M

1. The Special Senses
Touch
Dr. Binu Babu
Asso. Professor
Mrs. Jincy Ealias
Assi. Professor

2. Introduction
 Touch is a perception causing from the activation of
neural receptors in the skin, hair follicles, tongue, throat,
and mucosa. A variety of pressure receptors respond to
variations in pressure.
 Tactile anesthesia: is the loss or impairment of the ability
to feel touch
 Paresthesia: it is a sensation of tingling, pricking, or
numbness of the skin that may result from nerve damage
and may be permanent or temporary.

3.  Touch: The skin-based receptor system. The entire
surface of the body on which there is living tissue (skin) is
a potential receptive surface for the touch system.
 The most active and sensitive part of this receptive field
are the hands.
 Haptics : active exploratory touch strategies for acquiring
information from an object.
 Haptics includes not only touch information but also
kinesthesis

4. Types of sensory nerves

5. Classifications of touch
sensations
 Pain or nociception: Signals nerve and other tissue
damage.
 Balance or equilibrioception: Allows the sensing of
body movement, direction, and acceleration, and to
attain and maintain postural equilibrium and balance.
 Body awareness or proprioception: Provides the
parietal cortex of the brain with information on the
relative positions of the parts of the body.

6.  Temperature sensing or thermoception:
The sensation of heat and the absence of
heat.
 Interoceptive senses: if also considered,
sensation can be expanded to include
stretch oxygen and carbon dioxide
sensing, pH sensing, and etc.

7. Measuring touch sensitivity
 Von Frey hairs: small hairs, like those found on a paint
brush, of various diameter are pressed against different
parts of the body to see if it is felt. The thicker the
diameter hair needed to get a response, the less
sensitive the area.
 Different parts of the body vary in sensitivity to touch,
lips and hands are highly sensitive, back and buttocks,
much less so. Furthermore, for any area of body,
females tend to be more sensitive than males.

8. Measuring touch sensitivity
 Two point thresholds: a little different
procedure, but same general results. A compass-
type instrument is used which has two
adjustable points, points can be set a different
distances from each other. Sensitivity is
measured by determining how far apart points
must be sent before sub. can detect that there are
two points stimulating skin, not one.

9. Physiology of touch
 Skin receptors: at various depths under the skin are the
mechanoreceptors, which start the process of analyzing skin
sensations by responding to indentation or pressure on the
skin.
 In order of depth, nearest to surface to deepest:
 Meisnner Corpuscles: give strongest response to transient
stimulation such as a finger rubbing over a surface. RA-P
 Merkel Disks: give strongest response to steady pressure by
small object. SA-P
 Ruffini Endings: give greatest response to fairly strong, steady
pressure. Are also quite sensitive to movements which result in
stretching of skin. SA-D.
 Pacinian Corpuscles: respond best to initiation and termination
of diffused pressure against skin. RA-D

11. Physiology of touch
 Nerve Fibers: afferent fibers travelling from skin receptors to spin and
(for some) eventually brain. These fibers are of four distinct categories.
 Slow Adapting: fibers which carry messages about steady pressure on
skin. Not surprisingly these fibers are connected to Mekel disks and
Ruffini Endings in skin.
 Rapid Adapting: carry message about transient pressure changes on
skin. connected to Miessner and Pacinian Corpuscles.
 Punctate fibers: ones with distinct receptive fields (connect to
Miessner & Merkel)
 Diffuse fibers: ones with less disctict receptive fields. (connect to
Ruffini & Pacinian).
 Combination of these four types produce four types of nerve fibers.

12. Touch pathways
Touch pathway runs up dorsal
(back) of spinal column. Some
connect with interneurons and
motor neurons and mediate
reflexive arcs. 2 main pathways:
Lemniscal (red; newer) more
sophisticated aspects of touch.
Spinothalamic (older; blue) pain and
temperature.

13. Somatosensory cortex
The somatosensory cortex
is a region of the brain which
is responsible for receiving
and processing sensory
information from across the
body, such as touch,
temperature, and pain.

14. Pain perception
 Pain serves an important
adaptive function – it alerts the
organism to potential tissue
damage and compels withdraw
of affected area from pain
source.
 Chronic pain, however, often
makes life miserable for those
afflicted.
 Nociceptors: free nerve endings
that signal pain. Two locations:
skin surface – temperature;
Subcutaneous fat layer:
punctures.

15. Nerve fibers

16. Pain fibers
 A delta: myelinated; fast responding; sharp,
acute; thermal pain
 C-type: slow responding; building pain;
mechanical; thermal; chemical

17. Gate control theory of pain
 T cells send pain message. When
only fast (A-beta; A-alpha) fibers
active; no pain. Inhibitory message
send from SG to T cells. When both
fast and slow (C-fibers) respond,
SG cells are inhibited from sending
their inhibitory message to T cells,
T cells fire and pain message is
sent.
 Note also: T’s can be inhibited by
top-down messages from cortex.
“Meaning” of pain relevant.

18. Pain pathway
The stages of pain pathway
 Transduction
 Conduction
 Transmission
 Modulation
 Perception

19. TRANSDUCTION
 Transduction begins when peripheral terminals
of nociceptive (sensory receptors of pain)
 C fibers
 A-delta
 (Aδ) fibers
 Nociceptive fibers are depolarized by noxious
mechanical, thermal, or chemical energy. The
membranes of these terminals contain proteins
and voltage-gated ion channels that convert
thermal, mechanical, or chemical energy into an
action potential (AP).
 Nociceptor terminals are spread densely
throughout the skin. They are found less on
periosteum, joints, tendons, muscles, and least on
the surface of organs.

20. CONDUCTION
 Conduction of an Action Potential is
the second phase of nociception.
 An AP generated in nociceptor
terminals is conducted across the
peripheral process to the central
process were it depolarizes the
presynaptic terminal. The
presynaptic terminal interfaces with
a network of interneurons and
second order neurons in the dorsal
horn. Interneurons can facilitate or
inhibit transmission to second order
neurons

21.  These nociceptors projects from the
trigeminal ganglion and enter the CNS at the
level of the pons. Trigeminal Að and C fibers
pass down through the pons into the medulla
where they synapse on second order neurons
which then rise to decussate within the pons
and pass to the thalamus.

22. MODULATION
 Modulation of nociceptive
transmission is an adaptive
process involving both
excitory and inhibitory
mechanisms. The
responses of second order
neurons can be suppressed
or facilitated dependent on
importance of the event.

23. PERCEPTION
 Perception of nociceptive pain is dependant upon neural
processing in the spinal cord and several brain regions. Pain
becomes more than a pattern of nociceptive action potentials
when they reach the brain.
 Action potentials ascending the spinothalamic tract are decoded
by the thalamus, sensorimotor cortex, insular cortex and the
anterior cingulate to be perceived as an unpleasant sensation
that can be localized to a specific region of the body.
 Action potentials ascending the spinobulbar tract are decoded
by the amygdala and hypothalamus to generate a sense of
urgency and intensity. It is the intergration of sensations,
emotions and cognition that result in our perception of pain.

24. References
 Acree, T. E., & Beidler, L. M. (1971). Taste,. Berlin, New York,
Springer-Verlag.
 Per Brodal. (2010). The central nervous system : structure
and function. Oxford University Press, Cop.
 Nieuwenhuys, R., J Voogd, Voogd, J., & Christiaan Van
Huijzen. (2008). The Human Central Nervous System.
Springer.