Sensory Organs - Human Anatomy and Physiology

Sounds from the external environment are first met by the pinna which directs them towards and into the external auditory meatus (or opening of the ear). Sound travels towards the tympanic membrane or eardrum, which vibrates against the ossicles. The ossicles then transmit these vibrations to the cochlea which convert the vibrations into nerve impulses which travel to the brain through the vestibulocochlear nerve.

The internal auditory meatus is a canal within the petrous part of the temporal bone. The canal lies between the posterior cranial fossa and the inner ear. This canal provides passage through which the vestibulocochlear nerve, the facial nerve, and the labyrinthine artery pass from inside the skull to the inner ear and face. It also contains the vestibular ganglion. If a tumor were to grow in this area it would have a number of consequences including affecting taste (via chorda tympani of facial nerve), cause dizziness (via the vestibular ganglion), and cause dry eye (via facial nerve).

Chemoreceptors are used to sense taste and smell. Receptors in the nose and the mouth bind to chemicals that enter these regions. Once bound, the receptors send action potentials to the brain in order to stimulate the sensation of smell and taste. Depending on the type of receptor being bound, different sensations can arise.

Vision, hearing, and touch result from photoreceptors and mechanoreceptors. Photoreceptors in the eyes (namely rods and cones) generate electrical signals in response to light. Mechanoreceptors in the cochlea generate action potentials based on the vibrations of sound waves. Mechanoreceptors in the skin respond to pressure and other external stimuli to produce the sensation of touch.

The chorda tympani branch of the facial nerve (cranial nerve VII), which is carried by the lingual branch (of the trigeminal nerve), allow for special sensory taste fibers for the anterior two-thirds of the tongue. The general sensory innervation for the anterior two=thirds of the tongue is provided by the lingual branch of the mandibular nerve from the trigeminal nerve (cranial nerve V).

The glossopharyngeal nerve (cranial nerve IX) provides sensory and taste to the posterior one-third of the tongue. Hypoglossal nerve (cranial nerve XII) provides motor innervation for all the muscles of the tongue (except for palatoglossus which is innervated by the pharyngeal branch of the vagus nerve (cranial nerve X).

The dermatome responsible for sensation at the level of the nipple is T4.

C2 is responsible for sensation at the posterior half of the skull "cap" and C4 at the level of a low collar shirt. T10 is responsible for sensation at the level of the navel (to remember this think bellybutton...belly buTEN). L4 includes the kneecaps.

Astrocytes provide support and nutrition to neurons within the brain and spinal cord. Myelin sheath insulates the axons of some neurons which allows them to conduct impulses faster. Nerve fibers are single nerve axons that are surrounded by a sheath of connective tissue. Nerves themselves are cordlike structures that transmit nerve impulses.

C fibers are associated with chronic pain perception because they transmit "slow pain". This is due to the fibers being unmyelinated. In addition, these fibers send projections to the limbic system and the thalamus which causes the brain to develop chronic pain associations.

Both A delta and C fibers are associated with the perception of pain. This can be broken down more specifically to A delta fibers that are slightly mylinated and carry "Fast" sharp pain that is immediately felt. While C fibers carry "slow" pain due to being unmyelinated and is associated with chronic pain.

The semicircular canals detect rotation. They consist of three bony canals at right angles to each other. Each is filled with a fluid called endolymph and movement of the body causes the fluid to move. The fluid's movement against tiny hair cells within the canals allows the body to detect rotational acceleration. The cochlea is involved in hearing and the ossicles collectively transmit sound from the external environment/tympanic membrane to the cochlea.

Mechanoreceptors are associated with the perception of touch/proprioception. With this being said, free nerve endings give nociceptive sensory information to perceive pain and would not be considered mechanoreceptors, while all other options are.

Both Meissner's corpuscles and Merkel receptors are located superficially underneath the top layer of the skin, the epidermis. This allows these receptors to have a smaller receptor field where more sensitive sensory information can be picked up. To help remember this, use the following tip: "M&Ms (Meissner/Merkel) are small (small receptor fields) and lay on top (superficially) of your hand."

The gate theory suggests that mechanoreceptors inhibit nociception. This is done by the mechanoreceptors because they activate an inhibitory neuron that stops signaling of the nociceptors. This theory can apply to the human reaction to when we stub our toe, our natural reaction is to grab the hurt area which stimulates mechanoreceptors and inhibits the nociception.

Muscles spindles and GTO's are both receptors of A alpha fibers. These fibers are associated with proprioception within the body. Furthermore, muscle spindles respond to the stretch of a muscle while Golgi tendon organs respond to tension at the tendinous junctions.

Rods are located in every part of the retina except the fovea centralis, which is where the cones that allow us to see most colors and detail are located. Rods are responsible for our peripheral vision, and are very sensitive to motion. Rods do not provide very good spatial differentiation, but their sensitivity to light is essential to providing definition.

The ciliary muscle surrounds the lens in a circular fashion. When contracted, the ciliary muscle will decrease in size, which allows the lens to become more convex in shape. This allows the eye to focus on closer objects.

The iris, or pupillary sphincter, is capable of constricting and dilating the pupil. The main muscle responsible for raising the eyelid is the levator palpebrae superioris. The orbicularis oculi is responsible for closing the eyelid.