Special Senses Flashcards
1
Q
What is somatic sensation
A
This term means sensation by distal nerve endings of dorsal root ganglion neurons
2
Q
What happens when stimuli don’t evoke a conscious sensation
A
It should be noted that some stimuli do not evoke a conscious sensation, but rather induce a reflex
3
Q
What is the purpose of the reflex
A
The reflex serves the protection function and avoids tissue damage
4
Q
What does the spinal cord do
A
It transmits peripheral sensory information to higher brain regions, ultimately to the somatosensory cortex. It transmits motor signal from higher brain regions to the skeletal muscles to enable a movement in response to a stimulus.
5
Q
What are peripheral nerves
A
Most peripheral nerves are ‘mixed.’ This means that they contain bundles of sensory fibres and also of motor fibres.
6
Q
What are the fibres
A
Fibres are the axons of neurons which serve to propagate information via electrical signals called (Na+) action potentials.
7
Q
Where do peripheral nerves exit
A
Peripheral nerves exit between vertebra which are an important part of the skeleton. Specifically, vertebra keep the body in a stable axis which is ventral horn of the spinal cord and exit the spinal cord via the ventral roots.
8
Q
What is the motor endplate
A
This is the contact at the peripheral synapse. The motor axons release acetylcholine which activates receptors on the muscle fibre membrane. The resulting excitatory endplate potential elicits a Na+ action potential on the muscle fibre which ultimately leads to the contraction of the muscle fibre.
9
Q
How do sensory axons receive information
A
In the sensory axons, information received from sensory receptors propagates encoded as a ‘train’ of action potentials. The sensory axons enter the spinal cord via the dorsal nerve roots.
10
Q
How do sensory and motor nerves unite
A
Both sensory and motor axons unite close to the spinal cord to form the mixed peripheral nerve.
11
Q
What are Schwann cells
A
When a nerve ending is surrounded by connective tissue structure, most sensory axons are myelinated. This means that they are enwrapped by the cytoplasm of a certain type of glial cells called Schwann cells.
12
Q
What makes up the cytoplasm of Schwann cells
A
The cytoplasm of Schwann cells contains myelin. Myelin is a lipid that is very well suited to electrically isolate the axon from neighbouring axons. Moreover, the myelination enables saltatory action potential propagation that is faster than the continuous action propagation occurring in unmyelinated axons
13
Q
What are the nodes of Ranvier
A
The node of Ranvier is a small portion of non-isolated axon membrane in the cleft between 2 neighbouring Schwann cells.
14
Q
What kind of neuron are the dorsal root ganglion neurons
A
They have 2 processes that originated located next to each other from the soma, so they are bipolar neurons.
15
Q
What is white matter
A
The white matter in the central nervous system contains axons and oligodendrocytes type glial cells enwrappign them.
16
Q
What is gray matter
A
The gray matter contains the soma and dendrites of the central nervous system neurons which are neighboured by astrocyte-type glial cells.
17
Q
What are the epidermis
A
The epidermis consists of epithelial cells that secrete keratin in the outermost layers. It serves to isolate the skin from the environment for protection from damage.
18
Q
What are free nerve endings
A
The receptors are free nerve endings of dorsal root ganglion neurons that have specialized on responding to pain or temperature stimuli.
19
Q
What are Meissner corpuscles
A
They are in the touches between the enfolding of the epidermis. They have a coild-up free nerve endings that is embedded into a homogeneous layer.
20
Q
What are Merkel disk receptors
A
Merkel disk receptors are located at the tip of the epidermal protrusions and comprises endings with multiple cup-like connective tissue sheaths.
21
Q
What are Ruffini endings
A
Ruffini endings are located inside the dermis, but still relatively close to its boundary with epidermis. Their free nerve ending is coiled up inside a spindle-shaped layer of connective tissue.
22
Q
What are Pacinian corpuscles
A
The Pacinian corpuscle is located deep within the dermis. A single nerve ending per corpuscle is enwrapped by concentric layers of connective tissue.
23
Q
What do Meissner corpuscles feel
A
Meissner corpuscles sense a stroking or fluttering type of touch and respond to vibration occurring at 20-50 Hertz.
