Lecture 14 Outline Flashcards
What roles do senses play? Why do we have senses? What do they accomplish?
info about the environment (& communication)
- food, temp, osmolarity, potential mates
- 1st 3 are homeostatic
info about ourselves
- energy stores, temp, water/ion balance
- homeostasis
*regulation of homeostasis is a key point of sensory physiology
What are the special senses? (conscious)
- vision
- hearing
- taste
- smell
- balance
What are the somatic senses (somatosensory)? (conscious)
- touch (texture, vibration…)
- temp
- pain
- itch
- proprioception
What is proprioception?
means where is my body & where are my limbs (gives YOU a sense of the position of your body & your limbs in space & also a sense of the position of your body in regard to other things in your environment)
ex: imp. for reaching out & grabbing a coffee cup
- b/c you need to have a sense of where your arm is, before you even begin that movement
- otherwise you won’t be able to accurately grab that coffee cup
- & imp. for feedback (if something is in the way when you reach out to grab coffee then you need to detect the position of your limbs as you sneak around all those empty coffee cups on your desk to get the one you wnat
What are the somatic stimuli (unconscious)?
- muscle tension (when running for ex)
- proprioception
What are the visceral senses (unconscious)?
- blood pressure
- GI distension
- glucose
- osmolarity
- oxygen/CO2 content of blood
- many others
can’t get an accurate/conscious perception of it
- can’t tell if your circulatory blood glucose levels is 3MM, 4MM, 5MM etc.
What are the 5 general properties of sensory systems?
- Receptors are most SENSITIVE to certain forms of energy or stimuli (MODALITY)
- Sensory transduction CONVERTS stimuli to graded potentials
- Sensory neurons have RECEPTIVE FIELDS
- The CNS INTEGRATES sensory info
- CODING & PROCESSING DISTINGUISH stimulus properties
Receptors are sensitive to…
only certain kinds of energy/stimuli (modality)
The word receptor has 2 meanings in neuroscience:
- A protein that binds a ligand
- A “structure” that detects sensory info
- can be a # of kinds of things
- receptors in this context are often ION CHANNELS
What are modalities detected by “receptors”?
- chemoreceptor: for taste, smell, 02 concentration in blood for ex
- mechanoreceptor: somata sensor, finger tips feel rough or smooth, whether there is a vibration in something, angle of joint, hearing etc.
- thermoreceptor: heat/cold (determine whether something is colder or warmer than you body temp)
- photoreceptor: detect photons of light
Describe the Simple
- free nerve endings (that detect things like…
- ex: pH, O2, temp
What is the primary sensory neuron responsible for?
bringing sensory info back into the CNS (1st neuron)
Describe the Complex
- nerve ending ensheathed (AKA surrounded) in non-neuronal accessory cells/tissue
- ex: vibration
Describe the Special Senses
- specialized transducer cell (will detect its fav modality) forms a synapse with sensory neuron
- Ex: smell, vision
- requires 2 cells
- organization with these special transducer cells
- Primary sensory neuron
- Transducer cell
- part that will detect its fav modality (photo receptor for ex)
- called neural epithelial cells: neuron & endodermal in embryotic origin, but don’t really behave like a typical neuron (have some properties of neurons in that they can release these synaptic vesicles, but they are truly neural epithelial cell) - Synapse
- b/t transducer cell & primary sensory neuron
What are neural epithelial cells?
neuron & endodermal in embryotic origin, but don’t really behave like a typical neuron (have some properties of neurons in that they can release these synaptic vesicles, but they are truly neural epithelial cell)
Receptors transduce…
sensory signals into graded potentials
The transduction process involves:
changes in membrane potential of sensory neurons
- ligand gated channels (chemical) - would give us sensory info (olfaction for ex)
- mechano-, thermo- gated channels (touch, temp, pressure, hearing & balance)
- channels modulated through 2nd messenger pathways (vision)
Changing ion channel activity in the sensory receptor causes a graded potential called the…
receptor potential
The adequate stimulus is…
the preferred type of stimulus for a receptor
ex: preferred stimulus for photoreceptors is light
The intensity of the stimulus is…
encoded by the magnitude of the graded potential produced
The minimum stimulus to activate a receptor is the…
receptor threshold
ex: minimum intensity of light in order to cause a change in the release of N.T.
