Physiology Ch 47 p571-582
Somatic Sensations: Tactile and Positional Senses
Classification of Somatic Sensations - touch, pressure, vibration, tickle, and static position/rate
of movement
-somatic sensations can be grouped into classes, such as:
1. Exteroreceptive Sensations – arising from surface of body
2. Proprioceptive Sensations – relating to physical state: position, tendon, pressure, equilibrium
3. Visceral Sensation – those of viscera and internal organs
4. Deep Sensations – those that come from deep tissues, fasciae, muscles, and bone
Interrelations Among the Tactile Sensations of Touch, Pressure, and Vibration – these
sensations are classified as separate, but they are detected by same types of receptors with 3
principal differences
1. touch results from tactile stimulation of skin
2. pressure results from deformation of deeper tissues
3. vibration results from rapid repetitive sensory signals, but same receptors as above
Tactile Receptors – There are 6 different types of sensory receptors
1. Free Nerve Endings – found everywhere in skin and other tissues; detect touch and pressure
2. Meissner’s Corpuscle – touch receptor with great sensitivity; made of a large, encapsulated,
myelinated nerve fiber; present in non-hairy parts of skin such as fingertips, they are used
for light touch; FAST ADAPTING, also sense low-frequency vibration
3. Merkel Disc – a type of expanded tip tactile receptor found with Meissner’s Corpuscles in
fingertips and also hair parts of skin. Transmits strong/partially adapting signal and then a
continuing weaker signal that adapts slowly; responsible for detecting continual touch
a. Merkel discs often grouped together in a receptor organ called Iggo dome receptor
that projects up against epithelium of skin, protruding out
b. Entire group of Merkel’s discs is innervated by a single large myelinated nerve
c. Merkel’s discs localize touch sensations to specific body area and determine texture
4. Hair End-Organ – slight movement of any hair on body stimulates hair end-organ; a fast
adapting receptor detecting movement on surface of body and initial contact with body
5. Ruffini Endings – deeper in tissues, branched, and encapsulated receptors signaling
continuous states of deformation in issues such as prolonged touch/pressure. Also found in
joint capsules to help signal degree of joint rotation
6. Pacinian Corpucles – stimulated only by rapid local compression of tissues and are super
fast adapting; sense tissue vibration and other rapid changes
Transmission of Tactile Signals in Peripheral Nerve Fibers – all specialized sensory receptors
transmit signals in type Aβ fibers that transmit at 30-70m/s
-free nerve endings transmit through type Aγ fibers at velocities of 5-30m/s
-some tactile free nerve ending transmit through type C unmyelinated fibers at 2m/s,
signaling to spinal cord and lower brain stem the sensation of tickle
-those receptors transmitting precise location on skin are faster, while crude signals such as
pressure, poorly localized touch, and tickle are transmitted by slower fibers
Detection of Vibration – All tactile receptors detect vibration, but different receptors detect
different frequencies of vibration
-Pacinian corpuscles detect vibrations from 30-800Hz because they respond to rapid changes
and transmit over type Aβ fibers (can transmit up to 1000 impulses/second)
-low frequency vibrations from 2-80Hz stimulate Meissner’s corpuscles; less rapid adapting
Detection of Tickle and Itch by Free Nerve Endings – sensitive/rapid adapting
mechanoreceptive free nerve endings eliciting tickle and itch; found in superficial layers of skin
and transmitted by very small type C, unmyelinated fibers similar to those transmitting pain
-purpose of itch is to call attention to mild surface stimuli like flea crawling on skin or fly on skin,
to stimulate a scratch reflex to rid the host of the irritant
-scratch can induce pain, which suppresses the itch signals
Sensory Pathways for Transmitting Somatic Signals into CNS – all sensory information from
somatic segments of body enter the spinal cord in the dorsal horn, but travel through the brain
in two alternative sensory pathways:
1. Dorsal Column-Medial Lemniscal System – carries signals up to medulla of brain in