24
Q
What do Merkel disk receptors feel
A
Merkel disk receptors respond to pressure in the skin and enable to sense the texture of the surface. Merkel disk receptors sense vibrations occurring at 5-15 Hertz.
25
What do Pacinian corpuscles feel
The Pacinian corpsucle are specialzed to sense either deep constant pressure or high frequency vibrations at 60-400 Hertz.
26
What do Ruffini endings feel
The Ruffini endings have specialized in sensing when the skin is stretched, and they can sense gravity
27
What do Nociceptors feel
Nociceptors respond to different types of pain. Specifically, some of these receptors have specialized on sensing so-called sharp or pricking pain evoked by mechanical injury like with a knife.
28
What do thermal receptors feel
Thermal receptors provide information on the temperature of the skin. Specialized receptors respond optimally to skin cooling to or below 25 degrees Celsius. In contrast, warm receptors with a maximum frequency of action potentials at a skin temperature of 41 degrees.
29
What is adaptation
Attenuation of action potential frequency or receptor potential amplitude during a constant stimulus.
30
What is adaptation like in mechanoreceptors
Adaptation can also be caused by the connective tissue layer of a mechanoreceptor to serve the purpose of optimizing its response to a certain type of stimulus.
31
What causes fast adaptation in connective tissues
It can be concluded that the elastic properties of the connective tissue capsule causes the very fast initial adaptation and also reactivate, during swinging back, the mechanosensitive cation channels briefly during retraction from the stimulus.
32
What is the properties of Meissner corpuscles
There is no activity when no stimulus is applied. This receptor responds with a constant rate of action potentials to the ongoing pressure on skin. The receptor responds when the pressure is removed.
33
What are the properties of Merkel disk receptors
Merkel disk receptors are inactive before the onset of stimulus and respond with spiking to the movement of the stimulus. They do not respond to the removal of the stimulus.
34
What are the properties of Pacinian corpuscles
It responds with a single action potential to any change in stimulus amplitude
35
What are the properties of Ruffini endings
They are very slow adapting mechanoreceptors.
36
What is the receptive field of Meissner corpuscles
Its distal dendritic ends responds to stimulation of various, but highly localized spots. Meissner corpuscles respond to the same type of stroking or fluttering mechanical stimuli.
37
What is the receptive field of Pacinian corpuscles
A larger field that Meissner corpuscles that branch out in fewer collaterals.
38
What is the first synapse of pain or temperature receptors
Action potentials from pain or temperature receptor axons arrive at the 1st synapse, which is formed with a dorsal horn interneuron located on the ipsilateral side of the spinal cord. The axon of this postsynaptic ipsilateral spinal cord interneuron crosses the midline at the same level where they arrive sensory axon enters the spinal cord.
39
What is the second synapse of pain or temperature receptors
Consequence the 2nd synapse, in this system is primarily located in the thalamus. On the other hand, some axon collaterals of the spinal interneurons form synapses with neurons of the reticular formation
40
What is the third synapse of pain or temperature receptors
From the thalamus, the pain or temperature signals are projected via action potentials in a distributed manner to various area of the somatosensory cortex where the 3rd synapse is formed with a cortical neuron.
41
What is the ascending reticular activation system
The ascending reticular activation system which controls multiple autonomous functions such as alertness and arousal.
42
What is the first synapse of mechanoreceptors
The 1st synapse of the mechanoreceptors is in the brainstem. Specifically, it is located either in the nucleus cuneatus or the nucleus gracilis
43
What is the nucleus cuneatus
Receives information from the upper body area
44
What is nucleus gracilis
Received information from the lower body area
45
What is the second synapse of mechanoreceptors
The axons of the postsynaptic brainstem neurons cross the midline and project then on the contralateral side primarily to a specific subregion of the thalamus to form the 2nd synapse, but also via collaterals to the reticular formation
46
What is the third synapse of mechanoreceptors
From the thalamus, the sensory information is transmitted to a very specific spot within the somatosensory cortex. Here, is the 3rd synapse is formed with a cortical neuron for so-called somatotopic projection
47
What is the somatosensory cortex
Forms an extended area from the medial to the lateral part of the brain. It receives information from mechanoreceptors of the left body aspect as their fibres project ultimately onto the contralateral side.
48
What is the homunculus
A tiny human body that is mapped from the somatosensory cortex. It is based on the different density of skin mechanoreceptors and the size of their receptive fields.
