NEURO-ANATOMY ASCENDING AND DESCENDING TRACTS (1).ppt

Neurophysiology

1. Shadrack Asena.
Department of human anatomy.
The structure of the
Spinal Cord

2. Spinal Cord
Runs through the vertebral canal
Extends from foramen magnum to second
lumbar vertebra (L
2)
Regions
Cervical
Thoracic
Lumbar
Sacral
Coccygeal
Gives rise to 31 pairs of spinal nerves
All are mixednerves
Not uniform in diameter
Cervical enlargement: supplies upper limbs
Lumbar enlargement: supplies lower limbs
Conusmedullaris-tapered inferior end
Ends between L1 and L2
Caudaequina-origin of spinal nerves
extending inferiorly from conusmedullaris.

3. Spinal enlargements
The form and quantity of the gray matter vary at
different levels of the spinal cord .
The proportion of gray to white matter is greatest
in the lumbarand cervical enlargements.
In the cervical region, the dorsal gray column is
comparatively narrow and the ventral column is
broad and expansive, especially in the four lower
cervical segments.
In the thoracic region, both the dorsal and
ventral columns are narrow, and there is a lateral
column-sympathetics.
In the lumbar region, the dorsal and ventral
columns are broad and expanded. In the conus
medullaris, the gray matter looks like two oval
masses, one in each half of the cord, connected
by a wide gray commissure

4. Spinal cord levels in relation to age
During growth and development the vertebral
column lengthens faster than the spinal cord hence
Age Level of termination
3 months (iul) coccygeal
6 months (iul) 2
nd
sacral
At birth L
3
Puberty L
2
Adult L
1–L
2

5. Enlargements of spinal cord

6. Protection and Support of the Spinal Cord
Protection Support and anchorage
Vertebral column -Spinal nerves
Spinal meninges -Continuation
with brain stem
Cerebrospinal fluid -Denticulate
ligaments

7. Meninges
Connective tissue membranes
1.Dura mater (patchymeniges): outermost
layer; continuous with epineuriumof the
spinal nerves
2.Arachnoidmater (leptomeninges): thin and
wispy
3.Piamater: bound tightly to surface
Forms the filumterminale
•anchors spinal cord to coccyx
Forms the denticulate ligaments that attach the
spinal cord to the dura
Spaces
1.Epidural:external to the dura
1.Anesthesticsinjected here
2.Fat-fill
2.Subdural space: serous fluid
-it is a potential space
1.Subarachnoid:between piaand arachnoid
1.Filled with CSF

8. Cross Section of
Spinal Cord
It has an Anterior median fissure and
posterior median sulcus.
deep clefts partially separating left and
right halves
The Gray matter: contains neuron cell
bodies, dendrites, axons(un-myelinated)
Divided into horns
1.Posterior (dorsal) horn
2.Anterior (ventral) horn
3.Lateral horn
White matter: it contains Myelinated
axons
Divided into three columns(funiculi)
1.Ventral
2.Dorsal
3.lateral
Each of these divided into sensoryor
motor tracts

9. Cross-section of the S.C

10. Cross section of Spinal Cord
(A) Commissures: these are connections
between left and right halves of the
spinal cord
1.Gray with central canal in the
center
2.White commissure
(B) Roots
Spinal nerves arise as rootlets
then combine to form dorsal and
ventral roots
Dorsaland ventral roots merge
laterallyand form the spinal
nerve

11. spinal nerve and their functional components
I. General somatic efferents-(GSE) motorto skeletal muscle. The
neurons in the ventral hornsof the spinal cord.
II. General visceral efferent (GVE) -autonomic or visceral motor, to
smooth muscle, cardiac muscle and glands of the viscera. The
neurons reside on the intermediolateralcell column and their axons
i.e. pre-ganglionicsympathetic nervefibres, also join the spinal
nerve forming the white communicating ramusonly at T1-L3
(sympathetic) and S1-S2 (parasympathetic).
iii. General somatic afferent (GSA)-sensory to somatic tissue
(muscle, skin and joints). The cell bodies are located in the dorsal
root ganglia (DRG), and their central processes enter the dorsal
hornof the spinal cord.
IV. General visceral afferent (GVA)-sensory from the visceral
structures. GVA fibresusually accompany the GVE fibres, but
eventually spilt off to go to the dorsal horn, via the dorsal root
ganglion.

