Somatosensory System of the Human body. This is a course presentation for a neuroscience class. Ref: Neuroscience, 6th edition by Dale Purves

1. The Somatosensory
System: Touch and
Proprioception
Clinton Chayan Dhar
2021-06-18
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2. Introduction
• THE SOMATOSENSORY SYSTEM is arguably the most diverse of the sensory systems, mediating a range of sensations—
touch, pressure, vibration, limb position, heat, cold, itch, and pain—that are transduced by receptors within the skin,
muscles, or joints and conveyed to a variety of CNS targets.
Divided into functionally distinct subsystems with distinct sets of peripheral receptors and central pathways:
• One subsystem transmits information from cutaneous mechanoreceptors and mediates the sensations of fine touch,
vibration, and pressure.
• Another originates in specialized receptors that are associated with muscles, tendons, and joints and is responsible for
proprioception—our ability to sense the position of our own limbs and other body parts in space.
• A third subsystem arises from receptors that supply information about painful stimuli and changes in temperature as
well as non-discriminative (or sensual) touch. 2

3. Afferent nerves:
Periphery to CNS
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4. Afferent nerves: Periphery to CNS
• Sensory transduction—the process of converting the energy of a
stimulus into an electrical signal—is similar in all somatosensory
afferents: A stimulus alters the permeability of cation channels in the
afferent nerve endings, generating a depolarizing current known as a
receptor (or generator) potential.
• Magnitude of the depolarization --> the receptor potential reaches
threshold-->action potential.
• Afferent fibers often encapsulated by specialized receptor cells that
tune the afferent fibers to particular features of somatic stimulation.
• Without the specialized receptors other afferent nerves are called
free nerve endings. Those are important in the sensation of pain.
• Afferents that have encapsulated endings generally have lower
thresholds for action potential generation and are thus are more
sensitive to sensory stimulation than are free nerve endings.
Recently, the first family of mammalian mechanotransduction channels was identified. It consists of two members: Piezo1
and Piezo2 (Greek piesi, “pressure”)
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5. Somatosensory Afferents Convey Different Functional Information
• The receptive fields in regions with dense innervation
(fingers, lips, toes) are relatively small compared with
those in the forearm or back that are innervated by a
smaller number of afferent fibers.
• Regional differences in receptive field size and
innervation density are the major factors that limit the
spatial accuracy with which tactile stimuli can be
sensed. Thus, measures of two-point discrimination—
the minimum interstimulus distance required to
perceive two simultaneously applied stimuli as
distinct—vary dramatically across the skin surface.
Slowly and rapidly adapting mechanoreceptors provide
different information. Slowly adapting receptors continue
responding to a stimulus, whereas rapidly adapting
receptors respond only at the onset (and often the offset)
of stimulation. These functional differences allow
mechanoreceptors to provide information about both the
static (via slowly adapting receptors) and dynamic (via
rapidly adapting receptors) qualities of a stimulus. 5

