Sensory Transduction physiology AND exteroception

Question 1

Sensory nerves in the muscle spindle have what type of sensory receptor?

Answer 1

• Stretch sensitive NSC channel

* Depolarizes on stretch, sending the signal

Question 2

The receptor cells that function through hyperpolarizing receptor potentials have a resting membrane potential around where?

Answer 2

• Btw. OmV and Ek (-70mV)
  
  • Ends up being btw -30 and -40mV
  • The general principle here is that there is pretty good resting cation conductance and the stimulus will result in closing of those channels

Question 3

What kind of receptor cell is the rod photoreceptor?

Answer 3

• Hyperpolarizing short receptor cell
  
  • Normally has cGMP-gated ion channels open that keep the membrane more depolarized (-30 or -40mV)
  • Light hitting transducin (protein that changes conformation when it is hight by phone) will activate a cGMP phosphodiesterase that will start closing channels
  • The more transducins hit, the more hyperpolarized, thus the concept of photobleaching

Question 4

What is the molecular phenomenon that is the “eye seeing light”?

Answer 4

• Photon hits 1-cis-retinal (in the context of the protein rhodopsin)
  
  • Energy flips it to 1-trans-retinal
  • Conformational change in rhodopsin into metarhodopsin
  • Metarhodopsin stimulates the GPCR transducin which turns on the cGMP phosphodiesterase

Question 5

How is hyperpolarization of rod cells transmitted to the second neuron?

Answer 5

• Normally a constant flow of NT
  
  • Hyperpolarized cell means less NT
  • The rod cell is short enough that hyperpolarization is transmitted from receptor end to synaptic end by electronic transmission

Question 6

What five attributes of a stimulus is transmitted by sensory systems?

Answer 6

• Modality
  
  • Intensity
  • Quality
  • Duration/frequency
  • location

Question 7

What is the “modality” that is interpreted by sensory systems?

Answer 7

• Different forms of energy are converted by the nervous system into different sensations or sensory modalities
  
  • Vision, hearing, smaell, taste, touch and thermoreception

Question 8

What is “intensity” in interpretation by the sensory systems?

Answer 8

• Intensity or perceived amount of a sensation depends on the stimulus strength

Question 9

What is the “quality” received by the sensory systems?

Answer 9

• Intensity or perceived amount of a sensation depends on the stimulus strength

Question 10

A sensory system can interpret duration or frequency how?

Answer 10

• Duration is the cell’s way of knowing how long the stimulus took place (like time of open receptors for stretch)
  
  • Frequency can also be reported if the cell conveys information about how often the stimulus was experienced

Question 11

How is a sensory modality determined?

Answer 11

• The molecular and physical structure of the sensory cell/organ
  
  • Sometimes the shape is obvious as to the function, but sometimes (cutaneous sensation) there is a mix of adequate stimuli that could activate a group of receptors
  • All about the receptor proteins and how the channels open and close

Question 12

In general, information from the sensory systems that becomes conscious is relayed through where?

Answer 12

• Thalamus
  
  • Thalamic nuclei
  • Keep in mind where the 2nd and 3rd order nuclei are for a given sensory system

Question 13

The concept of “labeled lines”…

Answer 13

• The nervous system organizes certain senses by the kind of information they are carrying
  
  • These separate chains are separate, labeled lines that are attributed to a certain sensory system
  • The stimulus modality is coded by which nerve cells are active

Question 14

The LGN relays what information?

Answer 14

• LGN - lateral geniculate nucleus
• Thalamic nucleus
• Visual information to visual cortex in occipital lobe
The MGN relays what information?
• MGN - medial geniculate nucleus
• Auditory information to auditory cortex in temporal lobe

Question 15

The olfactory system uses what thalamic nucleus as a relay?

Answer 15

• Trick question, it doesn’t use the thalamus

* It’s all kinda structurally built into its own olfactory bulb that goes into the olfactory cortex

Question 16

Sensory receptors work in an all-or-none fashion when it comes to the necessary stimulus. How is intensity encoded then?

