Chapter 10 (PART ONE)
Chapter 10: The Senses
10.1 Introduction to the Senses
Definition: Senses arise from sensory receptors that detect environmental changes. This detection stimulates neurons to send nerve impulses to the central nervous system (CNS) for processing, leading the body to respond with feelings or sensations.
Categories of Senses:
General Senses:
Characteristics: Widely distributed in skin, joints, and visceral organs; structurally simple.
Examples: Touch, pressure, temperature, pain.
Special Senses:
Characteristics: Involves complex specialized sensory organs located primarily in the head.
Examples: Vision, hearing, smell, taste, equilibrium.
10.2 Receptors, Sensations, and Perception
Action Potentials: All action potentials are identical; different sensory events are recognized due to various types of receptors.
Types of Sensory Receptors:
Chemoreceptors: Sensitive to changes in chemical concentrations.
Nociceptors (Pain receptors): Detect tissue damage.
Thermoreceptors: Respond to temperature differences.
Mechanoreceptors: Respond to changes in pressure or movement.
Photoreceptors: Respond to light, located in the eye.
Sensation vs. Perception:
Sensation: Occurs when receptors are stimulated and send impulses to the brain.
Perception: The conscious awareness of stimuli. The brain employs projection, sending the sensation back to its point of origin, enabling location identification.
The type of sensation is determined by which brain region receives the impulses.
10.3 Sensory Adaptation
Overview: The brain prioritizes incoming sensory impulses to avoid overwhelming stimuli.
Definition of Sensory Adaptation: The ability of the nervous system to become less responsive to a constant stimulus over time.
Mechanisms: This results from:
Receptors becoming unresponsive.
Inhibition along the CNS pathway.
Receptor Examples:
Thermoreceptors and olfactory receptors exhibit significant adaptation, while pain receptors show little to no adaptation.
10.4 General Senses
Characteristics: General senses have widely distributed receptors associated with skin, muscles, joints, and viscera, sensing:
Touch
Pressure
Temperature
Pain
Receptors for Touch and Pressure: They sense tissue deformation or tissue displacement (mechanoreceptors). Types include:
Free Nerve Endings: Associated with itching and other sensations; located in epithelium.
Tactile (Meissner's) Corpuscles: Flattened connective tissue sheaths around nerve fibers responding to motion and fine touch; abundant in hairless areas like fingertips and palms.
Lamellated (Pacinian) Corpuscles: Large structures that detect deep pressure and vibrations; commonly found in deep dermis/subcutaneous layers.
10.5 Temperature Senses
Temperature Receptors: Include two groups of free nerve endings in the skin:
Warm Receptors: Respond to temperatures between 25°C (77°F) and 45°C (113°F); above this range, pain receptors are activated, causing a burning sensation.
Cold Receptors: Respond to temperatures between 10°C (50°F) and 20°C (68°F); below this range, pain receptors are stimulated, causing a freezing sensation.
Both receptor types adapt quickly; continuous stimulation for one minute leads to fading sensations.
10.6 Body Position, Movement, and Stretch Receptors
Proprioception: Mechanoreceptors that relay the sense of body position and spatial location.
Types of Proprioceptors:
Muscle Spindles:
Bundles of specialized skeletal muscle fibers with sensory neuron fibers wrapped around them.
Monitor muscle contraction states, aiding in maintaining upright posture.
Golgi Tendon Organs:
Located in tendons near muscle attachments.
Detect tendon stretching during muscle contraction.
10.7 Sense of Pain (Nociceptors)
Pain Receptors:
Mainly free nerve endings stimulated by tissue damage; widely distributed, except the brain's nervous tissue lacks pain receptors.
Overstimulation of cold receptors can send pain signals.
Communication via neurotransmitters: Substance P (in spinal cord) and Glutamate (in the brain).
Tissue damage results in prostaglandin release, increasing receptor sensitivity and pain intensity.
Pain-relieving substances include Aspirin and Ibuprofen (which inhibit prostaglandin synthesis) and Morphine (which inhibits Substance P release).
