Sensory Receptors and Pathways

• Sensory pathways involve receptors that detect stimuli and transmit signals to the brain for processing.

Types of Sensory Receptors

• Somatic senses include touch, pressure, temperature, and pain, found in the skin and deep tissues.
• Special senses include smell, taste, hearing, equilibrium, and vision.

Categories of Sensory Receptors

• Chemoreceptors are stimulated by chemical concentrations of substances, such as in smell and taste.
• Pain receptors are stimulated by tissue damage, triggering pain sensations in all senses, and do not adapt.
• Thermoreceptors are stimulated by changes in temperature, located in the skin.
• Mechanoreceptors are stimulated by pressure or movements, found in the skin and ears.
• Photoreceptors are stimulated by light, essential for vision.

Vocabulary

• Sensation occurs when the brain becomes aware of a sensory impulse.
• Perception occurs when the brain interprets the sensory impulse.
• Sensory information enters the brain similarly, with results depending on the brain area receiving the info.
• Sensory adaptation is the brain's ability to ignore unimportant stimuli, resulting from receptor unresponsiveness or nerve impulse inhibition.

Sense of Touch and Pressure

• Sensors for touch and pressure are in the skin, muscles, joints, and viscera, along with temperature and pain sensors.

Touch and Pressure Receptors

• Free nerve endings are found in epithelial tissue and detect itching, temperature, and pain.
• Tactile corpuscles are abundant in hairless skin areas like lips, palms, fingertips, soles, and nipples, responding to light touch.
• Lamellated (Pacinian) corpuscles are abundant in deep subcutaneous tissue, tendons, and ligaments, responding to heavy pressure.
• Sensitivity to pressure increases with depth in the integumentary system.
• The order from lightest to deepest sensation is tactile corpuscles, free nerve endings, then lamellated corpuscles.

Temperature Receptors

• Warm receptors are sensitive to temperatures between 77°F and 113°F.
• Cold receptors are sensitive to temperatures between 50°F and 68°F.
• Warm and cold receptors adapt rapidly with continuous stimulation.
• Temperatures outside the specified ranges stimulate a pain response.

Sense of Pain

• Pain receptors consist of free nerve endings found in the skin and internal tissues, serving a protective function to promote healing, and adapt poorly.
• The thalamus is responsible for pain awareness.
• The cerebral cortex determines pain intensity, location, emotional response, and motor response.

Pain Types

• Referred pain is when pain is felt in one body part, but originates from another.
• Acute pain is sharp, immediate, and localized, usually from the skin.
• Chronic pain is dull, aching, diffused, and felt in deep tissue, making it difficult to pinpoint.

Importance of Touch

• Positive touch is vital for bonding, calming, reassurance, thriving, and learning in babies, children, and adults.

Sense of Smell

• Olfactory receptors are chemoreceptors located in the upper nasal cavity.
• Humans have about 12 million olfactory receptor cells, while bloodhounds have about 4 billion.
• Odor molecules must dissolve in watery fluid surrounding cilia for receptors to detect them.
• Smell and taste are closely related, both using chemoreceptors, being in close proximity, and being interpreted in the temporal lobe.
• The sense of smell adapts quickly and decreases with age, linked to memories via the limbic system and temporal lobe.

Sense of Taste

• Papillae are bumps on the tongue where taste buds are located.
• Taste buds are special organs for taste on the tongue's surface, the mouth's roof, and the throat's walls, totaling about 10,000.
• Taste hairs are the sensitive parts of the receptor cells within taste buds.
• Each taste bud has 50-150 receptor cells, replaced every three days.
• Chemicals must dissolve in saliva before they can be tasted.
• As people age, taste buds disappear from the sides and roof of the mouth.
• Remaining taste buds become less sensitive and are not always replaced.

Taste Sensations

• Five primary taste sensations include sweet (table sugar), sour (lemon), salty (table salt), bitter (caffeine or quinine), and umami (savory, monosodium glutamate).
• Spicy sensations (capsaicin) stimulate pain receptors.

Sense of Hearing

• The ear is the organ of hearing and equilibrium, with three parts: external, middle, and inner.
• Vibrating objects produce sound waves for hearing.

External Ear

• The auricle (pinna) is an outer, funnel-like structure (ear lobe) used to collect and direct sound waves.
• The external acoustic meatus/auditory canal is an external canal leading inward, carrying sound waves to the middle ear.

Middle Ear

• The eardrum is a semitransparent membrane moving back and forth in response to sound wave vibrations, attached to the malleus.
• Auditory ossicles consist of the body’s three smallest bones: malleus, incus, and stapes.
• These bones bridge the eardrum and inner ear, transmitting and amplifying vibrations.
• The auditory (eustachian) tube regulates air pressure, maintaining equal pressure on both sides of the eardrum.
• It connects the middle ear to the throat and equalizes pressure during altitude changes.

Inner Ear

• The inner ear contains communicating chambers and tubes called a labyrinth.
• The three semicircular canals provide a sense of equilibrium but do not affect hearing.
• The cochlea contains hearing receptors, fluid, and hair cells (snail shell).
• The cochlear nerve and vestibular nerve send impulses to the brain.
• Static equilibrium involves motionlessness, while dynamic equilibrium relates to the body in motion.

Visual Accessory Organs

• Visual accessory organs include eyelids, lacrimal apparatus, and extrinsic eye muscles.
• Eyelids have four layers: skin, muscle, connective tissue, and conjunctiva.
• Eyelid skin is the thinnest in the body and is moved by the orbicularis oculi muscles.
• The lacrimal apparatus contains lacrimal glands that secrete tears to moisten, lubricate, and provide antibacterial protection.

Structures of the Eye

• The eye wall has outer, middle, and inner tunics (layers).

Outer Tunic

• The cornea helps focus incoming light rays and is transparent.
• The sclera is the white part of the eye, continuous with the cornea, and provides protection.
• The optic nerve transmits electrical impulses from the retina to the brain.
• The blind spot is due to the absence of sensory receptor cells.

Middle Tunic

• The choroid coat contains blood vessels and pigment-producing cells (melanin) to absorb excess light.
• The ciliary body produces aqueous humor, a clear fluid filling the front of the eye.
• The lens, behind the iris, focuses light onto the retina.

Accommodation

• Accommodation is the lens' ability to change shape for close or distant vision.
• With age, the lens hardens, reducing the ability to accommodate.
• The cornea first bends light, and the lens bends it again for focusing.
• Focusing on distant objects requires a thin, elongated lens with relaxed ciliary muscles.
• Focusing on close objects requires a thick lens with contracted ciliary muscles.
• Hyperopia results in farsightedness.
• Myopia results in nearsightedness.

Middle Tunic Continued

• The iris is the colored part of the eye (brown, blue, green).
• Aqueous humor is a watery fluid nourishing the cornea and lens, giving the eye its shape.
• The pupil is the circular opening in the iris's center, controlling the amount of light entering the eye.

Inner Tunic

Retina

• The retina is a multi-layered sensory tissue lining the back of the eye, containing photoreceptors that convert light rays into electrical impulses.
• The fovea centralis is the retinal region producing the sharpest vision.
• The optic disc is the opening where nerve fibers leave the eye and join the optic nerve.
• Vitreous humor is a jelly-like fluid that helps the compressed eye return to its normal shape. Light refraction involves the cornea and lens bending light waves to focus an image on the retina. Visual receptors include rods and cones.

Visual Receptors

• Rods are responsible for colorless vision in dim light and are more sensitive to light.
• Cones provide color vision and sharper images.
• Bipolar neurons signal messages.
• Color blindness is X-linked, affecting more males.