spinal cord anatomy,physiology and disorders

INTRODUCTION

The spinal cord is the long cylindrical lower part of central nervous system . It is the main pathway for information connecting the brain and peripheral nervous system . It occupies upper two - thirds of vertebral canal and is enclosed in the three meninges . It gives rise to 31 pairs of spinal nerves and retains the basic structural pattern .

Spinal cord Features

The spinal cord is 18 inches or 45 cm in an adult male and 42 cm in adult female . The weight of spinal cord is 30 g . It is surrounded by the three meninges.

It extends from upper border of atlas vertebra to the lower border of first lumbar vertebra in an adult.

In children , it extends up to L3 vertebra . Superiorly , it is continuous with the medulla oblongata , inferiorly it terminates as conus medullaris.

As the spinal cord is much shorter than the length of the vertebral column , the spinal segments do not lie opposite the corresponding vertebrae . In estimating the position of a spinal segment in relation to the surface of the body.

it is important to remember that a vertebral spine is always lower than the corresponding spinal segment.

MENINGEL COVERINGS

The spinal cord is surrounded by three meninges . The outermost is the dura mater , the middle one is arachnoid mater and the innermost is the pia mater.

The space between dura mater and arachnoid mater is called subdural space . The arachnoid and pia maters are separated by subarachnoid space which contains cerebrospinal fluid .

The medical procedure known as a lumbar puncture or spinal tap involves use of a needle to withdraw cerebrospinal fluid from the subarachnoid space , usually from the lumbar region of bas Isqytigloo of the spine .

The spinal cord extends in the lower part of 1st lumbar vertebra as conus medullaris . Below the level of conus medullaris only pia mater is continued as a thin fibrous cord , the filum terminale .

The spinal cord is enclosed only by the meningeal layer of dura mater . The space between the meningeal layer and endosteum of the vertebral canal is called epidural space , where epidural anaesthesia can be given .

The epidural space is filled with adipose tissue and it contains a network of blood vessels .

The spinal pia mater undergoes modification as follows to keep the spinal cord in position during the movements of the vertebral column.

Ligamenta denticulata are 21 pairs of teeth - like projections. They fuse laterally with the arach noid and dura mater midway between the exits of the roots of adjacent spinal nerves. The highest process attaches immediately superior to foramen magnum. The ligamentum denticulatum keeps the spinal cord in position.
Linea splendens is a thickening seen at the antero median sulcus in the lower part of the spinal cord.
The filum terminale is 20 cm long and after leaving through sacral hiatus ends by getting attached to the periosteum of dorsal surface of first segment of coccyx . It consists of two parts :

Filum terminale internum , the upper part which is 15 cms long which extends up to lower border of second sacral vertebra.
Filum terminale externum , the lower part which is outside the dura mater and is attached to the first segment of the coccyx . Between the lower border of L1 and S2 vertebrae , the subarachnoid space contains spinal nerve roots which constitute the cauda equina . It is due to this feature that lumbar puncture is done below L2 vertebra without any danger to spinal cord .

The dura and arachnoid along with subarachnoid space containing CSF extend up to 2nd sacral vertebra.

ENLARGEMENT

Limbs form the appendages of the trunk . Their muscles have to be supplied by neurons of spinal cord . Neurons at appropriate levels form enlargements to be able to supply increased musculature . It presents

Cervical enlargement supply of upper limb muscles . This extends from C4 to T1 spinal . segments with maximum diameter of 38 mm at level of C6 segment .
Lumbar enlargement for supply of muscles of lower limb . It extends from level of L2 to S3 segments . Its maximum diameter is 35 mm at level of S1 segment .

CAUDE EQUINA

Dorsal and ventral nerve roots of right and left sides of L2 to L5 , S1 to S5 and Col nerves lie almost vertically around filum terminale. These are called cauda equina as these resemble a horse's tail .

Dorsal and ventral nerve roots of one segment join together at respective intervertebral foramen to exit as the spinal nerve . There are 40 nerve roots at the beginning of cauda equina . These are dorsal and ventral nerve roots of right and left sides for each segment . So each segment has 4 nerve roots .

Thus , there are 4 x 4 = 16 lumbar nerve roots ; 4 x 5 = 20 sacral nerve roots and 4 x 1 = 4 coccygeal nerve roots , making it to 40 nerve roots . One dorsal root and one ventral root joins to form one spinal nerve and leaves through the foramen on one side .

So at every intervertebral foramen 4 nerve roots exit the cauda equina ; leaving it thinner . In the end only filum terminale remains to be attached to the coccyx .

