Ch 16: Nervous System Senses
Tactile Disc
-Expanded nerve terminal that synapses with Merkel cell -Sensitive to fine touch
16.1 Introduction to Sensory Receptors
16.1 Introduction to Sensory Receptors
16.1 b General Structure of Sensory Receptors
16.1 b General Structure of Sensory Receptors
16.1a General Function of Sensory Receptors
16.1a General Function of Sensory Receptors
16.1c Sensory Information Provided by Sensory Receptors
16.1c Sensory Information Provided by Sensory Receptors
16.1d Sensory Receptor Classification
16.1d Sensory Receptor Classification
16.5b Hearing
16.5b Hearing
Conjunctiva
A specialized stratified columnar epithelium termed the conjunctiva (kon-jŭnk-tī′vă) forms a continuous, transparent lining over the anterior surface of the sclera ("white") of the eye (the ocular conjunctiva) and the internal surface of the eyelid (the palpebral conjunctiva). The junction of the ocular conjunctiva and palpebral conjunctiva is called the conjunctival fornix (fōr′niks; vault or arch). (This junction is what prevents a contact lens from moving behind the eye.)
Bulbous Corpuscle
Detects continuous deep pressure and skin distortion
End Bulb
Detects light pressure and low-frequency vibration
Tactile Corpuscle
Discriminative touch for distinguishing texture and shape of an object; light touch
Lamellated Corpuscle
Functions in coarse touch; detects continuous deep pressure and high-frequency vibration
Gustatory Discrimination and Physiology of Taste
In contrast to the large number of olfactory receptors we have in our nose, our tongue detects just five basic taste sensations: sweet, salty, sour, bitter, and umami: Sweet tastes are produced by organic compounds such as sugar or other molecules (e.g., artificial sweeteners). Salt tastes are produced by metal ions, such as sodium (Na+) and potassium (K+). Sour tastes are associated with acids in the ingested material, such as hydrogen ions (H+) in vinegar. Bitter tastes are produced primarily by alkaloids such as quinine, unsweetened chocolate, nicotine, and caffeine. Umami stimuli: Umami (u'ma-mē) is a Japanese word meaning "delicious flavor." It is a taste related to amino acids, such as glutamate and aspartate, to produce a meaty flavor.
transducers
It is because sensory receptors transduce stimulus energy to electrical energy that sensory receptors are referred to as transducers Two features are critical to allow sensory receptors to function as transducers: (1) Sensory receptors, like neurons and muscle cells, establish and maintain a resting membrane potential (RMP) across their plasma membrane (see section 4.4). (2) Sensory receptors contain modality gated channels within their plasma membranes. A modality gated channel opens in response to a stimulus other than a neurotransmitter or a voltage change at the plasma membrane. (Recall that chemically gated channels open in response to a neurotransmitter and voltage-gated channels open in response to a voltage change; see section 12.6a). The specifics for the various types of sensory receptors and the opening of their specialized modality gated channels are explained in detail throughout the later sections of this chapter.
Lens
The lens is a strong yet deformable, transparent structure. It is composed of precisely arranged layers of cells that have lost their organelles and are filled completely by a protein called crystallin, which are enclosed by a dense, fibrous, elastic capsule. The lens focuses incoming light onto the retina, and its shape determines the degree of light refraction. The suspensory (sŭs-pen′sŏ-rē; suspendo = to hang up) ligaments attach to the lens capsule at its periphery, where they transmit tension that enables the lens to change shape. The relative tension in the suspensory ligaments is altered by relaxation and contraction of the ciliary muscles in the ciliary body. When we view objects greater than 20 feet away, the ciliary muscles relax, the ciliary body moves away from the lens, and so the tension on the suspensory ligaments increases. This constant tension causes the lens to flatten (figure 16.14a). This flattened shape of the lens is the "resting" position of the lens.
