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Special Senses (Code: MC2-9)
Introduction
▶ Senses may be subdivided into two categories:
- General/ Somatic senses – pain, touch, pressure, cold, and heat
- Special senses – taste, smell, vision, hearing and equilibrium (balance) Sensations ▶ Interpretation of impulses reaching sensory areas of the cerebral cortex ▶ The greater the intensity of a stimulus, the greater the frequency of impulse formation by receptor Projection ▶ The cerebral cortex projects back to the body region the sensation where the impulses originate so the sensation seems to come from that region
Adaptation
▶ Receptor repeatedly stimulated by the same stimulus, rate of impulse formation may
decline until impulses may not be formed at all
▶ Adaptation prevents overloading the NS with unimportant stimuli
▶ Once a receptor is adapted, a stronger stimulus is needed to form impulses
5 types of Sensory Receptors
1. ▶ Mechanoreceptors : respond to mechanical stimuli (bending or stretching of
receptors)
2. ▶ Chemoreceptors : respond to chemical concentration changes (odor molecules)
3. ▶ Photoreceptors – respond to light
4. ▶ Thermoreceptors – respond to temperature changes
5. ▶ Nociceptors – sensation of pain
I. General/ Somatic Senses
Receptors widely distributed in the skin, muscles, tendons and visceral organs
➢ Temperature
➢ Pressure
➢ Touch
➢ Pain
General/ Somatic Senses
1. TEMPERATURE: (Thermoreceptors)
▶ Heat receptors – located deep in the dermis; sensitive to temperatures above 25C
(77F);
▶ If above 45C (113F) interpreted as painful, burning sensations
▶ Cold receptors – located closer to surface of dermis; sensitive to temperatures below
20C (68F)
▶ If below 10C (50F), stimulate pain receptors produce
freezing sensation
▶ Temperature receptors adapt very quickly to constant stimulation
2. PRESSURE & TOUCH : (mechanoreceptors)
➢ Pacinian corpuscles – pressure receptors located deep in the dermis, ligaments,
tendons of joints
➢ Touch receptors include:
▶ Meissner’s corpuscles – dermal papillae;
➢ abundant in fingertips, palms, soles, nipples, external genital organs and lips;
➢ sensitive to very light touch (motion of objects that barely touch the skin)
▶ Free nerve endings – epithelial and connective tissues;
➢ internal organs;
➢ Act as pain receptors; also for touch, itch and temperature; some around base of
hair follicles and stimulated by movement of the hair shaft
II. Special Senses ▶ Receptors for special senses are localized ▶ Respond to only certain types of stimuli ▶ Three kinds of receptors ➢ Chemoreceptors – taste and smell ➢ Mechanoreceptors – hearing ➢ Photoreceptors – vision
1. Sense of Taste ▶ Receptors located in specialized microscopic organs called taste buds – organs of taste; located on the tongue in small, raised structures called papillae ▶ 10,000 taste buds; about 1, scattered in the roof of mouth and walls of throat ▶Taste bud consists of several taste receptors called taste cells ▶ Taste bud is spherical, with an opening à taste pore ▶ Hairlike projections from taste cells - > taste hairs - > exposed to chemicals on the tongue ▶ For a substance to activate the taste cells, it must be dissolved in a liquid – usually saliva ▶ Four primary taste sensations: ➢ Sweet - tip of tongue ➢ Sour - margins of tongue ➢ Bitter - back of tongue ➢ Salty - margins of tongue (anterior) ▶Other taste sensations: alkaline, metallic, umami (detects MSG & other flavor enhancers) ▶Chili peppers and ginger à may stimulate pain receptors - > as burning sensation/ heat receptors PHYSIOLOGY ▶ Taste receptors à Facial, glossopharyngeal and vagus nervesà medulla oblongataà thalamus à gustatory cortex of parietal lobe ▶ Taste receptors adapt rapidly to repeated stimulation ▶ Experiencing flavors involves tasting, smelling and feeling the texture and temperature of foods
