• receive stimuli from inside the body
• involved in homeostasis and regulated by a negative feedback system. Respond to changes in:
  • blood pressure
  • temperature
  • touch
  • pain
  • stretch
  • water-salt balance

• receive stimuli from outside the body
• include those in the eye (vision), ear (hearing), tongue (taste), and nasal cavity (smell), and equilibrium

1. Sensory receptors respond to environmental stimulib y generating nerve signals after carrying out integration (the summing up of signals)
   • Stimulation causes the axon to reach its threshold
   • If it exceeds a threshold value, an action potential is produced
   • A local depolarization then occurs in the dendrites
   • on-off pulses in these neurons are itnerpreted as the appropriate type of information in the brain in specialized sensory regions for each of the senses.
2. When the nerve signals arrive at the cerebral cortex of the brain sensation (the conscious perception of stimuli) occurs.
3. The brain integrate this info with other info received from other sensory receptors (i.e. When you burn yourself on the stove and quickly remove your hand from it, the brain receives info not only from your skin, but also from your eyes, nose, etc.)

• After a while receptors cease to respond
• Pressure receptors in skin show adaptation, pain receptors do not
• complex senses like vision have more complex patterns of adaptation

• Many internal organs have pain receptors
• These receptors are stimualted by chemicals released by damaged tissue. They respond to excessive pressure, inflammation, and extremes in temperature.
• Nerve impulses from these pain receptors travel to the spinal cord and synapse with neurons also receiving impulses from the skin. They are interpreted as pain in the CNS.

• Pain originating from trauma in internal organs (stimulation of pain receptors) is sometimes felt as pain from the skin or from skeletal muscles remote from the organ.
• Believed that this is because neurons from pain receptors travel in the same spinal nerves that carry impulses from the skin.
• i.e. heart attacks often manifest themselves as pain in the left shoulder or the arm 

• Involved in reflex actions that maintain muscle tone and thereby the body's equilibrium and posture.
• Muscles contain specialized "muscle spindles" which are modified muscle cells wrapped with sensory nerve endings.

1. When the muscle relaxes and its length increases (stretching), the muscle spindle is stretched and sensory nerve signals are generated and sent to the spinal cord (CNS). (The more the muscle stretched the faster the muscle spindle sends its signals.)
2. A reflex action then occurs, motor nerve impulses from the spinal cord (CNS) results in the contraction of muscle fibers adjoining the muscle spindle, knee-jerk reflex/reaction.

• The information sent my muscle spindles to the CNS is used to maintain the body's equilibrium and posture.
• Tendons and joints have similar structures which prevent over-stretching.

• Approx. 80-90% of what we perceive as "taste" is actually due to smell receptors that have detected chemicals in the air and in the food we eat.
• Our sense of smell depends on specialized olfactory receptors, located in epithelial cells that line a/b a 1 sq. in. area on the roof of the nasal cavity. 
• Olfactory cells are modified neurons. Their cell endings, olfactory hairs (cillia), contain receptor proteins for odor molecules. They project into the mucus layer of the nasal cavity.

• Odor chemicals dissolve in the mucus.
• Chemicals bind to the receptors in the olfactory cillia.
• Causes local depolarization in the receptor cells which synapse with sensory neurons in a projection of the brain called the olfactory bulb

• Taste receptors are located ins structures called taste buds.
• Taste buds located within the numerous small projections called papillae that cover the surface of the tongue.
• Taste buds are mainly around the front, back, and sides

• taste buds open at a taste pore with supporting cells and a number of elongated taste cells that end in microvilli containing taste receptor proteins.
• When a molecule binds to taste receptor proteins, nerve signals are generated (synapse) in sensory nerve fibers that go to the brain.

• sweet
• sour
• salty
• bitter
• umami

• receptors for chemicals that give these taste sensation are located in specific parts of the tongue
• salty = various metal cations
• sweet = sugars
• sour = acid
• bitter = various alkaloids

• Some complex tastes are due to stimulating more than one of these different receptors at once.
• More sensation we interpret as tastes are due to vaporization of chemicals within food particles that diffuse to the olfactory receptors in the nose.

• eyeball is roughly spherical, 2.5 cm diameter
• two anterior fluid filled chambers containing aqueous humor (front) and vitreous humor (back) respectively separated by the lens.
• The sclera is the outer layer made of a tough white connective tissue layer that protects the eyeball.

• Inside the sclera is a thin layer of darkly pigment cells called the choroid
• The pigments absorb scattered light reducing blur and flair
• Contains blood vessels that nourish the other tissues of the eye.

• Toward the front, the choroid becomes a donut shaped iris
• Lies behind the cornea
• Contains pigments that give us eye color.
• Consists of smooth muscle Relaxing and contracting of these smooth muscles cause the pupil to open and close.
  • These smooth muscles are controlled by the autonomic division of the PNS

• During visual accomodation, the lens changes its shape to bring the image to focus on the reina.
• The shape of the lens is controlled by ciliary muscles, smooth muscle attached by fine ligaments to the lens.
• Can't focus on both near and far objects at the same time

When we focus on a distant object the ciliary muscle is relaxed, causing the suspensory ligament to put tension on the lens.

• the innermost layer making up the eyeball
• It contains three layers of cells:
  • ganglion cells (innermost layer)
  • bipolar cells 
  • rods & cons

1. The rod cells and the cone cells synapse with the bipolar cells.
2. Signals from the bipolar cells stimulate ganglion cells and integration occurs.
3. Ganglion cells' axons converge to form the optic nerve which transmits information to the visual cortex of the brain.
4. Additional integration occurs in the visual cortex.

• Rods function in dim light and are not responsive to color.
• Three different kinds of cones, each kind is sensitive to either blue, green, or red light.

