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Chapter 15 – Sensory Pathways,
Somatic Nervous System!
Chapter 15!
Neural Integration I:
Sensory Pathways and the
Somatic Nervous System!
SECTION 15-2!
Sensory receptors connect our internal and
external environments with the nervous system!
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Sensation and Receptors!
Transduction!
•  Translation of stimulus into action potential!
•  No transduction, no sensation!
Perception!
•  Conscious awareness of sensation!
Senses!
General senses!
•  Temperature, pain, touch, pressure, vibration,
proprioception!
•  Receptors distributed throughout body!
•  Receptors relatively simple in structure!
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1!
Chapter 15 – Sensory Pathways,
Somatic Nervous System!
Sensation!
Special senses!
•  Olfaction, vision, gustation, equilibrium, audition!
•  Receptors found in special sense organs!
•  Receptors more complex in structure!
Receptor specificity = “Law of specific nerve
energies” or “Modality specificity”!
•  You can’t see with your ears.!
Specificity results from:!
•  Structure of receptor cell and/or!
•  Presence of accessory structures!
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Specificity Examples!
1. Pain receptors are free nerve endings!
•  Respond to tissue damage from:!
Pressure, chemicals, heat!
•  i.e., simple structure; somewhat non-specific!
2. Photoreceptors!
•  Respond to light!
•  Complex receptor cells!
•  Surrounded by specialized pigment and CT
cells!
•  i.e., complex structure; very specific!
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Receptive Field!
Receptive field = area monitored by a single
receptor!
•  Smaller receptive field → more precise
localization!
e.g. on finger tips!
•  Larger receptive field → less precise
localization!
e.g. on back!
!
“Two-point discrimination test”!
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2!
Chapter 15 – Sensory Pathways,
Somatic Nervous System!
Receptors and Receptive Fields
Figure 15-2!
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Receptor and Generator Potentials – 1!
Stimulus transduced into action potentials!
•  Stimulus causes a change in membrane
potential of receptor!
•  Change = receptor potential, a graded
potential!
Stronger stimulus → larger change!
Can be de- or hyperpolarization!
A graded depolarization in a neuronal receptor is
called generator potential.!
The greater the generator potential, the more
frequently action potentials are generated.
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Receptor (Generator) Potential!
Generator (receptor)
potential
Guyton and Hall, 1996!
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3!
Chapter 15 – Sensory Pathways,
Somatic Nervous System!
Interpretation of Sensory Information!
Labeled line:!
•  Specific sensory info sent from specific
receptor in specific part of body to specific
area of cortex!
•  Neuronal pathway from receptor to cortex is
the “labeled line”!
Cortex interprets:!
1. Type of stimulus based upon labeled line!
2. Location of stimulus based upon where in
cortex labeled line arrives (homunculus)!
3. Intensity, duration of stimulus based upon
action potential frequency and pattern!
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General Types of Receptors!
Rapidly-adapting (phasic) receptors!
•  Reduce firing rate during constant stimulation
•  Fire rapidly at the beginning and end of
stimulation!
•  Conveys information about changes in stimulus
intensity!
•  E.g. Pacinian corpuscle, thermoreceptor!
Slowly-adapting (tonic) receptors!
•  Fire at some level all the time!
•  Continue firing during constant stimulation
•  Conveys information about the duration of the
stimulus!
•  E.g. pain receptors, muscle spindles!
Tonic and Phasic Receptors
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Figure 15-3!
Tonic Receptors!
Phasic Receptors!
•  Pain receptors!
•  Muscle spindle!
•  Merkel disc!
•  Pacinian corpuscle!
•  Thermoreceptor!
•  Meissner’s corpuscle!
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4!
Chapter 15 – Sensory Pathways,
Somatic Nervous System!
Pacinian Corpuscle – Rapid Peripheral Adaptation!
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Adaptation to a Stimulus!
Adaptation = ↓ sensitivity to constant stimulus
1. Peripheral adaptation!
•  Receptor sensitivity changes!
•  ↓ amount of info reaching CNS!
2. Central adaptation!
Occurs within CNS, along sensory pathways!
A. Subconscious adaptation - e.g. olfaction!
•  Brain signals inhibitory cells in olfactory bulb!
B.  Conscious adaptation!
e.g., “tuning out” noise during an exam!
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SECTION 15-3!
General sensory receptors can be classified
by the type of stimulus that excites them!
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5!
Chapter 15 – Sensory Pathways,
Somatic Nervous System!
General Senses – Receptors!
•  Exteroceptors!