EXTERNAL FEATURES OF SPINAL CORD

Anteriorly , the spinal cord reveals a deep anterior median fissure lodging the anterior spinal artery. Posterior median sulcus is a thin longitudinal groove from which a septum runs in the depth of spinal cord .

Each half is subdivided into anterior , lateral and posterior regions by anterolateral and posterolateral sulci . Ventral or motor nerve roots emerge from the anterolateral sulcus . Dorsal or sensory nerve roots enter spinal cord from posterolateral sulcus.

INTERNAL STRUCTURE OF SPINAL CORD

White matter , i.e. nerve fibres lie outside and grey matter lies inside . In the centre of grey matter is the central canal containing . The canal is lined by single layer of ependymal cells .

The grey matter is in the form of " H " with a grey commissure joining the grey matter of right and left sides . Grey matter comprises one posterior horn and one anterior horn on each side in the entire extent of the cord . Only in T1 - L2 and S2 - S4 segments , there is an additional lateral horn for the supply of the viscera This horn is a part of autonomic nervous system .

The dorsal horn is found at all spinal cord levels and is comprised of sensory neuron that receive and process incoming somatosensory information . From there ascending projections emerge to transmit the sensory information to the diencephalon .

The ventral horn comprises motor neurons that innervate skeletal muscle . Nerve cells in the grey substance are multipolar varying much in their morphology . Many of them are Golgi type I and Golgi type II nerve cells .

The axons of Golgi type I are long and pass out of the grey matter into the ventral spinal roots or the fibre tracts of the white matter . The axons and dendrites of the Golgi type II cells are largely confined to the neighboring neurons in the grey matter .

CLINICAL ANATOMY OF SPINAL CORD

Conus medullaris syndrome : Due to injury to S2 , S3 , S4 segments of spinal cord . Features are :
- a . Anaesthesia in the perineum . The region is supplied by these three segments.
- b. Involvement of bladder and bowel is early S2 , S3 , S4 segments carry sacral component of the parasympathetic system which supplies the bladder and lower bowel .
- Sexual functions are affected as same nerves carry out sexual functions as well .
Cauda equina syndrome : Damage to cauda equina results in :
- a . Lower motor neuron type of paralysis in the lower limbs due to compression of ventral nerve roots.
- b . Root pain is an important symptom due to involvement of dorsal nerve roots .
- c. Bladder and bowel involvement is late .
Poliomyelitis : It is a viral disease which involves anterior horn cells leading to flaccid paralysis of the affected segments . It is a lower motor neuron paralysis . If poliomyelitis affects the upper cervical segments of spinal cord , it may be fatal because of the involvement of C4 segment which supplies the diaphragm through phrenic nerve .
Tabes dorsalis : It occurs during tertiary stage of syphilis . There is degenerative lesions of dorsal nerve roots and of posterior white columns . Its feature is severe pain in lower limbs , as the disease occurs in lower thoracic and lumbosacral segments . The lower limbs are mainly affected .

SPINAL NERVE

Spinal nerves arise in pairs. There are 31 pairs of spinal nerves as 8 cervical , 12 thoracic , 5 lumbar , 5 sacral and 1 coccygeal .

Each spinal nerve arises by a series of six to eight dorsal and ventral nerve rootlets . These rootlets unite in or near the intervertebral foramen to form the spinal nerve .

Dorsal Root Ganglion

As the dorsal rootlets converge , there is a swelling , the dorsal or posterior root ganglion , which houses the cell bodies of all the sensory neurons in that particular nerve . The neurons are pseudounipolar type .

Branches of a Typical Nerve

Dorsal Ramus

It supplies the dorsal one - third of the body wall . Dorsal rami do not supply the limbs.

Ventral Ramus

It supplies the ventral two - thirds of the body wall including the limbs.

BRANCHES OF SPINAL NERVES

After formation by dorsal and ventral roots , each spinal nerve divides into four branches.

1. Posterior or Dorsal Ramus

This ramus carries somatic motor fibers , somatic sensory fibers and visceral motor fibers . Posterior rami are smaller than anterior rami and supply the muscles , joints and skin of posterior part of the trunk.

2. Anterior or Ventral Ramus

Anterior ramus carries somatic motor fibers , somatic sensory fibers and visceral motor fibers . Anterior rami are larger than posterior rami and supply the muscles , joints and skin of anterior and lateral parts of trunk and upper and lower limbs.

Intercostal nerves

Intercostal nerves are formed by anterior rami of 2nd 12th thoracic spinal nerves . These nerves do not form a plexus but directly supply the structures in the intercostal in between ribs ) spaces.

