IS

Chapters 23 & 24: General and Special Senses

General Senses (Chapter 23)

  • Objectives

    • Compare the functions of general and special sense organs.
    • Explain the receptor response process and the functional characteristic of adaptation.
    • Describe the distribution of sensory receptors in the body.
    • Classify receptors by location and structure, and by the types of stimuli that activate them.
    • Describe receptors for pain, temperature, touch, and proprioception.

  • Introduction

    • Sensory receptors alert the CNS to potential dangers in the external environment and monitor internal conditions to maintain homeostasis.
    • The CNS effects changes in the PNS to avoid injury and sustain homeostasis.

  • Sense Organs

    • Sense organs (sensory receptors) enable the body to respond to stimuli from internal or external environment.
    • Two categories of senses: General (somatic) and Special.

  • Receptor Response and Potential

    • Receptor response: Receptors convert stimuli into nerve impulses.
    • Receptor potential: Local potential that develops in a receptor’s membrane when an adequate stimulus acts on the receptor.
    • Impulses (action potentials) travel along sensory pathways to the brain and spinal cord.
    • Adaptation: Decrease in receptor potential over time in response to a continuous stimulus.
    • See text page 523, Figure 23-1 for illustration.

  • Distribution of Receptors

    • Special senses (smell, taste, vision, hearing, equilibrium) are grouped into localized areas or complex organs.
    • General sense organs (somatic senses) are microscopic receptors widely distributed throughout the body (see text page 524, Figure 23-2).

  • Classification of Receptors by Location (Somatic Receptor Classifications; see Tables 23-1, 524–525)

    • Exteroceptors
    • On or near body surface; often called cutaneous receptors.
    • Visceroreceptors (Interoceptors)
    • Located internally, often within viscera; monitor internal environment.
    • Proprioceptors
    • A special type of visceroceptor.
    • Location: skeletal muscle, joint capsules, tendons.
    • Function: provide information on body movement, orientation in space, muscle stretch.
    • Two types: tonic (slow-adapting) and phasic (rapid-adapting).
    • Provide positional information about the body.

  • Classification of Receptors by Stimulus

    • Mechanoreceptors
    • Chemoreceptors
    • Thermoreceptors
    • Nociceptors
    • Photoreceptors
    • Osmoreceptors

  • Classification of Receptors by Structure

    • Free nerve endings
    • Most widely distributed sensory receptor.
    • Called nociceptors; primary receptors for pain.
    • Pain sensations
    • Tactile sensations (skin movement, light touch, stretch, pressure, vibration, itch, tickle, discriminative touch)
    • Root hair plexuses
    • Tactile (Merkel) disks

  • Sense of Touch

    • Encapsulated nerve endings governing touch and pressure.
    • Touch and pressure receptors include:
    • Meissner corpuscles (tactile): Large numbers in hairless skin areas (e.g., nipples, fingertips, lips).
    • Pacinian corpuscles (Lamellar): Deep in dermis and in joint capsules.
    • Mechanoreceptors tend to adapt quickly; the sensations they evoke often do not last long.

  • Sense of Proprioception (Chapter 23 reference: page 530, Fig. 23-6)

    • Muscle spindle
    • Large-diameter, rapid-conducting Type Ia afferent fibers.
    • Smaller-diameter, slower-conducting Type II afferent fibers.
    • Carry information to the brain about changes in muscle length.
    • Golgi tendon organs
    • Type Ib sensory neurons.
    • Activated by excessive contraction; trigger muscle relaxation to protect the muscle.

Special Senses (Chapter 24)

  • Chapter 24 Objectives

    • Discuss structures and functions of olfactory and taste sense organs.
    • Describe major anatomical components of external, middle, and inner ear.
    • Identify sense organs involved in balance.
    • Discuss the sense of vision.
    • Discuss select pathologies of the special senses.

  • Olfactory Receptors

    • Olfactory sense organs consist of epithelial support cells and olfactory sensory neurons.
    • Components include:
    • Olfactory cilia
    • Olfactory cells
    • Olfactory epithelium
    • Olfactory receptors
    • Olfactory epithelium detects odorants; signals are transmitted via olfactory nerves.

  • Olfactory Pathways (text page 536, Fig. 24-2)

    • Threshold for odorants triggers a receptor potential and then an action potential.
    • Impulse is passed to the olfactory nerves in the olfactory bulb.
    • Signal travels through the olfactory tract to the thalamic and olfactory centers for interpretation, integration, and memory storage.

  • Sense of Taste (Gustation)

    • Taste sense organs respond to gustatory stimuli; chemoreceptors are stimulated by chemicals dissolved in saliva.
    • Gustatory cells are sensory cells in taste buds; taste buds are structurally similar.
    • Each taste bud typically responds best to one of five primary tastes (not enumerated here).
    • Neural pathway for taste:
    • Anterior two-thirds of the tongue → facial nerve (CN VII).
    • Posterior one-third of the tongue → glossopharyngeal nerve (CN IX).
    • Signals relayed to the medulla oblongata, then to the thalamus, and finally to the gustatory area of the cerebral cortex.

