Physiology receptors and somatic sensations

1. Receptors and somatic
sensations..

3. • Sensation is a conscious perception of particular feeling caused by
stimulation of certain type of receptor by its adequate stimulus.
• Sensory Receptors-Specialized cells to detect specific stimuli

4. How does sensation occur:
a)Detection
occurs when environmental changes, such as pressure to the fingertips
or light to the eye, stimulate sensory receptors.
b)Sensation
occurs when nerve impulses arrive at the cerebral cortex of the brain.
c)Perception
occurs when the brain interprets the meaning of stimuli.

7. Signal transduction:
How is a stimulus converted into a neural signal?
The stimulus opens ion channels in the receptor membrane, either
directly or indirectly (through a second messenger).
In most cases, channel opening results in net influx of Na + into the
receptor, causing a Depolarization of the membrane.
In a few cases, the response to the stimulus is hyperpolarization when
Na+ channels are closed and K+ leaves the cell.
Sensory transduction converts stimuli into graded potentials. Such
changes in receptor membrane potential are known as the receptor
potential and the generator potential.

8. Different receptors can be excited in one of several ways to cause
receptor potentials:
(1) by mechanical deformation of the receptor, which stretches the
receptor membrane and opens ion channels;
(2) by application of a chemical to the membrane, which also opens ion
channels;
(3) By change of the temperature of the membrane, which alters the
permeability of the membrane;
(4) by the effects of electromagnetic radiation, such as light on a retinal
visual receptor,

9. differential sensitivities:
how do two types of sensory receptors detect different types of sensory stimuli?
each type of receptor is highly sensitive to one type of stimulus
for which it is designed and is almost nonresponsive to other types of sensory
stimuli.
Thus, the rods and cones of the eyes are highly responsive to light but are almost
completely nonresponsive to normal ranges of heat, cold, pressure on the eyeballs,
or chemical changes in the blood.
pain receptors in the skin are almost never stimulated by usual touch or pressure
stimuli but do become highly active the moment tactile stimuli become severe
enough to damage the tissues.

10. Modality of Sensation—The “Labeled Line”
Principle
Each receptor type is most sensitive to a particular type of stimulus.(This specificity of
nerve fibers for transmitting only one modality of sensation is called the labeled line
principle. )
The brain thus associates a signal coming from a specific group of receptors with a specific
modality.
pain, touch,sight, sound—is called a modality of sensation.
each nerve tract terminates at a specific point in the CNS
the type of sensation felt when a nerve fiber is stimulated is determined by the point in the
nervous system to which the fiber leads.
For instance, if a pain fiber is stimulated, the person perceives pain regardless of what type
of stimulus excites the fiber.
The stimulus can be electricity, overheating of the fiber, crushing of the fiber, or stimulation
of the pain nerve ending by damage to the tissue cells.
In all these instances, the person perceives pain.

11. Stimulus location:
each sensory receptor is most sensitive to stimulation of a specific area,
which defines the receptor’s receptive field.
When action potentials are elicited from a sensory neuron, the
neuron’s receptive field codes the stimulus location.
Sensory receptive fields vary in size and frequently overlap.
Convergence of inputs onto a single sensory neuron enhances that
neuron’s sensitivity, but reduces its spatial resolution.

12. convergence

13. The size of neuronal receptive fields representing a given area
determines our capacity to discriminate stimuli in this area.

14. • Lateral inhibition enhances the contrast between the stimulus and its
surrounding, facilitating its perception and localization.
• Sensory neuronal receptive fields are orderly organized in cortical sensory
areas to form topographical maps
• The location of a stimulus is coded according to which group of neurons is
active.
• Auditory and olfactory information is the exception to the topographical
localization rule.
• For these sensory modalities, the brain uses the timing difference in
receptor activation to compute the source location of sounds or odors.

