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Thursday
Neuroanatomy
3-3-2011
Lecture 4
Descending tracts
Tracts of spinal cord
Ascending tracts
descending tracts
1- Ascending tracts : bundles of ascending nerve fibers (sensory nerve fibers) that
ascend from the spinal cord to higher centers to connect the spinal cord with the
brain .
The ascending tracts carry afferent information that's divided into 2 main groups:
- Exteroceptive information: originates from outside the body such as pain, touch
and temperature.
- proprioceptive information: originates from inside the body such as muscles and
skin.
2- Descending tracts : bundles of descending nerve fibers (motor nerve fibers) that
descend in the white matter from different supraspinal nerve centers in the brain.
The descending tracts carry efferent information .
Simple form of descending motor pathway from the cerebral cortex
to the skeletal muscle, note the 3 neurons involved (1) (2) (3) .
Simple form of the ascending sensory pathway from the sensory
nerve ending to the cerebral cortex, note the 3 neurons involved.
1
The motor neurons
Upper motor neurons
lower motor neurons
1- Lower motor neurons: the motor neurons situated in the anterior gray columns of
the spinal cord (alpha and gamma motor neurons) that send axons to innervate
skeletal muscles through the anterior root of the spinal nerves and they constitute
the final common pathway to the muscles .
Lower motor neurons affected by nervous impulses that descend from the
medulla, pons, midbrain and cerebral cortex .
2- Upper motor neurons : the neurons above lower motor neurons as the
supraspinal neurons and their tracts.
Descending tracts
From brain stem and pass through
from cerebral cortex
Lateral part of white column
anterior or medial part
Rubrospinal tract
1- tectospinal tract
2- Vestibulospinal tract
3- Reticulospinal tract
Pontine
medullary
2
Corticospinal tract
1- Corticospinal tract
**Origin : cerebral cortex (in common)
1/3 of the fibers originate from primary motor cortex (area 4)
1/3 originate from secondary motor cortex (area 6)
1/3 originate from parietal lobe (area 3,1, and 2 )
**termination :anterior gray horn (alpha and gamma motor
neurons)
** function : carry motor information from cerebral cortex to
alpha and gamma motor neurons that innervates muscles
through the anterior root of spinal cord .
**path :
1- The Corticospinal tract pass from cerebral cortex without
crossing until the lower part of medulla oblongata where the
bundles become grouped together along the anterior
portion of the medulla oblongata to form a swelling known
as pyramid and that's why corticospinal tract is called
pyramidal tract .
2- At the junction of the medulla oblongata and the spinal
cord:
** most of the fibers cross the midline at the decussation of
the pyramids and enter the lateral white column of the
spinal cord to form the lateral corticospinal tract .
** the remaining fibers don't cross in decussation but
descend in the anterior white column of spinal cord as the
anterior spinal corticospinal tract.
3- These fibers terminate eventually in the anterior gray
column of the spinal cord .
4- Alpha and gamma motor neurons send axons to innervate
skeletal muscle through the anterior root of spinal nerves .
Note : motor decussation is crossing of the right and left
parts of lateral corticospinal tract at the lower border of
medulla oblongata .
3
2- Rubrospinal tract :
**origin : red nucleus in midbrain
**termination : anterior gray column
** function : facilitates the activity of flexor muscles and inhibits the activity of extensor
muscles .
** Contralateral (there's crossing in this tract)
3-Vestibulospinal tract :
**origin : vestibular nuclei
** termination : the anterior gray column
**function: the inner ear and the cerebellum by this tract facilitate the activity of extensor
muscles and inhibit the activity of the flexor muscle in association with the maintenance of
balance.
**ipsilateral (this tract descends without crossing)
4-Tectospinal tract :
**origin : Superior colliculus of midbrain
**termination : the anterior gray column of the upper cervical segments of the spinal cord
(don't forget that just the upper cervical segments).
**function : coordination of movement of head, neck and eyes in response to auditory or
visual stimulus .
**Contralateral (there's crossing )
-notice that tectospinal tract and Rubrospinal tract originate from midbrain.
5- Reticulospinal tract :
**origin : reticular formation that is (group of nerve cells and nerve fibers exist throughout
the brain stem = midbrain+pons+medulla oblongata.
**termination: anterior gray column
**function : influence voluntary movement and reflex activity .
4
(Dr said that its function isn't that important to memorize )
**parts :
1- Pontine reticulospinal tract : formed by neurons in pons that send axons (pontine
reticulospinal tract) which are mostly uncrossed down into the spinal cord.
2- Medullary reticulospinal tract : formed by similar neurons in medulla that send
axons(medullary reticulospinal tract) which are crossed and uncrossed and mostly
crossed.
- I put pictures of all the tracts in the last page so that u can compare between them ..
- you can visit this website(http://neuroscience.uth.tmc.edu/s3/iii2-3.html) that show the
path of each tract when u press (play) beside the picture of each one ..
DONE BY : Do7a n3erat
Dedicated to :
*my Brother Seif (wish u the best in the world )
* my Loyal sisters & friends : Lara abu 7ammour & Hind mihyar
*my sweet friends : Samar 3abid – Lara mazahreh - Maryam Bader – Zein Abu zeitoon
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