Sensory Receptors and Their Functions

Sensory Receptors

• Sensory receptors are specialized cells that transmit information about the type, location, and intensity of a stimulus.
• Each sensory receptor responds to a specific type of stimulus.
• Each sensory neuron has a receptive field, an area where it responds to stimuli.
• The brain interprets the intensity of a sensation based on the number of nerve fibers firing.

Classification of Receptors

• Chemoreceptors: Respond to chemicals.
• Mechanoreceptors: Respond to changes in position, pressure, stretch, or vibration.
• Thermoreceptors: Respond to temperature changes.
• Nociceptors: Respond to tissue damage.
• Photoreceptors: Respond to light.

Adaptation

• The firing frequency of a nerve slows down when a stimulus is continuous, leading to a diminished sensation.
• This is known as adaptation.

Proprioceptors

• Proprioceptors are specialized receptors found in muscles, joints, and tendons.
• They provide information about body movement and orientation.

The General Senses

• The general senses include pain, pressure, touch, stretch, and temperature.
• Receptors are widely distributed in the skin, muscles, tendons, joints, and viscera.

Pain

• Nociceptors or pain receptors are free nerve endings that transmit pain impulses to the brain.
• They are found in the skin, mucous membranes, and most organs.
• Fast pain fibers produce a sharp, localized pain.
• Slow pain fibers produce a dull, aching pain.
• Referred pain occurs when pain originating in a deep organ is perceived as coming from the body's surface.

The Special Senses

• The special senses include taste, smell, hearing, equilibrium, and vision.
• Receptors are grouped together or clustered in specialized organs.

Taste (Gustation)

• Taste is caused by chemicals contacting taste buds.
• Taste buds are found on the tongue, in the lining of the mouth, and on the soft palate.
• The four primary tastes are salty, sweet, sour, and bitter.
  • Umami, a "meaty" taste caused by amino acids, is a potential fifth category.
• All regions of the tongue can detect all tastes, but some areas are more sensitive to certain tastes.

Smell (Olfaction)

• The sense of smell is located within the nasal cavity.
• Receptor cells are neurons that gather to form the olfactory nerve (cranial nerve I).

Hearing

• Ears are responsible for hearing and balance.
• The ear comprises the outer, middle, and inner ear.

Outer Ear

• The auricle (pinna) funnels sound into the auditory canal.
• The auditory canal leads to the eardrum (tympanic membrane).
• The canal secretes earwax (cerumen).

Middle Ear

• Contains the three auditory ossicles: malleus, incus, and stapes.
• The ossicles transmit sound from the outer ear to the inner ear.
• The oval window connects the middle ear to the inner ear.
• The eustachian tube connects the middle ear to the nasopharynx, helping to equalize pressure.

Inner Ear

• Within the temporal bone, contains the bony labyrinth filled with perilymph and lined with the membranous labyrinth filled with endolymph.
• The bony labyrinth has three structures: semicircular canals, vestibule, and cochlea.
• The Organ of Corti, located within the cochlea, is the hearing sense organ.
• The cochlea contains hair cells and the tectorial membrane.

How Hearing Works

• Sound waves vibrate the eardrum.
• The ossicles transmit these vibrations to the oval window.
• Vibrations in the perilymph stimulate the hair cells in the Organ of Corti.
• Nerve impulses travel along the cochlear nerve to the auditory cortex in the temporal lobe.

Hearing Loss

• Conductive hearing loss is caused by interference with vibration transmission to the inner ear (e.g., fluid, earwax, ossicle fusion).
• Sensorineural hearing loss is caused by damage to the hair cells in the Organ of Corti, typically from exposure to loud noises.

Balance (Equilibrium)

• The vestibule and semicircular canals of the inner ear are responsible for balance.
• The semicircular canals lie at right angles to each other, allowing them to be stimulated by different movements.
• The utricle and saccule, located in the vestibule, also contribute to balance.