Somatic Nervous System

  1. Nervous Sys. Organization
    1. Anatomical - covered in A&P I
      1. CNS
      2. Peripheral
        1. afferent - sensory - to CNS
        2. efferent - motor - from CNS
    2. Functional - involves both CNS & peripheral
      1. Autonomic Nervous System - involuntary - not under conscious control covered in A&P I
        1. smooth musc. cardiac musc. glands
      2. Somatic Nervous System - voluntary - conscious control
        1. CNS -> Skel. musc.
        2. sensory -> CNS

  2. Sensory Perception
    1. Bkg.
      1. input from environment -> CNS
      2. detected by sensory neurons
        1. many diff. types
      3. part of peripheral nervous sys.
    2. Structural receptor types
      1. structural
        1. free nerve ending - dendrite embedded in tissue
          1. receive sensation
          2. ex. pain & temp.
        2. encapsulated ending - within connective tissue
          1. enhances sensitivity
          2. ex. pressure & touch
        3. specialized receptor cells
          1. ex. eye photoreceptors
      2. locational
        1. exteroceptor - near external environment stimulants
        2. interoceptor - signals from internal organs
        3. proprioceptor - near moving part of body
      3. functional - what detect
        1. ions & macromolecules
        2. physical variations
        3. electromagnetic (ex. light)
      4. type of stimuli
        1. chemoreceptors
        2. osmoreceptors - solute []s of body fluids
        3. nociceptor - pain - chemicals from tissue damage or similar intense stimuli
        4. mechanoreceptors - physical stimuli - pressure, vibration, sound, body position
        5. thermoreceptor
    3. Sensory modalities
      1. general sense - through out body with receptors within organs
        1. done through mechanoreceptors in skin, musc. & blood vessel walls
        1. proprioception - body position
        2. kinesthesia - body movement
        3. visceral - internal organs (autonomic)
      2. special sense - specific organ devoted to sense
        1. ex. eye, inner ear, tongue, nose

    Special Senses

  3. Background
    1. Special senses = vision, smell, taste, hearing, & equilibrium
      1. touch is the combined activity of the general senses C13
      2. What makes them special is their structure - they have distinct receptor cells
        1. confined to the head region
    2. Job = detect stimuli and respond to those stimuli by generating impulses on sensory neurons for transmission to CNS
      1. Identical input to the CNS - All receptors convert stimulus to a nerve impulse on a sensory neuron
        1. All impulses are identical
        2. it is not the receptor that determines the sensation/perception
        3. the area of the cerebral cortex that receives & analyzes the impulses determines the perception
  4. Gustation = Taste
    1. taste buds - sensory organs for taste - 10,000 or so
      1. few on soft palate, cheeks, pharynx, & epiglottis
      2. most on papillae - peg-like projections on the tongue
        1. consists of 50 - 100 epithelial cells - three types
          1. supporting cells - bulk of taste bud
          2. gustatory epithelial cells = taste (receptor) cells
            1. each has associated dendrites of neurons
            2. sense chemicals in solution (usu. dissolved in saliva)
            3. replaced every 7 - 10 days
          3. basal epithelial cells = stem cells
            1. divide & differentiate into gustatory epithelial cells
            2. helpful cause lose gustatory epithelial cells from friction & burning
    2. taste sensations
      1. Sweet - org. cpds bound via G protein receptor (glucose, fructose, artificial)
      2. Sour - acids specifically the H+ via channels
      3. Salt - metal ions - Na+ channel entry
      4. Bitter - alkaloids = N org. cpds usu. of plant origin via G protein receptors
        1. in coffee, beer, wine tannins, tea
      5. Umami - aa glutamate = 'beef taste', aging cheese, & MSG via G protein receptors
      6. ? long-chain fatty acids - why like fatty foods
  5. Olfaction = smell
    1. olfactory epithelium = location
      1. a yellow tinged patch in the roof of the nasal cavity
        1. 5 cm2 - small
        2. pseudostratified epithelium
      2. air must make a hairpin turn to reach
        1. why sniffing (drawing in more air) helps
    2. cells
      1. olfactory sensory neurons - bipolar neurons
        1. dendrites extend from epithelium into the mucus that lines the nasal cavity
        2. where bind cpds dissolved in mucus
      2. supporting cells - bulk of epithelial membrane
        1. contain a yellow-brown pigment
      3. olfactory stem cells - at base of epithelium, short cells
        1. olfactory sensory neurons only live for 30 - 60 days
        2. replaced by olfactory stem cell differentiationreceptors = detect odor chemicals in soln. - G protein receptors
      4. no good classification of basic smells
      5. most are combinations of chemicals
      6. 400 "smell genes" = code for a unique receptor protein
        1. receptor protein responds when bind cpd.
