The document describes the anatomical organization and pathways of the spinal cord related to sensory and motor functions. It discusses:
1) The ascending tracts (sensory tracts) in the posterior and lateral columns that transmit pain, temperature, touch and proprioceptive information to the brain. This includes the spinothalamic, medial lemniscus and spinocerebellar tracts.
2) The descending tracts (motor tracts) in the anterior and lateral columns that transmit signals from the brain to the spinal cord and lower motor neurons. This includes the corticospinal tract.
3) The organization of neurons involved in sensory and motor pathways, including first, second and third order neurons
20. APPLIED ANATOMY-
• INJURY TO LATERAL SPINO-THALMIC TRACT–
• It produces contra lateral loss of pain and thermal senses below the level of
lesion.
• The patient does not respond to pinprick or recognize hot and cold objects
placed in contact with skin.
• INJURY TO ANTERIOR SPINO-THALAMIC TRACT–
• It produces contra lateral loss of light touch and pressure below the level of
lesion.
• Discriminative touch is present as it is conducted by fasiculus gracilis and
fasiculus cuneatus.
• INJURY TO FASICULUS GRACILIS AND FASICULUS CUNEATUS–
• There is loss of vibratory sense, to discrimination, stereognosis and
conscious proprioception on the side of the lesion but if medial lemniscus is
involved there is loss below the level of lesion on the opposite side.
21. APPLIED ANATOMY OF MOTOR TRACTS-
• LOWER MOTOR NEURON LESION (FLACCID PARALYSIS)
• Trauma, infection, poliomyelitis vascular disorders,
degenerative diseases and neoplasm may produce lower
motor neuron by destroying cell body in anterior grey
column or its axons in the anterior root or spinal nerve.
• Following clinical signs are seen
• Flaccid paralysis of muscle and no resistance to passive
movements.
• Loss or diminished muscle tone (atonia or hypotonia) due
to involvement of stretch reflex from muscle spindle.
• Segmental paralysis of all movements voluntary and reflex.
• Segmental loss of all reflexes i.e. superficial and deep.
22. • After 2/3rd weeks after denervation the paralysed muscle contract
spontaneously forming fibrillation and fasciculation.
• Atrophy (wasting) of affected muscles takes place. Muscular
contracture i.e.-shortening of the muscle occurs mostly in
antagonist where there is no opposition by the paralyzed muscle.
• When preganglionic autonomic fibres are involved trophic
disturbances are observed with dry and cyanotic skin, brittleness
of finger, nails, loss of hair and lysis of bones and joints.
• Involvement of preganglionic fibres of S2, 3, 4 produces disturbances
of functions of urinary bladder and rectum.
• Lesions of preganglionic outflow in the ventral roots of T1 and T2
segments result in HORNER’S SYNDROME on the affected side.
• It includes pin point pupil, fixed pupil ptosis of upper eyelid due to
involvement of Muller's muscle (sup tarsal) of LPS, enopthalmos,
warm and dry skin on the affected side of the face.
23. • UPPER MOTOR NEURON LESION-(SPASTIC PARALYSIS)
• Interruption of fibres of pyramidal tract and other
descending supraspinal tract produces motor disturbances
collectively called as Upper Motor Neuron Paralysis.
• When the lesion takes place above the pyramidal
decussation the manifestations are seen on the opposite
side. If the lesion is located below the decussation; same
side is affected.
• Immediately after the lesion the affected muscle becomes
paralysed and deep tendon reflex temporarily disappears.
• Within few days the acute effects pass over and the
following manifestations are seen-
24. • Loss of voluntary movements which is most severe in upper
extremity. Paralysis of upper and lower extremities of one side
is known as hemiplegia.
• When all four limbs are involved it is called as the quadriplegia.
• Loss of movement of one limb is called as monoplegia.
• Bilateral paralysis of lower extremities is called as paraplegia.
• The muscle becomes spastic with exaggerated tendon reflexes
and increased muscle tone, hence termed as spastic paralysis.
