Section 1, Chapter 12

The Senses

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Introduction

• General senses• Receptors are widely distributed throughout the body• Skin, various organs and joints• Touch, pain, temperature, pressure, ect.

• Special senses• Specialized receptors confined to structures in the head • Vision, Taste, Smell, Hearing

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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

(smell, taste, pH)

• Pain receptors (nociceptors)• Respond to tissue damage

• Thermoreceptors• Respond to changes in temperature

• Mechanoreceptors• Respond to mechanical forces• Stretch receptors, proprioceptors, baroreceptors

• Photoreceptors• Respond to light

• Projection• Process in which the brain projects the sensation back

to the apparent source• It allows a person to pinpoint the region of stimulation

Sensations and Perception

• Ability to ignore unimportant stimuli

• Involves a decreased response to a particular stimulus from the receptors (peripheral adaptation) or along the CNS pathways leading to the cerebral cortex (central adaptation)

• Sensory impulses become less frequent and may cease

• Stronger stimulus is required to trigger impulses

Sensory Adaptation

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12.3: 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

Lamellated (Pacinian) corpuscles• Common in deeper subcutaneous

tissues, tendons and ligaments• Detect heavy pressure and vibrations

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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

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b, c: © Ed Reschke

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Temperature Senses• Warm receptors

• Sensitive to temperatures above 25oC (77o F)• Unresponsive to temperature above 45oC (113oF)

• Cold receptors• Sensitive to temperatures between 10oC (50oF) and 20oC

(68oF)

• Pain receptors• Respond to temperatures below 10oC• Respond to temperatures above 45oC

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Sense of Pain

• Free nerve endings

• Widely distributed

• Nervous tissue of brain lacks pain receptors

• Stimulated by tissue damage, chemical, mechanical forces, or extremes in temperature

• Adapt very little, if at all

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Visceral Pain

• Pain receptors are the only receptors in viscera whose stimulation produces sensations

• Pain receptors respond differently to stimulation• Pain receptors are not well localized• Pain receptors may feel as if coming from some other part

of the body• Known as referred pain…

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Referred Pain• May occur due to sensory impulses from two regions

following a common nerve pathway to brain

Appendix

Ureter

Lung and diaphragm

Heart

Stomach

Pancreas

Colon

Kidney

Urinary bladder

Liver andgallbladder

Smallintestine

Ovary(female)

Liver andgallbladder

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Pain Nerve Pathways

• Acute pain fibers• A-delta fibers • Thin, myelinated• Conduct impulses rapidly• Associated with sharp pain• Well localized

• Chronic pain fibers• C fibers • Thin, unmyelinated• Conduct impulses more

slowly• Associated with dull, aching

pain• Difficult to pinpoint

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Regulation of Pain Impulses

• Thalamus • Allows person to be aware of pain

• Cerebral cortex • Judges intensity of pain • Locates source of pain• Produces emotional and motor responses to pain

• Pain inhibiting substances:• Enkephalins• Serotonin• Endorphins

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Proprioception

• Mechanoreceptors

• Send information to spinal cord and CNS about body position and length, and tension of muscles

• Main kinds of proprioceptors:• Pacinian corpuscles – in joints• Muscle spindles – in skeletal muscles*• Golgi tendon organs – in tendons*

*considered to be stretch receptors

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Stretch Receptors

(a)

Muscle spindleSkeletal muscle fiber

Golgi tendon organ

Tendon

(b)

Sensorynerve fiber

Sensorynerve endings

Sensorynerve fiber

Connectivetissue sheath

Intrafusalfiber

Skeletal musclefiber

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Visceral Senses

• Receptors in internal organs

• Convey information that includes the sense of fullness after eating a meal as well as the discomfort of intestinal gas and the pain that signals a heart attack

Summary of Receptors of the General Senses

End of Section 1, chapter 12