Central Nervous System

2. The spinal cord
• The spinal cord, enclosed in the
vertebral column, extends from the
foramen magnum of the skull to the first
or second lumbar vertebra, just inferior
to the ribs.
• About 42 cm (17 inches) long and 1.8 cm
(3/4 of an inch) thick and the spinal
cord provides a two-way conduction
pathway to and from the brain.

3. • The spinal cord is protected by bone,
meninges, and cerebrospinal fluid.
• Cerebrospinal fluid fills the
subarachnoid space between the
arachnoid and pia mater meninges.

4. Gross Appearance
• Cylindrical in shape
• Foramen magnum  L1/L2 (adult)
• L3 (newborn)
• Occupies upper ⅔ of vertebral canal
• Surrounded by 3 layers of meniges:
– dura mater
– arachnoid mater
– pia mater
• CSF in subarachnoid space

5. Longitudinal depressions
a. Posterior Median Sulcus - PMS
b. Anterior Median Fissure - AMF
Enlargements of the spinal cord
a. Cervical enlargement (biggest)
- pectoral girdle – C4-C8
b. Lumbar enlargement -
supplies and innervates lower limbs

6. • Conus medullaris:- the spinal cord
terminates in a tapering cone-shaped
structure called the conus medullaris.
• Filum termniale
• Anterior median fissure
• Posterior median sulcus
• 31 pairs of spinal nerves attached to it
by the anterior roots & posterior roots

7. • The filum terminale:-
a fibrous extension of
the conus covered by
pia mater, extends
inferiorly from the
conus medullaris to
the coccyx, where it
anchors the spinal
cord so it is not jostled
by body movements

8. • Denticulate ligaments:-
The pia mater of the
spinal cord has a pair of
denticulate ligaments
with 21 attachments per
side which attach it to the
arachnoid and dura
mater.

9. Gray Matter
• H-shaped pillar with anterior &
posterior gray horns
• United by gray commissure
containing the central canal
• Lateral gray column (horn) present in
thoracic & upper lumbar segments
• Amount of gray matter related to the
amount of muscle innervated
• Consists of nerve cells, neuroglia,
blood vessels

10. Nerve cells in the anterior gray columns
• Large and multipolar
– Axons pass out in the anterior nerve roots
as α-efferents
• Smaller nerve cells are multipolar
– Axons pass out in anterior roots as ɣ-
efferents

11. Nerve cells in the
posterior gray columns
• 4 nerve cell groups
1. Substantia gelatinosa
2. Nucleus proprius
3. Nucleus dorsalis
(Clark’s column)
4. Visceral afferent
nucleus

12. 1. Substantia gelatinosa
– situated at the apex
– throughout the length of spinal cord
– composed mainly of Golgi Type II
neurons
– receives afferent fibres concerning with
pain, temperature & touch from
posterior root

13. • Nucleus proprius
– anterior to substantia gelatinosa
– present throughout the whole length
of spinal cord
– main bulk of cells in posterior gray
column
– receives fibers from posterior white
column that are assoc with
proprioception, 2-point discrimination
& vibration

14. • Nucleus dorsalis (Clark’s column)
– base of posterior column
– C8 – L3 / L4
– associated with proprioceptive endings
(neuromuscular spindles & tendon
spindles)
• Visceral afferent nucleus
– lateral to nucleus dorsalis
– T1 – L3
– receives visceral afferent informations

15. The gray matter
• The gray matter of the cord looks like the
letter H or like a butterfly. It consists of
mirror image lateral gray masses connected
by a crossbar of gray matter, the gray
commissure, that encloses the central
canal. The two dorsal projections of the gray
matter are the dorsal (posterior) horns,
and the ventral pair are the ventral
(anterior) horns. The thoracic and
superior lumbar segments of the cord have
an additional pair of gray matter columns,
the small lateral horns.

