ASCENDING PATHWAYS
Ascending Pathways

Three-neuron pathways:
Primary sensory neurons:
From external receptors
Travel through dorsal roots of spinal cord
Secondary neurons:
Make up tracts in spinal cord and brainstem
Tertiary neurons:
From thalamus to primary sensory cortex
Travel through internal capsule
Ascending Pathways

For conscious perception:
Spinothalamic system
Medial Lemniscal system

For unconscious perception:
Spinocerebellar
Spino-olivary
Spinotectal
Spinoreticular
Spinothalamic System
Lateral spinothalamic tract
 Anterior spinothalamic tract

Lateral Spinothalamic Tract
Carries pain and temperature
 Primary fibers ascend or descend 1-2
spinal cord segments before synapsing
with secondary fibers.

Lateral Spinothalamic Tract
Secondary axons decussate through
anterior gray and white commissures.
 Secondary axons make up the lateral
spinothalamic tract traveling in the lateral
column of the spinal cord.

Lateral Spinothalamic
Lateral Spinothalamic Tract

Secondary fibers are joined in brainstem
by fibers of the trigeminothalamic tract:
(Pain and temperature from face and teeth.)
Lateral Spinothalamic Tract

Secondary fiber collaterals project to
reticular formation:
Stimulate wakefulness and consciousness.

Secondary fibers project to ventral
posterolateral (VPL) nucleus of thalamus.
Lateral Spinothalamic Tract
Secondary fibers synapse with tertiary
fibers in VPL.
 Tertiary fibers (corticopetal fibers) synapse
in postcentral gyrus:

Somatic sensory areas 3, 1, 2

Tertiary fibers form part of internal capsule.
Anterior Spinothalamic Tract
Carries light touch (crude touch), pressure,
tickle, itch
 Primary neurons may ascend 8-10 spinal
cord segments before synapsing with
secondary neurons.
 Secondary fibers decussate in anterior
gray or white commissures.

Anterior Spinothalamic Tract
Secondary fibers ascend to synapse with
tertiary fibers in VPL nucleus of thalamus.
 Tertiary fibers ascend through internal
capsule to primary sensory cortex.

Lateral Spinothalamic Tract
• Red
– 1°
• Blue
– 2°
• Green
– 3°
Lateral Spinothalamic Tract
• Red
– 1°
• Blue
– 2°
• Green
– 3°
Anterior Spinothalamic
Anterior Spinothalamic Tract
• Red:
– 1°
• Blue
– 2°
Anterior Spinothalamic Tract
Medial Lemniscus System
Also called posterior column system.
 Carries sensations for two-point sensation
(fine touch), pressure, and vibration.

Medial Lemniscus System

Primary fibers ascend entire length of
spinal cord and synapse with secondary
neurons in medulla:
Fasciculus gracilis
Fasciculus cuneatus
Medial Lemniscus
• Red:
– Gracilis
• Blue
– cuneatus
Medial Lemniscus System

Fibers of fasciculus gracilis synapse in
nucleus gracilis:
Convey sensations from below midthoracic level.

Fibers of fasciculus cuneatus synapse in
nucleus cuneatus:
Convey sensations from above midthoracic level.
Also conveys proprioceptive sensation from arms to
cerebellum.
Medial Lemniscus System
Secondary fibers decussate.
 Secondary fibers ascend to synapse in
VPL of thalamus.
 Tertiary fibers ascend through internal
capsule to primary sensory cortex.

Medial Lemniscus
Medial Lemniscus
Posterior Spinocerebellar Tract
Originates in thoracic and upper lumbar
regions.
 Consists of uncrossed fibers that enter
cerebellum through inferior cerebellar
peduncles.
 Transmits ipsilateral proprioceptive
information to cerebellum.

Anterior Spinocerebellar Tract
Originates in lower trunk and lower limbs.
 Consists of crossed fibers that recross in
pons and enter cerebellum through
superior cerebellar peduncles.
 Transmits ipsilateral proprioceptive
information to cerebellum.

Spinocerebellar Tracts
Spino-Olivary Tracts
Project to accessory olivary nuclei and
cerebellum.
 Contribute to movement 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.
Stretch (myotactic) Reflex

Muscle spindle = receptor:
3-10 small, specialized intrafusal muscle fibers:
Contractile only at ends.
Non-contractile center.

Afferent neurons from center of intrafusal fibers
travel through dorsal root of spinal nerve to
synapse directly with alpha motor neurons of
extrafusal fibers in which muscle spindle is
embedded.
Stretch (myotactic) Reflex

Afferent neurons from muscle spindle also
synapse with ascending fibers within
spinal cord.
Gamma motor neurons supply intrafusal
fibers of muscle spindle:
Regulate sensitivity of intrafusal fibers.
Gamma neurons are modulated by
descending fibers within spinal cord.

Refer to syllabus for specific stretch
reflexes.
Golgi-Tendon Reflex

Golgi tendon organs:
Encapsulated nerve endings:
End with numerous terminal branches with
small swellings associated with individual
tendon fascicles.
Lie within tendons near the muscle-tendon
junction.
Stimulated when tendon is stretched.
Golgi-Tendon Organs/Reflex

Afferent neurons from Golgi organs pass
through dorsal root of spinal nerve and
synapse with inhibitory association
neurons in posterior gray matter of spinal
cord.
Golgi-Tendon Organs/Reflex
Association neurons synapse with alpha
motor neurons that innervate muscle fibers
associated with tendon.
 Causes relaxation of associated muscles
and prevents damage to the tendon due to
excessive tension.
