Somatic and Special Senses

Senses constantly provide us with information about our surroundings Grouped into two major categories: - general senses - special senses

Sensory Pathway Includes: - receptors - sensory neurons - sensory tracts - sensory area

Receptors Sensory Neurons detect stimuli specific with respect to changes to which they respond Sensory Neurons transmit impulses from receptors to central nervous system found in both spinal and cranial nerves (each carries only one type of receptor)

Sensory Tracts Sensory Areas white matter in spinal cord or brain transmit impulses to a specific part of brain Sensory Areas most are in cerebral cortex feel and interpret sensations learning to interpret sensations begins in infancy without awareness and continues throughout life

General Senses Somatic: - tactile - touch, pressure, vibration, itch, etc. - thermal - hot and cold - pain - acute and chronic - proprioceptive - muscle, tendon, joint Visceral - distension of viscera - internal organs - chemical composition of extracellular fluid

Special Senses Somatic: - visual - sight - auditory - hearing - equilibrium - static and dynamic equilibrium Visceral: - olfactory - smell - gustatory - taste

Skin Receptors

Tactile Sensations Touch receptors: - root hair plexuses - tactile discs - type II cutaneous mechanoreceptors - corpuscles of touch - (Meissner’s corpuscles)

Root Hair Plexuses dendrites arranged around hair follicles receptors that rapidly adapt to detect movements when hair is disturbed

Tactile Discs expanded (flattened) nerve endings slowly adapting touch receptors for discriminative touch

Type II Cutaneous Mechanorecptors also called end organ for Ruffini expanded nerve endings embedded in dermis receptors that adapt slowly to heavy and continuous touch

Corpuscles of Touch (Meissner’s Corpuscles) small, oval, encapsulated nerve endings rapidly adapting touch receptors recognize exactly what point to which body is touched abundant in hairless portions of skin

Corpuscles of Touch (cont.) rapidly adapting receptors that respond to low frequency vibrations also respond to pressure and touch stimuli

Tactile Sensations Pressure and vibration receptors: - corpuscles of touch (Meissner’s) - lamellated corpuscles (Pacinian)

Lamellated Corpuscles (Pacinian) oval structures composed of connective tissue layered like an onion enclose a dendrite rapidly adaptive receptors that respond to pressure and high frequency vibrations

Tactile Sensations (itch and tickle receptors) free nerve endings are receptors for both tickle and itch sensations

Thermal Sensations (thermoreceptors) heat receptors most sensitive to temperatures above 25oC (77oF) and become unresponsive at temperatures above 45oC (113oF) cold receptors most sensitive to temperatures between 10oC (50oF) and 20oC (68oF)

Thermal Sensations (cont.) intermediate temperature sensory input from combination of cold and heat receptors both heat and cold receptors rapidly adapt to continuous stimulation

Pain Sensations (Nociceptors) free, naked nerve endings located between cells of epidermis respond to all types of stimuli

Referred Pain pain that feels as if it originated from a part other than site being stimulated Example: - pain from heart attack (myocardial infarction) may be felt in left shoulder or inside of left arm - pain from gallstones may be felt in right shoulder