13. Nerve Pathways into the Spinal Cord
sensory
pathway
motor
pathway

16. Organization of Spinal Cord Gray Matter
Recall that the gray matter is divided into 3 horns
Dorsal, ventraland lateral-(only in thoracic region).
Dorsal half –sensory roots and ganglia
Ventral half –motor roots
Based on the type of neurons/cell bodies located in each
horn, it is specialized further into 4 regions
1.Somatic sensory (SS) -axons of somatic sensory neurons
2.Visceral sensory (VS) -neurons of visceral sensory neurons.
3.Visceral motor (VM) -cell bodies of visceral motor neurons
4.Somatic motor (SM) -cell bodies of somatic motor neurons

17. Gray Matter: Organization
Figure 12.31

18. Organization of the spinal lamina of the gray matter
the gray matter of the spinal cord shows a number of laminas (layers of nerve cells),
termed Rexed'slaminaeI-X
1.Lamina I : This is thin marginal layer contains neurons that respond to noxious stimuli and send axons to the
contralateralspinothalamictract.-posterolateraltracts of lissauer
2.Lamina II : Also known as substantiagelatinosa,this lamina is made up of small neurons, some of which respond to
noxious stimuli. Substance P,a neuropeptideinvolved in pathways mediating sensibility to pain, is found in high
concentrations in laminas I and II.
3.Laminas III and IV: These are referred to together as the nucleus proprius.Their main input is from fibers that
convey positionand light touch sense.
4.Lamina V:-This layer contains cells that respond to visceral afferent stimuli.
5.Lamina VI : This deepest layer of the dorsal horn contains neurons that respond to mechanical signals from joints and
skin.
6.Lamina VII:-This is a large zone that contains the cells of the dorsal nucleus (Clarke's column)medially as well as a
large portion of the ventral gray column. Clarke's column contains cells that give rise to the posterior spinocerebellar
tract.Lamina VII also contains the intermediolateralnucleus(or intermediolateralcell column) in thoracic and upper
lumbar regions. Preganglionicsympathetic fibers project from cells in this nucleus, via the ventral roots and white rami
communicantes, to sympathetic ganglia.
7.Laminas VIII and IX:-These two represent motor neuron groups in the medialand lateral portions of the ventral gray
column.
The medial portion (also termed the medial motor neuron column) contains the LMNs that innervate axial
musculature (ie, muscles of the trunk and proximal parts of the limbs).
The lateral motor neuron columncontains LMNs for the distal muscles of the arm and leg.
NB> In general, flexor muscles are innervated by motor neurons located close to the central canal, whereas extensor muscles
are innervated by motor neurons located more peripherally

20. White Matter in the Spinal Cord
It is Divided into three funiculi(columns) –
1.posterior,
2.lateral, and
3.Anterior
Each Columns contain 3 different types of fibers
1.Ascending fibres–ascending tracts
2.Descending fibres–descending tracts, or
3.Transverse fibres-commissure/decussations.
Fibers run in three directions
Ascending fibers -compose the sensory tracts
Descending fibers -compose the motor tracts
Commissural (transverse) fibers -connect opposite sides of
cord

21. White Matter Fiber Tract generalizations
1.Pathways decussate in most cases
2.Most consist of a chain of two or three neurons-
(1
st
order, 2
nd
order, 3
rd
order neurons)
3.Most exhibit somatotopyi.e. (precise spatial
relationships)
4.All pathways are paired
1.one on each side of the spinal cord

22. Blood supply to the spinal cord
Derived from:-
1.The anteriorand posteriorspinal arteries
2.Radicular arteriesderived from segmental vessels at
various level i.e. ascending cervical, deep cervical,
inferior thyroid; intercostal, lumbar and sacral arteries.
These provide the principal blood supply to thoracic,
lumbar, sacral and coccygeal spinal segments
3.Arteriaradiculariesmagna(artery of adamkiewicz) left
4.The veins drain to both the internal and external vertebral
venous plexuses

