CoverModule 1.0. Homeostasis, Membranes, Electrophysiology and ANS1.1. Homeostasis1.1.1. Homeostasis Defined1.1.2. Homeostatic Control Systems1.1.3. Feedback Response Loop1.2. Cell Transport; Water & Solutes1.2.1. Fluid Compartments1.2.2. Osmosis1.2.3. Diffusion of Solutes1.2.4. Active Transport1.2.5. Bulk Transport1.3. Electrophysiology1.3.1. Ions and Cell Membranes1.3.2. Membrane Potentials1.3.3. Graded Potential1.3.4. Action Potentials1.3.5. Refractory Periods1.3.6. Propagation of an Action Potential1.4. The Synapse1.5. The Autonomic Nervous System1.5.1. Organization of the Nervous System1.5.2. Structural Organization of the ANS1.5.3. The SNS and the PNS1.5.4. The Enteric Nervous System1.5.5. Physiology of the ANS1.5.6. Neurotransmitters of the ANS1.5.7. Receptors of the ANS1.5.8. Actions of the Autonomic Nervous System1.5.9. Table of Actions for the SNS and PNS and Some Common DrugsModule 2.0. Skeletal Muscle and Special Senses2.1. Structural Organization of Skeletal Muscle2.2.1. Neuromuscular Junction, Excitation-Contraction Coupling2.2.2. Muscle Contractures and Cramps2.3. Whole Muscle Contraction, Fiber Type, Fatigue and Muscle Pharmacology2.3.1. Motor Units2.3.2. Factors that Influence the Force of Contraction2.3.3. Energy Source for Muscle Contraction2.3.4. Skeletal Muscle Fiber Types2.3.5. Fatigue2.3.6. Muscle Pharmacology2.4. Smooth Muscle2.4.1. Smooth Muscle Contraction2.5. Control of Body Movement2.5.1. Voluntary Control of Muscle2.5.2. Reflexes2.6. Taste and Smell2.6.1. Taste2.6.2. The Sense of Smell2.7. Vision2.7.1. Structure of the Eye2.7.2. Focusing Light on the Retina2.7.3. Converting Light to Action Potentials2.7.4. The Retina2.7.5. Phototransduction2.7.6. Receptive Fields2.8. Hearing and Equilibrium2.8.1. The Nature of Sound2.8.2. The Hearing Apparatus2.8.3. Sound Vibrations to Action Potentials2.8.4. The Sense of Balance and EquilibriumModule 3.0. Cardiovascular System3.1. Structure of the Heart3.1.1. Chambers and Circulation3.2. Cardiac Cell Action Potentials3.2.1. Action Potentials in Cardiac Muscle Cells3.2.2. Action Potentials in Cardiac Autorhythmic cells3.2.3. Cellular Mechanisms of Inotropy and Chronotropy3.3. Electrophysiology of Heart Muscle3.3.1. Heart Conduction System3.3.2. Electrocardiogram (ECG)3.3.3. Abnormal ECG - Current of Injury3.4. The Cardiac Cycle3.4.1. Cardiac cycle3.4.2. Cardiac Measurements and Pressure Volume Loops3.5. Blood vessels and Blood Pressure3.5.1. Arteries and Veins3.5.2. Capillaries3.5.3. Blood Pressure Regulation and Shock3.5.4. Capillary Exchange3.5.5. Myogenic and Paracrine Regulation of Vasoconstriction and Vasodilation3.6. Blood3.6.1. Composition of Blood3.6.2. Hematopoeisis3.6.3. Breaking Down Red Blood Cells3.6.4. HemostasisModule 4.0. Urinary and Respiratory Systems4.1. Function and Structure of the Kidney4.1.1. Urinary System Function4.1.2. Functional Anatomy of the Urinary System4.1.3. The Nephron: Functional Unit of the Kidney4.1.4. The Renal Corpuscle: Bowman's Capsule4.2. Physiology of Urine Production4.2.1. Filtration4.2.2. Renal Clearance4.2.3. Tubular Reabsorption4.2.4. Urine Concentration and Dilution4.2.5. Hormonal Regulation of Urine Production4.3. Acid/Base Balance4.3.1. Buffers4.3.2. Acid/Base Disturbances4.4. The Respiratory System4.4.1. Respiratory System Structure and Function4.4.2. Respiratory Membrane4.4.3. Respiratory pressures and Inspriation/Expiration4.4.4. Alveoli and Surfactant4.4.5. Pneumothorax4.4.6. Pressure-Volume Loops and the Work of Breathing4.5. Gas Exchange and Transport4.5.1. Gas Laws4.5.2. Partial Pressure Gradients in the Lung4.5.3. Alveolar Gas Equation4.5.4. Oxygen and Carbon Dioxide Transport in the Blood4.5.5. Alveolar Ventilation4.5.6. Ventilation/Perfusion Ratio4.6. Chronic Bronchitis and Emphysema4.6.1. Respiratory Control by the Medulla Oblongata4.6.2. Chemicals that Regulate VentilationModule 5.0. Digestive, Endocrine and Reproductive Systems5.1. Functional Anatomy of the Digestive System5.1.1. Layers of the Digestive Tract5.1.2. Enteric Nervous System5.1.3. Organs of the Digestive System5.2. Digestion5.2.1. Carbohydrates5.2.2. Proteins5.2.3. Lipids5.2.4. Lipoproteins5.3. Regulation of Digestive Secretions5.4. Endocrine System5.4.1. Overview of the Endocrine System5.4.2. Hormone Receptors5.4.3. Hormones of the Body5.4.4. Other Hormones: Melatonin and Pheromones5.5. The Hypothalamus and Pituitary Gland5.5.1. Structure and Function of the Hypothalamus and Pituitary Gland5.5.2. The Posterior Pituitary5.5.3. The Anterior Pituitary5.5.4. Growth Hormone5.5.5. Prolactin5.5.6. Thyroid Hormones5.5.7. Adrenal Hormones5.6. Pancreas5.6.1. Insulin and Glucagon5.6.2. Diabetes Mellitus5.7. Reproductive System Anatomy5.7.1. Female Reproductive Anatomy5.7.2. Male Reproductive Anatomy5.7.3. Sexual Development at Puberty5.7.4. Male Reproductive Endocrine Axis5.7.5. Spermatogenesis5.7.6. Female Reproductive System: Oogenesis5.7.7. Ovulation and Fertilization5.7.8. The Ovarian Cycle5.7.9. The Uterine Cycle5.7.10. PregnancyAppendix A. GenderAppendix B. The Placebo EffectB.2.1. The Placebo EffectB.2.2. Examples of the Placebo EffectB.2.3. How do Placebos Work?B.2.4. Are Placebos Ethical?B.2.5. How do we validate actual effectiveness of placebosB.2.6. Tips for evaluating scientific evidenceB.2.7. What about Faith Healings