49
How was the homunculus map obtained in the past
The homunculus map was originally obtained by extracellular microelectrode recordings of the cortex of patients that had to undergo neurosurgery, mostly for removal of a part of the brain to treat epilepsy or for removing a tumour.
50
How do we maintain the map of the homunculus in present time
Now, a highly sophisticated non-invasive imaging technique called magnetic resonance imaging is used instead.
51
How do MRIs work
This technology measures the distribution and energy states of protons which are H+ ions.
52
What is 2-point discrimination
It measures differences in the sensation 2 mechanical stimuli of equal strength that are applied at the same time to two different spots in the skin vs. sensation of only 1 stimulus due to the lack of discrimination.
53
What is the outer ear
Comprises the pinna, the auditory canal, and the tympanic membrane.
54
What is the pinna
The shape of the pinna serves to increase the sensitivity of the ear to a particular frequency.
55
What is the auditory canal
After the sound wave propagates through the auditory canal, it hits the tympanic membrane which is the outer boundary of the middle ear.
56
What is the middle ear.
The middle ear is also filled with air and contains the ossicles. These structures are 3 (malleus, incus, and stapes) are the smallest bones in the human body
57
What are the ossicles
They are linked to each other and connect to the tympanic membrane with the membrane of the oval window
58
What is the oval window
Separates the inner boundary of the inner ear from the fluid-filled space of the inner ear.
59
What do the ossicles do
To amplify the vibration energy that the air sound waves contains before it hits the oval window. The absence of the auditory ossicles would constitute a moderate-to-server hearing loss.
60
Where does the fluid-filled space of the inner ear go
To the semicircular canal and the vestibule of the vestibular system which is involved in sensation of balance, posture, and gravity.
It extends into the cochlea which contains the hair cells that sense audio signals.
61
What is the cochlea
Contains 3 scales. The scales are compartments that are filled with aqueous solution, specifically the scala vestibula, scala media, and the scala tympani
62
What are the scalas of the cochlea
The scala vestibuli and the scala media are separated from each other by Reissner's membrane which is a layer of connective tissue cells. Another layer of connective tissue separates the scala media and the scala tympani.
63
What is the organ of Corti
Contains the sound-receptive hair cells on top of the basilar membrane which is a cell-free layer of connective tissue
64
What do the glial cells do in the organ of Corti
Particularly the glial cells serve to mechanically stabilize the organ of Corti and they also interact with the hair cells to optimize their functions.
65
What do the inner hair cells do in the organ of Corti
The row of inner hair cells is synaptically connected with afferent nerve fibres form the spiral ganglion. These axons form the auditory nerve. This indicates that the inner hair cells are the main receptors for auditory signals even though their number is smaller than that of the outer hair cells.
66
What do the outer hair cells do in the organ of Corti
The outer hair cells seem to serve to fine-tune the audio response of the inner hair cells. The upper part of the hair cells forms stereocilia. These are small protrusions of the plasmalammel membrane that makes contact with the cell-free gelatinous layer of the tectorial membrane.
67
What does the cochlea look like
Usually the cochlea is coiled up like the shell of a land snail. After isolation, the cochlea has been uncoiled for the section. This revealed that the apex, which is the tip, of the cochlea is located opposite to its base.
68
What is the apex of the cochlea.
At the apex, an opening called helicotrema provides a direct connection between the scala vestibuli and the scala tympani.
69
What does the scala vestibuli connect to
The scala vestibuli contacts one ossicle of the middle ear, the stapes, at the oval window
70
What does the scala tympani connect with
The scala tympani contacts the middle ear at the round window which is closed by a cell-free membrane
71
How are acoustic signals sensed in the ear
Acoustic signals of a specific frequency are sensed only by hair cells on a particular spot on the longitudinal extension of the basilar membrane that extends between its base that is located close to the middle ear and its tip, called the apex, that is located on top of the usually coiled-up cochlea.
72
Where does sound pressure first arrive
The sound pressure wave arrived firstly at the tympanic membrane. Here the sound wave in air is transformed with the help of ossicles into a pressure wave within the extracellular liquid space of the scala vestibule.