External Ear
The most visible portion of the external ear is a skin-covered, elastic cartilage-supported structure called the auricle (aw′ri-kl; auris = ear), or pinna (pin′ă; wing). The auricle is funnel-shaped, and it both protects the entry into the ear and directs sound waves into the bony tube called the external acoustic meatus (or external auditory meatus; see section 8.2b). This canal, which is about 2.5 centimeters (1 inch) in length and 7.5 millimeters (0.3 inch) in diameter, extends slightly superiorly from the lateral surface of the head to the tympanic membrane. The narrow external opening in the external acoustic meatus prevents large objects from entering and damaging the tympanic membrane. Near its entrance, fine hairs help guard the opening. Deep within the canal, ceruminous glands produce a waxlike secretion called cerumen (sĕ-rū′men; cera = wax), which combines with dead, sloughed skin cells to form earwax. This material may help reduce infection within the external acoustic meatus by impeding microorganism (e.g., bacteria) growth. The tympanic (tim-pan′ik; tympanon = drum) membrane, or eardrum, is a funnel-shaped membrane (approximately 1 centimeter in diameter) composed of fibrous connective tissue sandwiched between two epithelial sheets. It serves as the boundary between the external and middle ear. The tympanic membrane vibrates when sound waves hit it, and its vibrations provide the means for transmission of sound wave energy from the external ear to the middle ear. Pain associated with trauma to the tympanic membrane is transmitted to the brain along sensory neurons within both the vagus and trigeminal nerves
accommodation
The process of making the lens more spherical to view close-up objects is called accommodation (ă-kom′ŏ-dā′shŭn; accommodo = to adapt) (figure 16.14b). Accommodation is controlled by autonomic motor neurons of the parasympathetic division that extend within the oculomotor nerve
olfactory epithelium
The sensory receptor organ for smell is the olfactory epithelium. This epithelium lines the superior region of the nasal cavity, covering both the inferior surface of the cribriform plate and superior nasal conchae of the ethmoid bone (see figure 8.12a). The olfactory epithelium is composed of three distinct cell types (figure 16.5): Olfactory receptor cells (also called olfactory neurons), which detect odors Supporting cells (also called sustentacular cells), which sustain the olfactory receptor cells Basal cells, which function as neural stem cells to continually replace olfactory receptor cells
Five cranial nerves innervate the eye
These include the optic nerve (CN II), which is a sensory nerve that relays input from the retina when it is stimulated by light, and the trigeminal nerve (CN V), which relays sensations from the cornea. Then there are three primarily motor nerves that relay motor output to the eye muscles. The oculomotor nerve (CN III) innervates four of the six extrinsic eye muscles (see figure 11.7) to control eye movement and the intrinsic eye muscles (i.e., the iris and ciliary muscles) (see figure 16.10); the trochlear nerve (CN IV) and abducens nerve (VI) each innervate an extrinsic eye muscle.
Cells of the Neural Layer
Three distinct layers of neurons form the neural layer: photoreceptor cells, bipolar cells, and ganglion cells. The outermost layer of cells in the neural layer is composed of photoreceptor (phot = light) cells, which contain pigment molecules that react to light energy. The two types of photoreceptor cells are rods, which have a rod-shaped outer portion and function in dim light, and cones, which have a cone-shaped outer portion and function in high-intensity light and in color vision
Tactile Receptors
are the most numerous type of sensory receptor (figure 16.2). They are mechanoreceptors located in the skin and mucous membranes. The dendritic endings that compose these sensory receptors are either unencapsulated or encapsulated
optic disc
contains no photoreceptors. This is where axons of the ganglion cells extend from the back of the eye as the optic nerve (figure 16.12a). It is commonly called the blind spot because it lacks photoreceptor cells, and no image forms there.
Free Nerve Ending
dendrites that lack any obvious structural specialization; used to generate sensations of warmth, coolness, pain, tickle, and itch
Proprioceptors
detect body and limb movements and include only the somatic sensory receptors within joints, muscles, and tendons.
Eyelashes
extend from the free margins of the eyelids and help prevent particulate matter from entering the eye. Sensory receptors associated with the base of an eyelash trigger the blink reflex when the eyelash is touched.
ora serrata
is a jagged margin between the photosensitive posterior region of the retina and the nonphotosensitive anterior region of the retina. This nonphotosensitive portion continues anteriorly to cover the ciliary body and the posterior aspect of the iris
receptive field
is the area within which the dendritic endings of a single sensory neuron are distributed. The concept of a receptive field and its significance is most clearly shown with a comparison of receptive fields within the skin (figure 16.1). Note the relative amount of area that sensory neurons of the skin are distributed in two different regions of the body—the skin of the fingertips and the skin of the upper back. The size of the receptive field will determine the ability of the CNS to identify the exact location of a stimulus. A small receptive field provides us with the ability to identify the stimulus location more specifically. In contrast, a large receptive field allows us to determine only the general region of the stimulus.