Receptive Fields

The ability to produce sharp vision that distinguished between various contrasts and edges is a property of receptive fields. The three major cell types of the retina (rods/cones, bipolar, ganglion) work together to form receptive fields. Receptive fields of photoreceptors are circular, with some photoreceptors located directly in the center and others making up the peripheral edges of the circle.

Light can hit any part of the circular receptive field and induce hyperpolarization and that results in reduced glutamate release.

The circular nature of the photoreceptor field causes the bipolar cell to behave as if it were also circular but with an added level of complexity. This complexity is due to the fact that a bipolar cell can have one of two types of post synaptic potentials created by different glutamate receptors.

Some bipolar cells contain non-specific (cation) ionotropic glutamate receptor channels called AMPA channels. These channels open in response to the glutamate ligand.  Bipolar cells that contain these channels are excited by glutamate release or dark conditions (see bold statement above).

However, keep in mind that bipolar cells will be named by what light does to them. In this case, since dark conditions (more glutamate) cause excitement (or EPSPs) on the bipolar cells, light must do the opposite.  Therefore, since these bipolar cells are excited by lack of light and inhibited by the presence of light, we call them off center bipolar cells.

In contrast, other bipolar cells contain metabotropic glutamate receptors called mGLUR6 receptors. These receptors also respond to glutamate ligand but work through a second messenger signaling system that causes inhibition of otherwise constitutively (always open) calcium channels called TRPM1. These bipolar cells would then “turn on” when glutamate release is reduced (light) and “turn off” when glutamate release is increased (dark) therefore we call them on center bipolar cells.

For this next section you may find it helpful to refer to the following picture. Numbers on the picture will be referenced in the text. Also, as you interpret the picture, please remember that the plus and minus signs represent what the neurotransmitter coming from the presynaptic cell would do to the post synaptic cell.

Receptor field
Image by JS, 2013

In addition to synapsing directly on bipolar cells, each photoreceptor also synapses with horizontal cells.

Horizontal cells are always depolarized by glutamate.

This is represented by the green plus sign (This means that glutamate from the surround photoreceptor will cause depolarization of the horizontal cell).

