Somatosensory (back
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Readings: Shepherd Ch 12
The somatic senses are those that are relayed to the CNS through receptors
located in the body covering (integument). Since the function of
the integument of different animals is diverse we will limit our initial
discussion to the somatosensory system of vertebrates particularly mammals.
pg 268-9 Covering or integument in mammals is colloquially referred
to as skin. Two types of skin can be found in mammals: glabourous
(hairless) and hairy. Both are divided into two
main layers: epidermis (outer) -derived from ectodermal germ layer of embryo
and the dermis -derived from the mesoderm.
The epidermis gives rise to hair, feathers, claws
and glands. It is composed of a deep layer called the Stratum
Germinativum and a superficial layer called the Stratum Corneum.
Cells in the Stratum Germinativum continually undergo mitosis and migrate
toward the where they form the flattened cells of the Stratum Corneum.
Cells in the Stratum Corneum contain a high concentration f a stable fibrous
protein (keratin) and melanocytes which contain melanin.
The dermis is a soft flexible structure that can
also be divided into two layer structure. The more superficial layer,
papillary layer contains collagen and elastic fibers
while the deeper reticular layer contains fat cells.
Discussion will proceed by focusing on the different
types of receptors cells (cutaneous receptors) found in the integument
of humans. There are those found in both Glabrous and Hairy skin: Free
nerve endings, Pacinian Corpuscles, Merkel's discs, those found only in
Glabrous tissue: Meisnner Corpuscles and those found only in Hairy tissue:
Hair follicles and Ruffini endings.
We will discuss each of these with respect to 1)
location and structure of end organ, 2) activating stimuli and size of
receptive field, 3) transduction mechanism, 4) type of nerve that the signal
is conveyed through and 5) the type of signal that is generated in
the nerve when the receptor is activated.
Activating stimuli can be chmical, mechanical, temperature. Receptive
fields describe the area in which a particular stimulus will still be able
to activate a particular sensory receptor. Types of nerves are defined
by their diameter and/or conduction velocity (sensory nerves are classified
as Aalpha, A-beta, A-delta and C fibers- other classifications are used
for similar fibers that are found in other modalities or systems eg. motor
system). Type of signal in response to a constant stimulus can be
sustained or adapting and within the classification of adapting can be
either fast or slow.
I. Receptors
A. Free nerve endings- this is the most
difficult receptor type to discuss because, with respect to what stimuli
activate this receptor, it is the most non-specific of the somatosensory
receptors . However, since it is probably the oldest
evolutionarily we will begin here.
-
Structure/Location - They are found in both glaborous and
hairy skin and branch in the dermis or deeper layers of the epidermis.
These receptors are simply unmyelinated terminals of A-delta, and
C fibers.They are also found in muscles and in the gut. Fig
12.1 pg 269
-
Activation - this receptor is activated by mechanical
(light touch, tickle) and noxious stimuli as well as heating
and cooling. Some endings respond to only one modality (unimodal)
while others respond to 2 or 3 (polymodal). Those that respond to painful
or noxious stimuli are called nociceptors.Receptive fields are variable.
-
Transduction Mechanism - (pg 271 col 2)
-
Nociceptors will respond to chemical mediators that result from the
inflammatory response to injury ( histamine, serotonin, bradykinin).
Activation of nerve endings from A-delta fibers by these chemicals results
in the immediate sharp pain (pricking pain) while activation of the C-fibers
produces a sustained sensation of pain (burning pain or itching).
-
Temperature - cooling relative to normal body temperature is sensed by
free nerve endings from a subpopulation of A- delta fibers while warming
is sensed by free endings from a subpopulation of C-fibers. Individual
receptors will be activated along a broad range of temperatures and therefore
individual cells will have a broad sensitivity. this enhances the ability
to discriminate small changes in temperature near body temperature.
-
Light touch - stretch receptors that may be similar to the receptors found
in hair cells of the auditory system.
-
Activation/Encoding - all free nerve endings are slowly adapting.
day2
B. Pacinian Corpuscle FAI
-
Structure/location - They are found in both Glabrous and Hairy
skin and are situated deep in the dermis. The end organ itself seems to
be composed of a free nerve terminal of A-deltanad A-beta fibers
surrounded by an incomplete shell of cell processes and collagen fibers
enclosed in an outer complete lamellae. These receptors are
also located in other structures such as connective tissue of muscles,
the periosteum of bones and the mesentery of the abdomen.
-
Activation - this receptor is activated by mechanical stimuli
(Pressure, Vibration - 250-300 Hz). Displacement of 10um can produce a
maximal response. Large receptive fields
-
Transduction Mechanism - This is thought to occur in response to
activation of a 'stretch-activated channel in the sensory membrane.
-
Activation/Encoding - rapidly adapting based on studies on receptor
potential. activation of free ending by a stimulus produced a slowly,
not a fast adapting response.
C. Merkel's Discs (Merkel's nerve complex) SA
I
-
Structure/Location - They are found in both Glabrous and Hairy
skin and are situated on the border of the dermis and epidermis.
End organ is a free nerve ending associated with other cells.