Root Hair Plexus
of sensory nerves surrounds the base of each hair follicle and detects the movement of the shaft.
End bulbs
or Krause bulbs, are dendritic endings of sensory neurons ensheathed in connective tissue. They are located both in the dermis of the skin and in the mucous membranes of the Page 617oral cavity, nasal cavity, vagina, and anal canal. End bulbs are tonic receptors that detect light pressure stimuli and low-frequency vibration
Unencapsulated tactile receptors
simply dendritic endings of sensory neurons with no protective covering. The three types of unencapsulated receptors are free nerve endings, root hair plexuses, and tactile discs.
16.2 The General Senses
16.2 The General Senses
16.2a Tactile Receptors
16.2a Tactile Receptors
16.2b Referred Pain
16.2b Referred Pain
16.3 Olfaction and Gustation
16.3 Olfaction and Gustation
16.3a Olfaction: The Sense of Smell
16.3a Olfaction: The Sense of Smell
16.3b Gustation: The Sense of Taste
16.3b Gustation: The Sense of Taste
16.4 Visual Receptors
16.4 Visual Receptors
16.4a Accessory Structures of the Eye
16.4a Accessory Structures of the Eye These structures include the extrinsic eye muscles, eyebrows, eyelids, eyelashes, conjunctiva, and lacrimal glands. The extrinsic eye muscles include six skeletal muscles that are attached externally to each eye and function in eye movement. These muscles are described in detail in section 11.3b
16.4b Eye Structure
16.4b Eye Structure
16.4c Physiology of Vision: Refraction and Focusing of light
16.4c Physiology of Vision: Refraction and Focusing of Light
16.4d Physiology of Vision: Phototransduction
16.4d Physiology of Vision: Phototransduction
16.4e Visual Pathways
16.4e Visual Pathways
16.5 Hearing and Equilibrium Receptors
16.5 Hearing and Equilibrium Receptors
16.5a Ear Structure
16.5a Ear Structure
16.5c Auditory Pathways
16.5c Auditory Pathways
16.5d Equilibrium and Head Movement
16.5d Equilibrium and Head Movement
Papillae of the Tongue
On the dorsal surface of the tongue are epithelial and connective tissue elevations called papillae (pă-pil′ē; papula = a small nipple), which are of four types: filiform, fungiform, foliate, and vallate (figure 16.6a, b): Filiform (fil′i-fōrm; filum = thread) papillae are short and spiked; they are distributed on the anterior two-thirds of the tongue surface. These papillae do not house taste buds and thus, have no role in gustation. Instead, their bristlelike structure serves a mechanical function; they assist in detecting texture and manipulating food.Page 621 Fungiform (fŭn′ji-fōrm; mushroom-shaped) papillae are blocklike projections primarily located on the tip and sides of the tongue. Each contains only a few taste buds. Foliate (fō′lē-āt; leaflike) papillae are not well developed on the human tongue. They extend as ridges on the posterior lateral sides of the tongue and house only a few taste buds during infancy and early childhood. Vallate (val′āt; vallo = to surround) papillae, or circumvallate papillae, are the least numerous (about 10 to 12) yet are the largest papillae on the tongue. They are arranged in an inverted V shape on the posterior dorsal surface of the tongue. Each papilla is surrounded by a deep, narrow depression. Most of our taste buds are housed within the walls of these papillae along the side facing the depression.
sensation
Only nerve signals that reach the cerebral cortex of the brain result in our conscious awareness. A stimulus that we are consciously aware of is called a sensation characteristics include its modality, location, intensity, and duration
gustation
Our sense of taste, called gustation (gŭs-tā′shŭn; gusto = to taste), occurs when we come in contact with the taste-producing molecules and ions of what we eat and drink (called tastants). Gustatory cells are chemoreceptors located within taste buds on the tongue and soft palate. The tongue and soft palate also house mechanoreceptors and thermoreceptors to provide us with information about the texture and temperature of our food, respectively.
