Ascending Tracts
Types of Tracts
Ascending tracts


o
o

o
o
o
Conveys sensory afferent
impulses
Ist order neuron
Always pseudounipolar
Location of cell bodies

Enter spinal nerve

Dorsal root ganglia

Enter cranial nerve

Respective cranial
nuclei

Medulla
2nd order neuron
Location of cell bodies

Enter spinal nerve

Substancia Gelatinosa
at the posterior horn of
grey matter

Enter cranial nerve

Brain stem
Fibers of 2nd order neuron

Concious tracts

Always cross to
contralateral side

Always relay on
thalamus

Unconcious tracts

Never cross to
contralateral side

If crossed, crosses back
to ipsilateral side

Never reaches the
thalamus
3rd order neuron

Absent in unconcious tract

Cell bodies on the VentroPostero-Lateral (VPL)
nucleus of the thalamus
Subdivisions
Concious Sensation
Tracts

o

o
o
o

Stimuli from external
environment
Extroceptive
impulses
Sends concious
sensations
Pain
Temperature
Touch
Consist of 3 order
neurons
1st order neuron
Anterior Spinothalamic Tract (AST)
Senses
o
Pressure
o
Light touch

Location
o
Ventral white column


Lateral Spinothalamic Tract (LST)



o
o



Dorsal Column/
Fasciculus Dorsalis
Senses
Vibration
Fine touch
Sense of position
and movement

Location
o
Fasciculus Gracilis

Medial aspect
of white dorsal
column
o
Fasciculus
Cuneatus

Lateral aspect
of white dorsal
column
Stimuli from within the
body
o
Interoceptive/
proprioceptive
impulses
Sends unconcious
sensations
o
State of muscle
contraction
o
Position of joints
Consist of 2 order
neurons
2nd order neuron

o
o
Senses
o
Pain
o
Temperature

Location
o
Lateral white column
Fasciculus
Gracilis

o
o
o

Pseudounipolar in
nature
Proximal part of fiber
synapse with 2nd
order neuron at the
Substantia
Gelatinosa


Unconcious
Sensation Tracts

Sensation of
o
Lower
limbs
o
Lower
part of
thorax
Fasciculus
Cuneatus



Sensation of
o
Upper
limbs
o
Upper
part of
the
thorax
Anterior Spinocerebellar Tract
Senses**
o
Uconcious proprioceptive impulses
from

Muscle spindle

Golgi Tendon Organ

Location**
Lateral white column

Posterior Spinocerebellar Tract
**

Pseudounipolar in
nature
Enter spinal cord
and directly ascend
upward without
o
Synapse to 2nd
order neuron
o
Crossing to
other side
Synapse to the 2nd
order neuron at the
lower part of
medulla







Pseudounipolar in
nature
Proximal part of fiber
synapse with 2nd
order neuron at the
Substantia
Gelatinosa
Cell bodies located at the Subtantia
Gelatinosa
Fiber crosses to the contralateral side
LST - In front of the central canal
AST – further in front of central
canal
Reaches
AST – ventral white column
LST - lateral white column
Turns upward to the thalamus
Fibers end at the VPL nuclei in the
thalamus
Nuclei located at the lower part of
medulla
Fasciculus gracilis synapse at the
Nucleus gracilis
Fasciculus cuneatus synapse at the
Nucleus cuneatus
Fibers
o
Curved upward to form Internal
Arcuate Fiber
o
This fiber will cross to the
contralateral side
o
Crossing of fibers produced the
Sensory Decussation
Fibers ascend upward to the VPL as
Medial Lemniscus
Cell bodies located at the Nucleus
Dorsalis (Clarke’s column)

Crosses to the contralateral side

Ascends immediately at the anterior
part of the lateral white column

Enter the Superior Cerebellar
Peduncle

Can be either
o
Terminated at the contralateral
hemisphere of cerebellum

Cross back though the Middle
Cerebellar Peduncle t the
contralateral hemisphere of
Cerebellum

Cell bodies located at the Nucleus
Dorsalis (Clarke’s column)

Ascends immediately at the posterior
part of lateral white column

Passes through the Inferior Cerebellar
Peduncle

Reaches the same side of Cerebellar
hemisphere

3rd order neuron




N/A
N/A
Cell bodies
located at the VPL
nuclei of thalamus
Projects fibers to
the Primary Sensory
cortex (Area 3,1,2)
at the Precentral
gyrus
Cell bodies
located at the VPL
nuclei of thalamus
Projects fibers to
the Primary Sensory
cortex (Area 3,1,2)
at the Precentral
gyrus
Descending Tracts
Types of Tracts
Pyramidal Tracts
 Consists of (90%)
o 1st order neuron
 Upper motor
neuron
o 2nd order neuron
 Interneuron/
Internuncial neuron
o 3rd order neuron
 Lower motor
neuron
 The rest 10%, upper
motor neuron directly
synapse with lower
motor neuron
Functions
 Influence the motor
neurons for
o Voluntary movement
o Rapid skilled
movements
 Modulate the function
of ascending system
Subdivisions
1st order neuron



Ventral

Corticospinal
Tract
Lateral


Corticonuclear/
Corticobulbar Tract


1st
order neuron is called the Pyramidal/Betz cells
Cell bodies located at the 5th layer of cortical cell layers of
cerebrum
Locations
o Infero-lateral part of cereberal cortex

Neurons concern with

Head

Neck
o Supero-medial part

Neurons concern with

Upper limbs

Lower limbs
Fibers
o Converge into the Conrona Radiata to the Posterior
Limb of the Internal Capsule
o Upon reaching the midbrain, passes thorugh the 3/5th of
the Cerebral Peduncle
o After reaching the Pons

Fibers located at the ventral part

Separated into small bundles/ fasciculi by Nuclei
Pontis
o Then at the Medulla, fibers again converge and
descends through the Pyramid

1/3 of Medulla, it can be either

85% cross to the contralateral part – Lateral
Corticospinal Tract
o Descends thought the posterior part of the
lateral white column

15% descends directly without crossing to other
part – Ventral Corticospinal Tract
o Descends through the ventral white column
1st order neuron arised from the Betz cells of the
cerebral cortex
Coverged downward through the Corona Radiata to
the Genu of the Internal Capsule
Reaching the Pons will pass through the 3/5 th of
Cerebral Peduncle
Upon reaching to the brainstem, will synapse to
respective 2nd order neuron of respective cranial
nerves
2nd order neuron
 Interneuron at
the dorsal grey
matter
 1st order
neuron needs
to cross to the
contralateral
part of dorsal
grey mater
 Located at the
respective
level
 Interneuron at
the dorsal grey
matter
 1st order
neuron directly
synapse to the
3rd order
neuron at the
same side
 Located at the
respective
level

Located at the
brainstem in
the respective
cranial nuclei
3rd order neuron
 Cell bodies are
located at the
ventral grey
horn of the
respective
level

Most of 1st
order neuron
crosses to the
other side and
synpase with
the 3rd lower
motor neuron
of respective
cranial nuclei