Physiology Test #2 Topic 7

info from body to CNS

is subconscious info sent from internal viscera to the CNS.

afferent input that does reach level of conscious awareness is called sensory info, and the pathway is called sensory afferent.

• Somatosensory system
• special senses

• body surface sensations
• skin, muscles, joints, inner ear, limb position

vision, touch, hearing, taste, smell

sound, color, shape, texture, smells, tastes, temp

magnetic fields (birds can), light polarization (birds can), radio waves, x rays

• - we can't hear high frequencies that dogs can
• - some features of stimuli are accented or ignored during precortical processing
• - cerebral cortex further manipulates data to "complete the picture"
• - thus our perceptions of NOT replicate reality

• a. sensitivity of receptors to stimuli 
• b. responses of receptors to stimuli
• c. Adaptation of receptors to simuli
• d. Fate of information transmitted by receptors

• 1. Transduction
• 2. A receptor is specialized for specific stimuli 
• 3. Tupes of receptors
• 4. Compound sensations also occur

a receptor functions by converting stimulus energy to an action potential (AP)

• eyes se but do not hear
• but if you are hit in the eye (hitting is a mechanical stimulus) you "see stars"

• a)  photoreceptors-vision
• b)  thermoreceptors
• (1)  warmth
• (2)  cold
• c)  mechanoreceptors
• (1)  osmoreceptors (ECF osmolarity)
• (2)  baroreceptors (blood pressure)
• (3)  hair cells (sound, balance)
• d)  chemoreceptors
• (1) taste, smell, blood oxygen, blood pH
• e) nociceptors (pain)

e.g wet= mechano + thermo

• receptor structure
• when receptor stimulates
• conversion of receptor and generator potentials into APs

• a) separate cell, produces a receptor potential, which is a graded potential
• (1) most special senses are like this

• b) modified ending of afferent neuron, produces a generator potential, which is also
• a graded potential
• (1) olfactory is only special sense like this

• b)  usually results in the net influx of Ca++ and/or Na+ ions
• c)  which causes a membrane depolarization
• d)  this is a graded potential, not an action potential, so the bigger the stimulus, the bigger the change in potential

• (1)  local current flow occurs from end of afferent neuron to axon of same afferent neuron
• (2)  causes opening of Na+ channels
• (3)  if threshold is reached in the axon, an AP occurs

• b)  separate cell (Fig 10.2b)
• (1)  separate receptor cell stimulated which opens Ca++ channels
• (2)  influx of Ca++ causes release of chemical messenger
• (3)  messenger binds to protein receptor on membrane of afferent axon
• (4)  causes Na+ channels to open on afferent axon
• (5)  if enough Na+ channels open, threshold is reached, and an AP occurs

more APs are produced

the higher the frequency of AP that occur in afferent neuron

• Slow adapting receptors
• fast adapting receports

• Do not adapt to stimuli
• continue to produce APs as stimuli continue 
• e.g. muscle stretch receptors

rapidly adapt to stimuli, stop producing APs even though stimuli continue,

• Receptor causes AP in afferent neuron (the first order neuron)
• Afferent AP reaching spinal cord either...

• becomes part of a reflex arc
• or is relayed toward brain by an interneuron (second order neuron)

• receptors are chemoreceptors 
• stimulation of taste and smell receptors can cause "pleasurable" or "objectionable" sensations
• important in finding good food, avoiding toxins, finding mates

• 10,000 buds in mouth.. mostly on tongue
• each taste bud has a single opening
• consists of about 50 receptor cells

• (2)  binding a chemical causes depolarization of receptor membrane
• (3)  can initiate APs in afferent neurons with which they synapse
• (4)  taste receptors has lifespan of 10 days

• (1)  Caused by acids (H+)
• (2)  H+ blocks K+ channels, which reduces K+ leaking out of cell, which
• depolarizes membrane
• (3)  When membrane depolarizes, Ca++ channels open and Ca++ enters cell
• (4)  Entry of Ca++ causes release of neurotransmitter which bind to taste afferents and can cause AP in taste afferent.

• (2)  When membrane depolarizes, Ca++ channels open and Ca++ enters cell
• (3)  Entry of Ca++ causes release of neurotransmitter which bind to taste afferents and can cause AP in taste afferent.

• (1) Glucose or related sugar bind receptor
• (2)  Activiates a “G protein” system that involves several enzymes
• (3)  Ultimately results in blocking K+ channels, which reduces K+ leaking out

of cell, which depolarizes membrane

(4)  When membrane depolarizes, Ca++ channels open and Ca++ enters cell

(5)  Entry of Ca++ causes release of neurotransmitter which bind to taste

afferents and can cause AP in taste afferent.

Many chemicals can bind to bitter receptors (caffeine, nicotine, morphine,

strychnine)



(2)  Bitter molecule blocks K+ channels, which reduces K+ leaking out of cell,

which depolarizes membrane



(3)  When membrane depolarizes, Ca++ channels open and Ca++ enters cell



(4)  Entry of Ca++ causes release of neurotransmitter



(5)  Note: Some bitter taste buds apparently use a G protein sytem (as

described for sweet) which ultimately causes relsease of neurotransmitter

which bind to taste afferents and can cause AP in taste afferent.

Amino acids, especially glutamate, bind receptors



(2)  Causes net influx of Na+ which depolarizes membrane and ultimately

causes release of neurotransmitter which bind to taste afferents and can

cause AP in taste afferent.

brain stem to third order neuron in thalamus to gustatory cortex

• olfactory receptors
• smell sensations
• neural pathway

•   Only special sense receptor that is modified endings of afferent neurons (instead
• of separate cell)

b)  Axons of olfactory receptors collectively form olfactory nerve (cranial nerve I)

• c)  Receptor cells constantly replaced; only neurons known that do this
• d)  5 million receptors of 1000 different kinds (compared to only 3 receptor types for

• color vision and 4 for taste)
• e)  each receptor responds to specific components of odors

• b) Cascade of intercellular reactions that open Na+ and Ca++ channels
• c) Can thus generate APs in the afferent axon
• d) High frequency of binding = high frequency of APs

• bulb in forebrain to olfactory tubercle (in cerebrum, not in thalamus) to olfactory cortex
• and to limbic system (both in cerebral cortex) via third order neurons. This is an
• evolutionarily ancient pathway. A recently discovered evolutionarily young pathway
• associated with conscious awareness of smell does seem to utilize the thalamus.