1. Chapter 21 Somatic Senses

2. 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

3. 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

4. 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

5. Classification of Sensory Receptors
Structural classification
Type of response to a stimulus
Location of receptors & origin of stimuli
Type of stimuli they detect

6. 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

7. Structural Classification
Compare free nerve ending, encapsulated nerve ending and sensory receptor cell

8. 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

9. 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

10. 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

11. 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

12. 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

13. Hair Root Plexus
Free nerve endings found around follicles, detects movement of hair

14. Merkel’s Disc Flattened dendrites touching cells of stratum basale
Used in discriminative touch (25% of receptors in hands)

15. Ruffini Corpuscle Found deep in dermis of skin
Detect heavy touch, continuous touch, & pressure

16. Pacinian Corpuscle
Onion-like connective tissue capsule enclosing a dendrite
Found in subcutaneous tissues & certain viscera
Sensations of pressure or high-frequency vibration

17. Thermal Sensations
Free nerve endings with 1mm diameter receptive fields on the skin surface
Cold receptors in the stratum basale respond to temperatures between degrees F
Warm receptors in the dermis respond to temperatures between 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.

18. 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

19. Types of Pain Fast pain (acute) Slow pain (chronic)
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

20. 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. 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

22. 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.

23. 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

24. 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

25. 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

26. 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)

27. 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

28. 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

29. 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

30. 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

31. 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

32. 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

33. 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

34. 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

35. Location of Direct Pathways
Lateral corticospinal tract
Anterior corticospinal tract
Corticobulbar tract

36. 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

37. 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

38. 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

39. 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

40. 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

41. 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