Neurobiology Exam 2

What is hemifield neglect?

• A left-side deficit in vision perception

• caused by damage to the right posterior parietal cortex

• The neglect is typically contralateral to the side of the brain damage

Is hemifield neglect mainly a problem with sensation or perception?

Perception

Representational neglect (and explain Piazza del Duomo in Milan case)

Patients knew the Piazza del Duomo in Milan. They were asked to close their eyes/imagine themselves standing at point A and describe the buildings around them. They tended to report buildings on one side of their imagined view while leaving out buildings on the neglected side.

Then the researchers said, essentially: “Now imagine you're standing at point B, facing the other way.”

The important part is that the physical buildings did not change. Only the person's imagined viewpoint changed. From point B, some buildings that had previously been on the neglected side were now on the attended side of their imagined space. Those buildings could suddenly be recalled. Meanwhile, buildings that shifted to the neglected side of the imagined view could now be omitted.

Why is the retina considered a layered structure?

• Light must pass through several layers of retinal cells before reaching the outer segments of the photoreceptors

• The photoreceptors are the initial light receptors

What is the optic nerve?

The optic nerve is where axons and blood vessels travel between the retina and the rest of the CNS

What are photoreceptors?

• The sensory receptor cells of the visual system

• Two major types:

  • rods

  • cones

• They release glutamate

• They do not have an axon because the retina doesn’t have to travel long distances

• Their outer segments are specialized for phototransduction

• initial light receptors

What are the characteristics of rods?

• High sensitivity

• Low acuity

• Black-and-white vision

• Best for dim light

• Contain rhodopsin

What are the characteristics of cones?

• Lower sensitivity than rods

• High acuity

• Function best in bright light

• Responsible for color vision

• Three different types of cones

• Each type contains a different opsin

Why are there three types of cones?

• Each type contains a different opsin

• The three cone types therefore have different wavelength/light sensitivities (red, blue, green)

What is the fovea? Why does it have such high visual acuity?

• The fovea is a depression in the retina, where there are only close-packed cones. The point where you are looking/fixating corresponds to the fovea, and it is where the highest visual acuity for day vision.

• It has such high visual acuity because the cell layers are pushed aside, meaning light has a direct pathway to the photoreceptors and less scattering.

• There are no blood vessels in the foveal region

What is the blind spot/optic disc?

• The point where retinal ganglion cell axons and blood vessels enter/exit the eye

• Because there are no photoreceptors there, it creates a blind spot

What do photoreceptors do in the dark?

• Photoreceptor is depolarized

• Voltage-gated Ca2+ channels at the synaptic terminal stay more open

• More Ca2+ enters

• More glutamate is released

What happens when light hits the photoreceptor?

• the photoreceptor hyperpolarizes

• Voltage-gated Ca2+ channels at the terminal open less

• Less Ca2+ enters

• Less glutamate is released

What neurotransmitters are released by horizontal cells vs photoreceptors/bipolar cells?

Horizontal cells release GABA. The others release Glutamate.

What are the 2 types of bipolar cells? What receptors does each one have?

ON-center bipolar cells

• center light excites/depolarizes them

• surround light inhibits/hyperpolarizes them

• The ON bipolar cell has mGluR6 receptors.

OFF-center bipolar cells

• center light inhibits/hyperpolarizes them

• surround light excites/depolarizes them

• OFF-center bipolar cells have AMPA/kainate receptors

Compare the 2 retinal ganglion cells

P-type RGCs:

• smaller cells

• smaller receptive fields

• slower AP conduction

• slower adapting

• sensitive to wavelength/color

• process form and fine detail

• high-resolution vision

Good for: detail shape/form color

M-type RGCs:

• stimulus movement

• low-resolution vision

• faster/transient response

Good for: movement changes lower-detail vision

Where do retinal ganglion cell axons project?

