1. Somatosensory
Systems and
Receptors
Department of Neurology
Loyola University Medical Center
2012.

2. The primacy of touch
And I found that of all the senses the
eye was the most superficial, the
ear the most haughty, smell the
most voluptuous, taste the most
superstitious and inconstant, touch
the most profound and
philosophical.
Diderot, ‘Letter on the Blind,’ 1749

3. Somatosensory Systems
Information
 Input from the somatosensory systems informs
the organism about events impinging on it.
 Sensation can be divided into four types:
superficial, deep, visceral, and special.

4. Types of sensation
Superficial sensation
touch, pain, temperature, and two-point discrimination.
Deep sensation
muscle and joint position sense (proprioception), deep muscle pain,
and vibration sense.
Visceral sensations
relayed by autonomic afferent fibers and include hunger, nausea,
and visceral pain
Special senses—smell, vision, hearing, taste, and equilibrium

5. Categories of sensation
 Conscious sensation
perceived at the level
of cerebral cortex.
 Non conscious
sensation not
perceived, they have
reference to the
cerebellum

6. Conscious sensations
 Exteroceptive: external word.
Impinge either somatic receptors : body surface (touch,
pressure, heat, cold, pain)
Or, telereceptors: vision and hearing.
 Propioceptive: arise within the body.
locomotor system, or vestibular labyrinth. Position sense
(stationary) and kinesthetic sense (movement)

7. Non conscious sensations
 Non conscious proprioception:
Smooth motor coordination, spinocerebellar
pathways (afferent information)
 Interoception:
Unconscious afferent signals involved in visceral
reflexes

8. Receptors

9. Receptors
 Receptors are specialized cells for detecting particular changes in the
environment.
 Exteroceptors include receptors affected mainly by the external
environment
Meissner's corpuscles, Merkel's corpuscles, and hair cells for touch;
Krause's end-bulbs for cold; Ruffini's corpuscles for warmth; and free nerve
endings for pain.
Receptors are not absolutely specific for a given sensation; strong stimuli
can cause various sensations, even pain, even though the inciting stimuli
are not necessarily painful.
 Proprioceptors receive impulses mainly from pacinian corpuscles, joint
receptors, muscle spindles, and Golgi tendon organs. Painful stimuli are
detected at the free endings of nerve fibers.

10. Sensory units
 Stem fiber and the “family of endings”
constitute a sensory unit
 The territory from which a sensory unit can
be excited is its receptive field.
 Inverse relationship between sized of
receptive fields and sensory acuity.
Example, 2cm2 upper arm, 1cm2 wrist,
5mm2 finger pads.

11. Adaptation
 Each efferent fiber from a receptor relays stimuli that originate in a receptive
field and gives rise to a component of an afferent sensory system.
 Each individual receptor fires either completely or not at all when stimulated.
 The greater the intensity of a stimulus, the more end-organs that are
stimulated, the higher the rate of discharge is, and the longer the duration of
effect is.
 Adaptation denotes the diminution in rate of discharge of some receptors
on repeated or continuous stimulation of constant intensity; the sensation of
sitting in a chair or walking on even ground is suppressed

12. Nerve endings

13. Nerve endings
Free nerve endings.
Run toward skin surface.
Schwann cells sheaths open to permit
naked axons to terminate between
collagen bundles (dermal nerve endings)
or within epidermis (epidermal nerve
endings)

14. Free nerve endings

15. Free nerve endings
 Thermoreceptors: supply either “warm spots” or
“cold spots”
 Nociceptors (pain-transducing):
1. mechanical deformation, finely myelinated, Aδ
fibers
2. polymodal nociceptors, C-fibers: transduce
mechanical deformation, intense heat or cold,
irritant chemicals, and are responsible for the
axon reflex.

16. Follicular nerve endings
 Myelinated fibers apply a “palisade” of
naked terminals along the outer root
sheath epithelium of the hair follicles.
Circumferential set of terminals.
 Territorial overlap = each unit supplies
many follicles. Rapidly adapting.

17. Follicular nerve endings

18. Merkel cell neurite complexes
 Expanded nerve terminals
in the basal epithelium of
epidermal pegs and
ridges.
 Slowly adapting
 Discharge continuously in
response of sustained
pressure (wear glasses,
holding a pen)

19. Encapsulated nerve endings
 Capsule:
outer coat: connective tissue
middle coat: perineural epithelium
innter coat: modified Schwann cells
(teloglia)
 All mechanoreceptors

20. Meissner’s corpuscles.
 Most numerous in
finger pads.
 Ovoid receptors, with
zigzag axons.
 Rapidly adapting.
 Textured surfaces,
braille text, elevation in
5 um can be detected

21. Ruffini endings
 In hairy and glabrous
skin.
 Respond to drag
 Slowly adapting
 Resembling Golgi
tendon organ,
collagenous core and
several axons branch
liberally

22. Pacinian corpuscles
 Size of rice grains.
About 300 in the
hand.
 Rapidly adapting and
especially responsive
to vibration. (bone
vibration)

23. Delmas. Molecular mechanisms of mechanotransduction in mammalian sensory neurons Nature Reviews Neuroscience. 2011

24. Tactile discrimination and
stereognosis (spatial sense)
 3D objects, perceptually
evaluated by: muscle afferents
(muscle spindles) articular
afferents (joint capsules) and
skin.
 Cutaneous, muscular and
articular afferents relay
information independently to
the contralateral somatic
sensory cortex (posterior part
parietal lobe)

25. Somatosensory system pathways
1. Posterior (dorsal) column-medial
leminscal pathway.
2. Spinothalamic (anterolateral) pathway.

27. Posterior (dorsal) Column-medial lemniscal
pathway
 1st order: largest somas in the
posterior root ganglia.
 Process largest sensory receptors.
touch, joint sensation, two-point
discrimination, and vibratory sense
from receptors to the cortex
 Lower limb and lower trunk give
branches (fasciculus gracilis) to
gray matter to reach gracile
nucleus in medulla oblongata
 Upper limb and upper trunk
cuneate fasciculus to reach
cuneatus nucleus.

28. Posterior (dorsal) Column-medial lemniscal
pathway
 2nd order afferents, start
nucleus gracilis and nucleus
cuneatus. Ventrally in medulla
oblongata for sensory
decusation and the fibers turn
rostrally in medial lemniscus
 Ending lateral part ventral
posterior nucleus of the
thalamus, also ending medial
part same nucleus, trigeminal
lemniscus.
 3rd order afferents from the
thalamus to the somatic
sensory cortex.

29. Spinothalamic pathway
 Second order neurons, projecting from
laminae I-II, IV-V posterior gray horn,
to the contralateral thalamus.
 Cells receive excitatory and inhibitory
synapses from neurons of the
substantia gelatinosa (modulatory
effects)
 Cross midline anterior comissure at all
segmental levels. Anterior
spinothalamic tract, and lateral
spinothalamic tract.
 In the brainstem is spinal lemniscus
 Spinal lemniscus join trigeminal
afferents to the VP nucleus thalamus

32. And I found that of all the senses the eye was the most
superficial, the ear the most haughty, smell the most
voluptuous, taste the most superstitious and inconstant,
touch the most profound and philosophical.
Diderot, ‘Letter on the Blind,’ 1749
Thank you.