Sensory System

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General senses• Receptors that are widely distributed throughout the body

• Skin, various organs and joints

Special senses• Specialized receptors confined to structures in the head

• Eyes, ears, nose and mouth

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Receptors, Sensation, and Perception

• Sensory receptors• Specialized cells or multicellular structures that

collect information from the environment

• Stimulate neurons to send impulses along sensory fibers to the brain

• Sensation• A feeling that occurs when brain becomes aware of

sensory impulse

• Perception• A person’s view of the stimulus; the way the brain

interprets the information

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Receptor Types• Chemoreceptors

• Respond to changes in chemical concentrations

• Pain receptors (nociceptors)• Respond to tissue damage

• Thermoreceptors• Respond to changes in temperature

• Mechanoreceptors• Respond to mechanical forces

• Photoreceptors• Respond to light

• Osmoreceptors• Respond to changes in solute concentration

Heat Light

sound pressure

chemical change

Modalities

• Receptors transform an external signal into a membrane potential

• Receptor Potential - separate receptor

• Generator Potential - a specialized ending

of an afferent neuron

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Sensory Impulses

• Stimulation of receptor causes local change in its receptor potential

• A graded electrical current is generated that reflects intensity of stimulation

• If receptor is part of a neuron, the membrane potential may generate an action potential

• If receptor is not part of a neuron, the receptor potential must be transferred to a neuron to trigger an action potential

• Peripheral nerves transmit impulses to CNS where they are analyzed and interpreted in the brain

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Sensory Adaptation

The ability to diminish the extent of their depolarization despite sustained stimulus strength

Types of receptors according to their speed of adaptation

• Tonic(Do not adapt or adapt slowly e.g. muscle stretch receptors and joint proprioceptors)

• Phasic(Rapidly adapting receptors e.g. Tactile receptors)

Mechanism of adaptation

Nerve Fiber Classification

General classification scheme– A fibers: Myelinated

• Subtypes: , , , , a b g d some overlap in ranges • Fastest conducting and largest diameter – (120 a

m/sec, 20 )m

– B fibers: Slower myelinated– C fibers: Un myelinated

• Slower conducting than As and smallest diameter (0.5 m/sec, 0.5 m)

Nerve Fiber Classification

Sensory nerve classification – I, II, III fibers: Myelinated

• Subtypes: Ia, Ib • Fastest conducting and largest diameter – Ia

– IV fibers: Unmyelinated• Slower conducting than IIIs and smallest diameter

Summation• Spatial• TemporalRelaying of signals through neuronal pools• Threshold---Subthreshold stimuli• Excitation or FacilitationDivergence and Convergence of signals passing through neuronal

pool

After Discharge• Synaptic after discharge• Reverberatory circuit• Continuous signal output• Rhythmical signal output

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General Senses

• Senses associated with skin, muscles, joints and viscera

• Three (3) groups:• Exteroceptive senses (exteroceptors)

• Senses associated with body surface such as touch, pressure, temperature, and pain

• Visceroceptive senses (interoceptors)• Senses associated with changes in the viscera such as

blood pressure stretching blood vessels and ingestion of a meal

• Proprioceptive senses• Senses associated with changes in muscles and

tendons such as at joints

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Touch and Pressure Senses Free nerve endings

• Common in epithelial tissues• Simplest receptors• Sense itching

Tactile (Meissner’s) corpuscles• Abundant in hairless portions of

skin and lips• Detect fine touch; distinguish

between two points on the skin• Adapt rapidly

Lamellated (Pacinian) corpuscles• Common in deeper

subcutaneous tissues, tendons and ligaments

• Detect heavy pressure and vibrations

Expanded Tip receptors (Merkel’s discs---- Iggo dome receptors

• Initially strong but partially adapting

• Localize touch and determine texture

Hair end organ• Adapts readily• Touch (movement of object on

surface of body + initial contact)

Ruffini’s endings• Slow adaptation• found in joint capsules

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Touch and Pressure Receptors

Epidermis

Dermis

(a)

(b)

(c)

Section ofskin

Free nerveendings

Epithelialcells

Sensorynerve fiber

Epithelialcells

Tactile (Meissner’s) corpuscle(touch receptor)

Sensory nervefiber

Lamellated(Pacinian) corpuscle(pressure receptor)

Connective tissuecells

Sensory nervefiber

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

b, c: © Ed Reschke

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Summary of Receptors of the General Senses

Dorsal column Medial leminiscal system

• Fine touch• Vibration• Position • Pressure

Anterolateral System

• Pain• Temperature• Crude touch• Tickle and itch• Sexual sensations

Layers of somatosensory cortex

• Incoming sensory signals excite layer VI• I & II Receive signals from lower centers of brain(Controls excitability of respective area stimulated)• II &III send axons to related portions on opposide

side of cortex through corpus callosum• V & VI Send axons to deeper parts of brainV ( more distant areas)VI ( Thalamus)

http://piclib.nhm.ac.uk/piclib/www/comp.php?img=87494&frm=med&search=sensory&first=1

Functions of somatosensory Area I

• Localize• Determine Pressure against body• Judge weights of objects• Judge shapes or forms• Judge texture

• Two point discrimination• Vibratory sense• Position sense

Two point discrimination

Vibratory sense

30-800 cycles/sec (Pacinian)2- 80 cycles/sec(Meissners)

Position senseMultiple types of receptors for

joint angulation

Spindle (mid ranges of motion)

Pacinian, Ruffinis, Golgi tendonExtremes of joint angulation