PSYC2274 Chapter Notes - Chapter 13: Lamellar Corpuscle, Dorsal Root Ganglion, Bulbous Corpuscle
1
What exactly is touch?
- Touch refers to the sensations caused by mechanical displacements of the skin, which
occurs any time you make contact with an object.
- Includes perception of temp changes, sensation of pain, itchiness, and internal
sensations arising from muscles/tendons/joints that inform us of the positions and
movements of our limbs in space (remember kinesthesis from before).
- Important → can be used to identify/manipulate objects that can't be
seen/heard.
- The adjective tactile refers to these mechanical interactions taking place.
- Proprioception incorporates the sensory inputs received from locations internal to the
body and is mediated by kinesthetic and internal receptors.
- “Body sense” tells us where our limbs are in space.
- This is mediated by muscle spindles, which respond to uncommanded muscle
lengthening. They tell us the position of limbs relative to expectation based on
motor output.
- Somatosensation further encompasses the input received from touch receptors in the
skin and the proprioceptive system. Collectively, it receives signals from the skin,
muscles, tendons, joints, and internal receptors.
- What is the purpose of pain? It serves as a warning system!
- Our temperature sensations enable us to seek/create a thermally safe environment.
- Our mechanical sensations play an important role in our intimate sexual/reproductive
activities and provide powerful means of communicating our thoughts and emotions
nonverbally.
Now for a little touch physiology…
- You can find touch receptors all over your body in both hairless and hairy skin, as well as
our mouths, muscles, tendons and joints.
- Tactile receptors are located in both the outer layer (epidermis) and underlying layer
(dermis).
- The epidermis is the outermost layer, and also the thinnest.
- The dermis is the middle layer, and it contains nerve fibers.
- The hypodermis anchors the skin to muscles and other tissues.
- There are also many different types of tactile receptors/mechanoreceptors, and each can
be characterized by 3 attributes:
- Type of skin movement that causes a response (pressure, vibration, temp
change, etc.) → this is determined by the casing around the nerve ending.
- Size of receptive field (the extent of the body area that elicits a response)
- Rate of adaptation (FA vs. SA)
- Mechanoreceptors consist of an axon and a myelin sheath.
- All are considered A-beta fibers and have wide diameters that allow for fast
neural conduction.
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2
- There are 4 types of endings for the nerve fibers, all of which can be independently
classified according to their adaptation rates and sizes of their RFs:
- Pacinian corpuscle responds to high-frequency vibration (50 to 700 Hz). Good for
detecting initial contact with an object. Combined with fast adaptation. Associated with
FA I fibers that have large RFs.
- Meissner’s corpuscle responds to low frequency vibration (5 to 50 Hz). Fast-adapting,
so good for detecting dynamic stimuli - motion of object across the skin. Associated with
FA I fibers that have small RFs.
- Merkel’s discs (cell neurite complexes) responds to steady downward pressure. Slow
adapting so good for careful inspection of surfaces. Associated with SA I fibers that have
small RFs.
- Ruffini endings stretch and are slow adapting, so good for detecting grasping.
Associated with SA II fibers that have large RFs.
- Endings of Meissner and Merkel = located at the junction of the epidermis and dermis.
- Pacinian and Ruffini receptors are embedded more deeply in the dermis and underlying
subcutaneous tissue.
- The cell bodies of mechanoreceptors are in the dorsal root ganglia, outside the spinal
cord.
- 2 dimensions lead to a second set of labels for the mechanoreceptor types:
- SA I: respond best to steady downward pressure, fine spatial details, and low
frequency vibrations of less than about 5 Hz. These are important for texture and
pattern perception. When a single SA I is stimulated, people report feeling
“pressure.”
- SA II: respond to sustained downward pressure, particularly to lateral skin stretch.
For stimulation to be detected, more than one must be stimulated.
- FA I: respond best to low-freq vibrations from about 5-50 Hz.When a single fiber
is stimulated, people report a localized sensation of “wobble” or “flutter.”
- FA II: respond best to high-frequency vibrations from about 50-700 Hz. When
one is stimulated, people report a “buzz” feeling.
- The 4 types always work together.
How about the thermoreceptors?
- These are located in both the epidermal and the dermal layers of the skin and inform us
about changes in skin temperature. There are 2 distinct populations:
- Warmth fibers fire when skin temperature increases, and cold fibers fire when
skin temperature decreases. Cold outnumber warmth by about 30:1.
- Somatosensory neurons vary systematically in conduction velocity. 2 factors are axon
diameter and myelination.
And the nociceptors?
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Document Summary
Touch refers to the sensations caused by mechanical displacements of the skin, which occurs any time you make contact with an object. Includes perception of temp changes, sensation of pain, itchiness, and internal sensations arising from muscles/tendons/joints that inform us of the positions and movements of our limbs in space (remember kinesthesis from before). Important can be used to identify/manipulate objects that can"t be seen/heard. The adjective tactile refers to these mechanical interactions taking place. Proprioception incorporates the sensory inputs received from locations internal to the body and is mediated by kinesthetic and internal receptors. Body sense tells us where our limbs are in space. This is mediated by muscle spindles, which respond to uncommanded muscle lengthening. They tell us the position of limbs relative to expectation based on motor output. Somatosensation further encompasses the input received from touch receptors in the skin and the proprioceptive system. Collectively, it receives signals from the skin, muscles, tendons, joints, and internal receptors.
