The document summarizes the ascending and descending spinal tracts. There are ascending tracts that conduct sensory information from the body to the brain, including the lateral and anterior spinothalamic tracts for pain and temperature, and the posterior white columns for proprioception. There are also descending tracts that convey motor commands from the brain to the spinal cord to control skeletal muscles, including the lateral and anterior corticospinal tracts. In addition, descending autonomic fibers help regulate the autonomic nervous system.

1. Dr. Bikal Lamichhane
1st year Resident
NAMS
Ascending and Descending Spinal Tracts

2.  Tracts serve as neuronal pathway
to join spinal cord and brain.
 Nerve fibers of different sizes and
functions are sorted out and
segregated into nerve bundles or
tracts in white matter.
 These are
a. Ascending tract
b. Descending tract
c. Intersegmental tracts

3. Ascending tracts of spinal cord
 Conduct afferent information
i. Exteroceptive information:
 Originates outside body- pain , temperature, touch
ii. Proprioceptive information
 Originates inside body – from muscles and joints

4. First-order neuron
 Has its cell body in the posterior root ganglion of the
spinal nerve
 Peripheral process connects with a sensory receptor
ending
 Central process enters spinal cord through posterior
root to synapse on second-order neuron.
Second-order neuron
 Gives rise to an axon that decussates (crosses to
opposite side) & ascends to higher level of CNS where
it synapses with third-order neuron.
Third-order neuron
 Usually in thalamus & gives rise to a projection fiber
that passes to a sensory region of cerebral cortex

5. Lateral Spinothalamic tract
 Pain & temperature impulses travel to spinal cord in fast-conducting delta A-type fibers &
slow-conducting C-type fibers.
 Axons entering spinal cord from posterior root ganglion proceed to tip of posterior gray
column & divide into ascending & descending branches, travel for a distance of one or two
segments of spinal cord & form posterolateral tract of Lissauer.
 Terminate by synapsing with cells in posterior gray column.
 Axons of second-order neurons cross to opposite side in anterior gray & white commissures
within one spinal segment of the cord, ascending in contralateral white column.

6. As the tract ascends through spinal cord, new fibers are added to
anteromedial aspect of tract.
 In upper cervical segments of the cord, sacral fibers are lateral
and cervical segments are medial.
 Fibers carrying pain are situated slightly anterior to those
conducting temperature.
As the lateral spinothalamic tract ascends through medulla
oblongata, accompanied by anterior spinothalamic tract &
spinotectal tract: spinal lemniscus.

7.  It further ascends through posterior part of pons, midbrain &
tract end by synapsing with third-order neuron in ventral
posterolateral nucleus of thalamus.
 Further it course through posterior limb of internal capsule &
corona radiata to reach somesthetic area in postcentral gyrus
of cerebral cortex.

8.  Contralateral half of body is represented as
inverted, with the hand and mouth situated
inferiorly
 leg situated superiorly
 foot and anogenital region on medial surface
of hemisphere.
 Sensory Homonculus.
 Then transmitted to other regions of
cerebral cortex to be used by motor areas
and parietal association area.
 Role of cerebral cortex is interpreting
quality of sensory information at level of
consciousness.

10. Anterior spinothalamic tract
 The axons enter spinal cord from posterior root ganglion &
proceed to tip of posterior gray column.
 Second-order neuron now cross very obliquely to opposite side
in anterior gray & white commissures within several spinal
segments & ascend in opposite anterolateral white column as
anterior spinothalamic tract.
 New fibers are added to medial aspect.
Upper cervical segments of the cord, the sacral fibers are
mostly lateral & the cervical segments are mostly medial.

