Principles of Sensation, Touch Flashcards
what is the difference between sensation and perception?
- usually sensation and perception are parts of one continuous process
- sensation - how the brain represents external energy/signals
- perception - organization and interpretation of sensory information
- neuroscientists argue that sound is just the interpretation of energy and wavelength changes
- if a tree falls in the woods and nobody is there, it doesn’t make any sound
what are the different types of processing?
bottom-up and top-down processing
what is bottom-up processing?
- flow of information from sensory input to higher-level cognitive processes
- starts with raw data (sensory input) that the brain organizes and interprets to form a perception or understanding
- look at external world → signal is sent to the brain → sent to specialized areas → becomes a fully perceived image
- most of our perception happens through this
what is top-down processing?
- involves using pre-existing knowledge, expectations, or experiences to interpret and organize incoming sensory information
- we form representations of the world in our brain and when we look at the world, we look for features related to our representations
- ex. we have expectations about the environment/situations
- going to a new classroom, we expect to find desks and tables
- unique feature of humans perceptual system
what is transduction?
process of turning external energy into nervous system signals
how do our senses generally work?
- we receive sensory stimulation (external energy) using specialized receptor cells
- some specialized to pick up fluctuations in air pressure, others to pick up changes in electromagnetic spectrum
- transduction turns energy into NS signal
- turn into receptor potentials, fluctuations in resting membrane potential, not necessarily at a synapse
- neural information is delivered to the brain
what are the characteristics of receptor potentials?
- similar to post-synaptic potentials, but not at synapse
- can be via ionotropic or metabotropic receptors
- ionotropic receptors are faster than metabotropic receptor
what is the process of hearing?
- hair cell is found in inner ear and has cilia on it
- sound is changes in air pressure → arrives at ear → causes eardrum to flap and bones to move → causes liquid inside cochlea to move → ripples cause cilia on hair cells to move back and forth → channels are pulled open as the cilia sways
do we have five sense?
- don’t necessarily have five senses
- many more subcategories of senses
- there are many other senses in animals
- birds can orient themselves to N/S alignment even in a room
what is an example of how our sensory systems have a restricted range of responsiveness?
- the way we sense the world is evolutionarily useful
- visible light is a small part of the electromagnetic spectrum, we cannot see all aspects of the electromagnetic spectrum
- we see a specific band of the electromagnetic spectrum because those bands of light bounce off us and is reflected
- factors such as the type of sun and the light it gives off make this possible
- gamma rays would not provide meaningful information about the world if we could sense them
why do some animals see different bands of the electromagnetic spectrum?
- some animals can see different bands of light because it is evolutionarily important for them
- even closely related species can have differences in sensory systems
- for example, cats can hear higher frequencies, which helps them hunt mice - despite differences, there are still similarities in sensory systems between mammals, fish, and birds.
how does the human eye provide proof of human’s evolution? in what ways is this not optimal?
- we started from light sensitive cells that developed based on evolution
- human eyes are full of fluid because they evolved from when we were an aquatic species
- when light travels from one medium to another (water to air), it is distorted
- our eyes would’ve been better if they weren’t filled with fluid
- axons and other cells and blood vessels are in front of photoreceptors
- light has to bounce off of all these things before reaching photoreceptors
- we use metabotropic receptors in the eye, which are slower
- the place in the eye where all the axons leave is a blindspot
how did octopus’ eyes evolve?
- octopus had a different lineage of visual evolution
- water outside the octopus and water on the inside of the octopus’ eye
- photoreceptors are the most external part of the eye
- all the blood vessels and axons are behind photoreceptor (no blindspot)
- their evolution was much more optimal than ours
if all senses are just changes in electricity, how do we know which is for what sense? what are the two theories for how we decode action potentials?
- doctrine of specific nerve endings
- labelled lines
what is the doctrine of specific nerve endings? what are its limitations?
- specialized sensory cells respond only to their specific type of energy (touch-sensitive cells respond to touch, not light or sound)
Limitation:
- if you close your eye and push on it, you may see light or dark spots even without light entering
- this occurs because light-sensitive cells interpret any stimulus (pressure) as a change in light, despite no actual light change
what is the idea of labelled lines?
- specialized sensory cells respond only to their specific type of energy (touch cells respond only to touch) AND they stay segregated as signals travel to the brain
- each sense has separate pathways leading to specific brain areas, ensuring signals remain distinct
- crossed pathways can cause mixed sensory experiences like synesthesia (seeing sounds, hearing colors)
- if pathways are rewired to different targets, the brain can adapt and repurpose areas for new functions (sound-processing areas adapting to process taste)
what are the different layers of skin? what aspects of somatosensation do they detect?