• Through the optic nerve to:

  • lateral geniculate nucleus (LGN) of the thalamus

    • primary visual pathway

• midbrain

  • specifically the superior colliculus

• hypothalamus

  • involved in the circadian clock

LATERAL vs MEDIAL retinal ganglion

The LATERAL retinal ganglion cell axons do NOT cross the midline. They send their information to ipsilateral thalamus.

The MEDIAL retinal ganglion cell axons do cross the midline. They send their information to contralateral thalamus.

What is the order of the primary visual pathway?

→ photoreceptors
→ bipolar cells
→ retinal ganglion cells
→ LGN of the thalamus
→ primary visual cortex (Brodmann area 17)
→ secondary visual cortex and other association cortex

optic nerve VS optic chiasm VS optic tract

Optic nerve = axons coming from one eye, before the chiasm.

Optic chiasm = where the crossing fibers cross.

Optic tract = axons after the chiasm. At this point they are organized by visual hemifield, not simply by eye.

What happens at the LGN? How many layers does it have?

• The LGN keeps visual information organized and segregated because it is processed in parallel.

• ALL LGN neurons are monocular. This means each side gets information about that visual field from both eyes, but keeps the two eyes' inputs separated.

  • Left LGN receives information about the RIGHT visual field.

  • Right LGN receives information about the LEFT visual field.

• The LGN has 6 layers

LGN 6 Layers

The layers are numbered: 1, 2, 3, 4, 5, 6

And the M-type and P-type RGC information is kept separate.

• Layers 1–2: Receive the M / magnocellular pathway

• Layers 3–6: Receive the P / parvocellular pathway

Primary Visual Cortex 6 Layers— Visual information enters which layer? What can it also be called?

• right eye inputs and left eye inputs are separate in layer IV, as they are in the LGN

• This is why it’s called “striated cortex”

• Also known as Brodmanns area 17, or V1

Ocular Dominance Columns

An ocular dominance column is a vertical group of cortical neurons that responds more strongly to one eye than the other.

• Important detail: In some layers outside layer IV, neurons can respond to both eyes but they can still prefer one eye.

Descending Regulation

Information can also go from the visual cortex to the LGN

Orientation Selectivity

orientation selectivity = preference for a bar/edge at a particular angle

• Instead of a simple circular center-surround receptive field, a cortical neuron's receptive field can have an elongated region.

• There is a line/oval of excitation with inhibitory regions bordering it

• So a bar of light positioned at the right angle lines up perfectly with the excitatory region and causes lots of APs.

• Rotate the bar away from the preferred orientation and the response becomes weaker.

• M-type RGC pathway contributes much of the information used for orientation selectivity

Orientation columns VS Ocular dominance columns VS blobs

columnar organization in primary sensory cortex

Which eye is telling me about it?
→ ocular dominance columns

What angle is the edge?
→ orientation columns

What color is it?
→ blobs

Dorsal VS Ventral Streams

After the primary visual cortex, the information travels into different areas using a “stream”

DORSAL stream

• into parietal association cortex

• critical for processing object motion

• portions arranged in direction-of-motion columns

VENTRAL stream

• into temporal association cortex

• critical for object recognition, including shape, color, and face perception

• few direction-selective cells

What are the two major somatosensory subsystems?

• Mechanical stimuli

  • touch

  • pressure

  • vibration

  • proprioception

• Pain and temperature stimuli

What is proprioception?

Sensation of the position of the body in space

What are somatosensory receptors? What information do they receive?

A type of neuron that converts stimulus information into the language of the nervous system through sensory transduction.

They receive information about the:

• stimulus quantity/size/strength (number/frequency of AP)

• stimulus location (sensory receptor and cortex location)

• stimulus quality/nature/type (sensory receptor and CNS neuron properties)

What are the three functional categories of somatosensory receptors?

• Mechanoreceptors (proprioceptive and tactile; Merkel’s disks and Pacinian corpuscles)

• Nociceptors (pain - all free nerve endings)

• Thermoreceptors (heat/cold - are all free nerve endings)

What is a receptive field?

