Chapter 21
Somatic Senses
21-1
Sensory Modalities
• Different types of sensations
– touch, pain, temperature, vibration, hearing, vision
• Each type of sensory neuron can respond to
only one type of stimuli
• Two classes of sensory modalities
– general senses
• somatic are sensations from body walls
• visceral are sensations from internal organs
– special senses
• smell, taste, hearing, vision, and balance
21-2
Process of Sensation
• Sensory receptors demonstrate selectivity
– respond to only one type of stimuli
• Events occurring within a sensation
– stimulation of the receptor
– transduction (conversion) of stimulus into a
graded potential
• vary in amplitude and are not propagated
– generation of impulses when graded potential
reaches threshold
– integration of sensory input by the CNS
21-3
Sensory Receptors
• Selectively respond to only one kind of stimuli
• Have simple or complex structures
– General Sensory Receptors (Somatic Receptors)
• no structural specializations in free nerve endings that
provide us with pain, tickle, itch, temperatures
• some structural specializations in receptors for touch,
pressure & vibration
– Special Sensory Receptors (Special Sense
Receptors)
• very complex structures---vision, hearing, taste, & smell
21-4
Classification of Sensory Receptors
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•
•
•
Structural classification
Type of response to a stimulus
Location of receptors & origin of stimuli
Type of stimuli they detect
21-5
Structural Classification of Receptors
• Free nerve endings
– bare dendrites
– pain, temperature, tickle, itch & light touch
• Encapsulated nerve endings
– dendrites enclosed in connective tissue capsule
– pressure, vibration & deep touch
• Separate sensory cells
– specialized cells that respond to stimuli
– vision, taste, hearing, balance
21-6
Structural Classification
• Compare free nerve ending, encapsulated nerve
ending and sensory receptor cell
21-7
Classification by Location
• Exteroceptors
– near surface of body
– receive external stimuli
– hearing, vision, smell, taste, touch, pressure, pain,
vibration & temperature
• Interoceptors
– monitors internal environment (BV or viscera)
– not conscious except for pain or pressure
• Proprioceptors
– muscle, tendon, joint & internal ear
– senses body position & movement
21-8
Classification by Stimuli Detected
• Mechanoreceptors
– detect pressure or stretch
– touch, pressure, vibration, hearing,
proprioception, equilibrium & blood pressure
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•
•
•
Thermoreceptors detect temperature
Nociceptors detect damage to tissues
Photoreceptors detect light
Chemoreceptors detect molecules
– taste, smell & changes in body fluid chemistry
21-9
Adaptation of Sensory Receptors
• Change in sensitivity to long-lasting stimuli
– decrease in responsiveness of a receptor
• bad smells disappear
• very hot water starts to feel only warm
– potential amplitudes decrease during a maintained,
constant stimulus
• Receptors vary in their ability to adapt
– Rapidly adapting receptors (smell, pressure, touch)
• adapt quickly; specialized for signaling stimulus changes
– Slowly adapting receptors (pain, body position)
• continuation of nerve impulses as long as stimulus persists
21-10
Somatic Tactile Sensations
• Touch
– crude touch is ability to perceive something has
touched the skin
– discriminative touch provides location and texture of
source
• Pressure is sustained sensation over a large area
• Vibration is rapidly repetitive sensory signals
• Itching is chemical stimulation of free nerve
endings
• Tickle is stimulation of free nerve endings only by
someone else
21-11
Meissner’s Corpuscle
• Dendrites enclosed in CT in dermal papillae of hairless skin
• Discriminative touch & vibration-- rapidly adapting
• Generate impulses mainly at onset of a touch
21-12
Hair Root Plexus
•Free nerve endings found around follicles, detects
movement of hair
21-13
Merkel’s Disc
• Flattened dendrites touching cells of stratum basale
• Used in discriminative touch (25% of receptors in hands)
21-14
Ruffini Corpuscle
• Found deep in dermis of skin
• Detect heavy touch, continuous touch, & pressure
21-15
Pacinian Corpuscle
• Onion-like connective tissue capsule enclosing a dendrite
• Found in subcutaneous tissues & certain viscera
• Sensations of pressure or high-frequency vibration
21-16
Thermal Sensations
• Free nerve endings with 1mm diameter receptive
fields on the skin surface
• Cold receptors in the stratum basale respond to
temperatures between 50-105 degrees F
• Warm receptors in the dermis respond to
temperatures between 90-118 degrees F
• Both adapt rapidly at first, but continue to
generate impulses at a low frequency
• Pain is produced below 50 and over 118
degrees F.