• The spasticity makes the muscle firm and stiff, and affects the
flexor muscle of upper limb and extensor muscles of lower limb.
• Hypertonicity and exaggerated reflexes are due to increased
activity of fusimotor fibres.
25. • Muscular resistance to passive movements is exaggerated; the
resistance is strong at the beginning and shows suddenly a clasp
knife resistance as more force is applied against resistance.
• Clonus – It is a sustained series of rhythmic jerks instead of
single contraction when a quick stretch is applied to the tendon.
• Ankle clonus is observed in upper motor neuron lesion by
sudden dorsiflexion of foot.
• Loss of superficial reflexes – (abdominal or cremasteric reflex)-
• The superficial abdominal reflexes are absent. The abdominal
muscles fail to contract when the skin of abdomen is scratched.
• Cremasteric Reflex – It is absent. The cremaster muscle fails to
contract when the skin on the medial side of thigh is stroked
this reflex are passes through the L1 segment of spinal cord.
26. • The Babinski sign (reflex) is positive – in which when the
lateral side of sole of foot is scratched with a blunt object the
great toe is dorsiflexed and the other toes spread fanwise.
• Normal response is the plantar flexion of all the toes. But in
nonfunctional state of these tracts, the influence of other
descending tracts becomes apparent and a kind of withdrawal
reflex takes place in response to stimulation of the sole, with
the great toe dorsally flexed and other toes fanning out.
• It is positive in first year of life because corticospinal tract is
not myelinated until the end of 1st year.
• There is loss of performance of skilled and fine voluntary
movements mostly in the distal end of limbs.
27. • DESCENDING TRACTS–
• The motor neurons which lie in the anterior grey column
send axons to innervate the skeletal muscles; through the
anterior root of spinal nerve.
• These neurons are called as the LOWER MOTOR NEURONS
• These L.M.N. are continuously bombarded by nervous
impulse that descends from pons, midbrain and cerebral
cortex.
• The nerve fibres that descend in the white matter from
different supraspinal nerve centers are segregated into
nerve bundles and are called as descending tracts.
• The supraspinal neurons are called as the UPPER MOTOR
NEURONS.
28. • ANATOMICAL ORGANIZATION –
• ABOUT ASCENDING TRACTS– It consist of three sets of neurons as follows –
• FIRST ORDER NEURONS–
• They lie on the same side of the environment and consist of bipolar or unipolar
neurons, which are situated outside CNS except the mesencephalic nucleus of
trigeminal nerve.
• They are dorsal root ganglion of the spinal nerves and the sensory ganglion of some
cranial nerves.
• SECOND ORDER NEURONS–
• They are located in the posterior grey column of spinal cord, or in the nucleus
gracilis, nucleus cuneatus of the lower medulla.
• The axons of 2nd order of neurons cross the middle line but the level of crossing
differs. They terminate in the ventral nucleus of thalamus, or specialized part of
thalamus.
• THIRD ORDER NEURONS–
• It is usually the thalamus. The axons are mostly projected to the sensory cortex
and evoke consciousness.
• The three neuronic concept of conscious sensory pathway is violated by the
olfactory system.
• All the sensory modalities terminate into the thalamus except olfaction.
29. • ABOUT DESCENDING TRACTS–
• The descending pathway consists of three orders of neurons.
• FIRST ORDER NEURONS–
• It has its cell body in the cerebral cortex. Its axon descends to synapse on the 2nd
order neurons.
• SECOND ORDER NEURONS–
• It is an internuncial neuron situated in the grey column of spinal cord.
• THIRD ORDER NEURONS–
• The lower motor neuron in anterior grey column. The axons innervate the skeletal
muscle through the anterior root and spinal nerve.
30. TRACTS IN THE SPINAL CORD–
• In the transverse section of spinal cord –
• The sensory tracts are present in posterior and lateral
column.
31.
32. • The motor tracts are present in anterior and lateral
column.
