Neuro-Anatomy of the Spinal Cord

Spinal Cord
Presenter - Dr. Rahul Jain
Moderator - Dr. Jyoti Garg

Anatomy of Spinal Cord
• Extends from the “cranial border of the
atlas” where it is continuous with medulla
above to the “lower border of first lumbar
vertebrae”
• Present in the vertebral canal of the
vertebral column, cylindrical

• Avg. Length of SC - 45 cm in adult males,
42-43 cm in adult females

• Avg. length of the vertebral column - 70 cm
Early fetal development - SC extends to
lower end of sacrum
20 weeks - at L4 - L5 level/ lower border of
L4
40 weeks/ term - at upper border of L3 level
2 months - reaches the adult level, i.e. lower
border of L1
Source - Localisation in Clinical Neurology, Paul Brazis, 7th ed
DeJong;s Neurologic Examination 7th ed, Internet
Bradley’s Neurology in Clinical Practive, 7th ed
“The lower level varies slightly
from individual to individual, but
usually lies between lower
border of L1 and upper border
of L2”
10,8 mm
13,9 mm
12,8.5 mm

Spinal Meninges
Source -Netters Atlas of Anatomy

IIdentify the green areas
Spaces

Spinal Meninges
SC - lower border of L1
Dura (tough, avascular) - lower
border of S2
Arachnoid (thin, avascular) -
lower border of S2
Pia (thin, highly vascular) -
extends below as film
terminale
FIG. 7.3 Sagittal section through lumbosacral region showing conus medullaris, filum
externum), lower end of subarachnoid space and site of lumbar puncture.
Filum Terminale
•Thin, thread like elongation of
the Pia mater below the spinal
cord
•FT Internum - 15 cm
•FT externum - 5 cm
•It stabilises the spinal cord, by
attaching to the coccyx below.
(sometimes also known as
coccygeal ligament)
Source - Clinical Neuroanatomy by Vishram Singh

• “Although SC is a continuous
and non segmental structure,
but the 31 pairs of nerves
originating from it give it a
Segmental Appearance”

• Hence, SC is considered to
have 31 spinal segments.

• “Spinal segment - part of SC
to which the pair of Spinal
Nerve is attached”
Source - Localisation in Clinical Neurology,
Paul Brazis, 7th ed, Internet
Spinal Segments and Nerves

31 pairs of spinal nerves

33 vertebrae

Cervical spinal nerves
leave the vertebral canal
above their
corresponding vertebrae.
(C 1 to C 7)
C8 - emerges between
C7 vertebrae and T1
vertbrae.
Rest all emerge below
their corresponding
vertebrae.
Source - Carpenters Human Neuroanatomy, 9th ed

• Growth rate of Vertebral
Column > SC

• Hence the spinal nerves of
more caudal SC (ie. Lumbar
and Sacral) have to travel a
longer distance in the Sub
Arachnoid Space ..

• To reach their corresponding
intervertebral foramina.
Source - Localisation in Clinical Neurology,
Paul Brazis, 7th ed
Spinal Vertebral
Segment Column
Cervical SC (C1-C8) C1-C7
Thoracic SC (T1-T12) T1-T8
Lumbar SC (L1-L5) T9-T12
Sacral SC (S1-S5) L1
Source -Netters Atlas of Anatomy,
Localisation in Clinical Neurology,
Paul Brazis, 7th ed

dorsal horn, and ascend for one or two levels as they cross anterior
to the central canal to join the opposite spinothalamic tract. Lesions
that transect the descending corticospinal and other motor tracts
cause paraplegia or quadriplegia with heightened deep tendon
reflexes, Babinski signs, and eventual spasticity (the upper motor
neuron syndrome). Transverse damage to the cord also produces
TABLE 434-2 Spinal Cord Levels Relative to theVertebral Bodies
SPINAL CORD LEVEL CORRESPONDING VERTEBRAL BODY
Upper cervical Same as cord level
Lower cervical 1 level higher
Upper thoracic 2 levels higher
Lower thoracic 2–3 levels higher
Lumbar T10-T12
Sacral T12-L1
duce a mixed clinical pictu
and conus medullaris synd
also discussed in Chap. 14.
Special Patterns of Spinal C
ascending and descending p
in Fig. 434-1. Most fiber tr
and the spinocerebellar and
side of the body they innerv
pain and temperature sensa
contralateral to the side the
of these tracts produce char
to the underlying disease pr
Brown-Sequard Hemicord S
weakness (corticospinal trac
tory sense (posterior colum
temperature sense (spinotha
e_Part13_p3025-p3296.indd 3173
Source - Harrison’s Principles of Internal Medicine, 20th ed
Carpenter’s Human Neuroanatomy, 9th ed

Conus Medullaris
Also known as conus terminalis
Lower most tapered end of spinal cord.
Present usually at the lower border of L1
vertebrae.
“There is marked variability in location of
tip of conus among subjects”
“Upper border of conus is not well
defined”
Conus contains sacral spinal segments
• “Conus consists of S2 segments
and below”
• “In low conus lesions, S1 may be
spared”
• “Segments above the conus is called
epiconus consisting of L4,L5,S1
spinal segments” / T11/12 vertebral
level
Source - Localisation in Clinical Neurology, Paul Brazis, 7th ed
Bradleys Neurology in Clinical Practice 7th ed
Spinal Cord Injuries : Management and Rehabilitation 2009 Ch1
Toribatake, Yasumitsu & Baba, Hisatoshi & Kawahara, Norio & Mizuno, Katsunori (1997).
The epiconus syndrome presenting with radicular-type neurological features.
Spinal cord. 35. 163-70. 10.1038/sj.sc.3100369.

