VPB 121: Veterinary Physiology - II (2+1)

CLASSIFICATION OF SENSORY RECEPTORS

Sensory receptors are classified based on

• Function of the receptors
• Location of receptors
• Source of the stimulus
• Type of stimulus or modality
• Location of receptors

Function of the receptors

• Phasic receptors:  Produce action potential during part of stimulation hence they initiate response. It does not provide clue for duration of stimulus.
• Tonic receptors:  Generate action potential upon the application of stimulus and propagation of action potential goes on till the stimulus is removed. This gives information regarding the duration of stimulus. In this type, the possibility of weakening of the impulse irrespective of the maintenance of constant strength of stimulus due to prolonged exposure leading on to a phenomenon known as RECEPTOR ADAPTATION.

Location of receptors

Exteroceptors:  Detect stimuli from outside the body in animals’ micro or macro environment (change in temperature, air pressure). They can be divided into:

• Superficial cutaneous sensations: Touch, light superficial pressure, temperature, cold, warmth, and pain.
• Deep sensations:  Receptors in the muscle tendon and joints together known as kinesthetic sensation mediated through the muscle spindle receptor and Golgi tendon organ to sense position and movement.
  • Special Sensory receptors
  • Skin receptors.
• Interoceptors:  Detect stimuli within the body (Blood pressure, O2 uptake). They are,
  • Proprioceptors:  Convey information from the muscles, tendons, and joints.
  • Visceroceptors:  Carrying information from the viscera.
  • Specialized receptors:  Baroceptors, osmoceptors, chemoceptors, and volume receptors.
  • Baroceptors:  Present in carotid sinus and aortic arch to detect BP change.
  • Osmoreceptors:  Present in hypothalamus to detect osmotic change.
  • Chemoreceptors:  Sensitive to chemical change.  Present in tongue to detect taste and in nose to detect smell.  Hypothalamus central chemoreceptors detect pH change, blood glucose, and hormonal levels.  

Source of the stimulus

• Telereceptors:  Detect the stimuli at a distance (Photo and auditory receptors)

Stimulus or modality

• Photoreceptors:  Detect change in intensity of light mediating sense of vision.
• Thermoreceptors:  To detect change in temperature.
• Electroceptors and Magnetoceptors:  (Mostly in insects and birds).  They detect electrical and magnetic field respectively.
• Mechanoreceptors:  Involved in sensing touch, pain, hearing, balance (equilibrium).
• Chemoreceptors:  Detect chemical signals, sense of gustation, olfaction, blood O2, tension, and change in pH (internal environment).

Location of receptors

• Superficial cutaneous sensations: Touch, light superficial pressure, temperature, cold, warmth, and pain.
• Deep sensations:  Receptors in the muscle tendon and joints together known as kinesthetic sensation mediated through the muscle spindle receptor and Golgi tendon organ to sense position and movement.