Therefore you could say that horizontal cells are active in dark (more glutamate release) and inactive in light (less glutamate release). When a horizontal cell depolarizes (dark), the opposite end of this cell releases a neurotransmitter called GABA onto the terminal ends of the center photoreceptor. GABA then acts to inhibit the release of glutamate from the photoreceptor which results in less glutamate going from photoreceptor to bipolar cell.

With this complicated arrangement you will see an explanation below that explains how light focused only on the center of the receptive field associated with an “on center” bipolar cell but not the surround cells, will almost double the effect of the EPSP on the bipolar cell.

You may have to think about this a couple of times.

What we are saying is:

Light hitting the center but not the surround will result in excitation of the horizontal cells with glutamate (because its dark on the surround cells) Causes increase GABA release from the horizontal cell The terminal ends of the center photoreceptor receive the GABA and experience an IPSP which further lowers the amount of glutamate released from the central photoreceptor With less Glutamate in the synapse between the central photoreceptor and the bipolar cell the on center bipolar cell will experience less inhibition This means that the constitutively open cation channels will be uninhibited and depolarization of the bipolar cell will commence.

Even though the center photoreceptor cell is releasing less glutamate (due to light), the effect of the surround photoreceptors (due to no light and through the horizontal cell) is to slow down the release further.

Yeah, you might want to stare at the picture a few times and realize that you can have different effects with light and dark on center and surround.  In fact, these different effects are what we will try to “Own” now…

To illustrate how receptive fields work we will need to make some assumptions about how the bipolar and horizontal cells work together. To do this will use the figure below. Let’s assume that a photoreceptor in the center of the receptive field has a 60% effect on what the bipolar and then ganglion cell sends to the brain and that the surround located photoreceptors have a 40% effect. We can take one step further and divide the circle into three equal portions with each surrounding edge contributing 20% and the center still at 60%. In addition, let’s simplify further by also assuming that there are just three photoreceptors, one in the center and one on each side (see figure). Taking this approach will “hopefully” allow us to explain the frequency of action potentials that are ultimately sent to the brain via the ganglion cell.

1 For on center off surround receptive fields, light hitting the entire receptive field will inhibit glutamate release from all three photoreceptors. This means that the bipolar cell will be activated as a result of the center photoreceptor (reduced glutamate) but since the horizontal cells will not be active (reduced glutamate) they won’t help further reduce glutamate release from the center. Remember that activation of horizontal cells results in GABA release which further reduces glutamate. Therefore, of the 100% effect from the receptive field, 60% of the bipolar cells influence will come from the center photoreceptor and 40% from the surround (which in this case are not active).  Thus, only 60% of the 100% possible influence will make it to the ganglia cell and then brain.  This is represented by 6 tick marks representing an imaginary frequency of 6 action potentials.  

2 If light strikes just the center and one of the surround edges, we see a different frequency. In this case, the bipolar cell will be stimulated as a result of the center photoreceptor (less glutamate) and activation of one of the horizontal cells (dark and to the left…also less glutamate). However, the surround edge to the right gets light and ends up not stimulating the bipolar cell. This results in more glutamate release from the center photoreceptor.  Long story short, of the 100% effect, 60% from the center and 20% from the edge (dark) will contribute while 20% from the other edge (still in the light) will not contribute. The result is 80% of the available signal being transmitted to the brain. This is represented by 8 tick marks on the imaginary action potential frequency chart.

3 This image shows light striking only one edge. In this case, the center (dark) and one edge (light) will not contribute. Only one activated horizontal cell (dark) will contribute to the inhibition of glutamate release to the bipolar cell.  Therefore, the other 80% are causing more glutamate release to the bipolar cell. This results in 20% of the available signal being transmitted to the brain. This is represented by 2 tick marks (one at beginning and one at end) on the imaginary action potential frequency chart.

4 This image shows that the entire receptive field is in the dark. In this case 40% (both activated horizontal cells) will result in the observed action potential frequency. The center photoreceptor is in the dark and will not contribute its 60% inhibition of glutamate release to the photoreceptor. This is represented by 4 tick marks on the imaginary action potential frequency chart.

The opposite effect occurs for off center on surround receptive fields. Remember that in these cells glutamate is stimulatory to the bipolar cell (notice the green plus sign on the bipolar cell representing the different EPSP creating inotropic glutamate receptor).  Having an EPSP on the Bipolar cell instead of an IPSP receptor basically makes all the action potential frequency charts have the opposite effect. Try to work your way through the off center on surround receptive fields and prove to yourself that the action potential frequency graphs are correct.

Image by  BYU-I JS F15

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