Cauda Equina
• In latin - ‘Horse’s tail’

• B/L nerve roots of L
(2,3,4,5) , S (1,2,3,4,5) and
Coccygeal nerve

• These spinal nerves arise
from their spinal segments
higher up, and travel down,
to reach their respective
intervertebral foramina.

• form a bunch of nerve fibres
around the filum terminale

Fissures and Sulci of SC
ANTERIOR - Ant. Median Fissure, 2 Antero Lateral Sulci (anterior roots of spinal nerve
pass through this)
POSTERIOR - Post. Median sulcus, 2 Postero Lat sulci (posterior roots of spinal nerve
pass through this), 2 postero intermediate sulci (only in Cervical and upper thoracic)

Cervical enlargement - from
C4 to T2 segments (brachial
plexus) 13mm transverse, 9
mm sagittal

Lumbar enlargement - from L2
to S3 (Lumbo sacral plexus) 12
mm transverse, 8.5 mm
saggital

These enlargements are
produced due to presence of
large number of motor neurones
in these regions to supply the
musculature of upper and lower
limbs.
Source - Clinical Neuroanatomy by Vishram Singh
Enlargements of SC

Cross Sectional
Anatomy
PF
LF
AF
Bell Magendie Law - states ant.
Roots are motor, post. Roots -
sensory,
However now some aﬀerent
fibres in ant. root, upto 3% in
post root are eﬀerent

Amount of gray matter correlates with
the mass of the tissue it supplies. Max -
at cervical and lumbar enlargements
(they have largest horns)
Amount of white matter progressively
increases from below upwards because
• Ascending fibres keep getting
added to the cord from below
upwards
• Descending fibres keep getting
terminated at their respective spinal
levels, from upwards to below.
Post. Intermedio Sulcus - divides white
matter into fasiculus gracilis and
fasiculus cuneatus.
FG & FC - C1-C7, T1-T6
FG only - below T6 Source - Carpenter’s Human Neuroanatomy, 9th ed
Localisation in Clinical Neurology, Paul Brazis, 7th ed,

Gray Mater of SC
3 types of neurons in Gray Mater
• Motor Neurons - Ant. Horn

Alpha Motor Neurons (supply extrafusal muscle fibres)
LARGE Neurons,

Gamma Motor Neurons (supply intrafusal muscle fibres)
SMALL neurons

Beta Motor

• Sensory Neurons - Post. Horn

• Interneurons

Rexed Laminae
N.B. Apart from the central grey matter, there are strands of grey matter in
column adjacent to the base of the posterior horn, which are termed reticul
Laminar architecture of grey matter (Rexed laminae) (Fig. 7
The cytoarchitecture of grey matter of spinal cord is alternatively divided int
by Rexed (Table 7.6). These are numbered consecutively by Roman numerals,
posterior horn and moving ven-trally into the anterior horn.
Table 7.6
Rexed laminae and nuclear groups
Laminae Corresponding grey column nuclei
I Posteromarginal nucleus
II Substantia gelatinosa
III and IV Nucleus proprius
V and VI Base of dorsal column
VII Nucleus dorsalis (Clarke's column) and intermediolateral and intermediomedial nuclei of lateral horn
VIII and IX Medial and lateral groups of nuclei of anterior grey column
X Surrounds the central canal and composed of the grey commissure and substantia gelatinosa centralis
FIG. 7.14 The laminae of Rexed and related nuclear groups.
This concept of laminae is useful in experimental works only and provid
localization of terminal degenerating fibres after section of posterior ne
nerve tracts.
White Matter

Tracts of Spinal Cord
• “Nerve fibre bundles having the same origin and
termination are known as TRACTS”
• “In general, long tracts tend to be located more
peripherally in the white matter, shorter tracts found
more centrally”

• Dorso Lateral - phylogenetically newer (fine), Antero
Medial - Older (axial, trunk)

Anterior horn
(motor neurons)
Lateral
corticospinal
(pyramidal) tract
Dorsal root
Dorsal
spinocerebellar
tract
Ventral
spinocerebellar
tract
Lateral
spinothalamic
tract
C
T
L
S
Ventral
spinothalamic
tract
Pressure, touch
(minor role)
Ventral
(uncrossed)
corticospinal
tract
Tectospinal
tract
S L T C
C
T L S
Fasciculus
cuneatus
Rubrospinal
tract
Lateral
reticulospinal
tract
Vestibulospinal
tract
Ventral
root
Axial and
proximal
limb
movements
(Joint Position, Vibration, Pressure)
Posterior Columns
Distal limb
movements
(minor role)
Pain,
temperature Ventral
reticulospinal
tract
Fasciculus
gracilis
S
L
T C
Distal limb
movements
L/
S
L/
S
P
E
D
F
Source - Harrison’s Principle of Internal Medicine 20th ed.