-
Activation - this receptor is activated by light mechanical
stimuli (touch pressure ). Receptive field is small
-
Transduction Mechanism - Most likely mechanoreceptor/stretch receptor.
-
Activation/Encoding - Slowly adapting. Slight difference in that
Merkel's disc shows an overshoot during the initial phasic part of the
indentation (thus can provide info about changes in stimulus intensity
as well as steady state values - pg 274 col 2 - a bit confused here!).
Merkel's discs have small receptive fields and Ruffini endings have large
receptive fields.
D. Meissner Corpuscle FA II
-
Structure/location - Found only in Hairy skin. The end organ
itself is found at the junction between the dermis and epidermis.
-
Activation - this receptor is activated by mechanical
stimuli such as light touch, and vibration (30-40 Hz). High threshold of
activation. Small receptive fields.
-
Transduction Mechanism - Most likely mechano/stretch receptors.
-
Activation/Encoding - rapidly adapting (less so than Pacinian Corpuscles)
based on Ake Vallbo's technique called neurography. The fact that
it does not adapt as rapidly does not allow it to be sensitive to very
high frequency stimulation (contrast with Pacinian Corpuscles).
Summarize using Fig 12.6-
E. Ruffini endings SA II
-
Structure/Location - Found in hairy skin. End organ is situated
in the reticular layer of the dermis.
-
Activation - this receptor is activated by light mechanical
stimuli (touch pressure). Large receptive fields.
-
Transduction Mechanism - mechano/stretch receptors.
-
Activation/Encoding - Slowly adapting.
F. Hair follicle endings
-
Structure/Location - Found only in hairy skin. Free nerve
endings are wrapped around hair follicle in the papillary layer of the
dermis.
-
Activation - this receptor is activated by light mechanical
stimuli (30-40 Hz vibration).
-
Transduction Mechanism - Most likely mechano/strech receptors.
-
Activation/Encoding - fast adapting
* Takes the functional place of Meisnner Corpuscles found in Glabrous
skin.
G. Special muscle receptors: these are
included here because they contribute to both the reflex pathway and the
ascending somatosensory pathways.
-Somatosensory receptors found in muscle/tendons
Fig 14.3 (Delcomyn). These include Ruffini endings, Pacinian Corpuscles
and free nerve endings that are activated by noxious stimuli.
-Golgi tendon organs
transmits information about tension. Knee reflex.
-Muscle spindles
intrafusal muscle fibers contain bag and chain fibers. Ia fibers
innervate both bag and chain and convey info about changes in tension;
while type II fibers transmit information from only chain cells and convey
information about tension. Bag fibers receive a motor signal through
gamma motoneurons. This signal allows the the bag fibers to me maximally
active over a wide range of tension.
day 3
II. Pathways
note: You should be able to keep track of where
information enters i.e level of cord, where it travels within fiber pathways
and where it crosses (decussates) and synapses.
All sensory receptors from the body have cell bodies in Dorsal root
ganglian (DRG). DRG fibers enter the cord through the Dorsal Entry
Zone (Tract of Lissauer).
-
reflex pathway - (Ch 19) - spinal cord - muscles
-
Ascending pathway
a) Spinothalmic pathway aka anteriolateral system. (mostly conveys
pain information). Spinal cord (superficial layers I-II)- contralateral
white matter of cord (lateral spinothalamic tract)- thalamus ()- sensory
cortex. Conveys mostly pain and temperature information but
also contain some tactile and joint information. Give off collaterals
in brainstem that activate reticular formation and contribute to arousal
and consciousness.
paleospinal cerebellar - synapses in reticular nuclei.
neospinal cerebellar - synapses in thalamus-neocortex.
b) Lemniscal Pathway (mostly conveys information about touch). Spinal
cord - ipsilateral white matter to Nucleus Cuneatus and Gracile (dorsal
columns)- contralateral white matter (medial lemniscus) - thalamus (ventral
posterior lateral -VPL- nucleus of the thalamus) - somatosensory cortex.
Conveys mostly complex information about touch and pressure. Give off collaterals
in brainstem that activate reticular formation and contribute to arousal
and consciousness.
c) Spinocerebellar (mostly conveys proprioceptive information. Spinal
cord - ipsilateral and contralateral white matter to cerebellum.
day 4
Review of idea that topography or somatotopy is maintained through
most levels of the nervous system.
-
Spinal Cord -
-
Thalamus - somatosensory information from the body terminates in a nucleus
called the ventral posteriolateral nucleus of the thalamus (VPL).
Information from the face terminates in an adjacent region referred to
VPM in subprimates these regions are combined and called the ventrobasal
complex (VBC).
-
Somatosensory cortex.
*Facial information comes through cranial nerves and travels together
with the sensations from the body in various nuclei. Touch information
arrives in the VPM of the thalamus and travels to the cortex with touch
information from the body that has synapsed in the VPL. Pain/Temperature
takes a similar path.
III. Perception
Figure 12.6 Receptive Fields vs Adaptation.
(pg 276 col 1 )
Feeling of diffuse pain in the abdomen. Mediated by free nerve
endings in the gut. Activation of these nerves does not produce a
topographical representation of the gut at various levels of the anterolateral
pathway.
Hot and cold