1. Sensory System
Prof. K. Sivapalan

2. Receptor Mechanism.
Receptor is a transducer: it converts various energies into action potential.
Example- touch receptor: Pacinian corpuscle.
Receptor potentials are proportional to energy of touch.
When threshold is reached action potential is generated and transmitted in the nerve.
When the first node of Ranvier is blocked by anaesthetics, action potential is not generated.
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Sensory system

3. Receptor Mechanism ctd.
Receptor can be part of a neuron or a specialized cell.
One receptor responds to one form of energy at low strength.
Other forms of energy will require very high intensity to stimulate the same receptor.
Adequate stimulus- particular form of energy to which a receptor is most sensitive.
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Sensory system

4. Receptive field One axon is connected to several receptors.
Receptors of one axon will be spread in the receptive field. Receptive fields of axons can overlap.
The size of the receptive field varies – small in highly sensitive areas and large in less sensitive areas.
Mild stimulus excites few receptors and axons and strong stimulus excites more axons.
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Sensory system

5. Stimulus and response.
Larger the stimulus higher the receptor potential and frequency of action potentials.
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Sensory system

6. Some sensory modalities and receptors.
Sensation
Receptor
Organ.
Vision-light
Rods and cons
Eye
Hearing-sound
Hair cells
Ear
Taste-chemical
Taste receptors
Taste bud
Acceleration-mechanical
Hair cell
Internal ear
Pressure-Mechanical.
Nerve endings
Skin
Cold- thermal
Blood pressure- mechanical
Baroreceptor
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Sensory system

7. Tonic and phasic receptors.
All receptors generate action potential on application of stimulus.
Some continue to discharge as long as the stimulus is applied- tonic receptors.
Others adapt and stop action potentials- phasic receptors.
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Sensory system

8. Somatic sensation. Skin has several different receptors.
Encapsulated and expanded tips sense touch or pressure.
Meissner’s and Pacinian corpuscles are rapidly adapting. Merkel’s disks and Ruffini endings are slowly adapting.
Naked nerve endings can sense all cutaneous sensations.
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Sensory system

9. Sensory modalities. Cutaneous senses are: Crude touch. Fine touch.
Pressure.
Cold.
Warmth.
Sharp pain.
Vague pain.
Vibration.
Proprioception (sense of position and movement).
Itch and tickle.
Sexual sensation.
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Sensory system

10. Dermatome.
Axons from a dermatome enter the spinal cord through the respective dorsal root.
Lesions in the spinal cord result in loss of sensation of the dermatomes of the respective segments.
Lesions of nerves out side the spinal cord will result in loss of sensation in areas supplied by the damaged nerve.
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Sensory system

11. Transmission to Spinal Cord
Somatic sensory nerves have the cell body in the dorsal root ganglion.
Axons enter through dorsal root and synapse in the dorsal gray.
This results in local reflexes and transmission to higher centers.
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Sensory system

12. Sensory pathway.
Pain, temperature and crude touch- cross in the same segment and ascend in the spinothalamic tract.
Fine touch and proprioception ascend in the dorsal columns in the same side without synapse.
They synapse in the nuclei gracilis and cuneatus in Medulla and cross over and ascend in medial leminiscus.
All second order neurons synapse in thalamus.
Third order neuron synapses in the sensory cortex- sensory area I.
Projections from SI and thalamus go to Sensory area II in the superior wall of the Sylvian fissure- head in inferior end and leg in the bottom.
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Sensory system

13. Transmission in the spinal cord.
Sensory fibers in the spinal cord are arranges in layers.
Spinothalamic fibers from lower parts are outer most and the upper fibers are placed inner layer as they cross over in the anterior commisure.
In the dorsal columns, fibers from lower parts are most medial
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Sensory system

14. Sensory pathway in Spinal Cord
Sacral segments carrry sensation of only a small area.
Lumbar segments conduct more.
The upper cord shows the dorsal tract divided in to two to accommodate fibers coming in.
The gray matter also shows variation according to the amount of muscles innervated
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Sensory system

15. Sensory Decussation in Medulla
Nucleus gracillis and cuneatus contain the second order neurons of the dorsal columns.
Fibers arising in these nuclei pass anteriorly, cross over and form the medial leminiscus.
Fibers in spinothalamic fibers accompany the medial lemisiscus laterally.
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Sensory system

16. Higher Medulla.
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17. Upper Pons
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18. Lower Mid Brain
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19. Upper Mid Brain
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20. Thalamus All sensory fibers synapse in the thalamus.
Third order fibers pass from the thalamus to the cerebral cortex in the corona radiata.
Fibers ascending to and descending from the cortex pass lateral to thalamus and the caudate nucleus medial to lentiform nucleus.
This is known as internal capsule.
Sensory fibers occupy the genu of the internal capsule
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Sensory system

