Somatic and Special Senses Practice Flashcards

Sensory Receptors and Stimuli

• Receptor Definition: A receptor is defined as any specialized structure used to detect a stimulus.
• Stimulus Definition: A stimulus is anything that causes a response.
• Range of Receptors: Receptors range from simple free nerve endings to complex sense organs.
• Major Function: The primary role of a receptor is transduction, which involves changing chemical, thermal, visual, or mechanical sensory information into a nerve impulse.

Classification of Receptors by Action

• Chemoreceptors: These respond to chemicals and are primary for sensations like smells and taste.
• Thermoreceptors: Located in the skin, these respond specifically to temperature changes.
• Nociceptors: These respond to painful stimuli and irritating touch/temperature stimuli (such as an itch).
• Mechanoreceptors: These respond to physical changes.
• Photoreceptors: These respond specifically to light.
• Osmoreceptors: These respond to the osmotic pressure of body fluids.

Structural and Location-Based Classification

• Structural Classification:
  • General or Somatic Senses: These are distributed widely and classified as either Free/Unencapsulated or Encapsulated.
  • Complex or Special Senses: These comprise specialized organs including the tongue, ear, eye, and nose.
• Origin/Location of Stimulus:
  • Visceroceptors: Internal receptors that pick up changes in internal organs.
  • Externoceptors: Receptors located in external areas.
  • Proprioceptors: Receptors that detect position changes in skeletal muscle, joints, and tendons.

Somatic (General) Senses: Unencapsulated Receptors

• Free Nerve Endings: Responsible for sensing pain and temperature.
• Merkle cells/discs (Merkel cells): Associated with light pressure and tactile sensations.
• Root Hair Plexus: Also known as hair follicle receptors, these are found around the hair bulb and detect hair movement.
• Nocioceptors: Stimulated by irritating touch or temperature sensations, such as an itch.

Somatic (General) Senses: Encapsulated Receptors

• Meissner’s Corpuscle: Responsible for light touch; located in the papillary layer of the dermis.
• Pacinian Corpuscle (Pacini's corpuscle): Responsible for deep pressure and vibrations; located in the reticular layer of the dermis.
• Krause’s End Bulb: Responsible for light touch and temperature; may be specifically responsible for cold mechanoreception.
• Ruffini Corpuscle (Ruffini ending): Responsible for touch and temperature; may be specifically responsible for hot temperature.

Stretch Receptors (Proprioceptors)

• Definition: Special types of mechanoreceptors located in muscles and tendons that serve as protective features.
• Muscle Spindles: Mechanoreceptors in skeletal muscles that monitor the length of muscle fiber stretch.
• Golgi Tendon Receptors: Located in tendons; they monitor the contraction of tendons and skeletal muscles to assess muscle tension.
• Joint Kinesthetic Receptors: Proprioceptors that monitor stretch in articular capsules.

Summary of Special Senses Organs and Receptors

• Vision:
  • Organ: Eye.
  • Receptors: Rods (monochrome) and Cones (color).
  • Type: Photoreceptors.
• Hearing:
  • Organ: Ear.
  • Receptor: Organ of Corti.
  • Type: Mechanoreceptors.
• Equilibrium:
  • Organ: Ear.
  • Receptor: Cristae ampullaris.
  • Type: Mechanoreceptors.
• Smell:
  • Organ: Nose.
  • Receptor: Olfactory cells.
  • Type: Chemoreceptors.
• Taste:
  • Organ: Taste buds.
  • Receptor: Gustatory cells.
  • Type: Chemoreceptors.

The Olfactory Sense (Smell)

• Anatomy of the Olfactory System:
  • Structures include the olfactory epithelium, nasal conchae, olfactory tract, olfactory bulb, mitral cells, glomeruli, and the cribriform plate of the ethmoid bone.
  • Cell types: Olfactory receptor cells (with olfactory cilia), supporting cells, and basal cells.
  • Olfactory glands in the lamina propria connective tissue produce mucus.
• Olfactory Pathway:
  1. Odor molecules dissolve in the mucus surrounding olfactory cilia.
  2. If the threshold level is reached, an Action Potential (AP) is generated.
  3. The impulse passes through the Olfactory Nerves ($I$) to the olfactory bulb.
  4. The signal travels through the olfactory tract to thalamic and olfactory centers in the temporal lobe for interpretation and memory.

