Sensory Organs Sensory System

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Sensory Organs

Sensory System

  • Sensory system allows us to experience the world.

  • External information

      • Sound of a dog barking

  • Internal information

      • Sudden change in blood pressure

Five Types of Sensory Receptors

  • Receptor: Specialized area of a sensory neuron that detects a specific stimulus

    • Chemoreceptors

    • Pain receptors (nociceptors)

    • Thermoreceptors

    • Mechanoreceptors

    • Photoreceptors

Four Components of Sensation

  • StimulusReceptorSensory NerveBrain

    • Example:

Two Characteristics of Sensation: Projection and Adaptation

  • Projection: Brain refers sensation back to its source

    • Ordinary injury

    • Phantom limb pain

  • 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.

Five General Senses

  • Pain, Touch, Pressure, Temperature, Proprioception

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

Sites of Referred Pain

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

  • Gallbladder attack may present with shoulder pain.

Touch and Pressure Receptors

  • Touch (tactile) receptors

    • Mechanoreceptors

    • Found mostly in skin

  • Pressure receptors

    • Mechanoreceptors

    • Located in the skin, subcutaneous tissue, and deep tissue


  • 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


  • Proprioception: Sense of orientation or position in space

  • Receptors

    • Located in muscles, tendons, joints, and inner ear

    • Sensory information  parietal lobe, cerebellum

Five Special Senses

Smell, Taste, Sight, Hearing, Balance

Gustatory Sense: Taste

  • Taste receptors are chemoreceptors.

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

Olfactory Sense: Smell

  • Chemoreceptors in nasal tissue

  • Nerve impulses travel on Olfactory cranial (CN I) nerve to temporal lobe for interpretation

Vision: Sense of Sight

  • Visual accessory structures

  • Primary visual structures are the eye and visual pathway.

  • Visual Accessory Structures

  • Eyebrows

  • Eyelids

  • Conjunctiva

  • Eyelashes

  • Lacrimal apparatus

  • Extrinsic eye muscles

  • Eye: Organ of Vision

  • Eyeball: Three Layers

  • Sclera

  • Choroid

  • Retina

  • Sclera

  • Tough outer layer in posterior eyeball

  • Forward extension becomes cornea

  • Extrinsic eye muscles attach here

  • Choroid

  • Middle layer in the posterior eyeball

  • Forward extension becomes ciliary body and iris

  • Highly vascular to nourish retina

  • Retina

  • Inner layer in posterior eyeball

  • Site of photoreceptors

 Rods

 Cones

  • Optic disc

 Exit of CN II

  • 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

  • Cavities of Eyeball

  • Posterior cavity

  • Anterior cavity

    • Between lens and cornea

    • Contains aqueous humor

  • Formation and Drainage of Aqueous Humor

  • Formed by ciliary body

  • Circulates through pupil behind cornea

  • Drains through canals of Schlemm

  • Muscles of the Eye

  • Extrinsic muscles: Move eyeball in its bony orbit

  • Intrinsic muscles: Move structures within eyeball

    • Iris

    • Ciliary muscles

  • Extrinsic Muscles of the Eye

  • Four rectus muscles

  • Two obliques

  • Primary innervation

from CN III

  • Three Intrinsic Eye Muscles

  • Iris

 Circular muscle

      • Miosis

      • Muscarinic receptors

 Radial muscle

      • Mydriasis

      • Alpha 1 receptors

  • Ciliary muscles

  • Eye Disorders

  • Refraction: Lens

  • Ciliary muscles pull on suspensory ligaments.

  • Suspensory ligaments pull on lens.

  • Lens changes shape.

  • Refraction

  • Bending light rays to focus on retina

  • Lens, primary refracting structure

  • Focal point on retina

  • Errors of Refraction

  • Myopia, focal point in front of retina

  • Hyperopia, focal point behind retina

  • Astigmatism, result of irregularly curved cornea

  • Visual Pathway

  • Photoreceptors generate nerve impulse

  • Nerve impulse travels along CN II to occipital lobe

  • Occipital lobe “sees” Rover

  • 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.

  • 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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