Transcript Chapter 15: Special Senses

Chapter 15
Sensory, Motor & Integrative Systems
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Levels and components of sensation
Pathways for sensations from body to brain
Pathways for motor signals from brain to body
Integration Process
– wakefulness and sleep
– learning and memory
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Is Sensation Different from Perception?
• Sensation is any stimuli the body is aware of
– What are we not aware of?
• X-rays, ultra high frequency sound waves, UV light
– We have no sensory receptors for those stimuli
• Perception is the conscious awareness &
interpretation of a sensation.
– precisely localization & identification
– memories of our perceptions are stored in cortex
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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
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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
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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
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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
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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
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Structural Classification
• Compare free nerve ending, encapsulated nerve
ending and sensory receptor cell
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Classification by Response to Stimuli
• Generator potential
– free nerve endings, encapsulated nerve endings &
olfactory receptors produce generator potentials
– when large enough, it generates a nerve impulse in a
first-order neuron
• Receptor potential
– vision, hearing, equilibrium and taste receptors produce
receptor potentials
– receptor cells release neurotransmitter molecules on firstorder neurons producing postsynaptic potentials
– PSP may trigger a nerve impulse
• Amplitude of potentials vary with stimulus intensity
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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
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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
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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
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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
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Meissner’s Corpuscle
• Dendrites enclosed in CT in dermal papillae of hairless skin
• Discriminative touch & vibration-- rapidly adapting
• Tortora
Generate
impulses mainly at onset of a touch
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Hair Root Plexus
•Free nerve endings found around follicles, detects
movement of hair
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Merkel’s Disc
• Flattened dendrites touching cells of stratum basale
• Used in discriminative touch (25% of receptors in hands)
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Ruffini Corpuscle
• Found deep in dermis of skin
• Detect heavy touch, continuous touch, & pressure
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Pacinian Corpuscle
• Onion-like connective tissue capsule enclosing a dendrite
• Found in subcutaneous tissues & certain viscera
• Sensations of pressure or high-frequency vibration
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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.
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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
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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)
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begins more slowly & increases in intensity
aching or throbbing pain of toothache
in both superficial and deeper tissues
smaller C nerve fibers
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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
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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
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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.
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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
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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
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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
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Joint Receptors
• Ruffini corpuscles
– found in joint capsule
– respond to pressure
• Pacinian corpuscles
– found in connective tissue around the joint
– respond to acceleration & deceleration of joints
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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)
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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
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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
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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
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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
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Tertiary Syphilis
• Sexually transmitted disease caused by
bacterium Treponema pallidum.
• Third clinical stage known as tertiary syphilis
• Progressive degeneration of posterior portions
of spinal cord & neurological loss
– loss of somatic sensations
– proprioceptive impulses fail to reach cerebellum
• People watch their feet while walking, but still
uncoordinated and jerky
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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
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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
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– vocal cords, tongue, lips,
fingers & thumb
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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
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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
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– cortex to nuclei of CNs ---III, IV, V,
VI, VII, IX, X, XI & XII
– movements of eyes, tongue,
chewing, expressions & speech
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Location of Direct Pathways
• Lateral corticospinal tract
• Anterior corticospinal tract
• Corticobulbar tract
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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
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Indirect Pathways
• All other descending motor
pathways
• Complex polysynaptic circuits
– include basal ganglia, thalamus,
cerebellum, reticular formation
• Descend in spinal cord as 5
major tracts
• All 5 tracts end upon
interneurons or lower motor
neurons
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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
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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
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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
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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
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Wakefulness and Sleep
• Circadian rhythm
– 24 hour cycle of sleep and awakening
– established by hypothalamus
• Awake means to be able to react consciously to
stimuli
• EEG recordings show large amount of activity
in cerebral cortex when awake
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Reticular Activating System
• RAS has connections to
cortex & spinal cord.
• Many types of inputs
activate the RAS---pain,
light, noise, muscle
activity, touch
• Produces state of wakefulness
called consciousness
• Coma is sleeplike state
– deep coma has no reflexes
– death if cardiovascular reflexes are
lost
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Sleep
• State of altered or partial consciousness from
which a person can be aroused
• Triggers for sleep are unclear
– adenosine levels increase with brain activity
– adenosine levels inhibit activity in RAS
– caffeine prevents adenosine from inhibiting RAS
• Two types of normal sleep
– NREM = non-rapid eye movement sleep
– REM = rapid eye movement sleep
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Non-Rapid Eye Movement Sleep
• Stage 1
– person is drifting off with eyes
closed (first few minutes)
• Stage 2
– fragments of dreams
– eyes may roll from side to side
• Stage 3
– very relaxed, moderately deep
– 20 minutes, body temperature & BP have dropped
• Stage 4 = deep sleep
– bed-wetting & sleep walking occur in this phase
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REM Sleep
• Most dreams occur during REM sleep
• In first 90 minutes of sleep:
– go from stage 1 to 4 of NREM,
– go up to stage 2 of NREM
– to REM sleep
• Cycles repeat until total REM sleep totals 90 to 120
minutes
• Neuronal activity & oxygen use highest in REM
sleep
• Total
sleeping & dreaming time decreases with
age
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Learning & Memory
• Learning is acquiring new knowledge
• Memory is retaining that knowledge
– short-term memory
• recall phone number while dialing
• depends upon electrical events (reverberating circuits)
– long-term memory
• frequent retrieval of specific information helps with memory
consolidation (learning)
• structural or biochemical changes occurs
– increase in dendrites, enlarge endbulbs, increase in presynaptic
terminals or formation of additional membrane receptors
• Recently acquired memory lost first with coma or
shock treatments
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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
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Cerebral Palsy
• Loss of motor control and coordination
• Damage to motor areas of the brain
– infection of pregnant woman with rubella virus
– radiation during fetal life
– temporary lack of O2 during birth
• Not a progressive disease, but irreversible
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Parkinson Disease
• Progressive disorder striking victims at age 60
• Environmental toxins may be the cause
• Neurons from the substantia nigra do not release
enough dopamine onto basal ganglia
– tremor, rigidity, bradykinesia (slow movement) or
hypokinesia (decreasing range of movement)
– may affect walking, speech, even facial expression
• Treatments
– drugs to increase dopamine levels, or to prevent its
breakdown, surgery to transplant fetal tissue or
removal of part of globus pallidus to slow tremors
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