General Sensory Physiology

Sensory Receptors - The Body's Detectives

• Sensation: Stimulus awareness. Perception: CNS interpretation.
• Sensory Unit: Afferent neuron + receptors.
• Adequate Stimulus: Most effective stimulus.
• Muller's Law: Sensation by receptor path.

  ⭐ Muller's Law: sensation evoked is that for which receptor is specialized, regardless of stimulus.

• Classification by Modality:
  • Mechanoreceptors (touch, pressure)
  • Thermoreceptors (temperature)
  • Nociceptors (pain)
  • Photoreceptors (light)
  • Chemoreceptors (chemicals)
• Classification by Location:
  • Exteroceptors (external stimuli)
  • Interoceptors (internal stimuli)
  • Proprioceptors (body position)
• Key Receptors & Characteristics:

  Type	Modality	Adequate Stimulus	Example(s)
  Free Nerve Endings	Multiple	Pain, Temp, Touch	Skin, Cornea
  Meissner's Corpuscle	Mechanoreceptor	Light Touch, Low Freq Vib.	Glabrous Skin
  Pacinian Corpuscle	Mechanoreceptor	Deep Pressure, High Freq Vib.	Dermis, Joints
  Ruffini's Ending	Mechanoreceptor	Stretch, Sust. Pressure	Dermis, Joint Caps.
  Krause's End Bulb	Thermoreceptor	Cold	Mucous Memb., Skin
  Muscle Spindle	Proprioceptor	Muscle Stretch	Skeletal Muscles
  Golgi Tendon Organ	Proprioceptor	Muscle Tension	Tendons

Sensory Transduction & Adaptation - Signal Conversion Crew

• Sensory Transduction: Conversion of stimulus energy into an electrical signal.
  • Process: Stimulus energy → Change in receptor membrane ionic permeability → Receptor Potential (RP).

• Receptor Potential (Generator Potential):
  • Characteristics: Graded, local, summable, non-propagated. Does NOT obey all-or-none law.
  • ↑ RP amplitude → ↑ Action Potential frequency.
• Adaptation: Decreased receptor response to a continuous, unchanging stimulus.
  • Significance: Prevents sensory overload, allows focus on new/changing stimuli.
  • Tonic (Slowly Adapting): Respond for duration of stimulus (e.g., Muscle spindles, baroreceptors, nociceptors).
  • Phasic (Rapidly Adapting): Respond quickly at onset/offset of stimulus (e.g., Pacinian corpuscles, Meissner's corpuscles, hair receptors). 📌 Phasic is Fast adapting.

  ⭐ Phasic receptors are excellent at detecting changes or movement of a stimulus.

• Mechanisms of Adaptation:
  • Pacinian corpuscle: Redistribution of viscous fluid in the capsule.
  • Photoreceptors: Change in concentration of light-sensitive photopigments (e.g., rhodopsin bleaching).

Awaiting image generation for this query.

Sensory Coding - Interpreting the Message

• Modality (What): Labeled Line Principle: Specific neural pathways for each sense.
• Intensity (How Strong):
  • Frequency Coding: ↑ stimulus → ↑ AP frequency.
  • Recruitment (Number Coding): ↑ stimulus → ↑ active sensory units.
• Psychophysical Laws:
  • Weber-Fechner: $R \propto \log S$ (Sensation vs. log stimulus).
  • Stevens' Power Law: $R = k S^A$.
• Duration (How Long): Duration of AP firing; receptor adaptation (phasic/tonic).
• Location (Where):
  • Receptive Field: Neuron's sensory area. Size inversely related to acuity (fingertips small, back large). Overlap helps.
  • Two-Point Discrimination: Min. distance for 2 points. Fingertips: 2-4 mm; Back: ~40 mm.
  • Lateral Inhibition: Via inhibitory interneurons; sharpens contrast & spatial resolution.

    ⭐ Lateral inhibition is crucial for sharpening perception of stimulus location and edges. oka

Sensory Pathways - Highway to the Brain

• General Organization (3-Neuron Chain): Sensory signals travel via:
  • 1st Order Neuron: From receptor in periphery to CNS (spinal cord/brainstem).
  • 2nd Order Neuron: Within CNS; decussates (crosses midline), ascends to Thalamus.
  • 3rd Order Neuron: From Thalamus to primary Somatosensory Cortex.
• Major Ascending Pathways (Examples):
  • Dorsal Column-Medial Lemniscal (DCML): Transmits fine touch, vibration, proprioception.
  • Anterolateral System (Spinothalamic): Carries pain, temperature, crude touch.
• Thalamus: The major sensory relay station; processes and gates information ("gateway to the cortex").

  ⭐ All sensory information, with the exception of olfaction, passes through the thalamus before reaching the cerebral cortex.

• Somatosensory Cortex:
  • Location: Postcentral gyrus of parietal lobe.
  • Organization: Somatotopic (Sensory Homunculus map).
  • Plasticity: Cortical representation can change with experience.

High‑Yield Points - ⚡ Biggest Takeaways

• Receptor adaptation: Phasic (rapid changes), Tonic (sustained stimuli) is crucial for filtering information.
• Labeled line principle: Modality-specific pathways ensure accurate brain interpretation of sensations.
• Weber-Fechner law: Sensation intensity is proportional to the logarithm of stimulus intensity.
• Sensory transduction: Stimulus energy is converted into electrical signals (receptor potentials).
• Smaller receptive fields and lateral inhibition mechanisms enhance stimulus localization and acuity.
• Key stimulus attributes encoded: Modality, intensity, duration, and location.
• Stimulus intensity is coded by recruitment of more receptors and increased action potential frequency.