15. • Stimulus intensity is coded by:
1) the number of receptors activated (population coding from low-
threshold receptors to high-threshold ones.
2) the frequency of action potentials (frequency coding), following not
a linear but a power relationship

16. Classification:
Interoceptors
- detect stimuli inside body:- Include receptors for blood pressure,
blood volume, and blood pH
-Directly involved in homeostasis, regulated by negative feedback
Exteroceptors
- detect stimuli outside body- Include receptors for taste, smell, vision,
hearing, and equilibrium
-Function to inform CNS about environmental state

17. 1) Special sensations:
Vision, hearing, taste, smell and equilibrium
2) General sensations:
Arise from receptors distributed allover the body
Are classified into;
a) Somatic sensations: from somatic structures e.g. skin
b) Visceral sensations: from viscera
3) Organic senses: e.g. thirst, hunger and sexual desire

18. Special senses:
1. Vision-electromagnetic receptors
2. Hearing
3. Taste
chemoreceptors
4.Smell
5. Equilibrium

19. Somatic sensations:
Def:
• These sensations arise from somatic structures of all the body
i.e. skin and deep tissues-joints/muscle/viscera.
Types:
• They include according to their adequate stimulus:
1. Mechanoceptive sensations: 2 types;
• Tactile e.g. touch, pressure, and vibration sensations.
• Proprioceptive sensations e.g. sense of position and movement
2. Pain sensation/nocireceptors.
3. Thermal sensation; cold and warm

20. TACTILE SENSATIONS:
PRESSURE,TOUCH,VIBRATION.
RECEPTORS IN SKIN:MEISSNER CORPUSCLES,HAIR END ORGANS,MERKEL
DISCS,RUFFINI CORPUSLES,PACINIAN CORPUSCLES AND FREE NERVE
ENDINGS.
THERMORECEPTORS:
Free dendritic endings in hypodermis.
Adaptation only between 20 and 40C
Nociceptors activated if T > 45°C

21. SOMATIC SENSES:
PRIMARY SENSORY NEURONES:FROM RECEPTORS TO SPINAL CORD OR
MEDULLA
SECONDARY SENSORY NEURONES:ALWAYS CROSS OVER IN CORD OR
MEDULLA TO THALAMUS
TERTIARY SENSORY NEURONES: SOMATOSENSORY CORTEX

22. • Pathways for somatic sensations:
(1) the dorsal column–medial lemniscal system
(2) the anterolateral system-(spinothalamic tract /ventrolateral
system).has two components:
.lateral spinothalamic tract-pain/temp
Anterior spinothalamic tract/ventral:crude touch/firm pressure.

23. • dorsal column–medial lemniscal system:
carries signals upward to the medulla mainly in
the dorsal columns of the cord.
signals synapse and cross to the opposite side in
the medulla, they continue upward through the
brain stem to the thalamus by way of the medial
lemniscus.
Composed of large, myelinated nerve fibers that
transmit signals to the brain at velocities of 30
to 110 m/sec
has a high degree of spatial orientation of the
nerve fibers with respect to their origin thus
sensory information that must be transmitted
rapidly and with temporal and spatial fidelity is
transmitted here.
• Anterolateral system:
Signals immediately after entering the spinal
cord from the dorsal spinal nerve roots, synapse
in the dorsal horns of the spinal gray matter,
then cross to the opposite side of the cord and
ascend through the anterior and lateral white
columns of the cord. They terminate at all levels
of the lower brain stem and in the thalamus
smaller myelinated fibers that transmit signals
at velocities ranging from a few meters per
second up to 40 m/sec.
much less spatial orientation
ability to transmit a broad spectrum of sensory
modalities— pain, warmth, cold, and crude
tactile sensations

26. TOUCH

27. TOUCH SENSATION:
Def.,
• Feeling produced by application of light mechanical pressure to the skin
Types:
They include 2 types :
1. Crude touch-Poorly localized touch sensation produced by touching
the skin with diffuse ill defined object e.g. a piece of cotton or the
touch of clothes.
2. Fine touch- Highly localized touch sensation produced by application
of a well localized object to the skin e.g. a tip of a pencil or a head of
of a pin or teeth of a comb

28. RIGHT SIDE
CRUDE TOUCH.
Receptors:
• Free nerve endings
• Hair end organs
Pathway:
• Ventral or anterior spinothalamic
tract

29. FINE TOUCH:
Receptors:
• Meissner's corpuscles
• Merkel's discs
Pathway:
• Dorsal column medial leminiscal
system or gracile and cuneate tracts

30. FINE TOUCH:
Types:
• It includes :
1. Tactile localization: is the ability to localize the point of touch
with eyes closed
2. Tactile discrimination : is the ability to perceive 2 points of
touch with eyes closed as 2 separate points of touch
3. Stereognosis: is the ability to recognize a familiar object e.g. key
with eyes closed