24. Clinical Correlateson spinal cord
1. Vulnerable spinal segments :
-These are cord segments which receive blood supply predominantly from one source,
-They suffer vascular insufficiency if the source is occluded-spinal cord infarction.
-They include (T1-T2) and first lumbar spinal segments –the T1 and T2 and L1 arteries do
not interconnect with other arteries, in the same way as the arteries of the cervical and
lumbar regions do. Thus occlusion of one intercostal artery in a vulnerable region can result
in.
2. Spread of malignancy to the central nervous system
There are free venous connections between the external segmental veins and the internal vertebral
venous plexus which is devoid of valves. Therefore, malignancies for example of the prostate, the
breast and the thyroid may freely spread haematogenously, to the vertebral column, and central
nervous system, including the brain.
3. Localization of spinal cord lesions
Can be done in two ways:
(a) Testing for impairment or loss of cutaneous sensation.alongthebdermatomes .
(b) Reflex contraction of muscles. The segments involved in the more commonly tested
stretch or tendon reflexes are as follows:
1.Biceps reflex, C5 and C6;
2.Triceps reflex, C6 through C8;
3.Quadriceps reflex, L2 through L4;
4.Gastrocnemius reflex, S1 and S2.

25. Body dermatomes

26. ascending and
descending tracts-
(Sensory & Motor
Pathways)
SHADRACK ASENA.
Department of human anatomy.

27. Descending (Motor) Pathways
Descending tracts: they relay motorinstructions
from the brain to the spinal cord
They are divided into two groups
1.Pyramidal, or corticospinal, tracts
2.Indirect or extrapyramidalpathways, essentially
all others
The Motor pathways involve two neurons
1.Upper motor neuron (UMN)
2.Lower motor neuron (LMN)
aka ‘anterior horn motor neuron” (also, final
common pathway)

28. Extrapyramidal Motor Tracts
1.They Includes all the other motor pathways that are not part of the pyramidal
system
2.Upper motor neuron (UMN) originates in nuclei deep in cerebrum (notin
cerebral cortex)
3.Upper Motor Neuronesdoes notpass through the pyramids!
4.Lower Motor Neuronesexit through the anterior horn motor neuron
5.This system includes
1.Rubrospinaltract -facilitate flexor muscles and inhibit extensors
2.Vestibulospinaltract-facilitate extensors muscles and inhibit flexors
3.Reticulospinaltract -fibers from reticular activating system
4.Tectospinaltracts-audio-visual reflex activities in tilting the head
6.They Regulate:
1.Axial muscles that maintain balance and posture
2.Muscles controlling coarse movements of the proximal portions of limbs
3.Head, neck, and eye movement

29. Origins of Extrapyramidal(Multineuronal)
Pathways
Reticulospinaltracts –originates at reticular formation of brain;
maintain balance
Rubrospinaltracts –originate in ‘red nucleus’ of midbrain;
control flexor muscles
Tectospinaltracts -originate in superior colliculiand mediate
head and eye movements towards visual targets (flash of light)
Vestibulospinaltracts:-originate from the vestibular nuclei
concerned head movomentstowards audio targets

30. Extrapyramidal Tract-
general features
Note:
1. UMN cell body location (red
nucleus or other deep nuclei)
2. UMN axon decussates in pons
3. Synapse between UMN and LMN
occurs in anterior horn of s.c.
3. LMN exits via ventral root
4. LMN axon stimulates skeletal
muscle

31. White Matter: Pathway
Generalizations

32. Cortical spinal tracts
General features of the Pyramidal or the (Corticospinal) Tracts
1.They Originate in the precentralgyrusof brain(aka, primary motor area)
I.e., cell body of the UMN located in precentralgyrus
2.Pyramidal neuron is the upper motor neuron(UMN)
Its axon forms the corticospinaltract
3.UMN synapses in the anterior horn with LMN
Some UMN decussate in pyramids = Lateral corticospinaltracts
Others decussate at other levels of s.c. = Anterior corticospinaltracts
4.LMN (anterior horn motor neurons)
Exits spinal cord via anterior root
Activates skeletal muscles
5.Regulates fast and fine (skilled) movements
6.They are in two tracts (anteriorand lateral cortiospinaltracts)

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

34. (Ascending tracts)
Sensory Pathways

35. General features of the Sensory Pathways
1.Monitor conditions both inside the body and in the
external environment
2.Sensation-stimulated receptor passes information
to the CNS via afferent (sensory) fibers
3.Most sensory information is processed in the
spinal cord , thalamus, or brain stem. Only 1%
reaches the cerebral cortex and our conscious
awareness
4.Processing in the spinal cord can produce a rapid
motor response (stretch reflex)
5.Processing within the brain stem may result in
complex motor activities (positional changes in the
eye, head, trunk)