Tarsal glands
Several glands are associated with the eyelids and eyelashes. Tarsal glands, which are sebaceous glands located within the tarsal plates of the eyelid, release an oily secretion at the edge of the eyelid. Both a sebaceous gland and a modified sweat gland are located at the base of each eyelash. These glands contribute to the gritty, particulate material often noticed around the eyelids after waking.
adaptation
The CNS is able to determine the duration of stimulus because all sensory receptors become less sensitive to a constant stimulus and initiate a progressive decrease in nerve signals. This decrease in sensitivity to a continuous stimulus is called adaptation. However, the rate of decrease is different for the various types of sensory receptors. This difference in adaption is used to categorize sensory receptors as either tonic receptors or phasic receptors. Tonic receptors demonstrate limited adaptation. In response to a constant stimulus, tonic receptors continuously generate nerve signals and only slowly decrease the number relayed to the CNS. Examples of tonic receptors include sensory receptors within the inner ear that determine head position and proprioceptors in the joints and muscles that provide information of where your body is in space. In addition, all pain receptors are tonic receptors so as to provide the motivation to address the cause of the pain and hopefully eliminate it so that the pain will stop. In comparison, phasic receptors exhibit rapid adaptation to a constant stimulus. Phasic receptors generate nerve signals only in response to a new (or changing) stimulus and quickly decrease the number of nerve signals relayed to the CNS. Examples include the deep pressure receptors that sense the increased pressure when we first sit down in a chair. We are immediately aware of the pressure increase wherever our body contacts the chair. But soon, we do not notice this pressure because adaptation has occurred in these receptors. You may have experienced adaptation after placing your glasses on the top of your head and then forgetting that they were there. It is advantageous for us to not be continuously bombarded by this type of sensory information.
Cochlea
The cochlea is a snail-shaped, spiral chamber within the bone of the inner ear. Figure 16.26a depicts how this chamber "wraps" approximately 2.5 times around a spongy bone axis called the modiolus (mō-dī′ō-lŭs; hub of a wheel), giving the cochlea its snail-shaped appearance. The membranous labyrinth called the cochlear duct is housed within the cochlea. The roof of the cochlear duct is formed by the vestibular membrane, and the floor is formed by the basilar membrane (figures 16.26b and 16.27). These membranes partition the bony labyrinth of the cochlea into two smaller chambers on either side of the cochlear duct; both are filled with perilymph. The chamber adjacent to the vestibular membrane is the scala vestibuli (vestibular duct), and the chamber adjacent to the basilar membrane is the scala tympani (tympanic duct). The scala vestibuli and scala tympani merge at the helicotrema (hel′i-kō-trē′mă; helix = spiral, trema = hole) at the apex of the cochlea
ear
The ear is the organ that detects both sound and movements of the head. These stimuli are transduced into nerve signals that are transmitted by the vestibulocochlear nerve (CN VIII) (see section 13.9), resulting in the sensations of hearing and equilibrium.
Fibrous Tunic
The external layer of the eye wall is called the fibrous tunic, or external tunic. It is composed of the posterior sclera and the anterior cornea. Most of the fibrous tunic (the posterior five-sixths) is the tough sclera (sklĕr′ă; skleros = hard), a part of the outer layer that is called the "white" of the eye. It is composed of dense irregular connective tissue. The sclera provides for eye shape, protects the eye's delicate internal components, and serves as an attachment site for extrinsic eye muscles. Posteriorly, the sclera is continuous with the optic nerve sheath, which is an extension of dura mater that surrounds the optic nerve. ("Bloodshot" eyes occur with vasodilation of the scleral blood vessels, which become visible through the transparent conjunctiva.) The cornea (kōr′nē-ă) is a convex, transparent structure that forms the anterior one-sixth of the fibrous tunic; its convex shape refracts (bends) light rays coming into the eye. The cornea is composed of an inner simple squamous epithelium, a middle layer of collagen fibers, and an outer stratified squamous epithelium, called the corneal epithelium. (Think of the cornea as a collagen protein sandwich with epithelial layers as the bread.) The cornea contains no blood vessels. Nutrients and oxygen are supplied to the internal epithelium of the cornea by aqueous humor within the anterior cavity of the eye, whereas the surface corneal epithelium receives its oxygen and nutrients from lacrimal fluid. The cornea merges with the sclera at its outer edge; this region is called the limbus (lim′bŭs), or the corneal scleral junction. The corneal epithelium forming the external portion of the cornea is continuous with the ocular conjunctiva that covers the sclera. Thus, the entire eye is covered with an epithelium.
