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Senses

General Function of Sensory Receptors

  • Sensory Receptors: Specialized structures that provide information about both external and internal environments.

    • Respond specifically to different types of stimuli (e.g., light for eyes, sound for ears).

    • Transducer Function: They convert stimulus energy into electrical impulses.

    • Resting membrane potential is present among receptors.

    • Modalities such as modality gated channels respond specifically to their respective stimuli.

    • Convey action potentials to the central nervous system (CNS) for interpretation.

General Structure of Sensory Receptors

  • Sensory receptors send signals to the CNS via sensory neurons.

    • Receptive Field: Area denoting the distribution of sensory neuron endings.

      • Smaller receptive fields enable precise localization of stimuli.

Sensory Information Provided by Sensory Receptors

  • Sensation: Awareness of a specific stimulus; must reach the cerebral cortex to enter consciousness.

    • Not all stimuli result in conscious sensations; many are processed subconsciously.

    • Provide information regarding stimulus modality, location, intensity, and duration.

Sensory Receptor Classification

Categorization by Distribution

  • General Sense Receptors: Simple, widespread structures throughout the body.

    • Somatic Sensory Receptors: Tactile receptors in skin/mucous membranes and proprioceptors in joints/muscles.

    • Visceral Sensory Receptors: Located in internal organ walls, monitor conditions like stretch, chemicals, and pain.

  • Special Sense Receptors: Complex receptors in sense organs of the head.

    • Five primary senses: olfaction (smell), gustation (taste), vision, audition (hearing), and equilibrium (balance).

Categorization by Stimulus Origin

  • Exteroceptors: Detect stimuli from the external environment (skin, mucous membranes).

  • Interoceptors: Monitor internal organ stimuli (visceral sensory receptors).

  • Proprioceptors: Detect body and limb movements (muscles and joints).

Categorization by Modality of Stimulus

  • Five Types:

    1. Chemoreceptors: Detect chemicals in fluids (e.g., smell, oxygen levels).

    2. Thermoreceptors: Sense temperature changes.

    3. Photoreceptors: React to light (retina of the eye).

    4. Mechanoreceptors: Respond to physical distortion (touch, pressure).

    5. Nociceptors: Detect painful stimuli (can be somatic or visceral).

Tactile Receptors

  • Mechanoreceptors found abundantly in skin and mucous membranes.

    • Unencapsulated:

      • Free Nerve Endings: Simplest tactile receptors for pain and temperature.

      • Root Hair Plexuses: Detects hair displacement, located deeper in dermis.

      • Tactile Discs (Merkel Cells): Tonic receptors for light touch.

      • Various Encapsulated types:

      • End (Krause) bulbs: Detect pressure and low-frequency vibration

      • Lamellated (Pacinian) corpuscles: Detect deep pressure, coarse touch, high-frequency vibration

      • Bulbous (Ruffini) corpuscles: Detect deep pressure and skin distortion

      • Tactile (Meissner) corpuscles: Discriminative light touch—allow recognition of texture, shape

Referred Pain

  • Referred Pain: Misinterpretation of sensory signals from viscera as originating from skin or muscle.

Clinical View: Phantom Pain

  • Pain sensations from an amputated limb due to remaining sensory neuron pathways.

Overview of Sensory Systems

Smell (Olfaction)

Olfaction: sense of smell

  • detected odorants

Olfactory Cells: chemoreceptors and olfactory nerve goes directly to the cerebral cortex

Taste (Gustation)

  • Gustatory Cells: Chemoreceptors within taste buds that detect tastants.

  • Supporting cells: sustain gustatory cells

  • Basal cells: neural stem cells that replace gustatory cells

  • Types of Taste Buds:

  • Filiform: No taste buds, aids in food manipulation.

  • Fungiform: Few taste buds. Mushroom shaped

  • Foliate: Few taste buds during childhood only. Leaflike edges

  • Vallate: Largest and contain the most taste buds.

Gustatory Pathways

  • Cranial Nerves: Facial nerve (VII), Glossopharyngeal nerve (IX), Vagus nerve (X) contribute to taste sensation.

  • anterior parts of the tongue: sensory neurons part of facial

  • posterior two-thirds of the tongue: sensory neurons are part of the glossopharyngeal nerve

Basic Taste Sensations

  • Five categories identified: Sweet, Salt, Sour, Bitter, Umami.

  • molecules: sweet, bitter, umami

  • ions: salt and sour

Eye Structure

  • Lens: Changes shape for light focus on retina; conditions like hyperopia and myopia affect lens ability.

  • Sclera: White of eye. Protects and provides an attachment site

  • Cornea: no blood vessels and refracts light

  • vascular tunic: houses blood vessels, lymph vessels, intrinsic muscles

  • Choroid: capillaries nourish retina - extensive, posterior region

  • Ciliary muscles: brans of smooth muscle connected to lens

  • Pupil: opening of iris

  • Pigmented Layer: absorbs stray light to prevent scatter

  • Neural Layer: Houses photoreceptors and associated neurons. Receives light and converts it to nerve signals

  • Ora Serrata: jagged edge

  • Retina: cells of the neural layer

    • Bipolar cell layer: dendrites receive synaptic input from rods and cones

    • Photoreceptor cells: contains rods and cones

    • Ganglion cells: axons gather at the optic disc and form the optic nerve. capable of action potentials

  • Macula lutea: contains fovea cetralis - the highest proportion of cones and sharp vision

  • Peripheral retina: primarily rods and functions in low light

  • Rods: longer and narrow, more numerous - sensitive to dim lights

  • Cones: concentrated at fovea centralis and allow color vision

Clinical Conditions Related to the Eye

  • Cataracts: Opacities hindering clear vision.

  • Glaucoma: Increased intraocular pressure affecting vision.

  • Macular Degeneration: Vision loss primarily in the center visual field.

  • Detached Retina: outer pigments and inner neural layers separate

  • Accommodation: viewing objects closer then 20 feet

  • Presbyopia: age-related vision change - lens not spherical

  • Astigmatism: unequal curvatures

  • Myopia: concave lens

Visual Processing

  • Phototransduction: Process involving rods and cones in the retina to convert light into nerve signals.

  • Pathways: Signal travels from photoreceptors through bipolar and ganglion cells to the optic nerve, then to the brain.

  • Optic nerves: converge at the optic chiasm

  • Retina visual pathway: photoreceptors → bipolar cells → ganglion cells

Hearing and Balance

Conductive deafness: interference of wave transmission in external or middle ear

Sensorineural deafness: malfunction in inner ear or cochlear nerve

Hearing Mechanism

  • Cochlear Hair Cells: Stimulation through movement, sending signals through auditory pathways.

  • Perception of Sound: Through vibrations creating pressure waves, frequency correlates with pitch.

Equilibrium

  • Vestibular Apparatus: Responsible for equilibrium; includes semicircular ducts for rotation and maculae for static equilibrium.