Sense Organs and Sensory Receptor Cells

• Sense organs are specialized anatomical structures that receive particular kinds of stimuli.
• Receptor cells within sense organs contain membrane-bound proteins that receive specific stimuli.
• Sensory transduction is the process of turning stimulus energy into an electrical signal.
• Sensory cells can produce their own action potential or excite a nearby neuron using neurotransmitters.
• Sensory adaptation is the decrease in the frequency of action potentials in response to continuous, constant stimulation over time.

Phasic Receptor Cells

• Phasic receptor cells respond rapidly to a stimulus.
• Their response to a maintained stimulus decreases rapidly and stops, even if the stimulus is still present.
• They are good at signaling changes in stimulus.

Tonic Receptor Cells

• Tonic receptor cells respond slowly to a stimulus.
• Their response to a maintained stimulus decreases slowly and incompletely until the stimulus stops.

Classifying Sensory Receptor Cells by Sensory Modality

• Classical senses include hearing, taste, touch, smell, and vision.
• Some animals sense balance, temperature, pain, electrical fields, and magnetic fields, not limited to the five classical senses.

Classifying Sensory Receptor Cells by Stimulus Energy

• Sensory receptor cells respond to electromagnetic energy (light, electrical fields, magnetic fields).
• Sensory receptor cells respond to mechanical energy (sound, touch, balance).
• Sensory receptor cells respond to chemical energy (smell, taste).

Classifying Sensory Receptor Cells by Electrical Stimuli Transduction

• Ionotropic transduction occurs when the receptor molecule is an ion channel.
• The receptor molecule receives the signal and transduces it by changing the cell membrane's permeability to ions.
• Metabotropic transduction occurs when signal receptors receive the signal and activate intracellular signaling cascades via G protein-coupled receptors.

Classifying Sensory Receptor Cells by Location

• Exteroceptors respond to stimuli outside the body.
• Interoceptors respond to internal stimuli.
• Sensory cells can be separated by sensory modalities with focus on hearing, taste, touch, smell, and vision.
• Sensory receptor cells for touch and smell are neurons with axons that enter the CNS.
• Sensory receptors for light are specialized neurons that synapse on other neurons outside of the CNS.
• Sensory cells that detect sound and taste have no axons, but synaptically excite sensory neurons by releasing neurotransmitters.
• Insects, fish, and mammals have well understood sensory receptors.
• The central nervous system receives signals from sensory cells and integrates that input.
• Signals are received by particular parts of the brain from different types of sensory neurons.

Mechanoreceptors in Touch

• Mechanoreceptors are specialized cells that respond to mechanical stimuli, like touch and sound.
• Sensillum are sense organs found in insects.
• Many sensillum are mechanoreceptors (some are chemosensory).
• Bristle of sensillum is part of exoskeleton.
• Dendrites of the mechanoreceptor cell are attached to the inside of the bristle shaft at its base.
• Movement of the bristle opens stretch-activated ion channels in the dendrites, causing a depolarization of the cell and starting an action potential.
• The stronger the stimulus, the more action potentials.
• In mammals, the skin is the main sensory organ for touch.
• Dorsal root ganglion contains sensory neurons with cell bodies in the dorsal root ganglia adjacent to the spinal cord.
• They have stretch-activated ion channels (Piezo channels).
• These cells have four types of touch-sensitive endings in the skin.

Merkel Disc

• Formed by the association of a Merkel cell with a branched nerve ending.
• senses texture, shape, edges.
• Tonic receptors.

Meissner Corpuscles

• Has two to six sensory neuron endings.
• Touch sensitive.
• Phasic receptors

Ruffini Endings

• Pressure Sensitive
• Tonic receptors

Pacinian Corpuscles

• Located deep in the skin.
• Senses only vibration.
• This has an extremely phasic response, and only responds to rapid changes in mechanical pressure.
• This is encased in lamellae.

Hearing

• Animals use mechanoreceptors to sense sound with sound waves consisting of compression of air or water.
• Insects have a tympanal membrane that is displaced by sound waves.
• Mechanosensory cells are attached to the tympanum and are stimulated by its movement.
• Sensory organs evolved repeatedly in different groups of insects.
• Tympanal organs may occur at any of several locations on an insect’s body.
• Noctuid moths have a tympanic membrane that mainly sense ultrasonic frequencies (>20,000 Hz).
• Moths are preyed upon by echolocating bats, which produce ultrasonic cries.
• Have paired thoracic tympanic membranes each connected to a single sensory neuron.
• Vertebrates use hair cells to detect sound.
• Hair cells are epithelial cells with microvilli (sterocilia) arranged in a line from shortest to longest.
• Hair cells release neurotransmitter onto neurons, which conduct action potentials to the CNS.
• As microvilli are pushed further toward the longest microvillus, more neurotransmitter is released.
• Tetrapod ears have an external, middle, and inner ear.
• The external ear helps collect sound waves.
• The external ear consists of the pinna (not found in all vertebrates) and ear canal.
• The middle ear uses ossicles to transmit sound vibrations from the eardrum to the inner ear via the oval window.
• Ossicles include the malleus, incus, and stapes in mammals.
• Only the stapes are in reptiles, birds, and amphibians.
• The inner ear has a cochlea, a coiled tube containing fluid and hair cells in the organ of Corti.
• Hair cells sit on top of the basilar membrane, which separates the cochlea into upper and lower chambers.
• The oval window transmits sound, causing the basilar membrane to move, which pushes hair cells against the tectorial membrane causing hair cells to release neurotransmitters on neurons.
• High frequency sounds vibrate the stiffer and narrower beginning of the cochlea; low frequency sounds vibrate the wider end.
• The semicircular canals of the ears also help with balance.
• There are three semicircular canals that are arranged at roughly at right angles to each other.
• The superior duct allows forward and backward motion.
• The posterior duct allows shoulder to shoulder motion.
• The lateral duct allows left to right motion.
• Fluid-filled Canals detect angular movement in all directions in a region called the ampulla.
• Head movement causes fluid and microvilli on hair cells to move.
• Moving microvilli on hair cells causes the hair cells to release neurotransmitters.
• All jawed vertebrates have semicircular canals.