ascending sensory pathways

Ascending Sensory Pathways

Dorsal Column-Medial Lemniscus systemfine touch

position sense

Anterolateral systemtemperaturecoarse touch

pain

James Bisley ([email protected])

Dorsal Column-Medial Lemniscus system

Conveys mechanosensory information from the periphery to the cortex

• Cutaneous Mechanoreceptors (fine touch)

• Proprioception & Kinesthesia (position)

Fine touch

receptor

afferent

PainTemperatureCoarse touch

Kinesthesia is the “awareness” of body position

and movement

Proprioception is the “sub-concious”

information used in the feed-back control of

posture and precise movements.

Position sense

Position sense information comes from:

Muscle spindles

Golgi tendon organs

Joint receptors

Cutaneous mechanoreceptive afferents

Efference copy

Position sense

Proprioceptors

Motor unit(controlled by efferent)

Muscle spindle

Golgi tendon organ

Joint receptor

Afferent fiber information

Fiber type Class Diameter

(μm)

Conduction Vel. (m/s)

Types of receptors

Aα Ia

Ib

13-20 80-120 Primary muscle spindle

Golgi tendon organ

Aβ II 6-12 35-75 Skin mechanoreceptors

Secondary muscle spindle

Aδ III 1-5 5-30 Coarse touch, temperature, and pain

C

(no myelin)

IV 0.2-1.5 0.5-2 Coarse touch, temperature, and pain


Some terminology

We use the terms first, second and third order

neurons to describe the steps of the

pathway to cortex.

receptor

First orderneuron

Secondorder

neuron

Thirdorder

neuron


Afferents have their cell bodies in the DORSAL

ROOT GANGLIA.

Called pseudo-unipolar neurons.


The DRG axons enter through the dorsal horn of

the spinal cord

Spinal reflexes, Clarke’s Nucleus, etc


Fibers that convey information from lower limbs and body (below

spinal segment T6) travel ipsilaterally along the GRACILE TRACT.


Fibers that convey information from upper limbs and body (above

spinal segment T6) travel ipsilaterally along the CUNEATE TRACT.

GRACILE TRACT

There is a topographic representation of the body in the dorsal columns


Fibers in the Gracile Tract have their first synapse in the GRACILE NUCLEUS.

Fibers in the Cuneate Tract have their first

synapse in the CUNEATE NUCLEUS.

Caudal medullaThere is a topographic representation of the body in the dorsal column nuclei


Axons from the second order neurons form the INTERNAL ARCUATE FIBERS in the caudal

medulla, which decussates becoming the

contralateral MEDIAL LEMNISCUS.

Caudal medullaThere is a topographic representation of the body in the medial lemniscus


The representation of the body shifts as the medial lemniscus runs rostrally.


The axons of the second order neurons terminate

in the VENTRAL POSTERIOR LATERAL

NUCLEUS of the thalamus (VPL).

There is a topographic representation of the body in the VPL (lower extremities are lateral)

Dorsal Column-Medial Lemniscus systemG

raci

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un

eate

What about the face?


Pseudo-unipolar neurons have their cell bodies in

the TRIGEMINAL GANGLION.


Mid-pons Except for Proprioception

Pseudo-unipolar neurons have their cell bodies in the

MESENCEPHALIC NUCLEUS inside the CNS.



the TRIGEMINAL GANGLION.


Except for Proprioception

Pseudo-unipolar neurons have their cell bodies in the

MESENCEPHALIC NUCLEUS inside the CNS.


Axons project to second order neurons in the

PRINCIPAL (SENSORY) NUCLEUS OF THE

TRIGEMINAL COMPLEX in mid-pons.


Mid-pons

There is a topographic representation of the face in the principal (sensory) nucleus


Axons of the second order neurons decussate

and join the TRIGEMINOTHALAMIC

TRACT (which runs adjacent to the medial

lemniscus).


Mid-pons


The axons of the second order neurons terminate

in the VENTRAL POSTERIOR MEDIAL

NUCLEUS of the thalamus (VPM).


There is a topographic representation of the face in the VPM

Mid-pons

Mid-pons


Neurons in the VP complex project to

PRIMARY SOMATIC-SENSORY CORTEX via the POSTERIOR LIMB of

the INTERNAL CAPSULE.

The whole body is represented in the ventral posterior complex.


Neurons in the VP complex project to

PRIMARY SOMATIC-SENSORY CORTEX via the POSTERIOR LIMB of

the INTERNAL CAPSULE.