<lfil------- Medial lemniscus
~------Nucleus cuneatus
Internal arcuate fibers---+----:'-'-l~'-g// -+----- Spinal trigeminal nucleus
(neuron 11) ~--+------ Decussation of medial
Neuron I lemniscus
(dorsal root ganglion cell ....,_-<'TT.,,-.,~.:::----- -- Fasciculus gracilis
1 - - . , . . . - - - - - - Fasciculus cuneatus
Pacinian
corpuscle
Golgi-Mazzoni
corpuscle
Meissner's
corpuscle
ca
T4
~'11r-,.,,,_______ Fasciculus gracilis
L3
S4
re 11 .1. Formation and course of the posterior white columns in the spinal cord and the medial lemnis
Sacro-Lumbar fibres
Ascend medially, dorsally
PF
Arrangement (medial to
lateral)
FG - S, L, lower 6 T
FC - upper 6 T, C
Vibration & Pressure
sense
Joint Position sense
Fine Touch,
Discrimination
Posterior Columns

Sensory
cortex
Cerebral cortex
(postcentral gyrus)
V:
Corpus callosum---t-,-;;--~~;;:;;~"#J"
Thalamus--------,..,;:;ri-'-,-*-'<--
Putamen------+--'---./.
Globus pallidus------.-~
·- ---- -- -vent. posterolateral
nucleus (VPL)
MIDBRAIN
PONS
MEDULLA
<lfil------- Medial lemniscus
~------Nucleus cuneatus
Internal arcuate fibers---+----:'-'-l~'-g// -+----- Spinal trigeminal nucleus
(neuron 11) ~--+------ Decussation of medial
Neuron I lemniscus
Post.
Columns
continued..
1st Order -
Central
processes of
spinal ganglion
cells
2nd order -
internal arcuate
fasiculus, medial
lemniscus
3rd order -
Thalmocortical
fibre

Spino Cerebellar Tracts
Ant. spinocerebellar--s:---<:D
tract (axon of neuron 11)
C4
ca
Nucleus dorsalis
(of Clarke)
L3
S2
Neuromuscular spindle
(trapezius M.)
A t:>---,--- Post. spinocerebellar tract
(axons of neuron II)
Neuromuscular spindle
(ext. dig. communis M.)
Golgi tendon organ
(quadriceps femoris M.)
Dorsal root ganglion cells
(neuron I)
Golgi tendon organ
(Hamstring MM.)
Origin of P SCT (2nd order)
Origin of A SCT (2nd order),
Not well defined
Lumbar, sacral and coccygeal segments
(ie. coordination of LOWER LIMB mainly)
Above C8
Nucleus dorsalis
absent, therefore
C8 and above -

Anterior spinocerebellar tract
on surface of superior cerebellar
peduncle
UPPER PONS
Dentate nucleus-------,,-.,;--t;'
_'<
Sup. cerebellar peduncle--,1.'-i.:.:..=
PONS
MEDULLA
~----Cerebellorubral fibers
"------ Dentatoreticular fibers
":><;::----.!----- Decussation of superior
cerebellar peduncle
""",,______ Post. spinocerebellar fibers
in inf. cerebellar peduncle
_.....,___ Middle cerebellar
peduncle
~ - - - - - Accessory cuneate
nucleus
80% terminate C/L
Upper limb equivalent of Post SCT

• Hence only 2 orders of neurons.

• These tracts relay proprioceptive impulses (joint position,
muscle length and contraction) from muscle spindles and
golgi tendon organs - - - to the cerebellum

• Hence play important role in coordination.

• “None of the impulses conveyed by these REACH conscious
levels”

• Impulses transmitted by SCA are used for fine coordination
of posture and movement of individual limb muscles.

Anterior horn
(motor neurons)
Lateral
corticospinal
(pyramidal) tract
Dorsal root
Dorsal
spinocerebellar
tract
Ventral
spinocerebellar
tract
Lateral
spinothalamic
tract
C
T
L
S
Ventral
spinothalamic
tract
Pressure, touch
(minor role)
Ventral
(uncrossed)
corticospinal
tract
Tectospinal
tract
S L T C
C
T L S
Fasciculus
cuneatus
Rubrospinal
tract
Lateral
reticulospinal
tract
Vestibulospinal
tract
Ventral
root
Axial and
proximal
limb
movements
Posterior Columns
Distal limb
movements
(minor role)
Pain,
temperature Ventral
reticulospinal
tract
Fasciculus
gracilis
S
L
T C
Distal limb
movements
L/
S
L/
S
P
E
D
F
Cuneo cerebellar
tract - Cervical

MEDULLA
ca
Temperature------t--'s,c,,
Pain------'-:-
, -_-_
-----,r ---'
<D- ~ - - - Lateral spinothalamic
tract
~--- Sacral fibers
'.}----- Lumbar fibers
-'<'"'-----Thoracic fibers
~---- Cervical fibers
Pain receptors (free nerve
endings in skin of o "1~ :-"""l;~J______ Dorsolateral fasciculus
dermatomes
C 8 and T 4
Cold receptor in
skin of dermatome
L3
Heat receptor in
skin dermatome s 2
82 ~ -
(zone of Lissauer)
;.~ - - - - - - Cells of substantia gelatinosa
and nuc. centrodorsalis
--'!::==-------- Axons crossing to opposite
side in anterior white commissure
Lateral spino thalamic tract
1st order neuron
(pseudounipolar) , enters
spinal cord posteriorly,
ascends for 1-2 segments
in zone of Lissauer, then
enters sub. gelatinosa
2nd order

Sensory
cortex
Corpus callosum--t.>...-----,-€~<!ec:=-
Thalamus----7<C'".:-"-+-,'-;'-'.,.....,-
Internal capsule - --->--~-'-.....---+-...._,
MIDBRAIN
PONS
Cerebral cortex
Arm Hand (postcentral gyrus)
1'11!!>"=--- - r - Axons of neurons
in posterior limb of
internal capsule
~-~--vent. posterolateral
nucleus (VPL)
2nd order
3rd order

FIG. 7.20 Schematic diagram to show posterior (in red) and anterior (in blue) cerebellar trac
• Posterior (dorsal) spinocerebellar tract: The cell bodies of the first orde
the dorsal root ganglia of the spinal nerves. The central processes of these
7.19).
lis and fasciculus cuneatus (posterior column—medial lemniscus pathway).
Source - Textbook of Clinical Neuroanatomy by Vishram Singh