16. Lateral horns
• Formed by the intermediolateral
group of cells
• T1 – L2 / L3
• Cells give rise to preganglionic
sympathetic fibres
• In S2, S3, S4; they give rise to
preganglionic parasympathetic
fibres

17. • The gray commissure and central canal
– connects the gray on each side
– central canal in the centre
– posterior gray commissure
– anterior gray commissure
– central canal present throughout
– superiorly continuous with the central canal
of medulla oblongata
– inferiorly, expands as terminal ventricle
– terminates within the root of filum terminale

18. a. Dorsal Root - posterior - sensory nerves
b. Dorsal Root Ganglion (contains sensory
nerve cell bodies).
c. Ventral Root - anterior - motor neurons
"AMPS" anterior/motor and posterior/sensory.
Spinal nerves start where the dorsal root
ganglia and ventral root fuse or come together
and join to become a mixed nerve/exit the
spine at the intervertebral foramen.

20. • The spinal gray matter can be divided further
according to its neurons’ relative involvement in
innervating the somatic and visceral regions of the
body.
• Spinal gray matter has the following four zones.
– Somatic sensory (SS)
– Visceral sensory (VS)
– Visceral (autonomic) motor (VM)
– Somatic motor (SM).

21. White Matter
• The white matter of the spinal cord is composed
of myelinated and nonmyelinated nerve fibers
that allow communication between different
parts of the spinal cord and between the cord
and brain. These fibers run in three directions:
– Ascending — up to higher centers (sensory
inputs)
– Descending — down to the cord from the
brain or within the cord to lower levels
(motor outputs)
– Transverse — across from one side of the
cord to the other (commissural fibers)
• Consists of nerve fibres, neuroglia, blood vessels

24. • The white matter on each side of the
cord is divided into three white
columns, or funiculi named
according to their position as
– Dorsal (posterior) funiculi
– Lateral funiculi
– Ventral (anterior) funiculi

25. Tracts
• Ascending
• Descending
• Transverse

26. Ascending Tracts
• Fibres that ascend from spinal cord
to higher centres
• Conduct afferent information which
may or may not reach consciousness
• Information may be
– exteroceptive (pain, temperature, touch)
– proprioceptive (from muscles & joints)

27. • The nonspecific and specific ascending
pathways send impulses to the sensory
cortex
– These pathways are responsible for
discriminative touch (2 pt. discrimination) and
conscious proprioception (body position
sense).
• The spinocerebellar tracts send impulses
to the cerebellum and do not contribute to
sensory perception

28. Ascending Pathway

30. Lateral spinothalamic tract
• Pain & temp pathways
• 1st-order neurons
• Pain conducted by A-
type fibres & C-type
fibres
• 2nd-order neurons
– decussate to the opposite
side
– ends in thalamus (ventral
posterolateral nucleus
• 3rd-order neurons
– ends in sensory area in
postcentral gyrus

31. Anterior (ventral)
spinothalamic tracts
• Light (crude) touch
and pressure pathways

32. Posterior (Dorsal)
spinocerebellar tract
• Muscle joint sense pathways
to cerebellum
• Unconscious proprioception
• Muscle joint info from
muscle spindles, GTO, joint
receptors of the trunk &
lower limbs
• Info is used by the
cerebellum in the
coordination of movements
& maintenance of posture

33. Anterior (Ventral)
spinocerebellar tract
• Majority of 2nd-order
neurons cross to the
opposite side
• Enter cerebellum
through superior
cerebellar peduncle
• Info from trunk, upper
& lower limbs
• Also carries info from
skin & subcut tissue

34. Posterior (Dorsal) white
column
• Discriminative touch, vibratory sense,
conscious muscle joint sense (conscious
proprioception)

35. Descending Tracts
• Descending tracts deliver motor
instructions from the brain to the
spinal cord
• Divided into two groups
– Pyramidal, or corticospinal tracts:-
concerned with voluntary, discrete, skilled
movements
– Indirect pathways, essentially all others
• Motor pathways involve two neurons
– Upper motor neuron (UMN)
– Lower motor neuron (LMN)
• aka ‘anterior horn motor neuron” (final
common pathway)