3. Meningeal Branch

This branch carries somatic motor , somatic sensory and visceral motor fibers . All the meningeal branches arise from initial part of spinal nerves and run back through intervertebral foramen to supply spinal meninges , ligaments , dura , blood vessels , facet joints , intervertebral disks and periosteum of the vertebra.

3.Rami Communicantes

Rami communicantes ( singular = ramus communicans are the branches communicating between spinal nerves and sympathetic division of autonomic nervous system . There are two rami communicantes namely , gray ramus communicans and white ramus communicans . Gray rami communicantes carry fibers from sympathetic chain to all the 31 pairs of spinal nerves . White rami communicantes carry fibers from all thoracic and first two lumbar spinal nerves to sympathetic chain . Rami communicantes carry visceral motor and visceral sensory information to and from visceral organs .

SPINAL NERVE PLEXUSES

Spinal nerve plexus is a bundle of fibers from adjacent spinal nerves that forming a network . There are five plexuses which consist of both sensory and motor nerves fibers.

1. Cervical Plexus

Cervical plexus is formed by the anterior rami of 1st to 4th cervical spinal nerves with a small contribution from the 5th cervical nerve . It supplies head , neck , shoulder and chest.

2. Brachial Plexus

It is formed by anterior rami of 5th to 8th cervical and 1st thoracic nerves . It gives rise to five nerves , viz . muscu locutaneous nerve , axillary nerve , radial nerve , median nerve and ulnar nerve . It supplies hand , arm , chest and shoulder.

3. Lumbar Plexus

It is formed by anterior rami of 1st to 4th lumbar nerves . Femoral nerve and obturator nerve are the two impor tant nerves arising from this plexus . Lumbar plexus supplies anterolateral part of abdominal wall , external genitalia and part of lower limb.

4. Sacral Plexus

Sacral plexus is formed by anterior rami of 4th and 5th lumbar nerves and 1st to 4th sacral nerves . Important nerves arising from this plexus are sci atic nerve , pudendal nerve and gluteal nerves . Sacral plexus supplies buttocks , perineum and lower limb .

5. Coccygeal Plexus

It is formed by anterior rami of 4th and 5th sacral nerves and coccygeal nerve . It supplies coccygeal region .

INTERNAL STRUCTURE OF SPINAL CORD

Neural substance of spinal cord is divided into :

Inner gray matter.
Outer white matter.

GRAY MATTER OF SPINAL CORD

Gray matter of spinal cord is the collection of nerve cell bodies , dendrites and parts of axons It is placed central ly in the form of wings of the butterfly and it resembles the letter ' H ' . Exactly in the center of gray matter , there is a canal called the spinal canal.

Ventral and the dorsal portions of each lateral half of gray matter are called ventral ( anterior ) and dorsal posterior ) gray horns respectively .

In addition , the matter forms a small projection in between the anterior and posterior horns in all thoracic and first two lumbar segments . It is called the lateral gray horn .

Part of the gray matter anterior to central canal is called the anterior gray commissure and part of gray matter posterior to the central canal is called posterior gray commissure .

NEURONS IN GRAY MATTER OF SPINAL CORD

Gray matter contains two types of multipolar neurons

1. Golgi Type I Neurons

Golgi type I neurons have long axons and are usually found in anterior horns . Axons of these neurons form the long tracts of spinal cord.

2. Golgi Type II Neurons

Golgi type Il neurons have short axons , which are found mostly in posterior horns . Axons of these neurons pass towards anterior horn of same side or opposite side.

Organization of Neurons in Gray Matter

Organization of neurons in the gray matter of spinal cord is described in two methods :

Nuclei or columns.
Laminae or layers.

NUCLEI OF SPINAL CORD

Clusters of neurons are present in the form of nuclei or cell columns in gray matter . Advantage of this method is that different nuclei are easily distinguished . Disadvantage is that some neurons like internuncial neurons , which are outside the distinct nuclei are not included.

Nuclei in Anterior Gray Horn

Anterior gray horn contains the nuclei of lower motor neurons , which are involved in motor function . These nuclei are present in almost all the levels of spinal cord . Three types of motor neurons are present in lower motor neuron nuclei .

1. Alpha motor neurons

Alpha motor neurons are large and multipolar cells . Axons of these neurons leave the spinal cord through the anterior root and end in groups of skeletal muscle fibers called extrafusal fibers.

2. Gamma motor neurons

Gamma motor neurons are smaller cells scattered among alpha motor neurons . These neurons send axons to intrafusal fibers of the muscle spindle.

3. Renshaw cells

Renshaw cells are the inhibitory neurons , which play an important role in synaptic inhibition at the spinal cord.

