Sensory physiology 1 Flashcards
In general, how are electrical signals interpreted by your brain?
A stimulus (energy) is detected by a receptor. This energy is then transduced into action potentials, which are sent to an afferent sensory neuron. The action potentials are then sent from the afferent sensory neuron to the brain.
Define sensation
The process by which external energy is received at sensory organs, and transduced into electrical (neural) signals
- Mechanical process
Define perception
Our interpretation of neural signals
- e.g. hey that sound’s nice :3
Despite the vast diversity of sensations we experience, sensory systems use 4 types of information to help us differentiate between sensations. What are these 4 types of information?
- Modality
- Location
- Intensity
- Timing
Define modality as a type of information used to differentiate between sensations
Type of energy
- e.g. visible light vs. sound waves
Define location as a type of information used to differentiate between sensations
Set of sensory receptors that are active (e.g. where on the body certain things come into contact)
- e.g. dermatomes: area of skin where all the energy arriving at this site gets sent to the same area of the spinal cord
What is the labeled-lines principle with respect to location as a type of information used to differentiate between sensations
When you have a particular receptor type that gets activated, there are going to be isolated pathways of neurons going from that receptor to different parts of the brain
- energy is isolated/contained/separate from other forms of energy
Define intensity as a type of information used to differentiate between sensations
Total amount of stimulus energy delivered to the receptor
Define timing as a type of information used to differentiate between sensations
When the stimulus starts and stops
True or false: humans are generally very good at perceiving the same thing, but sometimes, we can be fooled.
True
- e.g. optical illusions
True or false: perception differs among different people because sensations change
False
- pre-existing states can alter perception, sensation will not change
What are the two mechanisms of transduction?
- Ionotropic
- Metabotropic
Describe the ionotropic mechanism of transduction
Sensory receptor directly opens an ion channel which causes the ions to flow into the cell
Most receptors are (ionotropic/metabotropic)
Ionotropic
Describe metabotropic mechanism of transduction
Not directly linked to opening an ion channel
- invovles 2nd messenger cascade
What type of ion channels is mechanoreception mediated by?
Stretch activated ion channels
- With increasing stretch on the membrane a channel opens more often and for longer duration (see increased current as pressure increases and creates a vacuum that stretches the membrane)
How does stimulus intensity affect frequency of action potentials?
Greater intensity stimuli = greater frequency of action potentials
What is receptor adaptation?
Explains how a receptor behaves when a stimulus is introduced
What are the two types of receptors?
- 2 names for each type
- Tonic receptor aka slow-adapting receptor
- Phasic receptor aka fast-adapting receptor
Describe tonic receptors aka slow-adapting receptors
When we introduce a stimulus, it starts to fire action potentials and only stops when the stimulus goes away
Describe phasic receptors aka fast-adapting receptors
Good at depicting the onset and offset of a stimulus
- If a stimulus remains constant, get decrease in the number of action potentials quickly.
- action potentials also quickly fire when stimulus is removed.
What are the 6 mammalian mechanoreceptors?
- Pacinian corpuscles
- Ruffini endings
- Meissner’s corpuscles
- Merkel discs
- Free nerve endings
- Hair cell
Glabrous skin
Skin with no hair follicles
- e.g. fingertips
Epidermis vs dermis
Epidermis: thin layer, outermost layer of the skin
Dermis: Rest of the skin
What do hair cells detect and how?
Detect if the hair is moved/disturbed in any way
- Stretch-activated ion channels (ionotropic) are connected by cytoskeletal strands.
- As the hair moves, cytoskeletal strands are pulled on -> opens up the channels and causes Na+ influx into hair cell -> depolarizes the cell and causes an action potential
- Ion channels open due to conformation change from the stretch
Describe Pacinian corpuscles (3)
- Located deep in dermis
- Respond to vibration and deep pressure (because the receptors are located deep in the skin, light pressure won’t activate these)
- Rapidly adapting (phasic)
Describe how pacinian corpuscles detect stimulus and why they’re phasic
If something presses deep into the skin -> makes contact with receptor ending -> waves of energy travel down from outer surface of receptor to central nerve core through fluid -> stretch-activated channels caused by disrupted nerve core, causing a graded potential and eventually an action potential if the threshold is met
- if stimulus is applied and remains constant, fluid redistributes and central nerve core moves back to initial position
- detects the onset of the stimulus, but action potentials stop if stimulus is constant
- fluid moves again when object is removed which triggers actino potentials
Describe Ruffini endings (5)
- Located deep in the dermis
- Respond to stretch (anchored to deep dermis: as the skin stretches, the dermis pulls on the membrane of the Ruffini endings)
- Slow adapting (as long as the skin is being stretched, it sends action potentials and the skin remains stretched)
- Indicate limb position
- Indicate object shape (object causes a characteristic pattern of stretch: brains can use the specific patterns to determine the shape of the object)
Describe Meissner’s corpuscles (4)
- Located superficially
- Physiologically similar to Pacinian Corpuscle (since they’re also fluid-filled sacs)
- Rapidly adapting
- Fine, light, touch
Describe Merkel discs (4)
- Located superficially
- Slow adapting
- Semi-rigid epithelial cell (anchored to the skin, fire action potentials as long as skin is stretched)
- Compression (e.g. when holding a pen, small indents in the skin remain compressed as long as you’re holding the pen)
Free nerve endings are also known as…
Nociceptors (pain receptors)
- Detect damage or potential damage to the skin
What are the 4 types of free nerve endings?
