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Somatic & Special Senses

Bio 250Human Anatomy & Physiology

Sensory Neurons

Sensory neurons are extensions of the CNSSensory neurons initiate impulses to the y pCNSSensory neurons have receptorsReceptors typically are sensitive to environmental changesEach type of receptor is primarily sensitive to a specific stimulus

Detection of StimuliMost sensory receptors show receptor specificity, e.g. touch receptors are very sensitive to pressure but relatively insensitive to chemical stimulito chemical stimuli.Specificity may result from the structure of the receptor cell or from the presence of accessory cells or structures that protect the receptors from other stimuli. (pain vs photo)Receptive field is the area monitored by a single receptor cell and is important in location of stimuli on the body.

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Classification of Receptors for General Senses

Exteroceptors: provide information about the external environmentProprioceptors: report the positions ofProprioceptors: report the positions of skeletal muscles and jointsInteroceptors: monitor visceral organs and functions

Types of Receptors (more detailed)

Chemoreceptors are sensitive to changes in chemical concentrations:

gustatory receptorsgustatory receptorsolfactory receptorsoxygen receptorscarbon dioxide receptorshydrogen ion receptors

Pain receptors (nociceptors) are sensitive to tissue damage

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Types of Receptors

Thermoreceptors are sensitive to temperature changesMechanoreceptors are sensitive to mechanical forces:forces:

touch receptorspressure receptorsauditory receptorsstretch receptorsproprioceptorsbaroreceptors (pressoreceptors)

Types of Receptors

Photoreceptors are sensitive to certain wavelengths of light

rods are sensitive to light and dark and arerods are sensitive to light and dark and are sensitive to low intensities of light (night vision)cones are sensitive to certain colors of light but require high intensities of light (color day vision)

Adaptation

Adaptation is a reduction in sensitivity in the presence of a constant stimulusPeripheral adaptation occurs when receptors or

lt th i l l f ti itsensory neurons alter their levels of activityCentral adaptation occurs inside the CNS along sensory pathways and usually involves inhibition of nuclei along a pathway, e.g. smellSome adaptation is fast and nearly complete like vision and smell and some adaptation is almost non-existent like pain.

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Adaptation as Filter

Adapation acts as a sensory filter to restrict information from reaching cortexOnly about 1% of the information provided by afferent fibers reaches the cortex and our by a e e t be s eac es t e co te a d ouconscious awareness.Output from higher centers can increase receptor sensitivity.RAS helps to focus our attention and can heighten or reduce our awareness of incoming sensations.

Somatic Receptors-Touch, Pressure & Temperature

Tactile or Meissner’s corpuscles are touch receptors and are abundant in hairless portions of the skin like lips, fingertips, palms, soles nipples and genital areassoles, nipples and genital areasLamellated or Pacinian corpuscles are pressure receptors found in deeper subcutaneous area of the skin and may also detect vibrations in tissues Two types of free nerve endings called heat receptors and cold receptors

Somatic Receptors-Pain ReceptorsOther free nerve endings are stimulated when tissues are damagedUsually protective because they signal y p y gan unpleasant sensation that makes us stop the action causing the damagePain receptors adapt very littleExtreme stimulation of touch and temperature receptors is perceived as pain by them.

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Special Senses

Olfactory senseGustatory senseAuditory senseAuditory senseVisual sense

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Sense of Smell- Olfaction

Due to a patch of chemoreceptors about the size of a quarter that are sensitive to chemicals in solution in the

f th l itmucus of the nasal cavityHumans have 10-20 million olfactory cells (Bloodhound has 4 billion cells)Can detect one molecule of green pepper smell in mixture of 3 trillion air molecules

Olfaction

Olfactory receptors adapt rapidly50% of intensity of an odor lost within one secondSensitivity to other odors remains intact

We can discriminate about 2 000 4 000 scentsWe can discriminate about 2,000-4,000 scentsImpulses travel from olfactory cells to olfactory bulb to olfactory tracts to portions of the limbic system to perceptive areas in the temporal lobe bypassing the thalamusAbout 1% of the olfactory receptors lost per year

Gustation

Taste buds in papillae located mostly on surface of the tongue (adult has about 3,000)Chemicals must be in solution in saliva to be sensedse sedFour primary tastes, salt, sweet, bitter, sourReceptor locations:

sweet on tip of tonguesour along margins of tonguesalt on tip and upper front portionbitter on back of tongue

Travels nerves VII, IX & X to thalamus to limbic system and parietal cortex

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Auditory Sense- Hearing

External ear (pinna and external meatus) collect sound waves from vibrating objectsvibrating objectsMiddle ear’s ossicles transmit sounds from tympanic membrane (eardrum) to inner earAuditory tubes drain fluid and equalize pressure on either side of eardrums

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Auditory Sense- Hearing

Inner ear consists of the cochlea with its nerve transducer, the

f C tiorgan of CortiDifferent frequencies vibrate different sets of receptors of the organ of CortiHumans can hear from 20 to 20,000 vps

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Sense of Sight- Vision

Eyes are the sense organs for the sense of sightEyelids and lacrimal apparatus areEyelids and lacrimal apparatus are accessory organs that protect the eyeExtrinsic muscles move the eye

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Structure of the Eye

Eyeball has three (3) layersScleroid coat is tough protective layer that is transparent in the front and is called the corneaChoroid coat is vascular and contains pigments to keepChoroid coat is vascular and contains pigments to keep the inside of the eye darkRetina contain the actual receptor cells (rods and cones)

Lens and ciliary musclesIris controls amount of light entering eyeballSpaces within eye are filled with fluid to maintain shape (aqueous and vitreous humors)

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Light Refraction

Light waves are refracted (or bent) primarily by the cornea and are focused by the lens onto the retinaLens must thicken to view close objectsCiliary body contracts to relieve tension on the suspensory ligaments to allow elastic lens to become thickerEye glasses or contacts can correct for defects in the focusing of images onto the retina

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Retina Receives Light-Converts To Nerve Impulses

Visual receptors are rods and conesRods are responsible for colorless vision in relatively dim illuminationyCones provide color vision but need higher levels of light

Visual receptors contain light-sensitive pigments that decompose in the presence of light and trigger nerve impulse

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Stereoscopic vision

Provides perception of distance and depthOccurs because of formation of twoOccurs because of formation of two slightly different retinal images that the brain superimposes and interprets as a single image in 3-DA one-eyed person uses relative sizes and positions of familiar objects to judge distance and depth

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Visual Nerve Pathways

Nerve fibers from the retina form the optic nerveMedial visual fields decussate at theMedial visual fields decussate at the optic chiasmaMost fibers enter the thalamus and synapse with others that continue to visual area in the occipital areaSome fibers pass into the brain stem and function in various visual reflexes