Lecture 5: Sensory Systems Flashcards
(90 cards)
1
Q
sensation
A
process where physical or chemical stimuli from external environment or from body itself are transformed into neural signals (action potentials)
2
Q
Perception
A
brain processes neural signals to understand the stimuli
3
Q
Difference between sensation and perception
A
sensation is detecting a stimulus while perception is understanding the stimulus
4
Q
Three major steps of sensation
A
- stimuli are captured by specialized cells (sensory receptor cells)
- Captured signals are transformed into neural signals
- neural signals are passed to the CNS
5
Q
What’s different about sensory receptor cells for touch (aka somatosensation)?
A
Nerve endings serve as sensory receptors
6
Q
What does stimulation of sensory receptors by physical or chemical stimuli cause?
A
Selective ion channels open which results in changes to the membrane potential. If the integrated effect of local potentials is large enough, APs are triggered (temporal and spatial summation applies)
7
Q
How are sensory cells connected to the brain?
A
sensory nerves project to spinal cord or brainstem then synapse with spinal dorsal horn neurons which convey signals to primary sensory cortex via thalamus
8
Q
What do the dorsal horns of the spinal cord contain?
A
Cell bodies of sensory neurons
9
Q
What do the ventral horns of the spinal cord contain?
A
Cell bodies of motor neurons
10
Q
Primary sensory cortex
A
receive inputs mainly from thalamic relay nuclei (except olfactory cortex)
11
Q
Secondary sensory cortex
A
receive inputs mainly from primary cortex within the same sensory system
12
Q
association cortex
A
receive inputs from multiple sensory systems usually from the secondary sensory cortex .
13
Q
Transduction
A
captured signals are transformed into neural signals (generation of APs)
14
Q
Sound stimuli
A
Vibration of air molecules. Properties include: amplitude, frequency, and complexity.
15
Q
What contributes to complexity of a sound
A
sound waves of different amplitude and frequency
16
Q
What is the tympanic membrane and what is it attached to?
A
Tympanic membrane is the eardrum. It’s attached to a series of bones called ossicles.
17
Q
Structures of the ossicles
A
malleus, incus, stapes
18
Q
Function of the ossicles
A
To amplify the vibrations by 20x
19
Q
Oval window
A
membrane covered opening to the cochlea
20
Q
What does vibration of the oval window do?
A
Moves the fluid in the cochlea
21
Q
How is sound converted into neural signals?
A
waves of air are converted to waves of fluid which is converted into AP spikes within cochlea when hair cells bend
22
Q
Why are the ossicles needed to amplify sound waves?
A
Soundwaves travel faster in fluid than in air but it takes much more energy to start the vibrations
23
Q
How many chambers in the cochlea
A
three, fluid filled chambers
24
Q
Membranes in the cochlea
A
Tectorial and basilar membranes
25
What are the auditory sensory receptor cells and where are they found?
Hair cells. Found embedded in the basilar membrane.
26
Describe a hair cell
Hair cells themselves have hair-like structures called stereocilia. The tops of the stereo cilia are attached to the tectorial membrane while cell bodies are embedded in basilar membrane.
27
How do hair cells pick up sound stimuli?
Fluid vibration dislocates the tectorial and basilar membranes causing stereocilia to bend.
28
What does the bending of the stereocilia do?
Opens mechanosensory ion channels called TRPA1 (transient receptor potential A1 channel) and K+ and Ca2+ ions enter depolarizing the hair cell.
29
Does the hair cell itself fire action potentials?
No! The influx of K+ and Ca2+ triggers opening of voltage gated calcium channels. Calcium ions enter and trigger release of NTs which excite auditory nerves contacting the hair cell bottom. The nerves fire pattern of APs encoding stimulus.
30
Cochlea is organized...
tonotopically (based upon frequency)
31
What region of the cochlea corresponds to higher pitched sounds? Lower pitched sounds?
Higher frequency stimulates hair cells closer to the oval window (narrower, stiffer base). Lower frequency sounds stimulate hair cells near tip of cochlea (wider, floppy apex).
