Ascending pathways These are the Pathways that connect the peripheral sensory stimuli to the CNS

1. ASCENDING PATHWAYS
(PHYSIOLOGY)

2. 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

3. Ascending Pathways
 For conscious perception:
Spinothalamic system
Medial Lemniscal system
 For unconscious perception:
Spinocerebellar
Spino-olivary
Spinotectal
Spinoreticular

4. Spinothalamic System
 Lateral spinothalamic tract
 Anterior spinothalamic tract

5. Lateral Spinothalamic Tract
 Carries pain and temperature
 Primary fibers ascend or descend 1-2
spinal cord segments before
synapsing with secondary fibers.

6. 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.

7. Lateral Spinothalamic

8. Lateral Spinothalamic Tract
 Secondary fibers are joined in
brainstem by fibers of the
trigeminothalamic tract:
(Pain and temperature from face and
teeth.)

9. Lateral Spinothalamic Tract
 Secondary fiber collaterals project to
reticular formation:
Stimulate wakefulness and
consciousness.
 Secondary fibers project to ventral
posterolateral (VPL) nucleus of
thalamus.

10. 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.

11. 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.

12. 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.

13. Lateral Spinothalamic Tract
 Red
◦ 1°
 Blue
◦ 2°
 Green
◦ 3°

14. Lateral Spinothalamic Tract
 Red
◦ 1°
 Blue
◦ 2°
 Green
◦ 3°

15. Anterior Spinothalamic

16. Anterior Spinothalamic Tract
 Red:
◦ 1°
 Blue
◦ 2°

17. Anterior Spinothalamic Tract

18. Medial Lemniscus System
 Also called posterior column system.
 Carries sensations for two-point
sensation (fine touch), pressure, and
vibration.

19. Medial Lemniscus System
 Primary fibers ascend entire length of
spinal cord and synapse with
secondary neurons in medulla:
Fasciculus gracilis
Fasciculus cuneatus

20. Medial Lemniscus
 Red:
◦ Gracilis
 Blue
◦ cuneatus

21. 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

22. 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.

23. Medial Lemniscus

24. Medial Lemniscus

25. 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.

26. 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.

27. Spinocerebellar Tracts

28. Spino-Olivary Tracts
 Project to accessory olivary nuclei and
cerebellum.
 Contribute to movement coordination
associated primarily with balance.

29. Spinotectal Tracts
 Project to superior colliculi of midbrain.
 Involved in reflexive turning of the
head and eyes toward a point of
cutaneous stimulation.

30. Spinoreticular Tracts
 Involved in arousing consciousness in
the reticular activating system through
cutaneous stimulation.

31. 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.

32. 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.

33. 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.

34. 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.

35. 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.

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