36. Pyramidal (Corticospinal) Tracts
• Originate in the precentral gyrus of brain (aka, primary
motor area)
– I.e., cell body of the UMN located in precentral gyrus
• Pyramidal neuron is the UMN
– Its axon forms the corticospinal tract
• UMN synapses in the anterior horn with LMN
– Some UMN decussate in pyramids = Lateral corticospinal tracts
– Others decussate at other levels of s.c. = Anterior corticospinal
tracts
• LMN (anterior horn motor neurons)
– Exits spinal cord via anterior root
– Activates skeletal muscles
• Regulates fast and fine (skilled) movements

37. Corticospinal
tracts
1. Location of UMN cell
body in cerebral cortex
2. Decussation of UMN
axon in pyramids are at
exit level of LMN
3. Synapse of UMN and
LMN occurs in anterior
horn of s.c.
4. LMN axon exits via
anterior root

38. Extrapyramidal Motor Tracts
• Includes all motor pathways not part of the pyramidal system
• Upper motor neuron (UMN) originates in nuclei deep in
cerebrum (not in cerebral cortex)
• UMN does not pass through the pyramids!
• LMN is an anterior horn motor neuron
• This system includes
– Rubrospinal
– Vestibulospinal
– Reticulospinal
– Tectospinal tracts
• Regulate:
– Axial muscles that maintain balance and posture
– Muscles controlling coarse movements of the proximal portions of limbs
– Head, neck, and eye movement

39. Reticulospinal tract
• The two recticulospinal
tracts have differing
functions:
• The medial reticulospinal
tract arises from the pons.
It facilitates voluntary
movements, and increases
muscle tone.
• The lateral reticulospinal
tract arises from
the medulla. It inhibits
voluntary movements, and
reduces muscle tone

40. Tectospinal tract
• This pathway begins at
the superior colliculus of the
midbrain. The superior
colliculus is a structure that
receives input from the optic
nerves. The neurones then
quickly decussate, and enter
the spinal cord. They
terminate at the cervical
levels of the spinal cord.
• The tectospinal tract
coordinates movements of
the head in relation to vision
stimuli.

41. Rubrospinal tract
• The rubrospinal tract
originates from the red
nucleus, a midbrain
structure. As the fibres
emerge, they decussate
(cross over to the other side
of the CNS), and descend into
the spinal cord. Thus, they
have
a contralateral innervation.
• Its exact function is unclear,
but it is thought to play a role
in the fine control of hand
movements

42. Vestibulospinal Tracts
• There are two vestibulospinal
pathways; medial and lateral.
They arise from the vestibular
nuclei, which receive input from
the organs of balance. The tracts
convey this balance information
to the spinal cord, where it
remains ipsilateral.
• Fibres in this pathway
control balance and posture by
innervating the ‘anti-gravity’
muscles (flexors of the arm, and
extensors of the leg), via lower
motor neurones.

45. Other Tracts
• Spinotectal tract
– The tectospinal tract (also known as colliculospinal tract) is
a nerve pathway that coordinates head and eye movements.
This neural tract is part of the indirect extrapyramidal tract. To be
specific, the tectospinal tract connects the midbrain tectum
and cervical regions of the spinal cord.
• Spinoreticular tract
– The spinoreticular tract is an ascending pathway in the white matter
of the spinal cord, positioned closely to the lateral spinothalamic tract.
The tract is from spinal cord to reticular formation to thalamus.
– It is responsible for automatic responses to pain, such as in the case of
injury.
• Spino-olivary tract
– This is a non-specific indirect ascending pathway and is connected to
olivary nuclei in the brain. It is located in the ventral funiculus of
the spinal cord. This tract carries proprioception information
from muscles and tendons as well as cutaneous impulses to
the olivary nucleus.

