Bio 168 Sensory Systems Study Guide
Fall Semester 2004
Learning Objectives, Topics & Keywords
Readings: Martini (5th ed) 481 487; Chapter 17.
Overview: Sensory information answers three questions about a stimulus: What?, Where?, How strong?
1. Understand the fundamental properties of sensation
stimulus --> sensory receptor --> generator potential --> action potentials --> CNS
external energy leads to depolarization of sensory neuron membrane potential
sensory receptor converts the generator potential to action potentials
= generator potential - graded with stimulus intensity
depolarization of receptor generates action potentials
stronger stimulus --> larger generator potential --> greater rate of action potentials
intensity of stimulus is coded in frequency of action potentials (the "how strong")
receptive field
some senses: receptor potentials in sensory cells which signal by release of transmitter (e.g., photoreceptors)
adaptation - response weakens with time even though have stimulus continues
2. Know the properties of the general senses (also called, somatosensory system)
peripheral sensors (each is specialized for particular stimuli its modality the "what")
mechanoreceptors (touch, pressure, vibration, proprioceptors)
thermoreceptors (heat, cold)
nociceptors (pain)
three different sensory pathways in CNS (ascending pathways)
(1) posterior column pathway (modalities: fine touch, vibration, proprioception)
primary afferent fibers enter dorsal root
send out collateral which synapse in dorsal horn
segmental input --- not on main ascending path (recall spinal reflexes)
ascend in dorsal (posterior) columns
first synapse is ipsilateral in dorsal column nuclei of medulla
pathway decussates in medulla and syanpses within contralateral thalamic nuclei
thalamic neurons project to somatosensory cortex and synapse there
somatosensory cortex (postcentral gyrus)
sensory homunculus (body mapped to brain surface -- the "where")
2) spinothalamic pathway (modalities: pain, crude touch, temperature)
first synapse is contralateral in cord
ascend in lateral and anterior tracts of cord
synapse next in thalamic nuclei which send axons to synapse in somatosensory cortex
(3) spinocerebellar pathway (modalities: unconscious proprioception: muscle spindles,
Golgi tendon organs)
synapse in dorsal gray -- axons ascend in either ipsi- or contralateral tracts
3. Understand the structural and functional basis of olfaction (sense of smell)
olfactory receptors
in olfactory membrane - each receptor cell specialized to sense a single odorant (~10,000 different types of olfactory receptors)
axons from olfactory receptors are CN I
pass thru cribriform plate in ethmoid bone
olfactory bulb
synapes on association neurons in olfactory bulb --> olfactory tract --> forebrain
4. Understand the structural and functional basis of gustation (sense of taste)
gustory receptors (chemoreceptors) within tastebuds (release neurotransmitters)
taste buds in papillae (3 kinds) on dorsal surface of tongue
at least 6 primary taste sensations: salt, sweet, sour, bitter, umami, water
5. Understand the structural and functional basis of vision (sense of sight)
lacrimal system
sclera
extrinsic muscles
cornea
main bending (refraction) of light here
covered by transparent (avascular) epithelium (conjunctiva)
iris
radial and circumferential smooth muscle
balance of radial vs circumferential contraction controls pupil size
lens
anterior compartment
posterior compartment
ciliary muscle
contraction focuses lens for near vision (accommodation)
presbyopia -- loss of accommodation with age as lens hardens
aqueous humor (secreted by ciliary body)
excessive pressure (glaucoma) from mismatch of secretion and drainage --> blindness
vitreous humor
retina
photoreceptors - two types
dual system:
rods (dim (blue) light -- high sensitity)
cones (bright light -- low sensitivity, 3 types of cones (red, green, blue sensitive)
fovea is pure cone region)
photoreceptor signals processed by neurons in retina
retina output neurons are ganglion cells
choroid
optic nerve (CN II)
made of axons from ganglion cells; project to thalamus (and mid-brain) as spatial map
relay from thalamus to primary visual area (occipital cortex)
orderly spatial map of retina on cortex
6. Understand the structural and functional basis of audition (sense of hearing)
overview: outer, middle, and inner ear; sound is a pressure wave (a mechanical input)
outer: pinna to auditory canal to tympanic membrane (eardrum)
cerumen in canal
sound vibrates tympanic membrane
middle ear:
ossicles (ear bones)
maleus, incus, stapes
Eustachean tube
otitis media
inner ear:
cochlea
oval window
divided by basilar membrane
vibration moves basilar membrane
frequency coded by place of maximum wave amplitude (high freq. near oval window)
hair cellss stereocilia get bent -->
depolarize -->
action potentials in CN VIII axons -->-->--> perception in temporal lobe
7. Understand the structural and functional basis of equilibrium (sense of balance)
vestibule - static sense (head angle)
otoliths (literally, "ear stones" made of calcium carbonate) gravitationally load the hair cells
semicircular canals - dynamic sense (head rotation)
endolymph bends stereocilia only during acceleration
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Durham Technical Community College
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Last updated 16 November 2004