• Rods and cones contain many stacked membranous disks which contain photopigments.
• They are molecules that absorb light and undergo a chemical transformation that ultimately results in an electrical stimulus.

• The visual pigment in rods is a deep purple pigment called rhodopsin
• When a rod absorbs light, rhodopsin temporarily splits into opsin and retinal. This sets in motion a cascade of reactions that cause ion channels in the plasma membrane to close, thereafter nerve signals go to the brain
• Light is absorbed in a double bond within the retinal molecule causing a cis-trans isomerization.

• If exposed to bright light, all rhodopsin in rods breaks apart.
• It takes a while for our eyes to adapt to bright light, during this time rhodopsin is recombining.

• All cones contain retinal, but have variations of opsin-like proteins which cause it to preferentially absorb red, green, or blue light
• Perception of color depends on the relative proportions of red, green, or blue cones that are stimulated and send impulses to the visual centers in the brain.

• Cones yield more detailed images than rods and are much less sensitive to light.
• Cones are concentrated in one part of the retina called the fovea which has the highest visual acuity.

• Color blindness can result from lack of pigment in particular cones (opsin-like proteins).
• Red or green deficiency most common

• Either of two cranial nerves that carry nerve impulses from the retina to the brain.
• Sensory neurons from the ganglionic cells in the retina leave the eye at the optic nerve.
• There are no rods and cones where the optic nerve exits the retina. Therefore no vision is possible in this area, "blind spot"

• The cornea does most of the bending of the light rays from far objects (greater than 20 ft away) so that they converge on the retina to form a clear image.
• Closer objects cannot be focused by the cornea, instead the lens becomes rounder by action of muscles in the ciliary body.

• myopia occurs when the eyeball is too long and rays come to a focus in front of the retina so that distant objects are blurry
• divergent (concave) lenses correct this problem

• Hyperopia occurs when the eyeball is too short and light rays from nearby objects come to a focus behind the retina 
• Converging (convex) lenses can correct this problem

when lenses become elastic in middle age

• Results from irregularities in the cornea so that not all light rays from all angles focus at the same point on the retina.

• Since close vision requires work on part of of the eye, long periods of reading, looking at computer screens, etc., can result in eyestrain
• People become progressively near-sighted

• Sound travels in waves of vibrating air molecules
• Things that create sound waves create air pressure changes that alternatively compress and decompress the surrounding air
• This disturbance propagates as a traveling wave at the speed of sound.
• The ear needs to convert these air pressure changes to nerve impulses.

• Consists of a structure called the pinna, visible outer flap, which connects to the auditory canal ending in the tymphanic membrane (eardrum).
• The opening of the auditory canal is lined with fine hairs and sweat glands that secrete earwax, a substance that helps guard the ear against the entrance of foreign materials.
• These structures direct sound waves to the eardrum, causing it to vibrate.

• An air-filled chamber within the temporal bone of the skull
• Vibrations in the tympanic membrane are transmitted via 3 tiny bones called the hammer (maleus), anvil (incus), and stirrup (stapes).
• These bones transmit vibrations from the typanic membrane to a small membrane in the inner ear called the oval window.
• Eustachian tubes connect the middle ear to the upper pharynx.
• This allows equalization of air pressure on either side of the eardrum with changes in altitude.

• Consists of a structure called the cochlea, located within the temporal bone.
• Cochlea consists of 3 fluid filled compartments arranged in a spiral: typmanic canal, vestibular canal, and cochelar canal.
• Cochlea is concerned with hearing

• located within the coachela
• hair cells, basilar membrane, and tectorial membrane is collectively called the organ of Corti.
• The cochelar canal is bounded on the bottom by the basilar membrane.
• Hair cells attached to the basilar membrane rub against the tectorial membrane above.
  • Results in generation of nerve impulses that cause hearing

• Sound waves enter the auditory canal and are transmitted to the oval window by the stirrup.
• Waves propagate through the vestibular canal and the tympanic canal.
• The round window at the end of the typanic canal bulges to absorb the pressure.
• The basilar membrane moves up & down in the response to the sound wave.
• Cilia rubbing against the tectorial membrane generate impulses that are transmitted to the brain by the auditory nerve.

• Cilia at different locations are sensitive to different frequencies.
• Near the base, high frequencies; near the tip, low frequencies.
• High volume or intensity of sound causes the basilar membrane to vibrate with greater amplitude causing more and more frequent nerve impulses from the hair cells.

• "vestibular apparatus" = semicircular canals + vestibule
• Two kinds of senses related to balance: rotational/dynamic & gravitational/static equilibrium

• semicircular canals contain structures called ampulla.
• During angular or rotational motion fluid in the semicircular canals flow over ampulla.
• This distorts gelatinious material (cupulla)
• Stimulates the hair cells embedded in a ridge-like structure called crista
• Nerve impulses sent to brain

• Hair cells in the vestibule are responsible for gravitational equilibrium.
• Utricle and saccule are similar to ampulla.
• When the body is at rest, otoliths (small CaCo₃ crystals) rest on the gelatinous material and the hair cells are not stimulated.
• If the body moves either horizontally or vertically hair cells are stimulated and nerve signals sent to the brain.

• Congennita

• Cilia in the Organ of Corti get worn away with either normal aging or continuous exposure to loud sound (>~120 dB)
• Trauma due to exposure to very loud sounds (i.e. explosions)
• Auditory nerve may degenerate. 
• Tumors, strokes may damage auditory cortex

1. Small microphone picks up sounds and transmits to a receiver.
2. Electrical impulses directly stimulate the vestibulocochlear (auditory) nerve
3. People with implants can learn to recognize many sounds
4. Helps deaf people regulate their voices and provide more cues for lip reading

stimulus: sound waves

category: mechanoreceptor

stimulus: motion

category: mechanoreceptors