Eye Structure
The eye is an almost spherical organ that measures about 2.5 centimeters (1 inch) in diameter. Most of the eye is receded into the orbit of the skull (figure 16.10), a space also occupied by the lacrimal gland, extrinsic eye muscles, numerous blood vessels, and the cranial nerves that innervate the eye and other structures in the orbit. Orbital fat cushions the posterior and lateral sides of the eye (see figure 16.8b), providing support and protection and facilitating oxygen and nutrient delivery by the blood through its associated blood vessels.
Retina
The internal layer of the eye wall, called the retina (ret′i-nă; rete = a net) also is known as the internal tunic or neural tunic. It is composed of two layers: an outer pigmented layer and an inner neural layer (figure 16.12). The pigmented layer is immediately internal to the choroid and attached to it. Two primary functions are associated with the pigmented layer. It provides vitamin A for the photoreceptor (light-detecting) cells of the neural layer and absorbs extraneous light to prevent it from scattering within the eye (a function it shares with the choroid). The inner neural layer (or neural retina) houses all of the photoreceptor cells and their associated neurons. This layer of the retina is responsible for vision by absorbing light rays and converting them into nerve signals that are transmitted to the brain.
Middle Ear
The middle ear contains an air-filled tympanic cavity (figure 16.24). Medially, a bony wall separates the middle ear from the inner ear. Two membrane-covered openings are located within this wall: the oval window and round window (discussed in detail in section 16.5b). Inferiorly, the auditory tube (also called the pharyngotympanic tube or Eustachian tube), which is approximately 3.5 centimeters (1.5 inches) in length, serves as a passageway that extends from the middle ear into the nasopharynx (portion of upper throat posterior to the nasal cavity; see figure 23.5). This tube is normally closed. Air movement through this tube occurs as a result of chewing, yawning, and swallowing, which equalize pressure on either side of the tympanic membrane—allowing the tympanic membrane to vibrate freely. The tympanic cavity, auditory tube, and nasopharynx are lined with a continuous mucous membrane. Middle ear infections result when infectious agents (e.g., a cold virus) move from the nasopharynx through the auditory tube into the middle ear The tympanic cavity of each middle ear houses the three smallest bones of the body, the auditory ossicles (os′i-kl) (see section 8.3). These three bones, which are positioned between the tympanic membrane and Page 643the oval window, are, from lateral to medial, the malleus (hammer), the incus (anvil), and the stapes (stirrup). (You can remember the order with the acronym MIS.) The malleus (mal′ē-ŭs), with its long handle and expanded end, resembles a large hammer in shape; the long handle has contact with the tympanic membrane. The incus (ing′kŭs), which is approximately triangular in shape and resembles an anvil, is the middle auditory ossicle. The stapes (stā′pēz), composed of an arch and a plate of bone, resembles a stirrup on a saddle. Its cylindrical, disclike footplate fits into the oval window. The ossicles are anchored by various small ligaments to the surrounding structures. Two tiny skeletal muscles, the tensor tympani (attached to the malleus) and the stapedius (attached to the stapes), are located within the middle ear. These muscles reflexively restrict ossicle movement when loud sounds occur (including when we are speaking) and thus protect the sensitive sensory receptors within the inner ear. This reflexive response that involves contraction of the tensor tympani and stapedius to loud sounds takes approximately 40 milliseconds and thus is not able to protect the Page 644sensory receptors from blasts of sounds, as occurs with a gunshot. For this reason, loud blasts of sound are especially damaging to the sensory receptors within the inner ear.
