Sensory System - University of Texas at Arlington - Human Anatomy

Exteroreceptors

Sensitive to stimuli arising outside the body. Accordingly, most of these are located at or near the body surface and include receptors for touch, pressure, pain, and temperature in the skin and most receptors of the special sense organs.

Interoceptors

Also called visceroceptors, receive stimuli from the internal viscera, such as the digestive tube, bladder, and lungs. Different types of these monitor a variety of stimuli, including changes in chemical concentration, taste stimuli, the stretching of tissues, and temperature. Their activation causes us to feel visceral pain, nausea, hunger, or fullness.

Propriceptors

Located in the musculoskeletal organs, such as skeletal muscles, tendons, joints, and ligaments. These receptors monitor the degree of stretch of these locomotory organs and send input on body movements to the CNS.

Mechanoreceptors

Respond to mechanical forces such as touch, pressure, stretch, and vibrations. One type of of these receptors, called a baroreceptor, monitors blood pressure.

Thermoreceptors

Respond to temperature changes.

Chemoreceptors

Respond to chemicals in solution (such as molecules tasted or smelled) and to changes in blood chemistry.

Photoreceptors

Respond to light, mainly located in the eye.

Nociceptors

Respond to harmful stimuli that result in pain.

Free Nerve Endings of Sensory Neurons

Located on the outside of the body, in visceral organs, and in muscles and joints. Respond to pain, hot and cold, pressure, and chemical changes. Concentrated mainly in connective tissues like tendons, the periosteum of bones found at joints, the skin, cornea, mucus membranes, and glands. One way to characterize them functionally is to say that they monitor the affective senses, those to which people have an emotional response—and people certainly respond emotionally to pain!

Itch Receptors

Free nerve endings located in the dermis which are activated by chemicals present at inflamed sites, such as histamine.

Merkel Discs (Epithelial Tactile Complexes)

Located on the basal layer of the epidermis. Responds to light pressure.  These complexes are slowly adapting mechanoreceptors.

Slowly Adapting Mechanoreceptors

Mechanoreceptors that continue to respond and send out action potentials even after a long period of continual stimulation.

Hair Follicle Receptors

Free nerve endings that wrap around hair follicles and are mechanoreceptors for light touch that monitor the bending of hairs. Unlike epithelial tactile complexes, they are rapidly adapting.

Rapidly Adapting Mechanoreceptors

Mechanoreceptors where the sensation disappears quickly even if the stimulus is maintained.

Encapsulated Nerve Endings

Receptors which consist of one or more end fibers of sensory neurons enclosed in a capsule of connective tissue. All seem to be mechanoreceptors, and their capsules serve either to amplify the stimulus or to filter out the wrong types of stimuli. These receptors vary widely in shape, size, and distribution in the body.

Meissner’s Corpuscle (Tactile Corpuscle)

Located in the dermal papillae of hairless skin, particularly the nipples, external genitalia, fingertips, and eyelids. Respond to light pressure, discriminative touch, and vibration of low frequency. They are rapidly adapting. In these receptors, a few spiraling nerve endings are surrounded by Schwann cells, which in turn are surrounded by an egg-shaped capsule of connective tissue. They perform the same “light touch” function in hairless skin that hair follicle receptors perform in hairy skin. 

Lamellar Corpuscles (Pacinian Corpuscles)

Located in the dermis and hypodermis, as well as the periosteum, tendons, ligaments, and joint capsules. Most abundant on fingers, soles of feet, external genitalia, and nipples. They are rapidly adapting receptors that are best suited to monitor vibration, an on/off pressure stimulus. These corpuscles are large enough to be visible to the unaided eye—about 0.5–1 mm wide and 1–2 mm long. In section, this corpuscle resembles a cut onion: Its single nerve ending is surrounded by up to 60 layers of flattened Schwann cells, which in turn are covered by a capsule of connective tissue.

Bulbous Corpuscle (Ruffini Endings)

Located deep in the dermis, hypodermis, and joint capsules. contain an array of nerve endings enclosed in a thin, flattened capsule. Like lamellar corpuscles, they respond to pressure and touch. However, they adapt slowly and thus can monitor continuous pressure placed on the skin as well as stretch sensations.

Proprioceptors

Encapsulated nerve endings that monitor stretch in the locomotory organs.