33. ASCENDING TRACTS- (SENSORY TRACTS )
• Pain and Temperature Pathway – Lateral Spinothalamic tract –
• It is the main central pathway for the pain and temperature sensations.
• The axons enter the spinal cord through the posterior root ganglion, these fibres of
1st order neurons terminate in the nucleus of substantia gelatinosa and the nucleus
proprius.
• The axons of 2nd order neurons now cross obliquely to the opposite side in the
anterior grey column infront of the central canal in the anterior white column .
• They ascend through opposite lateral white column as the lateral spinothalamic
tract.
• Traced above the tract ascends through the medulla and forms the spinal
lemniscus.
• The fibres mainly terminate in the posterolateral part of ventral nucleus of
thalamus (VPL).
34. • Anterior Spinothalamic tract –
• It conveys simple touch and pressure.
• The fibres of 1st dorsal root ganglion which terminate in the
posterior grey column.
• The fibres of 2nd order neurons arise in posterior grey column
and cross to the opposite side in the anterior grey and anterior
white column; and ascend in opposite anterolateral white
column as anterior spinothalamic tract.
• Traced above the tract join in the lower medulla with the fibres
of medial lemniscus and end in VPL nucleus of thalamus.
35. Pathway for discriminative touch, vibratory sense, pressure and conscious muscle and joint
senses and stereognosis-
Posterior White Funiculus OR Medial Lemniscus OR Fasciculus Gracilis and Fasciculus Cuneatus
• Both the fasciculus gracilis and fasciculus
cuneatus in the spinal cord are
represented as tract of Gall and the tract
of Burdach.
• They run along the entire length of spinal
cord.
• The tract of Gall-
• It carries the ipsilateral fibres from lower
extremity and part of the body below mid
thoracic level.
• The fibres are somatotopically arranged
and in cervical region the arrangement of
the fibres from medial to lateral side are
sacral, lumbar, thoracic and cervical.
• This tract of Gall continues upwards as
fasciculus gracilis.
36. • The tract of Burdach-
• It lies lateral to tract of Gall
and occupies the upper part
of the cord upto mid thoracic
level.
• It carries fibres from
ipsilateral part of the trunk
above the mid thoracic level
and from upper extremity.
• Fibres are arranged
somatotopically; the tract
continues upwards as
fasiculus cuneatus.
• The functions of these
fasiculi are to carry
information about
discriminative touch, sense
of position, vibration,
pressure and stereognosis, as
well as conscious
proprioception.
37. • The fibres ascend
ispsilaterlly and terminate
on second order neurons in
the nucleus gracilis and the
nucleus cuneatus.
• The axons of these nuclei are
called as internal arcuate
fibres which cross the
median plane decussating
with corresponding fibres of
opposite side in the sensory
decussation.
38. • The fibres then ascend in a single
compact bunch called as medial
lemniscus; which ascends through
medulla, pons and mid-brain.
• The fibres then terminate by
synapsing with 3rd order neurons
VPL nucleus of thalamus.
• From the thalamus they reach the
somesthetic area in the posterior
central gyrus of the cerebral cortex
in area number 3, 1, 2.
• In the somesthetic area the opposite
half of the body is represented as
inverted known as homunculus.
• The pharynx and jaw in the inferior
part, followed by the face, finger,
hand, arm, trunk and thigh.
39. MUSCLE JOINT SENSE PATHWAYS TO THE CEREBELLUM-
• Posterior Spinocerebellar tract –
• This tract conveys unconscious proprioception,
touch, and pressure sensation from lower
extremity and lower half of the body to the
cerebellum.
• It is concerned with fine co-ordination of individual
lower limb muscles during posture and movement.
• It lies in the posterior part of lateral funiculus.
• The first order neurons lie in the dorsal root
ganglion; the axons of which terminate in posterior
grey column and are known as Clarke’s column or
Nucleus dorsalis which extend from C8 to L1 or L2
segments of the cord.
• The axons of 2nd order of neurons enter the
posterolateral part of the lateral column of the
same side and ascend as posterior spinocerebellar
tract.
