Lab#3
Date:2/3/2011
Ascending tracts
As we took in the last lecture that:
The spinal tracts are divided into:
a) Ascending tracts b) descending tracts
ascending tracts = are sensory tracts that carry information
from the periphery to the brain.
those information may be divided into two main groups:
(1) exteroceptive information:, which originates from outside
the body, such as pain, temperature, and touch.
(2) proprioceptive information: which originates from inside
the body, for example, from muscles, tendons and joints.
Information from the peripheral sensory endings is conducted
through the nervous system by a series of neurons (three
neurons):
1st order neuron = dorsal root ganglia (fixed)
2nd order neuron =found inside the spinal cord or brain stem
(changes according to the type of tract)
3rd order neuron =inside the thalamus (fixed)
Termination =cerebral cortex in S1 area
**thalamus is full of nuclei so it's a grey matter ,so one of
those nuclei is called ventral posterolateral (VPL)nucleus
where the 3rd order neuron is found.
The nomenclature of some tract identifies the beginning and
ending of the tract
Ex: spinothalamic tract starts in the spinal cord and
terminates in the thalamus.
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Spinothalamic tract (REVIEW)
IT'S devided into:
A) anterior Spinothalamic tract →it carries pressure and light
touch.
b) posterior Spinothalamic tract → it carries pain and
temperature.
The dendrites of the 1st order neuron work as free nerve
endings in the skin (receptors)
Posterior white column-medial lemniscus pathway
Modality:
1) Discriminative touch 'that includes vibration':
It's the ability to differentiate between two stimuli happening
at the same time even if those stimuli are close to each other.
2) Conscious proprioception:
Sensation from muscles, tendons and joints situation .
Receptors:
Most receptors except free nerve endings; free nerve endings
are the receptors of spinothalamic tract.
The neurons associated in this tract:-
* 1st order neuron = dorsal root ganglia (fixed)
* 2nd order neuron =found inside the nuclei of brain stem
(specifically in medulla oblongata)
* 3rd order neuron =inside the thalamus (fixed)
* Termination
=cerebral cortex in S1 area
“Take the first step in faith. You don’t have to see the whole
staircase, just take the first step.”
~ Dr. Martin Luther King Jr.
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The pathway:
1- The axons enter the spinal cord from the posterior root
ganglion
2- They pass directly to the posterior white column of the
same side (no synapse or crossing in the gray mattar)
3- The ascending fibers travel upward in the posterior white
column as the* fasciculus gracilis and* fasciculus cuneatus to
the medulla oblongata where the first synapse takes place.
*inside the medulla ablongata there are two nuclei on each side
that differ not only in location but by the nerves that synapse
in them.
(Those nuclei contain the cell body of the 2nd order neuron)
Gracilis: located medially
Cuneatus: located laterally
The fasciculus gracilis is present throughout the length of the
spinal cord and contains the ascending fibers from the sacral,
lumbar, and lower six thoracic spinal nerves.
The fasciculus cuneatus is situated laterally in the upper thoracic
and cervical segments of the spinal cord and contains the long
ascending fibers from the upper six thoracic and all the
cervical spinal nerves.
4-The fibers of the fasciculus gracilis and fasciculus cuneatus
ascend ipsilaterally and terminate by synapsing in the secondorder neurons in the nuclei gracilis and cuneatus of the medulla
oblongata
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Ipsilateral:
Walk at the same side no change in the
direction
Condrolateral: crossing has happened and the
nerve walks on the opposite side
The axons of the second-order neurons are called the internal
arcuate fibers.
5-internal arcuate fibers will go condrolaterally (crossing) to
form sensory decussation
Sensory decussation: the place where the arcuate fiber of the
left side meet the arcuate fiber of the right side.
6-The fibers then (after crossing has happened) ascend to
form, the medial lemniscus, through the medulla oblongata, the
pons, and the midbrain
7- The fibers then synapse in the third-order neurons in the
ventral posterolateral nucleus of the thalamus, to terminate in
s1 area in the cerebral cortex.
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Any addition of neurons in the posterior white column tact will
be laterally because the additional neurons will be added from
the upper regions (cervical and upper 6 thoracic)
Which are added to fasciculus cuneatus laterally.
While in the spinothalamic tract the addition of neurons is
medialy.
Lesion on the right side of posterior white column tact will
affect the area below and on the right side of the lesion.
This injury will affect "discriminative touch,conscious
proprioception" but not light touch because it's controlled by
spinothalamic tract.
For ex:
If the cut affected the posterior white column tract the
person will feel the needle prick "light touch", but can't feel 2
needle pricks at the same time "discriminative touch".
If there is a tumor above the medulla oblongata (after the
crossing area) it'll become condrolateral
(ex: lesion in the right side will affect the left side).
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Spinocerebellar Tract
Starts from the spinal cord and ends in the cerebral cortex.
Modality= unconscious proprioception
Proprioception= as we know concerning muscle spindle,
tendon…..
Unconscious= because it didn't reach the cerebrum.
** This tract affects the movement but not our ingestion of
the movements.
Function of the cerebellum:
1- Coordination of movements (smoothing the movement)
2- Maintenance of posture (balance).
Receptors of this tract: are inside the organ, joint "golgi
tendon organ" ,"muscle spindle".
This tract is an exception because there is no 3rd order
neuron because it'll terminate in the cerebellum and doesn't
reach the thalamus.
The neurons associated in this tract:* 1st order neuron= dorsal root ganglia (fixed)
* 2nd order neuron=clarke's column (nucleus dorsalis)
Devided into:
a) Posterior Spinocerebellar Tract
b) Anterior Spinocerebellar Tract
*Termination = cerebellar cortex
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The pathway:1)The axons entering the spinal cord from the posterior root
ganglion enter the posterior gray column and synapse in the
second-order neurons at the base of the posterior gray column
in the nucleus dorsalis (Clarke's column).
2) The axons of the second-order neurons will ascend to
terminate in the cerebellar cortex.
Nucleus dorsalis (Clarke's column) extends only from
the eighth cervical segment to the third or fourth lumbar
segment.
♦ Axons entering the spinal cord from below the level of L3/L4
they will ascend in the white matter till they reach the third
or fourth lumbar segment, where they enter the nucleus
dorsalis and synapse there.
♦While the axons above the level of C8 will descend in the
white matter to reach C8 and enter the nucleus dorsalis and
synapse there.
The difference between
Posterior Spinocerebellar &Tract Anterior Spinocerebellar
Tract
Is:
That the axons of the second-order neurons of Anterior
Spinocerebellar tract cross to the opposite side and ascend
anterolateral as the anterior spinocerebellar tract.
While posterior there is NO crossing, it'll ascend at lateral as
the posterior spinocerebellar tract from the same side.
“All that we are is the result of what
we have thought. The mind is
everything. What we think we
become.”
DONE BY:
LARA AL LAHHAM
Buddha
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