dorsal columns of the cord. Synapses arrive at medulla, cross to the opposite side, and
continue up to brain stem and thalamus through the medial lemniscus
a. Composed of large, myelinated fibers transmitting at 30-110m/s
b. Has high degree of spatial orientation of nerve fibers with respect to origin
c. Transmits touch with high localization, vibrations, movement against skin,
sensations from joints, pressure sensations related to degrees of judgement of
pressure intensity (fine senses)
2. Anterolateral system – signals entering the dorsal horns from periphery synapse IN the
dorsal horns, THEN cross to the other side of dorsal horn and ascend through anterior
and lateral white columns of cord; terminate at levels of lower brainstem and thalamus
a. Composed of smaller, myelinated fibers transmitting up to 40m/s
b. Low degree of spatial orientation of nerve fibers with respect to origin
c. Can transmit a broad spectrum of sensory modalities: pain, warmth, cold, crude
tactile, tickle/itch, sexual sensations (all crude senses)
Anatomy of Dorsal Column-Medial Lemniscal System - on entering spinal dorsal roots, large
myelinated fibers from specialized mechanoreceptors divide to form medial branch and lateral
branch
-Medial branch turns medially first and then upward in dorsal column  brain
-Lateral branch enters dorsal horn and divides many times to synapse in the
intermediate/anterior portions of cord gray matter for 3 functions: give off fibers
entering dorsal column to travel upward to brain; many fibers are small and elicit local
reflexes; and others give rise to spinocerebellar tracts
-fibers in dorsal columns pass uninterrupted to dorsal medulla to synapse in the dorsal column
nuclei (cuneate and gracile nuclei)
-from here, second-order neurons decussate immediately to opposite side of brain stem
and continue up through medial lemnisci to thalamus, where fibers are joined by
sensory nuclei of the trigeminal nerve
-in thalamus, medial lemniscal fibers terminate in the thalamic sensory relay area called
ventrobasal complex
-from here. Third-order neurons project mainly to postcentral gyrus of cortex,
called the somatic sensory area I, but also to somatic sensory area II
Spatial Orientation of Nerve Fibers in Dorsal Column – Medial Lemniscal System – there is
distinct spatial orientation of nerve fibers from individual parts of body that is maintained
throughout; in dorsal columns of cord, fibers from lower parts of body lie toward the center of
cord whereas those that enter the cord at progressively higher segmental levels form successive
layers laterally
-in thalamus, distinct spatial orientation is maintained, with lower body represented by most
lateral portions of ventrobasal complex and head/face represented by medial areas of complex
-because of crossing in medial lemnisci – the left side of body is represented in right side
of thalamus, and right side of body in the left side of thalamus
Somatosensory Cortex – map of human cortex is divided into 50 areas called Brodmann’s areas
referred to by number to denote functional areas
-sensory signals generally terminate in cerebral cortex just posterior to central fissure, and
generally, anterior half of parietal lobe receives and interprets somatosensory signals
-visual signals terminate in occipital lobe and auditory signals terminate in temporal lobe
-anterior to central fissure you have the motor cortex for muscle contractions and movements
Somatosensory Areas I and II – these are two areas in anterior parietal lobe that have distinct
and separate spatial orientation of different parts of body; somatosensory area I is more
important than somatosensory area II (area I is called somatosensory cortex)
-area I has high degree of localization from every body area, whereas area II is poor
-area II has face represented anteriorly, arms centrally, and legs posteriorly
Spatial Orientation of Signals from Different Body Parts in Somatosensory Area I – area I is
behind central fissure in postcentral gyrus of cerebral cortex; each lateral side of cortex receives
sensory information from opposite side of body
-lips are represented by the largest area of somatic cortex followed by face and thumb; whereas
trunk and lower extremity are represented by the smallest areas.