•  Interoceptors!
•  Proprioceptors!
Note: There are no proprioceptors in visceral
organs of thoracic and abdominopelvic cavities. !
Why is this a good thing?!
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Nociceptors (Pain Receptors)!
Slow-adapting Few are in deep tissues or viscera!
Are free nerve endings of sensory neurons!
Have large receptive fields!
Subtypes based on sensitivity to:!
1.  Extreme temperature!
2.  Mechanical damage!
3.  Chemicals!
4.  https://www.eurekalert.org/pub_releases/2016-11/uclpss111016.php!
A strong stimulus can excite all three types!
Neurotransmitters: Glutamate, Substance P (for pain)!
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Nociceptors – 2 – Fiber Types!
1. Type A fibers!
•  Myelinated - “fast pain” or prickling pain!
•  Relayed to primary sensory cortex
Conscious, localized pain!
e.g. deep cut, pin prick!
2. Type C fibers!
•  Unmyelinated - “slow pain” = burning, aching!
•  Relayed to thalamus and reticular formation
Conscious, but not precisely localized!
e.g. uterine cramps!
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6!
Chapter 15 – Sensory Pathways,
Somatic Nervous System!
Nociceptors – 3!
Central adaptation to pain!
•  May decrease perception of pain!
e.g. endorphins, enkephalins prevent release
of substance P!
Part of the mechanism of pain!
1.  Tissue damage →!
2.  Arachadonic acid released from cell membranes!
3.  Converted to prostaglandins by cyclo-oxygenase!
4.  Prostaglandins stimulate pain receptors!
(Aspirin, Ibuprofen - block cyclo-oxygenase)!
5. Info ascends in cord via lateral spinothalamic tract!
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Thermoreceptors!
Are free nerve endings!
Phasic (fast-adapting)!
•  But background firing rate depends on
temperature!
There are 3 to 4 times more cold than warm
receptors!
Information ascends in lateral spinothalamic tract
(with pain information)!
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Thermoreceptors – 2
Handout!
Constant !
temperature!
Changing
temperature!
Cold fiber!
Warm fiber!
Kandel, Schwartz and!
Jessell, 2000!
Temperature!
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7!
Chapter 15 – Sensory Pathways,
Somatic Nervous System!
Tactile Receptors in the Skin
Figure 15-4!
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Mechanoreceptors!
Have mechanically-gated ion channels!
A.  Tactile receptors!
Touch, pressure (deep touch), vibration!
•  Fine touch and pressure!
Precisely localized - small receptive fields!
(Posterior column pathway)!
•  Crude touch and pressure!
Poorly localized - larger receptive fields!
(Anterior spinothalamic tract)!
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Tactile (Touch and Pressure) Receptors – 1!
1. Free nerve endings!
•  Between epithelial cells, in corneal epithelium!
•  Touch and pressure!
•  Tonic (slow-adapting)!
•  Small receptive fields!
2. Root hair plexus!
•  Dendrites surround hair root!
•  Movement of hair!
•  Phasic (fast-adapting)!
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8!
Chapter 15 – Sensory Pathways,
Somatic Nervous System!
Touch and Pressure Receptors – 2!
3. Merkel’s (tactile) discs!
•  Dendrites contact Merkel (epidermal) cells!
•  Fine touch and pressure - very sensitive!
•  Tonic (slow adapting)!
•  Small receptive field!
4.  Meissner’s (tactile) corpuscles!
•  Papillary layer of dermis!
•  Capsule = modified Schwann cells!
•  Fine touch and pressure, low freq. vibration!
•  Phasic (fast adapting)!
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Touch and Pressure Receptors – 3!
5. Pacinian (lamellated) corpuscles!
•  Dermis, joint capsules, fasciae, serous
membranes, some viscera!
•  CT capsule!
•  Deep pressure, high freq. vibration!
•  Phasic (fast adapting)!
•  Large size → large receptive field!
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Touch and Pressure Receptors – 4!
6. Ruffini corpuscles (end organs)!
•  Reticular layer of skin!
•  Capsule covers dendrites innervating
collagen fibers of skin!
•  Senses stretch, distortion of dermis!
•  Tonic (very slowly adapting - if at all)!
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9!
Chapter 15 – Sensory Pathways,
Somatic Nervous System!
Mechanoreceptors – Baroreceptors!
B.  Baroreceptors!
• 
Monitor pressure (actually monitor stretch
due to fluid pressure)!
• 
Free nerve endings within elastic CT!
• 
Stretch/recoil → change in AP frequency!