2. Sense of Smell ▶ Olfactory organs à upper portion of nasal cavity ▶ Olfactory receptor cells (bipolar neurons)à supported by columnar epithelium covered with cilia à harbor 400 types of olfactory receptor proteins ▶ Chemicals in inhaled air are in gaseous state (odorant molecules) à must dissolve in the film of fluid covering the receptors before detected ▶ Hundred types of olfactory receptors can detect around 10,000 different odors (brain interprets information as a combination olfactory code) ▶ PHYSIOLOGY: Impulses formed - >axons to olfactory bulb à olfactory tract à olfactory centers in the brain 3. Sense of Hearing ▶ Ear – organ of hearing; also the organ of equilibrium ▶ Subdivided into three major parts: external ear, middle ear and inner ear A. External (Outer) ear: ➢ Auricle (pinna) – outer, funnel-like structure; cartilage and skin attached to side of head ➢ External auditory canal (meatus) – short tube from auricle through temporal bone (2.5cm) ➢ Eardrum- semitransparent; moves back and forth in response to sound waves and reproduces the vibrations
▶ Cochlea – coiled portion ▶ Lower compartment of bony labyrinth extends from cochlea to a membrane covered opening called round window ▶ Cochlear duct ▶ Apex is separated by two membranes: Vestibular and Basilar membranes ▶ Upper surface of basilar membrane- Organ of Corti - contain receptors for sound stimuli called hair cells with cilia ▶ PHYSIOLOGY: Vibrations formed by sound ➢ waves/impulses - > external auditory canal – >eardrum - > ossicles - > oval window - > perilymph - > round window - > vestibular and basilar membranes - > hair cells - > CN VIII - > hearing centers of temporal lobe ▶ Human ear can detect sound waves with frequencies ranging from 20 – 20,000 vibrations per second ▶ Range of greatest sensitivity is 2,000 – 3,000 vibrations per second ▶ Units called DECIBELS (dB) measure sound intensity; ▶ Scale begins at 0 dB (least perceptible); 10 dB is 10x intense than the least perceptible sound; 20 dB (100x); 30 dB (1,000x) ▶ Whisper – 40 dB; ▶ normal conversation – 60 - 70 dB; ▶ heavy traffic or ringing telephone – 80 dB; ▶ 120 dB (rock concert) ▶ 140dB (jet plane take off) causes pain ▶ Prolonged exposure to sounds above 85dB – damage hearing receptors; cause permanent hearing loss ➢ Equilibrium : ➢ Static equilibrium controlled by the vestibule (membranous labyrinth); sense the position of the head, maintaining stability and posture when the head and body are still/motionless (gravitational)
➢ Dynamic equilibrium controlled by the semicircular canals detect motion of the head and aid in maintaining balance (rotational)
4. Sense of Vision ▶ Eye is a hollow, spherical organ; 2.5cm in diameter ▶ Eyelids – exposed anterior surface; blinked frequently to spread tears and mucus to keep eyes moist; inner surface lined by a mucous membrane, the conjunctiva (helps lubricate the eye) ▶ Eyelashes – keep airborne particles from reaching eye surface; provide protection from excessive light ▶ Eyebrows – shield eyes from light from above ▶ Lacrimal Apparatus – production and removal of tears ➢ Tears secreted continuously by lacrimal gland ➢ Tears are carried to surface of eye by tiny lacrimal ducts then flow downward and medially and collected at inner corner of eye by the superior and inferior canaliculi à lacrimal sac à nasolacrimal ductà into nasal cavity ➢ Tears keep anterior surface of eye moist and wash ➢ away foreign particles ➢ Antibacterial enzyme ( lysozyme) in tears help reduce chance of eye infections ➢ Extrinsic muscles – 4 recti muscles and 2 obliques
▶ Internal cavities: Anterior cavity
- space between the cornea and the lens.
- filled with a watery fluid called aqueous humor à helps maintain shape of cornea and is responsible for the internal pressure within the eye. ➔ secreted and absorbed at the same rate to maintain intraocular pressure Posterior cavity ➢ Behind the lens; ➢ filled with a clear, gel-like substance called vitreous humor; presses retina firmly against the wall of the eye and helps maintain shape of the eye ▶ PHYSIOLOGY: ▶ Light waves (refraction) - > cornea àpupil (constrict in near objects and bright light/dilates in far objects and dim light) - > aqueous humor à lens (accommodation – flattened in distant objects/convex in near objects) - > vitreous humor - > retina - > optic nerves - > optic chiasma - > half of nerve fibers to nasal side (cross over) and temporal side (no cross over) - > optic tract à thalamus
optic radiations à visual cortex of occipital lobes
