Physiology of Special Senses and Membranes

Special Senses Sensory Physiology

• Sensory physiology describes how special senses like taste, smell, hearing, balance, and vision function.
• Specialized receptor proteins detect stimuli (e.g., chemicals, pressure, light).
• Receptor cells convert stimuli into changes in membrane potential (graded potentials).
• Signals travel to integrating centers (e.g., brain).
• Perception occurs at these centers, leading to sensory experience.

Membrane Physiology Recap

• Plasma membrane separates the body's fluid compartments.
• It exhibits selective permeability, controlling substance entry/exit.
• Structural components include glycocalyx (carbohydrates), lipid bilayer, proteins (integral & peripheral), cholesterol, glycolipids, and glycoproteins.
• Electrochemical gradients influence substance movement.

Membrane Transport

• Passive diffusion: Lipid-soluble molecules cross the membrane directly.
• Facilitated diffusion: Carrier proteins or channels aid transport.
• Active transport: Energy required to move substances against their concentration gradient.

Transport Proteins – Channels

• Channels are transmembrane proteins.
• Ligand-gated channels open in response to specific molecules.
• Voltage-gated channels open in response to changes in membrane potential.
• Mechanically-gated channels open in response to physical forces.

G Protein-Coupled Receptor Signal Transduction

• G protein-coupled receptors are linked to intracellular signaling pathways.
• A stimulus triggers a conformational change in the receptor, activating a G protein.
• The activated G protein activates an amplifier enzyme, modifying second messengers.
• Second messengers initiate downstream intracellular reactions.

Graded Potentials

• Occur in excitable and non-excitable cells.
• Brief, localized changes in membrane potential.
• Vary in magnitude and duration.
• Decays over distance.
• Can be depolarizing or hyperpolarizing.

Action Potentials

• Occur in specialized cells (neurons & some muscle cells).
• All-or-none response: Threshold must be reached for generation.
• Propagates rapidly along the membrane without decreasing strength.
• Involves voltage-gated ion channels (e.g., Na+ channels).

Reminder: Nervous System

• Afferent neurons transmit signals to the CNS.
• Sensory receptors at the peripheral end of afferent neurons.
• Efferent neurons carry signals from the CNS to effectors (muscles/glands).
• Interneurons connect neurons within the CNS.

The Special Senses

• Special senses are localized in the head region.
• Sensory receptors include distinct cells like photoreceptors (vision), hair cells (hearing/balance), olfactory cells (smell), and taste cells (taste).
• Touch is mediated by general receptors (modified nerves).

Sensory System Example (Visual System)

• Visual system components: Photoreceptors (rods & cones), accessory tissue (iris, cornea), optic nerve, and primary visual cortex.
• Sensory receptor cells are specialized to detect incoming stimuli.
• Sense organs comprise receptor cells and accessory tissues.
• Sensory systems include sense organs, afferent neurons, and CNS processing areas.

Overview of Sensory Reception

• Sensory reception involves stimulus reception, transduction (conversion to electrical signal), transmission to integrating centers, and perception.

Sensory Receptor Cells

• Specialized cells to detect specific stimuli (e.g., chemicals, light, pressure).
• Exhibit receptor proteins for stimulus transduction.
• Respond to specific stimuli and transmit signals to integrating centers.
• Classified by modality (e.g., photoreceptors, chemoreceptors, mechanoreceptors).

Sensory Receptor Proteins

• Ion channels, mechanically- or voltage-gated.
• G protein-coupled receptors.
• Receptor protein conformation changes initiate signals.

Sensory Receptor Cells (Continued)

• Stimulus triggers a receptor potential (graded potential).
• Receptor potentials can lead to action potentials in neurons.
• In non-neuronal cells, receptor potentials trigger neurotransmitter release.

Check Your Knowledge

• Neuronal and epithelial sensory receptors differ in signal transduction mechanisms.
• Sensory receptor cells are both neuronal and non-neuronal.
• Receptor potentials are graded potentials, not action potentials.
• Stimulation of epithelial receptors triggers an action potential at the trigger zone of an afferent neuron.