Overview of main sensory pathways

1. ASCENDING PATHWAYS

2. Contents
• Nervous system communication
• Structures of spinal cord
• Ascending pathways
• Spinothalamic pathways
• Dosal column pathway
• Organization and functional components of ascending tract
• Somatosensory cortex
• Sensory homunculus
• Clinical significance

3. Nervous system communication
• Receives information, transform it into impulses (transduction)
transmit impulses to the CNS correlate / coordinate transmit to the
effector organs response / action.
• CNS accomplishes its functions through four major processes:
1. Recognition of stimulus and transmission of information in the form
of nerve impulse
2. Processing of information in CNS and perception of sensations.
3. Generation of appropriate command signal in CNS.
4. Specific responses in accordance to the command signal conveyed by
CNS.

4. CENTRAL NERVOUS SYSTEM
PERIPHERAL NERVOUS SYSTEM
Integration/ processing/ modulating
Lower motor neuron
Receptor neuron
Stimulus Effector organ/ response
PNS transmission
motor/ descending tracts
Sensory/ ascending tracts

5. Structures of the spinal cord
• White matter
Anterior columns/funiculi
Lateral columns
Posterior columns
• Gray matter
Dorsal horn (sensory input)
Lateral horn (ANS)
Ventral horn (motor output)
• Central canal

7. Ascending pathways
• Located in the white matter.
• Conduct afferent information which may or may not reach
consciousness.
• There are two types of information:
 Exteroceptive information
Proprioceptive information

8. Anatomical organization
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
• Sometimes a higher order of neuron originates
from the terminals of third order of neurons in
the sensory cortex and project to other areas of
the brain, especially to the cortical sensory
association area.

9. • For conscious perception:
Spinothalamic system / Anterolateral system
Medial lemniscal system / Posterior column / Dorsal column
• For unconscious perception:
Spinocerebellar
Spino-olivary
Spinotectal
Spinoreticular

10. The main somatosensory pathways to consciousness

11. Functions of the ascending tract
• Painful and thermal sensations ascend in the Lateral spinothalamic
tract.
• Crude touch and pressure ascend in the anterior spinothalamic tract.
• Discriminative touch, two-point discrimination ascends in posterior
white columns.
• Ascending in the posterior white columns is information from muscles
and joints pertaining to movement and position of different parts of the
body.

12. Spinothalamic tract / Anterolateral tract
• It is divided into two parts:
1. The anterior spinothalamic tract carries the sensation of crude touch and
pressure
2. The lateral spinothalamic tract carries the sensation of pain and
temperature.
• First Order of Neuron are primarily the afferent fibers originating from nociceptors,
thermoreceptors, and mechanoreceptors.
• The fibers enter the spinal cord through dorsal root and the cell bodies are present
in the DRG.
• In the spinal cord, fibers terminate on the second order of neurons that are
present on the same side of the dorsal horn of the spinal cord.
• The axons cross the midline in the same spinal segment and ascend up in the
opposite side of the anterolateral funiculus to reach the thalamus.
• Third order of neurons originate from the VPL, midline and intralaminar nuclei of
the thalamus and project to the specific areas in the sensory cortex.

13. Lateral spinothalamic tract
• The axons entering the spinal cord from the
posterior root ganglion proceed to the tip of
the posterior gray column and divide into
ascending and descending branches.
• These branches travel for a distance of one or
two segments of the spinal cord and from the
posterolateral tract of Lissauer.
• These fibers of the first-order neuron
terminate by synapsing with cells in the
posterior gray column, including cells in the
substantia gelatinosa.
• The axons of the second-order neurons now
cross obliquely to the opposite side in the
anterior gray and white commissures within
one spinal segment of the cord, ascending in
the contralateral white column as the lateral
spinothalamic tract

14. • As the lateral spinothalamic tract ascends through the medulla oblongata it
is accompanied by the anterior spinothalamic tract and the spinotectal tract;
together they form the spinal lemniscus.
• Many of the fibers of the lateral spinothalamic tract end by synapsing with
the third-order neuron in the ventral posterolateral nucleus of the thalamus.
It is believed that here crude pain and temperature sensations are
appreciated and emotional reactions are initiated.
• The axons of the third-order neurons in the ventral posterolateral nucleus of
the thalamus now pass through the posterior limb of the internal capsule to
reach the somesthetic area in the postcentral gyrus of the cerebral cortex
Lateral spinothalamic tract