Muscle Spindles

Located in the skeletal muscles, particularly those in the extremities. They measure the changing length of a muscle as that muscle contracts and is stretched back to its original length. An average muscle contains some 50 to 100 muscle spindles, which are embedded in the perimysium between the fascicles. Structurally, each spindle contains several modified skeletal muscle fibers called intrafusal muscle fibers surrounded by a connective tissue capsule. Intrafusal muscle fibers have fewer striations than the extrafusal muscle fibers, that is, the ordinary muscle cells outside the spindles.

Intrafusal Muscle Fibers

Modified skeletal muscle fibers surrounded by a connective tissue capsule. 

Extrafusal Muscle Fibers

Ordinary muscle fibers outside the spindles.

Anulospiral Endings

Also known as primary sensory endings. One of the sensory endings that innervates intrafusal fibers. They twirl around the noncontractile middle of the intrafusal fibers innervating the spindle center. These receptors are stimulated by the rate and degree of stretch of the muscle spindle.

Flower Spray Endings

Also known as secondary sensory endings. One of the sensory endings that innervates intrafusal fibers. They monitor the spindle ends (the only contractile parts of the spindle) and respond only to degree of stretch.

Alpha Efferent Neurons

Cause the entire muscle (extrafusal fibers) to generate contractile force and resist further stretching after being activated by the CNS which receives messages from the primary and secondary sensory endings that innervate the spindle when the muscle is stretched. This response can be initiated by a monosynaptic spinal reflex that rapidly prevents a fall; alternatively, the response can be controlled by the cerebellum regulating muscle tone, the steady force generated by noncontracting muscles to resist stretching.

Gamma Efferent Neurons

Preset the sensitivity of the spindle to stretch. When the brain stimulates the gamma motor neurons to fire, the intrafusal muscle fibers contract and become tense so that very little stretch is needed to stimulate the sensory endings, making the spindles highly sensitive to applied stretch. They are most active when balance reflexes must be razor sharp, as for a gymnast on a balance beam or a rock climber on a vertical face.

Tendon Organs (Golgi Tendon Organs)

They are proprioceptors located near the muscle-tendon junction, where they monitor tension within tendons. Each consists of an encapsulated bundle of tendon fibers (collagen fibers) within which sensory nerve endings are intertwined. When a contracting muscle pulls on its tendon, these receptors are stimulated, and their sensory neurons send this information to the cerebellum. These receptors also induce a spinal reflex that both relaxes the contracting muscle and activates its antagonist. This relaxation reflex is important in motor activities that involve rapid alternation between flexion and extension, such as running.

Joint Kinesthetic Receptors

Located in the joint capsules of the synovian joints. Proprioceptors that monitor stretch in the synovial joints. Specifically, they are sensory nerve endings within the joint capsules. Four types of joint kinesthetic receptors are present within each joint capsule: 

1. Lamellar (Puscinian) Corpuscles - Measure acceleration and rapid movement of joints 

2. Bulbous Corpuscles (Ruffini Endings) - measure the positions of nonmoving joints and the stretch of joints that undergo slow, sustained movements.

3. Free nerve endings - may be pain receptors

4. Receptors resembling tendon organs - function unknown

Send information on body movements to the cerebellum and cerebrum, as well as to spinal reflex arcs.

Taste Buds

Bulb-shaped sensory organs on and around the tongue that house the receptor cells for taste. There are 10,000 of these and a majority are located on the surface of the tongue. The rest are on the posterior region of the palate, the inner surface of the cheeks, the posterior wall of the pharynx, and the epiglottis. The cells in these receptors are replenished every 7–10 days by the division of the basal epithelial cells, replacing the gustatory epithelial cells that are scraped and burned off during eating. If an entire one is destroyed, a new one will form after its nerve ending grows back into the regenerating epithelium.

Papillae

Peglike projections of the tongue mucosa

Fungiform Papillae

Small papillae scattered over the entire surface of the tongue with taste buds on the apical surface.

Vallate Papillae

Papillae arranged in an inverted V near the back of the tongue with taste buds in the side walls.

Foliate Papilla

Papilla located on the posterolateral surface of the tongue with taste buds in the side walls

Gustatory Epithelial Cells

Generate impulses in the sensory nerve fibers that innervate them after receiving chemical signals from the microvilli at their end.

