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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

General Senses: Touch, Temperature, Pain, Vibration, Proprioception

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

 

Special Senses: Olfaction, Vision, Gustation, Audition, Equilibrium

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 cells’s 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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Last updated 16 November 2004