46. Spinal Meninges
• Dura mater
• Arachnoid mater
• Pia mater

47. Dura mater
• Dense, strong fibrous membrane
• Encloses the spinal cord & cauda
equina
• Continuous above with meningeal
layer of dura covering the brain
• Ends at the level of S2
• Separated from wall of vertebral
canal by the extradural space
• Contains loose areolar tissue and
internal vertebral venous space

48. Arachnoid mater
• Delicate impermeable membrane
• Lies between pia and dura mater
• Separated from pia mater by
subarachnoid space
• Continuous above with arachnoid
mater covering the brain
• Ends on filum terminale at level of
S2

49. Pia mater
• Vascular membrane
• Closely covers spinal cord
• Thickened on either side between
nerve roots to form the ligamentum
denticulatum

50. Blood supply
Arteries of the spinal cord
• Anterior spinal artery
• Posterior spinal artery
• Segmental spinal arteries

51. Anterior spinal artery
• Formed by the union of 2 arteries
• From vertebral artery
• Supply anterior ⅔ of spinal cord
Posterior spinal arteries
• Arise from vertebral artery or
posterior inferior cerebellar arteries
(PICA)
• Descend close to the posterior roots
• Supply posterior ⅓ of spinal cord

53. Segmental spinal arteries
• Branches of arteries outside the
vertebral column
• Gives off the anterior & posterior
radicular arteries
• Great anterior medullary artery of
Adamkiewicz
• Arise from lateral intercostal artery
or lumbar artery at any level from
T8 – L3

55. Venous drainage
• Venous drainage largely follows arterial
supply. That is, there
are anterior and posterior spinal
veins and anterior and posterior radicular
veins, which freely communicate with
the internal vertebral plexus in the epidural
space. This is in turn drains to the
cerebral dural venous sinuses and cerberal
veins as well as the external vertebral
plexus.
• The veins of the spinal cord and vertebral
column are valveless.

57. Applied anatomy
Spinal shock
• Follows acute severe damage to the
spinal cord
• All cord functions below the level of
the lesion become depressed or lost
• Sensory impairment and flaccid
paralysis occur
• Segmental spinal reflexes are
depressed
• Persists for less than 24 hours (may
be as long as 1 – 4 weeks)

58. Poliomyelitis
• Acute viral infection of the neurones
of anterior gray column
• Motor nuclei of cranial nerves
• Death of motor neurone cells →
paralysis & wasting of muscles
• Muscles of lower limb more often
affected

59. Spinal Nerves
• A spinal nerve is a mixed nerve, which carries
motor, sensory, and autonomic signals between
the spinal cord and the body. In the human body
there are 31 pairs of spinal nerves, one on each
side of the vertebral column.
• These are grouped into the corresponding
– Cervical nerves 8 pairs
– Thoracic nerves 12 pairs
– Lumbar nerves 5 pairs
– Sacral nerves 5 pairs
– Coccygeal nerves one pair
• The spinal nerves are part of the peripheral
nervous system.

60. Structure of spinal nerve
• 3 layers of Connective tissues
– Epineurium
• Outermost layer
• Consists of dense network
of collagen fibers
– Perineurium
• Middle layer
• Divide nerve into series of
compartments which
contain bundles of axons
(fascicles)
– Endoneurium
• Innermost layer
• Surround individual axons

61. • Spinal nerves consist of two types of
nerves:
– Sensory nerves
– Motor nerves

62. • Sensory nerves deliver
information to spinal
cord from muscles and
joints about body
position and also
transmit sensations such
as touch, pressure, pain
and temperature.

63. Motor nerves
Motor nerves pass information received from
brain through spinal tracts to the skeletal
muscles to direct precise voluntary movements.
Spinal nerves are linked to specific muscles:
• Cervical spinal nerves supply the muscles of
neck, shoulders, arms and hands, and
diaphragm.
• Thoracic spinal nerves supply truck muscles
and muscles involved with breathing.
• Lumbar and sacral spinal nerves supply hip,
leg and foot muscles.
• Sacral nerves supply anal and urethral
sphincters.