Answer 16

• The time that these channels are open
  
  • Stretch receptor example - open for a small amount of time with weak force, but longer with strong force
  • The stimulus reaches a threshold and opens the channel, there is no channel-level partial opening.

Question 17

Receptor cells would mess up intensity transduction if they had voltage gated cation channels to propagate their action potential. But how do long sensory receptor cells do it?

Answer 17

• The long cells encode intensity by frequency of stimulation
  
  • The more action potentials close together, the more intense the stimulus

Question 18

The numerical peripheral nerve scheme is used for what?

Answer 18

• Typically the numerical classificaiton is used for afferent axons from proprioceptors

Question 19

The alphabetical peripheral nerve scheme is used for what?

Answer 19

• The alphabetical scheme is used for cutaneous sensory axons

Question 20

A-alpha nerves are further classified how?

Answer 20

• Ia and Ib
  
  • Ia - muscle spindle afferent
  • Ib - tendon, organ afferent

Question 21

Nerve fibers in the category IV are how big and conduct how fast?

Answer 21

• 0.2 - 1.5um

* 0.4-2m/sec

Question 22

Nerve fibers in the category III are how big and conduct how fast?

Answer 22

• 1-5um

* 4-30m/sec

Question 23

Nerve fibers in the category II are how big and conduct how fast?

Answer 23

• 5-10um

* 30-60m/sec

Question 24

Nerve fibers in the category I are how big and conduct how fast?

Answer 24

• 10-20um

* 60-120m/sec

Question 25

C fibers are further classified how?

Answer 25

* IV * Warm temperature * Burning pain * Itch * Crude touch

Question 26

A-delta fibers are further classified how?

Answer 26

* III * Sharp pain * Cool temperature * Extreme hot temperature

Question 27

A-beta nerves are further classified how?

Answer 27

* II * Mechanoreceptors of skin * Secondary muscle spindle afferents

Question 28

Somatosensory system has two major subdivisions….?

