chp 16

Chapter 16: Anatomy and Physiology Lecture Outline

16.1 Function and Structure of Sensory Receptors

16.1a General Function of Sensory Receptors

  • Definition of Sensory Receptors: Special structures that provide information about both external and internal environments of the body.

  • Stimulus Response:

    • Receptors react to specific stimuli related to their function.

    • Different types of receptors are sensitive to different types of stimuli, such as:

      • Light energy for visual receptors in the eyes.

      • Sound energy for auditory receptors in the ears.

  • Transducers:

    • Convert various forms of stimulus energy (e.g., light, sound) into electrical energy signals that can be interpreted by the nervous system.

  • Resting Membrane Potential:

    • Receptors maintain a resting membrane potential, which can be altered by stimuli.

  • Modality Gated Channels:

    • Specialized channels in the receptor membranes that are activated by specific stimuli types.

  • Action Potentials:

    • Action potentials generated by the receptors are sent to the central nervous system (CNS) for interpretation.

16.1b General Structure of Sensory Receptors

  • Signal Conveyance:

    • Sensory receptors convey signals to the CNS through sensory neurons.

  • Receptive Field:

    • The geographic area over which a sensory receptor (or neuron) can detect stimuli.

    • Smaller receptive fields allow for more precise localization of stimulus.

  • Visual Example:

    • Refer to Figure 16.1 for comprehensive visual representation.

16.1c Sensory Information Provided by Sensory Receptors

  • Sensation:

    • The awareness generated by stimuli reaching the conscious brain.

    • For a sensation to occur, the signals must reach the cerebral cortex.

    • Not all stimuli lead to conscious sensations (e.g., blood pressure).

  • Important points to note:

    • Sensory information conveyed includes:

    • Modality (type of stimulus): Decoded through connections in the brain—e.g., visual signals via the optic nerve to the occipital lobe and auditory signals via the cochlear nerve to the temporal lobe.

    • Location of Stimulus: Determined by which neuron’s receptive field is activated. The postcentral gyrus contains a body map represented by a homunculus.

    • Intensity: Determined by the frequency of nerve signals. Stronger stimuli increase the number of neurons firing and the frequency of the firing.

    • Duration: (Receptor Adaptation)

    • Adaptation: A decrease in sensitivity to a continuous stimulus.

      • Tonic Receptors: Show limited adaptation; e.g., pain receptors, where constant stimulation can lead to persistent pain signals.

      • Phasic Receptors: Adapt quickly; e.g., receptors for pressure that stop firing unless the stimulus changes.

16.1d Sensory Receptor Classification

  • Broad Categories:

    • Sensory receptors can be classified based on:

    • Distribution:

      • General Sense Receptors: Simple structures, such as somatic sensory receptors in skin and proprioceptors.

      • Special Sense Receptors: Specialized receptors found in complex organs (e.g., eyes, ears).

    • Stimulus Origin:

      • Exteroceptors: Detect external stimuli (e.g., skin receptors).

      • Interoceptors: Monitor internal stimuli (e.g., visceral sensory receptors).

      • Proprioceptors: Detect body and limb movement (e.g., muscle and joint receptors).

    • Modality:

      • Five main types:

      • Chemoreceptors: Detect dissolved chemicals (e.g., taste and smell).

      • Thermoreceptors: Detect temperature changes (e.g., skin receptors).

      • Photoreceptors: Detect light (found in the retina).

      • Mechanoreceptors: Activate by physical distortion (e.g., touch, pressure, vibration).

      • Nociceptors: Detect pain from mechanical, chemical, or thermal damage.

16.2 Tactile Receptors

16.2a Tactile Receptors Overview

  • Description: Tactile receptors are mechanoreceptors found primarily in the skin and mucous membranes.

  • Types:

    • Unencapsulated Tactile Receptors:

    • Dendritic endings without protective coverings, e.g., free nerve endings, root hair plexuses, tactile discs.

    • Encapsulated Tactile Receptors:

    • Comprise neuron endings wrapped in connective tissue, e.g., tactile corpuscles, end (Krause) bulbs, lamellated (Pacinian) corpuscles.

    • Refer to related figures for identification and location of tactile receptors.

16.2b Proprioceptors

  • Definition of Proprioceptors:

    • Specialized mechanoreceptors located in muscles, tendons, and joints that relay body position and movement information.

  • Function: Proprioception, termed the “sixth sense”, helps detect stretch and position of body parts.

  • Types of Proprioceptors:

    • Muscle Spindle:

    • Detects stretch in skeletal muscles; tonic receptor.

    • Golgi Tendon Organ:

    • Monitors tension in tendons; tonic receptor.

    • Joint Kinesthetic Receptor:

    • Senses stretch in articular capsules; tonic receptor.

16.2c Referred Pain

  • Definition of Referred Pain:

    • Occurs when signals from viscera are perceived as originating from the skin or muscles.

  • Clinical Relevance:

    • Heart attack may manifest as pain in the arm or jaw due to shared neural pathways.

    • Common sources include:

    • Heart attack pain referred to pectoral region and medial arm.

    • Kidney pain may refer to the inferior abdomen.

    • The bladder pain may manifest in the buttocks.

16.2d Phantom Pain

  • Definition and Relevance:

    • Sensation of pain felt from an amputated limb due to active sensory neuron pathways remaining, leading to phantom pain.

16.3 Olfaction: The Sense of Smell

16.3a Olfactory Function

  • Definition of Olfaction: The biochemical detection of odorants dissolved in the air.

  • Olfactory Epithelium:

    • Sensory organ located in the superior nasal cavity comprised of:

    • Olfactory Receptor Cells: Major sensory cells detecting odorants via chemoreceptors.