• 
Phasic (fast adapting), respond to change!
• 
Some locations:!
Digestive tract, urinary bladder!
Carotid sinus and aortic sinus!
Lungs!
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Proprioceptors!
Do not adapt to constant stimulation
•  Muscle spindles (chapter 13)!
•  Golgi tendon organs (chapter 13)!
Free nerve endings!
Sense tension in tendon!
•  Joint capsule receptors!
Free nerve endings!
Pressure, tension, movement of joint!
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Chemoreceptors!
Respond to water- and lipid-soluble substances!
I.e. must be dissolved to be sensed!
•  Adapt rapidly (seconds)!
General chemosensation:!
•  Send info to brain stem (not sensory cortex)!
e.g. CNS sensors in pons, medulla!
[H+], [CO2] in CSF!
e.g. Peripheral sensors!
[H+], [CO2], [O2] in blood!
Info to brain via CN IX and X!
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10!
Chapter 15 – Sensory Pathways,
Somatic Nervous System!
SECTION 15-4!
Separate pathways carry somatic and visceral
sensory information!
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Organization of Sensory Pathways!
1. First-order neuron – connected to receptor!
•  Cell body in dorsal root ganglion or cranial nerve
ganglion!
•  Synapses with second-order (interneuron) in brain
stem or cord!
2. Second-order neuron !
•  Crosses over to contralateral side!
•  Usually synapses with third-order neuron in thalamus!
3. Third-order neuron!
•  Synapses in post-central gyrus (primary sensory
cortex)!
•  Does not cross over!
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Sensory/Ascending Tracts/Pathways
Fig. 15-5!
Labeled line:
Neuronal pathway from receptor to cortex !
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11!
Chapter 15 – Sensory Pathways,
Somatic Nervous System!
Posterior Column Pathway
Spotlight 15-6c!
•  Fine touch!
•  Vibration!
•  Pressure!
•  Proprioception!
(from contralateral!
side of body)!
Midbrain!
Third-order!
neuron!
Second-order!
neuron!
Medulla!
First-order!
neuron!
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Posterior Column – Medial Lemniscus – 1!
Sensations!
•  Fine (highly localized) touch, pressure,
vibration, conscious proprioception!
Tracts!
•  Fasciculus gracilis!
Lower half of body - trunk and lower limbs!
•  Fasciculus cuneatus!
Upper half of body - neck, arms, chest!
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Posterior Column – Medial Lemniscus - 2!
1. First-order neuron!
•  Receptor → synapse in nucleus gracilis or
cuneatus in medulla!
2. Second-order neuron!
•  Cross over to other side in medulla!
•  Enter medial lemniscus (“ribbon”)!
•  Synapse in thalamus!
3. Third-order neuron!
•  Thalamus → somatosensory cortex!
•  Thalamus processes info according to stimulus
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type and location!
12!
Chapter 15 – Sensory Pathways,
Somatic Nervous System!
Anterior Spinothalamic Tract
Spotlight 15-6a!
Crude touch and pressure!
(from other side of body)!
!
“crude” = poorly localized!
Thalamus
Third-order!
neuron!
Midbrain
Medulla
Second-order!
neuron!
First-order!
neuron!
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Lateral Spinothalamic Tract
Figure 15-5b!
Pain and Temperature!
(from other side of
body)!
Thalamus
Third-order!
neuron!
Second-order!
neuron!
Medulla
First-order!
neuron!
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Anterior and Lateral Spinothalamic Tracts!
Sensations!
A. Anterior spinothalamic tract!
•  Crude (poorly-localized) touch and pressure!
!
B. Lateral spinothalamic tract !
•  Pain and temperature!
!
!
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13!
Chapter 15 – Sensory Pathways,
Somatic Nervous System!
Anterolateral Pathways!
BOTH Anterior and Lateral Pathways!
1. First-order neurons!
•  Synapse in dorsal horn!
2. Second-order neurons!
•  Cross over in spinal cord!
•  Synapse in thalamus!
3. Third-order neurons!
•  Synapse in primary sensory cortex!
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The Spinocerebellar Pathway
Spotlight 15-6d!
Unconscious
proprioception!
!
No third-order
neuron
First-order!
neuron!
Second-order!
neuron!
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Spinocerebellar Pathways!
Sensation!
Unconscious proprioception (cerebellum!)!
Input from Golgi tendon organs, muscle
spindles, joint capsules!
Pathway!
1.  First-order neurons!
•  Synapse in dorsal horn!