Pain serves as a protective warning to the body; adaptation is minimal.
10.8 Visceral Pain
Visceral Pain Receptors:
The only receptors in the viscera. Their responses to damage differ from surface tissue receptors, with stretch/spasms generating strong pain despite minimal damage.
Pain signals may arise from mechanoreceptors, decreased blood flow, or chemicals activating chemoreceptors.
Referred Pain: Occurs when pain from an internal organ is perceived in another body area due to common nerve pathways between skin and internal organs.
Example: Cardiac pain is often felt in the left shoulder/arm.
10.9 Pain Nerve Fibers
Two types of pain-conducting fibers:
Fast (Acute) Pain Fibers: Myelinated fibers that carry rapid impulses, producing sharp pain sensations that cease with stimulus removal.
Slow (Chronic) Pain Fibers: Unmyelinated fibers that convey slow impulses, resulting in dull, achy sensations difficult to localize. They continue to send impulses post-stimulus.
Simultaneous stimulation from both fiber types leads to a sharp followed by an aching pain.
10.10 Pain Pathways
Head Pain Impulses: Reach the brain via cranial nerves; other body pain travels through spinal nerves.
Pain impulses entering the spinal cord are processed in gray matter of the posterior horn.
Major pathways include:
Terminations in reticular formation, thalamus, and limbic system.
The limbic system responds emotionally to pain, while the cerebral cortex locates the pain source and assesses its intensity.
10.11 Special Senses
Overview: Special senses are characterized by sensory receptors located in complex organs in the head, namely:
Smell (olfactory organs)
Taste (taste buds)
Hearing (ears)
Equilibrium (ears)
Sight (eyes)
10.12 Sense of Smell (Olfaction)
Olfactory Organs: Composed of olfactory receptor cells located in the roof of the nasal cavity.
Olfactory Receptor Cells: Bipolar neurons with cilia supported by columnar epithelial cells; each neuron expresses one specific type of olfactory receptor membrane protein.
Function: Chemoreceptors respond to inhaled odorants that must dissolve in liquids to stimulate receptors; interconnected with the sense of taste (approximately 75-80% of taste perception).
10.13 Olfactory Pathways
Upon stimulation, olfactory receptors connect with neurons in the olfactory bulbs adjacent to the crista galli of the ethmoid bone.
The axons form Cranial Nerve I (Olfactory Nerve). Sensory impulses are analyzed in the olfactory bulbs and further interpreted in the temporal and frontal lobes of the cerebrum, often interfacing with the limbic system for emotional responses to scents.
10.14 Olfactory Stimulation
Mechanism: Each odor corresponds with specific receptor sets in olfactory receptor cell membranes, leading to a sodium influx and potential action potential generation.
The brain interprets various combinations of activated receptors as distinct odors. Adaptation occurs quickly, leading to fading smells over time.
10.15 Sense of Taste (Gustation)
Taste Buds: Approximately 10,000 taste buds predominantly located around the tongue's papillae. Each bud contains 50-100 taste cells, which function as chemoreceptors and are replaced every 10 days.
Taste Cells: Modified epithelial cells with hair-like structures protruding through taste pores; they function by binding specific chemicals (must dissolve in saliva) to membrane protein receptors.
10.16 Taste Sensations
At least five primary taste sensations:
Sweet
Sour
Salty
Bitter
Umami (savory)
Other tastes like alkaline and metallic can be perceived; spicy foods can activate pain receptors in addition to taste cells.
Adaptation rates for taste sensations mirror those of olfactory sensations.
10.17 Taste Pathways
Taste impulses travel via facial, glossopharyngeal, and vagus nerves to the medulla oblongata, which coordinates salivation and gastrointestinal secretions. The signal proceeds through the thalamus and is understood in the gustatory cortex of the parietal lobe of the cerebrum.