        2. usu. a single receptor can respond to more than a single cpd.
          1. and the same type of cpd can bind to diff. receptors
        3. only 1 receptor protein type per cell
      7. very sensitive - binding only a few molecules can initiate an action potential
    3. signal transduction
      1. the action potential of the olfactory epithelium travels up the olfactory sensory neuron axons which extend through the ethmoid bone to the . . .
      2. olfactory bulb - on the ventral surface of the frontal lobe
        1. here are the olfactory sensory neuron terminal branches which synapse with . . .
      3. mitral cell dendrites
        1. pick up neural transmitters from olfactory epithelium bipolar neurons
        2. the mitral cell axons then travel to brain
        3. some to temporal lobe
        4. some to limbic system & hypothalamus
          1. where smells become associated with long-term memory & emotional response
      4. refine & amplify signals from olfactory sensory neurons
  6. The Ear: Hearing & Balance
    1. Structure of the ear for hearing
      1. Outer (external) Ear
        1. auricle (pinna) = what most people call the ear
          1. shell-shaped projection surrounding the opening of the external auditory canal
          2. composition - elastic cartilage covered with thin skin & occasional hair
          3. function = direct sound waves into the external auditory canal
        2. external auditory canal (acoustic meatus) = short curved tube that extends from the auricle to the eardrum
          1. lined with skin bearing
            1. hairs, sebaceous glands, &
            2. ceruminous glands (se-roo' mi-nus) - modified apocrine sweat glands
            • cerumen = ear wax - yellow secretion keeping foreign materials/insects out
          2. external auditory canal ends at the . . .
        3. tympanic membrane = eardrum
          1. thin, translucent, CT membrane, covered by skin on external surface
          2. shaped like a flattened cone with apex protruding into middle ear
          3. sound waves make the ear drum vibrate
          4. divides the external ear with the . . .
      2. Middle Ear = air-filled cavity with three small bones
        1. ossicles - span the middle ear
          1. malleus (mal'e-us) = hammer - secured to the eardrum
          2. incus (ing'kus) = anvil
          3. stapes (sta'pez) = stirrup
        2. Eustachian tube
          1. connect the middle ear with the nasopharynx
          2. norm. tubes flattened & closed - opened up when yawn or swallow
          3. function - equalize pressure in middle ear
            1. impt. to equalize pressure so ear drum function properly (not distort sounds)
          4. otitis media = middle ear inflammation
            1. common in children with shorter more horizontal tubes
            • infection treated with antibiotics
        3. the ossicles transmit the vibratory motion of the ear drum to oval window
          1. on the medial side of the middle ear
          2. on the other side of the oval window is the ...Inner ear = labyrinth - complicated shape
          3. lies deep within the temporal bone - behind the eye socket
            1. very protected, delicate structures
          4. fluid filled
          5. as stapes moves, it moves the oval window, which creates waves in the fluid
          6. the waves travel into the . . .
        4. cochlea - Latin = "snail" because it is snail shaped - about 2½ turns
          1. responsible for hearing
          • transferring the liquid waves into neural signals
          1. if uncoil -long tube with three inner tubes
          1. scala vestibuli - begins at the scala vestibule & winds around inner contour (superior) of the cochlea duct
            1. ends at the apex
          2. scala tympani - open to and begins at the end of scala vestibuli = apex
            • open (shares fluid) with scala vestibuli
            • follows the outer contour (inferior)
            • ends at the ...
            1. round window = membrane btw. inner & middle ear but not assoc. with stapes
            • remember inner ear is not fluid filled
            • so as round window vibrates - it only moves against air & produces nothing
            • this is the end - it vibrates only with sounds too  or  to hear
          3. cochlear duct (scala media) - between the scala vestibuli & scala tympani
            • closed fluid filled chamber containing . . .
            1. tectorial membrane
            • runs length of cochlear duct on scala vestibuli side (superior)
            1. basilar membrane
            • also runs length of cochlear duct but on scala tympani side (inferior)
            • narrow & thick toward the oval window
            • widens and thins toward the cochlear apex
            1. organ of Corti - composed of hair cells
            • attached to basilar membrane so that it is btw tectorial & basilar membrane
            • actual sense organ for hearing
            • stereocilia on hair cells - actin stiffened microvilli
            • the stereocilia (hairs) stick up and touch the tectorial membrane
            • the other end of the hair cells are associated (very near, synapsed) with. . .
