Internal Structure of
Spinal Cord
Laminae of Rexed
• 10 divisions based on cellular architecture
of the spinal cord. It is numbered from
dorsal to ventral
• Lamina I. top of dorsal horn - thin layer of small
stellate cells & large neurons
– receive incoming Dorsal Root fibers
– send some axons to contralateral spinothalamic tract
• Lamina II. substantia gelatinosa - very small,
dense cells (gelatinosa cells) with highly branched
dendrites
– receive afferent dorsal root axon collaterals &
descending fibers from reticular formation
– send unmyelinated axons to dorsolateral tract - ascend
or descend a few segments
– give off branches at several levels to other laminae of
dorsal horn
– thought to be involved especially in modulation &
throughput of pain perception
• Lamina III. interneurons - receive dorsal root
axons; send axon branches to other Dorsal Horn
laminae
– II and III (substantia gelatinosa) functions in
regulating afferent input to the spinal cord.
• Lamina IV. tract cells - long dendrites extend into
Lamina II, III;
– axons cross midline to contralateral spinothalamic tract
– projects to the lateral cervical nucleus, the posterior
column nuclei and the thalamus (spinothalamic tract)
• Lamina I-IV all receive inputs from dorsal root,
IV most
• V-VI. base of Dorsal Horn (indistinguishable in
humans)
– has long-dendrite tract cells similar to IV; and many
various interneurons
– afferent fibers = some dorsal root afferent; and
especially. descending corticospinal fibers (their major
target)
– Tracts cells from Lamina IV, V, and VI are generally
referred as nucleus proprius
– Lamina 5 and 6 receives proprioceptive input AND
sensory information relayed by lamina 4. These are the
sites of origin of ascending projections to higher centers
• Intermediate zone:
– VII. largest region of the spinal gray matter
• many interneurons with collaterals within their segment and long
axons that go out into the white matter & travel up/down to other
segments’ gray matter
• VII also contains a few other cell columns (not part of the Lamina
system)
– Nucleus dorsalis (nucleus thoracicus /Clarke’s column:)
• in medial/base of dorsal horn/upper intermediate, T1-L3 big neurons,
axons make up dorsal spinocerebellar tract
• ventral spinocerebellar tract originates from lamina V, VI and VII and
from neurons from the edge of the ventral horn in lumbar region
(spinal border cells)
– Intermediolateral cell column: in lateral horn of T1-L2
• Cell bodies of preganglionic neurons of sympathetic nervous system
• Anterior / Ventral horn: (motor & interneurons)
• VIII. mostly in medial side of ventral horn ( covers
width of ventral horn in part of thoracic segments)
– receives terminals of descending fibers from vestibulospinal
& reticulospinal tracts
– sends to ipsilateral & contralateral to lamina VII & IX at their
level & a few other close levels
– participates in movements of muscles in the head and neck
• IX. columns of motor neurons within boundary of VII
or VIII
– axons make up ventral roots: alpha-motor neuron to skeletal
muscle (gamma motor neuron to muscle spindles) four motor
neuron columns = dorsolateral; central; ventromedial;
ventrolateral . The axial muscles (neck, trunk) are
represented medial and ventrally whereas limbs are
represented in lateral and dorsal part of the spinal cord
• In Cervical cord (L IX):
– a. Phrenic nucleus: in ventromedial column, C3-C5,
innervates diaphragm muscle
– b. Spinal accessory nucleus: C1-C5, lateral VH
• axons in rootlets join to form spinal root, ascends through
foramen magnum; joins with cranial root (XI cranial nerve) to
form accessory nerve - exits through jugular foramen
• spinal root innervates trapezius & sternocleidomastoid
• Nucleus of Onuf at S2 level
– most ventral part of ventral horn
– contribute to pudendal nerve - pelvic floor, including
sphincters of fecal and urinary continence.
• Commissural area:
• X. immediately around central canal, smaller
neurons than surrounding laminae
– receives some afferents from dorsal root
– contains decussating (crossing) fibers, glia cells
– contains neurons that project to the opposite side of the
cord
• Dorsal afferent fibers predominately terminate in
the dorsal horn. Impulses concerned with pain,
temperature, and touch reach the tract cells from
which spinothalamic tract originates. Pain signals
are modified mainly in Lamina II. Dorsalis
nucleus of thoracic and upper lumbar gives off
dorsal spinocerebellar tract.