73
How does sound become the traveling wave
This transformation of rhythmic air pressure changes into a rhythmic fluid pressure change results in a vibration of the basilar membrane. This vibration induces a 'traveling wave' of up and down movements of the basilar membrane toward the apex.
74
What is the maximum amplitude of the traveling wave
Depending on the specific frequency of the acoustic stimulus, the traveling wave has a maximal amplitude on a particular spot along the basilar membrane. This is due to the fact of that this membrane is narrow and stiff at the base and becomes progressively wider and flopper toward the helicotrema at apex.
75
What is the range of frequencies for the stiff and floppy small membranes
Stiff small membranes are well suited to reproduce high frequency signals in the range above 2-4 kilohertz, while floppy membranes optimally transmit bass frequencies in the range below 2-4 kilohertaz.
76
What is the maximal movement of the basilar membrane and the cochlea.
Thus, the maximal movement of the basilar membrane associated with the traveling wave is at the apex for frequencies of less than 0.5 kilohertz. In contrast, frequencies of above 16 kilohertz provide a maximal up and down movement of the basilar membrane at the base of the cochlea.
77
How does sound-induced compression impact the vertical tympanic membrane
Sound-induced compression of the vertical tympanic membrane causes a downward deflection of the horizontal basilar membrane while rarefaction induces an upward movement
78
How does the bending of the stereocilia impact the tectorial membrane and the organ of Corti
During such sound-related oscillatory movements, the tectorial membrane within the organ of Corti moves laterally with regards to the basilar membrane due to its specific anatomical architecture.
79
What is the bending of the stereocilia
This lateral movement evokes a sidewise bending of the hair cell stereocilia which makes contact with the gelatinous membrane which covers them.
80
What happens when hair cells bend to the left side and the right side
A bending of the cilia to the left side evokes a hyperpolarization of their resting Vm by <1 mV while bending to the right side evokes a depolarization of ~3 mV.
81
What is the bending range of the hair cells
Note that the bending distance covers the range of only +/- 20 nm which is very small absolute movement.
82
What is the receptor potential of hair cells
The receptor potential can precisely follow back and forward stereocilia movements even when they occur at a rate of >10 kilohertz. This indicates that the hair cell mechanoreceptor channels can open and close very rapidly.
83
How is the depolarization of the hair cells triggered
In the left part that the mechanosensitive channels of neighbouring cilia are coupled via so-called tip-links which are elongated spiral-shaped protein fibres of the cadherin family.
84
What happens when stereocilia are bent to the left
Bending of the stereocilia to the left side reduces the tension in the tip-links and this results in complete closure of K+ channel by a blocking ‘lid’ which is in fact a part of the subunit of the channel polypeptide.
85
What happens to the stereocilia when they are absent of stimulus
The mechanosensitive channels are partly open in the absence of a stimulus
86
What happens when stereocilia are bent to the right
During bending of the stereocilia to the right, the tension within the cadherin protein increases. This removes the blocking lid from the mechanosensitive K+ channel protein. Consequently, more K+ enters the stereocilia compared to the resting state where the pore is already partly open.
87
How does potassium ions impact the stereocilia
K+ ions enter the stereocilia and thus the intracellular cytosolic space of the hair cells. This means in this unique sensory neuronal system, an atypical influx of K+ occurs into the cell. This K+ influx induces a depolarization that results in activation of voltage gated Ca2+ channels.
88
How does calcium ions impact the stereocilia
The resulting influx of Ca2+ leads to a rise of cytosolic Ca2+ and this mediates the release of the neurotransmitter glutamate from the hair cell.
89
How is glutamate stored in hair cells
Glutamate is stored in vesicles at the base of the hair cells like in a presynaptic terminal of a central nervous system neuron.
90
Do hair cells have axons
Hair cells do not have an axon-like dendrite like dorsal root ganglion neurons possess and have instead the mechanosensitive stereocilia. They do neither have an axon to transmit the auditory information to the central nervous system like the dorsal root ganglion somatic sensor neurons have.
91
Since hair cells lack axons what is required of them
Therefore, it is required that afferent axons from follower neurons form the postsynaptic membrane and transmit the information via a train of action potentials to higher brain regions, ultimately to the auditory cortex. In the auditory system, the follower neuron is located in the spiral ganglion.
