nervous s, sensory, lect 4

8/14/2019 Nervous S, Sensory, Lect 4

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Dr Sherwan R Shal 2008-2009

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A receptor is any structurespecialized to detect a stim

Some receptors are simple nerveendings, whereas others are seorgansnerve endings comwith connective, epithelial, ormuscular tissues that enhance

moderate the response to a14:59


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All sensory receptors aretransducers.A transduceris anydevice that

converts one form of energy t

anothera microphone, light bugasoline engine, for example.Sensory transducers convert stimu

energy into electrochemical enelectrochemical enaa meaningful pattern of action

potentials.14:59


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TheThe effecteffect of a stimulus on a recepof a stimulus on a recepto produce a type of local potential cto produce a type of local potential c

receptor potentialreceptor potential.. The receptor potential mayThe receptor potential may causecause

receptor cell (which is not always a nreceptor cell (which is not always a ntoto release a neurotransmitterrelease a neurotransmitter thatthat

stimulates an adjacent neuron.stimulates an adjacent neuron. If the receptor itself is a neuron aIf the receptor itself is a neuron an

voltage reaches threshold, the neurovoltage reaches threshold, the neuro

impulses to CNS.impulses to CNS.14:59


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This may result in a sensationconscious awareness of the stimbut much of the sensory informareaching the CNS produces no

sensation.We are seldom aware ofinformation concerning muscletension and blood pH, for examplit is vitally important to our

homeostasis for the CNS to monit14:59


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Modality refers to the type ofstimulus or the sensation itproduces.

Vision, hearing, and taste areexamples of sensory modalitie

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Location is also encoded by wnerve fibers are firing.

A sensory neuron receives input

an area called its receptive fieThe brains ability to determin

location of a stimulus depends the

size of this field.

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Intensity can be encoded in three wIntensity can be encoded in three w(a) As stimulus(a) As stimulus intensity risesintensity rises, th, thfiringfiring frequenciesfrequencies of sensory neof sensory nefibers risefibers rise

(b) Intense stimuli recruit(b) Intense stimuli recruit greatergreaternumbers of nerve fibersnumbers of nerve fibers to fireto fire(c) weak stimuli activate only the m(c) weak stimuli activate only the m

sensitive nerve fibers, whereassensitive nerve fibers, whereas stststimuli can activate a lessstimuli can activate a less

sensitive group of fiberssensitive group of fibers with with 14:59


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Duration is encoded in the way nerfibers change their firing frequencietime.

Phasic receptors generate a burst ofpotentials when first stimulated, then

quickly adaptquickly adapt and sharply reduce osignal transmission even if the stimucontinues.

Lamellated corpuscles, tactile receLamellated corpuscles, tactile recehair receptors, and smell receptohair receptors, and smell recepto

rapidly adapting phasic receptorrapidly adapting phasic receptor14:59


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Tonic receptors adapt slowlyadapt slowlygenerate nerve impulses mogenerate nerve impulses mo

steadilysteadily..

ProprioceptorsProprioceptors are among theslowly adapting tonic receptorsslowly adapting tonic receptorsbecause the brain must alwaysaware of body position, muscle

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All receptors, however, exhibitsensory adaptationif thestimulus is prolonged, firing

frequency and conscious sensadecline.

Adapting to hot bath water is aexample.

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Receptors can be classified by severaloverlapping systems:1. By stimulus modality: Chemoreceptors respond to chem

including

odors, tastes, and body fluid compositi Thermoreceptors respond to heacold.

Nociceptors1 (NO-sih-SEP-turs) apain receptors; they respond to tissu

damage resulting from trauma (blow14:59


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Mechanoreceptors respond physical deformation caused bvibration, touch, pressure,stretch, or tension.

They include the organs of hearinand balance and many receptothe skin, viscera, and joints.

Photoreceptors, the eyes,14:59


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2. By the origins of the stimuli: Interoceptors detect stimuli in thinternal organs and produce feelingvisceral pain, nausea, stretch, andpressure.

Proprioceptors sense the positionmovements of the body or its parts. Toccur in muscles, tendons, and jointcapsules.

Exteroceptors sense stimuli exte

the body; they include the receptors14:59


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3. By the distribution of receptors in the body. Ttwo broad classes of senses: General (somesthetic) senses, with recep

that are widely distributed in the skin, muscltendons, joint capsules, and viscera.

These include the sense of touch, pressure, stre

heat, cold, and pain, as well as many stimuli tdo not perceive consciously, such as blood prand composition.

Special senses, which are limited to the hand innervated by the cranial nerves.

The special senses are vision, hearing, equilibriu

taste, and smell.14:59


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Unencapsulated nerve endingare sensory dendrites that anot wrapped in connective tissu

They include free nerve endings,tactile discs, and hair receptors

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Free nerve endings include:warm receptors, which respond to rtemperature;

cold receptors, which respond to fallitemperature; and

nociceptors (painnociceptors (pain receptors).receptors).They are bare dendrites that have no s

association with specific accessory cetissues. They are most abundant in

epithelia and connective tissue.14:59


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Tactile (Merkel2) discs are treceptors for light touch, thoto sense textures, edges, and

shapes. Hair receptors (peritrichialendings) monitor the movemof hairs.

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Encapsulated nerve endings anerve fibers wrapped in gliaor connective tissue. Most of th

are mechanoreceptors for toucpressure, and stretch.They include muscle spindles a

Golgi tendon organs.14:59


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Tactile (Meissner4) corpusclephasic receptors for light touchtexture.

They occur in the dermal papillae oskin, especially in sensitive hairleareas such as the fingertips, palmeyelids, lips, nipples, and genitals

Tactile corpuscles enable you to tedifference between silk and sand

for example by light strokes of yo14:59


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Krause end bulbs are similatactile corpuscles but occur mucous membranes rather tha

the skin. Lamellated (pacinian) corpuare phasic receptors for deepressure, stretch, tickle, and

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Ruffinicorpuscles are tonicreceptors for heavy touch,pressure, stretching of the skin

joint movementsThey are located in the dermis,subcutaneous tissue, ligamentstendons, and joint capsules.

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nervous s, sensory, lect 4

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