Module 4 - Sensory Flashcards
L4.1 - Define the somatosensory and special sensory systems.
The somatosensory systems mediates the feeling of touch, pain, temperature and proprioception -> the somatic sense
visual, auditory, taste, smell
We also have visceral systems necessary for homeostasis and the vestibular for balance and head movement.
L4.1 - Describe the afferent pathways of somatosensory systems, ie. the anterolateral system (pain and temperature) and the dorsal-columns-medial lemniscal system (touch and proprioception).
L4.1 - Describe sensory receptors and explain their activation
Sensory receptors are generally specialized structures, that only respond to a certain modality of stimuli (adequate stimuli) with certain thresholds for activation. They can be found in accessory structures, which are specialized epithelium that are sense organs (like the nose, eyes, skin, etc.)
They are either primary (neurons), that signal through APs and have axons or they are secondary (epithelia), that signal with NTs and have no axons.
L4.1 - Define the adequate stimulus
An adequate stimulus is a stimulus that can activate a receptor based on its modality - not related to the intensity!
L4.1 - Describe receptor potential and the sensory cascade (transduction)
The receptor will receive the sensory input and create an electrical receptor potential, which refers to the amplitude and duration of the electrical signal (depends on stim intensity). The transduction is the conversion into electrical signaling, where the signal is then propagated to other cells.
Receptor Potential: Change in membrane potential produced by transduction currents.
Transduction: the conversion of stimulus energy into electro-physiological response.
Can be direct (as in hair cells) or indirect through metabotropic receptors
L4.1 - Describe regulation of repetitive firing
Response of the receptor declines under constant stimulation, proving that change is more important than steady state
L4.1 - Describe adaptation and explain adaptation mechanism
Response of the receptor declines under constant stimulation, change is more important than steady state.
Sensory receptors detect contrasts, thus changes in temporal and spatial patterns of stimulation why the constant stimulation receives less attention. E.g., you don’t feel the clothes you wear, but you might initially when you put it on.
L4.1 - Define slowly and rapidly adapting receptors
Slow adapting receptors will respond as long as a signal will be present (good to keep the the body aware of the stim being there), but rapidly adapting receptors decrease their firing and adapt to a present stimulus - they will fire again once the stim is ended or changed to signal that change.
L4.3 (touch) - To explain transmission of sensory information from receptor to the cerebral cortex
Peripheral receptor (DRG) -> Dorsal column (synapse on gracile or cuneate nucleus) -> crosses to the medial lemniscus pathway (synapse onto the VPL of the thalamus) -> project to the primary somatosensor
L4.3 (touch) - To describe distribution and classification of receptors according to adequate stimulus and adaptation
Mercel cells: in the dermis, slowly adapting - Deformation of skin, hair
bending
Meissner’s corpuscles: in the epidermis, rapidly adapting - feel surface structures
Ruffini corpuscles: Ruffini in the dermis, slowly adabting, Stretch of skin/joints
Pacanian corpuscles: in subcutanius(?), rapidly adapting - deep fast vibrations
L4.3 (touch) - To explain receptive field and lateral inhibition
Receptive field: the area of skin one receptor detects a stim from - smaller for surface receptors (the Germans)
Lateral inhibition: The primary input neurons inhibits its neighbors create contrast and thereby enable 2-point discrimination
L4.3 (touch) - To describe localization of sensory areas and processing of sensory information in the cerebral cortex
There is the primary somatosensory in the postcentral gyrus and the posterior parietal area.
the primary somatosensory cortex is divided into areas
1 (rapidly adapting receptors - surface structures - Meissner - required for 2D),
2 (deep receptors for size and shape - required for the depth - 3D),
3a (muscle receptors and nociceptors - pain & proprioception),
3b (Cutaneous receptors - touch)
the 3’s get 70% of VLP and VPM input and 1 and 2 30%
All project to the secondary somatosensory cortex and then onwards to amygdala/hippocampus
L4.4 (touch) - Describe nociceptors and nerve fibers conducting pain impulses
Nociceptors respond to noxious stimuli and sit on free nerve endings, and the fibers are c-fibers (unmyelinated, thin, glutamatergic) and a-delta (myelinated, faster). Nociceptors can be mechanoreceptors, thermal receptors or chemical receptors.
L4.4 (pain)- Describe the differences between discriminative and affective components in pain
Discriminative pain is used to act on and is mediated by a delta fibers (first pain), affective pain is mediated by c-fibers and relates to 2nd pain, which projects to affective areas ACC and the insula.
L4.4 (pain) - Define first and second pain and their central representation
First pain goes the primary somatosensory and secondary pain (more diffuse in time and location) to the ACC and the insula
L4.4 (pain) - Describe that perception of pain differs from the objective painful stimulus
Pain is the experience; nociception is the transduction through pain fibers - pain is a subjective experience that should be respected, nociception can result in pain.