92
What is the fluid that fills the scala vestibuli and scala tympani
The scala vestibuli and the scala tympani are filled with intersitial fluid called perilymph which has the typical high extracellular concentration of Na+ and low concentration of K+
93
What is the fluid that fills the scala media
The composition of the fluid within the scala media, called endolymph, is like that of the cytosol in the intracellular space. The high K+ concentration is due to K+ secretion into the scala media by endothelial cells of the stria vascularis. Conversely, the Na+ concentration in the scala media is quite low which is a typical intracellular value for neurons.
94
How does the scala media impact potassium ions
This particular ion composition in the scala media affects the K+ gradients in the hair cells and how that affects the direction of the K+ flux through the K+ channels in the stereocilia.
95
What is the first step of the afferent auditory pathway
The afferent axons for transmitting the information from the hair cells are from neurons in the spiral ganglion.
96
What is the second step of the afferent auditory pathway
Once the spiral ganglion neurons are excited, they release glutamate onto afferent axons of the cochlear branch of the vestibulocochlear nerve which is the cranial nerve 8.
97
What is the third step of the afferent auditory pathway
Within this nerve, the action potentials propagate, on the one hand, on the ipsilateral side to the cochlear nuclei in the medulla oblongata.
98
What is the fourth step of the afferent auditory pathway
When the neurons in that ipsilateral nucleus are activated, they excite neurons in the superior olivary nucleus in the pons.
99
What is the fifth step of the afferent auditory pathway
Once these pontine neurons get excited, they send action potentials via the so-called lateral menisci axon tracts to neurons in the inferior colliculus in the midbrain.
100
What is the sixth step of the afferent auditory pathway
When these neurons get excited, they activate neurons in the medial genicular nucleus of the thalamus.
101
What is the final step of the afferent auditory pathway
Once the thalamic neurons get excited, they activate neurons in the primary auditory cortex. While this complex multi-synaptic pathway is exclusively ipsilateral, a second pathway is activated in parallel on the contralateral side. The crossing occurs already in the nuclei of the medulla oblongata.
102
What is visible light
Visible light constitutes only a very small fraction of the overall spectrum of electromagnetic waves.
103
How does the amplitude and intensity work with electromagnetic waves
The amplitude of an electromagnetic weave represents the intensity of this signal whereas the wavelength is the inverse value of its frequency. This means that an electromagnetic wave of low frequency has a large wavelength.
104
What do we know about ultraviolet light
Since ultraviolet electromagnetic waves have more energy, they are toxic and can destroy cells.
105
What do we know about infrared light
Infrared light can be therapeutic
106
What is the conjunctiva
The conjunctiva is a mucous membrane which comprises the inner surface of the eyelids and covers the forepart of the sclera. The conjunctiva helps to lubricate the eye by producing mucus and tears, although a smaller volume of tears than the lacrimal gland.
107
What is the sclera
The sclera is the hard connective tissue that forms the major portion of the almost round eyeball.
108
What is the pupil
The pupil is not a structure, but rather a hole that is covered by a transparent layer through which light enters the eye.
109
What is the iris
The round iris muscle surrounds the pupil, and its diameter can be adjusted via the pupil reflex.
110
What is the pupil reflex
The pupil reflex makes sure that the diameter of the pupil is reduced during exposure of the eye to high intensity light signals to protect the photoreceptive structures and other cells within the retina.
111
What is the cornea
The transparent cornea protects the front portion of the eye. It also contributes to a major portion to focusing the light via refraction onto the retina.
112
What is the optic nerve
The optic nerve is what through which the visual signals propagate encoded as a train of action potentials to higher brain regions, it is a big strand at the back of the eyeball.
113
What is the extraocular muscles
The extraocular muscles are attached to the middle of the eyeball pointing backwards. Their contraction enables the movement of the eye towards objects in case the head can not be turned for this purpose.
114
How is light processed by the eye
1. The light passes the outer eye chamber that is filled with aqueous humor fluid.
2. It then passes through the pupil that is formed by the iris muscle.
3. It then passes through the transparent lens that is attached via the so-called zonular fibers comprising connective tissue to the ciliary muscle.
4. The light passes through the main inner space of the eyeball that is filled with vitreous humor whose composition is slightly different from that of the aqueous humor in the front chamber.