21-17
Pain Sensations
• Nociceptors = pain receptors
• Free nerve endings found in every tissue of
body except the brain
• Stimulated by excessive distension, muscle
spasm, & inadequate blood flow
• Tissue injury releases chemicals such as
K+, kinins or prostaglandins that stimulate
nociceptors
• Little adaptation occurs
21-18
Types of Pain
• Fast pain (acute)
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–
–
–
occurs rapidly after stimuli (.1 second)
sharp pain like needle puncture or cut
not felt in deeper tissues
larger A nerve fibers
• Slow pain (chronic)
–
–
–
–
begins more slowly & increases in intensity
aching or throbbing pain of toothache
in both superficial and deeper tissues
smaller C nerve fibers
21-19
Localization of Pain
• Superficial Somatic Pain arises from skin areas
• Deep Somatic Pain arises from muscle, joints,
tendons & fascia
• Visceral Pain arises from receptors in visceral
organs
– localized damage (cutting) intestines causes no pain
– diffuse visceral stimulation can be severe
• distension of a bile duct from a gallstone
• distension of the ureter from a kidney stone
• Phantom limb sensations -- cells in cortex still
21-20
Referred Pain
• Visceral pain that is felt just deep to the skin overlying the
stimulated organ or in a surface area far from the organ.
• Skin area & organ are served by the same segment of the
spinal cord.
– Heart attack is felt in skin along left arm since both are supplied
by spinal cord segment T1-T5
21-21
Pain Relief
• Aspirin and ibuprofen block formation of
prostaglandins that stimulate nociceptors
• Novocaine blocks conduction of nerve
impulses along pain fibers
• Morphine lessen the perception of pain in
the brain.
21-22
Proprioceptive or Kinesthetic Sense
• Awareness of body position & movement
– walk or type without looking
– estimate weight of objects
• Proprioceptors adapt only slightly
• Sensory information is sent to cerebellum &
cerebral cortex
– from muscle, tendon, joint capsules & hair cells
in the vestibular apparatus
21-23
Muscle Spindles
• Specialized intrafusal muscle
fibers enclosed in a CT capsule
and innervated by gamma
motor neurons
• Stretching of the muscle
stretches the muscle spindles
sending sensory information
back to the CNS
• Spindle sensory fiber monitor
changes in muscle length
• Brain regulates muscle tone by
controlling gamma fibers
21-24
Golgi Tendon Organs
• Found at junction of tendon & muscle
• Consists of an encapsulated bundle of collagen fibers
laced with sensory fibers
• When the tendon is overly stretched, sensory signals
head for the CNS & resulting in the muscle’s relaxation
21-25
Somatic Sensory Pathways
• First-order neuron conduct impulses to
brainstem or spinal cord
– either spinal or cranial nerves
• Second-order neurons conducts impulses from
spinal cord or brainstem to thalamus--cross over
to opposite side before reaching thalamus
• Third-order neuron conducts impulses from
thalamus to primary somatosensory cortex
(postcentral gyrus of parietal lobe)
21-26
Posterior Column-Medial Lemniscus
Pathway of CNS
• Proprioception, vibration,
discriminative touch, weight
discrimination & stereognosis
• Signals travel up spinal cord in
posterior column
• Fibers cross-over in medulla to
become the medial lemniscus
pathway ending in thalamus
• Thalamic fibers reach cortex
21-27
Spinothalamic Pathways
• Lateral spinothalamic tract
carries pain & temperature
• Anterior tract carries tickle,
itch, crude touch & pressure
• First cell body in DRG with
synapses in cord
• 2nd cell body in gray matter of
cord, sends fibers to other side
of cord & up through white
matter to synapse in thalamus
• 3rd cell body in thalamus
projects to cerebral cortex
21-28
Somatosensory Map of Postcentral
Gyrus
• Relative sizes of cortical areas
– proportional to number of
sensory receptors
– proportional to the
sensitivity of each part of
the body
• Can be modified with learning
– learn to read Braille & will
have larger area
representing fingertips
21-29
Sensory Pathways to the Cerebellum
• Major routes for proprioceptive
signals to reach the cerebellum