L4.4 (pain) - Explain that descending tracts modulate the pain perception of a painful stimulus
The top down control (raphe nucleus and locus coeruleus) allow inhibition of the sensory cells in the dorsal horn through NE, serotonin and endogenous opioids released in the synaptic cleft and can bind to pre or post synapse to hyperpolarize the sensory neuron (can also excite an inhibitory interneuron)
L4.7 (vision) - Describe the five major excitable cell types in retina and their synaptic connections
Photoreceptors: Cones & rods - cones are for color vision (3 types: red, green and blue) and high acuity vision found primarily in the fovea. Rods are more sensitive due to convergence (many rods synapse onto one bipolar cell) to light than the cones but can’t show color, found in the periphery (discs are closed in the outer segment)
Bipolar cells can be “on” (mGlu - fires for light) or off (AMPA - fires in the dark). They get input from photoreceptors)
Horizontal cells support transmission and influence synaptic integration between the photoreceptors and the bipolar cells
Ganglion cells are known to be the first cells to fire APs, leading out the signal to the optic nerve. The “p” (petite) subtype is found in the fovea, the “m” (mega) subtype in the rest of the retina.
Amacrine cells support transmission and influence synaptic integration between the bipolar cells and ganglion cells
L4.7 (vision) - Explain phototransduction (molecular steps from photon absorption to photoreceptor hyperpolarization)
Within the discs, you have the rhodopsin (the photopigment of rods) where you will find retinal in the 11-cis form. When light hit this, it will become alt-trans. The G-protein transducence is in it’s inactive bound to GDP. However, when it becomes active, GDP is exchanged for GTP. transducence is activated by the change in retinal. The GTP takes the alpha subunit of transducence and activates a phosphodiesterase in the disc membrane. The phosphodiesterase hydrolyzes cGMP to GMP. Normally, cGMP holds the sodium channel open (why we have depolarization at rest), but when light hit the rods, these molecular processes removes cGMP and the channel closes. The many steps allow for amplification and therefore fast acting of the closure of channels.
The continued depolarization at rest of the cell allows for glutamate to be released until a bright light is present.
To return, the activated rhodopsin is phosphorylated by rhodopsin kinase, which enables protein arrestin to bind to rhodopsin (block the ability to activate transducence). We hydrolize transducence turns it off, and without it, the phosphodiesterase also turns off. Retinal is returned to its 11-cis state and in the dark more cGMP is being build up.
L4.7 (vision) - Describe the distribution of rods and cones in the retina
In the fovea you only have cones, more rods in the periphery - cones get larger receptive fields the less there is
L4.7 (vision) - Define a receptive field and describe center-surround receptive fields
The receptive field of a sensory cell is the area in which stimulation changes its signaling activity, reflected as excitation or inhibition.
The center surrounding receptive fields belong to bipolar and ganglion cells, which gets projected to the LGN
These receptive fields are the best way to create contrasts - the firing of the cells will depend on how much of the plus is being hit by light vs minus
L4.7 (vision) - To describe the trichromatic theory and the most common forms of color blindness
The idea is that the 3 color that the cones can detect can create all colors - if there is a cross over of genes during mitosis, you might end up without the gene for one of the cones, which could lead to color blindness. The gene that most often gets left out in cross over is for red and green, which is why that’s the most common. The red cone is missing for those that have the red/green color blindness. Men have higher risk of color blindness, as the mutations are recessive and many of these (including the red cone) are on the x chromosomes.
L4.7 (vision) - To explain retinal circuits for light/dark discrimination, including ON-center and OFF-center neurons
The molecular pathway for the question above shows, that when light hits retinal, it goes from 11-cit to all-trans and cGMP become GMP and the ion channels close, meaning that less glutamate is released as a result of light hitting the photoreceptor
For the on-cells, we will have inhibitory receptors (the decrease in glutamate will cause disinhibition of cells there in the dark are inhibited by glutamate) glutamate release from on-center bipolar cell to the on-ganglion cell to indicate that there is light on the center
For the off-center cells have AMPA receptors, so a decrease in glutamate will lead to a decrease in activation decrease in signaling to the off-center ganglion cell to signal that there is light in the middle
Opposite is true for less light
This also explains why there is less signaling in the resulting V1 cells when the entire receptive field is covered by light vs just the center.
Horizontal cells support the lateral/surround inhibition of the on-center photoreceptor cells if there is also light in the surrounding to give a higher contrast image in the end. Here we see that the activation of one photoreceptor can cause the horizontal cells to inhibit others to better detect edges in the environment and give sharper contrast
TLDR: the horizontal cells provide signal integration and lateral inhibition of photoreceptors to improve the contrast.
L4.7 (vision) - Describe the central pathways from retina to the primary visual cortex