65. Distribution of
Spinal Nerves
Spinal Nerves:
– Consist of dorsal root and ventral root
– Branch to form pathways to
destination
– Includes motor and sensory nerves
Dermatomes:
a. Each pair of spinal nerves controls a region
of body surface sensation - the exception
to this is C1, which does not.
b. From dorsal and ventral rami fibers
c. Damage to the spinal nerve results in loss
of sensation to a region of skin
d. This is a helpful diagnostic tool, sometimes
pain is referred from one nerve to a
corresponding region of skin.

66. Spinal Nerves: Rami
• The short spinal nerves branch into
three or four mixed, distal rami
– Small dorsal ramus – to back
– Larger ventral ramus – to
plexuses/intercostals
– Tiny meningeal branch – to meninges
– Rami communicantes at the base of the
ventral rami in the thoracic region –
to/from ANS

67. Nerve Plexuses
• All ventral rami except T2-T12 form interlacing
nerve networks called plexuses
• Plexuses are found in the cervical, brachial,
lumbar, and sacral regions
• Each resulting branch of a plexus contains
fibers from several spinal nerves
• Fibers travel to the periphery via several
different routes
• Each muscle receives a nerve supply from
more than one spinal nerve
• Damage to one spinal segment cannot
completely paralyze a muscle

68. Spinal Nerve Innervation:
Back, Anterolateral Thorax, and
Abdominal Wall
• The back is innervated by
dorsal rami via several
branches
• The thorax is innervated
by ventral rami T1-T12 as
intercostal nerves
• Intercostal nerves supply
muscles of the ribs,
anterolateral thorax, and
abdominal wall

69. The 4 Major Plexuses of
Ventral Rami
1. Cervical plexus
2. Brachial plexus
3. Lumbar plexus
4. Sacral plexus

70. Cervical Plexus
• The cervical plexus is
formed by ventral rami of
C1-C4 (C5)
• Most branches are
cutaneous nerves of the
neck, ear, back of head,
and shoulders
• The most important nerve
of this plexus is the
phrenic nerve
• The phrenic nerve is the
major motor and sensory
nerve of the diaphragm

71. Brachial Plexus
• Formed by C5-C8 and T1
(C4 and T2 may also
contribute to this
plexus)
• It gives rise to the
nerves that innervate
the upper limb

72. Trunks and Cords of
Brachial Plexus
• Nerves that form brachial plexus originate from:
– superior, middle, and inferior trunks
– large bundles of axons from several spinal nerves
– lateral, medial, and posterior cords
– smaller branches that originate at trunks

73. Brachial Plexus: Nerves
• Axillary – innervates the
deltoid and teres minor
• Musculocutaneous – sends
fibers to the biceps brachii
and brachialis
• Median – branches to most
of the flexor muscles of
forearm
• Ulnar – supplies the flexor
carpi ulnaris and part of the
flexor digitorum profundus
• Radial – innervates
essentially all extensor
muscles

74. Lumbar Plexus
• Arises from (T12) L1-L4 and
innervates the thigh,
abdominal wall, and psoas
muscle
• The major nerves are the
femoral and the obturator

75. Sacral Plexus
• Arises from L4-S4 and
serves the buttock,
lower limb, pelvic
structures, and the
perineum
• The major nerve is the
sciatic, the longest and
thickest nerve of the
body
• The sciatic is actually
composed of two
nerves: the tibial and
the common fibular
(perineal) nerves

76. Nerve plexuses - Summary
• Cervical – C1-C4
– Phrenic nerve
• Brachial – C5 – T1
(roots/trunks/divisions/cords)
– Axillary, MC, median, ulnar, radial
• Lumbar – L1-L4
– Femoral, obturator
• Sacral – L4-S4
– Sciatic (common peroneal/tibial), pudendal

77. Figure 13–13a
5 Patterns of Neural Circuits in
Neuronal Pools
1. Divergence:
– spreads
stimulation to
many neurons
or neuronal
pools in CNS
2. Convergence:
– brings input
from many
sources to
single neuron