Gustatory Hairs

Long microvilli which project from the gustatory epithelial cells. They are bathed in saliva containing the dissolved molecules that stimulate taste. These molecules bind to the plasma membrane of the microvilli and induce the gustatory epithelial cells to generate impulses

Taste Pore

A pore on the surface of a taste bud which allows saliva and taste molecules to enter the taste bud.

Umami

A distinct flavor identified in the 1980s, it is elicited by a substance called glutamate that is naturally found in meat, aged cheese, and tomatoes. 

The Facial Nerve (VII)

Transmits impulses from taste receptors in the anterior 2/3 of the tongue.

Glossopharyngeal Nerve (IX)

Carries sensations from the tongue’s posterior third, as well as from the few buds in the pharynx.

Vagus Nerve (X)

Carries taste impulses from the few taste buds on the epiglottis and lower pharynx.

Gustatory Pathway 

All the sensory neurons that carry taste information synapse in a nucleus in the medulla called the solitary nucleus. From there, impulses are transmitted to the thalamus and ultimately to the gustatory area of the cerebral cortex in the insula lobe. 

Olfactory Epithelium

A sensory receptor region in the superior lining of the nasal cavity; this epithelium contains olfactory neurons that respond to odors in the air. Covers the superior nasal concha and the superior part of the nasal septum and is bathed by swirling air that has been inhaled into the nasal cavity. Sniffing draws more air across the epithelium and thus intensifies the sense of smell. A pseudostratified columnar epithelium.

Olfactory Sensory Neurons

Bipolar neurons in the olfactory epithelium. Among the few neurons in the body that undergo replacement throughout adult life.

Supporting Epithelial Cells

Columnar cells in the olfactory epithelium.

Olfactory Stem Cells

Located in the base of the olfactory epithelium, they are undifferentiated neuroepithelial cells that continually form new olfactory sensory neurons.

Olfactory Cilia

“Hairs” that radiate from the ends of dendrites of olfactory sensory neurons into the apical surface of the epithelium. At the surface these “hairs” act as the receptive structures for smell by binding odor molecules to receptor proteins located in the plasma membrane of them. Largely immotile.

Olfactory Glands

Secrete mucus which covers the apical surface of the olfactory epithelium. This mucus, which captures and dissolves odor molecules from the air, is renewed continuously, flushing away old odor molecules so that new odors always have access to the olfactory cilia.

Filaments of The Olfactory Nerve (I)

Axons of the olfactory sensory neurons that have gathered into bundles in the lamina propria and penetrate the cribiform plate of the ethmoid bone.

Olfactory Bulb

Located above the cribiform plate of the ethmoid bone. In this structure, the olfactory nerve axons branch profusely and synapse with neurons called mitral cells. 

Mitral Cells

Relay the olfactory information from the olfactory bulb to other parts of the brain such as the limbic system, where smells elicit emotions, and the primary olfactory cortex.

Glomeruli

Complex synaptic clusters

Olfactory Cortex

Processes olfactory information into a conscious perception of odor; it also sends this information through a thalamic relay to the orbitofrontal cortex, where the smells are analyzed and compared to other smells.

Eye

A spherical structure with a diameter of about 2.5 cm (1 inch). Only the anterior one-sixth of it’s surface is visible; the rest of it lies in the cone-shaped bony orbit, where it is surrounded by a protective cushion of fat. Behind it, the posterior half of the orbit contains the optic nerve, the arteries and veins to this structure, and the extrinsic muscles.

Eyebrows

Consist of coarse hairs in the skin on the superciliary arches (brow ridges of the skull). They shade the eyes from sunlight and prevent perspiration running down the forehead from reaching the eyes.

Palpebrae

Also known as eyelids.

Sclera

Opaque, white, tough outer layer that forms the posterior 5/6ths of the fibrous layer. Protects the eyeball and provides shape and a sturdy anchoring site for the extrinsic eye muscles.

Cornea

Transparent anterior sixth of eye. through which light enters the eye. This round window bulges anteriorly from its junction with the sclera. Consists of a thick layer of dense connective tissue sandwiched between a superficial corneal epithelium and a deep corneal endothelium.