36. Ascending sensory Tracts
Transmit impulses to two destinations:-
a) To the cerebral cortex:-Concerned with specific
sensory modalities: pain, temperature, touch,
proprioception, that reach a conscious level
I.Fasciculus gracile
II.Fasciculus cuneatus
III.Lateral spinothalmictracts
IV.Anterior spinal thalamic tracts
(b) To the cerebelum:-concerned with tactile and
stretch receptors to subconscious centers
located in the cerebelum
I.Anterior Spinocerebellartracts.
II.posterior spinal cerebellartracts.
III.cuniocerebellartracts

37. There is a continuous flow of information between the
brain, spinal cord, and peripheral nerves. This
information is relayed by sensory (ascending)and
motor (descending)‘pathways’.
Generally the pathways exhibit the following
characteristics:
1.Consists of a chain of tracts,associated nucleiand
varying number of relays (synapses)
2.Consist of two or three neurons
3.Exhibit somatotopy(precise spatial relationships)
4.Decussate -(contralateralrepresentation)
5.Involve both the brain and spinal cord
6.Are paired (bilaterally and symmetrically)

38. Organization of Ascending Tracts
3. The 3
rd
order Neuron: start from Thalamus VN-PL-T
and Terminate in the Cerebral Cortex” postcentralgyrus”
2. 2
nd
order Neuron: are located in the Spinal Cord
(serve as inter-neurones)
1. 1
st
order Neuron: Dorsal Root Ganglion (Spinal Ganglion)
from the sensory receptors.

39. Ascending Spinal Tracts
1.Anterior spinothalamic-transmits pressure and non discriminative
touch information
2.Lateral spinothalamic-transmits pain and temperature impulse
3.Anterior spinocerebellar-transmits information on muscle joint sense
4.Posterior spinocerebellar-transmits information from muscle spindles
and tendon spindles
5.Cuneocerebellar-direct root on muscle joint sencefrom upper trunk
6.Spinotectal–information on the head position following visual tagets
7.Spinoreticul-reticular activating pathway for emergency and alertness
8.Spino-olivary-information on the head position following audio targets
9.Visceral sensory tracts-information from the organs and glands

40. Sensory Pathways
Contain a sequence of THREE
neuronsfrom the receptor to the
cerebral cortex
First order neuron:
Sensory neuron that delivers
information from the receptor to
the CNS.
The Cell body is located in the
dorsal root ganglion.
The Axon (central process)
passes to the spinal cord
through the dorsal root of spinal
nervegives many collaterals
which take part in spinal cord
reflexes runs ipsilaterallyand
synapses with second-order
neurons in the cord and medulla
oblongata
1
2
3

41. Second order neuron:
Has cell body in the
spinal cord or medulla
oblongata
Axon decussate &
Terminate on 3rd order
neuron
Third order neuron:
Has cell body in the
thalamus
Axon terminates on
cerebral cortex
ipsilaterally

42. White Matter: Pathway Generalizations
Ascending and descending fibers are organized in
distinct bundles which occupy particular areas and
regions in the white matter
Generally long tracts are located peripherally in the white
matter, while shorter tracts are found near the gray
matter
•The TRACTis a bundle of nerve fibers (within CNS)
having the same origin, course, destination &
function
•The name of the tract indicates the origin and
destination of its fibers
•The axons within each tract are grouped according
to the body region innervated

43. Tracts of the Spinal Cord
Tracts that serve to join brain to the spinal
cord
Ascending
Descending
Fibers that interconnect adjacent or distant
segments of the spinal cord
Intersegmental(propriospinal)

44. Three major pathways carry sensory information
Posterior column pathway (gracile& cuneate
fasciculi)
Anterolateralpathway (spinothalamic)
Spinocerebellarpathway

45. Dorsal Column
Contains two tracts, Fasciculus
gracilis(FG)& fasciculus cuneatus
(FC)
Carry impulses concerned with
proprioceptionand discriminative
touch from ipsilateralside of body
Contain the axons of primary
afferent neuronsthat have entered
cord through dorsal roots of spinal
nerves
FGcontains fibers received at sacral, lumbar and lower
thoraciclevels, FCcontains fibers received at upper thoracic
and cervical levels

