Transcript 03Tracts

SENSORY
OR
ASCENDING
TRACTS
Dr. Jamila
El-Medany
Dr. Essam
Eldin Salama
OBJECTIVES
• By the end of the lecture, the student will be
able to:
• Define the meaning of a tract.
• Distinguish between the different types of
tracts.
• Locate the position of each tract.
• Describe the sensory pathway.
• Identify the different sensory spinal tracts and
their functions.
• Identify the course of each of these tracts.
 Grey matter of the spinal cord is completely surrounded by white matter,
which consists of Ascending and Descending Nerve Fibers.
 The White matter is divided into Dorsal, Lateral & Ventral Columns or
Funiculi.
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 Definition:
• Bundles or fasciculi of
fibers that occupy more
or less definite
positions in the white
matter.
• They have the same
Origin, Termination and
carry the same
Function.
• They serve to join the
brain to the spinal cord.
• They are classified into:
1- Short Tracts
(intersegmental).
2-Long Tracts:
Ascending (sensory or
afferent).
Descending (motor or
efferent).
WHITE MATTER TRACTS
Definition:
• Fibers that
interconnect
adjacent or distant
segments of the
spinal cord.
Position:
• They lie close to
the grey matter
Intersegmental (propriospinal)
tracts
 Ascending Tracts;
• Carry impulses from pain, thermal, tactile, muscle and joint receptors to the
brain.
• Some of this information eventually reaches a conscious level (the cerebral
cortex),
• while some is destined for subconscious
centers (e.g. the cerebellum).
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1- Dorsal column tracts:
(Gracile & Cuneate)
Function: Transmit
a. Proprioceptive (deep)
sensations (sense of
movement, position,
vibration).
b. Fine touch sensations (tactile
localization, tactile
discrimination &
stereognosis).
These senses reach a Conscious
level (cerebral cortex).
2- Spinothalmic tracts:
Function: Transmit impulses
concerned with specific sensory
modalities: pain, temperature,
touch, that reach a Conscious
level (cerebral cortex).
3- Spinocerebellar tracts:
Function: Transmit impulses from
tactile and stretch receptors to
Subconscious centers (cerebellum)muscle tone and coordinations.
Ascending
Or
Sensory Tracts
• Pathways that
carry
information to a
conscious level
share certain
common
characteristics.
• There is a
sequence of
three neurones
between the
peripheral
receptor and
the cerebral
cortex.
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The first-order neurone or primary
afferent neurone) enters the spinal
cord through the dorsal root of a
spinal nerve and its cell body lies in
the dorsal root ganglion.
The main fiber remains on the ipsilateral
side of the cord and terminates in synaptic
contact with the second neurone either in
the spinal grey matter or in the medulla
oblongata of the brain stem.
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•
•
•
•
The second order neurone has
its cell body in the cord or
medulla oblongata.
Its axon crosses over (decussates)
to the opposite side of the CNS
and ascends to the thalamus,
where it terminates upon the
third neurone.
The third-order neurone has its
cell body in the thalamus.
Its axon passes to the
somatosensory cortex of the
parietal lobe of the cerebral
hemisphere.
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 Two main tract systems in the spinal cord fit into this pattern:
•
Dorsal (posterior) columns and
•Prof. Makarem
Spinothalamic tracts.
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1. Dorsal column Tracts
• Function: Carry proprioceptive (deep) & fine
touch sensations to sensory area of the
cerebral cortex.
• The dorsal column contains the axons of the
neurones that have entered the cord through
the dorsal roots of spinal nerves.
• Fibers of Fasciculus Gracilis enter via the
sacral, lumbar and lower thoracic levels;
(lower limbs).
• Fibers of Fasciculus Cuneatus enter via the
upper thoracic and cervical levels; (upper
limbs).
• Fibers ascend to the Medulla oblongata
where they terminate upon second-order
neurones,
(Nucleus Gracilis and nucleus Cuneatus in the
medulla).
Prof. Makarem
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• Axons of second-order
neurones decussate in the
medulla as the internal arcuate
fibers (sensory decussation).
