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Anatomy of the Somatosensory System
FROM WIKIBOOKS
1
Our somatosensory system consists of sensors in the skin
and sensors in our muscles, tendons, and joints. The re-
ceptors in the skin, the so called cutaneous receptors, tell
us about temperature (thermoreceptors), pressure and sur-
face texture (mechano receptors), and pain (nociceptors).
The receptors in muscles and joints provide information
about muscle length, muscle tension, and joint angles.
Cutaneous receptors
Sensory information from Meissner corpuscles and rapidly
adapting afferents leads to adjustment of grip force when
objects are lifted. These afferents respond with a brief
burst of action potentials when objects move a small dis-
tance during the early stages of lifting. In response to
Figure 1: Receptors in the hu-
man skin: Mechanoreceptors can
be free receptors or encapsulated.
Examples for free receptors are
the hair receptors at the roots of
hairs. Encapsulated receptors are
the Pacinian corpuscles and the
receptors in the glabrous (hair-
less) skin: Meissner corpuscles,
Ruffini corpuscles and Merkel’s
disks.
Hairy skin Glabrous skin
Epidermis
Dermis
Pacinian
corpuscle
Papillary Ridges
Septa
Ruffini’s
 corpuscle
Hair receptor
Meissne r’s
corpuscleSebaceous
gland
Free nerve
ending
Merkel ’s
receptor
1 The following description is based on lecture notes from Laszlo Zaborszky, from Rutgers University.
1
Figure 2: Mammalian muscle
spindle showing typical position
in a muscle (left), neuronal con-
nections in spinal cord (middle)
and expanded schematic (right).
The spindle is a stretch receptor
with its own motor supply con-
sisting of several intrafusal mus-
cle fibres. The sensory endings of
a primary (group Ia) afferent and
a secondary (group II) afferent
coil around the non-contractile
central portions of the intrafusal
fibres.
rapidly adapting afferent activity, muscle force increases
reflexively until the gripped object no longer moves. Such
a rapid response to a tactile stimulus is a clear indication
of the role played by somatosensory neurons in motor ac-
tivity.
The slowly adapting Merkel’s receptors are responsible
for form and texture perception. As would be expected for
receptors mediating form perception, Merkel’s receptors
are present at high density in the digits and around the
mouth (50/mm² of skin surface), at lower density in oth-
er glabrous surfaces, and at very low density in hairy skin.
This innervations density shrinks progressively with the
passage of time so that by the age of 50, the density in hu-
man digits is reduced to 10/mm². Unlike rapidly adapting
axons, slowly adapting fibers respond not only to the ini-
tial indentation of skin, but also to sustained indentation
up to several seconds in duration.
Activation of the rapidly adapting Pacinian corpuscles
gives a feeling of vibration, while the slowly adapting
Ruffini corpuscles respond to the lataral movement or
stretching of skin.
Nociceptors
Nociceptors have free nerve endings. Functionally, skin
nociceptors are either high-threshold mechanoreceptors
From Wikibooks
2
Rapidly adapting Slowly adapting
Surface receptor /
small receptive
field
Hair receptor, Meissner’s corpuscle: De-
tect an insect or a very fine vibration.
Used for recognizing texture.
Merkel’s receptor: Used for spa-
tial details, e.g. a round surface
edge or “an X” in brail.
Deep receptor /
large receptive
field
Pacinian corpuscle: “A diffuse vibra-
tion” e.g. tapping with a pencil.
Ruffini’s corpuscle: “A skin
stretch”. Used for joint position
in fingers.
Table 1
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or polymodal receptors. Polymodal receptors respond not
only to intense mechanical stimuli, but also to heat and
to noxious chemicals. These receptors respond to minute
punctures of the epithelium, with a response magnitude
that depends on the degree of tissue deformation. They al-
so respond to temperatures in the range of 40–60°C, and
change their response rates as a linear function of warm-
ing (in contrast with the saturating responses displayed by
non-noxious thermoreceptors at high temperatures).
Pain signals can be separated into individual compo-
nents, corresponding to different types of nerve fibers
used for transmitting these signals. The rapidly transmit-
ted signal, which often has high spatial resolution, is
called first pain or cutaneous pricking pain. It is well local-
ized and easily tolerated. The much slower, highly affec-
tive component is called second pain or burning pain; it is
poorly localized and poorly tolerated. The third or deep
pain, arising from viscera, musculature and joints, is also
poorly localized, can be chronic and is often associated
with referred pain.
Muscle Spindles
Scattered throughout virtually every striated muscle in the
body are long, thin, stretch receptors called muscle spin-
dles. They are quite simple in principle, consisting of a few
small muscle fibers with a capsule surrounding the middle
third of the fibers. These fibers are called intrafusal fibers,
in contrast to the ordinary extrafusal fibers. The ends of the
intrafusal fibers are attached to extrafusal fibers, so when-
ever the muscle is stretched, the intrafusal fibers are also
Anatomy of the Somatosensory System
3
Force
control
signal
Driving
signal
Length
control
signal
Load
External
forces
Tendon
organs
Muscle force
Muscle
length
Force feedback
Length &
velocity
feedback
Force (Golgi tendon organ)
Spindles
Gamma bias
Length (secondary muscle-spindel afferents)
Length error (primary muscle-spindel afferents)
Velocity (primary muscle-spindel afferents)
Muscle
Inter-
neurons
Figure 3: Feedback loops for proprioceptive signals for the perception and control of limb move-
ments. Arrows indicate excitatory connections; filled circles inhibitory connections.
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stretched. The central region of each intrafusal fiber has
few myofilaments and is non-contractile, but it does have
one or more sensory endings applied to it. When the mus-
cle is stretched, the central part of the intrafusal fiber is
stretched and each sensory ending fires impulses.
Muscle spindles also receive a motor innervation. The
large motor neurons that supply extrafusal muscle fibers
are called alpha motor neurons, while the smaller ones sup-
plying the contractile portions of intrafusal fibers are
called gamma neurons. Gamma motor neurons can regu-
late the sensitivity of the muscle spindle so that this sensi-
tivity can be maintained at any given muscle length.
Joint receptors
The joint receptors are low-threshold mechanoreceptors
and have been divided into four groups. They signal differ-
ent characteristics of joint function (position, movements,
direction and speed of movements). The free receptors or
type 4 joint receptors are nociceptors.
From Wikibooks
4