ch17: special senses

• Sensory receptors: specialized structures that respond to stimulus
• majority of sensory receptors are classified as “general senses” which are relatively simple in structure and include somatic and visceral sensory receptors.
• Somatic sensations receptors: tactile, thermal, pain, proprioceptive
• Visceral Sensations: detect information from internal organs that is not consciously perceived (changes in blood pressure, blood chemistry, gut stretch etc)
• special senses have relatively complex sensory organs found in specific areas of the head
• special sense have complex neural pathways (innervated by cranial nerves, have specific sensory areas located in cerebral cortex)
• Sense organ: structure composed of nervous tissue along with other issues that enhance its response to a certain type of stimulus.
• Eyebrows

  • shades from sunlight
  • catches perspiration from forehead
• Eyelids

  • skin-covered folds
  • protection from excess light and foreign objects
  • spread lubricating secretions over the eye
  • eyelashes trap and sweep away foreign particles
• Conjunctiva

  • transparent vascular mucus membrane
  • lines the inner surfaces of the eyelids and covers the anterior surface of the sclera
  • secrete mucus to prevent desiccation(extreme dryness) of the eye
• Extrinsic eye muscle

  • 6 muscles come from the bony orbit and insert on the sclera
  • maintain the shape of the eyeball, hold it in orbit, and provide precise eye movements
• Sty: infected hair follicle at base of eyelash
• Blood shot eyes: vessels in conjunctiva over sclera get irritated and dilated
• • Conjunctivitis: inflammation of the conjunctiva (by bacteria or virus)

  if sever it is highly contagious infection known as pinkeye
• Diplopia: is a problem with the extrinsic eye muscle

  • double vision; eyeballs can’t be focused on same visual field due to paralysis, weak muscle, alcohol consumption
• Strabismus: cross eyed; due to congenital weakness in eye muscles in which the affected eye rotates medially or laterally
• Lacrimal tear gland

  • in superior lateral region of each orbit
  • secretes lacrimal fluid(tear) continuously
• tears contain water, salt, mucus, antibodies and lysosome

  • clears, lubricates, and moistens the eye
• Lysozyme: an antibacterial enzyme that prevents infection
• Pathway of tears

  lacrimal gland → lacrimal ducts→ lacrimal fluid flows over the eye→ lacrimal punctum → lacrimal canaliculus→ lacrimal sac→ nasolacrimal duct→ nasal cavity
• wall of teh eye consists of 3 layers (tunic)
• Fibrous tunic

  • outermost avascular(no blood supply) layer
  • consists of sclera and cornea,
• Sclera -posterior portion

  • ‘white’ of the eye
  • the anchoring place for the extrinsic eye muscle
  • maintains the shape of the eye
  • protects inner surface
• Cornea- anterior transparent portion (to let light in) with a fixed curvature

  • The majority of light bending (refraction) occurs at the cornea to focus light rays
• The lens is the structure that bends light. can be altered by changing shape.
• Corneal transplants are successful because the corneal is avascular.

  • Rejection is rare
• Vascular tunic

  • the highly vascularized middle layer
  • consists of choroid, ciliary body, and iris
• choroid- darkly pigmented that can absorb excess light

  → contains many blood vessels

  →supply nutrients to the retina (only the posterior 1/3 part)

  *the central artery of the retina supplies the rest
• Ciliary body- muscular ring around the lens

  • consists of ciliary muscles and ciliary processes
• Ciliary muscles: linked up to the lens.

  • The smooth muscle that regulates the shape of the lens
• Ciliary processes: folds that contain vessels that produce aqueous humour that fill up the anterior cavity

  • suspensory ligaments extend from ciliary processes to hold the lens in position
• Irises: pigmented part of the eye

  • the amount of melanin will determine the eye colour
  • contains circular and radial smooth muscles
  • Iris's muscles adjust the pupil of the eye
• pupil: the opening in the iris

  • it is where the light enters
• Radial muscle make the pupil larger to get more light in

  = pupil dilates as radial muscles contract
• Circular muscle makes the pupil smaller to get less light in.

  • found close to the pupil

  = pupil constricts as circular muscles contract
• Retina

  • innermost delicate layer of the eye
  • consists of 2 main layers= pigmented and neural layer
• Pigmented layer: closest to choroid(the dark layer that absorb excess light)

  → absorbs excess light to reduce scattering
• neural layer: visual part of the retina

  → subdivided into 3 distinct layers ( photoreceptor layer, bipolar cell layer, ganglion cell layer
• Photoreceptor layer: consists of rods and cones that are used to see colour.
• Rods: 120mil dim light receptors

  →excited by low light intensity

  → produce images in shades of grey