Somatic Nervous System - Sensory Organs

Sensory Perception

• Objectives:

  • Describe different types of sensory receptors

  • Describe the structures responsible for the special senses of taste, smell, hearing, balance, and vision

  • Distinguish how different tastes are transduced

  • Describe the means of mechanoreception for hearing and balance

  • List the supporting structures around the eye and describe the structure of the eyeball

  • Describe the process of phototransduction

Sensory Receptors: Structural Types

• Free nerve endings

  • Pain and temperature

• Encapsulated endings

  • Pressure and touch

• Specialized Receptor cell

  • Photoreceptors

Sensory Receptors: Location

• Classification by location:

  • Exteroceptor

    • Located near stimulus

  • Interoceptor

    • Internal organs

  • Proprioceptor

    • Near moving body part

Sensory Receptors: Functional Types

• Classification by function:

  • Chemoreceptor

    • Chemical stimuli

  • Osmoreceptors

    • Solute concentrations

  • Nociceptor

    • Pain

  • Mechanoreceptor

    • Physical stimuli / sound / balance

  • Thermoreceptor

    • Temperature

  • Photoreceptor

    • Rods & cones for vision

Sensory Modalities

• General sense

  • Distributed throughout body

• Special sense

  • Specific organ dedicated to it

• Modality

  • Refers to how information is encoded

Gustation

• Sweet

  • Dissolved glucose

  • G protein-coupled receptors

• Salty

  • Perception of Sodium in saliva

• Sour

  • Hydrogen ion perception

• Bitter

  • G protein- coupled receptors

  • Depolarize or hyperpolarize

• Umami

  • G protein-coupled receptors

  • L-glutamate

Gustation

• Taste buds

  • Within papillae

• Gustatory receptor cells

• Supporting cells

• Basal Cells

  • Can activate:

    • Facial nerve

    • Glossopharyngeal nerve

    • Vagus nerve

Olfaction

• Olfactory epithelium

• Olfactory receptor neurons

  • Dendrites on apical surface

• Odorant molecules

  • Airborne, pass over olfactory epithelial region

  • Bind to proteins in mucus

  • Transported to dendrites

  • Graded potential

Olfaction

• Axons extend:

  • From basal surface to brain

  • Connect to olfactory bulb

  • Axons then split:

    • Olfactory cortex

      • Inferior and medial area of temporal lobe

    • Hypothalamus and Limbic system

      • Long term memory / Emotion

Accessory Eye Structures

• Accessory structures:

  • lacrimal gland (tears),

  • lacrimal duct leads to the eye and conjunctiva.

  • Four straight and two oblique extrinsic eye muscles.

  • Eyelids, eyebrows, and eye lashes for protection.

Eye Anatomy

• Tissue layers:

  • Fibrous tunic

    • Sclera & Cornea

  • Vascular tunic

    • Choroid

    • Ciliary Body

    • Iris

  • Neural tunic (Sensory Layer)

    • Retina

    • Pigmented Epithelium

    • Rods & Cones

    • Optic Nerve

Lens

• Lens – biconvex, flexible structure to help focus light on the retina, held by suspensory ligaments which attach to ciliary muscles

Rods and Cones

• 130 million rods vs. 6.5 million cones

• Rods are cylindrical stacks covered in proteins, they react to light, more rods the further from the fovea

• Cones sense color (blue, green, and red) and are concentrated in the fovea

Ey /Vision Problems

• Hyperopia – eyeball is short, light focuses after the retina (farsightedness)

• Myopia – eyeball is long, light focuses before retina (nearsightedness)

• Diplopia – eyes don’t focus on same spot (double vision)

Vision Processing

• Vision is processed right to left, upper to lower, and is inverted, but the image is integrated to the proper positioning

• The ventral stream recognizes an object's significance, uses temporal lobe structures

• The dorsal stream locates objects in space and guides movements in response, uses parietal lobe structures and interacts with somatosensory cortical areas

Ear

• Outer – auricle, external auditory meatus, external auditory canal with ceruminous glands, and tympanic membrane (ear drum)

• Middle – begins at tympanic membrane, 3 ossicles (malleus, incus, and stapes), and ends at oval window. Middle ear is filled with air unless there is an ear infection.

• Inner – passages in the temporal bone. Vestibule, cochlea (audition), and 3 semicircular ducts (equilibrium). Perilymph surrounds the membranous labyrinth and endolymph fills the membranous labyrinth.

Auditory Tube

• Known as Eustachian tube

• Connects middle ear to the pharynx

• Allows air to enter or leave to equalize pressure

• Fluid can drain through tube during infections

Cochlea

• Has 3 ducts – scala vestibuli (attached to oval window), scala media (organ of Corti), and scala tympani (attached to round window).

• Vestibular and tympanic canals are filled with perilymph

• Scala media is filled with endolymph and is where sensory function occurs

• Each duct is separated by a membrane

• Organ of Corti attaches to the basilar membrane, has hair cells with stereocilia, and hairs are anchored to a tectorial membrane

Audition

• Audition is a physical/mechanical sensation

• Sound waves cause tympanic membrane to vibrate, vibrations make ossicles move, perilymph moves basilar membrane which bends the hairs, and now channels can open for depolarization.

• Nerve signal can now be conducted through the vestibulocochlear nerve (CN VIII) to the brain

• Ossicles amplify the sound waves

Sound

• High frequency for high pitch at the base (start) of the cochlea

• Intensity – more hair cells move because of louder sounds

• Hearing aid amplifies the sound if the system isn’t working well enough, but won’t do anything if hairs are compromised

• Cochlear implant – should conduct sound to nerve and used if hairs are no longer working

Equilibrium/Balance

• Static equilibrium is detected by mechanoreceptor in the vestibule

• If moving, cells slide in the 3 semi circular ducts and bend hairs to signal to the brain where the head is in space, filled with perilymph

Depolarization

• Both auditory and balance work by mechanoreceptors

• Hair is pulled to open the channel and $K^+$ rushes in (Not $Na^+$

• Balance is different in that hair cells bend one way causing depolarization and the opposite way causes hyperpolarization