- hypodermis (deepest layer): anchors tissue to muscles and the rest of the body; detects stretch and vibration
- dermis (middle layer): contains most sensory cells; responsible for fine touch discrimination
- epidermis (outermost layer): contributes to fine touch discrimination
what is the pacinian corpuscle (lamellated corpuscle)? where is it located?
- specialized skin receptor that detects vibrations
- input layer located in the hypodermis, the deepest layer
in what way is the pacinian corpuscle’s structure unique?
- they are pseudounipolar neurons - have a single process that splits into branches for sensory input and signal transmission to the central nervous system (output)
- doesn’t have dendrites
- it’s input layer is deep in the skin and connects directly to the output layer without a cell body in between
- axon travels to the spinal cord; the cell body is located in the dorsal root ganglion
- exception to the typical neuron structure (dendrite → soma → axon): instead, input → axon → soma
how do receptor potentials from the pacinian corpuscle work?
- receptor potentials are also called graded potentials (similar to PSPs)
- small vibrations produce small graded potentials; large vibrations produce large graded potentials
- graded potentials on the input layer can depolarize the cell, then an action potential occurs in the axon
- action potential is the same as what we know, but means of input are different
how does transduction happed in the pacinian corpuscle?
- vibration transduction occurs via stretch receptors with mechanically gated channels
- found in the skin and muscles, including muscle spindles (receptors for proprioception)
how do cells with differing thresholds contribute to sensory perception?
- neurons can transmit a max of 250 action potentials (APs) per second, limiting their ability to detect the full range of stimuli
- one neuron type cannot detect the entire range of a stimulus
- some cells are sensitive to small changes and reach their firing ceiling early, detecting only the difference between no signal and a weak signal
- these cells cannot distinguish between weak and moderate signals
- a range of cells with varying sensitivities is needed for full sensory perception
- population coding is necessary to interpret even simple touch stimuli.
what is intensity coding?
- the brain decodes sensory information by counting the total number of action potentials reaching the next neuron (summation of responses)
- multiple neurons work in parallel to convey the signal
- as the stimulus strengthens, more neurons are recruited to respond
- range fractionation: some neurons are specialized to be more sensitive to specific ranges of stimuli
what is sensory adaptation? what are the different kinds of sensory adaptation?
adaptation: progressive loss of response to maintained stimulus
- when we are touched, there is more action potentials at the beginning, even if the stimulus continues
- adaption happens at every level of sensation
- tonic receptors (slow-adapting) - persist in firing for longer
- phasic receptor (fast-adapting) - tell you about a change, then firing decreases
what is the benefit of sensory adaptation?
- adaptation is beneficial because constant pieces of information don’t provide any new information
- not going to be particularly useful to us
- nervous system is more interested in changes than absolute values of stimuli
what are receptive fields? how would they differ for types of touch?
- area on the skin in which this neuron will respond
- modality specific - receptive field for visual neurons is part of visual field, receptive field for touch is a patch of skin
- areas deeper in the skin have larger receptive fields (receptive fields for vibration are bigger than those for fine touch)
what is an “on centre, off surround” receptive field for touch? how does this increase discrimination of touch?
- if you press in the centre, it’ll stimulate the cell and cause bigger potentials
- if you press in the periphery of the receptive field, it has an inhibitory effect
- helps with discrimination and precision
- the centre of one cell is the periphery of another cell, increasing discrimination
what are the four main skin receptors? what are they responsible for? what receptors are found in different layers of skin?
- pacinian corpuscle - vibration
- meissner’s corpuscle - light touch
- merkel’s discs - fine touch
- ruffini’s ending - stretch
- hypodermis (deepest): pacinian corpuscle and ruffini’s ending
- dermis (middle): meissner’s corpuscle
- epidermis: merkel’s discs
what types of receptors have the largest receptive fields? what type have the smallest?
in order from largest to smallest:
1. pacinian corpuscle
2. ruffini’s ending
3. meissner’s corpuscle
4. merkel’s discs
- areas deepers in the skin have larger receptive fields
which skin receptors are fast adapting and which are slow adapting?
fast adapting:
pacinian corpuscle - vibration
meissner’s corpuscle - light touch
slow adapting:
merkel’s discs - fine touch
ruffini’s ending - stretch
how do sensory axons differ from each other?
- there are different sizes and thickness of axons
- in order of thickest to thinnest: A alpha, A beta, A delta, C
- A axons are myelinated and C axons are not
- conduction speed is extremely fast in axons that are thick and myelinated
- can’t have this for every cell, reserved for most important signals
- proprioception has the thickest myelinated axons because they are important for knowing where your body is in space