            • cochlear nerve dendritesPhysiology of Hearing
      3. When vibrations approach the ear, they are gathered & funned by the pinna into the external auditory meatus
      4. which channels the vibrations to the ear drum
      5. as the ear drum is pushed & pulled by the pressure waves the movement is transferred
      6. to the ossicles (malleus -> incus -> stapes) which then transmit the movement to the oval window
        1. Because the oval window is smaller it vibrates about 20 X more vigorously than the ear drum
      7. the vibrations are transmitted though the fluid in the inner ear vestibule
        1. which transfers the waves to the scala vestibuli
      8. The sound waves now fluid waves can take two pathways . . .
        1. pass through the fluid of the scala vestibuli around the apex then through the scala tympani, culminating at the round window
          1. causing the round window to bulge out into the middle ear which contains air
          2. You do not hear these waves - too low (< 20 Hz)
          3. the round window is acting as a pressure valve
        2. pressure waves in the scala vestibuli take a 'shortcut' through the cochlear duct into the scala tympani & then to the round window
      9. In this second pathway the basilar membrane moves  &  (vibrate)
        1. since the organ of Corti rides on top of the basilar membrane the organ of Corti also vibrates
        2. Key here is the tectorial membrane does not vibrate
        3. So the "hairs" (microvilli) are bent back & forth with the oscillating basilar membrane
        4. The mechanical deformation of the hairs alternately opens & closes mechanically gated ion channels in the hair cell resulting in depolarizing & hyperpolarizing potential changes
        5. causing the hair cells to release (glutamate) a neurotransmitter to the cochlear nerve dendrites
        6. which initiate action potentials through their axons eventually reaching the brain and you "hear"
      10. Notes on physiology of hearing
        1. hearing can be viewed as a modified form of touch
          1. impulses generated by the bending of hairs.
        2. Pitch determination is provided by differences of the basilar membrane
          1. narrow & stiff near the oval window, wide & flexible at the apex
          2. So diff. regions vibrate to diff frequencies (pitches)
          3. 20 - 20,000 hertz
            1. 20 Hz near apex, 20,000 Hz near entrance
        3. Volume - translated into the force of the waves in the fluid
          1. the diff. btw. the  pressure & the  pressure areas of the wave
          2. translates into the force of which the 'hairs' are pushed against the tectorial memb.
          3. the 'hairs' can detect movement as small as the diameter of a H atom (smallest)
          4.  volume can shear off the 'hairs' or permanently distort them -> hearing lossStructure of the ear for balance - Equilibrium
      11. vestibule - other room off of inner ear
        1. divided into two structures (sub-rooms)
      12. utricle - (u'tri-kl) & saccule - first room(s)
        1. fluid filled sac off the vestibule of the inner ear
        2. responsible for head position
        3. this fluid is continuous with the . . .
      13. semicircular canals - off of utricle
        1. responsible for head movement
        2. fluid filled semicircular canals (tubes)
        3. each about 2/3 of a circle
        4. three canals - each oriented in one of three planes of space
          1. anterior, posterior, (right angle to each other) & lateral (horizontal)
        5. ampulla - swelling at end of each canal
          1. where the canals meet again with the vestibule
          2. where equilibrium receptors (hair cells) are located
    2. Physiology of equilibrium
      1. our sense of equilibrium not totally depend on input form the inner ear but also from:
        1. vision & info. from stretch receptors of muscles & tendons
        2. all help brain build a picture not only of body parts in relation to each other but also in relation to the outside world
      1. static equilibrium = position of the nonmoving head relative to gravity
        1. utricle & saccule
        2. gravity is constant - so static
          1. "tilt meter" - tells us "which way is up"
        3. macula tissue - within the utricle & saccule
          1. composed of hair cells and support tissue
        4. otoliths - calcium crystals within the saccule & utricle
          1. the saccule & utricle are lined with macula tissue = sensory hair cells like in the organs of Corti
          2. he otoliths are quite dense and always fall to the 'bottom'
          3. when they are resting on the bottom they bend the sensory hairs of the macula tissue which initiate nerve impulses to the brain
          4. the brain then interprets the sensory input as head position relative to gravity
      2. dynamic equilibrium = monitors the turning motions of the head
        1. involves the three semicircular canals
          1. remember each is oriented in each of the three principal axes
        2. within each semicircular canal is an ampulla & within each ampulla is a tuft of sensory hairs
          1. when the head is turned, the fluid in the canals lags behind by inertia
          2. this causes the sensory hairs to bend
          3. the bending causes the hairs to generate sensory impulses that travel to the brain
          4. and are interpreted as head movement
        3. when the head stops turning the fluid continues to flow, briefly
          1. this causes the hairs to bend in the opposite direction
          2. which causes impulses to the brain to be generated
          3. if you spin around too much and stop suddenly, the fluid continues to move but you are not -> dizziness
        4. thus the turning and stopping of the head provides information to the brain about turning motions of the head which is integrated by the brain.
