Lecture_10_notes__1_

Neuroscience 106: Lecture 10 - The Retina

I. Introduction to Systems Neuroscience

• Focus so far: cellular/molecular level of neuroscience, emphasizing the basic building blocks of the nervous system.

• Upcoming focus: systems level, facilitating a deeper understanding of how individual cells interact within networks to perform complex functions across various sensory modalities.

• Definition of systems: circuits of neurons interacting to perform functions such as visual processing, auditory perception, pain sensation, memory formation, and motor control.

• Challenges in understanding systems:

  • Greater unknowns and complexities arise due to the intricate nature of neural connections and functions.

  • Many systems operate in counterintuitive ways, complicating interpretations of neural data and behavior.

• Neural Codes:

  • Core concept: The meaning of neuronal action, such as action potentials, heavily relies on the specific neural circuit involved in a given task.

  • Example:

    • Action potentials in pain neurons are directly linked to the perception of pain, signaling discomfort and potential harm.

    • Action potentials in visual neurons are responsible for visual perception, processing light information to create a visual representation.

    • Action potentials in motor neurons initiate movement, facilitating voluntary and reflexive actions.

    • Action potentials in emotional systems evoke feelings such as fear or joy, influencing behavior and decision-making.

• Systems may consist of subsystems, where groups of neurons in different subsystems code for distinct but interconnected functions.

• Visual system example: Various groups of neurons are specialized for specific tasks, including form detection, color perception, motion detection, and the identification of object locations.

II. The Visual System: The RetinaA. Structure of the Retina:

• Composed of several thin layers of cells organized in a way that allows for efficient light processing within the eye.

• Fovea:

  • The fovea is the region of highest visual acuity; it is where light precisely focuses when looking directly at an object.

  • Contains a high concentration of cone photoreceptors, crucial for color vision and fine detail.

• Optic Disk:

  • Area where retinal ganglion cell axons exit the eye to form the optic nerve; it lacks photoreceptors, resulting in a blind spot in the visual field.

B. Receptive Fields:

• Defined as the specific stimulus area that, when stimulated, changes a neuron’s activity level.

• Example: Light at specific points influences the activity of Corresponding retinal cells, and different cells will respond selectively to edges, contrasts, or motion within their receptive fields.

C. Types of Retinal Cells:

1. Photoreceptors:

   • Represent the first stage in visual processing; these cells are highly sensitive to light and convert photons into electrical signals.

   • Two types of photoreceptors:

     • Rods:

       • Approximately 120 million rods; primarily sensitive to low light levels, contributing to night vision; they do not support high acuity vision and are achromatic, meaning they do not perceive color; predominantly located outside the fovea.

     • Cones:

       • Approximately 6 million cones; less sensitive to light than rods, yet they are critical for color perception and functioning in bright light; there are three subtypes of cones (red, blue, green), with a high concentration in the fovea to facilitate detailed vision.

2. Bipolar Cells (BPs):

   • Serve as intermediaries between photoreceptors and retinal ganglion cells, transmitting signals through graded potentials.

3. Retinal Ganglion Cells (RGCs):

   • The only output cell type of the retina; carry processed visual information to the brain via their axons, which converge to form the optic tract.

4. Horizontal Cells (HCs):

   • Help integrate and regulate the input from multiple photoreceptors, aiding in contrast enhancement.

5. Amacrine Cells (ACs):

   • Modulate the signal between bipolar cells and ganglion cells, contributing to temporal dynamics in visual processing.

D. Axons and Information Transfer:

• Only retinal ganglion cells have long axons responsible for communication over distances; no action potentials are necessary for close-range communication between nearby cells.

• Both RGCs and ACs can generate action potentials; meanwhile, other cell types utilize graded depolarization to facilitate neurotransmitter release.

• The degree of depolarization correlates directly with the amount of neurotransmitter released, affecting the intensity of the response.

E. Cell Layer Arrangement:

• The retina is organized in an 'inside-out' manner where photoreceptors are located deepest, thus light must pass through several layers of cells before reaching the photoreceptors, optimizing the capture of light.

• In the foveal pit, the arrangement of other cell types is altered to minimize obstruction to incoming light, enhancing visual acuity.

III. Phototransduction: Light Absorption by PhotoreceptorsA. Mechanism:

• Phototransduction refers to the process by which light is converted into a change in membrane potential.

• Membrane potentials:

  • The resting membrane potential (RMP) in darkness is approximately -30 mV.

  • Maximum hyperpolarization occurs with bright illumination, reaching about -65 mV.

• Neurotransmitter:

  • Glutamate, the primary neurotransmitter released by photoreceptors, decreases in concentration with increasing light intensity.

B. Light-Induced Changes:

• In darkness, ligand-gated Na+ channels remain open, leading to a depolarized state at -30 mV.

• Light binds to photopigments, altering the conformation and ultimately causing the channels to close.

C. cGMP Changes:

• When photopigments are in the dark, they appear purple; exposure to light leads to bleaching, resulting in a pale yellow appearance.

• Key Elements:

  • Rhodopsin: The photopigment found in rods, consists of opsin (a protein) and retinal (the light-sensitive molecule).

  • Retinal exists in two configurations: 11-cis (when in the dark) and all-trans (when exposed to light).

• cGMP Activation Steps:

  • Opsin activates a G-protein called transducin.

  • Transducin further activates an enzyme known as cGMP phosphodiesterase.

  • This enzyme converts cGMP to GMP, leading to reduced cGMP concentration in the cell.

  • The decrease in cGMP causes the closure of Na+ channels, ultimately resulting in the hyperpolarization of photoreceptor cells.

• Advantages of this system include increased surface area for enhanced sensitivity; a single photon can significantly alter the membrane potential in rods, and G-protein amplification greatly augments the signal detection capability of the phototransduction cascade.