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SYSTEMS NEUROBIOLOGY COURSE
SOMATOSENSORY SYSTEM LECTURE – FEB. 21, 2002
Areti Tsiola ([email protected])
OVERVIEW OF NERVOUS SYSTEM:

CNS – brain, spinal cord

Autonomic – functionally and anatomically distinct and overlapping to different extents
Sympathetic
Parasympathetic
Enteric
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PNS – (cranial and spinal) nerves and corresponding ganglia
NS & communication with the outside world. Today’s topic: somatosensory system.
Skin = largest organ of the human body.
Modalities of somatic sensibility:
1. Discriminative touch (// size, shape, texture of objects, movement across skin)
2. Temperature sense
3. Nociception (// perception of noxious stimuli – pain or itch)
4. Proprioception (// static position and movement of body)
Each modality has (morphologically and functionally) distinct Rs (neurons) and
pathways.
SENSORY NEURONS FAITHFULLY ENCODE STIMULI (Fig 21-8)
For cranial structures  (sensory part of) trigeminal neurons (# V)
For rest of body  DRG neurons
DEFINE GANGLIA
DRAW CROSS SECTION OF SPINAL CORD – point out dorsal and ventral horns (white, gray
matter)
DRG neurons: soma in ganglia
Axon w/ 2 branches  to CNS (spinal cord)
 to periphery:  (specialized) nerve terminal (sensory fn)
 1o afferent fiber (signal transmission)
Peripheral terminals of DRG neurons:
 bare endings: pain (nociceptors) & temperature sensations. Small diameter, unmyelinated or
thinly myelinated, slowly conducting axons.
 encapsulated: touch (mechanoRs) & proprioception. Large diameter, myelinated axons, fast
conducting.
I. TOUCH – MECHANO-Rs.
Greatest sensitivity on hairless skin. High density of mechano-Rs in ridges/ papillae, which
detect changes in contour of skin.
2 TYPES OF MECHANO-Rs ON HAIRY SKIN (Fig 22-2)
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All rapidly adapting.
 Hair follicle R – respond to hair displacement
 Field R – primarily over joints, sense skin stretch
 Merkel Rs
 Bare nerve endings
4 TYPES OF MECHANO-Rs ON GLABROUS SKIN (Fig 22-2)
In superficial skin layers:
1. Meissner’s corpuscle
Rapidly adapting.
Fine mechanical sensitivity due to mechanical coupling to papillary ridge.
R = fluid-filled structure w/ flattened epithelial cells.
2. Merkel disk R
Slowly adapting.
Semi-rigid structure transmitting strain from skin to nerve ending.
Found in clusters.
Flat surface – continuous firing. Shape of object translated by changes in firing rate. The
higher the curvature, the higher the increase in firing rate in a small # of Rs, and vice versa.
Superficial touch mechanoRs and Braille reading:
Increased sensitivity based on subsets of Rs converging to a single axon – Population
coding.
Deep subcutaneous tissue (fewer and larger):
3. Pacinian corpuscle (morphology slide, physiology slide)
Responds to rapid indentation or vibration, not to steady pressure.
Most sensitive mechanoR.
4. Rufini ending
Slowly adapting. Senses stretches of skin, shape of grasped objects, gravitational forces
(e.g. when lifting an object).
Slowly adapting Rs
Rapidly adapting Rs
Detect shape of objects and pressure
Detect motion of objects on skin
Higher touch threshold
Lower touch threshold
RECEPTIVE FIELDS OF MECHANO-Rs
(Re-)Define RF. Region of space from which a given R receives input.
Fig 22-3.
Superficial layer Rs – multiple Rs per afferent fiber. Small RFs. Finer sensitivity.
Deep layers – 1 R to 1 afferent fiber. Larger RFs. Coarse sensitivity.
CHANGES IN SENSITIVITY DUE TO DIFFERENCES IN R DISTRIBUTION (Fig 22-4).
DEGREE OF SPATIAL RESOLUTION DEPENDS ON CONTRAST BTWN ACTIVE AND
INACTIVE NERVE FIBERS.
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TWO-POINT DISCRIMINATION – Explain it wrt RFs.
SYNERGISTIC OPERATION OF ALL TOUCH MECHANO-Rs IN EVERYDAY LIFE.
II. TEMPERATURE SENSING
Humans recognize: cold, cool, warm, hot.
Warm and cold Rs.
Fire at rest.
Respond to a range of temperatures. Change in firing rate in response to temperature
change. Population coding.
Sensory adaptation: Thermal Rs reduce their firing rate w/in a few secs.
III. NOCICEPTION – PAIN
Chemical Rs to a large extent – respond to substances released from injured tissue.
3 CLASSES OF NOCICEPTORS
1. Mechanical nociceptors
 Fire in response to strong tactile stimuli (Draw exp: blunt end vs. pinprick vs. forceps pinch
and afferent fiber recording).
 Bare nerve endings and myelinated axons  fastest-conducting nociceptive afferents.
2. Thermal nociceptors
Respond to extreme heat (> 45 C) or extreme cold (< 5 C).
3. Polymodal nociceptors
Respond to all of the above.
Responsible for toothaches.
IV. PROPRIOCEPTION
2 SUBMODALITIES:
 Limb-position sense
 Kinesthesia (limb movement sense)
3 TYPES OF MECHANO-Rs – in muscles and joints
1. Muscle spindle R – specialized stretch R in muscle
2. Golgi tendon organs – @ junction btwn muscle fibers and tendon
3. Rs in joint capsules
Stretch Rs in skin also.
VISCERAL MECHANOSENSORY AND CHEMOSENSORY Rs
The gut has a mind of its own.
Similar to skin mechanical nociceptors.
Control of visceral activity.
CONDUCTION VELOCITIES
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Differences based on axon diameter and degree of myelination,
Compound AP.
FROM THE PERIPHERY TO THE CORTEX
Show 1o, 2o somatosensory ctx – Subdivisions. Serial and parallel processing in order to
perceive.
Neurons in 1o somatosensory ctx are at least 3 synapses away from the peripheral Rs.
RFs of cortical neurons are larger than RFs of peripheral Rs. Successful discrimination in the ctx
is accomplished by the fact that a cortical neurons responds BEST at a particular spot.
Integration of info for stimulus processing.
Convergence – divergence in relay nuclei before reaching ctx.
TOPOGRAPHIC ARRANGEMENT IN THE CTX. Preservation of location and modality.
Describe columns. (Show cortical layers.) More evident in L4. Visualized with radioactive
probes.
Technique: extracellular recordings.
Phantom limb pain
Humunculus: somatotopic map in the human ctx. Not necessarily correlated with surface area.
Criterion = innervation density.