Chapter 16: Sensory Systems – Anatomy & Physiology Study Notes

Sensory Systems Overview

Definitions

Sensory systems are essential for interpreting the environment and maintaining homeostasis. They rely on specialized structures to detect and transmit information about stimuli.

• Sensory input: Vital for personality and intellectual function.

• Sensory deprivation: Withholding sensory stimulation, which can affect mental health and cognition.

• Sensory receptor: A structure specialized to detect a stimulus. Can be a bare nerve ending or part of a sense organ (nerve tissue surrounded by other tissues such as epithelium, muscle, or connective tissue).

General Properties of Sensory Receptors

Transduction

Transduction is the conversion of one form of energy to another, which is the fundamental purpose of any sensory receptor.

• Converts stimulus energy (light, heat, touch, sound, etc.) into nerve signals.

• Examples of transducers: sense organs, gasoline engines, light bulbs.

Receptor Potential

• Small, local electrical change on a receptor cell brought about by an initial stimulus.

• Results in release of neurotransmitter or a volley of action potentials that generate nerve signals to the CNS.

Sensation

• Subjective awareness of the stimulus.

• Most sensory signals delivered to the CNS produce no conscious sensation (filtered out in the brainstem).

• Some do not require conscious awareness (e.g., pH and body temperature).

Receptors Transmit Four Kinds of Information

Modality

Refers to the type of stimulus or the sensation it produces (e.g., vision, hearing, taste).

• Labeled line code: All action potentials are identical; each nerve pathway from sensory cells to the brain is labeled to identify its origin, allowing the brain to interpret the modality.

Location

• Encoded by which nerve fibers are issuing signals to the brain.

• Receptive field: Area that detects stimuli for a sensory neuron. Receptive fields vary in size (e.g., fingertip vs. skin on back).

• Two-point touch discrimination: Ability to distinguish two closely spaced stimuli.

• Sensory projection: Brain identifies the site of stimulation.

• Projection pathways: Pathways followed by sensory signals to their ultimate destination in the CNS.

Intensity

• Encoded in three ways:

  • Which fibers are sending signals

  • How many fibers are doing so

  • How fast these fibers are firing

Duration

• How long the stimulus lasts, encoded by changes in firing frequency over time.

Receptive Fields

Spatial Resolution

Receptive fields determine the spatial resolution of sensory input. Smaller receptive fields allow for finer discrimination of stimuli.

• Example: Fingertips have small receptive fields for precise touch discrimination; the back has larger fields and less precision.

Classification of Receptors

By Modality

• Thermoreceptors: Detect temperature changes.

• Photoreceptors: Detect light (vision).

• Nociceptors: Detect pain.

• Chemoreceptors: Detect chemicals (taste, smell).

• Mechanoreceptors: Detect mechanical changes (touch, pressure, vibration).

By Distribution

• General (somesthetic) senses: Widely distributed throughout the body.

• Special senses: Limited to the head (vision, hearing, equilibrium, taste, smell).

Types of Nerve Endings

Unencapsulated Nerve Endings

• Dendrites not wrapped in connective tissue.

• Free nerve endings: Detect pain and temperature; found in skin and mucous membranes.

• Tactile discs: Detect light touch and texture; associated with Merkel cells at the base of the epidermis.

• Hair receptors: Wrap around base of hair follicle; monitor movement of hair.

Encapsulated Nerve Endings

• Dendrites wrapped by glial cells or connective tissue.

• Connective tissue enhances sensitivity or selectivity of response.

Pain

Pain Sensation

• Pain: Discomfort caused by tissue injury or noxious stimulation, leading to evasive action.

• Nociceptors: Two types provide different pain sensations:

  • Fast pain: Travels in myelinated fibers (12–30 m/sec); sharp, localized, stabbing pain perceived with injury.

  • Slow pain: Travels in unmyelinated fibers (0.5–2 m/sec); longer-lasting, dull, diffuse feeling.

• Somatic pain: From skin, muscles, and joints.

• Visceral pain: From the viscera (stretch, chemical irritants, or ischemia; poorly localized).

