The document discusses mechanosensation and the somatosensory system. It describes the basic organization, including labeled lines for different sensory functions. It outlines the peripheral mechanosensory elements like various receptor types, their locations and functions. It details the pathways for mechanosensation from the post-cranial body and face, including the dorsal column medial lemniscus system and trigeminal lemniscus pathway. It also discusses proprioception, muscle spindles, golgi tendon organs, and areas of the cortex involved in somatosensory processing.

1. Mechanosensation
Domina Petric, MD

2. Functions of somatosensory system
shape and texture of objects that interact with our skin
prevention of possible injury
monitoring of the forces that are acting upon the body
proprioception (self awareness)

3. Basic organisation of somatosensory system
labeled lines (labeled line system)
mechanosensation; pain and temperature
two pairs of pathways (pair of pathways for the back of
the head and body; pair of pathways for the face)

4. Peripheral mechanosensory elements
 Dorsal root ganglion cells are pseudomonopolar cells.
 Axon bifurcates and grows in two directions: one direction grows out to the
body and other grows out to the spinal cord.
 Peripheral process of pseudomonopolar cell is connected with specialised
receptor ending.
 The mechanosensory central process of the pseudomonopolar axon enters the
dorsal root and makes a sharp upward bend and enters the dorsal column of
white matter.
 The pain and temperature central process of pseudomonopolar axon enters the
dorsal horn and makes a synaptic connection.

5. Pseudomonopolar (pseudounipolar) neuron
Central
process
Peripheral
process https://images.fineartamerica.com
Peripheral process
Central process

6. Peripheral mechanosensory elements
Sensory
function
Receptor type Afferent axon type Axon diameter
(μm)
Conduction
velocity (m/s)
Proprioception Muscle spindle Ia, II (myelinated) 13-20 80-120
Touch Merkel, Meissner,
Pacinian, Ruffini
cells
Aβ (myelinated) 6-12 35-75
Pain,
temperature
Free nerve endings Aδ (myelinated) 1-5 5-30
Pain,
temperature,
itch
Free nerve endings
(unmyelinated)
C (unmyelinated) 0,2-1,5 0,5-2

7. Mechanosensation
Meissner corpuscle and Merkel-cell neurite complex are
responsible for light touch sensation.
Ruffini corpuscle is sensitive to stretch.
Pacinian corpuscles (encapsulated with many Schwann
cell lamellae) are sensitive to vibration

8. Afferent systems and their properties
Properties Small receptor field Large receptor field
Merkel Meissner Pacinian Ruffini
Location Tip of epidermal
sweat ridges
Dermal papillae (close to
skin surface)
Dermis and deeper tissues Dermis
Axon
diameter
7-11 μm 6-12 μm 6-12 μm 6-12 μm
Conduction
velocity
40-65 m/s 35-70 m/s 35-70 m/s 35-70 m/s
Sensory
function
Form and texture
perception
Motion detection, grip
control
Perception of distant
events through transmitted
vibrations, tool use
Tangential force,
hand shape, motion
direction
Effective
stimuli
Edges, points, corners,
curvature
Skin motion Vibration Skin stretch
Receptive
field area
9 mm2 22 m2 Entire finger or hand 60 mm2
Innervation
densitiy
100/cm 2 150/cm 2 20/cm2 10/cm2
Spatial
acuity
0,5 mm 3 mm 10+ mm 7+ mm

9. Muscle spindle, proprioception
 It is formed by specialized muscle fibres and certain types of receptor endings
that innervate those specialized muscle fibres.
 Muscle fibres within muscle spindle are called intrafusal muscle fibres.
 Intrafusal muscle fibres have collection of nuclei that are bundled near the
centre of spindle shape.
 Those muscle fibres have their contractile elements extending away from the
central region out to pole of the muscle spindle.
 Group Ia afferent axons innervate the central region of muscle spindle and
group II afferent axons end in a flower spray ending on the contractile
elements.

10. Muscle spindle
Wikipedia.org

11. Golgi tendon organ
Group Ib afferent axons innervate the junction of
the muscle fiber and the tendon.
Golgi tendon organ is sensitive on the muscle
force, while the muscle spindle is sensitive to
muscle stretching.

12. Golgi tendon organ
Wikipedia.org

13. Ventral posterior complex of the thalamus
It is divided into two main nuclei:
•ventral posterior lateral nucleus
(somatosensory pair of pathways from
posterior part of the head and body)
•ventral posterior medial nucleus
(somatosensory pair of pathways from face)

14. Primary somatosensory cortex in postcentral gyrus
https://faculty.washington.edu
/chudler/gif/twoptctx.gif

15. Primary somatosensory cortex (S1) somatotopy
 Areas 1, 2, 3a and 3b (Broadmann´s areas).
 Representation of contralateral somatosensation of foot is in the area
of paracentral lobule.
 Expansive representation for contralateral hand is in the S-shape area
of the central sulcus.
 The face is conveyed by inputs from the ventral posterior medial
nucleus of the thalamus which terminates in the inferior segment of
the postcentral gyrus, below the S-shape bend of the central sulcus.

16. https://www.studyblue.com/notes/note/n/lec-10/deck/7007528
(Neuroscience 5th edition. Sinauer Associates, Inc. 2012.)
Paracentral lobule
S-shape bend of the central sulcus
Thalamic
Ventral
Posterior
Lateral
Nucleus
Face
Thalamic
Ventral
Posterior
Medial
Nucleus

17. Cortical magnification
How much more does cortical
circuits magnify the
representation of the body that
is related to the density of
peripheral receptors and the
density of neurons at all the
antecendent stations in the
somatic sensory pathway.

