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General Exteroreceptors

Neelam Maurya

General Exteroreceptors

GENERAL EXTERORECEPTORS 

  • A type of “Exteroreceptors”.
  • Provide information about external environment.
  • Like touch, pressure, temperature, light, sound, taste, & smell.
TYPES OF GENERAL EXTERORECEPTORS:
  • Cutaneous mechanoreceptors/Tactile/Touch receptors.
  • Subtypes:
  • Superficial receptors.
  • Deep receptors

Thermoreceptors.

  • Subtypes:
  • Warmth receptors.
  • Cold receptors

Nociceptors.

  • Subtypes: 
  • Thermal nociceptors.
  • Mechanical nociceptors.
  • Polymodal nociceptors.
  • Silent nociceptors.
TYPES OF SENSATIONS RELATED TO EXTERORECEPTORS:

1. Epicritic sensations:

  • Involve finer aspects of touch, pressure, & proprioception.
  • Mediated by encapsulated receptors.

Eg:

  • Fine-touch
  • Two-point discrimination.
  • Pallesthesia (Detection of vibration).
  • Stereognosis (Recognition of shape).
  • Topognosis (Recognition of surface texture).

2. Protopathic sensations:

  • Involve pain & temperature sense.
  • Mediated by receptors with free nerve endings.
  • Protopathic sensations are cruder than epicritic sensations
  • Have a protective function of warning of injury.
CUTANEOUS MECHANORECEPTORS/TACTILE/TOUCH RECEPTORS:
  • General exteroreceptors for epicritic senses.

Classifications of receptors:

1. Based on location (skin proximity).

  • Superficial receptors.
  • Deep receptors

1a. Superficial receptors:

  • Present in epidermis or papillary layer of dermis.
  • In glabrous (nonhairy) skin, receptors are,
  • Merkel’s disc (slowly adapting)
  • Meissner’s corpuscle (rapidly adapting).
  • ln hairy skin – Hair follicle receptors.

1b. Deep receptors:

  • Present in deeper dermis or in subcutaneous tissues.
  • Receptors are same in both hair & nonhairy skin

Eg:

  • Ruffini’s end organ (slowly adapting)
  • Pacinian corpuscle (Rapidly adapting).
2. Based on receptor adaptability:
  • Whether a tactile receptor senses pressure or vibration depends on whether receptor is fastly adapting or slowly adapting.
  • Touch, pressure, & vibration are different forms of same sensation.
  • Pressure is felt when force applied on skin is sufficient to reach deep receptors.
  • Touch is felt when force is insufficient to reach deep receptors.
  • Hence, detected by superficial receptors (Merkel’s disc & Meissner’s corpuscle).
  • Vibrations are rhythmic variations in pressure.
  • I.e. Rhymic variations of force that reaches deep receptors.

Divisions:

2a. Slowly adapting:

  • Examples include, “One each from superficial & deep cutaneous receptors”
  • Ruffini’s end organ – 
  • Meant to detect sustained pressure.
  • Useless for vibrations.
  • Merkel’s disk – 
  • Detect two-point discrimination.

2b. Rapidly adapting:

  • Examples, 
  • “One each from superficial & deep cutaneous receptors”
  • Pacinian corpuscle – 
  • Stops discharge in response to sustained pressure.
  • Useful to detect vibrations – I.e., when pressure fluctuates rapidly.
  • Meissner’s corpuscle – 
  • Detect surface texture.
  • Hence, Higher the rate of receptor adaptation —-> Greater is detectable vibration frequency.
3. Based on type of tactile sensations detected:
3a. Superficial sensations:
  • Generally touch
  • By Meissner’s corpuscle (detect surface texture i.e. rough or smooth)
  • By Merkel’s disc (detect two-point discrimination).

3b. Deep sensations:

  • Pressure (Deep touch) – Detected by Rufini organ.
  • Vibrations – Detected by Pacinian corpuscle.
SUMMARY: 
1. Superficial cutaneous receptors:
  • Detect touch (Superficial sensation)
  • Merkel’s disk – Slowly adapting & detect two-point discrimination.
  • Meissner’s corpuscle – Rapidly adapting & detect surface texture.

2. Deep cutaneous receptors:

  • Detect deep touch, pressure, & Vibration.
  • Ruffini’s end organ – Slowly adapting & detect sustained pressure/deep touch.
  • Pacinian corpuscle – Rapidly adapting & detect vibrations.

PAIN RECEPTORS/NOCICEPTORS:

  • General exteroreceptors for protopathic senses.
  • Receptors with free nerve endings.