46. Fibers ascend without
interruption where they
terminate to the 2
nd
order
neuronsin nucleus gracilis
and nucleus cuneatus
The axons of the 2
nd
order
neurons decussatein the
medulla as internal arcuate
fibersand ascend through the
brain stem as medial
lemniscus.
The medial lemniscus
terminates in the ventral
posterior nucleus of the
thalamusupon 3
rd
order
neurons, which project to the
somatosensorycortex
(thalamocorticalfibers)

48. SpinothalamicTracts
Located lateral and ventral to
the ventral horn
Carry impulses concerned
with pain and thermal
sensations (lateral tract)and
also non-discriminative
touchand pressure (medial
tract)
Fibers of the two tracts are
intermingled to some extent
In brain stem, constitute the
spinal lemniscus
Fibers are highly somato-
topically arranged, with those
for the lower limblying most
superficially and those for the
upper limblying deeply
Information is sent to the
primary sensory cortex on
the opposite side of the body

49. Lateral SpinothalamicTract
Carries impulses concerned
with painand thermal
sensations.
Axons of 1
st
order neurons
terminate in the dorsal horn in
the Substanciageratinosa
Axons of 2
nd
order neuron
decussate within one segment
of their origin, by passing
through the ventral white
commissure& terminate on 3
rd
order neurons in ventral
posterior nucleusof the
thalamus
Thalamic neurons project to
the somatosensorycortex

50. Anterior SpinothalamicTract
Carries impulses concerned
with non-discriminative touch
and pressure
Axons of 1
st
order neurons
enter cord terminate in the
dorsal horn
Axons of 2
nd
order neuron
(mostly in the nucleus
proprius) may ascend several
segments before crossingto
opposite side by passing
through the ventral white
commissure& terminate on 3
rd
order neurons in ventral
posterior nucleusof the
thalamus
Thalamic neurons project to
the somatosensorycortex

51. Spino-reticulo-thalamic System
The system represents an additional route
by which dull, aching painis transmitted to
a conscious level
Some 2
nd
order neuronsterminate in the
reticular formationof the brain stem,
mainly within the medulla
Reticulothalamicfibersascend to
intralaminarnuclei of thalamus, which in
turn activate the cerebral cortex

52. SpinocerebellarTracts
The spinocerebellarsystem
consists of a sequence of
only two neurons
Two tracts: Posterior&
Anterior
Located near the
dorsolateraland
ventrolateralsurfaces of the
cord
Contain axons of the second
order neurons
Carry information derived
from muscle spindles, Golgi
tendon organs and tectile
receptors to the cerebellum
for the control of posture and
coordination of movements

53. Posterior SpinocerebellarTracts
Present only above level
L3
The cell bodies of 2
nd
order neuron lie in
Clark’s column
Axons of 2
nd
order
neuron terminate
ipsilaterally(uncrossed)
in the cerebellarcortex
by entering through the
inferior cerebellar
peduncle

54. Ventral SpinocerebellarTracts
The cell bodies of 2
nd
order
neuron lie in base of the dorsal
hornof the lumbosacralsegments
Axons of 2
nd
order neuron crossto
opposite side, ascend as far as
the midbrain, and then make a
sharp turn caudally and enter the
superior cerebellarpeduncle
The fibers cross the midline for a
second time within the cerebellum
before terminating in the
cerebellarcortex
Both spinocerebellartracts convey
sensory information to the same
side of the cerebellum

55. SpinotectalTract
Ascends in the anterolateral
part in close association with
spinothalamicsystem
Primary afferents reach dorsal
horn through dorsal roots and
terminate on 2
nd
order neurons
The cell bodies of 2
nd
order
neuron lie in base of the dorsal
horn
Axons of 2
nd
order neuron
cross to opposite side, and
project to the periaquiductal
gray matter and superior
colliculusin the midbrain

56. Spino-olivaryTract
Indirect spinocerebellar pathway (spino-
olivo-cerebellar)
Impulses from the spinal cord are
relayed to the cerebellum via inferior
olivary nucleus
Conveys sensory information on body
position/balancing to the cerebellum
Fibers arise at all level of the spinal cord