• It enters through the inferior cerebellar peduncle
and terminates into lower limb areas of
cerebellum.
• The information carried by this tract is used by
cerebellum in the co-ordination of limb
movements and the maintenance of posture.
40. • Anterior Spinocerebellar Tract –
• It lies infront of posterior spinocerebellar tract in the lateral column.
• The tract carries unconscious proprioception and exteroceptive information from
the lower extremity and lower part of the body.
• It is concerned with general status of posture and movements of entire lower
limbs.
• The first order neurons are represented by dorsal root ganglia the axons of which
relay in the tract cells which are second order neurons; the axons of which cross to
the opposite side and ascend in the lateral white column infront of posterior
spinocerebellar tract.
• It ascends through the medulla, pons and mid brain and enter cerebellum through
superior cerebellar peduncle.
• It decussates in the superior cerebellar peduncle and enters the ipsilateral
cerebellar peduncle.
41. • Cuneocerebellar tract OR Posterior External Arcuate fibres –
• This tract is similar to other spinocerebellar tracts but it carries unconscious
proprioceptive and exteroceptive information of upper limb and upper part
of trunk to the cerebellum.
• As the nucleus dorsalis does not extend above T1 cord segment, similar
fibres i.e. of proprioception, touch, and pressure of UL and upper part of
body ascend through ipsilateral fasiculus cuneatus of posterior column and
terminate in accessory cuneate nucleus of lower medulla.
• It acts as second order neurons; axons of which form cuneocerebellar tract
or posterior external arcuate fibres enter the inferior cerebellar peduncle
and into the cerebellum.
• This tract is the counter part of posterior spinocerebellar tract.
42. OTHER SPINAL TRACTS
• Spinotectal –
• It lies between anterior and lateral spinothalamic tracts.
• This pathway provides afferent information for spinovisual
reflexes and brings about the movements of the eyes and
head towards the source of stimulation.
• Spino-reticular –
• It is an afferent pathway which plays an important role in
influencing levels of consciousness.
• Spino-olivary –
• This tract conveys information to the cerebellum from the
cutaneous and proprioceptive organs.
43. • The tract then continues
through middle 3/5th of
basis pendunculi of
midbrain.
• On entering the pons the
tract is broken into many
bundles by the transverse
pontocerebellar fibres.
• In the medulla the bundles
are grouped together
along the anterior border
to form a swelling
anteriorly called pyramid.
44. • At the junction of medulla oblongata and
spinal cord, 90% fibres cross to the
opposite at the lower part of the pyramid
and enter the lateral white column of
spinal cord to form lateral corticospinal
tract.
• Remaining 10% fibres remain uncrossed
descend in the anterior white column as
the anterior corticospinal tract; but in the
lower segments of the cord they cross to
opposite side, thus ultimately 100% fibres
cross to opposite side.
• The lateral corticospinal tract descends
through the entire length of spinal cord
and its fibres terminate in the anterior
grey column of all the spinal cord
segments, i.e. in laminae IV to VII, and
through interneurons are connected to
alpha and gamma motor neurons of
lamina IX.
• It helps in smooth performance of skillful
activities.
45. • Myelination of corticospinal tracts-
• Myelination of corticospinal tract starts at
about 3 years after birth and is completed by
puberty.
• Therefore brisk and purposeless movements of
newborns are partly reflex in origin and partly
controlled by extra pyramidal pathways; due to
this the ‘Babinski Extensor Response’ is
observed in newborns.
• Volitional control of micturation appears usually
after 3 years with the onset of myelination of
pyramidal tract.
46. Extra pyramidal tracts-
• Rubrospinal Tract –
• It acts as an alternate route of pyramidal system to exert influence on lower
motor neurons.
• Tectospinal tract –
• These fibres are concerned with reflex and postural movements are response to
visual stimuli.
• Vestibulospinal tract -
• vestibulospinal tract facilitate the active extensor muscles and inhibit the flexor
muscle for maintenance of equilibrium and posture of the body and the limbs.