• Dorsal horn
– Each dorsal root branches into six to eight rootlets.
These axons enter the dorsallateral tract (Lissauer)
where they divide into ascending or descending
branches terminating in their own or immediately
adjacent segments.
• Ventral horn
– Lamina IX has two types of motor neurons, alpha
(numerous, large in diameter) and gamma (less
numerous, small). Alpha innervates muscle cells and
gamma innervates muscle spindles.
• Lateral horn
– Sympathetic efferent neurons, located in the thoracic
and upper lumbar segments only (T1 – L2).
•
Intermediate zone:
–
Sacral autonomic nucleus: lateral horn S2S4
•
–
Cell bodies of sacral PSN preganglionic
Intermediomedial cell column: just lateral to
X (through out the length of spinal cord
•
thought to be involved in visceral reflexes;
Internal structure of spinal cord
• A. White matter - stains black or dark blue
with Weigert's stain
– is composed of axons that form tracts or
funiculi
– ascending or descending - travel rostrally or
caudally
– divided into 3 areas: dorsal, ventral & lateral
funiculi
Dorsal Funiculus
• between dorsal gray horns
• axons from dorsal root ganglia (DRG) ascend
ipsilaterally;
• carry sensory discrimination for tactile,
proprioception, movement, vibration, synapse in
Nuclei gracile and nuclei cuneate in medulla then
cross over to the contralateral side.
• Different pathway for discriminative touch and
proprioception
Receptors
• Sensory receptors:
– exteroceptors: response to pain, temperature,
touch, pressure on skin
• like free nerve endings, Merkel endings, Pacinian
corpuscles
– proprioceptors: in muscles, tendons, joints,
provide information for awareness of position
and movement
• like neuromuscular spindles, Golgi tendon organs
(neurotendinous spindles)
Discriminative touch
• Receptors are tactile receptors (Meissner’s
corpuscles), sent to dorsal root ganglia
– fasciculus gracilis - medial - lower extremity,
somatotopic origins (lower = more medial)
– fasciculus cuneatus - lateral - appears above midthoracic level (information from the upper extremity)
– Both funiculi terminate to gracile and cuneate nucleus,
respectively at medulla level. Then by internal arcuate
fibers cross over to the contralateral side, project to
ventral posterior nucleus of thalamus by medial
lemniscus, then to the ipsilateral primary somesthetic
cortex (be aware the medial lemniscus turns 90, then
lower body information is on the lateral side now) .
Three levels of perception
• primary afferents neurons (pathway) at
dorsal root ganglia (spinal cord level)
• At the medulla, the primary afferents
finally synapse. The neurons receiving the
synapse are now called the secondary
afferents neurons.
• At thalamus, a third and final neuron will
go to cerebral cortex, the final target.
Proprioception
• Receptors are neuromuscular spindles,
neurotendeninous spindles,
• Upper limb:
– Same as above mentioned mechanism
Proprioception
• Lower limb:
– dorsal funiculus goes only up to mid-thoracic level,
terminates into nucleus dorsalis
– axons from nucleus dorsalis ascend ipsilaterally as
dorsal spinocerebellar tract. At medullar level, before
entering cerebellum by inferior cerebellar peduncle,
gives off collaterals entering nucleus Z (rostral to
gracile nucleus), which are concerned with conscious
proprioception from the lower limb.
– Cells from nucleus Z then give rise to internal arcuate
fibers that cross the midline and join the medial
lemniscus, then enter the ventral posterior nucleus of
thalamus, then primary somatosensory cortex
ipsilaterally.