32
Infrasound
sounds below 20 Hz (cannot be perceived by human ear)
33
Ultrasound
sounds above 20 000 Hz (cannot be perceived by human ear)
34
complex sounds stimulate...
multiple positions of the cochlea
35
What can damage to a particular region of the cochlea do?
deafness to certain frequencies
36
Describe auditory pathway (from ear to cortex)
1. auditory nerves innervate cochlear nuclei (medulla) through 8th cranial nerve
2. Cochlear nucleus innervates superior olives (medulla) at both right and left sides—innervates the contralateral side more than ipsilateral side.
3. Superior olives innervate inferior colliculus of tectum (midbrain)
4. Inferior colliculus innervates medial geniculate nucleus of thalamus
5. MGN innervates primary auditory cortex in temporal lobe
37
Tonotopic organization is maintained along...
the auditory pathway (at cochlear nucleus, superior olives, primary auditory cortex)
38
Somatosensory signals enter the spinal cord via...
dorsal roots. Action potentials are sent along axons that enter the dorsal roots.
39
Somatosensory nerve is part of...
the dorsal root ganglia—a cluster of neurons next to the dorsal root. These are unipolar neurons.
40
AB fibre
myelinated, non noxious mechanical stimulus
41
Ad fibre
myelinated, noxious mechanical stimulus
42
c fibre
not myelinated (AP transduction slow), noxious heat and chemical stimuli
43
Merkel's disks
gradual skin indentation (ex. pressure)
44
Pacinian corpuscles
sudden displacements of skin (ex. vibration)
45
Meissner's corpuscle
light touch
46
Ruffini endings
gradual skin stretch
47
Types of mechanosensory receptors on skin
- Merkel's disks
- Meissner's corpuscle
- Pacinian corpuscle
- Ruffini endings
- free nerve endings
48
How is mechanical stimuli captured by receptors?
mechanical stimulus leads to mechanical stretch of the axon membrane and mechanosensory ion channels ope. If a threshold is reached action potential is generated and propagated to spinal cord
49
nociceptors
free nerve endings captured by pain and temperature—located throughout body (skin, muscles, blood vessels, internal organs)
50
How are pain signals created by free nerve endings?
1. Direct deformation of free nerve endings causes pain (ex. pinching)
OR
2. In case of tissue injury, damaged tissues release a cocktail of chemicals around the free nerve endings
OR
3. Nerve endings release Substance P (neuropeptide) (histamine release from mast cells, blood vessel dilation, inflammation)
51
What causes the prolonged APs in free nerve endings (lasting pain)?
Potassium ions from damaged cells and bradykinin (neuropeptide) from blood vessels.
52
Which type of fibre senses temperature
A set of C fibres
53
TRP channels
Transient Receptor Potential channels are channels that display distinct thermal thresholds from noxious cold to harmful hot.
54
Which TRP channel does capsaicin activate? How about menthol?
Capsaicin activates TRPV1, menthol activates TRPM8.
55
Example of noxious cold and hot TRP channels
noxious cold - TRPA1
| harmful hot - TRPV2
56
Dermatome
stripe of skin innervated by a given segment of spinal cord. Sensory signals from the same dermatome enter the same level of spinal cord.
57
Two paths that somatosensory information travels upward once they reach the spinal cord
Dorsal-column medial lemniscus pathway and anterolateral pathway
58
Which spinal-neural pathway carries touch info and which carries pain and temp info?
dorsal-column medial lemniscus - touch
| anterolateral pathway - pain and temp
59
Dorsal-column medial lemniscus pathway
1. axon enters dorsal horn and joins dorsal column and ascends
2. in medulla, axon synapses innervating neurons of dorsal column nuclei
3. dorsal column nuclei neurons send their axon across midline up to thalamus
4. Thalamus will in turn send this info to the somatosensory cortex
60
When does the dorsal column medial lemniscus pathway cross the midline?
Between the medulla and thalamus part of the circuit
61
Anterolateral pathway
1. axons synapse on neurons of dorsal horn upon entering spinal cord
2. axons of dorsal horn neurons CROSS THE MIDLINE and ascend along the lateral column of spinal cord
3. ascending axons synapse on the thalamus
4. Thalamus sends signals to the somatosensory cortex
62
Where is pain info sent along the anterolateral pathway?