• Olivospinal tract –
• They influence the activity of motor neurons in the anterior grey column.
• Reticulospinal Tract -
• These tracts provide a pathway by which the hypothalamus can control the
sympathetic outflow and sacral parasympathetic outflow.
47. DESCENDING TRACTS (MOTOR TRACTS)-
• Corticospinal or Pyramidal tract
• It is commonly known as the pyramidal tract.
• The tract is concerned with skilful, voluntary
movements of non-postural type affecting
mainly the flexor muscle of upper and lower
extremity.
• About 1/3rd of fibres originate from the primary
motor cortex (area 4); 1/3rd from the secondary
motor cortex (area 6), 1/3rd from the parietal
lobe (area 3,1,2) thus 2/3rd fibres arise from the
precentral gyrus and 1/3rd from post central
gyrus.
• Majority of the corticospinal fibres are
myelinated and are relatively slow conducting
fibres.
• The descending fibres converge in the corona
radiata and then pass through the posterior
limb of internal capsule.
• The fibres in the limb are arranged
somatotopically.
Editor's Notes
On entering the spinal cord, the nerve fibres of different sizes and functions are sorted out and segregated into nerve bundles or tracts.
Thus a tract is defined as the collection of nerve fibres having the same origin, course, functions and termination.
Ascending or sensory tracts.
Descending or motor tracts.
Substantia Gelatinosa –
It lies at the apex of posterior grey column throughout the length of the spinal cord.
It receives afferents concerned with pain, temperature and touch from posterior roots.
Nucleus of Propius –
It lies anterior to Substantia Gelatinosa throughout the spinal cord.
It receives the afferent concerned with two point discriminative touch, sense of position, vibration and conscious proprioception.
Nucleus Dorsalis or Clarke’s Column –
It lies medial to the base of posterior grey column.
It extends from C8 to L2 segment.
It consists of interneurons and tract cells.
It receives UNCONSCIOUS exteroceptive and proprioceptive afferents from lower limb and lower part of tract.
Visceral afferent nucleus –
It is an elongated nucleus at the base of posterior grey column and extends from T1 to L2 segments and from S2 to S4 segments.
It receives visceral afferents from dorsal nerve roots and projects fibres to preganglionic visceral afferent nuclei of the autonomic system.
Intermediate or Lateral Grey Column -
It extends from T1 – L2 segments and is composed of neurons which act as preganglionic neurons of sympathetic system.
They pass to ganglia of sympathetic trunk.
The lateral column of sacral region extending from S2 to S4 segments.
They act as preganglionic parasympathetic system to form pelvic splanchnic nerves.
The entire grey matter of spinal cord is mapped out by Rexed into following ten laminae-
Laminae I to VI –
Confined to posterior grey column.
Lamina II corresponds to substantia gelatinosa. They receive exteroceptive sensation.
Lamina III to IV corresponds to nucleus of propius.
Lamina V to VI receive proprioceptive sensation.
Lamina VII –
It contains interneurons and is connected to cerebellum and brainstem.
Lamina VIII –
It lies in the medial part of anterior grey column.
It receives vestibulo-spinal and reticulospinal tracts.
Lamina IX –
It lies in lateral part of anterior grey column.
It contains alpha, gamma and the interneurons.
It receives the terminals of corticospinal, rubrospinal, vestibulo-spinal and reticulospinal tracts.
Lamina X –
It surrounds the central canal and consists mostly of neuroglial cells.
Central Canal –
The central canal is the continuation of aqueduct of midbrain and is lined by ependyma. It contains C.S.F, within the conus medullaris it is dilated to form terminal ventricle.
Exteroceptive – These arise outside the body such as pain, touch, temperature.
Proprioceptive – Which arise inside the body such as from the muscles and joints
Visceroceptive – These arise from the viscera.
The motor neurons which lie in the anterior grey column send axons to innervate the skeletal muscles; through the anterior root of spinal nerve.
These neurons are called as the LOWER MOTOR NEURONS. Anterior column contains motor neurons and interneurons and cranial nerve nulcei of brainstem are the exmples.