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Sensory system

22. Cerebrum
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23. Representation in cortex
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Sensory system

24. Cortical representation.
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25. Neural basis of somatic sensation.
Localization: depends on the area stimulated in the cortex.
Stimulation of points in post central gyrus projects specific sensation to particular cutaneous area.
Stimulation by anything anywhere in the pathway provokes the same sensation as that comes from the receptor site.
Coding sensory information depends on specificity of receptor and the cells stimulated in cortex.
Gate control theory- limiting sensory input by gates in spinal cord.
Transmission of sensation in spinal cord is limited by:
Collaterals from afferents.
Descending tracts from brain stem.
Circulating chemicals- endorphins
Cortical activity can modify perception of sensation.
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Sensory system

26. Lesions of somato-sensory cortex
Critical tactile sensation is lost.
Slight crude touch returns- thalamus has slight discrimination of touch.
Little effect on sense of pain and moderate effect on temperature.
Thalamus and brain stem have significant role in perception of these sensations.
Ablation of SII causes defects in learning based on tactile discrimination.
SII is concerned with further elaboration of sensory data.
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Sensory system

27. Characteristics of sensation.
Sensations conducted in the dorsal columns and antero-lateral systems differ in many aspects.
Dorsal columns:
Velocity of transmission- very fast.
Degree of spatial localization- best.
Gradation of intensity- good.
Ability to transmit rapidly changing and repetitive signals- good.
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Sensory system

28. Pain.
Sensory modalities generate emotional responses in addition to several other functions.
Pain has ‘built-in’ unpleasant effect
But it is important for protective function.
Several different signals are perceived as pain.
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Sensory system

29. Pain Receptors.
Naked nerve endings, found in almost every tissue – superficial layers of skin, periosteum, arterial walls and joint surfaces.
Not in brain tissue.
Fast pain: bright, sharp, localized sensation. It is conducted by Small Aδ fibers at m/s.
Slow pain: dull, intense, diffuse, and unpleasant sensation. It is conducted by unmyelinated C fibers at m/s.
Adequate stimulus- mechanical, thermal [45 °C, the temperature of tissue damage] and chemical energy.
Fast pain is elicited by mechanical and thermal stimuli and slow pain by all three.
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Sensory system

30. About pain sensation. Receptors don't adapt- especially for slow pain.
Pain impulses can be inhibited in dorsal horn by collaterals from touch fibers.
Brain’s opiate system – endophin and encephalins inhibit at spinal and central levels.
At the spinal level pain evokes withdrawal and at cortical level avoidance reactions.
Pain also activates mediofrontal cortex, insular cortex, cerebellum, and reticular system in brain stem.
Effects of these are complex- could be emotional and autonomic.
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Sensory system

31. Post injury and neuropathic pain
After moderate or severe injury, pain persists even after the healing of the injury.
Stimuli to injured area that usually cause mild pain evoke severe pain - hyperalgesia.
Other stimuli such as touch can cause pain [alldynia].
If the nerves to that area are damaged, the pain may persist and may become excruciating- neuropathic pain.
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Sensory system

32. Deep pain.
Poorly localized, nauseating and associated with sweating and changes in blood pressure.
In periosteum, ligaments, muscles, etc.
This initiates reflex contraction of nearby muscles.
Muscle spasm causes ischaemia, pain and more spasm- vicious circle.
Ischaemic pain - caused by substance P in skeletal and cardiac muscles. [uteress]
Colicky pain in intestines, biliary tract and urinary tract is associated with strong contractions.
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Sensory system

33. Visceral Pain.
Poorly localized, unpleasant, associated with nausea and autonomic symptoms and signs.
Viscera has few temperature and touch receptors. Pain fibers are also less than in skin. They reach CNS by autonomic afferents.
In the spinal cord, they travel in spinothalamic tract.
Cortical areas are inter mixed with somatic representational areas.
Stimulation: hollow viscera are sensitive to distension. When inflamed, miner stimuli causes severe pain.
Visceral pain stimulates contraction of nearby skeletal muscle reflexly- rigidity of abdominal wall in peritonitis.
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Sensory system

34. Referred pain:
Pain sensation originating in viscera is felt in somatic structure that may be in considerable distance away.
Such pain is referred to the somatic structure.
Pain seems to spread [radiate] from local to distant site when it is both local and referred.
Visceral pain stimulates contraction of nearby skeletal muscle reflexly- rigidity of abdominal wall in peritonitis.
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Sensory system

35. Surface areas of referred pain.
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36. Dermatomal rule.
Convergence: cutaneous and visaral afferents converge on same second order neuron.
Facilitation: visceral afferents send colateral that facilitates transmission of somatic sensation.
Somatic anesthesia- convergence no effect, facilitation reduces.
Usually pain is referred to structures from the same dermatomal segment.
Role of experience: pain can be referred to sits where pain was experienced- as in surgical scars.
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Sensory system