5. Finally, a light beam that proceeds in parallel through the center of the eye hits the retina at the point of sharpest vision called the Fovea.
115
What is the retina
The retina with its photoreceptors covers a major portion of the inner surface of the back of the eye. From all spots inside the retina, afferent axons project towards a point where the sclera is open and through which the optic nerve exits the eye.
116
How is the eye supplied with blood
The eye is well supplied with blood vessels because the process of detailed vision occurs the cost of a quite substantial consumption of energy.
117
What is the blind spot of the eye
The optic nerve and the blood vessels pass through the ‘blind spot’ which is basically a hole in the retina. It is obvious that in this area there are no photoreceptors can be located and therefore no visual signals can be detected.
118
What is refraction
The physical process by which rays from point sources of light are being focused on the retina. When diverging rays from a point source of light enters a dense transparent medium at an angle to its convex surface, refraction at the interface between the 2 materials ‘bends’ them inwardly.
119
What is light refraction in the eye
Light refraction is only shown for the cornea, where the greatest extend of refraction occurs. Refraction occurs to a minor part also in the lens and at other sites in the eye.
120
How is refraction processed by our nervous system
Incoming light from above and below an object is bent in opposite direction. This means that ultimately the cognitive processes within the central nervous system have to correct for this physical mirroring problem to make sure that one recognizes an object how it appears in reality.
121
What is myopia
Myopia meaning nearsightedness, the eyeball is too long for anatomical reasons; often this is inherited and gets worse with age. Accordingly, close-to-parallel light rays from far-away objects focus after the process of refraction within the eyeball in front of the retina.
122
How do we correct for myopia
This is corrected by positioning in front of the cornea glasses with a concave surface. Via such glasses, slight refraction in the opposite direction is induced such that the light rays now touch the cornea at the correct angle to make sure that they are focused on the retina.
123
What is hyperopia
Hyperopia meaning farsightedness, here, the eyeball is too short anatomical reasons.
124
How do you correct hyperopia
This is corrected by positioning in front of the cornea glasses with a convex surface. Via such glasses, a modest refraction in the same direction is induced such that the light rays now touch the cornea at the correct angle to make sure they are focused on the retina.
125
What is accommodation
The process of ‘accommodation’ for near vision. Light rays from distant objects are nearly parallel. Therefore, they focus on the retina when the lens is less curved. The lens is less curved when the ciliary muscle is relaxed
126
How does zonular fibres impact accommodation
The zonular fibres are, on the one side attached to the ciliary muscle and are on the other side, attached to the lens. Due to the angles of the circular array of ciliary muscle fibres, the zonular fibres are stretched and, therefore, under tension, when the ciliary muscle is relaxed. Because the zonular fibres are under tension, they pull at the lens that it becomes less curved than it would be when isolated. This is because the lens is quite flexible and naturally has a rather round shape.
127
How does accommodation increase the curvature of the lens
Accommodation increases the curvature of the lens. This increase in the curvature of the lens is since a contraction of the ciliary muscle is induced by reflex-like central nervous activation of parasympathetic nerves. As the result of the contraction of the ciliary muscle, the zonular fibres relax and, accordingly, the lens curvature increase. The increased rounding of the lens then causes an increased bending of the light rays when hitting its surface.
128
What is the final state of accommodation
This ultimately results in focusing of the light rays from near objects on the retina as a result of such accommodation.
129
Why does the retina need to focus the physical image
The physical image needs to be focused on the retina because here the light-sensitive cells are located. They can only transmit concise information about the size and shape of an object ultimately toward the visual cortex when the image of this object is not blurry when projected onto the retina.
130
What is the first layer of the retina
The light passes firstly through the layer of ganglion neurons whose axons form the optic nerve.
131
What is the second layer of retina
The next layer is the pigmental epithelium which serves to reflect the incoming light such that most of its photons can be detected by the photoreceptors.
132
What is the outermost layer of retina
The outermost layer is the choroid layer of the sclera that forms the hard outer surface of the eye.
133
What are the similarities between both photoreceptors
Both receptor subtypes, have on one end, a similarly looking synaptic terminal. Here, they release neurotransmitter at an amount that reflects the intensity of incoming photons of light. Both receptor subtypes have a similar inner segment. This segment includes mainly the soma with the nucleus and 2 processes that connect, on the one hand, with the synaptic terminal, and on the other hand, with the outer segments.