These motor neurons are called lower motor neurons; they are multipolar.
These L.M.N. are continuously bombarded by nervous impulse that descends from pons, midbrain and cerebral cortex.
The supraspinal neurons are called as the UPPER MOTOR NEURONS.
FIRST ORDER NEURONS–
They lie on the same side of the environment and consist of bipolar or unipolar neurons, which are situated outside CNS except the mesencephalic nucleus of trigeminal nerve.
They are represented by dorsal root ganglion of the spinal nerves and by the sensory ganglion of some cranial nerves.
SECOND ORDER NEURONS–
They are located in the posterior grey column of spinal cord, or in the nucleus gracilis nucleus cuneatus of the lower medulla.
The axons of 2nd order of neurons cross the middle line but the level of crossing differs. They terminate in the ventral nucleus of thalamus, or specialized part of thalamus.
THIRD ORDER NEURONS–
It is usually the thalamus. The axons are mostly projected to the sensory cortex and evoke consciousness.
The three neuronic concept of conscious sensory pathway is violated by the olfactory system.
All the sensory modalities terminate into the thalamus except olfaction.
FUNCTION OF THALAMUS-
The thalamus appreciates the quality of consciousness (crude) but is unable to analyze the details of sensation. This is done by the sensory cortex which handles detailed processes of consciousness and localizes, discriminates and analyzes the different modalities of the exteroceptive and proprioceptive sensations.
The ascending tracts which fail to reach the thalamus cannot induce consciousness.
Control of skeletal muscle activity from the cerebral cortex and other higher centers is conducted through the nervous system by a series of neurons.
The descending pathway consists of three orders of neurons.
FIRST ORDER NEURONS–
It has its cell body in the cerebral cortex. Its axon descends to synapse on the 2nd order neurons.
SECOND ORDER NEURONS–
It is an internuncial neuron situated in the grey column of spinal cord.
THIRD ORDER NEURONS–
The lower motor neuron in anterior grey column. The axons innervate the skeletal muscle through the anterior root and spinal nerve.
Pain and Temperature Pathway –
LATERAL SPINOTHALAMIC TRACT –
It is the main central pathway for the fast pain and temperature sensations.
The axons enter the spinal cord through the posterior root ganglion, proceeds to the tip of posterior grey column; these fibres of 1st order neurons terminate in the nucleus of substantia gelatinosa and the nucleus proprius.
The axons of 2nd order neurons now cross obliquely to the opposite side in the anterior grey column infront of the central canal in the anterior white column and ascend through opposite lateral white column as the lateral spinothalamic tract.
It lies deep to anterior spinocerebellar tract and dorsal to spinotectal tract. It extends along the entire length of spinal cord.
Traced above the tract ascends through the medulla and forms the spinal lemniscus.
The spinal lemniscus continues to ascend through the posterior part of the pons. In the midbrain it lies in the tegmentum lateral to the medial lemniscus. The fibres mainly terminate in the posterolateral part of ventral nucleus of thalamus (VPL).
The fibres in the spinothalamic tract are somatotopically arranged so that the fibres come from the lower segments of the cord are gradually pushed to the superficial surface.
Hence in the upper cervical segments the arrangement of fibres in the spinothalamic tract from superficial to deep is sacral, lumbar, thoracic and cervical. Also the temperature fibres are dorsal and fibres are ventral.
ANTERIOR SPINOTHALAMIC TRACT –
It conveys simple touch and pressure.
The fibres of 1st order neurons from the dorsal root ganglion terminate in the posterior grey column. The fibres of 2nd order neurons arise in posterior grey column and cross to the opposite side in the anterior grey and anterior white column; and ascend in opposite anterolateral white column as anterior spinothalamic tract.
It lies dorsal to vestibulospinal tract and separated from the lateral spinothalamic tract by spinotectal tract.
Traced above the tract join in the lower medulla with the fibres of medial lemniscus and end in VPL nucleus of thalamus.