Answer 28

``` • Dorsal coloumn/lemniscal system ○ Discriminative touch ○ Fine touch ○ proprioception • Antero-lateral system ○ Crude touch ○ Pain ○ temperature ```

Question 29

Rapidly adapting vs. slow adapting receptors

Answer 29

* rapidly adapting will, with a constant touch stimulus, fire one action potential then quit * slow adapting will continue to fire action potentials as long as the touch stimulus is in place * these two are rather equally distributed in the glabrous portion of the hand

Question 30

Define receptive field

Answer 30

• Area of skin in which a mechanical stimulus elicits a response from a cell

Question 31

What are the two basic classes of somatosensory receptors (relating to size of receptive field)

Answer 31

• Small fields, sharp borders ○ 2-8mm diameter • Large fields, poorly defined borders

Question 32

What do meissner's corpuscles do?

Answer 32

• Correspond to the rapidly-adapting afferents with small receptive fields

Question 33

Ruffini endigns and pacinian corpuscles differ how?

Answer 33

* Both large field * Ruffini - slowly adapting * Pacinian - rapidly adapting

Question 34

Pacinian corpuscles lie deep in subcutaneous tissues and dermis and therefore what's their receptive field status?

Answer 34

* Large field, poorly defined | * Rapidly adapting

Question 35

Merkel's discs are what kind of receptor?

Answer 35

• Small field, slowly adapting

Question 36

The hair follicle stretch receptors work how?

Answer 36

* They are myelinated untill they approach the hair follicle * They send out unmyelinated branches which wrap around the follicle or run up and down along it * Whenever the follicle is moved by the mechanical movement of the hair, the stretch receptors open * RAPIDLY ADAPTING

Question 37

A-beta fibers are what?

Answer 37

* Large, myelinated axons * Mechanorecptors fall in this category * Touch, pressure, vibration, hair bending, proprioception

Question 38

What kind of fibers are muscle proprioceptors?

Answer 38

• A-alpha

Question 39

What tract do mechanoreceptors follow?

Answer 39

* A-beta and A-alpha fibers * Cell body of receptor cell lin DRG ipsilateral side * Send off local interneuron for spinal reflex * Ascend in dorsal column either fasciculus cuneatus or fasciculus gracilis to the thalamus * Synapse on nucleus gracilis or cuneatus * This is the lemniscal system

Question 40

What tract do mechanoreceptors follow?

Answer 40

* A-beta and A-alpha fibers * Cell body of receptor cell lin DRG ipsilateral side * Send off local interneuron for spinal reflex * Ascend in dorsal column either fasciculus cuneatus or fasciculus gracilis up the spinal cord * Synapse on nucleus gracilis or cuneatus (medulla) * This is the lemniscal system

Question 41

What is the ventrobasal complex?

Answer 41

* Made of the VPM and VPL * All 2nd order neurons from the dorsal column/lemniscal pathway or the trigeminal nuclei * VPM - head sensation * VPL - trunk and limbs sensation

Question 42

What's the medial lemniscus? | • A structure formed from the crossing fibers of the 2nd order neurons in the dorsal column/lemniscal system

Answer 42

* These second-order fibers are joined in the midbrain by 2nd order fibers crossing from the trigeminal nuclei * Medial lemniscus fibers ascent to synaps in the ventro-basal complex of thalamus * Trunk and limbs are represented by cells in the ventral-posterior-lateral nucleus (VPL) * Head is represented in the ventral-posterio-medial (VPM) nucleus * The VPM and VPL form the ventrobasal complex

Question 43

Where do cells in the ventrobasal complex project?

Answer 43

* Areas 3,1, 2 * Posterior bank of the central sulcus * (primary somatosensory area) * From here cells go to SII or secondary somatosensory area in primary motor cortex and nearby association somatosensory cortical areas in parietal cortex

Question 44

What is a column in the cortex?

Answer 44

* Vertical stacks of related cells * Essentially they all have the same receptive fields and the same modality * Information in the cortex is organized in vertical stacks or columns

Question 45

Somatosensory information is organized in vertical columns. But what's with the cortical organization into layers of cells?

Answer 45

* Different layers relay information from and two different places * Layer IV recieves input from thalamus * Layer VI projects back to thalamus * Layer II and III project to other areas of somatosensory cortex

Question 46

The layered interconnections in the cortex tell us what about the columns?

Answer 46

• Cortical columns serve as computational modules that transform information received from the thalamus and redistribute it to other regions of the brain

Question 47

The trigeminal ganglion can be thought of as what?

Answer 47

• The dorsal root ganglion for the head, with motor fibers running through it

Question 48

The principal nucleus is the trigeminal ganglion version of what?

Answer 48

* Main sensory nucleus = principal nucleus of trigeminal ganglion * Analogue of the dorsal column nuclei * Afferents in fine touch, vibration and proprioception synapse here (from head and face)

Question 49

After the principal nucleus, where do 2nd order neuron's axons go?

Answer 49

* Second order neurons in the main sensory nucleus send axons across the midline to join the medial lemniscus * Some fraction of the cells in the principal nucleus project ipsilaterally

Question 50

What makes up the mesencephalic nucleus?

Answer 50

* Proprioceptive afferents like lmuscle stretch receptors that run in the trigeminal nerve * Involved in muscles of mastication and some mechanorecpeotors in the mouth * They make monosynaptic connections with motor neurons to the muscles of mastication in the motor trigeminal nucleus * Analogous to stretch receptors and reflex in spinal cord

Question 51

The medial lemniscus has fibers from the head, trunk and limbs. How do you know which ones came from the head and the principal ganglion?

Answer 51

* The neurons that go to the VPM are the neurons that come from the head * VPM 3rd order neurons relay to the cortex