Lateral and anterior spinothalamic tracts.
Source - Textbook of Clinical Neuroanatomy by Vishram Singh

ion IV Spinal Cord
Lateral corticospinal
tract
Corticospinal tracts
Corticospinal tract
2%
Uncrossed
lateral
corticospinal
tract
th
D
tr
st
th
sm
co
in
a
th
si
o
so
o
h
co
ci
b
w
1
h
d
2
la
a
co
ti
in
g
n
sm
ti
m
ti
n
la
C
u

Ant. limb of--------h
internal capsule
Claustrum-------==---~
External capsule------+-,"
Putamen _ ________,.......,...
Cortex of insula------+
Motor cortex
(precentral gyrus)
Pyramidal cells
¢>Ii,=-==_::,..:::._ Fibers to lower extremity
7'~'.-...,.--1c- - - Fibers to trunk
'+----- Fibers to upper extremity
Globus pallidus - - - - - - ~·?"- p 1
· b f · 1 1
Genu of internal capsule ,,_....___
_.,....,:.,,~---- OS!. im
O
interna capsu e
•• Corticospinal tract
MIDBRAIN +----Temporopontine tract
Crus cerebri--------'<-
Oculomotor (111) nerve --------'>ss::::::::=51,t:::H:Jl::H'i- - - - Frontopontine tract
PONS
MEDULLA
MEDULLA
Lateral corticospinal tract - - ----1 1
(crossed-axons of neuron I)
CB
Longitudinal fibers in
basilar portion of pons
_:.~,.st~:------ Anterior corticospinal tract
(uncrossed-axons of neuron I)

MEDULLA
Lateral corticospinal tract - - ----1 1
(crossed-axons of neuron I)
CB
To motor endings
in MM. of forearm l'.____
and hand ~ --
T4
To motor endings
in intercostal and 'i'...________
segmental back ~ - -
MM. ~ - L4
To motor endings in :_.1f
gluteus medius and ~ -
tibialis anterior MM. ~--
_:.~,.st~:------ Anterior corticospinal tract
(uncrossed-axons of neuron I)
aio.1,<-11- , - - t - - - - lnternuncial cell-neuron II
_,, ___ ___ Ventral root fiber
~1""".-7'--'..,._____ Anterior horn cell-neuron Ill
--------To sacral segments of cord
Figure 11 .13. Lateral and anterior corticosplnal tracts (red), which are the principal descending motor pathways
concerned with skilled, voluntary motor activity. Letters and numbers indicate corresponding segments of the spinal
cord.
It has been estimated that 55%
of all pyramidal fibres end in
cervical cord, 20 % in thoracic,
25 % in lumbosacral segments.
Therefore more fine control of
complex movement of upper
limbs than on lower limbs.

Anterior horn
(motor neurons)
Lateral
corticospinal
(pyramidal) tract
Dorsal root
Dorsal
spinocerebellar
tract
Ventral
spinocerebellar
tract
Lateral
spinothalamic
tract
C
T
L
S
Ventral
spinothalamic
tract
Pressure, touch
(minor role)
Ventral
(uncrossed)
corticospinal
tract
Tectospinal
tract
S L T C
C
T L S
Fasciculus
cuneatus
Rubrospinal
tract
Lateral
reticulospinal
tract
Vestibulospinal
tract
Ventral
root
Axial and
proximal
limb
movements
Posterior Columns
Distal limb
movements
(minor role)
Pain,
temperature Ventral
reticulospinal
tract
Fasciculus
gracilis
S
L
T C
Distal limb
movements
L/
S
L/
S
P
E
D
F

Tectospinal tract
• Fibres arise from neurons in deeper layers
of superior colliculus

• Go antero-medially along the peri
aqueductal gray matter, cross to opposite
side

• Then descends down upto the C8, in
anterior white matter funiculus, next to the
Ant. median fissure.

• Terminates in anterior gray matter, laminae.

• Mediates Reflex postural movements in
response to visual, and perhaps auditory
stimuli. SCM, TRAPEZIUS
3 4 5
_,, Auditory radiation
MIDBRAIN
,a;o-4,,_.._ _ _ Medial longitudinal
Decussation of sup.-..w~=
fasciculus
cerebellar peduncle
Tectospinal and-------= =------ Rubrospinal and
tectobulbar tracts ...-=:,, ,---.._ rubroreticular tracts
PONS
Facial (VII) nerve
Vagus (X) nerve-
MEDULLA
Nucleus ambiguus
Hypoglossal nerve
CB
T7
Middle cerebellar
peduncle
Rubroreticular fibers
=----- Rubrospinal tract
(axons of neuron I)
>
Ventral root nerve fibers
(axons of neuron Ill)
Tectobulbar tract
• Fibres arise from same point and
innervate oculomotor, trochlear and
abducent nuclei.
• Mediates eye movements in
response to visual stimuli.

Vestibulospinal Tract
• 4 vestibular nuclei on each side In
the floor of 4th ventricle in front of
pons and medulla.