Vascular Tunic
The middle layer of the eye wall is the vascular tunic, also called the uvea (ū′vē-ă; uva = grape). The vascular tunic houses an extensive array of blood vessels, lymph vessels, and the intrinsic muscles of the eye. It is composed of three distinct regions; from posterior to anterior, they are the choroid, the ciliary body, and the iris. The choroid (kōr′oyd) is the most extensive and posterior region of the vascular tunic, and it is composed of areolar connective tissue that houses both an extensive network of capillaries and melanocytes (cells that produce melanin pigment). Two primary functions are associated with the choroid. Its vast network of blood vessels supplies oxygen and nutrients to the retina (inner adjacent layer of the eye, which contains photoreceptors), and melanin produced by its Page 625melanocytes absorbs extraneous light to prevent it from scattering within the eye. The ciliary (sil′ē-ar-ē; cilium = eyelid) body is located immediately anterior to the choroid and is composed of both a ciliary muscle and ciliary processes. The ciliary muscle is a ring of smooth muscle. Extending from the ciliary muscle to the lens are suspensory ligaments, which anchor the lens. Relaxation and contraction of the ciliary muscles change the tension on the suspensory ligaments, thereby altering the shape of the lens. The ciliary processes contain capillaries that secrete aqueous humor (both functions of the ciliary body are discussed in detail in section 16.4b). The most anterior region of the vascular tunic is the iris (ī′ris; rainbow), which is the colored portion of the eye. The iris is composed of two layers of smooth muscle fibers, melanocytes, and an array of vascular and nervous structures. In the center of the iris is an opening called the pupil (pū′pil), which allows light to enter the eye to reach the retina. The iris controls pupil size, or diameter—and thus the amount of light entering the eye—using its two smooth muscle layers (figure 16.11). The sphincter pupillae (pyū-pil′ē) muscle (or pupillary constrictor) is arranged in a pattern that resembles concentric circles around the pupil. This muscle contracts (and the pupil becomes smaller) when stimulated by visceral motor Page 626neurons of the parasympathetic division of the ANS that are within the oculomotor nerve (CN III). In comparison, the dilator pupillae muscle (or pupillary dilator) is organized in a radial pattern extending peripherally through the iris. This muscle contracts (and the pupil becomes larger) when stimulated by neurons of the sympathetic division of the ANS (see section 15.7b). Only one set of these smooth muscle layers can contract at a time. The pupillary reflex is the ability of the iris to change the size of the pupil in response to varying amounts of light. When stimulated by bright light, the pupillary reflex involves the relaying of sensory input from the photoreceptors of the eye to the brain, which initiates nerve signals along the parasympathetic division fibers of the oculomotor nerve to stimulate the sphincter pupillae muscle to contract, which decreases pupil diameter. When stimulated by low light levels, the pupillary reflex ultimately involves initiating nerve signals along sympathetic division fibers to stimulate the dilator pupillae muscle to contract, which increases pupil diameter. This reflex is tested if brain trauma is suspected
eyelids
also called the palpebrae (pal-pē′brē; sing., palpebra), form the protective covering over the surface of the eye. Each eyelid is formed primarily by a fibrous core (the tarsal plate), the orbicularis oculi muscle (which closes the eyelid), and a thin covering of skin. A muscle associated only with the upper eyelid is the levator palpebrae superioris muscle, which pulls the upper eyelid to "open the eye." The space between the open eyelids is the palpebral (pal′pĕ-brăl) fissure (or eyeslit). The eyelids are joined at the medial and lateral palpebral commissures, or canthi. At the medial commissure is a small, reddish body called the lacrimal caruncle (kar′ŭng-kl; caro = flesh)
Sensory receptors
are components of the nervous system that provide us with information about our external and internal environments. Here we describe the general function and structure of sensory receptors, the type of information they provide, and how the various types of sensory receptors are classified.
Taste buds
are cylindrical sensory receptor organs containing cells that have the appearance of an onion (figure 16.6c, d). Each taste bud is composed of three distinct cell types: Gustatory cells (also called gustatory receptors), which detect tastants (taste-producing molecules and ions) in our food Supporting cells that sustain the gustatory cells Basal cells, which function as neural stem cells to continually replace the relatively short-lived gustatory cells Gustatory cells are regenerated every 7 to 9 days by basal cells within the taste bud. This process decreases with age, and our sensitivity to taste also decreases. Beginning at about age 50, our ability to distinguish between different tastes declines.
