Special Senses

• transduction: physical or chemical stimulus is converted into action potential that can be interpreted by brain
• stimulus alters resting membrane potential of sensory neurons to produce action potential that reaches CNS
• special senses also detect stimuli (light, chemicals, or sounds) and transduce them into action potentials

  • also propagated through axons of peripheral neurons to CNS
• stimuli (both general and most special ) are first processed by sensory nuclei and transmitted to thalamus and primary cortex areas for awareness and identification
• special senses: convey specific stimuli from specialized sensory organs in discrete locations of head

  1. smell (olfaction)
  2. taste (gustation)
  3. vision
  4. hearing (audition)
  5. vestibular sensation
• general senses involve detection of touch, pain, and temperature

  special sensory involve detection of light, sound waves, head movements, and chemicals
• any special senses rely on receptors that are not neurons except oflaction

  general sensory receptors are receptive ends of sensory neurons
• special sensory organs are confined to head (cranial nerves)

  general sensory information travels on axons of both cranial and spinal nerve
• accessory structures for vision

  1. eyebrows
  2. eyelids/palpebrae
  3. conjuctiva
  4. lacrimal apparatus
• eyebrows (vision)

  • overlie the supraorbital margins of the skull
  • shade the eye from sunlight
  • prevent perspiration from reaching the eyes
• eyelids/palpebrae (vision)

  • separated by palpebral fissure (eyelid slit)
  • meet at medial and lateral commissures
  • lacrimal caruncle
  • at medial commissure
  • sebaceous and sweat glands
  • produce oily secretion
  • epicanthic fold
  • vertical fold of skin that appears lateral to nose
  • tarsal plates
  • connective tissue sheets that support the eyelids
  • anchor to the orbicularis oculi and levator palpebrae superioris
  • eye lashes
  • innervated hairs at rim of eye
  • reflex blinking when stimulated
  • tarsal glands
  • embedded in the tarsal plates
  • modified sebaceous glands
  • prevents lids from sticking
• conjunctiva (vision)

  • transparent mucous membrane
  • lines eyelids
  • palpebral conjunctiva
  • folds over the anterior surface of the eyeball
  • bulbar conjunctiva
  • vascularized
  • conjunctival sac
  • space between the eyelid and eyeball
  • produces mucus that lubricates eye
• lacrimal apparatus (vision)

  • lacrimal gland
  • in the medial portion of the eye
  • produces lacrimal secretions/tears
  • contains mucus, antibodies, and lysozymes
  • help "clean" the eye
  • lubricates the eye
  • lacrimal canaliculi
  • via lacrimal punctata
  • lacrimal sac
  • drains the canaliculi into the nasolacrimal duct
• extrinsic eye muscles: allow eyes to follow objects

  • rectus muscles
  • originate from common tendinous ring
  • superior
  • inferior
  • lateral
  • medial
  • oblique muscles
  • moves the eye vertically when eye is turned
  • superior oblique
  • passes through the trochlea
  • fibrocartlagenous loop suspended by the frontal bone
  • rotates eye downward and laterally
  • inferior oblique
  • rotates the eye up and laterally
  • innervation
  • abducens (lateral rectus)
  • trochlear (superior oblique)
  • oculomotor (all others)
  • motor units
  • ~8-12 muscle fibers
  • highly precise
• pupillary sphincter muscle

  • parasympathetic control
  • in increased light intensity, pupil contract to make pupil smaller allowing less light in
• pupillary dilator muscle

  • sympathetic control
  • in decreased light intensity, pupil contracts to make pupil larger allowing more light in
• eyeball includes 3 layers

  1. fibrous layer
  2. vascular layer
  3. neural layer
• fibrous layer

  • fibrous layer: dense avascular connective tissue
  • sclera: toughened white portion of the eye
  • continuous with the dura mater of the brain
  • cornea: allows light into the eye
  • transparent layer
  • epithelia covering
  • external
  • stratified squamous
  • internal/deep corneal endothelium
  • simple squamous
  • contains active sodium pumps
  • maintains water content at low levels
  • no direct vascularization
• vascular layer

  • choroid
  • heavily vascularized & pigmented
  • vessels supply the rest of the eye
  • pigment reduces scattering & reflection of light
  • ciliary body: encircles the lens
  • ciliary muscles
  • smooth
  • encircles the lens & control shape
  • ciliary processes: secrete fluid that fills anterior portion of eye
  • suspensory ligament/ciliary zonule: hold the lens in an upright position
  • iris: colored portion of the eye
  • only contain brown pigment
  • coloration is the result of light scatter
  • pupil: opening allows light to enter
  • comprised of 2 smooth muscle layers
  • pupillary sphincter muscle
  • pupillary dilater muscle