Amplitude of graded potentials is translated to…
FREQUENCY of action potentials in higher order neurons
A MORE intense stimulus DOES NOT cause higher amplitude of APs…
(b/c APs are all or none), it causes HIGHER FREQUENCY of APs from a LARGER GP
Each sensory neuron has a…
receptive field
Define receptive field
defined as region of space in which the presence of a stimulus will alter the firing of a sensory neuron
- if you have sensory neuron in your finger-there’s going to be a limited amount of skin in your finger tip which is innovated by that primary sensory neuron
Primary sensory neurons converge onto…
secondary neurons - allows summation
- has an implication of how sensitive or the acquity a particular receptive field might be
The more convergence of primary sensory neurons onto secondary neurons the ______ the receptive field & ______ the acuity (the amount of sensitivity - ability to discriminate 2 separate stimuli)
LARGER
LOWER
Describe how convergence creates large receptive fields
- the receptive fields of 3 primary sensory neurons overlap to form one large secondary receptive field
- convergence of primary neurons allows simultaneous subthreshold stimuli to sum at the secondary sensory neuron & initiate an AP
- 2 stimuli that fall within the same secondary receptive field are perceived as a single point, b/c only one signal goes to the brain. Therefore, there is no 2-point discrimination
- if you only touch this purple region (primary sensory neurons) you’re going to get the exact same effect in the secondary sensory neuron
- b/c of the amount of convergence here, you may need to stimulate multiple receptive fields in order to bring the secondary sensory neuron to its threshold
What is the Secondary receptive field?
sum of all of those overlapping primary receptive fields
- b/c if you poke on the green circle vs. the purple the participate will still only be able to detect that as a single touch
- if you spread that so you touch both simultaneously it can still only tell as 1 single touch, the patient can’t tell that their being touched in 2 areas simultaneously - they can still only detect 1 & there is no spatial resolution - so if you poke on either spot it’s all going to be the same, so that is what we mean by lower acquity - there’s a lesser ability to be able to discriminate when your being touched by 2 points opposed to 1
Describe how small receptive fields are found in more sensitive areas
- when fewer neurons converge, secondary receptive fields are much smaller
- the 2 stimuli activate separate pathways to the brain. The 2 points are perceived as distinct stimuli & hence there is 2-point discrimination
- the size of the primary neuron receptor fields is the same but b/c each of them is innovating a secondary sensory neuron & there is no convergence - you get a lot MORE AQUITY
- b/c the secondary receptive field is the exact same size as the primary receptive field so its very easy to discriminate when this patient is being simultaneously touched on the green circle vs. purple circle
- when fewer neurons converge, the secondary receptive field is a lot smaller & get a lot greater 2-point discrimination
OF PRIMARY SENSORY NEURONS - BUT EACH OF THEM IS MAKING SYNAPSES ONTO ITS OWN SECONDARY NEURON (VERY LITTLE CONVERGENCE)
HIGHER degree of convergence means…
LESS acuity
The more an area is represented on a somatotopic map…
the greater the acuity
Areas of high acuity represent…
less convergence
A smaller area is taking up the same # of processing neurons in your cortex…
therefore less convergence of all of those sensory neurons here for your fingers then there is for your arm so it will represent a higher degree of acuity
Describe integration by CNS
some sensory info may be processed at the level of the spinal cord/brainstem
(SC - never makes its way to the cortex ex: muscle stretch)
(brainstem - heart rate & blood pressure (unconscious perception & its processed at the level of brainstem)
- directly to brain stem via cranial nerves
- visceral reflexes integrated in brain stem or spinal cord usually do not reach conscious perception
some sensory info processed in cortex
- if consciously aware, must be processed in cortex
- a stimulus may activate a primary sensory neuron, but may not have enough intensity to surpass PERCEPTUAL THRESHOLD (may be powerful enough to cause a GP (receptor potential) & might be powerful enough to cause an AP in the primary sensory neuron, but might not have enough intensity to actually surpass your perceptual threshold)
Define Perceptual threshold
level of stimulus necessary to be aware of particular sensation
Ex: sitting outside of light breeze moving hairs on skin b/c its not surpassing your perceptual threshold
- but if you turn on tv to watch documentary on spiders & you hate spiders then the smallest breeze that might move those hairs your gonna swat out b/c for some reason your perceptional threshold has changed & are now hyper aware of the movement of any of those hair cells
