Sense Organs and Stimuli

• Sense organs are groups of sensory cells responding to specific stimuli like light, sound, touch, temperature, and chemicals.
• Stimuli are changes in light, temperature, or pressure that cause a reaction in living organisms.
• Receptors are structures that detect stimuli, scattered through the skin or concentrated in sense organs like the eye and ear.
• Sequence of events: stimulus → receptor → coordination → effector → response
• Receptors detect stimuli by changing the stimulus's energy into electrical energy in nerve impulses.
• The eye converts light energy, and the ear converts sound energy into nerve impulses.

Receptor Types and Stimuli:

• Eye (retina): Light
• Ear (organ of hearing): Sound and body movement (balance)
• Tongue (tastebud): Chemical (taste)
• Nose: Chemical (smell)
• Skin: Pressure, touch, and pain (mechanical)
• Temperature: Heat

The Eye

• The eyeball is protected within the skull's orbit and attached by 6 extraocular muscles.
• The conjunctiva is a thin, protective layer that also controls the amount of light entering the eye, lubricated by tear glands.
• The eye wall has three layers: the outermost sclera, the choroid coat, and the retina.
• The sclera is a tough, white fibrous coat that protects the eye and becomes the transparent cornea.
• The cornea is transparent, allowing light to enter the eye and refract due to its density compared to air.
• The choroid coat protects the eye and nourishes it with a network of blood vessels, forming the iris and ciliary body.
• The iris is a circular structure with a central hole called the pupil, controlling the pupil's diameter to regulate light entry.
• The pupil is the opening in the iris that allows light to enter the eye and onto the lens.
• The lens is a transparent disc behind the pupil that changes shape to focus light onto the retina, controlled by ciliary muscles and suspensory ligaments.
• The retina, at the back of the eye, contains light-sensitive cells connected to the optic nerves, which receive light and are stimulated.

Light Receptors

• Rods detect intensity of dim light.
• Cones detect bright light and color.
• The fovea/fovea centralis has the highest density of light-sensitive cells.

Pupillary Reflex

• The pupillary reflex adjusts to different light intensities to prevent damage to light-sensitive cells.
• Antagonistic circular and radial muscles control this process.

Pupil Response to Light

• In bright light, circular muscles contract and radial muscles relax, constricting the pupil to reduce light entry.
• In dim light, radial muscles contract and circular muscles relax, dilating the pupil to increase light entry.

Accommodation

• Accommodation allows the eye to focus based on the object's distance.
• Accommodation is achieved by lens shape change to bend light as needed.

Focusing on Nearby Objects

• Ciliary muscles contract, decreasing the ring's diameter.
• Suspensory ligaments loosen, stopping the pulling on the lens to become fatter.
• Shorter focal length and light is refracted more.

Focusing on Faraway Objects

• Ciliary muscles relax, increasing the ring's diameter.
• Suspensory ligaments tighten.
• The suspensory ligaments pull on the lens, causing it to become thinner.
• Longer focal length and light is refracted less.

Skin Structure

• The skin includes the epidermis, dermis, and hypodermis.
• The epidermis is the outermost layer provides a waterproof barrier that protects from the environment and creates skin tone.
• The dermis is beneath the epidermis which contains hair follicles, sweat glands, blood capillaries, and nerves.
• The hypodermis is the innermost and thickest layer containing deposited and stored fat.
• The epidermis has three layers: the outer cornified layer, granular layer, and innermost Malpighian layer.
• The cornified layer's cells are dead, dry, flat, and horny due to keratin, preventing water loss and germ entry.
• The granular layer consists of living cells that become dry and horny, forming the cornified layer.
• The Malpighian layer contains melanin-producing cells that protect against UV light.

Dermis

• Most of the dermis is connective tissue with elastic and collagen fibers; as people age, their skin becomes loose and wrinkled.
• Sweat glands produce sweat, which is mainly water with small amounts of salt and urea, and helps regulate temperature.
• Sebaceous glands secrete sebum into hair follicles to lubricate the hair and skin, preventing dehydration and inhibiting bacteria growth.
• Nerve endings in the epidermis and dermis detect pain, pressure, or temperature changes.
• Numerous capillaries in the dermis are controlled by vasomotor nerves, which regulate vasodilation and vasoconstriction to control body temperature.
• The Malpighian layer of the epidermis forms hair follicles in the dermis, with blood capillaries and nerves at the base.

Hair erector muscles & hypodermis

• Hair erector muscles attached to hair follicles cause hairs to stand on end to trap warm air making "goose pimples".
• The hypodermis is beneath the dermis, where adipose tissue stores fat, providing insulation.
• Heat is produced through tissue respiration in muscles and liver and distributed by blood, and through eating hot food and from sun radiation.
• Heat is lost through skin by convection, radiation, conduction, evaporation of sweat, feces, urine, and expired air.

Homeostasis

• Homeostasis maintains a stable internal environment despite external changes.
• Negative feedback inhibits a change, maintaining homeostasis.
• Receptors in the skin detect temperature changes, signaling the hypothalamus to regulate body temperature with kidneys and skin.
• Homeostasis requires a stimulus, a receptor, and a self-regulatory corrective mechanism, which brings about negative feedback.

Body Temperature Regulation

• On a cold day, arterioles in the skin constrict and sweat glands become less active to conserve heat.
• Erector muscles contract, hairs stand on ends, trapping a thicker layer of air and shivering may be initiated.
• On a warm day, arterioles in the skin dilate, and sweat glands become active to lose heat.
• Erector muscles relax, hairs lie flat, Metabolic rate decreases and there is no shivering.