Four types: 

  • Thermal nociceptor.
  • Mechanical nociceptors.
  • Polymodal nociceptors.
  • Silent nociceptors.

1a. Thermal nociceptors:

  • Detect extreme temperature,.
  • I.e. < 50C (cold nociceptors) or >450C (warm nociceptors).

1b. Mechanical nociceptors:

  • Respond to intense pressure.
  • Both thermal & mechanical nociceptors – free nerve endings of Aδ fibers.

1c. Polymodal nociceptors:

  • Are free nerve endings of C-fibers
  • Respond to high mechanical, chemical/thermal stimuli.

1d. Silent nociceptors:

  • Activated by inflammation.
  • Not by noxious stimuli.

TEMPERATURE RECEPTORS/THERMOCEPTORS:

Cold receptors:

  • Active between 5°-40°C (maximum sensitivity at 25°C)

Warmth receptors:

  • Active between 29°C – 45°C (maximum sensitivity at 45°C).

Relationship between pain & temperature receptors:

  • Temperature below 5°C stimulates cold nociceptors.
  • Temperature above 45°C stimulates heat nociceptors. 
  • Hence, temperature < 5 °C or > 45°C causes pain.
NERVE FIBERS FOR PAIN & TEMPERATURE:
  • Pain and temperature are carried by Aδ (type III) & C (type IV) sensory neurons.

For pain – 

  • Aδ (Type III) neuron – Fast pain. 
  • C (Type IV) neuron – Slow pain.

For temperature

  • Aδ (Type III) neuron – Cold.
  • C (Type IV) neuron – Both cold & warmth.
Exam Question
 

GENERAL EXTERORECEPTORS

  • Provide information about external environment, like touch, pressure, temperature, light, sound, taste, & smell.
TYPES OF GENERAL EXTERORECEPTORS:
  • Cutaneous mechanoreceptors, Thermoreceptors & Nociceptors.
SUBTYPES:
1a. Cutaneous mechanoreceptors/Tactile/Touch receptors –
  • Superficial & deep receptors.

1b. Thermoreceptors/temperature receptors:

  • Warmth receptors & Cold receptors.

1c. Nociceptor/Pain receptors:

  • Thermal nociceptors, Mechanical nociceptors, Polymodal nociceptors & Silent nociceptors.
TYPES OF SENSATIONS RELATED TO EXTERORECEPTORS:
  • Epicritic sensations & Protopathic sensations.

Protopathic sensations:

  • Involve pain & temperature sense.
  • Mediated by receptors with free nerve endings.
CUTANEOUS MECHANORECEPTORS/TACTILE/TOUCH RECEPTORS:

1a. Superficial receptors:

  • Present in epidermis or papillary layer of dermis.
  • In glabrous (nonhairy) skin, receptors are,
  • Merkel’s disc (slowly adapting).
  • Meissner’s corpuscle (rapidly adapting).
  • 1b. Deep receptors:
  • Includes Ruffini’s end organ (slowly adapting) & Pacinian corpuscle (Rapidly adapting).
Receptors and its adaptability:

1. Slowly adapting:

  • Ruffini’s end organ – 
  • Meant to detect sustained pressure.
  • Useless for vibrations.

2. Rapidly adapting:

  • Pacinian corpuscle – 
  • Useful to detect vibrations.
  • Meissner’s corpuscle – 
  • Detect surface texture.
Receptors & sensation detected:
1. Superficial sensations:
  • By Meissner’s corpuscle (detect surface texture i.e. rough or smooth)

2. Deep sensations:

  • Pressure (Deep touch) – Detected by Rufini organ.
  • Vibrations – Detected by Pacinian corpuscle.
SUMMARY: 
1. Superficial cutaneous receptors:
  • Merkel’s disk – 
  • Slowly adapting & detect two-point discrimination.
  • Meissner’s corpuscle – 
  • Rapidly adapting & detect surface texture.

2. Deep cutaneous receptors:

  • Ruffini’s end organ – 
  • Slowly adapting & detect sustained pressure/deep touch.
  • Pacinian corpuscle – 
  • Rapidly adapting & detect vibrations.
PAIN RECEPTORS/NOCICEPTORS:
  • These are receptors with free nerve endings.
  • Thermal nociceptors important subtype.
 TEMPERATURE RECEPTORS/THERMOCEPTORS:
  • Cold & warmth receptors.
NERVE FIBERS FOR PAIN & TEMPERATURE:
  • Pain and temperature are carried by Aδ (type III) & C (type IV) sensory neurons.

For temperature:

  • C (Type IV) neuron – Both cold & warmth.
Don’t Forget to Solve all the previous Year Question asked on General Exteroreceptors