-size of the areas proportional to number of specialized sensory receptors in body area
Layers of Somatosensory Cortex and Their Function – cerebral cortex has six layers of neurons
beginning with layer I on surface and extending deeper to layer VI. Functions of these layers:
1. incoming sensory signal excited layer IV first, then spreads both superficially and deeply
2. Layers I and II receive diffuse, nonspecific input from lower brain centers to control overall
level of excitability of respective regions stimulated
3. Layers II and III send axons to opposite sides of brain through corpus callosum
4. Layers V and VI send axons to deeper CNS; layer V is larger and projects more distally such
as to basal ganglia, brain stem, and spinal cord
a. Layer VI – multiple fibers extend to thalamus to control excitatory levels of incoming
sensory signals entering thalamus
Sensory Cortex is Organized in Vertical Columns of Neurons; Each Column Detects a Different
Sensory Spot on Body with Specific Sensory Modality – neurons of somatosensory cortex are
arranged in vertical columns extending through 6 layers of cortex with 10,000 cell bodies each
-each column represents single specific sensory modality; some for stretch around joints, some
to stimulation of tactile hairs, others are pressure points, etc…
-at layer IV – where signal first enters, columns of neurons function separately from one
another. At other levels, interactions between neurons occur to initiate analysis of signal
-most anteriorly of central gyrus, deep in central fissure, vertical columns respond to muscle,
tendon, and joint stretch receptors (Brodmann’s area 3a)
-these signals spread anteriorly to motor cortex immediately forward of central fissure
to control effluent motor signals to activate muscle contraction
-as you move posteriorly in somatosensory area I, more vertical columns respond to adapting
cutaneous receptors and further back, columns are sensitive to deep pressure
-in the most posterior portion of somatosensory area I, 6% of vertical columns respond only
when stimulus moves across skin in particular direction  then signal moves posteriorly
Functions of Somatosensory Area I – widespread loss of somatosensory area I causes these:
1. people unable to localize different sensation from different parts of body, but they can
crudely associate sensations, such as which hand, major level of body trunk, or one of legs
2. person is unable to judge critical degrees of pressure against body
3. person is unable to judge weights of objects
4. person is unable to judge shapes or forms of objects (astereognosis)
5. person is unable to judge texture of materials because this judgment depends on critical
sensations caused by movement of fingers of surface to be judges
6. Pain and Temperature STILL PRESERVED with loss of somatosensory I, but poorly localized
Somatosensory Association Areas – Brodmann’s areas 5 and 7 of cerebral cortex are in parietal
lobe behind somatosensory area I plays a role in deciphering deeper meanings of sensory
information; called somatosensory association areas
-stimulation here can cause an awake person to experience a complex body sensation, such as
feeling an object like a knife or a ball
-this area combines information from multiple points to decipher meaning; receives input from
somatosensory area I, ventrobasal nuclei of thalamus, visual cortex, and auditory cortex
Effect of Removing Somatosensory Association Area – Amorphosynthesis – person loses ability
to recognize complex objects and complex forms felt on opposite side of body; person forgets
that other side of body exists, only focuses on one side of object when feeling it
Overall Characteristics of Signal Transmission from Dorsal Column/Medial Lemniscal System –
divergence occurs at each synaptic stage; neurons in the central part of cortical field discharge
to the greatest extent
-weak stimulus causes only centralmost neurons to fire, whereas stronger stimulus causes more
neurons to fire, but those at center fire the most rapidly
Two-point Discrimination – two needles pressed lightly against skin at the same time, person
says whether he feels one point or two; close on tips of fingers, really far on the back due to
different numbers of receptors in those areas
-stimulation causes cortical excitation in two ways, one with surround inhibition and one
without
Effect of Lateral Inhibition (Surround Inhibition) to Increase Contrast in Perceived Spatial
Pattern – every sensory pathway also gives rise to lateral INHIBITORY signals which spread o
sides of excitatory signals to inhibit adjacent neurons
-lateral inhibition blocks lateral spread of excitatory signals and increases degree of contrast in
sensory pattern perceived in cerebral cortex
-in dorsal column system, lateral inhibition takes place at every synaptic level: dorsal column
nuclei in medulla, ventrobasal nuclei in thalamus, and cortex itself
Transmission of Rapidly Changing and Repetitive Sensations – dorsal column system can
recognize changing stimuli that occur in as little as 1/400 of a second
Vibratory Sensation – vibratory sensations can be detected up to 700 Hz; higher frequency
come from pacinian corpuscles and lower frequency come from Meissner’s corpuscles
-signals transmitted ONLY in dorsal column pathway
Interpretation of Sensory Stimulus Information – to achieve a great degree of range in stimulus