134
What do rods look like
The rods have a larger outer segment. This segment contains the photopigment in the membrane of intracellular discs that are in the cytoplasm, like the ‘organelles’ mitochondria and endoplasmic reticulum in other cells.
135
What light do rods process
The rods have a very high sensitivity to dim, low intensity light. This optimizes them for seeing in grey light when it is getting close to dark in the evening or is still dark early in the morning.
136
What do cones look like
The outer segment of the cones is smaller, and the disc membranes are located closer to the plasmalemmal membrane. The cones comprise 3 subtypes that contain each a photopigment that responds to light of a particular colour.
137
What light do cones process
The cones are responsible for colour vision, and they are also organized such that they ‘see’ at high spatial and temporal resolution.
138
Where are cones found
The cones are concentrated in the fovea centralis. This is the spot in the retina with the sharpest image representation of an object.
139
Where is the highest and lowest concentration of cones
The number of cones is very high only in a spot 5 degrees along both sides of the center of the fovea. It then steeply decreases with further distance from the fovea center. At 30 degrees from the fovea center, there is only a negligible number of cones present in the retina.
140
What is the highest and lowest concentration of rods
There is a steep decrease of the number of rods toward the very center of the fovea where the number is close to 0. The number of rods is highest at 30 degrees away from the fovea center and the declines towards the periphery.
141
What is convergence
Convergence means that several, up to many, neurons synapse onto a smaller number of follow neurons.
142
Where is there low convergence
In the retina, there is a very low extend of convergence in the center of the fovea. Specifically, 1 photoreceptor synapses, via 1 intermediate bipolar cell, ultimately one 1 ganglion neuron.
143
Where is there high convergence
In the temporal retina several photoreceptors synapse onto the same number of bipolar neurons. This convergence is a major factor to explain why the fovea is the spot where the vision has the best temporal resolution
144
What is phototransduction at weak intensities
At weak light intensities, a certain type of cation channel is persistently kept open by the relatively high intracellular concentration the second-messenger cGMP into the outer segments of the photoreceptor.
145
What is phototransduction
When light strikes the chromophore of the opsin photopigment, it changes its conformation and the dissociates from the opsin. As a result, a cGMP phosphodiesterase in the disc membrane is stimulated via the intermediate action of the protein transducing. The activated cGMP phosphodiesterase changes cGMP into GMP. The causes a decrease of the concentration of cGMP in the cytosol and ultimately also close to the cation channel.
146
What is the receptive field of the retina
In the centre of the receptive field, about 10-20 photoreceptors converge directly onto the bipolar cell. A circular spot affecting the center of the receptive field induces a depolarization of the bipolar cell due to hyperpolarization of the photoreceptors.
147
What happens there is hyperpolarization in the receptive field
A ring-like light pulse affecting the peripheral photoreceptors results in a hyperpolarization of the bipolar cell due to a hyperpolarizing input from the horizontal cell that is hyperpolarized by the photoreceptors. The photoreceptor is depolarized in the dark and releases glutamate.
148
Is glutatmate inhibitory or excitatory in the bipolar cell of the retina
That released glutamate caused inhibition causes inhibition of the bipolar cell – this is one of the few occasions where glutamate acts inhibitory.
149
What is the left and right wiring of the visual field
The left visual field collects the visual information from the left eye whereas the right field collects the information from the right eye. Objects from the left rim of the visual field are projected onto the right side of the retina whereas objects from the right rim of the visual field are projected onto the left side of the retina.
150
How is the right side of the visual field processed
The right side of the visual field from both eyes is projected ultimately onto the left side of the visual cortex area of the occipital lobe of the left hemisphere.
151
How is the left side of the visual field processed
The left side of the visual field from both eyes is projected ultimately onto the right side of the visual cortex area of the occipital lobe of the right hemisphere
152
How is the visual field processed in the brain
About 50% of the axons in the optic nerve cross to the contralateral hemisphere. The optic chiasm is an area in the brain where the crossing of the axons takes place. From the optic nerve, the axons innervate neurons in a specific subregion of the thalamus. The thalamic neurons then activate the neurons within the visual cortex.
