Chapter 14

Sensory receptors play a critical role in helping organisms learn about their environment as well as the state of their internal environment.
They receive and transform stimuli from various sources and types into electrochemical signals that the nervous system uses.

Sensory transduction occurs when a stimulus alters the cell membrane potential of a sensory neuron, leading to the production of an action potential.
These action potentials are transmitted to the central nervous system (CNS).
In the CNS, sensory information is integrated and processed, potentially leading to a motor response.
Sensation vs. Perception:
- Sensation: Activation of sensory receptor cells in response to a stimulus.
- Perception: The interpretation and meaningful organization of sensory stimuli into perceptions, which relies on sensations, although not all sensations are consciously perceived.

Receptors are specialized cells or structures that detect stimuli. There are three main receptor classifications:
1. Cell Type: Neurons that have either free nerve endings or encapsulated endings.
2. Position: Position relative to the stimulus (exteroceptors, interoceptors, proprioceptors).
3. Function: Based on how stimuli are transduced into changes in membrane potential (e.g. chemical, physical, electromagnetic).

Free Nerve Ending: Neuron with dendrites embedded in tissue for sensations like pain and temperature.
Encapsulated Ending: Neurons with their sensory nerve endings encapsulated for enhanced sensitivity (e.g., lamellated corpuscles responding to pressure).
Specialized Receptor Cell: Neurons with specific structures for interpreting distinct stimulus types (e.g., photoreceptors in the retina).

Exteroceptors:
- Located near stimuli in the external environment (e.g., skin receptors).
Interoceptors:
- Located internally, interpreting stimuli from internal organs (e.g., blood pressure receptors).
Proprioceptors:
- Situated near moving body parts, sensing tissue position (e.g., muscle receptors).

Receptors transduce stimuli of various types:
1. Chemical stimuli: Detected by chemoreceptors; respond to taste and smell.
2. Physical stimuli: Detected by mechanoreceptors; respond to pressure, vibration, sound, and body position (balance).
3. Electromagnetic radiation: Detected by photoreceptors in the sight.

Commonly acknowledged senses include taste, smell, touch, hearing, and sight.
Balance is often overlooked in discussions about the senses.
Touch can be further classified into submodalities based on mechanoreceptors, including pressure, vibration, stretch, and hair-follicle sensation.
General vs. Special Senses:
- General Senses: Distributed throughout the body (e.g., touch, proprioception).
- Special Senses: Associated with specific organs (e.g., eye for vision, ear for hearing).

Traditionally recognized tastes: sweet, salty, sour, bitter. Later research identified umami (savory) and a potential sixth taste for fats.
The tongue's surface is covered with papillae, which house taste buds composed of gustatory receptor cells.
Mechanism of Taste:
- Salty: Detection of sodium ions (Na+) leading to depolarization.
- Sour: Detection of hydrogen ions (H+) leading to depolarization and perceived acidity.
- Sweet: Triggered by sugars or sweeteners binding to specific G protein-coupled receptors.
- Bitter: Many bitter compounds affect cell signaling variably; link to protective mechanisms against toxins (e.g., bitter alkaloids).
- Umami: Associated with L-glutamate, linked to protein-rich food perception.

Olfactory receptors located in the olfactory epithelium of the nasal cavity, consisting of bipolar sensory neurons.
Odorant molecules bind to receptor proteins on olfactory dendrites, triggering graded membrane potentials.
Pathway: Axons from olfactory neurons reach the olfactory bulb and connect to various brain areas (e.g., limbic system).
Unique characteristics:
- Smell does not undergo thalamic processing before reaching the cerebral cortex.
- Strong connection to memory and emotion due to limbic system involvement.

Hearing involves sound wave transduction into neural signals by structures of the ear.
Major components:
- Auricle (Pinna): Channels sound waves into the ear canal.
- Middle Ear: Contains the ossicles (malleus, incus, stapes).
- Inner Ear (Cochlea): Contains the sensory neurons responsible for sound transduction via hair cells in the organ of Corti.
Neural Pathway: Sound waves cause fluid vibrations in the cochlea, leading to hair cell activation which transduces sound into neural signals.

The vestibular system in the inner ear maintains balance and spatial orientation through hair cells within the utricle, saccule, and semicircular canals.
Movement and position are signaled by bending of stereocilia in response to fluid movement, transmitting information to the brain to detect head positions and movements.

Comprises various sensations including pressure, vibration, temperature, pain, and proprioception.
Receptors: Mechanoreceptors, thermoreceptors (cold and warm), nociceptors for pain, and proprioceptors for body positioning.
Specialized components include Merkel cells (light vibration detection), Pacinian corpuscles (deep pressure), and Ruffini endings (skin stretch).

Sensory pathways can be ascending pathways carrying information to the brain, specifically through the spinal cord and brainstem.
Major pathways are the dorsal column system for touch/proprioception and the spinothalamic tract for pain/temperature.

Different cranial nerves relay specific sensory information directly to the brain (e.g., trigeminal nerve for facial sensations).
Gustation follows pathways through cranial nerves to the gustatory cortex for taste processing.

Information from sensory receptors is first processed at the primary sensory cortex, then moves to association areas for deeper processing.
Interaction across modalities allows for a seamless perceptual experience of the environment, integrating visual and sensory inputs with context and memory.

The motor functions primarily originate in the frontal lobe. The prefrontal cortex coordinates higher-level executive functions influencing motor planning and responses.
Lower motor neurons in the spinal cord carry out voluntary movements through the corticobulbar (to brain stem) and corticospinal (to spinal cord) tracts.

Reflex actions can occur without higher cognitive involvement, enabling quick reactions to stimuli through spinal or cranial reflex pathways.
Examples include withdrawal reflexes to harmful stimuli and stretch reflexes to maintain muscle length; both demonstrate inhibitory inter-neuron functions to ensure swift and effective responses.