intensity for perception; at low stimulus, slight changes in intensity of pacinian corpuscle
increase the potential markedly – high levels of stimulus increases potential slowly; this way it
allows pacinican corpuscle to measure minute changes
-when sound stimulates specific point on basilar membrane, weak sound stimulates only those
hair cells at the point of maximum hair vibration, but as sound increases, many more hair cells in
each direction also become stimulated to carry information over many more fibers
Importance of Large Intensity Range of Sensory Reception – if not for this high range, our
sensory systems would probably be operating in the wrong range
Judgement of Stimulus Intensity – Weber-Fechner Principle: Detection of Ratio of Stimulus
Strength – gradations of stimulus strength are discriminated approximately in proportion to
logarithm of stimulus strength
-person holding 30g wright can barely detect 1g increase in weight; person holding 300g cannot
detect a 10g increase in weight; so the ratio of change in stimulus strength for detection is
constant, about 1-30 in this case
interpreted signal strength = log (stimulus) + constant
-only accurate for higher intensities of visual, auditory or cutaneous experiences
Power Law – another attempt states:
Interpreted signal strength = K * (stimulus – k)^y
K,k, y are constants
Position Senses – frequently called proprioceptive senses and can be divided into static
position (conscious perceptorion of orientation of body parts), and rate of movement sense
(also called kinesthesia or dynamic proprioception)
Position Sensory Receptors – skin tactile receptors and deep receptors near joints are used for
position sensation; in fingers, half of position recognition is thought to be through skin
receptors; for larger joints of body, deep receptors are more important
-for determining joint angulation in midranges of motion, muscle spindles are most important;
when angle of joint is changing, some muscles are stretched while others are loosened, and
these signals travel by way of muscle spindles through dorsal column system
-at extremes of joint angulation, stretch of ligaments and deep tissues is also a factor; helped by
pacinian corpuscles, ruffini endings, and golgi tendon organs
-pacinian corpuscles and muscle spindles are adapted to detect rapid rates of change
Processing of Position Sense in Dorsal Column-Medial Lemniscal Pathway – thalamic neurons
responding to joint rotation are of 2 categories: (1) those maximally stimulated when joint is at
full rotation, and (2) those maximally stimulated when joint is at minimal rotation
Transmission of Less Critical Sensory Signals in Anterolateral Pathway – signals do not require
highly discrete localization or discrimination of fine gradations of intensity
Anatomy of Anterolateral Pathway – spinal cord anterolateral fibers originate in dorsal horn
laminae I, IV, V, and VI (laminae are where many dorsal root sensory nerves terminate)
-anterolateral fibers cross immediately in the anterior commissure of cord to the opposite
anterior and lateral white columns were they turn upward toward brain by way of anterior
spinothalamic and lateral spinothalamic tracts
-upper terminus of the two tracts is (1) through reticular nuclei of brain stem and (2) in two
different nuclear complexes of thalamus – the ventrobasal and intralaminar nuclei
-in general, tactile signals are transported to ventrobasal complex terminating in same thalamic
nuclei as dorsal column tactile signals
-only small number of pain project directly to ventrobasal complex; instead, pain signals
terminate in reticular nuclei of brainstem and relayed to intralaminar nuclei of thalamus’
Characteristics of Transmission in Anterolateral Pathway – velocities of transmission are 1/3rd
to ½ of those in dorsal column pathway, ranging between 8-40m/s
-degree of spatial localization of signals is poor
-gradations of intensities is less accurate, up to 10-20 gradations of strength rather than 100 for
dorsal column system
-ability to transmit rapidly changing or repetitive signals is poort
-anterolateral system is crude type of transmission that carries pain, temperature, tickle, itch,
and sexual sensations
Function of Thalamus in Somatic Sensation – thalamus has slight ability to discriminate tactile
sensation, even though thalamus normally functions mainly to relay information to cortex
-loss of somatosensory cortex has little effect on one’s perception of pain and moderate effect
on perception of temperature; therefore lower brain areas like brainstem is associated with
discrimination of those senses
Cortical Control of Sensory Sensitivity – Corticofugal Signals – corticofugal signals are
transmitted in backward direction from cerebral cortex to lower sensory relay stations in
thalamus, medulla, and spinal cord to control intensity of sensitivity of sensory input
-almost entirely inhibitory to decrease intense transmission in the relay nuclei doing two things:
1. decreases lateral spread of sensory signals to adjacent neurons and increases sharpness in
signal pattern
2. keeps sensory system operating in a range of sensitivity that isn’t low and so that signals are
ineffectual or not so high that system is swamped
Dermatomes – each spinal nerve innervates segmental field of skin called dermatome