Sensory Physiology and Receptor Characteristics

How the Brain Encodes Stimuli

Once a receptor is activated, the nervous system must communicate the "what, where, and how much" of the stimulus to the brain. It does this by encoding three fundamental properties:

1. Location: The brain determines where a stimulus is occurring based on which specific receptors within a receptive field are activated.
2. Intensity: To signal how strong a stimulus is, the nervous system uses two methods: increasing the number of receptors activated and increasing the frequency of action potentials (the electrical impulses) generated by those receptors.
3. Duration: The length of time a stimulus lasts is coded by how long the action potentials are sustained.

However, not all receptors respond to a stimulus indefinitely. Some exhibit adaptation, a process where they cease to respond to a constant, unchanging stimulus over time. This allows the brain to ignore "background noise"—like the feeling of clothes against your skin—and focus on new, changing information.

Transduction: Converting Stimuli to Signals

Transduction is the core mechanism of sensation: the conversion of a physical or chemical stimulus into an electrical signal. This process varies significantly depending on the sensory system.

The Gustatory (Taste) System

Taste is mediated by taste buds containing chemoreceptors. The process of signal transduction in the taste system follows two distinct pathways depending on the chemical nature of the stimulus:

• G-Protein Coupled Receptor (GPCR) Pathway: For sweet, umami, or bitter tastes, specific ligands bind to receptors such as the Gustducin GPCR. This binding triggers intracellular signaling pathways.
• Ion Channel Modulation: For salty or sour tastes, the stimulus directly affects ion concentrations. Saltiness is often triggered by the influx of $Na^+$ ions, while sourness is triggered by $H^+$ ions.

Regardless of the pathway, the end goal is the same: the change in membrane potential causes an increase in cytoplasmic $Ca^{2+}$ levels. This calcium influx triggers the release of neurotransmitters (like serotonin) or ATP, which causes the primary gustatory neurons to fire action potentials toward the brain.

The Olfactory and Auditory Systems

While taste relies on chemical binding, other systems use different mechanical or chemical triggers:

• Olfactory System: Uses chemoreceptors to detect airborne chemicals.
• Auditory System: Uses mechanoreceptors in the cochlea to detect sound waves. Sound travels from the ear canal through the tympanic membrane and ossicles (malleus, incus, stapes) to the oval window, eventually moving fluid within the cochlea to trigger hair cells.
• Visual System: Uses photoreceptors (rods and cones) in the retina to detect light. In rods, light causes a cascade involving the protein transducin, which leads to a decrease in cGMP and the closing of sodium channels, ultimately hyperpolarizing the cell.