Sensory Transduction physiology AND exteroception Flashcards
Sensory nerves in the muscle spindle have what type of sensory receptor?
- Stretch sensitive NSC channel
* Depolarizes on stretch, sending the signal
The receptor cells that function through hyperpolarizing receptor potentials have a resting membrane potential around where?
- 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
What kind of receptor cell is the rod photoreceptor?
- 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
What is the molecular phenomenon that is the “eye seeing light”?
- 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
How is hyperpolarization of rod cells transmitted to the second neuron?
- 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
What five attributes of a stimulus is transmitted by sensory systems?
- Modality
- Intensity
- Quality
- Duration/frequency
- location
What is the “modality” that is interpreted by sensory systems?
- Different forms of energy are converted by the nervous system into different sensations or sensory modalities
- Vision, hearing, smaell, taste, touch and thermoreception
What is “intensity” in interpretation by the sensory systems?
• Intensity or perceived amount of a sensation depends on the stimulus strength
What is the “quality” received by the sensory systems?
• Intensity or perceived amount of a sensation depends on the stimulus strength
A sensory system can interpret duration or frequency how?
- 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
How is a sensory modality determined?
- 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
In general, information from the sensory systems that becomes conscious is relayed through where?
- Thalamus
- Thalamic nuclei
- Keep in mind where the 2nd and 3rd order nuclei are for a given sensory system
The concept of “labeled lines”…
- 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
The LGN relays what information?
• 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
The olfactory system uses what thalamic nucleus as a relay?
- 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
Sensory receptors work in an all-or-none fashion when it comes to the necessary stimulus. How is intensity encoded then?
- 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.
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?
- The long cells encode intensity by frequency of stimulation
- The more action potentials close together, the more intense the stimulus
The numerical peripheral nerve scheme is used for what?
• Typically the numerical classificaiton is used for afferent axons from proprioceptors
The alphabetical peripheral nerve scheme is used for what?
• The alphabetical scheme is used for cutaneous sensory axons
A-alpha nerves are further classified how?
- Ia and Ib
- Ia - muscle spindle afferent
- Ib - tendon, organ afferent
Nerve fibers in the category IV are how big and conduct how fast?
- 0.2 - 1.5um
* 0.4-2m/sec
Nerve fibers in the category III are how big and conduct how fast?
- 1-5um
* 4-30m/sec
Nerve fibers in the category II are how big and conduct how fast?
- 5-10um
* 30-60m/sec
Nerve fibers in the category I are how big and conduct how fast?
- 10-20um
* 60-120m/sec
C fibers are further classified how?
- IV
- Warm temperature
- Burning pain
- Itch
- Crude touch
A-delta fibers are further classified how?
- III
- Sharp pain
- Cool temperature
- Extreme hot temperature
A-beta nerves are further classified how?
- II
- Mechanoreceptors of skin
- Secondary muscle spindle afferents
Somatosensory system has two major subdivisions….?
• Dorsal coloumn/lemniscal system ○ Discriminative touch ○ Fine touch ○ proprioception • Antero-lateral system ○ Crude touch ○ Pain ○ temperature
Rapidly adapting vs. slow adapting receptors
- 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
Define receptive field
• Area of skin in which a mechanical stimulus elicits a response from a cell
What are the two basic classes of somatosensory receptors (relating to size of receptive field)
• Small fields, sharp borders
○ 2-8mm diameter
• Large fields, poorly defined borders
What do meissner’s corpuscles do?
• Correspond to the rapidly-adapting afferents with small receptive fields
Ruffini endigns and pacinian corpuscles differ how?
- Both large field
- Ruffini - slowly adapting
- Pacinian - rapidly adapting
Pacinian corpuscles lie deep in subcutaneous tissues and dermis and therefore what’s their receptive field status?
- Large field, poorly defined
* Rapidly adapting
Merkel’s discs are what kind of receptor?
• Small field, slowly adapting
The hair follicle stretch receptors work how?
- 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
A-beta fibers are what?
- Large, myelinated axons
- Mechanorecptors fall in this category
- Touch, pressure, vibration, hair bending, proprioception
What kind of fibers are muscle proprioceptors?
• A-alpha
What tract do mechanoreceptors follow?
- 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
What tract do mechanoreceptors follow?
- 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
What is the ventrobasal complex?
- 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
What’s the medial lemniscus?
• A structure formed from the crossing fibers of the 2nd order neurons in the dorsal column/lemniscal system
- 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
Where do cells in the ventrobasal complex project?
- 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
What is a column in the cortex?
- 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
Somatosensory information is organized in vertical columns. But what’s with the cortical organization into layers of cells?
- 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
The layered interconnections in the cortex tell us what about the columns?
• Cortical columns serve as computational modules that transform information received from the thalamus and redistribute it to other regions of the brain
The trigeminal ganglion can be thought of as what?
• The dorsal root ganglion for the head, with motor fibers running through it
The principal nucleus is the trigeminal ganglion version of what?
- 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)
After the principal nucleus, where do 2nd order neuron’s axons go?
- 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
What makes up the mesencephalic nucleus?
- 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
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?
- The neurons that go to the VPM are the neurons that come from the head
- VPM 3rd order neurons relay to the cortex
