General and Special Senses

General and Special Senses

Sensory Receptors

• Pick up sensory information.

• Simplest form: neuroendrites.

• Free nerve endings: sensitive to many types of stimuli (cuts, temperature changes, crushing).

• Sensory information arrives at the CNS as an action potential carried by afferent fibers (sensory).

• Conscious awareness of sensation: perception.

Touch, Taste, and Visual Receptors

• Covered in more detail later.

General Concepts

• Receptive Field:

  • Area monitored by a single receptor.

  • Larger field = poor ability to localize stimuli.

  • Example: Two-point discrimination test on the thigh demonstrates the concept.

• Adaptation:

  • Reduction in sensitivity due to constant stimulus.

  • Reduces information received in the cerebral cortex.

General vs. Special Senses

• General Senses:

  • Temperature, pain, touch, pressure, vibration, proprioception (body position).

• Special Senses:

  • Smell (olfaction), taste (gustation), vision, balance (equilibrium), hearing.

Receptor Types

• Nociceptors: Sense pain.

• Thermoreceptors: Sense temperature.

• Mechanoreceptors: Sense physical distortion from touch, pressure, body position.

• Chemoreceptors: Sense chemical stimuli.

Pain (Nociceptors)

• Located in skin, joint capsules, periosteal coverings of bones, and around blood vessel walls.

• Sensitivity:

  • Extremes in temperature.

  • Mechanical damage.

  • Dissolved chemicals released by injured cells.

• Axon Types:

  • Myelinated Fibers:

    • Quick, localizable pain (e.g., pin prick).

    • Increased speed of action potential.

  • Unmyelinated Fibers:

    • Slow, burning, aching pain; hard to localize.

    • Visceral organ pain.

• Referred Pain:

  • Pain perceived as originating on the body surface.

  • Cardiac pain felt in the upper chest and arms.

• Adaptation:

  • Perception of pain decreases over time.

Temperature (Thermoreceptors)

• Located in dermis, skeletal muscles, liver, and hypothalamus.

• Hypothalamus: Plays a large role in temperature regulation.

• Cold receptors are more numerous than warm receptors.

• Carried along the same pathway as pain.

• Icing an injury: Cold temperature competes with pain pathways.

Mechanoreceptors

• Sense touch, pressure, and body position.

• Sensitive to stretching, compression, twisting.

• Distortion of the receptor's cell membrane opens or closes ion channels.

• Tactile Receptors:

  • Touch, pressure, and vibration sensations.

  • Fine touch and pressure receptors: precise and detailed.

  • Crude touch and pressure receptors: poor detail and localization.

Receptor Types

• Free nerve endings: Touch and pressure located in skin cells.

Baroreceptors

• Provide information for autonomic activity regulation (not consciously thinking about).

• Monitor changes in pressure and respond immediately.

• Located in elastic tissues in the walls of distensible organs.

Locations and Functions:

• Carotid and Aortic Sinuses:

  • Information on blood pressure.

  • Communicate with cardiovascular and respiratory control centers.

• Lungs:

  • Information about respiratory rhythm.

  • Communicate with the respiratory system.

• Digestive Tract:

  • Responsible for moving materials along.

• Colon:

  • Information on fecal matter volume.

  • Trigger defecation reflex.

• Bladder Wall:

  • Information on urine volume.

  • Trigger urinary reflex.

Proprioceptors

• Monitor body position.

• Located in joints, tendons, ligaments, muscle contraction

• Do not adapt to constant stimulation.

• Information constantly sent to the CNS, processed subconsciously.

• Integrated with the inner ear for balance.

Chemoreceptors

• Respond to different chemical concentrations.

• Locations: Medulla oblongata, carotid bodies, and aortic bodies.

Functions and Locations:

• Medulla Oblongata:

  • Monitors hydrogen ion concentration (pH) and carbon dioxide in CSF (cerebrospinal fluid).

  • Important for respiratory system; changes depth and rate of respiration.

  • $pH$ too low (acidic) triggers increased breathing rate and depth to remove $CO_2$.

• Carotid Bodies:

  • Monitor $pH$, $CO_2$, and oxygen concentration.

  • Communicate with the respiratory center via the glossopharyngeal nerve.

  • Trigger changes in respiratory and cardiovascular activity.

• Aortic Bodies:

  • Same as carotid bodies but communicate via the vagus nerve.

Special Senses: Olfaction (Smell)

• Olfactory epithelium on either side of the nasal cavity.

• Mucus Function:

  • Prevents buildup of dangerous stimuli.

  • Keeps area moist and free of dust and debris.

• Sniffing: increases airflow across olfactory epithelium, intensifying stimulation.

Olfactory Pathways

• Chemicals diffuse into mucus.

• Bind to olfactory receptors, changing membrane permeability.

• Action potential is produced.

• Axons leave the olfactory epithelium and collect into bundles.

• Bundles penetrate the cribriform plate of the ethmoid bone (through the skull).

• Synapse occurs on the olfactory bulbs.

• Axons leave the olfactory bulb and travel along the olfactory tract.

• Reach the olfactory cortex (cerebrum, hypothalamus, limbic system) for identification, meaning, and emotional/behavioral response.

Special Senses: Taste (Gustation)

• Taste or gustatory receptors are found on the tongue, pharynx, and larynx.

• Decrease by adulthood.

• Taste buds are found along the sides of epithelial projections called papillae.

Taste Reception

• Dissolved chemicals contact taste hairs.

• Stimulates change in the membrane causing a membrane potential.

• Creates an action potential in the sensory neuron.

Primary Taste Sensations

• Sweet, salty, sour, and bitter.

• Humans also have umami (beef/chicken broth, Parmesan cheese) and water receptors.

• Water receptors are especially found in the pharynx and affect systems involved in water balance and blood volume regulation.

Nerve Involvement

• Facial, glossopharyngeal, and vagus nerves.

• Olfactory receptors have an overwhelming role in identifying taste.

• Tastes are dull and unappealing when smell is blocked.

Special Senses: Vision and the Eye

Accessory Structures of the Eye

• Eyelids, eyelashes, and exocrine glands: Keep the eye lubricated and free of debris.

  • Infections of glands cause styes (localized swelling).

• Superficial Epithelium (Conjunctiva):

  • Covers the inner surface of eyelids and outer surface of the eye proper.

  • Extends to the edges of the cornea.

  • Infection causes conjunctivitis (pink eye).

    • Redness due to dilation of blood vessels beneath the epithelium.

• Lacrimal Apparatus:

  • Produces, distributes, and removes tears.

  • Tear gland is located superior and lateral to the eyeball.

  • Tears empty into the lacrimal sac, into the nasolacrimal duct, and can be carried into the nasal activity, which is the reason that, when someone cries their nose runs also.

Function of Tears

*Keeps the surface clean and moist.
*Reduce friction.
*Reduce debris.
*Prevent bacterial infection (lysozyme).
*Provide nutrients and oxygen to the conjunctival epithelium.

Eye Muscles

• Six muscles control eye movement.

• Damage to muscles can impair eye movement.

Eyeball Anatomy

• Posterior Cavity:

  • Contains a gelatinous vitreous body.

• Anterior Cavity:

  • Subdivided into two chambers:

    • Anterior Chamber: contains aqueous humor which is a clear fluid that circulates nutrients and waste.

    • The canal of Schlemm carries this aqueous humor into veins in the sclera and returns all this to the venous system.

• Glaucoma: Interference of return causing increased pressure leads to blindness if untreated due to retinal and optic disc distortion and the increased pressure being put on these two structures, then causes vision to be distorted and blindness.

• Posterior Chamber: The other chamber of the anterior cavity.