**When you enter a heightened state of arousal, then your perceptual threshold for certain stimuli can change
When you enter a heightened state of arousal, then…
your perceptual threshold for certain stimuli can change
Describe sensory pathways in the brain
most pathways pass through the thalamus on their way to the cerebral cortex
- Olfactory pathways from the nose project through the olfactory bulb to the olfactory cortex
- Most sensory pathways project to the thalamus. The thalamus modifies & relays info to cortical centers
- Equilibrium pathways project primarily to the cerebellum
The properties of the stimulus are determined by…
coding & processing
Remember that all stimuli are converted to action potentials. How does the CNS determine the properties of each stimulus?
properties of stimulus that need to be determined
- modality of the stimulus (type of stimulus)
- location of the stimulus
- -> role of lateral inhibition
What is lateral inhibition?
process that allows us to discriminate b/t adjacent areas that become stimulated
How are the properties coded?
intensity & duration of the stimulus reflected by graded potentials & action potentials
Describe modality
remember that receptors have a PREFERRED type of stimulus or ADEQUATE STIMULUS
specific groups of neurons within the brain are associated with specific modalities (happens through development - you end up with a specific set of primary sensory neurons that only detect/pass along visual info etc.)
- perception of a stimulus depends on the neural pathway that brings it to the CNS, NOT the receptor that transduced the signal (called) LATERAL LINE PROCESSING
- EACH MODALITY (after dev.) HAS ITS OWN “WIRING SYSTEM” FROM THE RECEPTOR TO HIGHER PROCESSING CENTRE (in the cortex)
Describe Labeled Line Processing
perception of a stimulus depends on the neural pathway that brings it to the CNS, not the receptor that transduced the signal
Each modality has its own…
“wiring system” from the receptor to higher processing centre
The sensory processing is hard-wired to the pathway that brings it back to the CNS…
- each modality has its own “wiring system” from the receptor to higher processing centre
- so your perception depends on the pathway that brings it back into the CNS
Describe location
the location of a stimulus is determined by what receptive fields are activated
location of perceived stimulus determined by labeled line processing
1 issue that would arise is that:
- stimulus from adjacent areas of the body are most often processed in adjacent areas of the brain (somatotopic maps)
- sensory info from fingers are processed in adjacent areas
- adjacent areas of skin on 1 finger - that gets processed by adjacent areas of the sensory cortex
- can cause some issues with your ability to discriminate b/t 2 adjacent receptor fields
*lateral inhibition is a mechanism to increase contrast b/t activated receptive fields & non-activated ones
Same process can even occur at the level of sensory neurons in the PSSC…
- might be if you are activating areas encoded by the index finger for ex, then that may cause inhibition in these lateral areas so the ring finger or index finger
- you specifically get enhancement of the perception of what’s happening in that middle finger
Describe Lateral Inhibition
enhances contrast & makes a stimulus easier to perceive
- the responses of primary sensory neurons A, B, & C are proportional to the intensity of the stimulus in each receptor field
- secondary senses neuron B inhibits secondary neurons A & C, creating greater contrast b/t B & its neighbours
stimulus –> primary neuron response is proportional to stimulus strength –> pathway closest to the stimulus inhibits neighbours –> inhibition of lateral neurons enhances perception of stimulus
- if you can inhibit these later sensory neurons then by contrast it makes whats happening in neuron B seem a lot stronger (lateral inhibition)
- requires axon collaterals from B being able to inhibit secondary neurons from other adjacent areas
Describe Intensity & Duration
longer or stronger stimuli may release more neurotransmitter
primary sensory neuron is gonna be activated by a stimulus & the stimulus is gonna cause a GP
moderate stimulus - strong enough to cause APs in this primary sensory neuron
longer & stronger stimulus:
- receptor potential strength & duration vary with the stimulus
- receptor potential is integrated at the trigger zone
- frequency of APs is proportional to stimulus intensity
- duration of a series of APs is proportional to stimulus duration - neurotransmitter release varies with the pattern of APs arriving at the axon terminal
*Increased freq. of APs implies more release of neurotransmitter from primary sensory neuron
LONGER & STRONGER stimulus DOES NOT cause…
larger amplitude APs it CAUSES higher frequency of APs!