Proprioception
• - also carry descending fibers:
• a. from nucleus gracile, and nucleus cuneate modulation of sensations from higher centers
• b. from dorsal root ganglia afferents - help
coordinate / integrate sensory info from nearby
levels
• c. spinal gray - thought to coordinate upper &
lower limbs in reflexes
Lateral funiculus
• Ascending:
• 1. dorsal spinocerebellar tract:
– begins from above L3
– axons from ipsilateral Nucleus dorsalis
(Clarke’s column cells) to ipsilateral cerebellar
cortex via inferior cerebellar peduncl
– Participates the pathway for lower limb
proprioception (see above)
Lateral funiculus
• 2. ventral spinocerebellar tract:
– mostly from contralateral spinal border cells of ventral
horn at edge of lumbar sacral ventral horn
– largely crossed fibers ascend up to midbrain level turns
into superior cerebellar peduncle
– fibers cross again in cerebellum before entering to
cerebellar cortex
– therefore, deliver sensory info from same side of body
– Both spinocerebellar (ventral and dorsal) carry same
side of the information into cerebellum. Although,
dorsal tract entering the cerebellum via the inferior
cerebellar peduncle and ventral by the superior
cerebellar peduncle.
Lateral funiculus
• lateral spinothalamic tract: carry pain,
temperature, light touch sensations
• Receptors are nociceptor (nerve endings) for pain,
Merkel etc for light touch, unknown receptors for
temperature. Fibers enter the dorsal lateral tract.
Axons from dorsal horn (nucleus proprius) IV, VVI cross the midline in ventral white commissure,
then travels ipsilaterally in the ventral part of
lateral funiculus end in the ventral posterior
nucleus of the thalamus.
Spinothalamic
• upper limb is represented more medial
(close to the gray matter); lower is lateral,
superficially (somatotopically arranged)
• From medulla level up , spinothalamic
fibers constitute most of the spinal
lemniscus, which also includes fibers from
spinotectal tract
Lateral funiculus
• Other tracks in lateral funiculus
– spinotectal tract: same source as spinalthalamic
tract; cross, then up to superior colliculus,
midbrain reticular formation
– spinoreticular tract:
• probably pain perception for internal organs
• originates from laminae IV-VIII crossed midline to
pontine reticular formation (crossed)
• from ipsilateral side, terminates to medullary
reticular formation (uncrossed)
Lateral funiculus
• Descending: (modify ascending activities)
• lateral corticospinal tract:
– Originate from frontal and parietal lobes, pass
through internal capsule, basis pedunculi, midbrain,
pons
– Cross as medullar level to enter (become) lateral
funiculus.
• a. fibers from frontal lobe: end in ventral horn &
intermediate zone of gray matter
• b. fibers from parietal lobe : end in dorsal horn
gray matter
Lateral Corticospinal Tract
• Functionally, these tracts (lateral and anterior)
have a strong influence on voluntary motor
activity. Some of the fibers of the lateral
corticospinal tract terminate directly on the
motor neurons (anterior horn cells) of the
spinal cord, particularly those involved in fine
motor control of the fingers and hand. Most
others act via the interneurons of the anterior
horn, which then influence the motor neurons.
Lateral Corticospinal Tract
• Damage to the corticospinal tract results
in changes in modulation of the deep
tendon reflexes to cause hyperreflexia
and spasticity at segments below the
level of corticospinal damage. The three
clinical characteristics of hyperreflexia
are amplitude of reflex, speed of action
and spread of reflex.
Other descending tracts
• Rubrospinal tract
– from contralateral Red Neucleus - only extends to C2 in
humans
• Medullary reticulospinal tract:
–
–
–
–
from nucleus reticular formation
control motor neurons
mostly ipsilateral, but some fibers cross in medulla
control of un/ subconscious motor activity
(corticospinal tracts control skilled/conscious
movement)
Ventral funiculus
• Descending:
• 1. ventral corticospinal tract = uncrossed portion
of corticospinal tract
• These then cross in the anterior commissure of the
cord at the level where fibers synapse with the
anterior horn cell
Anterior Spinothalamic Tract
• Light (crude) touch and pressure
– Info to dorsal spinal ganglion, synapse with
substantia gelatinosa group (Lambda II, III)
– Second order cross to the opposite side and
ascend as anterior spinothalamic tract, new
fibers are added to the medial aspect of the
track as it goes up (lower lateral, upper medial)
– At medula level, joins with lateral
spinothalamic tract and spinotectal tract to form
spinal lemniscus
Anterior Spinothalamic Tract
• The spinal lemniscus continues to ascend to
the ventral posterolateral nucleus of
thalamus (third order neuron)
• Axons from third order neuron, then project
to post central gyrus
Other tracks in anterior funiculus
• (lateral) vestibulospinal tract: uncrossed fibers
from lateral vestibular nucleus of medulla
– terminates at Lamina VIII, & medial VII
– mediates equilibrium reflexes
• descending medial longitudinal faciculus: (also
called medial vestibulospinal tract)
– from medial vestibular nucleus of medulla
– mediates equilibrium-oriented head movements primarily in upper cervical levels
Spinal cord damage
• 1. This initial period of "hypotonia" after
upper motor neuron injury is called “Spinal
Shock” and reflects the decreased activity
of spinal circuits suddenly deprived of input
from the motor cortex and brainstem
Spinal cord damage
• After several days, however, the spinal cord
circuits regain much of their function for
reasons that are not fully understood.