The area of skin within which an appropriate stimulus causes the neuron to depolarize

How do Merkel’s disks and Pacinian corpuscles differ?

• Merkel’s disk = small receptive field + slow adapting (precise + persistent)

• Pacinian corpuscle = large receptive field + fast adapting (broad + change-sensitive)

What kinds of ion channels are used in the somatosensory system?

What are the three broad mechanisms that these ion channels detect?

How do these channels result in an electrical signal?

Mechanoreceptors detect movement/stretching

• Direct sensitivity to lipid membrane stretching

• Opening when a cytoskeletal protein is moved

• Opening when an extracellular protein is moved

When the mechanosensitive channels open, Na⁺/Ca²⁺ enter, cause AP. This converts mechanical energy → electrical signal.

Why do different primary sensory axons conduct information at different speeds?

Which sensory information tends to be carried by slower axons?

Explain how this relates to when you stub your toe.

Sensory axons vary in diameter and amounts of myelin

• larger + more myelinated → faster AP conduction speed

• smaller + less/no myelin → slower AP conduction speed

Aβ / Group II: mechanoreceptors of skin (FAST)

C / Group IV: temperature, pain, itch (SLOW)

You stub your toe. You can often immediately know: “I hit my toe.” Then a moment later: “OWWWW.” The mechanical/touch information can travel through faster axons like Aβ. Pain information can travel through slower C fibers.

What is two-point discrimination? Sensation or Perception? Does it map sensitivity or

acuity?

Ability to determine that two nearby objects touching the skin are two distinct points instead of one.

Perception

Acuity

What 2 things determine two-point discrimination? Give examples of each.

1. Receptor density

   • More sensory receptors packed into an area = better ability to distinguish nearby touches.

2. Receptive field size

   • Smaller receptive fields = better acuity

   • Larger receptive fields = worse acuity

So fingertips have:

• lots of receptors

• small receptive fields

• therefore very good two-point discrimination

The back has:

• fewer receptors

• larger receptive fields

• therefore poor two-point discrimination

How is two-point discrimination measured?

Determine the minimum distance between two simultaneous stimuli required for the person to perceive them as two separate points

Peripheral Somatosensory Neuron orientation in the spinal cord

• peripheral somatosensory neurons = primary or first-order sensory neurons = afferents

• The cell body of these neurons are in the dorsal root ganglion. The peripheral branch of the axon is in a spinal nerve, and the central branch of the axon is in the dorsal root.

• Which root carries somatosensory information, ventral or dorsal? dorsal

What is a dermatome?

A dermatome is the area of skin supplied by a particular spinal nerve.

• CN V = somatosensation of the face

• CN II = optic/retinal information

What are the 2 somatosensory systems? What do they carry?

Pathway 1: Dorsal column–medial lemniscal pathway

• Carries: touch, vibration, two-point discrimination, proprioception

Pathway 2: Spinothalamic pathway

• Carries: pain and temperature

Explain how mechanosensory afferents go through the dorsal column pathway IPSI then CONTRA. What does first-order vs second-order mean?

Suppose you touch the right hand.

The primary/first-order sensory neuron:

right hand receptor → DRG → dorsal root → enters right side of spinal cord

It does NOT immediately cross. Instead, it ascends RIGHT dorsal column

The first-order neuron reaches the dorsal column nuclei in the ipsilateral medulla.

There it synapses onto the second-order neuron. Then the second-order neuron crosses to the opposite side.

It begins ipsilateral and becomes contralateral at the medulla.

Explain how spinothalamic afferents go through the dorsal column pathway IPSI then CONTRA. What does first-order vs second-order mean?

Now suppose you hurt your right hand.

Again, the primary neuron:

right-hand nociceptor → DRG → dorsal root → spinal cord

But this pathway behaves differently.

The first-order neuron synapses on a second-order neuron in the spinal cord.