- the currency of info is the freq. of APs
- freq. of APs is what’s going to encode that particular property of the stimulus
Describe receptor adaptation: tonic vs phasic signaling
receptor adaption: receptors adapt to sustained stimulus
a) tonic receptors
b) phasic receptors
Tonic receptors
are slowly adapting receptors that respond for the duration of a stimulus
- APs are being stimulated the whole time the stimulus is present
- in some neurons, the stimulus might cause this GP, but its gonna stimulate APs in this neuron the whole time this stimulus is present
What are tonic receptors imp. for?
- joint receptors
- pain receptors
b/c we need to have that info all the time (always need to know the position of your joints for ex, otherwise if you go to pick up something your coordinated motion of your arm is not gonna be accurate
- need to know where your arm is at the very beginning in order to project it to the correct place
Phasic receptors
rapidly adapt to a constant stimulus & turn of
- main kind
- reflects a change in the stimulus - NOT just when a stimulus is present
- only see APs happening at beginning & sometimes at the end
- also known as accommodation
- even though a stimulus is present for a long period of time - we only see APs at the very beginning here
What are phasic receptors imp. for?
- olfactory - smell it quickly if you are there for a period of time it eventually goes away (*dependent on changes in whether its there)
- pacinian corpuscles (vibration) - sensory detectors in your skin (lead to detection of vibration)
Glabrous skin
ex: skin on your palm of your hands (no hairs)
Epidermis
outer layer
Ruffini ending (corpuscle)
found in deeper layers of the skin - detects stretch of the skin (when the skin stretches out it will become active)
- bunch of enlarged nerve endings & its attached to some of the connective tissue & when the CT stretches it stretches those enlarged nerve endings
- SLOWLY adapting or even tonic - reason for that is that its detecting the amount of stretch on that given area of skin - imp. for positioning of joints & other things that are touching skin
Dermis
deeper layer
Peripheral nerve bundle
contain # of diff. sensory neurons
= bundle of axons
Bare nerve ending
connected to unmyelinated axon
- multimot? - # of diff kinds of stimuli that can be detected by those
- heat receptors, temp receptors found in this
Meissner’s Corpuscle
located just below the layer of the epidermis
- a nerve ending thats encapsulated in connective tissue
- a lot of COLLAGEN that is surrounding this have nerve ending
- detects things like a FLUTTER or LIGHT STROKE on your skin
- since that is the modality of the stimulus - it rapidly adapts to those kinds of stimuli (if that light touch is present on your skin for a long period of time - it rapidly adapts
Merkel disk receptor
found superficially (super close to the epidermis) - detects things like STEADY PRESSURE & TEXTURES
- b/c its modality is for texture is why its found in very shallow levels of the skin - very near the epidermis
- just enlarged nerve endings
- adaptation of this is slow - able to provide constant info (useful if you run hand across a surface & you are trying to determine the texture)
- useful if it could constantly apply info opposed to adapting very quickly
Bare nerve ending
myelinated
- detects movement of this hair & sends info back to the primary sensory neuron
Pacinian Corpuscle
located deep within the dermis
- nerve ending thats surrounded by layers of CT like an onion
- rapidly adapting & its main modality are things like VIBRATION (if there is a vibration that goes on for a long period of time it eventually will stop activating this primary sensory neuron)
Temperature receptors are…
ion channels
can also be gated open by certain chemicals