Thereafter, a consistent pattern of motor
signs and symptoms emerges, including:
Upper motor neuron damage
• The Babinski
sign
– Seen in spinal
cord injury and
infants with
incomplete
upper motor
neuron control
Upper motor neuron damage
• Spasticity. Spasticity is increased muscle tone,
hyperactive stretch reflexes, and clonus (an
oscillatory motor response to muscle stretching).
Extensive upper motor neuron lesions may also be
accompanied by rigidity of the extensor muscles
of the leg and the flexor muscles of the arm (called
decerebrate rigidity; see below). Spasticity is
probably caused by the removal of inhibitory
influences exerted by the cortex on the postural
centers of the vestibular nuclei and reticular
formation.
Upper motor neuron damage
• Hyporeflexia of superficial reflexes. Further signs
are the decreased vigor (and increased threshold)
of superficial reflexes such as the corneal reflex,
superficial abdominal reflex (tensing of abdominal
muscles in response to stroking the overlying
skin), and the cremasteric reflex in males
(elevation of the scrotum in response to stroking
the inner aspect of the thigh). The mechanism of
this diminishment of superficial reflexes is not
well understood.
Upper motor neuron damage
• A loss of the ability to perform fine
movements. If the lesion involves the
descending pathways that control the lower
motor neurons to the upper limbs, the
ability to execute fine movements (such as
independent movements of the fingers) is
lost.
Lower motor neuron damage
• The cell bodies of the lower neurons are located in
the ventral horn of the spinal cord gray matter and
in the motor nuclei of the cranial nerves in the
brainstem.
• Symptoms include paralysis (flaccid), loss of or
paresis (weakness) of the affected muscles, loss of
reflexes (areflexia) due to interruption of the
efferent (motor) limb of the sensory motor reflex
arcs
Assessment of Spinal Cord Damage
• Dermatome
Assessment of Spinal Cord Damage
• Spinal reflexes:
–
–
–
–
Biceps reflex: C5 to C6
Triceps reflex: C6 to C8
Quadriceps reflex (Knee jerk reflex), L2 – L4
Gastrocnemius reflex (ankle reflex), S1 to S2
Biceps reflex
• The biceps reflex is
elicited by placing
your thumb on the
biceps tendon
• Biceps reflex: C5 to C6
Triceps reflex
• The triceps reflex is
measured by striking
the triceps tendon
directly with the
hammer while holding
the patient's arm with
your other hand
• Triceps reflex: C6 to
C8
Knee Jerk Reflex
• With the lower leg
hanging freely off the
edge of the bench, the
knee jerk is tested by
striking the quadriceps
tendon directly with
the reflex hammer
• Quadriceps reflex
(Knee jerk reflex), L2
– L4
Ankle Reflex
• The ankle reflex is
elicited by holding the
relaxed foot with one
hand and striking the
Achilles tendon with
the hammer and noting
plantar flexion
• Gastrocnemius reflex
(ankle reflex), S1 to S2
Lou Gehrig's disease
• Amyotrophic lateral sclerosis (ALS),
sometimes called Lou Gehrig's disease,
is a rapidly progressive, invariably fatal
neurological disease that attacks the
nerve cells (neurons) responsible for
controlling voluntary muscles. Both
upper and lower motor neurons are
damaged.
Lou Gehrig's disease
• Symptoms of upper motor neuron
damage include stiffness (spasticity),
muscle twitching (fasciculations), and
muscle shaking (clonus). Symptoms of
lower motor neuron damage include
muscle weakness and muscle shrinking
(atrophy).