• Injured tissues release chemicals that stimulate pain fibers, such as bradykinin (most potent pain stimulus known).

Pain Signal Destinations

Pain Pathways

Pain signals travel through specific neural pathways to reach the brain, where they are processed and perceived.

• First-order neurons transmit signals from nociceptors to the spinal cord.

• Second-order neurons relay signals to the thalamus and other brain regions.

• Third-order neurons project to the somatosensory cortex for conscious perception.

CNS Modulation of Pain

Analgesic Mechanisms

• Analgesic: Pain-relieving mechanisms of the CNS.

• Tied to receptor sites for opium, morphine, and heroin in the brain.

• Enkephalins: Two analgesic oligopeptides with 200 times the potency of morphine.

• Endorphins and dynorphins: Larger analgesic neuropeptides discovered later.

• Endogenous opioids: Internally produced opium-like substances (enkephalins, endorphins, dynorphins).

• Secreted by the CNS, pituitary gland, digestive tract, and other organs.

• Neuromodulators: Can block transmission of pain signals and produce feelings of pleasure and euphoria.

Chemical Sense – Taste

Gustation

Gustation is the sensation that results from the action of chemicals on taste buds.

• About 4000 taste buds, mainly on the tongue, but also inside cheeks, and on soft palate, pharynx, and epiglottis.

• Lingual papillae:

  • Filiform: No taste buds; important for food texture.

  • Foliate: No taste buds; weakly developed in humans.

  • Fungiform: At tips and sides of tongue.

  • Vallate (circumvallate): At rear of tongue; contains half of all taste buds.

Taste Bud Structure

• All taste buds look alike; lemon-shaped groups of 40–60 taste cells, supporting cells, and basal cells.

• Taste cells: Have apical microvilli (taste hairs) that serve as receptor surface for taste molecules; taste pores are pits in which taste hairs project; taste hairs are epithelial cells, not neurons; synapse with and release neurotransmitters onto sensory neurons at their base.

• Basal cells: Stem cells that replace taste cells every 7–10 days.

• Supporting cells: Resemble taste cells without taste hairs, synaptic vesicles, or sensory role.

Physiology of Taste

• Molecules must dissolve in saliva and flood the taste pore to be tasted.

• Five primary sensations:

  • Salty: Produced by metal ions (sodium and potassium).

  • Sweet: Associated with carbohydrates and other foods of high caloric value.

  • Sour: Acids such as in citrus fruits.

  • Bitter: Associated with spoiled foods and alkaloids (nicotine, caffeine, quinine, morphine).

  • Umami: 'Meaty' taste of amino acids in chicken or beef broth.

• Taste is influenced by food texture, aroma, temperature, and appearance.

• Mouthfeel: Detected by branches of lingual nerve in papillae.

• Hot pepper stimulates free nerve endings (pain), not taste buds.

Chemical Sense – Smell

Olfaction

Olfaction is the sense of smell, mediated by olfactory mucosa and olfactory cells.

• Olfactory mucosa: Contains 10–20 million olfactory cells (neurons), epithelial supporting cells, and basal stem cells; located in the mucosa of superior concha, nasal septum, and roof of nasal cavity (about 5 cm2).

• On average, 2000–4000 odors can be distinguished.

Olfactory Cells

• Neurons shaped like bowling pins; head bears 10–20 cilia called olfactory hairs with binding sites for odorant molecules; nonmotile.

• Lie in a tangled mass in a thin layer of mucus.

• Basal end of each cell becomes the axon; axons collect into small fascicles and leave cranial cavity through the cribriform foramina in the ethmoid bone.

• Fascicles are collectively regarded as Cranial Nerve I.

• Only neurons in the body directly exposed to the external environment; lifespan of only 60 days; basal cells continually divide and differentiate into new olfactory cells.

• Supporting cells and basal cells present.

Olfactory Projection Pathways

• Olfactory signals are transmitted to various brain regions, including the orbitofrontal cortex, olfactory bulbs, olfactory tract, insula, hypothalamus, amygdala, primary olfactory cortex, and hippocampus.