18. Primary somatosensory cortex (S1) somatotopy
Area 3a is concerned with proprioceptors (muscle spindle, golgi tendon
organ, joint receptors) and proprioception.
Area 3b is concerned with cutaneous receptors.
Area 1 is going to respond to complex stimuli activating multiple skin
surfaces.
Area 2 is concerned with the shapes of objects that we encounter.

19. Secondary somatosensory cortex
Secondary somatosensory cortex is in the inferior and
posterior aspect of the parietal lobe and recieves inputs
from primary somatosensory cortex.
Informations are then passed to amygdala and
hippocampus (creation of memories).
All areas project to secondary somatosensory cortex, but
area 2 also projects to parietal areas 5 and 7 (orientation
relative to the environment).

20. Area 3a:
proprioception
Area 3b:
cutaneous
receptors
Area 1:
complex
stimuli
Area 2:
shapes of
objects
Secondary somatosensory cortex (S2)
Amygdala,
hippocampus
Parietal areas 5
and 7: orientation

21. Mechanosensory
pathways
II.

22. Two pairs of somatosensory
pathways
Post-cranial body:
posterior part of the head
and body below the head
Dorsal column
medial lemniscal
system:
mechanical stimuli
Anterolateral
system: pain and
temperature
Face and anterior portion
of the head
Pathway through
the principal
sensory trigeminal
complex nucleus:
mechanical stimuli
Pathways through
the spinal
trigeminal
complex nucleus:
pain and
temperature

23. Dorsal column medial lemniscus system, mechanosensation
from the post-cranial body
 First order neuron is dorsal root ganglion neuron in the spinal cord.
 Central process enters the dorsal column in the white matter and runs
the longitudinal length of the spinal cord.
 Dorsal column nuclei in the dorsal part of tegmentum of medulla
oblongata: synapses for the second order neuron.
 Axons of the second order neuron cross the midline.
 Third order neuron is in the ventral posterior lateral nucleus of the
thalamus.

24. First order
neuron:
dorsal root
ganglion
neuron
Dorsal column
of the spinal cord
Second
order
neuron:
dorsal
column
nuclei in the
tegmentum
of medulla
Crossing the midline
Third order
neurons:
ventral
posterior
lateral nucleus
of the
thalamus
Dorsal column medial lemniscus system: mechanosensation from the post-cranial body.
www.bioon.com
Second order neurons
are in cuneate nucleus
and gracile nucleus.
Internal arcuate fibers are crossing
the midline.
After
crossing
the
midline,
fibres
continue
as medial
lemniscus.
Postcentral
gyrus
Fibers from gracile tract
(lower extremity) project to the
paracentral lobule.
Fibers from cuneate tract (upper
extremity) project to the middle
of the postcentral gyrus.

25. Trigeminal lemniscus pathway or V-lemniscus,
mechanosensation from the face
 First order neuron is the ganglion cell in the trigeminal ganglion.
 Axons enter the brainstem through the trigeminal nerve.
 Second order neuron is in the principal sensory trigeminal complex nucleus.
 Second order axon crosses the midline and ascends the remaining divisions of the brain
stem, the upper pons and the midbrain and enters the ventral posterior complex of the
thalamic nuclei.
 This pathway is trigeminal lemniscus and it is near the medial edge of the medial
lemniscus.
 Third order neuron is in the ventral posterior medial nucleus.
 Third order neuron projects to the inferior one third of the postcentral gyrus.

26. Trigeminal lemniscus pathway: mechanosensation from face and anterior portion of the head.
First order
neuron:
trigeminal
ganglion
Second
order
neuron:
principal
sensory
trigeminal
complex
nucleus Axons cross the midline.
Axons
continue
ascending
as
trigeminal
lemniscus.
Third order
neuron:
ventral
posterior
medial
thalamic
nucleus.
Inferior one third of
postcentral gyrus

27. Cerebellum
It helps to facilitate agile movements of the body and agile movements of
thoughts.
The cerebellum gets inputs from the cortex about what we are trying to do.
The cerebellum gets feedback from our sensory systems about what we are
actually doing.
If there is a problem, the cerebellum will generate an error signal that is sent
back into our motor system for an adjustment (correction can be made).

28. Spinocerebellar pathways: dorsal spinocerebellar tract
 Receptor is for example proprioceptor in skin of the foot.
 The first order neuron is in the dorsal root ganglion of the spinal cord.
 First order axon enters the dorsal column and ascends to the thoracic
spinal cord (from lower extremity).
 Clarke´s nucleus is in the intermediate gray matter of the thoracic
spinal cord: synapse occurs with first order axon.
 Clarke´s nucleus extends throughout the thoracic spinal cord and then
to the upper lumbar levels of the spinal cord.

29. Spinocerebellar pathways: dorsal spinocerebellar tract
Clarke´s nucleus gives rise to a second order axon that ascends in the dorsal
lateral white matter of the spinal cord.
This is dorsal spinocerebellar tract.
The dorsal spinocerebellar tract ascends to the cerebellum via the inferior
cerebellar peduncle.
This is the pathway for lower extremity.

30. Spinocerebellar pathway for upper extremity
From the upper extremity, first order axons ascend in
the dorsal column.
From the dorsal column fibres interact with external
cuneate nucleus in medulla oblongata
From external cuneate nucleus second order axons
project to the cerebellum.

31. Conscience awareness
Proprioceptive informations from
spinocerebellar tracts enter the
medial lemniscus via colateralls and
enter the conscience awareness.

32. Literature
 https://www.coursera.org/learn/medical-neuroscience/lecture: Leonard
E. White, PhD, Duke University
 https://faculty.washington.edu
 https://www.studyblue.com
 Neuroscience 5th edition. Sinauer Associates, Inc. 2012.
 https://images.fineartamerica.com
 Wikipedia.org
 www.bioon.com