Peripheral Sensory Nerve Endings
Peripheral nerve receptors can be divided into several categories:
Mechanoreceptors can be encapsulated or nonencapsulated. The nonencapsulated endings include:
The encapsulated mechanoreceptors include:
The eyelid contains a tarsal plate of dense connective tissue centrally. Anteriorly, the palpebral portion of the orbicularis muscle is present and over this there is keratinized stratified squamous epithelium containing hair follicles (eyelashes) and sebeaceous glands. Posteriorly, the eyelid has a reflection of the conjunctival epithelium that also covers the anterior sclera (the "white" of the eye). The conjunctiva is a low stratified squamous epithelium in which are scattered goblet cells. The mucinous secretion of the goblet cells forms part of the tear film. Meibomian glands on the posterior eyelid produce an oily secretion that becomes part of the tear film. Tears that form most of the tear film that protects the exposed sclera and cornea are produced from the lacrimal gland located at the superior lateral aspect of the eye socket.
The cornea has a superficial non-keratinized stratified squamous epithelium that can regenerate quickly. The squamous cells can migrate in hours to cover any defect, and the epithelium regrows, mainly from the periphery.
The eye is a globe that is surrounded by a tough connective tissue capsule called the sclera (or tunica fibrosa, the outer layer), which is modified anteriorly to form a light-transparent cornea. The anterior structures of the eye behind the cornea include the iris with the dilator muscle that determines the size of the pupillary opening to admit light that strikes the crystalline lens, which focuses the light. The lens is held in place by suspensory ligaments attached to the ciliary body which has muscle that contracts to change the focus. Anterior to the iris is the anterior chamber and posterior to the iris is the posterior chamber; both chambers are filled with aqueous humor that originates from the pigmented cells of the ciliary processes. This plasma-like liquid circulates from posterior to anterior chamber through the pupillary opening. The fluid is resorbed into the trabecular meshwork and canal of Schlemm which, if blocked, increases intraocular pressure and leads to glaucoma.
The uvea (also called the tunica vasculosa, or middle layer) is composed of the ciliary body, iris, and choroid. The choroid is a connective tissue layer that is highly vascular and pigmented from melanocytes within it. The choroid begins at the ora serrata anteriorly at the posterior margin of the ciliary body. Outside of the choroid is the sclera. Inside the choroid is Bruch's membrane of elastic and collagen fibers that separates the choroid from the retina.
The retina is a complex structure that covers the choroid and comprises the inner layer of the eye. The retina itself consists of many layers. The outer pigmented layer is adjacent to the sclera. Next is a layer of rods and cones. The photoreceptors of the rods and cones are loosely positioned in sleeves of microvilli of the pigmented epithelium, an arrangement which makes it possible to detach the retina with tension. Rods and cones contain the photosensitive pigment rhodopsin. Cones provide the best visual acuity, are sensitive to color, and are concentrated in an area called the fovea centralis, a depression in the macula lutea that appears as a yellowish disc on funduscopy. The rods are more numerous and sensitive to brightness.
The nerve fibers project to the inner surface of the retina and gather together at the optic disk to form the optic nerve that extends posteriorly to the brain. Since the optic disk has no rods or cones, it is a "blind spot".
Hearing and Balance
The ear is divided into the external ear, middle ear, and inner ear. The external ear has the visible pinna with skin covering elastic cartilage, which serves to gather sound waves and conduct them into the external auditory meatus, then down the auditory canal. The keratinized stratified squamous epithelium is hair bearing. In the very thin connective tissue layer below the epithelium of the auditory canal are ceruminous glands that form cerumen (ear wax). The brown color of the ear wax comes from the lipochrome pigment in cells of the ceruminous glands.
The thin tympanic membrane, which divides the external from middle year, lies at the end of the auditory canal functions like a diaphragm to oscillate when struck with sound waves. The outer aspect is covered by thin epidermis, with underlying elastic and collagen fibers, and an inner cuboidal lining. The sound wave oscillations are transmitted through the tiny bony ossicles (malleus, incus, stapes). These little bones are connected to give a lever action that amplifies the motion to increase sensitivity to sound waves. The stapes rests on the oval window, a connective tissue covered opening in the bony cochlea of the inner ear. The middle ear space is connected to the mastoid air cells and to the pharynx via the eustachion tube that helps to equalize pressure across the tympanic membrane.
The cochlea of the inner ear is one portion of the bony labyrinth that is composed of the semicircular canals, vestibule, and cochlea, all embedded in the petrous ridge of the temporal bone at the base of the skull. Within the bony labyrinth is a perilymphatic space filled with perilymph. The membranous labyrinth is suspended within the perilymph.
Two small sac-like structures of the membranous labyrinth are the utricle and the saccule, connected by a thin duct. There is a region in each structure known as the macula which contains sensory receptors. Those receptors in the saccule sense gravity and those of the utricle acceleration. The receptors of the maculae have an epithelium modified to form hair cells with overlying tiny concretions called otoliths whose movement in relation to the body provides the sensory input.
The semicircular canals have extensions from the utricle and are filled with endolymph. The three canals are oriented in three planes to provide sensory input regarding orientation of the head. Each canal has an enlarged area called an ampulla which contain the hair cell receptors. A gel called the cupula overlies the hair cells and functions similar to the otoliths.
The cochlear duct extends from the saccule and contains endolymph. The cochlea contains the organ of Corti with its hair cells that sense sound and transmit nerve impulses via the auditory nerve (8th cranial nerve). The organ of Corti contains two chambers, the scala vestibuli next to the oval window on which the stapes rests, and the scala tympani, both of which are filled with an incompressible fluid called perilymph that conducts the sound waves. The cochlear duct lies between these two chambers. Chronic exposure to loud noises will damage the hair cells, particularly those for higher pitch, and lead to deafness. If a condition such as ankylosis of the ossicles (otosclerosis) occurs, then conduction of sound is impaired--conduction deafness. If the auditory nerve is damaged, then nerve deafness results.
The receptors for the sense of smell are located in a 1 cm patch of epithelium on either side of the nasal cartilage high up in the nose. The olfactory epithelium overlies the cribriform plate of the ethmoid bone. There are bipolar neurons that serve as the sensory cells. They have supporting basal cells at the base of the olfactory epithelium and sustentacular cells at either side. The sustentacular cells have microvilli. The actual sensory endings of the bipolar neurons are bulb-like olfactory vesicles from which long, non-motile cilia extend. The axons extend through the basement membrane back through the cribriform plate and synapse with the olfactory bulbs ending of the olfactory nerves at the base of the brain on the inferior aspect of the frontal lobes. There are serous glands called Bowman's glands in the underlying submucosa, and the chemicals that impart smell are dissolved in the secretion for presentation to the cilia of the neurons.