    • Supporting Cells: Help support and sustain olfactory receptor cells.

    • Basal Cells: Precursor cells that differentiate into olfactory receptor cells.

    • Lamina Propria:

    • A connective tissue layer housing blood vessels, olfactory glands.

    • Olfactory Glands (Bowman’s Glands): Produce mucus that covers olfactory epithelium.

16.3b Gustation: The Sense of Taste

  • Definition of Gustation: The chemosensory function for detecting tastants.

  • Constituents of Taste Buds:

    • Comprised of gustatory cells (chemoreceptors), supporting cells, and basal cells (stem cells).

  • Types of Tongue Papillae:

    • Filiform Papillae: Most numerous but no taste buds.

    • Fungiform Papillae: Contain taste buds and located on the anterior tongue.

    • Foliate Papillae: Contain few taste buds, mainly in childhood.

    • Vallate Papillae: House most taste buds, located in a row at the posterior tongue.

16.3c Gustatory Pathways

  • Pathway for Gustation:

    • Sensory neurons connected to taste buds project to the CNS (specifically via CN VII for the anterior tongue and CN IX for the posterior tongue).

    • Sensory input from taste is relayed through the nucleus solitarius in the medulla to the thalamus, and then to the primary gustatory cortex located in the insula of the cerebrum.

16.3d Taste Sensations

  • Basic Taste Sensations:

    • Sweet: Stimulated by organic compounds; mainly sugars.

    • Salty: Resultant from metallic ions like Na+ and K+.

    • Sour: Caused by acids (e.g., citric acid).

    • Bitter: Associated with alkaloids (e.g., caffeine).

    • Umami: Related to amino acids such as glutamate; perceived as savory.

16.4 Structures and Functions of the Eye

16.4a Accessory Structures of the Eye

  • Description: Accessory structures assist the eye function and protection including six extrinsic muscles, eyelids, conjunctiva.

  • Eyebrows:

    • Protect against sweat and aid facial expressions.

  • Eyelids (Palpebrae): Formed of fibrous core with muscles and skin; contain tarsal (Meibomian) glands for lubrication.

  • Conjunctiva: A membrane that covers the eye and eyelid surfaces, secreting mucus for lubrication and tear production.

  • Lacrimal Apparatus:

    • Produces and drains lacrimal fluid (tears) to keep the eye moist and free of debris.

16.4b Eye Structure

  • General Description: The eye is nearly spherical and measures about 2.5 cm in diameter.

  • General Anatomy:

    • Wall composed of three layers (tunics): Fibrous (cornea and sclera), Vascular (choroid, iris, ciliary body), and Retina.

  • Cavities of the Eye:

    • Anterior cavity (aqueous humor) divided into rear (posterior) and front (anterior) chambers; Posterior cavity (vitreous humor).

16.4c Physiology of Vision: Light Refraction and Focusing

  • Refraction Principles:

    • Light must be refracted to focus properly on the retina; occurs at boundaries between air and cornea, and through the lens.

  • Focus Mechanism for Distant Objects:

    • Relaxed ciliary muscles flatten the lens; pupil dilation.

  • Focus Mechanism for Near Objects:

    • Ciliary muscles contract, lens becomes more rounded; pupil constricts.

16.4d Phototransduction Mechanism

  • Phototransduction: The conversion of light signals into electrical signals begins in photoreceptors (rods and cones).

  • Photoreceptors:

    • Rods sensitive to dim light, located mostly in peripheral retina. Cones allow for color vision, densely packed in the fovea centralis for sharp vision.

  • Photopigments:

    • Comprised of retinal (Vitamin A derivative) and protein opsins; each photoreceptor type contains specific photopigments responding to different wavelengths of light.

  • Transduction Process in the Dark:

    • High levels of cGMP keep cation channels open, allowing Na+ and Ca2+ ions to enter (dark current).

    • Photoreceptor is depolarized and continuously releases glutamate.

  • Reaction to Light:

    • Light causes hyperpolarization of rods, reducing glutamate release, thus activating bipolar cells.

16.4e Visual Pathways

  • Pathway of Visual Input:

    • Photoreceptors > Bipolar cells > Ganglion cells (form optic nerve).

    • Fibers from the optic tracts split at the optic chiasm, sending signals to the thalamus and up to the occipital lobe visual cortex.

16.5 Ear Structure and Function

16.5a Ear Components

  • Functions: The ear detects sound and provides balance information; signals are transmitted via the vestibulocochlear nerve (CN VIII).

  • Anatomy:

    • External Ear: Auricle structures capture sound.

    • Middle Ear: Contains auditory ossicles (malleus, incus, stapes) to amplify sound; tympanic membrane separates external and middle ear.

    • Inner Ear: Composed of the cochlea for hearing and vestibule for balance.

16.5b Hearing Physiology

  • Sound Pathway: Sound waves vibrate the tympanic membrane and are transmitted through the auditory ossicles to the oval window, creating pressure waves in the inner ear.

  • Hearing Mechanisms: Hair cells in the spiral organ transduce sound waves, releasing neurotransmitters to stimulate sensory neurons (CN VIII).

16.5c Auditory Pathways

  • Pathway for Sound Signal Reception: Nerve signals travel through the cochlear nucleus, inferior colliculus, thalamus, before reaching the auditory cortex for perception.

16.5d Equilibrium

  • Equilibrium Function: Awareness of head position detected by vestibular apparatus, which sends signals alongside visual and proprioceptive inputs to maintain balance.

  • Balance Mechanisms: Maculae detect static equilibrium and linear movements; semicircular canals detect rotational movements.

  • Neurotransmitter Release: Varies upon movement, modifying nerve signal transmission via the vestibular branch of CN VIII to the CNS.