Lecture Notes: Sensory Receptors and General Sense (16.1–16.2)

Overview: The Region Sensory Nervous System and Receptors

The region sensory nervous system includes structures inside and outside the body that detect stimuli. The eye is described as having a main layer of sensitive cells (retina) and additional layers including muscles and accessory tissues.
We will analyze sensor heads (receptors) to understand their anatomy and physiology and what receptors do: convert one form of energy into another and generate electrical signals.

Energy Forms and Receptor Transduction

Light striking the eye: light is electromagnetic energy; it is transduced into electrical signals (receptor potentials) in the retina and then conveyed to the visual cortex.
Form of energy: light is electromagnetic energy; conversion happens in the retina (visual transduction).
Sound waves: mechanical energy; displacement of the tympanic membrane and inner-ear structures transduces this into electrical signals that travel as nerve impulses to the temporal lobe.
Receptors convert energy forms to electrical signals (nerve impulses) to be interpreted by the brain.
Receptor potentials are small local electrical changes, not large signals. They are measured in the unit of microvolts (µV).
Receptor potential refresh: Neuronal membranes generate electrical signals via movement of ions across membranes governed by electrochemical gradients.

Neurons, Membranes, and Ion Gradients

Resting membrane potential: the inside of a neuron is negatively charged relative to the outside due to unequal ion distribution (outside more positive than inside).
Electrochemical gradient maintains ion movement through membrane channels.
The sodium–potassium ATPase (Na⁺/K⁺-ATPase) maintains this gradient by actively pumping ions:
- 3\,\mathrm{Na}^+\text{ (out)} and 2\,\mathrm{K}^+\text{ (in)} per ATP hydrolyzed.
When a neuron is stimulated, Na⁺ channels open and Na⁺ influx occurs, changing the membrane potential — depolarization.
This depolarization constitutes the receptor potential, a local change in the membrane potential triggered by sensory stimulation (e.g., light entering retina).

From Receptors to Synapses: Neurotransmission

The axon terminal transmits the electrical impulse to another cell across a synapse.
Neurotransmitters (chemical signals) are stored in vesicles in the presynaptic neuron.
When an action potential reaches the axon terminal, neurotransmitters are released by exocytosis into the synaptic cleft.
Neurotransmitters bind to receptor sites on the postsynaptic cell, opening ion channels and altering the postsynaptic membrane potential.
This process propagates the signal from the receptor to the brain (e.g., vision, hearing, smell).
The synaptic cleft is the space between presynaptic and postsynaptic neurons where neurotransmitters act.

Sensation, Perception, and Variability

Sensation: the subjective awareness of a stimulus.
Perception is the interpretation of those sensations; there can be variability in perception (e.g., pain sensitivity can vary among individuals and populations).
Sensory receptors are grouped by modality (the type of stimulus they respond to) and by location and duration.

Modality and the Labeled Line Code

Modality: the type of stimulus a receptor is sensitive to (e.g., vision for light, hearing for sound).
What is perceived is determined by modality; for example, even if you press on the eyeball, the modality remains light perception because retinal receptors are specialized for light.
Labeled line code: each nerve line (fiber) is dedicated to a single modality. If you stimulate a specific nerve line, you elicit the corresponding modality in the brain (e.g., stimulating an optic nerve line yields a visual percept).
Visual prosthetic example: If the eye is damaged but the optic nerve remains healthy, a retinal implant with a camera can bypass the damaged retina and stimulate the raw end of the optic nerve to evoke a basic form of vision.
- This approach relies on the labeled line code: stimulation of an optic-nerve fiber produces visual sensation.
Real-world example: Larry Hester (66) was blind from retinitis pigmentosa. An electronic stimulator implanted in his left eye was activated to provide a new form of vision by stimulating the healthy optic nerve; this is primitive vision but helps navigate environments and avoid obstacles.

Receptive Fields and Spatial Resolution

Receptive field: the area of the body (or retina) where stimulation affects the firing of a particular sensory neuron.
Two-point discrimination illustrates receptive field size:
- Fingertips have small receptive fields (high receptor density) and can distinguish two close points.
- Back of the shoulder has larger receptive fields (lower receptor density) and needs points to be farther apart to be perceived as two.
The degree of detail detected depends on receptor density and the number of sensory neurons innervating a region.
In the eye, central retina (fovea) has a high concentration of nerve cells (cones) for high acuity; periphery has a lower concentration.
The brain interprets the location and intensity of a stimulus based on which fibers fire, how many fibers fire, and how fast they fire.

Coding of Stimulus Intensity: Three Mechanisms

The brain distinguishes intensity via three mechanisms:
- Which fibers are active (fiber identity and modality).
- How many fibers are simultaneously firing (recruitment).
- The firing rate of active fibers (frequency coding).
Greater recruitment and higher firing frequency are interpreted as stronger stimuli.

Duration, Adaptation, and Receptor Types

Receptors differ in how they respond over time:
- Phasic receptors: respond quickly but adapt rapidly; they fade with sustained stimulation (e.g., wearing clothes and not feeling them after a while).
- Tonic (tonically responsive) receptors: adapt slowly or not at all; some sensations persist as long as the stimulus is present (e.g., ongoing pain from a wound).
Between phasic and tonic, receptors vary along a continuum of adaptation.
This topic covers the general properties of receptors (16.1) and sets the stage for 16.2 on general senses.

16.2 General Sense: Preview of Next Topics

Next topics focus on unique receptor types, mainly in the skin, with emphasis on pain.
Also to be discussed: chemical senses and laboratory activities.
A short break is mentioned in the source as part of the lecture structure.

Key Formulas and Quantitative Points

Resting membrane potential (typical): V_{rest} \approx -70\ \,\mathrm{mV}
Sodium–potassium ATPase stoichiometry: 3\,\mathrm{Na}^+\;\text{out},\;2\,\mathrm{K}^+\;\text{in} \;\text{per ATP hydrolyzed}
Receptor potential magnitude: \Delta V_{RP} \sim \text{a few to tens of }\mu\mathrm{V}
Conceptual relationships:
- Modality dictates the perceptual outcome via the labeled line code.
- Receptive fields determine spatial resolution and discrimination (e.g., two-point discrimination).

Connections to Foundational Principles and Real-World Relevance

The energy conversion chain (physical energy → electrical signals → brain representation) underpins all senses.
Understanding resting potentials, ion gradients, and Na⁺/K⁺-ATPase helps explain how neurons encode and propagate sensory information.
The labeled line concept explains why stimulating specific neural pathways yields particular percepts (vision, hearing, touch).
Visual prosthetics illustrate translational medicine: leveraging receptor modality properties to restore a basic form of function when primary organs are damaged.
Receptive field organization and receptor density explain why some body parts have higher tactile acuity than others.
Temporal dynamics (phasic vs tonic) explain why some sensations fade with time while others persist, shaping experiences like clothing feel vs. chronic pain.

Ethical and Philosophical Implications

Variability in pain perception across individuals and populations raises questions about pain assessment and treatment equity.
As with emerging neural prosthetics, ethical considerations include device safety, accessibility, and impact on quality of life.

References to Course Structure and Terminology

Concepts tied to sections 16.1 (general properties of receptors) and 16.2 (general sense).
The lecture previews topics on skins receptors, pain mechanisms, and chemical senses, followed by a lab session.