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Chapter 21Somatic Senses

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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

• 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 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

• 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

• Generate impulses mainly at onset of a touch

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•Free nerve endings found around follicles, detects movement of hair

Hair Root Plexus

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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)– 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

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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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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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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– 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

– 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– 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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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 FunctionAspects 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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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

– 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