Download Somatosensory 2

Quote of the Day
"Make sure you have finished speaking before your
audience has finished listening"
Assignments
-Read Chapters 10 and 11 in the Course Pack. Make a list of any terms
that you don!t understand and/or any questions that you may have.
-Start working on Study Guide 2 (Somatosensory system).
-Dorothy Sarnoff
- Exam 2 is coming up on Monday, November 9. This exam will cover
the somatosensory system, and whatever part of the visual system has
been covered by then.
submitted by Deborah Clark
Relationship between receptive field size and spatial acuity
2-point discrimination varies across different parts of the body
The more closely spaced
the receptors are and the
smaller their receptive
fields, the better the ability
to resolve two pressure
points on the skin.
Stretch receptors
Hair follicle receptors
Hairy skin contains hair follicle mechanoreceptors that sense movement
of the hair.
Animals! whiskers
are specialized
variations of hair
follicle receptors.
Muscles and tendons
have embedded within
them receptors that
respond to stretch
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Free nerve endings
Free nerve endings throughout the skin respond to
heat, cold, and/or pain.
“Paradoxical cold”
Cold receptors are also activated by high temperatures
Thermoreceptors
The skin contains receptors that are activated by warmth, and others
that are activated by cold.
Pain (Nociception)
The sensation of pain is caused by activation of very small diameter
nerve endings. When tissue is damaged, chemical substances are
released that stimulate these fibers.
Some stimuli that activate
nociceptors:
Thermal: high heat or
extreme cold
Mechanical: Intense
mechanical stimuli
Chemical: Irritants or
substances released by
injured tissue
Nerve fibers conveying different types of information have
different diameters
Key Concept: Neural latency
In any sensory pathway, different neurons respond to a stimulus with
different latencies.
This means that information about the same stimulus reaches its
destination(s) at different times.
This principle will later become important in understanding how neural
circuits in sensory systems analyze temporal patterns.
Larger fibers conduct information faster than small ones.
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Somatosensory pathways in the CNS
Nerve fibers that innervate skin receptors of the body have their cell
bodies in the dorsal root ganglion, a group of neuron cell bodies located
near the spinal cord.
Each spinal nerve has a dorsal root (sensory) and a
ventral root (motor)
• Motor neurons! cell bodies are in the
ventral horn of the grey matter. Their
axons exit via the ventral root of each
spinal nerve.
• Sensory neurons! cell bodies are located
in the dorsal root ganglion. Their axons
enter the spinal cord via the dorsal root of
each spinal nerve.
Cross-section through spinal cord
Information processing in the spinal cord
• Sensory information may
be transmitted directly to
motor neurons or indirectly
via interneurons. The
same information may be
relayed to the brainstem.
Each spinal nerve innervates a specific area of the body
Spinal nerves innervate body
segments
• Nerves from/to different
parts of the body enter the
spinal cord at different
places.
The dorsal columns transmit information about touch and
proprioception
• The dorsal columns are fiber tracts that send information to the thalamus
via nuclei in the brainstem and the medial lemniscus (a CNS fiber tract)
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The spinothalamic tracts transmit information about
temperature and pain
Different pathways for different somatic“modalities”
• The spinothalamic tracts are fiber tracts that send information to the
thalamus and from there to the cortex via the anterolateral pathway (a
CNS fiber tract)
Key Concept: Parallel Pathways
Different aspects of a sensory stimulus may be processed in different
neural pathways.
For a somatosensory stimulus, a single physical object could provide
information about pressure, temperature, pain, etc., which could be
thought of as different modalities.
Parallel pathways can also selectively process specific aspects of a
stimulus within a single modality.
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