31. PRESSURE AND VIBRATION

32. Pressure:
• It is a feeling produced by the application of heavy mechanical stimuli to
the skin
Vibration :
• It is a feeling of rhythmic pressure changes produced by the rapid
repetitive stimulation of certain mechanoreceptors e.g. Pacinian and
Meissner's corpuscles
• It is tested by use of tuning fork
Pathway: both sensations are carried by Gracile and Cuneate tract

33. PROPRIOCEPTIVE SENSATION

34. Def :
• Feeling produced by stimulation of proprioceptors in skeletal muscle and
joints
• Or It is the conscious perception of the position and movements of the
different parts of the body, particularly the limbs and joints.
Types:
a) Static or sense of position
b) Dynamic or sense of movement of joints
Pathway: Dorsal column medial leminiscal system or Gracile and
Cuneate tract

35. • Proprioceptors are mechanoreceptors
involved in reflex actions.
– Help maintain muscle tone
– Muscle spindles increase the degree of muscle contraction
– Golgi tendon organs decrease the degree of muscle contraction
Result is proper muscle length and tension (tone)

37. PAIN:

38. Def :
• Unpleasant sensory and emotional experience associated with actual
tissue damage.
Significance:
• Pain is a protective mechanism for the body.
• It occurs whenever the tissues are damaged and it initiates protective
reflex for removing the injurious stimulus
Receptors:
• Free nerve endings
Pathway: lateral spinothalamic tract

40. all pain receptors are free nerve endings (Three categories-mechanical,
thermal and polymodal that respond to all besides chemical)
can be stimulated by:
mechanical (stretch)
thermal
chemical
bradykinin, serotonin, histamine, potassium ions, acids,
acetylcholine and proteolytic enzymes prostaglandins and substance
P enhance the sensitivity of pain endings but do not directly excite
them

41. pain receptors do not adapt to the stimulus
the rate of tissue damage is the cause of pain (most individuals feel pain
at 450 C)
extracts from damaged tissue cause pain when injected under the skin
bradykinin causes the most pain and may be the single agent most
responsible for causing the tissue damage type of pain
also the local increase in potassium ion concentration and action of
enzymes can contribute to pain

42. Fast pain is transmitted by type A fibers (velocity 6-30 m/sec).
Slow pain is transmitted by type C fibers (0.5 - 2 m/sec).
Fast pain fibers are transmitted in the neospinothalamic tract.
Slow pain fibers are transmitted in the paleospinothalamic tract.

45. Neospinothalamic tract:
• On entering the cord, pain fibers may travel up
or down 1-3 segments and terminate on
neurons in the dorsal horn.
• 2nd neuron crosses immediately to the opposite
side and passes to the brain in the anterolateral
columns.
• Some neurons terminate in the reticular
substance but most go all the way to the
ventrobasal complex of the thalamus.
• 3rd order neurons go to the cortex.
• Fast-sharp pain can be localized well.
• However, fast pain fibers must be stimulated
with other tactile receptors for the pain to be
highly localized.
Paleospinothalamic tract:
o Type C pain fibers terminate in laminae II and III of
the spinal cord and make one or two local
connections before giving rise to 2nd order neurons
which cross immediately and pass to the brain in
the anterolateral columns.
o Only 10 to 25 % of the fibers terminate in the
thalamus.
o Most terminate diffusely in the:
o reticular nuclei of the medulla, pons and
mesencephalon
o tectal area of the mesencephalon
o periaqueductal gray region.
• lower terminations important to appreciate the
suffering type of pain
• from the lower reticular areas of the brain
stem neurons project to the intralaminar nuclei
of the thalamus, hypothalamus and other basal
brain regions
• poor localization of slow pain, often to just the
affected limb or part of the body

47. Appreciation of pain.
• Removal of the somatic sensory areas of the cortex does not destroy
the ability to perceive pain.
• Pain impulses to lower areas can cause conscious perception of pain.
• Therefore, cortex probably important for determining the quality of
pain.
• Stimulation of the reticular areas of the brain stem and intralaminar
nuclei of thalamus (where pain fibers terminate) causes widespread
arousal of the nervous system.