• They receive aﬀerents from
vestibular nerve (lateral nuclei),
and the cerebellum (rest 3)

• Lateral nucleus projects to VS
tract. Uncrossed. Runs entire
length of SC. Same somatotopic
organisation. CTLS (medial to
lateral)

• 3-4 fold more influence on
cervical and lumbar segments
than on thoracic
• Medial Nucleus projects as VS
fibres in the Medial longitudinal
fasciculus.
Thalamus
peduncles
Vestibulospinal
Medial longitudinal fasciculus
both the pons and medulla. The f
nuclei of this complex (i.e., lateral, m
perior, and inferior nuclei) (35) rec
ent fibers from the vestibular nerv
cerebellum which are distributed
tially (Figs. 11.19, 11.21, and 13
vestibulospinal tract, the principal
ing spinal pathway from this comp
principally from the lateral vestibul
(or Deiters' nucleus) (1 , 33, 34, 41,
lateml vestibular nucleus consists of
collection of giant cells in the late
the complex near the entry of the
nerve root. Practically all cells of
vestibular nucleus contribute fibe
formation of this tract, which
the length of the spinal cord in th
part of the lateral funiculus (Fi
11.21).
The vestibulospinal tract, like
spinal tract, is somatotopically orga
181). Cells in dorsolateral parts of th
lar nucleus project to lumbosacral

Vestibular nuclei
Medial - - - - ~
Superior ----~~
Lateral---------...
PONS-MEDULLA
JUNCTION
Spinal nucleus of
trigeminal (V) nerve-+--A~-
Facial (VII) nerve--+---'l~-7"-~
and nucleus
Vestibular nuclei
Medial ---------:::,,._
lnferior-------1--1--
MEDULLA
MEDULLA
- i - - - 1 - - - Inferior cerebellar
peduncle
Middle cerebellar
peduncle
,,..,......,,...__ Vestibular nerve
~---- Nucleus of superior
olive
~----Medial lemniscus
~---- Pyramid
--+------ Inferior cerebellar
peduncle
.-,..,.>,------ Inferior olivary nucleus
--------Pyramid
,---- Spinal accessory (XI)
nerve
~~.....,.~----- Pyramidal decussation
Medial longitudinal fasciculus -------= cb-- - - - - - - Vestibulospinal tract
(vestibular component) (axons of neuron I, uncrossed)

• The VS tract projects to Laminae 8 of gray matter anterior horn, which projects to
interneurons (2nd order) >> AHC (3rd order) neurons. Activate alpha and gamma motor
neurons both. (extensor muscle tone increased, increases in antigravity muscles, to
maintain posture)

• Fibres from medial nuclei project in midline in B/L MLF, they influence cervical motor
neurons so that head moves in such a way to assist in maintaining equlibrium and fixation of
gaze.
C3
Anterior horn cell-neuron Ill-_.,__,,.,.,
Ventral root nerve fiber
T4
L3
S2
l:c.}-Motor end plates in
trapezius and scalene MM.
-- ~ } -
Motor end plates in
intercostal and segmental
back MM .
} -
Motor end plates in
quadriceps femoris M.
} -
Motor end plates in
gastrocnemius M.
Figure 11 .21 . Vestibulospinal tract (blue) and descending vestibular fibers in the medial longitudinal fasciculus (re
Fibers of the vestibulospinal tract have a somatotopic origin in the lateral vestibular nucleus. descend the length
the spinal cord. and terminate predominantly in lamina VIII of Rexed. Descending vestibular fibers in the medial lon
tudinal fasciculus arise from the medial vestibular nucleus. In the lower brainstem, these fibers are bilateral, but in
cervical spinal cord they are ipsilateral. Letters and numbers indicate segmental spinal levels.

• Cerebellar influence on SC (through MLF) and vestibular
influence (through VS tract) the SC activity are mediated
by these tracts

• Lateral vestibular nucleus exerts facilitatory influences on
reflex activity of SC and SC mechanisms which control
muscle tone.

• It has been found, that stimulation of lateral vestibular
nucleus, causes increased muscle tone in EXTENSOR
group of muscles

• Modulation of VS tract, occurs with locomotor rhythm only
when cerebellum is intact. The Ant. Lobe of cerebellum
inhibits the lateral vestibular nucleus, and exerts a
controlling influence on labyrynthine activation of tone.

Reticulospinal tract
• One arises from pontine RF
(ponto reticulospinal),other from
Medullary RF (medullary
reticulopsinal)

• P RS Tract, is U/L,uncrossed,
extends as Ventral RS tract
next to midline. Descend in
entire length of SC. More
numerous than medullary

• PRS tract, terminates on
laminae 8, exerts influence on
axial muscles, particularly NECK
muscles

• Medullary RS arises from medial
2/3rds of Medullary RF, both
crossed and uncrossed. Forms
lateral RS tract. Is present in
entire length of cord
PONS
MEDULLA
Pontine
reticulospinal - - - - - - - , - -a:,
tract
C8
Pontine reticular
formation
Medullary reticulo-
spinal tract
Reticulospinal tracts indicating their regions of origin, course. and terminations. Pontine reticulospinal

• Medullary RS terminates in
laminae 7, 9.

Influence muscle tone via
Gamma motor neuron
(which innervates muscle
spindle) - Extensor muscles
Pontine/ ventral-medial -
excitatory to gamma -
hypertonia (not controlled
by cortex)
medullary/lateral RS -
inhibitory to gamma neuron
- hypotonia (controlled and
stimulated by cortex)
PONS
MEDULLA
Pontine
reticulospinal - - - - - - - , - -a:,
tract
C8
Pontine reticular
formation
Medullary reticulo-
spinal tract
Reticulospinal tracts indicating their regions of origin, course. and terminations. Pontine reticulospinal

Rubrospinal tract
• Fibres arise from red nucleus, in mid brain.