Nuclei in Lateral Gray Horn

Lateral gray horn has cluster of neurons called inter mediolateral nucleus . The neurons of this nucleus give rise to sympathetic preganglionic fibers , which leave the spinal cord through the anterior nerve root . Inter mediolateral nucleus extends between T1 and L2 seg ments of spinal cord.

Nuclei in Posterior Gray Horn

Posterior gray horn contains the nuclei of sensory neu rons , which receive impulses from various receptors the body through posterior nerve root fibers . There are four types of nuclei of sensory neurons.

Marginal nucleus

Marginal nucleus is also called posteromarginal nucleus marginal zone nucleus or border nucleus It covers the very tip of posterior gray horn and it is found in all levels of spinal cord.

2. Substantia gelatinosa of Rolando

Substantia gelatinosa of Rolando is a cap - like gelatinous material at the apex of posterior horn situated in all levels of spinal cord . It is formed by small neurons .

3. Chief sensory nucleus or nucleus proprius Chief sensory nucleus is situated in the posterior gray horn ventral to substantia gelatinosa . It is a poorly de fined cell column located in all segments of spinal cord 4. Dorsal nucleus of Clarke Clarke nucleus is also called Clarke column of cells and it is the collection of well - defined neurons . It occu pies the basal portion of posterior horn . This nucleus is found in spinal segments between C8 and L3 only ( Table 140.2 ) .

SPINAL SEGMENT

Segment or part of spinal cord to which a pair of dorsal nerve roots ( right and left ) and a pair of ventral nerve roots is attached is called a spinal segment .

Since length of spinal cord ( 45 cm ) is smaller than the length of vertebral column ( 65 cm ) , the spinal segments do not correspond to the vertebral levels . Spinal segments being shorter lie above the corresponding vertebrae.

During the third month of embryonic development , the spinal cord extends the entire length of the vertebral canal and both grow at about the same rate .

As development continues , the body and the vertebral column continue to grow at a much greater rate than the spinal cord proper . The outcome of this uneven growth is that the adult spinal cord extends to the level of the first or second lumbar vertebrae.

the nerves grow to exit through the same intervertebral foramina as they did during embryonic development . This growth of the nerve roots occurring within the vertebral canal , results in the lumbar , sacral , and coccygeal roots extending to their appropriate vertebral levels . All spinal nerves , except the first , exit below their corresponding vertebrae.

NUCLEI OF SPINAL CORD

The grey matter of spinal cord is arranged in three horns . Anterior is motor , lateral being visceral efferent and afferent in function , and posterior is sensory in function.

Nuclei in Anterior Grey Column or Horn

The anterior horn is divided into a ventral part , the head and a dorsal part , the base . The nuclei in anterior horn innervate the skeletal muscles.

Most prominent neurons are alpha neurons . Their axons leave the spinal cord through ventral nerve roots to innervate skeletal muscles . Smaller neurons are gamma neurons .

These supply intrafusal fibres of muscle spindles . The cells in the anterior horn are arranged in the following three main groups.

Medial group : It is present throughout the entire extent of spinal cord and innervates the axial muscles of the body .
Lateral group : Present only in the cervical and lumbar enlargements and supplies musculature of limbs . It is subdivided into three subgroups.
- Anterolateral supplying proximal muscles of limbs ( shoulder and arm / gluteal region and thigh.
- Posterolateral supplying intermediate muscles of limbs ( forearm / leg ).
- Post - posterolateral innervating the distal segment ( hand / foot ).
Central group : Only in upper cervical segments as phrenic nerve nucleus and nucleus of spinal root of accessory nerve.

Nuclei in Lateral Horn

Nuclei in lateral horn are as follows :

Intermediolateral nucleus : This acts as both efferent and afferent nuclear columns . This nucleus is seen at two levels.
- From T1 to L2 segments , giving rise to preganglionic sympathetic fibres ( thoracolumbar outflow ) .
- From S2 to S4 segments , giving rise to pre ganglionic parasympathetic fibres chiefly for the pelvic viscera. At these two levels , the intermediolateral cell column receives visceral afferent fibres
Intermediomedial nucleus : This is mostly internuncial neuronal column .

Nuclei in Posterior Grey Column Afferent Nuclear Group Column

The four main afferent nuclei are seen in this are :

Posteromarginal nucleus : Thin layer of neurons caps the posterior horn . It receives some of incoming dorsal root fibres.
Substantia gelatinosa : This is found at the tip of poste rior horn through the entire extent of spinal cord.
Nucleus proprius : It lies subjacent to the substantia gelatinosa throughout the entire extent of cord . 1-3 groups of nuclei are present in laminae I - IV
Nucleus dorsalis of Clarke also known as thoracic nucleus at the medial part of base of posterior horn extending from C8 to L3 segments . These are situated in laminae V and VI .