57. SpinoreticularTract
Originates in laminaeIV-
VIII
Contains uncrossed
fibers that end in
medullaryreticular
formation &crossed &
uncrossed fibers that
terminate in pontine
reticular formation
Form part of the
ascending reticular
activating system

58. IntersegmentalTracts
Extensive fiber connections
between spinal segments
Fasciculus proprius
Short ascending &
descending fibers
Both crossed &
uncrossed
Begin and end within the
spinal cord
Participate in
intersegmentalspinal
reflexes
Present in all funiculi
adjacent to gray matter

59. Intersegmental Tracts
Dorsolateraltract of
Lissauer: Primary sensory
fibers carrying pain,
temperature and touch
information bifurcate upon
entering the spinal cord.
Their branches ascend and
descend for several spinal
segments in the dorsolateral
tract, before synapsingin
the dorsal horn
Intersegmental fibers, establishing connections with
neurons in the opposite half of the spinal cord, cross
the midline in the anterior white commissure

60. Somatic Motor Pathways

61. Motor Pathways
CNS issues motor commands in response to
information provided by sensory systems, sent by
the somatic nervous system (SNS) and the
autonomic nervous system (ANS)
Conscious and subconscious motor commands
control skeletal muscles by traveling over 3
integrated motor pathways
The corticospinalpathway –voluntary control of
motor activity
Corticobulbartracts
Corticospinaltracts
Themedial and lateralpathways –modify or direct
skeletal muscle contractions by stimulating,
facilitating, or inhibiting lower motor neurons

62. Motor Pathways
•Contain a sequence of TWO
neuronsfrom the cerebral
cortex or brain stem to the
muscles
•Upper motor neuron: has cell
body in the cerebral cortex or
brain stem, axon decussates
before terminating on the
lower motor neuron
•Lower motor neuron: has cell
body in the ventral horn of the
spinal cord, axon runs in the
ipsilateralventral root of the
spinal nerve and supply the
muscle.
UMN
LMN

63. Descending Spinal Tracts
Originatefrom the cerebral cortex& brain
stem
Concernedwith:
Control of movements
Muscle tone
Spinal reflexes & equilibrium
Modulation of sensory transmission to
higher centers
Spinal autonomic functions

64. The motor pathways are
divided into two groups
Direct pathways
(voluntary motion
pathways) -the
pyramidal tracts
Indirect pathways
(postural pathways),
essentially all others -
the extrapyramidal
pathways

65. Direct (Pyramidal) System
Regulates fast and fine (skilled) movements
Originate in the pyramidal neurons in the
precentralgyri,
Impulses are sent through the corticospinal
tracts and synapse in the anterior horn
Stimulation of anterior horn neurons activates
skeletal muscles
Part of the direct pathway, called corticobulbar
tracts, innervates cranial nerve nuclei

66. Indirect (Extrapyramidal) System
Complex and multisynapticpathways
The system includes:
•Rubrospinaltracts: control flexor muscles
•Vestibulospinaltracts: maintain balance and
posture
•Tectospinaltracts: mediate head neck, and eye
movement
•Reticulospinaltracts-emergency and alertness
responces

67. Descending Spinal Tracts
Pyramidal
Corticospinal
Extrapyramidal
Rubrospinal
Tectospinal
Vestibulospinal
Olivospinal
Reticulospinal
Descending
Autonomic Fibers

68. Corticospinal Tracts
Concernedwith
voluntary, discrete,
skilled movements,
especially those of distal
parts of the limbs
(fractionated movements)
Innervate the
contralateralside of the
spinal cord
Providerapid direct
method for controlling
skeletal muscle

69. Origin: motor and sensory
cortices
Axons pass through corona
radiata, internal capsule, crus
cerebriand pyramidof
medulla oblongata
In the caudal medulla about
75-90%of the fibers
decussateand form the
lateral corticospinaltract
Rest of the fibers remain
ipsilateraland form anterior
corticospinaltract. They
also decussate before
termination

70. Distribution:
55% terminate at
cervicalregion
20% at thoracic
25% at lumbosacral
level
Termination: Ventral horn
neurons(mostly through
interneurons, a few fibers
terminate directly)
Corticobulbartracts end
at the motor nuclei of CNs
of the contralateralside