• Cross to opposite side in midbrain after
origin

• In SC, they lie just besides the lateral
corticospinal tracts, partially intermingled
with them

• They have similar somatotopic arrangement
(medial to lateral - CTLS)

• In Midbrain , it gives fibres to C/L facial
nerve nucles (rubrobulbar), C/L medullary
reticular formation (rubroreticular),
cerebellum (rubrocerebellar)

• Stimulation of red nucleus, produces
excitatory post synaptic potentials in C/L
flexor alpha motor neurons, and
inhibitory post synaptic potentials in C/L
extensor alpha motor neuron.
3 4 5
_,, Auditory radiation
MIDBRAIN
,a;o-4,,_.._ _ _ Medial longitudinal
Decussation of sup.-..w~=
fasciculus
cerebellar peduncle
Tectospinal and-------= =------ Rubrospinal and
tectobulbar tracts ...-=:,, ,---.._ rubroreticular tracts
PONS
Facial (VII) nerve
Vagus (X) nerve-
MEDULLA
Nucleus ambiguus
Hypoglossal nerve
CB
T7
Middle cerebellar
peduncle
Rubroreticular fibers
=----- Rubrospinal tract
(axons of neuron I)
>
Ventral root nerve fibers
(axons of neuron Ill)
Rubrobulbar fibres

Anterior horn
(motor neurons)
Lateral
corticospinal
(pyramidal) tract
Dorsal root
Dorsal
spinocerebellar
tract
Ventral
spinocerebellar
tract
Lateral
spinothalamic
tract
C
T
L
S
Ventral
spinothalamic
tract
Pressure, touch
(minor role)
Ventral
(uncrossed)
corticospinal
tract
Tectospinal
tract
S L T C
C
T L S
Fasciculus
cuneatus
Rubrospinal
tract
Lateral
reticulospinal
tract
Vestibulospinal
tract
Ventral
root
Axial and
proximal
limb
movements
Posterior Columns
Distal limb
movements
(minor role)
Pain,
temperature Ventral
reticulospinal
tract
Fasciculus
gracilis
S
L
T C
Distal limb
movements
L/
S
L/
S
P
E
D
F
MLF
Upto c8
Below -
separate
Descending tracts
• Mediation of somatic motor activity
• Control of muscle tone
• Maintainence of posture and equilibrium
• Suprasegmental control of reflex activity
• Modification of sensory input

Spinal Reflexes
• Five essential elements required -

A. Peripheral Receptor

B. Sensory Neuron - enter via dorsal root,
have cell bodies in dorsal root ganglia

C. Local Interneuron -

D. Motor Neuron - through ant. Root.

E. Terminal eﬀector
• Spinal reflexes can be

Monosynaptic/
Polysnyaptic

Intra segmental/
Intersegmental
FIG. 7.22 Polysynaptic spinal reflex arc involved in withdrawal reflex. Note the five components:
(2) an afferent or sensory neuron, (3) an association neuron, (4) an efferent or motor neuron, and
• Stretch reflex (see page 31)
• Golgi tendon reflex (see page 32).
The classification of spinal reflexes is given in Table 7.11.
Table 7.11
Classification of spinal reflexes
Source - Ganong’s Physiology
Clinical Neuroanatomy by Vishram Singh
Carpenter’s Human Neuroanatomy, 9th ed.

• Inputs on motor neuron have 3 consequences -

Voluntary acitivity,

adjust body posture

coordinate action of muscles to make movements smooth and
precise.

• These motor neurons (final common pathway to muscles) alpha
motor, are bombarded by impulses from brain, brain stem , above
by -

• either directly on alpha motor neuron, or mainly to internuerons,
then gamma motor neuron, To regulate the output. ie. more gamma
activity - more output.

• Muscle - Extrafusal and Intrafusal muscle fibres.

• Extrafusal - contractile force, supplied by alpha motor neurone

• Intrafusal - Pure sensory function, do not contribute to
contractile force, Have muscle spindles.
eflex. Asimple reflex arc consisting of an afferent neuron (arising from neurom
indle) and an efferent alpha motor neuron stimulating extrafusal muscle fibres.
urons carrying the nerve impulse synapse with the alpha (α)
the spinal cord. When alpha motor neurons are stimulat
stretched muscle (the stretch reflex). This, in turn, reduces te

Muscle spindles
• Present in intrafusal muscle fibres.
• Therefore serve a sensory function

• Receptor - muscle spindle stretch

• Aﬀerents - carried by type 1a and type 2 fast
myelinated fibres.

• Synapse with - alpha motor neurone

• Causing muscle contraction (extrafusal muscle fibre)

Meissner
corpuscle
Specialized quickly adapting
mechanoreceptor
Myelinated axons Touch
Krause end bulbs Specialized terminal axon ending Small myelinated axons Thermal sensation
Muscle spindles Specialized organ involving
intrafusal muscle fibers and
associated nerves
Large-diameter myelinated axons Muscle length and contraction
eTABLE 30.2 Sensory Afferents
Class (older terminology) Diameter (mm) Conduction velocity (m/sec) Modalities
Ia (Aα) 12–20 70–100 Proprioception (muscle spindles)
Ib (Aα) 12–20 70–100 Proprioception (Golgi tendon organs)
II (Aβ) 5–12 30–70 Touch and pressure from skin, proprioception from
muscle spindles
III (Aδ) 2–5 10–30 Pain and temperature; sharp sensation; joint and
muscle pain sensation
IV (C, unmyelinated) 0.5–2.0 0.5–2.0 Pain, temperature
NOTE: The terminology of sensory afferents has changed throughout the years. The older terminology, indicated in parentheses, spans motor and
sensory modalities, so the newer classification presented here for sensory fibers should be used. The corresponding terminology is presented only
for informational reference.

• Gamma motor neuron forms - 30 % of total output of ventral root,
rest by alpha motor neuron , beta motor neurone(both intra and
extrafusal)

• Supplies intrafusal muscle fibres, motor to muscle spindle.

• Generates stretch reflex, by causing the lengthening and stretching of
the muscle spindle. (doesnt produce detectable muscle contraction)

• Then the stretch reflex occurs. Hence increased Gamma activity
results in heightened muscle spindle sensitivity, heightened stretch
reflexes.

