Mechanoreceptors Classification

Mechanoreceptors

• Mechanoreceptors react to damaging stimuli, resulting in mechanical forces, including blood pressure, pain, extreme cold or heat, excessive pressure, stretching, and vibrations.
• They also respond to inflammatory chemicals.

Classification of Receptors - By Modality

• There are four classes of mechanoreceptors:
  • Tactile receptors
  • Proprioceptors
  • Baroreceptors
  • Organ of Corti and vestibular system hair cells

Tactile Receptors

• There are six types of tactile receptors in the skin that detect innocuous stimuli:
  • Meissner's corpuscles
  • Merkel's disks
  • Free nerve endings
  • Pacinian corpuscles
  • Ruffini corpuscles
  • C-fiber LTM (low-threshold mechanoreceptors)

Proprioceptors

• Proprioceptors are specialized mechanoreceptors that are usually present within muscle spindles, Golgi tendons, and joints.
• They report muscle length and movement information critical for motor control, limb position, and proprioception.

Baroreceptors

• Baroreceptors detect pressure.
• Types include:
  • Carotid sinus and aortic baroreceptors
  • Alveolar stretch receptors
  • GI stretch receptors
  • Urinary bladder stretch receptors

Organ of Corti and Vestibular System Hair Cells

• Hair cells of the inner ear are specialized mechanosensory cells that convert mechanical stimuli provided by sound waves or head movement into electrical signals.

Checkpoint

• Taste (gustation) uses chemoreceptors.
• Pain (nociception) uses both chemoreceptors and mechanoreceptors.
• Smell (olfaction) uses chemoreceptors.
• Touch uses mechanoreceptors.
• Vibration uses mechanoreceptors.
• Vision uses neither.
• Oxygen levels are detected by chemoreceptors.
• Pressure (baroreception) uses mechanoreceptors.

General Properties of Receptors

• Sensory receptors display properties common to almost all receptor types:
  • Excitability
  • Adequate stimulus is just enough strength of stimulus to excite receptors for the production of graded potential.
  • Receptor protein specificity: Sensory receptors are optimally selective for a single type of stimulus.

Transduction Mechanism in Sensory Receptors

• The fundamental purpose of any sensory receptor is the conversion of stimulus energy into nerve signals.
• Sensory transduction is the conversion of one form of energy to another.
• Sensory signals begin as graded potentials that lead to the activation of an action potential or release of a neurotransmitter.

Hair Cells Transduction

• The deflection of the hair bundle in the direction of the tallest stereocilia increases the opening of mechanoelectrical transduction (MET) channels at the tips of the stereocilia, allowing an inflow of K+, resulting in a depolarizing receptor potential.

Salt and Sour Taste Transduction

• Direct activation of ion channels mediates mechanosensation, thermosensation, sour (acid) and salt taste.

Sensation and Perception

• Sensation is the process where sensory receptors receive information from both the internal and external environment and encode the information for transmission to various areas of the nervous system.
• Perception refers to the process where the CNS receives and interprets the sensations based on the present state of the internal and external environment, and the memory of similar situations.

Stimulus Location, Intensity, and Duration

• Stimulus location is coded by the neuron's receptive field.
• Stimulus intensity is coded by the number of receptors activated (population coding) and the frequency of action potentials (frequency coding).
• Duration refers to how long the stimulus lasts; if a stimulus is prolonged, firing of the neuron gets slower over time (sensory adaptation).