LAMINAR ORGANISATION IN SPINAL CORD

In thick sections , spinal cord neurons appear to have a laminar ( layered ) arrangement . Ten layers of neurons are recognised , known also as laminae of Rexed.

These are numbered consecutively by Roman numerals starting at the tip of the dorsal horn and moving ventrally into ventral horn. Laminae I to IV , in general , are concerned with exteroceptive sensation and comprise the dorsal horn.

whereas laminae V and VI are concerned primarily with proprioceptive sensations . Lamina VII is equivalent to the intermediate zone and acts as a relay between muscle spindle to midbrain and cerebellum , and laminae VIII - IX comprise the ventral horn and contain mainly motor neurons.

The axons of these neurons innervate mainly skeletal muscle . Lamina X surrounds the central canal and contains neuroglia .

Lamina I : Corresponds to posteromarginal nucleus.

Lamina II : Corresponds to substantia gelatinosa.

Laminae III and IV : Correspond to nucleus proprius.

The neurons of laminae I - IV are related to extero ceptive sensations , i.e. crude touch , crude pressure pain and temperature . Their axons give rise to ventral and lateral spinothalamic tract .

Laminae V and VI : Correspond to base of dorsal column . Neurons of laminae V and VI are meant for reflex proproceptive impulses . Their axons give rise to dorsal and ventral spinocerebellar tract.

Laminae VII : Occupies the territory between dorsal and ventral horns . This lamina contains many cells that function as interneurons . Three clear cell columns are recognised within this lamina . These are inter mediolateral , intermediomedial and nucleus dorsalis ( nucleus thoracis or Clarke's column ) .

Nucleus dorsalis is present on the medial aspect of dorsal horn from C8 to L3 segments and its axons give rise to dorsal spinocerebellar tract . The sacral autonomic nucleus is an inconspicuous column of cells in the lateral part of lamina VII in segments S2 , S3 and S4 .

Lamina VIII : Corresponds to ventral horn in thoracic segments but at the level of limb enlargements of spinal cord , it lies on the medial aspect of ventral horn.

Lamina IX : Includes the lateral group of nuclei of the ventral horn . The axons of these neurons leave the spinal cord to supply the striated or skeletal muscles of limbs .

Lamina X : Surrounds the central canal . It is composed of decussating axons , neuroglia and some neurons in the grey matter surrounding central canal that have properties of interneurons.

SENSORY RECEPTORS

The peripheral endings of afferent fibres which receive impulses are known as receptors.

Functional classification

Exteroceptors : These respond to stimuli from external that is pain , temperature , touch and environment , pressure.
Proprioceptors : These respond to stimuli in deeper tissues that is contraction of muscles , movements , posi tion and pressure related to joints . These are responsible for coordination of muscles , maintenance of body posture and equilibrium . These actions are perceived both at unconscious level and at conscious level .
Interoceptors enteroceptors : These include receptor end - organs in the walls of viscera , gland , blood vessels and specialised structures in the carotid sinus carotid bodies and osmoreceptors . Also carry sensations of hunger , nausea and pain.
Special sense receptors : These are concerned with vision , hearing , smell , balance and taste .

TRACTS OF THE SPINAL CORD

A collection of nerve fibres that connects two masses of grey matter within the central nervous system is called a tract . Tracts may be ascending or descending.

They are usually named after the masses of grey matter connected by them . Some tracts are called fasciculi or lemnisci . The following tracts are seen in a transverse section through the spinal cord . Their location should be identified.

DESCENDING TRACTS

The descending tracts are of two types - pyramidal and extrapyramidal.

Pyramidal or Corticospinal Tracts

The pyramidal or corticospinal tract consists of two parts :

Lateral corticospinal tract , which lies in the lateral funiculus.
Anterior corticospinal tract , which lies in the anterior funiculus.

The pyramidal or corticospinal tract is for med by the axons of pyramidal cells predominantly lying in the motor area of cerebral cortex . There is some contribution to it from axon of cells in premotor and sensory areas.

From here , the fibres course through the posterior limb of internal capsule , midbrain , pons and medulla oblongata . At the lower level of medulla oblongata , 80 % of fibres cross to the opposite side.

This known as 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 inter nuncial neurons at the anterior horn cells.

The 15 % of fibres that do not cross enter anterior white column of spinal cord to form anterior 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 internuncial neurons similar those of lateral corticospinal tract . Only 5 corticospinal fibres supply muscles of the same side chiefly the neck muscles . Thus , neck muscles have bilateral controls .