• Gamma motor neuron discharge is controlled by supra spinal
descending pathways. Mainly under inhibitory control by Dorsal
Reticulospinal Tract (medullary), (which is under stimulatory
control of cerebral cortex through corticoreticular fibres)

• Alpha motor neurons - supply extrafusal muscle fibres in
skeletal muscles are the eﬀerent sides of most of the reflex
arcs.

• All neural influences aﬀecting muscular contraction ultimately
funnel through the Alpha motor neuron to the muscles. Hence
“final motor pathway”
• The inputs from other levels of spinal cord, long descending
tracts from the brain, relayed via interneurons, converge on and
determine the activity in the final common pathways.
FIG. 4.3 Stretch reflex. Asimple reflex arc consisting of an afferent neuron (arising from neuromuscular spindle or
neurotendi-nous spindle) and an efferent alpha motor neuron stimulating extrafusal muscle fibres.
The sensory neurons carrying the nerve impulse synapse with the alpha (α) motor neurons in
erior horn of the spinal cord. When alpha motor neurons are stimulated, they cause ra
ntraction of the stretched muscle (the stretch reflex). This, in turn, reduces tension in the intrafu
res.
The stretch reflex is used by clinicians to elicit the tendon jerks (see page 38).
amma reflex loop
ring active muscle contraction, a considerable proportion of motor fibres that arise from hig
ntres in the brain stimulate gamma (γ) motor neurons that innervate intrafusal muscle fib
using their contraction. When intrafusal muscle fibres contract, spindle sensory neurons respo
d through spinal reflex connection to alpha (α) motor neurons, extrafusal muscle fibres contr
e gamma reflex loop thus consists of gamma motor neuron, neuromuscular spindle, affer

Monosynaptic
stretch reflex
• Simplest reflex arc
• Monosynaptic
• Skeletal muscle with intact nerve supply is stretched, it contracts -
stretch reflex.
• 1. Peripheral receptor - muscle spindle located in fleshy part of muscle,
2. Afferent - 1a fast sensory fibres 3. Efferent - alpha motor neurone 4.
effector - muscle
• Absence of dtr/this reflex - abnormality in any of the 4 components (receptor,
sensory neuron, motor neuron, effector muscle).

• Hyperactive - signifies interruption of corticospinal and other descending
pathways that suppress the activity in the reflex arc.
FIG. 4.3 Stretch reflex. Asimple reflex arc consisting of an afferent neuron (arising from
neurotendi-nous spindle) and an efferent alpha motor neuron stimulating extrafusal muscle
The sensory neurons carrying the nerve impulse synapse with the alpha
nterior horn of the spinal cord. When alpha motor neurons are stim
ontraction of the stretched muscle (the stretch reflex). This, in turn, reduc
ibres.
The stretch reflex is used by clinicians to elicit the tendon jerks (see page
Gamma reflex loop

Reciprocal inhibition - Renshaw cell inhibitor
• Collateral from 1a sensory fibre - inhibitory interneuron -
alpha motor neurone of flexor muscle. (bisynaptic)

Inverse stretch reflex
• Mediated by golgi tendon
organs

• Light - moderate stretch -
muscle spindle. 1a contraction
of extensor muscles, extension
of knee. Inhibition of
antagonist flexor muscles of
knee.

• Excessive stretch - GT organ,
1b - bi synaptic - inhibition of
alpha motor. (extension of
knee)

• Activation of alpha motor
neurone of antagonist muscle
(eg. flexor of knee)
Muscle spindle
Golgi tendon
Activation of alpha motor of
flexor of knee

Tone
• Hypotonic - when gamma motor neurone discharge is low

• Hypertonic - when gamma motor neurone discharge is high.

• If all motor connection severed - flaccid.

• Hypertonic muscles - passive flexion of elbow, stretch of triceps, stretch
reflex increased in triceps, therefore increased muscle contraction of
triceps while flexing the elbow. (in initial part of movement)

• Further stretch (further part of movement) would activate Golgi tendon ,
inverse stretch reflex, sudden loosening of the triceps, (inhibition) therefore
resistance to flexing elbow suddenly collapses, and hence free movement.

• Continue passive flexion of elbow, again stretches the triceps muscle,
again causing stretch reflex activation in triceps, and sequence repeated.

• This sequence of resistance followed by a sudden decrease in resistance
when a limb is moved passively - clasp knife spasticity.

Clonus
• Regular repititive, rhythmic contractions of a muscle
subjected to sudden, maintained stretch. (sudden dorsiflexion
of ankle - stretch of post compartment plantar flexor muscles
- contraction (because gamma activity more) hence more
contraction - hence forceful plantar flexion - post
compartment muscles contracted very much - golgi tendon
activation - relaxation of plantar muscles - again stretch.

• Increased gamma motor neurone discharge.

• Stretch reflex - inverse stretch reflex sequence repeated.

• More than 5 - sustained, considered abnormal.

Spasticity
• Spasticity is not related to pyramidal system. Selective damage to pyramidal tract at
the level of medullary pyramids is not followed by spasticity. This kind of damage is a
rare phenomenon. (done in animal trials)

• Spasticity is due to loss of reduction of inhibitory influences of medullary RS tract (which
was inhibitory to gamma motor neuron), this causes increased gamma eﬀerent
discharge.

• SINCE ISOLATED DAMAGE TO PYRIMIDAL FIBRES IS RARE, HENCE BOTH
PYRIMIDAL AND EXTRAPYRIMIDAL TRACT INVOLVEMENT PRODUCES - SPASTICITY.