– anterior spinocerebellar tract
– posterior spinocerebellar tract
• Subconscious information used
by cerebellum for adjusting
posture, balance & skilled
movements
• Signal travels up to same side
inferior cerebellar peduncle
21-30
Somatic Motor Pathways
• Control of body movement
– motor portions of cerebral cortex
• initiate & control precise movements
– basal ganglia help establish muscle tone & integrate
semivoluntary automatic movements
– cerebellum helps make movements smooth & helps
maintain posture & balance
• Somatic motor pathways
– direct pathway from cerebral cortex to spinal cord & out
to muscles
– indirect pathway includes synapses in basal ganglia,
thalamus, reticular formation & cerebellum
21-31
Primary Motor Cortex
• Precentral gyrus initiates
voluntary movement
• Cells are called upper motor
neurons
• Muscles represented
unequally (according to the
number of motor units)
• More cortical area is needed if
number of motor units in a
muscle is high
– vocal cords, tongue, lips,
fingers & thumb
21-32
Direct Pathway (Pyramidal Pathway)
• 1 million upper motor neurons in cerebral cortex
– 60% in precentral gyrus & 40% in postcentral gyrus
• Axons form internal capsule in cerebrum and
pyramids in the medulla oblongata
• 90% of fibers decussate(cross over) in the medulla
– right side of brain controls left side muscles
• Terminate on interneurons which synapse on lower
motor neurons in either:
– nuclei of cranial nerves or anterior horns of spinal cord
• Integrate excitatory & inhibitory input to become
final common pathway
21-33
Details of Motor Pathways
• Lateral corticospinal tracts
– cortex, cerebral peduncles,
90% decussation of axons
in medulla, tract formed in
lateral column.
– skilled movements hands &
feet
• Anterior corticospinal tracts
– the 10% of axons that do
not cross
– controls neck & trunk
muscles
• Corticobulbar tracts
– cortex to nuclei of CNs --III, IV, V, VI, VII, IX, X, XI
& XII
– movements of eyes, tongue,
chewing, expressions &
speech
21-34
Location of Direct Pathways
•
Lateral corticospinal tract
•
Anterior corticospinal tract
•
Corticobulbar tract
21-35
Paralysis
• Flaccid paralysis = damage lower motor neurons
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no voluntary movement on same side as damage
no reflex actions
muscle limp & flaccid
decreased muscle tone
• Spastic paralysis = damage upper motor neurons
– paralysis on opposite side from injury
– increased muscle tone
– exaggerated reflexes
21-36
Final Common Pathway
• Lower motor neurons
receive signals from both
direct & indirect upper
motor neurons
• Sum total of all inhibitory
& excitatory signals
determines the final
response of the lower
motor neuron & the
skeletal muscles
21-37
Basal Ganglia
• Helps to program automatic movement sequences
– walking and arm swinging or laughing at a joke
• Set muscle tone by inhibiting other motor circuits
• Damage is characterized by tremors or twitches
21-38
Basal Ganglia Connections
• Circuit of connections
– cortex to basal ganglia
to thalamus to cortex
– planning movements
• Output from basal
ganglia to reticular
formation
– reduces muscle tone
– damage produces
rigidity of Parkinson’s
disease
21-39
Cerebellar Function
Aspects of Function
• learning
• coordinated &
skilled movements
• posture &
equilibrium
1. Monitors intentions for movements -- input from cerebral cortex
2. Monitors actual movements with feedback from proprioceptors
3. Compares intentions with actual movements
4. Sends out corrective signals to motor cortex
21-40
Spinal Cord Injury
• Damaged by tumor, herniated disc, clot or trauma
• Complete transection is cord severed resulting loss
of both sensation & movement below the injury
• Paralysis
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monoplegia is paralysis of one limb only
diplegia is paralysis of both upper or both lower
hemiplegia is paralysis of one side
quadriplegia is paralysis of all four limbs
• Spinal shock is loss of reflex function (areflexia)
– slow heart rate, low blood pressure, bladder problem
– reflexes gradually return
21-41