The Gustatory Sense (Taste)

• Anatomy of Taste:
  • Structures include the epiglottis, palatine tonsil, lingual tonsil, and various papillae (circumvallate and fungiform papillae).
  • Taste Bud Anatomy: Consists of gustatory receptor cells, gustatory hairs (microvilli), supporting cells, basal cells, and a taste pore.
• The Five Basic Taste Sensations:
  • Sweet: Elicited by sugars, saccharin, alcohol, and some amino acids; sensed primarily at the tip of the tongue.
  • Salt: Elicited by metal ions; found on the anterior sides.
  • Sour: Elicited by hydrogen ($H^{+}$) ions; found on the posterior sides.
  • Bitter: Elicited by alkaloids such as quinine and nicotine; found at the posterior of the tongue.
  • Umami: Elicited by the amino acid glutamate.
• Taste Pathway:
  1. An AP is created in the taste cell.
  2. The AP is generated to the brain via the Facial nerve ($VII$), Glossopharyngeal nerve ($IX$), and Vagus nerve ($X$).
  3. The signal travels to the medulla oblongata, then to the thalamus, and finally to the gustatory cortex of the parietal lobe.
  4. The hypothalamus and limbic system provide the appreciation of taste.

Anatomy and Physiology of the Ear

• Functions: Hearing and Equilibrium/Balance.
• Receptor Type: Both senses utilize mechanoreceptors, specifically hair cell receptors.
• Major Divisions:
  • External (Outer) Ear: Includes the auricle (pinna) with the helix and lobule, and the external auditory canal (meatus).
  • Middle Ear (Tympanic Cavity):
    • Features the tympanic membrane (eardrum), ossicles (malleus/hammer, incus/anvil, stapes/stirrup), and the pharyngotympanic (auditory/Eustachian) tube.
    • Also includes the oval window, round window, tensor tympani muscle, and stapedius muscle.
  • Inner (Internal) Ear (Labyrinth): Includes the vestibule, semicircular canals, cochlea, vestibular nerve, and cochlear nerve.

The Mechanism and Pathway of Hearing

• Stimulation Context: Hearing occurs when the auditory area of the temporal lobe is stimulated. Sound waves must propagate through air, membranes, bones, and fluids.
• Hearing Pathway:
  1. Sound waves enter the external auditory meatus with the help of the pinna.
  2. Waves strike the tympanic membrane.
  3. Vibrations move through the ossicles in order: Malleus $\rightarrow$ Incus $\rightarrow$ Stapes.
  4. The stapes presses against the oval window, initiating fluid conduction.
  5. Perilymph (fluid similar to CSF) ripples, moving the vestibular membrane.
  6. This ripples the endolymph inside the cochlear duct.
  7. The movement causes the tectorial membrane to deflect (bend) the cochlear hair cells in the Organ of Corti.
  8. The bending causes the release of ions and neurotransmitters, generating an AP.
  9. Nerve impulses travel via the cochlear nerve to the brain stem (passing through the medulla, pons, midbrain, and thalamus).
  10. Interpretation occurs in the auditory centers of the temporal lobe.

Equilibrium (Balance)

• Sense Organs: Located in the inner ear within the vestibule and semicircular canals, collectively known as the vestibular apparatus.
• Control Nerve: Equilibrium is controlled by the vestibular nerve, which is a branch of the vestibulocochlear nerve (cranial nerve $VIII$).

Anatomy and Physiology of the Eye

• Key Anatomical Structures:
  • Conjunctiva, Iris, Lens, Ciliary muscle, Retina, Choroid layer, Sclera.
  • Central retinal artery and vein, Optic nerve, Optic disc (the blind spot).
  • Anterior cavity (containing aqueous humor), Pupil, Cornea.
• Visual Pathway:
  1. Light enters through the Cornea.
  2. Passes through the Anterior cavity (Aqueous humor).
  3. Enters the Pupil and passes through the Lens.
  4. Passes through the Posterior cavity (Vitreous humor).
  5. Reaches the Retina to stimulate Photoreceptors (Rods and Cones).
  6. Light causes the breakdown of the chemical rhodopsin.
  7. This chemical breakdown creates a membrane potential that is transmitted as an Action Potential.
  8. The AP transfers to synapsed neurons connecting to the Optic nerve.
  9. The Optic nerve transmits the signal to the visual cortex in the occipital lobe.
• Image Formation: Requires light refraction, accommodation of the lens, constriction of the pupil, and convergence of the eyes.

Visual Focus and Disorders

• Distant Vision: The eyes are naturally adapted for distant vision ($20\,ft$ and beyond). The eye muscles are relaxed and the lens is flat. The "far point of vision" is the distance beyond which the lens shape does not need to change ($20\,ft$).
• Close Vision: Requires active adjustments to focus on the "near point":
  • Accommodation: Ciliary muscles change lens shape to increase refractory power (the lens bulges).
  • Constriction: The pupillary reflex constricts the pupil to block divergent light rays.
  • Convergence: Medial rotation of the eyeballs toward the object.
• Presbyopia: The change in close focus capability as one ages (typically around $50$ years).
• Vision Disorders:
  • Myopia (Nearsightedness): Distant objects are focused in front of the retina; the eyeball may be too long.
  • Hyperopia (Farsightedness): Distant objects are focused behind the retina; the eyeball may be too short.