2. Second-order neurons!
•  Synapse with Purkinje cells in cerebellum!
3. NO third-order neurons!
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14!
Chapter 15 – Sensory Pathways,
Somatic Nervous System!
Referred Pain!
Mechanisms of Pain!
Cleveland Clinic Journal of Medicine, 74(1): 2007:!
•  Pain of visceral origin is referred to somatic
regions that are innervated from the same spinal
segments as the heart.!
•  The pain is generally referred to proximal, but
not distal, somatic structures.!
•  The referred pain is experienced as deep (slow)
pain.!
•  Slides 44, 45, and 46 are FYI only
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Neuroscience at University of Texas!
1. Common dermatome hypothesis!
•  Structures develop from same embryonic
segment (dermatome)!
•  Axons from visceral (heart) and somatic (skin)
receptors synapse with same neurons in cord!
3. Facilitation hypothesis !
FYI Only!
2. Convergence hypothesis!
•  Signals from viscera “facilitate” perception of
pain from skin by higher centers!
4. Learned phenomenon hypothesis!
•  The brain is not “used to” receiving visceral pain
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signals so interprets them as being somatic!
Phantom Pain!
1. Peripheral mechanism!
Damage → changes inputs to cord!
•  “deafferentation” and neuroma growth!
•  Increased sensitivity → pain!
2. Central mechanism!
FYI Only!
•  Increased Na+ channel synthesis!
a. Central sensitization!
•  New growth of proximal end of cut neuron!
•  New growth of non-pain neurons into pain
areas of DRG!
•  New connections - increased receptive field!
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15!
Chapter 15 – Sensory Pathways,
Somatic Nervous System!
Phantom Pain – 2!
2b. “Windup effect”!
•  Increased (upregulation) of pain pathway receptors!
3. Brain changes mechanism!
a. “Cortical reorganization”!
FYI Only!
2c. Rewiring may cause loss of inhibitory signals
from higher centers.!
•  Areas formerly responsible for amputated limb taken
over by neurons from other areas!
b. Changes in “body schema”!
•  Disconnect between intended and actual movements
upsets body schema!
http://www.hindawi.com/journals/prt/2011/864605/!
Principal Sensory Pathways – 1
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Table 15-1!
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Principal Sensory Pathways – 2
Table 15-1!
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16!
Chapter 15 – Sensory Pathways,
Somatic Nervous System!
SECTION 15-5!
The somatic nervous system is an efferent
division that controls skeletal muscles!
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Descending (Motor) Tracts
Figure 15-8!
Note: Names indicate direction.!
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Motor Pathways!
Pathways involve at least 2 neurons!
1. Upper motor neuron (UMN)!
•  Cell body in CNS (e.g. primary motor cortex)!
•  Synapses with lower motor neuron!
(Excitatory or inhibitory synapse)!
•  Stays in CNS!
2. Lower motor neuron (LMN)!
•  Cell body in brain stem or cord!
•  Innervates skeletal muscle fibers of a motor
unit!
(Always excitatory synapse)!
•  Called “Final Common Pathway”!
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17!
Chapter 15 – Sensory Pathways,
Somatic Nervous System!
Corticospinal Pathways – 1!
A.K.A. “Pyramidal System” or “Direct Pathways”!
Conscious control of skeletal muscle!
1. Corticobulbar tract!
•  (“bulbar” refers to the brain stem)!
•  UMN cell bodies in primary motor cortex!
•  Synapse in cranial nerve nuclei
•  Conscious control of skeletal muscle!
e.g. eye, jaw, facial muscle movements!
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Corticospinal Pathways – 2!
2.  Lateral corticospinal tract !
•  (85% of motor corticospinal pathway neurons)!
• 
UMN in primary motor cortex!
Cross over at decussation of pyramids in
medulla!
• 
Synapse on LMN in cord!
• 
Conscious control of skeletal muscle!
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Corticospinal Pathways - 3!
3.  Anterior corticospinal tract!
•  15% of corticospinal neurons!
• 
UMN in primary motor cortex!
• 
Synapses on LMN in cord!
• 
Cross over in spinal cord (not medulla)!
• 
Conscious control of skeletal muscle!
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18!
Chapter 15 – Sensory Pathways,
Somatic Nervous System!
The Corticospinal Pathway
Figure 15-9!
Parts of
Corticobulbar
tract
Decussation of the pyramids
Lateral corticospinal tract ≈ 85%
Final Common Pathway
Principal Motor Pathways
Anterior corticospinal
tract ≈ 15%
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Table 15-2!
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19!