        5. since proprioceptive (visual, muscular) info also goes to the brain the brain may receive conflicting input leading to motion sickness, sea sicknessVision
    3. Accessory structures
      1. eyebrows - short, course hairs above the exposed portions of the eyes
        1. shade from sunlight
        2. channel sweat running down the forehead laterally away from the eyes
      2. eyelids - two on each eye
        1. upper = larger & lower
        2. thinnest skin of body
        3. can be closed completely to cover the eyes' exposed surfaces
        4. reflex blinking helps prevent drying by spreading oil, mucus, & saline soln.
      3. eyelashes - line the margins of the eyelids
        1. follicles have many nerve endings which respond to slight touches
          1. triggering reflex blinking
      4. conjunctiva - thin transparent mucous membrane that covers the exposed surface
        1. along with other glands it produces a lubricating mucus that prevents drying out
        2. conjunctivitis (pinkeye) = inflammation/infection of this membrane
      5. lacrimal gland = tear producing gland
        1. exocrine glands
        2. located in the upper outer (superior lateral) corners of the orbital cavities
          1. tears = dilute saline solution - tears
            1. contents = water, salt, mucus, antibodies, & lysozyme
            • so it cleans & protects while moisturizes & lubricates
        3. blinking spreads tears downwards & across eyeball
        4. excess tears follow a canal to the nasal cavity
      6. extraocular muscles
        1. six strap-like muscles control movement of each eyeball
        2. 4 - superior, inferior, lateral & medial rectus muscles
        3. 2 - oblique muscles - superior & inferior oblique muscles
          1. rotates the eye downward (inferiorly) or upwards (superiorly)
    4. Eye structure = complex organ housing photoreceptors
      1. located within the orbital cavity which provides protection
        1. surrounds 80% of eye
        2. 2.5 cm (1 inch) diameter slightly irregular sphere
      1. fibrous tunic (layer) = outermost coat of eye
        1. dense avascular CT with two diff. regions
        1. sclera - posterior portion
          1. bulk of fibrous layer
          2. tough glistening white & opaque = "white of eye"
        2. cornea = transparent anterior part of the eye
          1. bulges forward
          2. helps focus
          3. non-vasculated \ easily transplanted - beyond reach of immune sys.
          4. well supplied with many nerve endings - mostly detect pain
          5. what touch when putting in contacts
      2. vascular tunic (layer) = middle coat of eyeball
        1. also called the uvea (u've-ah "grape")
        2. pigmented
        3. 3 regions
        1. choroid (core roid) = posterior 5/6th of uvea
          1. highly vascular - provides nutrition to other layers
          2. dark brown - from melanocytes - absorb light so no scattering or reflecting
        2. ciliary body = muscle tissue, anterior
          1. thickened ring of tissue that encircles the lens
          2. contains smooth muscles that attach to the lens -> control lens shapeiris = visible colored part of eye
          3. most anterior part of the vascular tunic
          4. btw. cornea & lens
          5. shaped like a flattened doughnut
          6. double layer of smooth muscle - 1 opens & other  size of . . .