78. Figure 13–13c
3. Serial processing:
– moves
information in
single line
4. Parallel
processing:
– moves same
information along
several paths
simultaneously
5 Patterns of Neural Circuits in Neuronal Pools

79. Figure 13–13e
5. Reverberation:
– positive feedback
mechanism
– functions until inhibited

80. Reflex activity
• 5 components of
a reflex arc
– Receptor
– Sensory neuron
– Integration
center (CNS)
– Motor neuron
– Effector

81. 4 Classifications of Reflexes
1. By early development
– Innate or Acquired
2. By type of motor response
– Somatic or Visceral
3. By complexity of neural circuit
– Monosynaptic or Polysynaptic
4. By site of information processing
– Spinal or Cranial

82. Spinal Reflexes
• Range in increasing order of
complexity:
– monosynaptic reflexes
– polysynaptic reflexes
– intersegmental reflex arcs:
• many segments interact
• produce highly variable motor response

83. Monosynaptic Reflexes
• Have least delay
between sensory
input and motor
output:
– e.g., stretch reflex
(such as patellar
reflex)
• Completed in 20–
40 msec

84. Muscle Spindles
• The receptors in stretch
reflexes
• Bundles of small,
specialized intrafusal
muscle fibers:
– innervated by sensory
and motor neurons
• Surrounded by
extrafusal muscle
fibers:
– which maintain tone and
contract muscle

85. Postural Reflexes
• Postural reflexes:
– stretch reflexes
– maintain normal upright posture
• Stretched muscle responds by
contracting:
– automatically maintain balance

86. Polysynaptic Reflexes
• More complicated than
monosynaptic reflexes
• Interneurons control more than 1
muscle group
• Produce either EPSPs or IPSPs

87. The Tendon Reflex
• Prevents skeletal muscles from:
– developing too much tension
– tearing or breaking tendons
• Sensory receptors unlike muscle
spindles or proprioceptors

88. Withdrawal Reflexes
• Move body part away
from stimulus (pain or
pressure):
– e.g., flexor reflex:
• pulls hand away from
hot stove
• Strength and extent of
response:
– depends on intensity
and location of
stimulus

89. Reciprocal Inhibition
• For flexor reflex to work:
– The stretch reflex of antagonistic
(extensor) muscle must be inhibited
(reciprocal inhibition) by interneurons
in spinal cord

90. Crossed Extensor Reflexes
• Occur simultaneously,
coordinated with
flexor reflex
• e.g., flexor reflex
causes leg to pull up:
– crossed extensor reflex
straightens other leg
– to receive body weight
– maintained by
reverberating circuits

91. Integration and Control
of Spinal Reflexes
• Though reflex behaviors are automatic:
– processing centers in brain can facilitate or
inhibit reflex motor patterns based in spinal
cord
• Higher centers of brain incorporate lower,
reflexive motor patterns
• Automatic reflexes:
– can be activated by brain as needed
– use few nerve impulses to control complex
motor functions
– walking, running, jumping

92. Superficial reflexes
• Stroking of the skin elicits muscle contraction
– Involves functional upper motor pathways as well
as cord level reflex arcs
• Plantar reflex (L4-S2) Babinski is normal in
infants
– Usually indicative of CNS damage in adults
• Abdominal reflex (T8-T12)
– Absent with corticospinal lesion

93. Spinal Cord Trauma:
Transection
• Cross sectioning of the spinal cord
at any level results in total motor
and sensory loss in regions inferior
to the cut
• Paraplegia – transection between
T1 and L1
• Quadriplegia – transection in the
cervical region

94. Applied anatomy/physiology
Peripheral Neuropathies
• Regional loss of sensory or motor function
• Due to trauma or compression
• Example: if your foot “falls asleep”
Shingles
• Caused by varicella-zoster virus (chickenpox)
• After chickenpox, virus hides in neurons of spinal cord
• Later in life, attacks neurons in dorsal roots of nerves =
painful rash/blisters
• Distribution of rash corresponds to dermatome nerves
affected

95. THANK YOU