Mint activates…
cold receptors (ion channels opened by cold temps)
Capsaicin activates…
warm receptors (ion channels opened by warm/hot temps)
Family of ion channels called…
TRP channels
- Transient receptor potential channel
There is a family of TRP channels & these ion channels are found in primary sensory neurons & when they open…
they are gonna cause a GP & that can be stimulated by exposure to a range of temps (can detect various ranges of temps)
All temp receptors are found on…
free (bare) nerve endings
More ____ receptors than ____ receptors
cold
warm
Cold receptors
activated below body temp
Warm receptors
37 to 45 degrees celsius
Pain receptors
above 45 degrees celsius
2021 Nobel Prize for Physiology or Medicine awarded to…
David Julius & Ardem Patapoutian for their discoveries of receptors for temp & touch
Describe Skin Nociceptors
- nociceptors = pain
- bare nerve endings
- activated by strong noxious stimuli (things we generally find unpleasant - high temp, pokey like a pin or a cut for ex) that may damage tissue
- may be activated by “extreme temperature” (painful hot, painful cold) (unpleasant)
- Arachidonic acid & prostaglandins may activate TRPV2 channels & GPCR to cause graded potentials in nociceptive neurons
Take home message about skin nociceptors
nociceptor neurons can be activated by temp (extreme temps like painful hot or painful cold) or they can be activated by arachidonic acid & prostaglandins that either activate ion channels that cause a GP or GPCR to cause a GP
Different types of axons (=fibres) carry…
pain info
leads you to think that you can perceive pain in diff kinds of ways
Describe the 3 diff types of axons (=fibres) that carry pain info
- medium diameter myelinated (A-beta) (=fast pain)
- small unmyelinated (A-delta) (=fast pain)
- small unmyelinated (C-fibres) (=slow pain)
A-beta: Type of nerve fibres (axons)
- med (*bigger than delta)
- myelinated
- function: cutaneous mechano
A-delta: Type of nerve fibres (axons)
- med
- myelinated
- function: cold, nociceptors
C-fibres: Type of nerve fibres (axons)
- small
- UNmyelinated
- function: nociceptors
- slow, intense wave of pain (ex: from stabbing toe)
- wave is much slower b/c these fibers are small & not myelinated
- slowest conducting axons are going to be smaller & not myelinated
A-beta, A-delta & c-fibers are sensory neurons so will have…
pseudounipolar structure & the cell bodies will be found in the dorsal route ganglia
When you stub toe on door for ex:
- you know you’ve done it immediately, you sense it immediately
- that initial wave of sensation is not intensely painful (will be carried out by A-beta’s & A-delta’s)
- wait a fraction of a sec & you swear & get that wave of pain that comes from activation of C-fibers
- it takes longer for that C-fiber encoded info to make it to your cortex b/c your axons are unmyelinated
- that pain is more intense, longer lasting but it takes longer to get there & that is what’s getting encoded on these C-fibers
Describe the sensory pathway for: fine touch, vibration, & proprioception
Key here with these A-Beta fibres & these sensory neurons is that the axon of this primary sensory neuron (distinct pathway for carrying info back to the CNS):
- it ascends & makes its 1st synapse on the SAME side of the brainstem
- & the secondary neuron is gonna cross over to the synapse in the thalamus & that guy in the thalamus will ultimately make a synapse in the cortex
Ipsilateral
(described more on slide 38)
Ipsilateral
same side
Sensory pathway for: nociception & temp
A-delta fibers & C-fibers
Main differences in these 2 neural pathways are where the 1st synapse occurs & where the axon traveling to the brainstem or thalamus actually ascends
Contralateral (crosses over)
Contralateral
crosses over