• Then it ascend in (opposite side)
through brain stem as the
Medial Lemniscus.
• The medial lemniscus
terminates in the ventral
posterior (VP) nucleus of the
thalamus.
• Third-order neurone,
(thalamocortical, or sensory
radiations- run in the internal
capsule) project to the
somatosensory cortex.
Prof. Makarem
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Spinothalamic tracts lie
lateral and ventral to the
ventral horn of the spinal
grey matter.
II- Spinothalamic tracts
• They carry pain and thermal (temperature) sensations and also nondiscriminative (crude) touch and pressure.
– The Lateral spinothalamic tracts carry pain and temperature
sensations.
– The ventral spinothalamic tracts carry crude touch and pressure.
• But fibers carrying these modalities are probably intermingled, at least
to some extent.
Prof. Makarem
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A-Lateral Spinothalamic
Tract (pain & temperature)
 Function:
• Carries pain & Temperature to
thalamus and sensory area of the
cerebral cortex.
 Neurones: 3 Neurones
• Neurone I: Small cells in the
dorsal root ganglia.
• Neurone II: Cells of substantia
gelatinosa of Rolandi in the
posterior horn.
• Neurone III: Cells of (VP) nucleus
of the thalamus.
• The spinothalamic tract contains
second-order neurones, the cell
bodies of which lie in the
contralateral dorsal horn.
 Lesion:
•Syringomyelia, (widening of the
central canal) leads to Loss of pain
& temperature below the level of
the lesion.
• After leaving the parent cell bodies,
spinothalamic axons decussate to the
opposite side of the cord by passing
through the ventral white commissure,
which lies ventral to the central canal of
the cord, and, thus, enter the contralateral
spinothalamic tract.
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B- Ventral Spinothalamic
(Crude touch & Pressure)
 Function:
• Carries crude touch & pressure
to thalamus and sensory cortex.
 Neurones: 3 Neurones
• Neurone I:
Medium sized cells in the
dorsal root ganglia.
• Neurone II:
Cells of main sensory nucleus or
(nucleus proprius).
• Neurone III:
Cells of VP nucleus of thalamus.
 Effect of lesion:
Loss of crude touch sensation
below the level of the lesion.
Spinocerebellar
tracts
• Ascending pathways that
carry impulses to a
subconscious level are
represented by the
spinocerebellar tracts.
• Fibres of spinocerebellar
tracts form dorsal and
ventral tracts that are
located at the dorsolateral
& ventrolateral surfaces of
the cord, respectively.
• Both tracts carry
information derived from
muscle spindles, Golgi
tendon organs and tactile
receptors to the
cerebellum for the control
of posture & coordination
of movement.
Prof. Makarem
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• The spinocerebellar system
consists of a sequence of
only two neurones.
• Neurone I:Large cells of
dorsal root ganglia.
• Neurone II: cells of the
nucleus dorsalis (Clark's
nucleus.
• Both spinocerebellar tracts
contain second-order
neurones.
• The tract neurones
terminate directly in the
cerebellar cortex.
Prof. Makarem
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Dorsal (Direct)
Spinocerebellar tract
• Fibers of the dorsal
spinocerebellar tract
originate from the
cells of Clarke's
column at the base of
the posterior horn.
• The axons ascend
ipsilaterally to enter
the cerebellum
through the inferior
cerebellar peduncle.
Ventral (Indirect)
Spinocerebellar Tract
• Fibers of the ventral
spinocerebellar tract
decussate, ascend on
the contralateral side
of the cord and enter
the cerebellum via the
Superior cerebellar
peduncle.
• Some axons then
recross within the
cerebellar white
matter.
Spino-Olivary Tracts
• Project to accessory
olivary nuclei and
cerebellum.
• Contribute to
coordination &
associated primarily
with balance.
Spinotectal Tracts
• Project to superior
colliculi of midbrain.
• Involved in reflexive
turning of the head and
eyes toward a point of
cutaneous stimulation.
Spinoreticular Tracts
• Involved in arousing
consciousness in the
reticular activating
system through
cutaneous
stimulation.
Thank you