It ultimately goes to the somatosensory cortex via the thalamus but pain information is provided to various brainstem sites like superior colliculus and reticular formation to trigger visual reflex and arousal
63
Where is the primary somatosensory cortex located?
in the postcentral gyrus of the parietal lobe
64
How is the somatosensory cortex organized?
somatotopically organized—based on positions in the body (body parts that are more sensitive have more cortical area)
65
Where are olfactory receptor cells found?
embedded in the olfactory epithelium (thin sheet of cells embedded in nasal cavity)
66
Olfactory receptor cells are what type of neuron?
Bipolar neuron—two major processes. Dendritic process with olfactory cilia captures odorant molecules and axon projects to olfactory bulb.
67
Where does olfactory transduction take place? How?
in olfactory cilia....
1. Odorants bind to odorant receptors (metabotropic)
2. Activates g protein which in turn opens Na+ amd Ca2+ channels (membrane depolarization)
3. AP spikes travel to olfactory bulb
68
OR genes in humans vs mice
350 vs 1500
69
Olfactory receptor cells express only...
one type of odorant receptor (OR1, 2, or 3)
70
If there are only three types of odorant receptors how can we distinguish between smells?
Each odour activates a unique combination of OR cells. An "odour map" is generated in the olfactory bulb by activating a unique combination of glomeruli.
71
Describe neural circuit involved in olfaction
- Axons of olfactory receptor cells travel through small holes in skull bone and innervate olfactory bulbs
- axon terminals synapse on dendrites of MITRAL cells
72
Mitral cells
Main output cells of the olfactory bulb. Axons of these cells enter brain via olfactory tracts (axons bundled into cranial nerve I)
73
Olfactory glomerulus
a cluster of synaptic connections between axons of olfactory receptor cells and dendrites of mitral cells.
74
How many glomeruli per olfactory bulb?
2000 glomeruli
75
Relation between OR cell type and glomerulus
Cells expressing same OR converge onto the same glomerulus. Each odour activates a unique combination of glomeruli.
76
What is unique about the olfaction neural circuit?
The mitral cells (main output of olfactory bulb) bypass the thalamus and project directly to the olfactory cortex and limbic system.
77
Why are responses to odours so visceral and immediate?
Olfaction doesn't go to thalamus. Goes straight to olfactory cortex and generates immediate emotional response to odours.
78
Limbic system
consists of hippocampus, amygdala, fornix, cingulate cortex, septum, mammillary bodies. Involved in four Fs and learning/memory.
79
Papillae
Small bulges covering tongue. Contains several hundred taste buds.
80
Taste buds
Cluster of 100 taste receptor cells
81
How often are taste cells regenerated
every two weeks
82
Describe taste cell
top end has microvilli that are clustered in a pore and bottom end is innervated by gustatory nerves
83
How are taste receptor cells organized
each receptor cell is tuned to a single taste and innervated by a single gustatory nerve
84
Saltiness
Na+ conc in food. Sodium channels (not voltage gated super sensitive sodium channel) open NTs are released.
85
Sourness
H+ pass through sodium channel (same one that mediates taste of salt) depolarizing taste cells and NTs are released
86
Difference between transduction of salty/sour foods and sweet/umami/bitterness
salty and sour foods are mediated by sodium channels while sweet/umami/bitterness is mediated by metabotropic receptors (G-protein coupled receptors).
87
Families of taste receptor proteins
T1R -> T1R1, T1R2, T1R3
for sweetness and umami
T2R -> at least 30 distinct types for bitterness
88
T1R2 and T1R3
Dimer: together these detect sweet substances (sugars and artificial sweeteners)
89
T1R1 and T1R3
Dimer: detects various amino acids. Detects monosodium glutamate.
90
Describe gustatory neural circuit
- gustatory nerves synapse onto solitary nucleus of medulla
- neurons of solitary nucleus project to ventral posterior medial (VPM) nucleus of thalamus
- VPM neurons project to gustatory cortex in insula cortex of parietal lobe
