CBNS 106 Final Lecture 1 (Systems Neuroscience + Retina)

Systems Neuroscience

Systems Neuroscience

a) Study of networks/circuits of neurons that carry out specific functions

b) Focuses on how groups of neurons work together

c) Neural circuits = interconnected neurons that process information

Neural Circuits

a) Individual neurons form functional circuits

b) Different neuron types contribute differently:

• PYR = pyramidal neurons

• PV+ interneurons = parvalbumin interneurons

• SST+ interneurons = somatostatin interneurons

• VIP+ interneurons = vasoactive intestinal peptide interneurons

Why Study Neural Circuits?

a) Brain processing relies on population coding

b) Single action potentials usually have small effects

c) Many neurons firing together create meaningful signals

Neural Coding

1. AP Frequency (Rate Coding)

a) Strength of sensory stimulus coded by firing rate

b) More action potentials = stronger stimulus

• Example: Cold-sensing neurons increase firing frequency as the temperature drops more

2. Coordinated activity (Synchrony Coding)

a) Information can also be coded by timing precision between neurons

b) Coordinated firing = synchrony

c) Higher synchrony often linked to perception and cognition

• Example: EEG studies show stronger synchrony during face recognition task

3. Graded Potentials

a) Some neurons encode stimulus strength using membrane potential amplitude

b) Larger depolarization → more neurotransmitter release

c) Important in photoreceptor cell

Neural Systems (YK this)

Examples of Neural Systems

• Visual system

• Somatosensory system

• Motor system

• Auditory system

• Reward system

Brain Functional Specialization

• Specific brain areas perform specific functions

• Example:

  • Visual cortex processes vision

  • Somatosensory cortex processes touch/pain

Subsystems

• Each sensory system contains specialized subsystems

• Example in vision:

  • Shape processing

  • Color processing

  • Motion processing

Properties of Sensory Systems

1. Central Pathway

Sensory information travels:

1. Peripheral receptors

2. Subcortical structures

3. Cerebral cortex

2. Sensory Receptors (part of Peripheral receptors)

Characteristics:

• Modality specific

• Convert energy into electrical activity (transduction)

• Specialized for detecting particular stimulus features

3. Receptive Field

• Area of sensory space that changes a neuron’s activity

• Can refer to:

  • Visual space

  • Skin surface

  • Auditory frequency range

4. Topographic Map

• Nearby neurons represent nearby sensory locations

• Maintains orderly spatial organization in the brain

Example:

• Mouse whisker system has organized spatial mapping

Visual system overview

Main Visual Pathway

Light → Eye → Retina → LGN → Primary Visual Cortex

Light

• Electromagnetic radiation visible to humans

• Converted into neural activity through phototransduction

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Structure of the Eye

1. Pupil

• Opening that lets light enter eye

2. Iris

• Colored part of eye

• Controls pupil size

3. Cornea

• Transparent covering

• Refracts light

4. Sclera

• “White of the eye”

• Tough outer wall

5. Extraocular Muscles

• Move the eyeball

Retina

Overall

• Neural tissue lining back of eye

• Site of phototransduction

Five Major Retinal Cell Types

1. Photoreceptor cells (rods and cones)- where phototransduction occurs

2. Bipolar cells

3. Retinal ganglion cells

4. Horizontal cells

5. Amacrine cells

Retinal Information Linear and Lateral Pathway combined:

1. Light enters eye

2. Photoreceptors detect the light, begin phototransduction

3. Horizontal cells connect neighboring photoreceptors (compare, enhance contrast, and sharpen edges)

4. Bipolar Cells relay information and sends forward

5. Amacrine cells modify timing and regulate motion processing

6. RGCs axons from optic nerve and carry visual information to the brain

Photoreceptors

1. Rods

a) Rod Characteristics

• ~120 million rods

• Highly sensitive to light

• Function in dim light

• Low visual acuity

• Achromatic (no color)

• Rare in fovea

2. Cones

a) Cone Characteristics

• ~6 million cones

• Less sensitive to light

• High visual acuity

• Detect color

• Concentrated in fovea

b) Cone Types

• Red cones

• Green cones

• Blue cones

Fovea

What is it

• Center of visual field

• Light directly reaches photoreceptors

• Produces:

  • Sharp vision

  • High acuity

  • Best color vision

Blind Spot

• Region lacking photoreceptors

• Optic nerve exits eye here

Phototransduction

Dark Current

In darkness, photoreceptors are actually active

1. Rhodopsin is inactive

2. Transducin (G protein is inactive)

3. cGMP levels are high and keep sodium channels open

4. cGMP-gated Na channels stay open

5. Na continues to flow into the cell (dark current)

6. Cell stays depolarized (-30mV)

7. Glutamate is continuously released

Light Response

When light hits the photoreceptors, the opposite occurs

1. Light activates rhodopsin

2. Activated rhodopsin activates transducin (G protein)

3. Transducin activates phosphodiesterase (PDE)

4. PDE converts cGMP to GMP

5. cGMP decreases

6. Na channels close and don’t flow into cell (influx stops)

7. Cell is hyperpolarized (-65mV)

8. Glutamate release stops

***Photoreceptors signal light by REDUCING neurotransmitter release

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Opsins

1. Opsin

• G protein-coupled receptor (GPCR)

• Combines with retinal to detect light

2. Rhodopsin

• Found in rods

3. Cone Opsins

• Different opsins detect different wavelengths/colors

Color Vision

1. Red Cones

• Hyperpolarize most strongly to red light

2. Blue Cones

• Respond best to blue light

3. Green Cones

• Respond best to green light

4. Color Detection

• Based on comparing activity across cone types

Light intensity coding

1. Graded Response to Light

Increasing light intensity causes:

• Greater hyperpolarization

• Less glutamate release

2. Relationship

Bright light:

• Lowest glutamate release

• Greatest hyperpolarization

Darkness:

• Highest glutamate release

• Most depolarized state