section 1, chapter 12: general senses

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