Chapter 013

The Human Body in Health and Illness, 4th edition

Barbara Herlihy

Chapter 13:Sensory System

Lesson 13-1 Objectives

• State the functions of the sensory system.• Define the five types of sensory receptors.• Describe the four components involved in the

perception of a sensation.• Describe the five general senses.

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Sensory System

• Sensory system allows us to experience the world.

• External information• Sound of a dog barking

• Internal information• Sudden change in blood pressure

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Five Types of Sensory Receptors

• Receptor: Specialized area of a sensory neuron that detects a specific stimulus– Chemoreceptors– Pain receptors (nociceptors)– Thermoreceptors– Mechanoreceptors– Photoreceptors

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Four Components of Sensation

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Two Characteristics of Sensation

• Projection• Adaptation

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Projection

• Brain refers sensation back to its sourceA. Ordinary injury B. Phantom limb pain

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Adaptation

• Adaptation: With continuous stimulation, sensory receptors become less responsive.

• Receptors vary in their ability to adapt. Smell and temperature receptors adapt well.

Pain receptors do not adapt at all.

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Five General Senses

• Pain• Touch• Pressure• Temperature• Proprioception

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Pain Receptors or Nociceptors

• Consist of free nerve endings stimulated by tissue injury, chemicals, tissue hypoxia

• Widely distributed throughout the skin, viscera, other internal tissues

• Do not adapt

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Sites of Referred Pain

• Compare heart’s location with possible sites of pain during a heart attack.

• Gallbladder attack may present with shoulder pain.

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Touch and Pressure Receptors

• Touch (tactile) receptors– Mechanoreceptors– Found mostly in skin

• Pressure receptors– Mechanoreceptors– Located in the skin, subcutaneous tissue, and deep

tissue

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Thermoreceptors

• Receptors for cold and heat• Located in free nerve endings and other

specialized sensory cells in the skin• Quick adaptation• Temperature extremes experienced as pain

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Proprioception

• Proprioception: Sense of orientation or position in space

• Receptors– Located in muscles, tendons, joints, and inner ear – Sensory information parietal lobe, cerebellum

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• Describe the five special senses.• Describe the structure of the eye.• Explain the movement of the eyes.• Describe how the size of the pupils changes.

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Five Special Senses

• Smell• Taste• Sight• Hearing• Balance

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Olfactory Sense: Smell

• Chemoreceptors in nasal tissue

• Nerve impulses travel on CN I to temporal lobe for interpretation

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Gustatory Sense: Taste

• Taste receptors are chemoreceptors.

• Nerve impulses move along three cranial nerves to parietal and temporal lobes.

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Vision: Sense of Sight

• Visual accessory structures • Primary visual structures are the eye and

visual pathway.

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Visual Accessory Structures• Eyebrows• Eyelids• Conjunctiva• Eyelashes• Lacrimal

apparatus• Extrinsic eye

muscles

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Eye: Organ of Vision

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Eyeball: Three Layers

• Sclera• Choroid• Retina

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Sclera

• Tough outer layer in posterior eyeball

• Forward extension becomes cornea

• Extrinsic eye muscles attach here

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Choroid

• Middle layer in the posterior eyeball

• Forward extension becomes ciliary body and iris

• Highly vascular to nourish retina

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Retina

• Inner layer in posterior eyeball

• Site of photoreceptors Rods Cones

• Optic disc Exit of CN II

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Retinal Photoreceptors

• Rods– Located on periphery – Responsible for black and white or night vision

• Cones– Located on central part of posterior eye– Concentrated in fovea centralis in center of

macula lutea– Responsible for color vision

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Cavities of Eyeball• Posterior cavity

– Between lens and retina

– Contains vitreous humor

• Anterior cavity– Between lens and

cornea– Contains aqueous

Formation and Drainage of Aqueous Humor

• Formed by ciliary body

• Circulates through pupil behind cornea

• Drains through canals of Schlemm

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Muscles of the Eye

• Extrinsic muscles: Move eyeball in its bony orbit

• Intrinsic muscles: Move structures within eyeball– Iris– Ciliary muscles

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Extrinsic Muscles of the Eye

• Four rectus muscles• Two obliques• Primary innervation

from CN III

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Three Intrinsic Eye Muscles

• Iris Circular muscle

• Miosis• Muscarinic receptors

Radial muscle• Mydriasis• Alpha 1 receptors

• Ciliary muscles

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Eye Disorders

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Refraction: Lens

• Ciliary muscles pull on suspensory ligaments.

• Suspensory ligaments pull on lens.

• Lens changes shape.

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Refraction

• Bending light rays to focus on retina

• Lens, primary refracting structure

• Focal point on retina

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Errors of Refraction

• Myopia, focal point in front of retina

• Hyperopia, focal point behind retina

• Astigmatism, result of irregularly curved cornea

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Visual Pathway

• Photoreceptors generate nerve impulse

• Nerve impulse travels along CN II to occipital lobe

• Occipital lobe “sees” Rover

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Visual Pathway: Optic Chiasm • Lateral fibers of CN II

ascend to same side of brain.

• Medial fibers of CN II cross to opposite sides, forming the optic chiasm.

• The brain sees one image.

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How Seeing Occurs

• Pathway of light Cornea aqueous humor pupil lens

vitreous humor rods and cones

• Pathway of nerve impulses Rods and cones CN II occipital lobe

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• Describe the three divisions of the ear.• Describe the functions of the parts of the ear

involved in hearing.• Explain the role of the ear in maintaining the

body’s equilibrium.

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Sense of Hearing: Three Parts of Ear

• External to tympanic membrane

• Middle from tympanic membrane to oval window

• Inner behind oval window

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Three Parts of Ear: Contents

• External – Auricle, external auditory canal

• Middle– Malleus, incus, stapes– Eustachian tube

• Inner– Cochlea, vestibule, semicircular canals, origin of

CN VIII

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Eustachian Tube • Connects throat with

middle ear• Equalizes pressure

across tympanic membrane

• In young child, short and horizontal

• In adults, longer and more vertical

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Receptors : Organ of Corti

• Mechanoreceptors (hairs) in cochlea of inner ear

• Organ of Corti in endolymph

• Cochlear branch of CN VIII

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How Hearing Occurs

• Pathway of vibrations Sound waves tympanic membrane ossicles oval window organ of Corti

• Pathway of nerve impulses Organ of Corti CN VIII (cochlear branch) temporal lobe

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Recap: How Hearing Occurs

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Receptors for Balance

• Mechanoreceptors (hairs) in vestibule and semicircular canals of inner ear

• Hairs in endolymph• Vestibular branch of

CN VIII

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