48. Analgesia System of the Brain
and Spinal Cord
• The brain has the capability to suppress pain fibers.
• Periaqueductal gray area neurons send axons to the nucleus raphe
magnus and the nucleus paragigantocellularis.
• Raphe magnus and paragigantocellularis neurons send axons to the
dorsal horns of the spinal cord.
• These neurons activate a pain inhibitory complex in the spinal cord.
• Higher brain levels, the periventricular nuclei of the hypothalamus and
the medial forebrain bundle can activate the periaqueductal gray
region and suppress pain.

50. Pain Suppression Mechanism
• Nerve fibers in the periventricular nucleus and the periaqueductal gray
secrete enkephalin at their nerve endings.
• Nerve fibers from the raphe magnus secrete serotonin at their nerve
endings.
• The serotonin causes the local neurons to secrete enkephalin.
• Enkephalin is believed to cause both pre- and post-synaptic inhibition
of type C and type A pain fibers where they synapse in the dorsal
horns

51. Endogenous Opiate Systems
• Several “opiate-like” substances have been identified. All are breakdown
products of three large molecules; proopiomelanocortin, proenkephalin, and
prodynorphin
• The major opiate substances;  endorphin, met-enkephalin, leu-enkephalin,
dynorphin
• The enkephalins and dynorphin are found in the brain stem and spinal cord.
• The b-endorphin is found in the hypothalamus and the pituitary.
Functions:
• pain suppression during times of stress
• an important part of an organism’s response to an emergency is a reduction
in the responsiveness to pain
• effective in defense, predation, dominance and adaptation to
environmental challenges

52. • 3 centers located in the spinal cord that influence perception of pain:
Substantia gelatinosa in the dorsal horn:responsible for passing on info
which can be interpreted as pain.its the gate that prevents the brain
from receiving too much info too quickly
Dorsal column fibers.
Central transmission cells.

53. Pain and Tactile Fibers
• Smaller unmyelinated A delta and C sense pain such as sharp,burning and
aching feeling
• Large myelinated A beta carry touch,heat,cold and pressure
• A beta is faster and have priority which blocks out the pain message to the
brain and closes gate
• Stimulation of large type Ab sensory fibers from peripheral tactile receptors
can depress the transmission of pain signals, “the gate control hypothesis”.
• Mechanism is a type of lateral inhibition of the pain fiber by the sensory
fiber.
• Mechanism of action of massage, liniments, electrical stimulation of the
skin

54. • Stimulate large diameter fibers A fibers inhibits the transmission of
pain(closes the gate)
• Stimulate small diameter fibers(c fibers of pain(opens the gate)
• When gate is closed signals from small diameter pain fibers don’t
excite the dorsal horn transmission.
• When gate is open pain is felt.
• Gating system is affected by descending tracts from the
cortex(emotional factors)
• Also influenced by amount of activity in pain fibers/other peripheral
fibers.

55. Gate can be closed by:
• Physical pain-analgesic remedies
• Emotional pain-”good mood”
• Behaviour factors-conc on other stuff other than injury
• Relaxation and contentment
• activity
• Counter stimulation

58. Gate Control Theory of pain suppression

59. Refered and visceral pain:
o Pain from an internal organ that is perceived to originate from a distant area
of the skin.
o Mechanism is thought to be intermingling of second order neurons from the
skin and the viscera.
o Viscera have few sensory fibers except for pain fibers.
o Highly localized damage to an organ may result in little pain, widespread
damage can lead to severe pain.
• localized to the dermatome of embryological origin
• heart localized to the neck, shoulder and arm
• stomach localized to the above the umbilicus
• colon localized to below the umbilicus

60. Pain in organs is poorly localized.
May be displaced if Multiple 1°
sensory neurons converge on
single ascending tract.

61. Temperature:

62. o many more cold receptors than warm receptors
o density of cold receptors varies
o highest on the lips, lowest on the trunk
o freezing cold and burning hot are the same sensation because of stimulation of
pain receptors
o Cold receptors respond from 7 to 44o C with the peak response at 25o C.
o Warm receptors respond from 30 to 49o C with the peak response at 44o C.
o The relative degree of stimulation of the receptors determines the temperature
sensation.
o Thermal receptors adapt to the stimulus but not completely.
o Cold or warm is thought to change the metabolic rate of the receptor.
o This changes the rate of intracellular reactions.