Ch. 15: Special senses

Ch. 15: Special senses

1. Sensation: an awareness of a change in internal or external environment

2. Perception: concious interpretation of a sensation

3. stimulus --> receptor ---> conduction ---> interpretation

4. Orbit: bony socket around the eye protecting the posterior 80%

   • Eyebrows: help shade the eye from sunlight and prevent sweat from trickling into the eye
     • Eyelid/palpebrae: protect the front of the eye
       • Levator palpebrae superioris muscle- muscle that raises the eyelid
       • Lacrimal apparatus: ductwork and lacrimal gland which makes lacrimal fluid (tears)
         • - lacrimal fluid contains mucus, antibodies, and lysozyme - cleanses, lubricates, and protects the eye
         • Pathway of lacrimal fluid flow
           • 1. blinking spreads the tears from the lacrimal gland to the medial corner of the eye 2. tears flow from the lacrimal punctum to the lacrimal canaliculus, to the lacrimal sac, to the nasolacrimal duct 3. lacrimal fluid empties into the nasal cavity below the inferior nasal concha
     • Ciliary muscles: controls shape of the lens
       • - located behind the iris - forms muscular ring around the lens - suspensory ligaments connect ciliary body to the lens
       • Iris: controls the diameter of the pupil
         • - circular fibers constrict the pupil - radial fibers dilate the pupil (pupil: opening that light passes through)
         • Anterior cavity
           • - space between lens and cornea that contains aqueous humor. Aqueous humor is a thin, watery fluid produced by specialized capillaries in the processes of the ciliary body. it supplies nutrients and O2
           • scleral venous sinus
             • blockage in this canal results in glaucoma
             • lens
               • - bioconvex disc made of proteins called crystallins, and anchored to the ciliary body by suspensory ligaments. - functions to focus light on the retina. - Clouding of the lens= cataracts * most refraction occurs at the cornea and lens Accomodation of the lens: refers to the lens changing shape in response to contractions of ciliary muscles to focus images on the retina. Ciliary muscles contract to show near images and relax to show far images.
           • Posterior cavity
             • - between the lens and the retina - contains vitreous humor Vitreous humor is transparent, jellylike fluid that maintains the shape of the eye and holds the retina in place. It is formed prenatally. 
       • Choroid: darkly pigmented area surrounding most of the inside of the eye
         • - functions to nourish eye and absorb light
         • Retina
           • - the retina forms images upside down and right to left. The brain can sense the degree of accomodation and convergence to judge the distance. 3 neuron layers:1. photoreceptors- - pigmented epithelium on the edge- outermost nerve cells that can sense light and initiate a nerve impulse - rods vs cones: rods (1 type)- provide black and white vision, are more sensitive to light and respond in dim light conditions, and exhibit much neuron convergencecones (3 types- blue, green, and red)- provide color vision, are less sensitive to light and respond in bright light conditions, and exhibit less neuron convergence2. bipolar neurons- middle layer that connects photoreceptors to ganglion3. ganglion neurons- innermost layer that synapse with bipolar neurons; their axons combine to form the optic nerve
           • Optic disc: "the blind spot"
             • - where the fibers of the optic nerve come together to penetrate the choroid and sclera - no photoreceptors here
           • macula lutea
             • - oval spot directly behind the pupil - high amount of cones here fovea centralis: depression in the middle of the macula lutea which contains only cones - this is the point of clearest vision
           • Retina initiation of nerve impulse
             • 1. light passes through the ganglion and bipolar neurons before striking the photoreceptors 2. photoreceptor will convert the light to to an electric signal and send it back to the bipolar neurons 3. the bipolar neurons send the signal to the ganglion neurons 4. ganglion neurons generate an action potential and the optic nerves convey the visual sensory information to the CNS
             • optic nerve ---> optic chiasma ---> optic tract ---> thalamus ---> occipital lobe
     • Fibrous tunic
       • - includes sclera (white of the eye) and cornea (clear window over iris and pupil) - outermost, tough fibrous connective tissue- gives shape to eyeball and protects inner structures
       • Vascular tunic
         • - includes iris, ciliary body, and choroid - middle layer that has many blood vessels
         • Nervous tunic
           • - innermost layer consisting of nervous tissue - converts light to nerve impulse
   • Conjunctiva: mucous membrane lining the inside of the eyelid
     • - covers the sclera but not the cornea - prevents the eye from drying out
     • The six extrinsic eye muscles: causes movement of the eyeball
       • 1. superior oblique (trochlear nerve IV) 2. medial rectus 3. lateral rectus (abducens nerve VI) 4. inferior rectus 5. superior rectus 6. inferior oblique -others are innervated by occulomotor nerve III)

5. Taste (gustation)

   • - there are 5 distinct tastes: 1. sweet 2. sour 3. bitter 4. salty 5. umami - 80% of taste is smell
   • papillae: bumps on the surface of the tongue that increase friction
     • - receptors for taste, chemoreceptors called taste buds, are mostly located on papillae
     • taste buds
       • - contain gustatory taste cells that have long microvilli called gustatory hairs - When activated, taste cells release neurotransmitter to dendrites of a sensory neuron - then, they send sensory info to the brain  Facial nerve VII innervates the anterior 2/3 of the tongue Glossopharyngeal nerve IX innervates the posterior 1/3 of the tongue
       • facial nerve ---> medulla oblongata ---> thalamus ---> insula

6. Smell (olfaction)

   • - olfactory receptor cells are chemoreceptors called bipolar neurons. they are located within the olfactory epithelium in the roof of the nasal cavity. - olfactory receptor cells have axons that travel through the cribiform plate of the ethmoid bone and carry impulses to the olfactory bulbs in cranial nerve I. from there, they go to the olfactory tract. From the olfactory tract, they either go to the thalamus or limbic system. the thalamus route leads to the olfactory complex or frontal lobe.

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