- Mechanical nociceptors
- Thermoreceptors
- Chemically sensitive
- Polymodal
Mechanical nociceptors responds best to….
Physical stimuli
Thermoreceptors respond best to…
Extreme hot or cold
Chemically sensitive free nerve endings respond best to…
Extreme changes in pH
Polymodal free nerve endings respond best to…
All stimuli (maximally activated by all stimuli)
What is a receptive field?
A receptive field refers to the specific region of sensory space (like skin, for example) where a stimulus can trigger a response in a particular sensory neuron. It defines the area or range over which a single neuron is sensitive to stimuli.
What two types of mechanoreceptors have very small receptive fields?
Merkel disks and Meissner’s corpuscles
- located very superficially
What two types of mechanoreceptors have large receptive fields?
Ruffini endings and Pacinian corpuscles
- located deep so has a large receptive field
Merkel disks and Meissner’s corpuscles have (greater/smaller) overlap within their receptive fields
Smaller
Ruffini endings and Pacinian corpuscles have (greater/smaller) overlap within their receptive fields
Greater
Which three mechanoreceptors have a higher density near the fingertips compared to the palm?
Merkel discs, Meissner’s corpuscles and Pacinian corpuscles
- But pacinian corpuscles respond to bigger amounts of stretch
Which mechanoreceptor has a higher density near the palm than the fingertips?
Ruffini endings
- Higher density near palm compared to fingertips because fingertips don’t stretch as much as palm.
Define two point threshold and what it is dependant on
The distance between 2 physical stimuli (points) where you are no longer able to detect each point separately
- The size of the threshold is dependent on receptor density and receptive fields
What is the only mechanoreceptor that isn’t myelinated?
Free nerve endings
If two stimuli are applied to one receptive field, is 1 point or 2 points felt?
1 point
When two stimuli are placed in two different receptive fields, is 1 point or 2 points felt?
2 points
When two stimuli are applied to one receptive field and the overlap between two receptive fields, is 1 point or 2 points felt?
2 points
When two stimuli are applied to the overlap between two receptive fields and a region where there is no receptive field present, is 1 point or 2 points felt?
1 point
When a stimulus is applied to the overlap between two receptive fields, sensory information is sent down two lines to the brain. So do you feel two stimuli when this occurs?
No, only feel 1 stimulus even if both receptors are becoming active
- Brain is able to detect that this is only one point because of the timing and intensity of the stimulus.
Which regions have the smallest two point threshold in the body?
Fingers, thumb, palm, forehead, cheek, nose, upper lip, hallux (big toe)
Which regions have the largest two point threshold in the body?
Forearm, upper arm, shoulder, breast, belly, thigh, back, calf
Describe the receptor activity of Merkel’s discs as fingers run over Braille writing
- Located close to the surface
- Similar replica activity to bumps
- Really good at distinguishing between Braille letters
Describe the receptor activity of Meissner’s corpuscle as fingers run over Braille writing
A little bit less clear than Merkel discs, but still good at distinguishing between letters
Describe the receptor activity of Ruffini endings as fingers run over Braille writing
Still a little bit active, but not very good at making the distinction between letters
Describe the receptor activity of Pacinian corpuscles as fingers run over Braille writing
- Get almost universal activation
- Detect vibration, and paper isn’t smooth, so as you drag your finger across the page, you will get vibration
- Blank spots probably from smooth parts of paper -> causes finger to lift
What is the dorsal root ganglion (aka posterior root ganglion)?
- Dorsal root ganglion cell is the location of the cell bodies of all the somatosensory receptor cells entering the spinal cord at that location.
- Mechanoreceptors travel to the spinal cord via a “unipolar cell” that acts as both a dendrite and an axon
- First part of unipolar cell: axon initial segment, that has stretch-activated channels. If stretch-activated channels allow for depolarization + AP threshold to be met, AP is sent down unipolar cell.
Dermatomes
Represent the spinal segments where all the sensory information from that patch of skin travel to.