Then that second-order neuron crosses almost immediately in the spinal cord.

travels up LEFT side → left thalamus → left cortex

Why do we care about where the pathways cross from ipsilateral to contralateral?

SPINAL CORD DAMAGE!

Imagine damage to one side of the spinal cord.

At that spinal level:

Touch pathway has not crossed yet. So touch/proprioception information is still traveling on the same side as the body part it came from.

Pain pathway has already crossed.

So pain/temp information from that side of the body is now traveling on the opposite side of the spinal cord. Therefore, one-sided spinal-cord damage can affect: touch on one side of the body and pain/temperature on the opposite side

Where do BOTH pathways eventually end up?

Both ultimately send information to the contralateral ventral posterior (VP) nucleus of the thalamus.

The thalamus uses thalamocortical projections to send the information to the contralateral cerebral cortex through (more specifically, layer IV in S1 = primary somatosensory cortex)

• So right-body information ultimately gets represented in the left cerebral cortex.

T or F? descending projections outnumber ascending projections in the somatosensory system

True

How does the somatosensory system utilize descending projections for inhibiting pain?

Involves neurons in the PAG.

There are opioid receptors at multiple levels, so opioid signals can act at several places in the pathway. Also, the nervous system makes its own opioid chemicals (ex. endorphins)

Which sensory receptor system includes neurons with cell bodies in the intralaminar nuclei?

A. Photoreceptors
B. Somatosensory receptors
C. Auditory receptors
D. None of the above

Somatosensory receptors

If the dorsal column is damaged, which type of information is more likely to be lost?

A. Pain
B. Vision
C. Mechanosensation
D. Hearing

C. Mechanosensation

In somatotopic maps, each area of the body is proportionally represented compared to the actual size of that body part.

A. False
B. True

A. False

Where is the dorsal root ganglion located?

A. In the central gray matter of the spinal cord
B. On the dorsal root just outside the spinal cord
C. In the dorsal column
D. On the ventral root

B. On the dorsal root just outside the spinal cord

Which of the following are true of free nerve endings in the somatosensory system?

A. They are involved in detection of vibrational stimuli
B. Some are photoreceptors
C. None of the above
D. They typically have a specialized epithelial structure associated with them
E. Some are mechanoreceptors

C. None of the above

Which sensory receptor systems generate information that is relayed through the thalamus?

A. Photoreceptors
B. None of the above
C. Somatosensory receptors

A. Photoreceptors

C. Somatosensory receptors

Two-point discrimination is a measure of the weakest pressure stimulus detected by photosensory receptor neurons.

A. True
B. False

B. False

Where is the dorsal column located in the spinal cord?

A. In the ventral root
B. In the dorsal/posterior white matter of the spinal cord
C. In the dorsal root ganglion
D. In the central gray matter

B. In the dorsal/posterior white matter of the spinal cord

Tastant vs Odorant

Tastants, chemicals in foods, are detected by taste buds, which consist of special sensory cells.

Similarly, specialized cells in the nose pick up odorants, airborne odor molecules. Odorants stimulate receptor proteins found on hairlike cilia at the tips of the sensory cells.

Ultimately, messages about taste and smell converge, allowing us to detect the flavors of food.

Papillae VS Taste buds

Papillae are the taste sensitive visible on the tongue.

Each papillae contains between one to hundreds of taste buds.

Each taste bud contains 50-150 taste receptor cells. Taste buds also contain basal cells and a set of gustatory afferent axons.

What triggers an AP in a taste cell?

Different tastants (i.e., chemicals) will trigger depolarization and sometimes an action potential in the taste cells.

At threshold (i.e., when you are able to perceive a taste), most taste buds are sensitive to one basic taste: sweet, sour, salty, bitter, umami, (and MAYBE carbon dioxide)

What is the central taste pathway?

• tastant activates a taste receptor cell

• taste receptor cell depolarizes

• gustatory sensory neuron gets activated and carry the taste information toward the brain through cranial nerves.