15. Anterior spinothalamic tract
• The axons entering the spinal cord from the
posterior root ganglion proceed to the tip of the
posterior gray column where it terminate by
synapsing with cells in the posterior gray column,
including cells in the substantia gelatinosa.
• The axons of the second-order neurons cross
obliquely to the opposite side in the anterior gray
and white commissures within one spinal
segment of the cord, ascending in the opposite
anterolateral white column as anterior
spinothalamic tract.
• Crude awareness of touch and pressure is
believed to appreciated in the VPN.
• The axons of the third-order neurons in the VPN
of the thalamus now pass through the posterior
limb of the internal capsule to reach the
somesthetic area in the postcentral gyrus of the
cerebral cortex

16. Dorsal column /Medial lemniscus system
• Carries sensations for two-point sensation (fine touch), pressure and
vibration, proprioception, tactile localization, tactile discrimination and
stereognosis.
• Primary fibers ascend entire length spinal cord and synapse with secondary
neurons in medulla:
Fasciculus gracilis
Fasciculus cuneatus
• 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.

17. • The first order of neurons terminate in the nucleus gracilis
and nucleus cuneatus in the medulla.
• Second Order of Neuron- the cell bodies of these neurons
are present in the nucleus gracilis and cuneatus in the
medulla.
• The fibers originating from these nuclei cross the midline and pass
on to the opposite side in the medulla and ascend up in the
medial lemniscus to reach thalamus.
• Dorsal column nuclei: contains two regions
- Cluster and non cluster regions
• Third order of neurons originate from the specific nucleus
(the VPL nucleus) in the thalamus.
• The neurons project to the somatosensory areas of the cerebral
cortex.

18. Sensation Receptor First order
neuron
Second order
neuron
Third order
neuron
Pathways Destination
Pain and
temperature
Free nerve
endings
Posterior root
ganglion
Substantia
gelatinosa
Ventral
posterolateral
nucleus of
thalamus
Lateral
spinothalamic,
Spinal
lemniscus
Posterior
central gyrus
Light touch and
pressure
Free nerve
endings
Posterior root
ganglion
Substantia
gelatinosa
Ventral
posterolateral
nucleus of
thalamus
Anterior
spinothalamic,
Spinal
lemniscus
Posterior
central gyrus
Discriminatory
touch,
Vibratory
sense,
Conscious
muscle join
sense
Meissner’s
corpuscle,
Pacinian
corpuscles,
Muscle
spindles,
Tendon organs
Posterior root
ganglion
Nuclei gracilis
and cuneatus
Ventral
posterolateral
nucleus of
thalamus
Fasciculi gracilis
and cuneatus,
Medial
lemniscus
Posterior
central gyrus
The Main Somatosensory Pathways to Consciousness

19. • The primary sensory cortex is
located in the postcentral gyrus,
and handles signals coming from
the thalamus.
• Brodmann areas 3, 1, and 2 make
up the primary somatosensory
cortex.
• Relative sizes of cortical areas
• Proportional to number of
sensory receptors.
• Proportional to the sensitivity
of each part of the body.
• Can be modified with learning
• Learn to read Braille & will
have larger area representing
fingertips
Somatosensory cortex

20. Sensory Homunculus
• A sensory homunculus represents a
map of brain areas dedicated
to sensory processing for different
anatomical divisions of the body.
• The topography of the sensory
homunculus corresponds to the
contralateral side of the body.
• Starting from the medial aspect of
the postcentral gyrus, this area
corresponds to the sensory territory
of the genitals and lower extremities.

21. • Moving more laterally along the
postcentral gyrus, the superior
to superolateral regions of the
gyrus corresponds to the neck,
shoulder, and torso. While
along the lateral side of the
postcentral gyrus, this region
corresponds to the upper
extremity and hand. The
inferolateral region of the
postcentral gyrus corresponds
to the facial and bulbar
structures.

22. Front of sensory homunculus Rear of sensory homunculus

23. Clinical significance
• Brown sequard syndrome
• Lesions
Diagnostic test
• Romberg’s test
• Somatosensory Evoked Potential response study

24. References
• Richard S. Snell; Clinical Neuroanatomy, 7th edition
• G K Pal; Comprehensive textbook of Medical physiology
• Dr. A K Jain; Textbook of physiology
• William W Campbell DeJong's The Neurologic Examination (Campbell,
The NeurologicExamination) –Lippincott Williams & Wilkins(2005)
• Gandhoke GS, Belykh E, Zhao X, Leblanc R, Preul MC. Edwin Boldrey and
Wilder Penfield's Homunculus: A Life Given by Mrs. Cantlie (In and Out
of Realism). World Neurosurg. 2019 Dec;132:377-388. doi:
10.1016/j.wneu.2019.08.116. Epub 2019 Aug 27. PMID: 31470165.

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