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

Structure of the Eye

The eye is a hollow, fluid filled organ that is surrounded by three layers of tissue (see image below). The outermost layer is composed of connective tissue and there are no blood vessels penetrating this layer. It can be divided into two parts, the sclera, the white part of the eye comprising the posterior 5/6 of the eyeball, and the cornea, the clear window on the anterior surface of the eye. The sclera helps protect the eye and also provides a site of attachment for the six muscles responsible for the movement of the eye. The cornea is transparent and functions as the major refractor of the light as it enters the eye. Its transparency is due to the nature of the collagen and proteoglycan fibers that form it. Following are a couple of pictures that help orient us to the anatomy of the eye.

Anatomy of the eye
Structure of the Eye. Title: File:1413 Structure of the Eye.jpg; Author: OpenStax College; Site:; License: This file is licensed under the Creative Commons Attribution 3.0 Unported license.
Diagram of Human Eye. Title: File: Simple diagram of human eye multilingual.svg; Author: Jmarchn; Site:; License: This file is licensed under the Creative Commons Attribution-Share Alike 3.0 Unported license
  1. Conjunctiva
  2. Sclera
  3. Cornea
  4. Aqueous humour (in anterior and posterior chambers. See purple dotted line)
  5. Lens
  6. Pupil
  7. Uvea with
  8. Iris
  9. Ciliary body and
  10. Choroid
  11. Vitreous humor
  12. Retina with
  13. Macula or macula lutea
  14. Optic disc → blind spot
  15. Optic nerve

The middle layer of the eye is the vascular tunic. Most of the blood vessels of the eye can be found in this layer. The picture above shows blood vessels of the retina. The blood vessels of the vascular tunic are not shown. If they were shown, you would see them associated with the choroid in the image. The posterior portion of this layer is the choroid. Anteriorly the choroid is continuous with the ciliary body. The ciliary body is composed of a ring of smooth muscle, the ciliary muscle, and the ciliary processes. The ciliary muscle is a sphincter-like muscle that is attached to the lens capsule via the suspensory ligaments. It is responsible for adjusting the thickness of the lens. The ciliary processes are secretory structures that produce the aqueous humor that fills the compartment in front of the lens.

Anatomy of the Eye. Title: Blausen 0390 EyeAnatomy Sectional.png; Author: BruceBlaus; Site:; License: This file is licensed under the Creative Commons Attribution 3.0 Unported license

The most anterior part of the vascular tunic is the iris. The iris is composed primarily of smooth muscle containing varying amounts of the pigment melanin. The amount of melanin determines eye color, large amounts produce brown eyes, while smaller amounts result in blue or green eyes. The iris is actually two layers of muscle with a circular hole in the center, the pupil. The sphincter pupillae is a circular layer that causes the pupil to constrict (miosis) when it contracts and the dilator pupillae is a radial layer that causes the pupil to dilate (mydriasis) when it contracts. These layers are innervated by the autonomic nervous system, the dilator is under sympathetic control and the sphincter is under parasympathetic control.

The innermost layer is the neural tunic or retina. There are actually two distinct layers of the retina. The pigment epithelium is a layer of simple cuboidal epithelium that sits on the choroid. This layer has large amounts of melanin giving it a dark black color. One important function of the pigmented retinal is to absorb light that doesn’t strike the photoreceptors and prevent it from being reflected inside the eye. The neural layer is the innermost layer of the wall of the eye and contains the photoreceptors that are stimulated by the entering light. Two distinct anatomical structures on the retina are the optic disk and the fovea centralis. The optic disk, also called the blind spot, is the point where the optic nerve and blood vessels enter the eye. There are no photoreceptors in this area and hence light striking the optic disk cannot be detected. The fovea centralis (fovea = pit) is a small indention located in the center of a special area of the retina called the macula lutea (macula = body, lutea = yellow). The macula is roughly 5 mm in diameter, about the diameter of a pencil eraser, and the fovea is about the size of the head of a pin. When you look at an object the light coming directly from that object focuses on the fovea, it is the portion of the retina with the greatest visual acuity (clarity).

The lens is not technically part of any of these three layers but it is obviously extremely important in focusing light. It is a biconvex structure composed of transparent cells (epithelial cells). These cells have lost their nuclei and other organelles and are filled with transparent proteins called crystallines. It is surrounded by the very elastic lens capsule which, in turn, is attached to the ciliary muscles by the suspensory ligaments. When there is no tension on the suspensory ligaments (ciliary muscles are contracted) the lens assumes its natural shape, this is when it is at its thickest. When the ciliary muscles relax the tension on the suspensory ligaments increases and the lens flattens. Remember the ciliary muscle is a sphincter muscle so when it contracts its diameter decreases, reducing tension on the ligaments attached to the lens capsule.

The lens divides the eye into two fluid filled compartments. The anterior cavity is the space between the lens and the cornea. As was mentioned above, this cavity is filled with the aqueous humor produced by the ciliary processes. Aqueous humor is a watery fluid produced continually and circulates through the cavity before being reabsorbed into the blood. It is important in maintaining proper intraocular pressure as well as circulating nutrients and removing wastes to the cells of the lens and cornea. If the normal circulation is blocked it can result in an inappropriate increase in pressure, a condition known as glaucoma. If not treated, glaucoma can result in vision loss and blindness. You may see some anatomy texts divide the anterior cavity into two “chambers.” The anterior chamber of the anterior cavity would be between the cornea and the iris. The posterior chamber of the anterior cavity would be a very small space between the iris and the lens. The posterior cavity is the space behind the lens. This compartment is filled with vitreous humor. Vitreous humor is more of a gel, similar to egg white. It also is important in maintaining intraocular pressure, but unlike aqueous humor, turns over very slowly.

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