Sensory System and Sensory Receptors Study Notes

Overview of the Sensory System

The sensory system is an integral part of the nervous system responsible for collecting information from all parts of the body and the surrounding environment. This information is transmitted to the Central Nervous System (CNS) for processing, which results in changes to body activity, behavior, or reactions.

Ionic Basis of Membrane Potentials

Resting Membrane Potential (RMP)

At rest, the cell membrane is more permeable to K+K+ than to Na+Na+. While Na+Na+ can permeate with difficulty, it is effectively excluded by the Na+/K+Na+/K+ pump. Cl−Cl- is kept out due to the negative charges of impermeable anions. K+K+ diffuses out along its concentration gradient, creating a diffusion potential.

The magnitude of the potential is determined by the Nernst Equation:

Ek=RTZF×ln[K+]o[K+]iEk = \frac{RT}{ZF} \times \text{ln} \frac{[K+]_o}{[K+]_i}

Ek=61×log4140=−94 mVEk = 61 \times \text{log} \frac{4}{140} = -94 mV

The Action Potential (AP)

Upon stimulation, membrane permeability to ions changes:

  1. Resting Phase: Voltage-gated Na+Na+ and K+K+ channels are closed.
  2. Depolarization: Voltage-gated Na+Na+ channels open; K+K+ channels remain closed.
  3. Repolarization: Na+Na+ channels close while K+K+ channels open.
  4. Hyper-polarization: K+K+ channels remain open for an extended period.

Classification of Sensory Receptors

Sensory receptors are specialized structures that convert stimulus energy into electrical energy (action potentials).

  • Primary Receptors: Formed by peripheral nerve endings of a neuron.
  • Secondary Receptors: Specialized non-neural cells that generate action potentials in a neuron.

Categorization by Stimulus

  • Source of Stimulus:
    • Exteroreceptors (e.g., eyes, nose, ears, cutaneous).
    • Enteroreceptors (Proprioceptors and visceral receptors).
  • Anatomical Area: Special, Somatic, and Visceral sensory receptors.
  • Stimulating Agents:
    • Mechanoreceptors: Skin tactile (Free nerve endings, RuffinisRuffinis end organs, MerkelsMerkels discs, PacinianPacinian corpuscles, MeissnersMeissners corpuscles, hair end-organs), hearing (hair cells), equilibrium (hair cells), and baroreceptors.
    • Thermoreceptors: Cold and warm receptors.
    • Nociceptors: Pain detected via free nerve endings.
    • Photoreceptors: Rods and cones (electromagnetic).
    • Chemoreceptors: Taste, smell, PO2PO_2, PCO2PCO_2, and osmolarity.

Receptor Activation and Transduction

Receptors are adapted to respond to an adequate stimulus, which is the specific form of energy (e.g., light for rods and cones) to which they are most sensitive at a low threshold. Transduction occurs when stimulus energy alters the RMP to cause a receptor potential (generator potential).

Activation Mechanisms

  • Mechanoreceptors: Mechanical deformation stretches the membrane, opening mechanically gated ionic channels.
  • Chemoreceptors: Chemicals open ligand-gated channels.
  • Thermoreceptors: Temperature changes alter membrane properties and ionic permeability.
  • Photoreceptors: Electromagnetic waves change membrane characteristics directly or indirectly.

Local depolarization must reach an adequate magnitude (e.g., 10 mV10 mV for the PacinianPacinian corpuscle) to produce a conducted action potential.

Stimulus Strength and Adaptation

Stimulus Intensity

As stimulus pressure increases, the magnitude of the generator potential (GP) increases. The frequency of the resulting action potentials is proportional to the amplitude of the receptor potential.

Receptor Adaptation

Adaptation is the decrease in action potential frequency during a maintained, constant stimulus.

  • Rapidly Adapting Receptors: Suited for recording alterations in stimuli with high sensitivity (e.g., PacinianPacinian corpuscle, hair receptor).
  • Slowly Adapting Receptors: Detect long-term conditions (e.g., muscle spindle, joint capsule receptors).
  • Intermediate Adaptation: Temperature and light receptors.