11.  In medulla oblongata, it accompanies the lateral spinothalamic
tract and the spinotectal tract, forming spinal lemniscus.
 ascend through posterior part of pons, and the tegmentum of
the midbrain and terminate by synapsing with third-order
neuron in ventral posterolateral nucleus of thalamus.
 posterior limb of the internal capsule and corona radiata
 somesthetic area in postcentral gyrus of cerebral cortex

13. Posterior White Column: Fasciculus Gracilis & Fasciculus
Cuneatus
 Axons enter spinal cord from posterior root ganglion & pass directly to posterior white column of same side.
 Long ascending fiber-synapse with cells in posterior gray horn, with internuncial neurons, & with anterior horn cells
as fasciculus gracilis & fasciculus cuneatus.
 Short descending fiber are involved with intersegmental reflexes.
 Fasciculus gracilis:
 throughout length of spinal cord ;
 fibers from the sacral, lumbar & lower six thoracic spinal nerves
 Fasciculus cuneatus:
 situated laterally in the upper thoracic & cervical segments
 fibers from the upper six thoracic & all the cervical spinal nerves

14.  Ascend ipsilaterally and terminate by synapsing on second-order
neurons in nuclei gracilis and cuneatus of medulla oblongata.
 The axons of second-order neurons, called internal arcuate fibers,
cross median plane, decussating with corresponding fibers of
opposite side in sensory decussation.
 then ascend as a single compact bundle, medial lemniscus,
through medulla oblongata, pons and midbrain ventral
posterolateral nucleus of thalamus.

15.  Then it traverse via posterior limb of internal capsule &
corona radiata to reach somesthetic area in postcentral gyrus
of cerebral cortex.
 Function:
- fine touch, exact localization & two-point discrimination
- Vibratory sense & the position of the different parts of the
body

17. Posterior Spinocerebellar tract
 First order neuron:- Traverse from posterior root ganglion and enter posterior gray column;
 Synapse to nucleus dorsalis (Clarke's column).
 Second order Neuron :-Ascends through posterolateral part of the lateral white column on the same side
of medulla
 Here it joins inferior cerebellar peduncle & terminates in cerebellar cortex.
 Function:
- receive muscle joint information from the muscle spindles, tendon organs, and joint receptors of the
trunk & lower limbs.
- coordination of limb movements & the maintenance of posture

18. Anterior Spinocerebellar Tract
 Enters through posterior root, synapse to second-order neurons in
nucleus dorsalis at base of posterior gray column.
 Majority cross to opposite side and ascend as in contralateral white
column; minority of axons ascend as anterior spinocerebellar tract
in lateral white column of same side.
 Then it enters the cerebellum through cerebellar peduncle to synapse
into cerebellar nucelus.
 Function:
- information from muscle spindles, tendon organs, and joint
receptors of trunk and upper and lower limbs

19. Other ascending tracts
 Cuneocerebellar:
- Nucleus cuneatus to cerebellum,
- Conveys information of muscle joint sense to the cerebellum
 Spinotectal:
- To superior colliculus of midbrain
- afferent information for spinovisual reflexes & brings about movements
of eyes head toward source of stimulation
 Spinoreticular:
- reticular formation in medulla oblongata, pons, & midbrain
- influencing levels of consciousness
 Spino olivary: conveys information to the cerebellum from cutaneous &
proprioceptive organs

20. Descending tracts
 First-order neuron:
- has its cell body in cerebral cortex.
- descends to synapse on second-order neuron,
 Second order neuron: an internuncial neuron,
situated in anterior gray column of spinal cord.
 Third-order neuron:
- lower motor neuron, in the anterior gray column
- innervates the skeletal muscle through the
anterior root & spinal nerve.
 In some instances, the axon of the first-order neuron
terminates directly on the thirdorder neuron (as in
reflex arcs).
 Control of skeletal muscle activity

22.  At junction of medulla oblongata & spinal cord, most of fibers cross midline at
decussation of pyramids and enter lateral white column of spinal cord to form lateral
corticospinal tract.
 Remaining fibers do not cross in decussation but descend in anterior white column of
spinal cord as anterior corticospinal tract; cross midline and terminate in anterior gray
column of spinal cord segments in cervical & upper thoracic regions.
 Lateral corticospinal tract descends length of spinal cord; terminate in anterior gray
column of all spinal cord segments.
 synapse with internuncial neurons, which, in turn, synapse with alpha motor neurons &
some gamma motor neurons.

27. Descending Autonomic Fibers
 The higher centers of CNS associated with control of
autonomic activity are situated in cerebral cortex,
hypothalamus, amygdaloid complex, & reticular formation.
Although distinct tracts have not been recognized,
investigation of spinal cord lesions has demonstrated that
descending autonomic tracts do exist & probably form part of
reticulospinal tract.