6. Mechanoreceptors Specialized to Receive Tactile Information
• Merkel cell afferents are slowly adapting fibers
that account for about 25% of the
mechanosensory afferents in the hand
(especially Fingertips).
• Deleting Piezo2 selectively in Merkel cells
significantly reduces the sustained and static
firing of the innervating afferents.
• Meissner afferents also express Piezo2. They
are rapidly adapting fibers that innervate the
skin even more densely than Merkel afferents,
accounting for about 40% of the
mechanosensory innervation of the human
hand.
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7. Simulation of activity
patterns
in different mechanosensory
afferents in the fingertip.
• Only slowly adapting Merkel cell afferents (top
panel) provide a high-fidelity representation of
the Braille pattern—that is, the individual Braille
dots can be distinguished only in the pattern of
Merkel afferent neural activity. (After Phillips et
al., 1990.)
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8. Mechanoreceptors Specialized to
Receive Tactile Information (Cont.)
• Due at least in part to their close proximity to the
skin surface, Meissner afferents are more than
four times as sensitive to skin deformation as
Merkel afferents; however, their receptive fields
are larger than those of Merkel afferents, and
thus they transmit signals with reduced spatial
resolution.
• Information conveyed by Meissner afferents is
responsible for detecting slippage between the
skin and an object held in the hand, essential
feedback information for the efficient control of
grip.
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9. Mechanoreceptors
Specialized to Receive
Tactile Information (Cont.)
• Pacinian afferents are rapidly adapting fibers
that make up 10–15% of the mechanosensory
innervation in the hand.
• The most sensitive Pacinian afferents generate
action potentials for displacements of the skin
as small as 10 nanometers. Because they are
so sensitive, the receptive fields of Pacinian
afferents are often large, and their boundaries
are difficult to define.
• The properties of Pacinian afferents make
them well suited to detect vibrations
transmitted through objects that contact the
hand or are being grasped in the hand,
especially when making or breaking contact.
These properties are important for the skilled
use of tools (e.g., using a wrench, cutting
bread with a knife, writing).
• Ruffini afferents are slowly adapting fibers and are the least understood of
the cutaneous mechanoreceptors. Ruffini endings are elongated, spindle-
shaped, capsular specializations located deep in the skin, as well as in
ligaments and tendons.
• The long axis of the corpuscle is usually oriented parallel to the stretch lines
in skin; thus, Ruffini corpuscles are particularly sensitive to the cutaneous
stretching produced by digit or limb movements; they account for about 20%
of the mechanoreceptors in the human hand.
• Information supplied by Ruffini afferents contributes, along with muscle
receptors, to providing an accurate representation of finger position and the
conformation of the hand.
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10. Mechanoreceptors
Specialized for
Proprioception
• Innervation of the muscle spindle arises from two classes
of fibers: primary and secondary endings. Primary endings
arise from the largest myelinated sensory axons (group Ia
afferents) and have rapidly adapting responses to changes
in muscle length; in contrast, secondary endings (group II
afferents) produce sustained responses to constant muscle
lengths.
• Proprioceptors in the musculoskeletal system. These “self-
receptors” provide information about the position of the
limbs and other body parts in space. (A) A muscle spindle
and several extrafusal muscle fibers. The specialized
intrafusal muscle fibers of the spindle are surrounded by a
capsule of connective tissue. (B) Golgi tendon organs are
low-threshold mechanoreceptors found in tendons; they
provide information about changes in muscle tension. (A
after Matthews, 1964.)
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11. Central Pathways Conveying Tactile
Information from the Body: The Dorsal
Column–Medial Lemniscal System
Fig: The main touch pathways. (A) The dorsal column–medial
lemniscal pathway carries mechanosensory information from
the posterior third of the head and the rest of the body. (B) The
trigeminal portion of the mechanosensory system carries
similar information from the face.
*The term column refers to the gross columnar appearance of
these fibers as they run the length of the spinal cord.
*The word lemniscus means “ribbon”
Thus, the somatosensory cortex represents mechanosensory signals first generated in the cutaneous surfaces of the
contralateral body.
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12. Central Pathways Conveying
Proprioceptive Information
from the Body
Proprioceptive afferents for the lower part
of the body synapse on neurons in the
dorsal and ventral horn of the spinal cord
and on neurons in Clarke’s nucleus. Neurons
in Clarke’s nucleus send their axons via the
dorsal spinocerebellar tract to the
cerebellum, with a collateral to the dorsal
column nuclei. Proprioceptive afferents for
the upper body also have synapses in the
dorsal and ventral horns, but then ascend
via the dorsal column to the dorsal column
nuclei; the external cuneate nucleus, in turn,
relays signals to the cerebellum.
Proprioceptive target neurons in the dorsal
column nuclei send their axons across the
midline and ascend through the medial
lemniscus to the ventral posterior nucleus.
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13. Somatosensory
Components of
the Thalamus
Fig: Somatosensory portions of the thalamus and their cortical targets in the
postcentral gyrus. The ventral posterior nuclear complex comprises the VPM, which
relays somatosensory information carried by the trigeminal system from the face,
and the VPL, which relays somatosensory information from the rest of the body. The
diagram at the upper right shows the organization of the primary somatosensory
cortex in the postcentral gyrus, shown here in a section cutting across the gyrus
from anterior to posterior. (After Brodal, 1992 and Jones et al., 1982.) 13

14. Primary
Somatosensory
Cortex
(A) Diagram showing the region of the
human cortex from which electrical
activity is recorded following
mechanosensory stimulation of different
parts of the body. (B) Diagram showing
the somatotopic representation of body
parts from medial to lateral. (C) Cartoon
of the homunculus constructed on the
basis of such mapping. Note that the
amount of somatosensory cortex devoted
to the hands and face is much larger than
the relative amount of body surface in
these regions.
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15. Connections within the
somatosensory cortex
establish functional
hierarchies.
• Inputs from the ventral posterior
complex of the thalamus terminate in
Brodmann’s areas 3a, 3b, 1, and 2, with
the greatest density of projections in
area 3b. Area 3b in turn projects heavily
to areas 1 and 2, and the functions of
these areas are dependent on the
activity of area 3b. All subdivisions of
primary somatosensory cortex project to
secondary somatosensory cortex; the
functions of SII are dependent on the
activity of SI.
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16. Functional changes in the somatosensory cortex following amputation of
a digit.
(A) Diagram of the somatosensory cortex in the owl monkey,
showing the approximate location of the hand representation.
(B) The hand representation in the animal before amputation;
the numbers correspond to different digits. (C) The cortical
map determined in the same animal 2 months after
amputation of digit 3. The map has changed substantially;
neurons in the area formerly responding to stimulation of digit
3 now respond to stimulation of digits 2 and 4.
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17. Functional expansion of a cortical
representation by a repetitive behavioral task.
• (A) An owl monkey was trained in a task that
required heavy usage of digits 2, 3, and
occasionally 4. (B) The map of the digits in the
primary somatosensory cortex prior to training.
(C) After several months of “practice,” a larger
region of the cortex contained neurons
activated by the digits used in the task. Note
that the specific arrangements of the digit
representations are somewhat different,
indicating the variability of the cortical
representation in individual animals.
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18. Summary
• First-order neurons in this chain are the primary sensory neurons located in the dorsal root and cranial nerve
ganglia.
• The next set of neurons conveying ascending mechanosensory signals is in brainstem nuclei .
• The final link in the pathway from periphery to cerebral cortex consists of neurons found in the thalamus,
which in turn project to the postcentral gyrus.
• The result of this complex interaction is the unified perceptual representation of the body and its ongoing
interaction with the environment.
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19. THANAK YOU ALL
for your kind attention!
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