Hearing and Equilibrium

Hearing

• Response to vibrating air molecules.

• Both hearing and equilibrium senses reside in the inner ear, a maze of fluid-filled passages and sensory cells.

• Fluid is set in motion, and sensory cells convert this motion into an informative pattern of action potentials.

The Nature of Sound

• Sound: Any audible vibration of molecules.

• A vibrating object pushes on air molecules, which in turn push on other air molecules; air molecules hitting the eardrum cause it to vibrate.

Outer (External) Ear

• Outer ear: Funnel for conducting vibrations to the tympanic membrane (eardrum).

• Auricle (pinna): Directs sound down the auditory canal; shaped and supported by elastic cartilage.

• Auditory canal: Passage leading through the temporal bone to the tympanic membrane.

• External acoustic meatus: Slightly s-shaped tube, about 3 cm long.

• Guard hairs: Protect outer end of canal.

• Cerumen (earwax): Mixture of secretions of ceruminous and sebaceous glands and dead skin cells; sticky, coats guard hairs, contains lysozyme, water-proofs canal, protects skin, keeps tympanic membrane pliable.

Middle Ear

• Middle ear: Located in the air-filled tympanic cavity in temporal bone.

• Tympanic membrane (eardrum): Closes the inner end of the auditory canal; separates it from the middle ear; about 1 cm in diameter; vibrates in response to sound; highly sensitive to pain.

• Auditory (eustachian) tube: Connects middle ear cavity to nasopharynx; equalizes air pressure on both sides of tympanic membrane; normally flattened and closed, opens during swallowing or yawning.

• Auditory ossicles:

  • Malleus: Attached to inner surface of tympanic membrane.

  • Incus: Articulates between malleus and stapes.

  • Stapes: Footplate rests on oval window, where inner ear begins.

Inner Ear Anatomy

• Labyrinth: Vestibule and three semicircular ducts.

• Cochlea: Organ of hearing; 2.5 coils around a screwlike axis of spongy bone, the modiolus; threads of the screw form a spiral platform that supports the fleshy tube of the cochlea.

Cochlear Duct and Spiral Organ

• Contains specialized structures for sound transduction.

Stimulation of Cochlear Hair Cells

• Vibration of ossicles causes vibration of basilar membrane under hair cells (up to 20,000 times per second).

• Hair cells move with basilar membrane, generating action potentials.

Auditory Processing Centers

• Auditory signals are processed in the cochlear nucleus, superior olivary nucleus, inferior colliculus, thalamus, and primary auditory cortex.

Equilibrium

Vestibular Apparatus

• Constitutes receptors for equilibrium.

• Three semicircular ducts: Detect only angular acceleration.

• Two chambers: Anterior saccule and posterior utricle; responsible for static equilibrium and linear acceleration.

Static and Dynamic Equilibrium

• Static equilibrium: Perception of the orientation of the head when the body is stationary.

• Dynamic equilibrium: Perception of motion or acceleration.

  • Linear acceleration: Change in velocity in a straight line (elevator).

  • Angular acceleration: Change in rate of rotation (car turns a corner).

Saccule and Utricle

• Macula: 2 by 3 mm patch of hair cells and supporting cells in the saccule and utricle.

  • Macula sacculi: Lies vertically on wall of saccule.

  • Macula utriculi: Lies horizontally on floor of utricle.

• Each hair cell has 40–70 stereocilia and one true cilium (kinocilium) embedded in a gelatinous otolithic membrane.

• Otoliths: Calcium carbonate-protein granules that add to the weight and inertia and enhance the sense of gravity and motion.

Summary Table: Sensory Receptor Types

Type	Stimulus Detected	Location
Thermoreceptor	Temperature	Skin, hypothalamus
Photoreceptor	Light	Retina (eye)
Nociceptor	Pain	Skin, viscera
Chemoreceptor	Chemicals	Taste buds, olfactory mucosa
Mechanoreceptor	Touch, pressure, vibration	Skin, inner ear

Additional info: Academic context and definitions have been expanded for clarity and completeness. Table entries inferred from standard textbook knowledge.