10.18 Sense of Hearing
Ear Functions: The ear is responsible for hearing and equilibrium, comprising three sections:
Outer Ear: Includes the auricle (pinna), external acoustic meatus (external auditory canal), and tympanic membrane (eardrum). These parts collect and direct sound waves to the eardrum.
10.19 Middle Ear and Auditory Ossicles
The middle ear, or tympanic cavity, is an air-filled space within the temporal bone housing three auditory ossicles (malleus, incus, stapes).
Function: The tympanic membrane vibrates the malleus, which subsequently vibrates the incus and stapes, leading to fluid stimulation at the oval window of the inner ear, amplifying sound absorption.
10.20 Auditory Tube
The auditory (Eustachian) tube connects the middle ear to the nasopharynx, maintaining equal air pressure on both sides of the eardrum. Infectious mucus can travel through it, leading to middle ear infections.
10.21 Inner Ear Structure
Overview: The inner ear is a labyrinth comprising interconnected chambers and tubes consisting of bony and membranous labyrinths surrounded by perilymph fluid and containing endolymph fluid.
Key structures include the cochlea (for hearing) and the semicircular canals (for balance).
10.22 Cochlea and Hearing Mechanism
The cochlea has three spiraling chambers:
Scala vestibuli
Cochlear duct
Scala tympani
Mechanism: Movements of the stapes at the oval window produce vibrations in perilymph, transmitting them through the vestibular membrane into endolymph, activating specific receptor cells along the basilar membrane. This leads to action potential generation in hair cells extending into the endolymph, sending signals to the auditory cortex via the cochlear branch of the vestibulocochlear nerve.
10.23 Sound Intensity Measurement and Auditory Pathways
Measurement: Sound intensity is measured in decibels (dB), on a logarithmic scale:
0 dB: threshold of hearing.
30 dB: 1,000 times more intense than 0 dB.
85 dB: prolonged exposure can cause damage.
Auditory Pathways: Nerve fibers transmit impulses to auditory cortices in the temporal lobes. Crossover occurs so both brain sides process inputs from both ears. Hearing loss can be conductive or sensorineural, based on transmission or receptor damage.
10.24 Sensory Impulses Generation in the Ear
Step-by-step Process:
Sound waves enter the external acoustic meatus.
Waves cause the eardrum to vibrate.
Auditory ossicles amplify and transfer vibrations to the stapes' end.
Stapes' movement transfers vibrations to inner ear perilymph.
Vibrations traverse the vestibular membrane into cochlear duct endolymph, moving the basilar membrane.
Vibration frequencies stimulate different receptor cells.
Receptor cell depolarization increases calcium ion permeability.
Calcium influx releases neurotransmitter from receptor cell vesicles.
Neurotransmitter activates nearby sensory neurons.
Sensory impulses are triggered on cochlear nerve fibers.
Auditory cortices interpret the impulses.
10.25 Sense of Equilibrium
Divisions: Consists of two parts:
Static Equilibrium: Maintains head position, posture, and balance during stillness. Receptors are located in the vestibule of the inner ear.
Dynamic Equilibrium: Helps maintain balance during sudden motion. Receptors are found in the semicircular canals.
10.26 Static Equilibrium
The organs located within the vestibule (utricle and saccule) contain maculae (static equilibrium organs) consisting of:
Hair cells (sensory receptors).
Gelatinous material (which hair cells project into).
Otoliths (calcium carbonate grains embedded in the gelatinous mass).
Function: Gravity or head movement displaces gelatin and otoliths, bending hair cells and generating action potentials sent to the brain via the vestibular branch of the vestibulocochlear nerve.
10.27 Dynamic Equilibrium
Organs within the semicircular canals detect head motion and aid balance during sudden movements.
Cristae Ampullaris: Organs of dynamic equilibrium located in the ampulla of each semicircular canal, where hair cells extend into a dome-shaped gelatinous mass (cupula).
Mechanism: Rapid head movements move semicircular canals while the endolymph remains static, leading to bending of the cupula and hair cells, generating action potentials sent to the brain regarding head position, aided by proprioceptors and visual input.