          7. pupil = allows light to enter eye - defined by opening of iris
          8. only contain 1 pigment = brown - color depends on amt. of pigment
      3. neural tunic = retina = inner layer
        1. lines the posterior 3/4 of the eyeball
        2. spread out it is roughly the size of a postage stamp
        3. contains millions of photoreceptors - transduce light E to neuronal impulses
        4. this layer is held against the vascular tunic by fluid pressure in the eyeball
          1. detached retina - fluid gets btw two layers and separate neural layer from nutrients
      4. lens - biconvex, transparent (avascular), flexible struct. that can change shape
        1. focuses light on retina
        2. held in place by ligaments from ciliary body
      5. internal chambers - divided by lens - all fluid filled
        1. posterior segment - larger area btw lens & retina
          1. vitreous humor = clear gel that binds lg. amts. of water & fills cavity
            1. forms in embryo & lasts life time
        2. anterior segment - btw. cornea and lens
          1. contains the iris and ciliary body
          2. aqueous humor fills segment = clear fluid similar to blood plasma
            1. form & drain continuously
            2. supplies nutrients & oxygen to the lens & cornea & carries away wastes
            3. if drainage clogged -> glaucoma - results in blindness by damaging the optic nerve
    5. Physiology of Vision
      1. Light processing before reach retina
        1. refraction = the change in the speed of light as it passes from one medium to another
          1. 90% of refraction is performed by the smooth curve of the cornea
            1. acts as a fixed focus lens due to its fixed shape
          2. 10% of refraction is performed by the lens
            1. flexible \ can adjust the amount of remaining focusing to adjust for
            2. objects viewed at different distances
            3. accommodation - process that increases the refractory power of the lens
            • to focus on close objects the lens muscles contract - change the shape of the lens
            • when the muscles relax the lens returns to its stretched, thinnest, shape by itself
            • which allows things far away to be focused on
          3. read in text about myopia & hyperopia
      2. photoreceptors = modified neurons with tips imbedded in pigmented layer of retina
        1. about a quarter billion
        2. use pigments to detect light - Hphobic so imbedded in membrane by proteins
        1. phototransduction
          1. retinal = pigment with two conformations (shapes)
            1. 11-cis-retinal - bent FA chain
            • made from Vit. A in dark
            • without enough Vit. A in diet can not synthesize
            • so can not see in low light levels = night blindness
            1. all-trans-retinal - straight FA chain
          2. opsins = proteins that bind retinal forming . . .rhodopsin
          3. process
            1. when light strike 11-cis-retinal it changes shape to trans
            2. this causes opsin to change shape which triggers a G protein which causes membrane repolarization & neurotransmitter release from the photoreceptor cells to the bipolar neurons.
        2. rod cells - slender & rod shaped
          1. along rod part of cell are stacks of membrane with rhodopsin
          2. do not perceive color
          3. very sensitive to light so function at low light levels, night
            1. at high light levels rhodopsin is broken down faster than it can be resynthesized
            • so rod cells become bleached of pigments
            • so when enter dark area from a bright area ->
            • takes time to resynthesize rhodopsin -> why can't see right away
        3. cone cells - low sensitivity to light so only respond to high levels of light
          1. short & conical
          2. react more rapidly & do not get bleached (require  light levels)
          3. color
            1. three types of cone cells & each responds to a diff. color
            • all millions of colors composed of three colors
            • Primary colors = red, blue, green
      3. signal transduction
        1. the light sensing part of the photoreceptors (rods & cones) is embedded in the pigment cell
          1. Note: pointed away from the lens
          2. light receiving part of cell is constantly renewed
            1. old parts are phagocytized by pigmented cells ("inverted by design)
        2. the other end of rod & cone cells (pointed toward lens) is associated with bipolar neurons
          1. the bipolar cells then associate with ganglion cells
          2. the ganglion cells' axons run along the inner surface of the retina
            1. they combine to form the optic nerve
          3. blind spot (optic disk) = place where the optic nerve exits the eye
            1. contains no photoreceptors
    6. Photoreceptor distribution
      1. wiring
        1. cones - usu 1 cone associates with 1 bipolar cell
          1. why sharp detailed vision
        2. rods - usu multiple rods assoc. with 1 bipolar cell
          1. which makes even more rods feed into a ganglion cell
          2. why night vision is fuzzy & indistinct
      2. macula lutea (mak'u-lah lu'te-ah) = highly pigmented oval section of retina
        1. lateral to blind spot & precisely @ eye's posterior end
        2. point where the cornea & lens focus most precisely
        3. cones are the predominant receptors
        4. within the center of the macula lutea is the fovea centralis
          1. precise focusing point
          2. only cones are present
      3. moving away from the macula lutea the number of cones  & the number of rods 
        1. so that at the edges of the retina only rods are present
    7. Depth perception
      1. with our two eyes having overlapping fields of vision, we have the ability to accurately judge the distance of objects that we see (3-D or stereo vision)
      2. the visual pathway relaying sensory impulses from the retina to the brain crosses over along the route, such that each side of the brain receives input from both eyes
      3. within the brain, the two images are merged into a single picture with depth
      4. ex. stereopticon, View-Master