  • Senses of Hearing and Balance

    • Structure of the Ear
    • External Ear
      • Auricle (pinna)
      • External acoustic meatus
    • Middle Ear
      • Three auditory ossicles: malleus (hammer), incus (anvil), stapes (stirrup)
      • Openings: tympanic membrane (ear drum), oval window, round window, auditory (eustachian) tube
    • Inner Ear
      • Bony labyrinth and membranous labyrinth
      • Cochlea (involved with hearing)
      • Endolymph and perilymph fluids
    • Balance and Equilibrium (text page 543, Fig. 24-9)
    • Vestibule contains utricle and saccule (membranous structures within the vestibule).
    • Three semicircular canals (one in each temporal bone) arranged at right angles to detect movement in all directions; membranous semicircular ducts; ampulla near the junction with the utricle.
    • Static Equilibrium: Otoliths within the macula respond to head position changes by stimulating hair cells.
      • Otoliths are ear stones; gravity shifts the otolith-weighted matrix to stimulate hair cells.
      • Righting reflexes help restore body position.
    • Dynamic Equilibrium: Cupula of the semicircular ducts moves with endolymph flow; does not respond to gravity.
    • Pathway: Action potentials travel via the vestibular portion of CN VIII to the medulla oblongata.

  • The Sense of Vision

    • External structures of the eye
    • Eyebrows and eyelashes
    • Eyelids
    • Conjunctiva (mucous membrane lining)
    • Palpebral fissure (opening between eyelids)
    • Canthus (corner where eyelids meet)
    • Muscles of the Eye
    • Extrinsic (outside) muscles: named by position; superior, inferior, medial, and lateral rectus; superior and inferior oblique.
    • Intrinsic (within) muscles: Iris (controls pupil size); Ciliary muscle (controls lens shape).
    • Layers of the eyeball
    • Outer layer: Fibrous
    • Middle layer: Vascular
    • Inner layer: Retina; site of photoreception and the start of the visual pathway
    • Optic nerve: CN II extends from the eyeball to the brain
    • Retinal blood vessels: essential for retinal function
    • Formation of the retinal image
    • Refraction of light rays
    • Accommodation for near vision requires three coordinated changes:
      • Increase in curvature of the lens
      • Constriction of the pupils
      • Convergence of the two eyes
    • Change of lens shape involves:
      • Contraction of the ciliary muscle reduces tension on suspensory ligaments, allowing the lens to bulge for near vision.
      • Relaxation of the ciliary muscle increases tension on suspensory ligaments, flattening the lens for distant vision.
    • Pupil constriction during near vision (Near reflex) and bright light (Photopupillary reflex).
    • Convergence of the eyes increases as objects come closer.

  • Mechanisms of Disease (Special Senses)

    • Disorders of the ear
    • Otosclerosis: impaired conduction in the stapes with tinnitus.
    • Otitis: inflammation of the outer or middle ear.
    • Disorders of the eye
    • Refraction disorders: Hyperopia (farsightedness); Myopia (nearsightedness).
    • Cataracts: clouding of the lens.
    • Disorders of the Retina
    • Retinal detachment
    • Diabetic retinopathy: bleeding in retinal vessels with neovascularization (abnormal vessels).
    • Glaucoma: increased intraocular pressure.
    • Disorders of the visual pathway
    • Retinal pathway disorders.

  • Additional notes

    • References to Cycle of Life, The Big Picture, and Mechanisms of Disease, pages 555–558, are listed for further reading.
    • Pages and figures cited (e.g., text page 523, Fig. 23-1; page 524–525, Table 23-1; page 530, Fig. 23-6; page 535–541, Fig. 24-1 to 24-7; page 543, Fig. 24-9) indicate where to find corresponding diagrams and elaborations in the textbook.

  • Quick connections to core concepts

    • The distinction between receptor potential and action potentials underpins how stimuli are transduced into neural signals.
    • Adaptation allows sensory systems to filter persistent, uninformative stimuli, preserving attention for novel changes.
    • Spatial distribution of receptors (exteroceptors, interoceptors, proprioceptors) links anatomy to functional role in monitoring internal and external environments.
    • The eye and ear demonstrate multisensory integration: mechanical, chemical, and electrical signaling contribute to perception, posture, and balance in daily activities.

  • Practical and real-world relevance

    • Understanding receptor types helps explain why certain injuries or diseases (e.g., neuropathies, diabetic retinopathy, glaucoma) impact specific senses.
    • Knowledge of the pathways (e.g., taste via CN VII and CN IX; olfactory via olfactory bulb to cortex) informs clinical evaluation of sensory loss.
    • Awareness of reflexes (near response, photopupillary reflex, righting reflexes) highlights how the nervous system maintains orientation and safety during movement.

  • Formulas and specific notations used in study notes

    • Near vision changes involve coordinated actions:
    • Increase lens curvature (ciliary muscle contraction) ->
    • Pupil constriction ->
    • Convergence of eyes.
    • These can be summarized as a functional sequence:
    • Near response: {Lens curvature ↑, Pupil size ↓, Convergence ↑}.

  • Notable terms to review

    • Nociceptors, Exteroceptors, Interoceptors, Proprioceptors
    • Mechanoreceptors, Chemoreceptors, Thermoreceptors, Photoreceptors, Osmoreceptors
    • Meissner corpuscles, Pacinian (Lamellar) corpuscles, Merkel disks, Root hair plexuses
    • Muscle spindle (Type Ia and Type II), Golgi tendon organs (Type Ib)
    • Olfactory epithelium, Olfactory cilia, Gustatory cells, Taste buds
    • CN II (optic), CN VII (facial), CN IX (glossopharyngeal), CN VIII (vestibulocochlear)
    • Otoliths, Macula, Semicircular canals, Cupula
    • Endolymph, Perilymph, Cochlea, Ossicles (Malleus, Incus, Stapes)
    • Hyperopia, Myopia, Cataracts, Retinal detachment, Diabetic retinopathy, Glaucoma