in anterior motor neurons.
It is involved in reflex movements arising from proprioception.
Effects of Lesions of Spinal Cord
Lesion of Ascending Tracts
Lesion of ascending tracts produces loss of sensations carried by the affected tract.
Lesion of Descending Tracts
Lesion of descending tracts causes loss of voluntary movements, changes in muscle tone and reflex activity.
Changes in Complete Transection
Complete transection of spinal cord causes changes below the level of lesion.
| Situation | Sensory Changes | Motor Changes |
|---|---|---|
| At level of lesion | Complete anesthesia | Lower motor neuron changes: loss of muscle tone, flaccid paralysis, loss of reflexes, wastage of muscles and loss of vasomotor activity |
| Below level of lesion | Loss of fine touch, tactile localization, tactile discrimination, vibration sense, conscious kinesthetic sensation and stereognosis | Upper motor neuron changes: muscle tone increases, spasticity, exaggeration of reflexes, extensor plantar response, Babinski sign and loss of superficial reflexes |
Spinal Shock
Spinal shock is the condition in which there is temporary loss of all reflex activity below the level of complete spinal cord transection.
Stages of Spinal Shock
- Stage of complete loss of reflex activity
- Stage of recovery of reflex activity
- Stage of reflex failure
Stage of Complete Loss of Reflex Activity
Immediately after injury, complete loss of reflex activity occurs.
Common manifestations include:
- Loss of muscle tone
- Loss of reflexes
- Loss of vasomotor activity
- Loss of sweating
- Loss of bladder and bowel control
Stage of Recovery of Reflex Activity
After about 3 weeks, some reflex activity begins to recover.
Initially, the reflexes are very slow and become progressively exaggerated.
Flexor reflexes return first and later extensor reflexes appear.
Stage of Reflex Failure
Reflex activity may become depressed again in the later stage.
Hemisection of Spinal Cord
Hemisection of spinal cord is the condition in which one half of the spinal cord is injured.
Changes at the Level of Lesion
Sensory Changes
- Complete anesthesia
Motor Changes
- Lower motor neuron changes
- Loss of muscle tone
- Flaccid paralysis
- Loss of reflexes
- Wastage of muscles
- Loss of vasomotor activity
Changes Below the Level of Lesion
Changes on the Same Side
Sensations carried by the uncrossed tracts are lost.
- Fine touch
- Tactile localization
- Tactile discrimination
- Vibration sense
- Conscious kinesthetic sensation
- Stereognosis
Motor changes are due to involvement of upper motor neurons.
- Muscle tone increases
- Spasticity
- Exaggeration of reflexes
- Extensor plantar response
- Babinski sign
- Loss of superficial reflexes
Changes on the Opposite Side
Sensations carried by crossed tracts are lost.
- Crude touch
- Pain
- Temperature
There is no paralysis below the level of lesion if the motor tract is not involved on the opposite side.
Brown-Séquard Syndrome
Hemisection of spinal cord produces the characteristic combination of sensory and motor changes known as Brown-Séquard syndrome.
Applied Physiology
Causes of Dysfunction of Spinal Cord
- Direct injury due to bullet firing or accidents
- Complication caused by pressure from a tumor
- Ischemia due to rupture of spinal arteries
- Degeneration of spinal cord
Conditions When Dysfunction Occurs
- Complete transection
- Incomplete transection
- Hemisection
- Diseases of spinal cord
Complete Transection of Spinal Cord
Complete transection may occur due to:
- Bullet injury
- Accidents causing damage of spinal cord
- Occlusion of blood vessels
- Complete spinal cord transection
Stages
- Stage of spinal shock
- Stage of reflex activity
- Stage of reflex failure
Diseases of Spinal Cord
1. Syringomyelia
Syringomyelia is a spinal cord disorder characterized by the presence of fluid-filled cavities in spinal cord.
It usually occurs due to a congenital abnormality and accumulation of fluid.
Characteristic features are loss of sensation, particularly temperature and pain sensations.
Severity of loss of sensations depends on the extension of disease in spinal cord.
2. Amyotrophic Lateral Sclerosis
Amyotrophic lateral sclerosis (ALS) is a slowly progressive nervous disorder.
It occurs due to degeneration of posterior motor neurons and involves both motor and sensory functions.
3. Multiple Sclerosis
Multiple sclerosis (MS) is a chronic and progressive inflammatory disease characterized by demyelination in brain and spinal cord.
It affects the myelinated nerve fibers of brain and spinal cord.
It is associated with gradual destruction of myelin sheath (demyelination).
There is impaired transmission of impulses through brain and spinal cord.
The symptoms refer to scars in myelin sheath.
It is associated with weakness and disturbances in maintenance of posture and double vision.
Blindness, fatigue, depression, symptoms including difficulty in performing day-to-day activities, speech difficulty, dizziness, muscle spasm and paralysis may occur.
4. Disk Prolapse
Intervertebral or spinal disk is a cartilaginous structure of vertebral column that separates each vertebra.
It is part of a tough outer fibrous layer and a soft inner part.
Disk prolapse is the rupture of the spinal disk.
During disk prolapse, the soft inner material bulges out.
Bulging material may irritate or compress the nerve root that passes through the intervertebral foramen.
Severity of the condition depends upon the degree of bulging.
Symptoms include pain and weakness in the affected region.
Spinal Cord Disorders
Disorders of the spinal cord may affect sensory, motor and autonomic functions depending on the site and extent of involvement.
Important Spinal Tracts
Ascending Tracts
- Anterior spinothalamic tract
- Lateral spinothalamic tract
- Ventral spinocerebellar tract
- Dorsal spinocerebellar tract
- Spinotectal tract
- Fasciculus dorsolateralis
- Spinoreticular tract
- Spino-olivary tract
- Spinovestibular tract
- Fasciculus gracilis
- Fasciculus cuneatus
- Comma tract of Schultze
Descending Tracts
- Anterior corticospinal tract
- Lateral corticospinal tract
- Medial longitudinal fasciculus
- Anterior vestibulospinal tract
- Lateral vestibulospinal tract
- Reticulospinal tract
- Tectospinal tract
- Rubrospinal tract
- Olivospinal tract