71. RubrospinalTract
Controls the tone of limb
flexor muscles, being
excitatory to motor neurons
of these muscles
Origin: Red nucleus
Axons course ventro-
medially, cross in ventral
tegmentaldecussation,
descend in spinal cord
ventral to the lateral
corticospinaltract
Cortico-rubro-spinal pathway
(Extrapyramidal)

72. Tectospinal Tract
Mediates reflex movements of
the head and neck in response
to visual stimuli
Origin: Superior colliculus
Axons course ventro-medially
around the periaqueductalgray
matter, cross in dorsal
tegmentaldecussation,
descend in spinal cord near the
ventral median fissure,
terminate mainly in cervical
segments
Cortico-tecto-spinal pathway
(Extrapyramidal)

73. Vestibulospinal Tracts
Lateral Vestibulospinal
Tracts
Origin: lateral vestibular
(Deiter’s) nucleus
Axons descend ipsilaterallyin
the ventral funiculus
Terminate on ventral horn
cells throughout the length of
spinal cord
Has excitatory influences
upon extensor motor neurons,
control extensor muscle tone
in the antigravity maintenance
of posture

74. Vestibulospinal Tracts
Medial vestibulospinal
tract
Origin:medial vestibular
nucleus
Axons descend bilaterallyin
the ventral funiculus, with the
medial longitudinal fasciculus
Most of the fibers end in the
cervical region, some reaching
upper thoracic segments
Involved in movements of the
head required for maintaining
equilibrium

75. Reticulospinal Tracts
Influencevoluntary movement,
reflex activity and muscle tone by
controlling the activity of both
alpha and gamma motor neurons
Mediate pressorand depressor
effect on the circulatory system
Are involved in control of
breathing
Origin: pontine& medullary
reticular formation
Medial (pontine) reticulospinal
tract descends ipsilaterally
Lateral (medullary) reticulospinal
tract descends bilaterally
Both tracts located in the ventral
funiculus

76. Descending Autonomic Fibers
The higher centers
associated with the control of
autonomic activity are
situated mainly in the
hypothalmaus
The fibers run in the
reticulospinaltracts
Terminate on the autonomic
neuronsin the lateral horn of
thoracic & upper lumbar
(sympathetic) and sacral
segments (parasympathetic)
levels of the spinal cord

77. reflexes
.

78. Reflexes
Reflexes involving skeletal muscles and somatic motor neurons are
called somatic reflexes.
Reflexes controlled by autonomic neurons are autonomic reflexes .
Spinal reflexes are integrated w/ithe spinal cord while cranial reflexes
are integrated in the brain.
Reflexes may be inborn or learned.
Reflexes may be monosynapticor polysynaptic.
Difference?

79. Reflex Arcs
A reflex is a rapid,
predictable motor response
to a stimulus. Unlearned
and involuntary.
Example?
Components of a reflex arc:
Receptorsite of stimulus
Sensory neurontransmits
afferent info to CNS
Integration center1 or
more interneurons
Motor neurontransmits
efferent signals to effector
Effectormuscle or gland

81. Somatic Reflexes
Let’s look at the
muscle spindle reflex
and the Golgi tendon
reflex and figure out:
What they are?
Why are they somatic?
Are they mono-or
polysynaptic?
Are they ipsilateral or
contralateral reflexes?

82. Muscle Spindle Reflex

83. Golgi Tendon Reflex

84. Autonomic Reflexes
May be spinal (e.g.,
urination and defecation)
or modified by higher
brain structures.
The thalamus,
hypothalamus and brain
stem are in charge of
multiple reflexes –HR,
BP, breathing, eating,
osmotic balance,
temperature, vomiting,
gagging, sneezing.
All are polysynaptic.

86. Brain Stem
Located btwn the
cerebrum and the SC
Provides a pathway for
tracts running btwn higher
and lower neural centers.
Consists of the midbrain,
pons, and medulla
oblongata.
Each region is about an inch
in length.
Microscopically, it
consists of deep gray
matter surrounded by
white matter fiber tracts.
Produce automatic
behaviors necessary for
survival.