• Spasticity is maintained by pontine (medial RS tract) which is facilitatory to gamma
motor neuron.

• Vestibulospinal Tract has minor role. Its sectioning did not have a major role in spasticity.

• Along with this, the stimulation of pre central gyrus anterior edge has also found to
inhibit stretch reflex and hence spasticity. This is called Suppressor strip. Damage to
this area results in spasticity.

• In initial SC lesion, only the inhibitory Reticulospinal
tracts are damaged, - increased gamma activation -
spasticity

• In complete SC lesions, facilitatory and inhibitory both
damaged, withdrawl reflexes which were mildly inhibited
by these RS tracts, become more active - which results in
paraplegia in flexion, flexor spasms, etc.

Withdrawal reflexes
• Polysynaptic reflex.

• Crossed extensor reflex

• Intersegmental, multisegmental reflex

Spinal shock
• All spinal reflexes profoundly depressed

• Humans it usually lasts for a minimum of 2 weeks

• Cessation of tonic bombardment of spinal neurons by excitatory impulses in
desecnding pathways

• Spinal inhibitory interneurons that normally are controlled by inhibited by
descending tracts, are then disinhibited, which causes inhibition of motor neurons.

• The recovery of reflex excitability may be due to the development of denervation
hypersensitivity to the mediators released by the remaining spinal excitatory nerve
endings, sprouting of collaterals from the existing neurons, formation of additional
excitatory endings on interneurons and motor neurons

• “bulbocavernous and cremastric reflexes, return first, f/b ankle, babinski,
knee jerk”

• Spasticity develops in antigravity muscles
UL
• adductor and internal
rotators of shoulder
• Flexors of elbow, wrist
and digits
• Pronators of forearm
LL
• Adductors of hip
• Extensors of hip and
knee
• Plantar flexors of foot
and toes

What are anti gravity
muscles ?
• Those muscles which act by stretch reflexes, to counterbalance
the eﬀect of gravity and maintain the body in an upright posture.

• In standing posture, centre of gravity is in front, as if the body is
falling forward. So the antigravity muscles, - extensors of neck,
back, hip and leg muscles should be in contracted state.

• They have a higher intrinsic tone as compared to their antagonistic
muscles.

• Slowly contracting, not easily fatigued, more of red fibres.

• To maintain the posture, these muscles, are activated with the help
fo spinal reflex arc.

• Ant. Spinal
Artery -
Antero
Lateal cord

• PSA -
Post. 1/3rd
of cord

• Segmental
arteries

terial supply of the interior part of spinal cord.
r spinal artery supplies the anterior two-third of the cord, while two
The anastomosis arteria corona is less eﬃcient at the region of lateral coloumns

• Segmental arteries are spinal branches
of (2 cervical, 1 Thoracic, 1 lumbar, 1
sacral)

1. Deep cervical

2. Ascending cervical

3. Posterior intercostal

4. Lumbar

5. Lateral sacral arteries

Enter through the intervertebral foraminas

There are about 8 ant radicular arteries
and 12 post radicular arteries

After reaching the SC they re enforce the
ASA, PSA to form five longtidunal trunks.

• Segmental arteries are spinal
branches of (2 cervical, 1 Thoracic,
1 lumbar, 1 sacral)

1. Deep cervical

2. Ascending cervical

3. Posterior intercostal

4. Lumbar

5. Lateral sacral arteries

Enter through the intervertebral
foraminas

There are about 8 ant radicular arteries
and 12 post radicular arteries

After reaching the SC they re enforce
the ASA, PSA to form five longtidunal
trunks.

• Anterior radicular arteries are larger, but lesser in number.

• Largest Anterior Radicular artery, Artery of Adamkiewicz,
arises between T9- L2, usually on left side, and supplies
lumbar enlargement.
• The caliber of ASA is largest at level of lumbar and cervical
enlargments.

• “ Upper thoracic segments specially at T4 - most vulnerable
to ischemia”

•

• Drain into

1. Vertebral vein

2. Post. Intercostal
vein

3. Lumbar Vein

4. Lateral sacral vein

Intervertebral venous
plexus communicates
with the basilar vein
above - basivertebral
venous plexus
FIG. 7.28 Venous drainage of the spinal cord.
• Two median longitudinal, one in the anterior median fissure and the oth

Radiology
MRI basics – T1 vs T2
T1:
Black (low intensity)
Fluid (e.g. urine, CSF)
Gray (intermediate intensity)
Muscle
Gray matter
White (High intensity)
Fat
White matter
T2
Black (low intensity)
White matter
[Fat; if fat saturated image]
Gray (intermediate intensity)
Muscle
Gray matter
White (High intensity)
Fluid (e.g. urine, CSF)
[Fat; if NOT fat saturated image]

Mid sagittal view is best for visualising SC, Cauda, CSF, vertebral bones
However as most disc herniations are postero-lateral as opposed to straight posterior
its not optimal for disc herniation
1
2
3
4
5
6
7

Mid-Sagittal MRI of the Lumbar Spine. Left (T1-weighted image); Right (T2-weighted image). On this view centered o
spine, one can see all five lumbar vertebrae in addition to the sacrum and lower thoracic vertebrae. Note on the T1 imag
dark and on the T2 image that CSF is bright. Also note the subcutaneous fat which is bright on both T1- and T2-weighted
mid-sagittal view is the optimal view to visualize the spinal cord, cauda equina, CSF and vertebral bones. However, as mo
herniations are posterior lateral (as opposed to straight posterior), the mid-sagittal view may not the optimal view to see

Neuro-Anatomy of the Spinal Cord

Neuro-Anatomy of the Spinal Cord

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