Area 3aPrimarily proprioception

input

Area 3bPrimarily tactile input

Area 1Primarily tactile input, but

receptive fields usually cover several digits

Area 2Combination of tactile and

proprioception. Hand configuration & stimulus shape are both important

Dorsal Column-Medial Lemniscus systemThe whole body is represented in each area of SI

Owl Monkey


The somatosensory homunculus

Anterolateral system

Conveys pain, temperature and coarse touch information from the periphery to the cortex

Fiber type Class Diameter

(μm)

Conduction Vel. (m/s)

Types of receptors

Aα Ia

Ib

13-20 80-120 Primary muscle spindle

Golgi tendon organ

Aβ II 6-12 35-75 Skin mechanoreceptors

Secondary muscle spindle

Aδ III 1-5 5-30 Coarse touch, temperature, and pain

C

(no myelin)

IV 0.2-1.5 0.5-2 Coarse touch, temperature, and pain

Central Pain Pathways:Sensory discriminative component

As with the tactile system, the cell bodies are

located in the DORSAL ROOT GANGLIA.

Pseudo-unipolar neurons.


The DRG axons enter through the dorsal horn of

the spinal cord

Upon entering, the axons branch into ascending

and decending collaterals forming the

DORSOLATERAL TRACT of LISSAUER.


The axons run up or down several spinal cord segments in Lassauer’s tract before synapsing in

the gray matter of the dorsal horn.


The second order neurons decussate

immediately and form the SPINOTHALAMIC TRACT (aka the

anterolateral tract).

anterior white commissure





There is a topographic representation of the body in the spinothalamic tract anterior white commissure


Neurons in the spinothalamic tract

terminate in the VENTRAL POSTERIOR

LATERAL NUCLEUS (VPL) of the Thalamus.

Just like the tactile systemThere is a topographic representation of the

body in the VPL (lower extremities are lateral)

Some simple differences between the pathways

Test the pathway

Light touch

Vibration

2-point discrimination

Sense of position

Dorsal column Anterolateral

X

X

Test the pathway

Pain

Temperature

Coarse touch




the TRIGEMINAL GANGLION and ganglia associated with nerves VII (Facial), IX (Glosso-

pharyngeal) & X (Vagus).

Anterolateral tract


After entering the brain stem, the fibers descend

in the SPINAL TRIGEMINAL TRACT to the medulla, where they synapse onto neurons in the SPINAL NUCLEUS of

the TRIGEMINAL COMPLEX (primarily the

pars caudalis).

There is a topographic representation of the head

in the pars caudalis

Anterolateral tract


Axons from the second order neurons decussate immediately and then join

the ascending anterolateral tract in the

brain stem.


Axons from the second order neurons terminate

in the VENTRAL POSTERIOR MEDIAL

NUCLEUS (VPM) of the Thalamus.

There is a topographic representation of the face in the VPM

Anterolateral tract

The whole body & all somatic senses are

represented in the ventral posterior complex.

Neurons in the VP complex carrying pain information project to

PRIMARY and SECONDARY SOMATIC-

SENSORY CORTEX.



Cortex

localization of pain

Sub-corticalperception of pain

Paleospinothalamic pathways

suffering component of pain (reduced by benzodiazepines)

Central Pain Pathways:Descending Control of Pain

Stimulation of PAG results in analgesia.

The same holds true for the pars caudalis of the

spinal nucleus of the trigeminal complex

Central Pain Pathways:Descending Control of Pain

Endogenous opioid

Opioids play a role in the descending control of pain

In the dorsal horn or the pars caudalis

Central Pain Pathways:Local Control of Pain

Interaction between dorsal column and anterolateral systems regulates pain perception. This is why rubbing a wound after

sharp pain helps a bit.

Cutaneous mechanoreceptor

Cutaneous nociceptor

Stimulation of dorsal columns can antidromically

induce analgesia

Central Pain Pathways:Local & Descending Control of Pain

What you should know

Aα and Aβ fibers excite interneurons that reduce the transmission of pain

information

Descending fibers excite interneurons that reduce the transmission of pain

information


• Information content• Fine touch, vibration and sense of position

• The path to cortex• Locations & projections of 1st, 2nd & 3rd order neurons

• Where decussation occurs

• Differences between DRG inputs & Vth nerve inputs

• Basic arrangement of topography throughout the system

• The organization of somatosensory cortex• 4 areas

• Basic arrangement of topography

Anterolateral system

• Information content• Coarse touch, temperature & pain

• The path to cortex• Locations & projections of 1st, 2nd & 3rd order neurons

• Where decussation occurs

• Differences between DRG inputs & Vth nerve inputs

• Basic arrangement of topography throughout the system

• The path from cortex• Main areas involved in descending control of pain

• 2 ways that pain can be modulated in dorsal horn

ascending sensory pathways

Documents

contralateral medial

dorsal horn

order neurons

body shifts

topographic representation

dorsal root ganglia

cell bodies

awareness of body position