• stop at gustatory nucleus in the medulla

• the information goes up to the VPM = ventral posterior medial nucleus of the thalamus

• VPM sends info to primary gustatory cortex

Is the central taste pathway IPSI or CONTRA?

Ipsilateral

What is the central olfactory pathway?

• Odorants bind receptors on the cilia of olfactory receptor cells in the olfactory epithelium.

  • They can bind directly or be carried by odorant-binding proteins.

• Odorant binding activates the odorant-specific G-protein Gₒlf.

• This activates an intracellular signaling pathway → depolarization of the olfactory receptor cell.

• The axons of olfactory receptor cells form the olfactory nerve and project to the olfactory bulb.

• Different odorants activate different patterns of receptor cells.

  • The overall pattern across many receptor cells provides odor specificity.

  • This is called population coding.

• In the olfactory bulb, receptor axons converge into glomeruli.

  • Glomeruli are about 50–200 μm in diameter.

  • About 25,000 primary olfactory neuron axons converge within each glomerulus.

  • Glomeruli show an orderly representation related to the odorant being detected.

• From the olfactory bulb, information can follow the usual pathway:

  • MD nucleus of the thalamus → orbitofrontal neocortex

• Olfaction also has an unusual direct projection to piriform cortex.

  • Piriform cortex is in the temporal lobe/part of parahippocampal cortex.

  • It is involved in odor identification.

• Additional projections go to:

  • amygdala → social functions

  • entorhinal cortex → memory

What is the central auditory pathway?

• The sensory receptor cells of the auditory and vestibular systems are hair cells.

• Hair cells are located in specialized fluid-filled organs, such as the cochlea.

• Sound causes movement of the surrounding fluid → bends the stereocilia on hair cells.

• Stereocilia movement mechanically opens/closes K⁺ channels.

  • Movement in one direction → K⁺ enters the hair cell → depolarization.

  • Movement in the opposite direction → hyperpolarization.

  • K⁺ enters because the surrounding fluid has an unusually high K⁺ concentration.

• The resulting auditory signal then travels through multiple brain regions before reaching auditory cortex.

• A key feature of the auditory system is the large amount of processing before auditory cortex.

• The auditory pathway is bilateral.

  • Auditory information from each ear projects to BOTH sides of the brain.

  • Therefore, cutting/damaging one side of the central auditory pathway would NOT cause complete deafness in one ear

• There are also many descending projections throughout the auditory system.

  • Higher areas can send information back down to regulate lower auditory processing areas.

Broca’s aphasia

impaired language production, NOT due to impaired motor control (damage to Brodman #44,45)

Wernicke’s aphasia

impaired language comprehension, despite normal auditory function (Brodman #22)

astereognosia

inability to recognize objects by feeling them, despite normal sense of touch (can be limited to contralateral hand) (posterior parietal, often contralateral deficits)

hemifield neglect

lack of awareness of left half of world, despite being able to “see” (right posterior parietal cortex)

prosopagnosia

inability to recognize faces, “face blindness” despite normal vision (damage to ventral visual stream)

What type of orderly sensory map is used in each sensory system?

• Visual system → retinotopic map

  • organized representation of the visual field/retina

• Somatosensory system → somatotopic map

  • organized representation of the body

• Auditory system → tonotopic map

  • organized representation of sound frequencies

• Olfactory system → orderly representation in glomeruli based on odorants

• Gustatory system → may be gustotopic, but organization is not yet clear

What would happen if left optic nerve was cut? Right optic nerve? Optic chiasm? Left optic tract? Right optic tract?

• Left optic nerve cut → lose input from the left eye; on the combined visual field, lose left peripheral vision

• Right optic nerve cut → lose input from the right eye; lose right peripheral vision

• Optic chiasm cut → lose peripheral vision on both sides because the crossing fibers are cut

• Left optic tract cut → lose the entire right visual field

• Right optic tract cut → lose the entire left visual field