87. Midbrain
Located btwn the
diencephalon and the pons.
2 bulging cerebral peduncles
on the ventral side. These
contain:
Descending fibers that go to
the cerebellum via the pons
Descending pyramidal tracts
Running thru the midbrain is
the hollow cerebral aqueduct
which connects the 3
rd
and 4
th
ventricles of the brain.
The roof of the aqueduct ( the
tectum) contains the corpora
quadrigemina
2 superior colliculithat
control reflex movements of
the eyes, head and neck in
response to visual stimuli
2 inferior colliculithat
control reflex movements of
the head, neck, and trunk in
response to auditory stimuli

88. •Cranial nerves 3&4
(oculomotor and trochlear)
exit from the midbrain
•Midbrain also contains the
headquarters of the
reticular activating system

89. Midbrain
On each side, the midbrain
contains a red nucleusand
a substantia nigra
Red nucleus contains
numerous blood vessels and
receives info from the
cerebrum and cerebellum
and issues subconscious
motor commands concerned
w/ muscle tone & posture
Lateral to the red nucleus is
the melanin-containing
substantia nigra which
secretes dopamine to inhibit
the excitatory neurons of
the basal nuclei.
Damage to the substantia
nigra would cause what?

90. Pons
Literally means “bridge”
Wedged btwn the midbrain &
medulla.
Contains:
Sensory and motor nuclei for 4
cranial nerves
Trigeminal (5), Abducens (6),
Facial (7),
and Auditory/Vestibular (8)
Respiratory nuclei:
Apneustic& pneumotaxic centers
work w/ the medulla to maintain
respiratory rhythm
Nuclei & tracts that process and
relay info to/from the cerebellum
Ascending, descending, and
transverse tracts that interconnect
other portions of the CNS

91. Medulla Oblongata
Most inferior region of the
brain stem.
Becomes the spinal cord at
the level of the foramen
magnum.
Ventrally, 2 ridges (the
medullary pyramids) are
visible.
These are formed by the
large motor corticospinal
tracts.
Right above the medulla-SC
junction, most of these
fibers cross-over
(decussate).

92. Medulla Oblongata
Nuclei in the medulla are
associated w/ autonomic
control, cranial nerves, and
motor/sensory relay.
Autonomic nuclei:
Cardiovascular centers
Cardioinhibitory/cardioaccele
ratory centers alter the rate
and force of cardiac
contractions
Vasomotor center alters the
tone of vascular smooth
muscle
Respiratory rhythmicity
centers
Receive input from the pons
Additional Centers
Emesis, deglutition,
coughing, hiccupping, and
sneezing

93. Medulla Oblongata
Sensory & motor nuclei of 5
cranial nerves:
Auditory/Vestibular (8),
Glossopharyngeal (9), Vagus (10),
Accessory (11), and Hypoglossal
(12)
Relay nuclei
Nucleus gracilisand nucleus
cuneatuspass somatic sensory
information to the thalamus
Olivary nucleirelay info from the
spinal cord, cerebral cortex, and the
brainstem to the cerebellar cortex.

94. What brainstem
structures are
visible here?

95. Limbic System
 Includes nuclei and tracts along the
border btwn the cerebrum and the
diencephalon.
 Functional grouping rather than
anatomical
 Functions include:
1.Establishing emotional states
2.Linking conscious cerebral cortical
functions w/ unconscious functions of
the brainstem
3.Facilitating memory storage and
retrieval
•Limbic lobe of the cerebrum consists of 3 gyri that curve along the corpus
callosum and medial surface of the temporal lobe.
•Limbic system the center of emotion –anger, fear, sexual arousal, pleasure,
and sadness.

96. Reticular Formation
Extensive network of neurons
that runs thru the medulla and
projects to thalamic nuclei that
influence large areas of the
cerebral cortex.
Midbrain portion of RAS most
likely is its center
Functions as a net or filter for
sensory input.
Filter out repetitive stimuli. Such
as?
Allows passage of infrequent or
important stimuli to reach the
cerebral cortex.
Unless inhibited by other brain
regions, it activates the cerebral
cortex –keeping it alert and
awake.
How might the “sleep centers”
of your brain work? Why does
alcohol make you tired?

97. Protection
What is the major protection for the
brain?
There are also 3 connective tissue
membranes called the meninges:
Cover and protect the CNS
Protect blood vessels
Contain cerebrospinal fluid
The 3 meninges from superficial to
deep:
Dura mater
Arachnoid mater
Pia mater

98. Skin
Galea Aponeurotica
Connective Tissue
Bone
Dura Mater
Arachnoid mater