Amacrine (am′ă-krin) cells
are positioned between the bipolar and ganglion cells and help process and integrate electrical signals between bipolar and ganglion cells. The electrical signals are either action potentials or graded potentials (see section 12.9a). Only the amacrine and ganglion cells in the retina produce action potentials; the other cells generate graded potentials.
Horizontal cells
are sandwiched between the photoreceptor and bipolar cells in a thin web. These horizontal cells regulate and integrate the electrical signals sent from the photoreceptor cells to the other cell layers
eyebrows
are slightly curved rows of thick, short hairs at the superior edge of the orbit along the supraorbital ridge. They function in both nonverbal communication associated with facial expressions and prevention of sweat from dripping into the eyes.
Root hair plexuses
are specialized dendritic endings of sensory neurons that form a weblike sheath around hair follicles in the reticular layer (deeper layer) of the dermis. Any movement or displacement of the hair changes the arrangement of these dendritic endings, initiating nerve signals. These phasic receptors quickly adapt; thus, although we feel the initial contact of a long-sleeved shirt on our arm hairs when we put on the garment, our conscious awareness subsides immediately until we move and the root hair plexuses are restimulated.
Free nerve endings
are the least complex of the tactile receptors and reside closest to the surface of the skin, usually in the papillary layer (superficial layer) of the dermis (see section 6.1b). Often, some branches extend into the deepest epidermal strata and terminate between the epithelial cells. Free nerve endings are also located in mucous membranes. These tactile receptors primarily detect temperature and pain stimuli, but some also detect light touch and pressure. Free nerve endings can be either tonic receptors (adapt slowly) or phasic receptors (adapt quickly).
Encapsulated tactile receptors
dendritic endings of sensory neurons that are wrapped either by connective tissue or by connective tissue and specialized glial cells called neurolemmocytes (previously called Schwann cells; see section 12.4b). Encapsulated tactile receptors include end bulbs, lamellated corpuscles, bulbous corpuscles, and tactile corpuscles.
Exteroceptors
detect stimuli from the external environment. Exteroceptors include the somatic sensory receptors of the skin and mucous membranes, as well as the receptors of the special senses. All of these types of sensory receptors respond to a stimulus that is outside of the body.
Interoceptors
detect stimuli from within our internal environment. Interoceptors include the visceral sensory receptors within the wall of internal organs and blood vessels. Interoceptors keep our CNS informed about the changes that are occurring within our bodies.
ear is partitioned into three distinct anatomic regions
external, middle, and inner (figure 16.23). The external ear is located mostly on the outside of the body, and both the middle ear and inner ear are housed within the petrous part of the temporal bone
macula lutea
is a rounded, yellowish region just lateral to the optic disc (figure 16.13). Within the macula lutea is Page 628a depressed pit called the fovea centralis (fō′vē-ă sen′tră′lis; fovea = pit, centralis = central), which contains the highest proportion of cones and almost no rods. This pit is the area of sharpest vision; when you read the words in your text, they are precisely focused here. Although the other regions of the retina also receive and interpret light rays, no other region can focus as precisely as can the fovea centralis because of its high concentration of cones. The remaining most extensive region of the retina is called the peripheral retina, which contains primarily rods and functions most effectively in low light.
Aqueous humor
is a transparent, watery fluid that circulates within the anterior cavity (figure 16.15). It is continuously produced by the ciliary processes. The circulation of aqueous humor provides nutrients and oxygen to both the avascular cornea (specifically, its inner epithelium) and the lens. Blood plasma (see section 18.2) is filtered across the walls of capillaries of ciliary processes and enters the posterior chamber to form aqueous humor. (This process is similar to the formation of cerebrospinal fluid by the choroid plexuses within the ventricles of the brain; see section 13.2c.) The aqueous humor circulates from the posterior chamber through the pupil and into the anterior chamber. Aqueous fluid drains from the anterior chamber into a circular canal at the limbus called the scleral venous sinus (previously called the canal of Schlemm). This fluid then drains into nearby veins. Thus, as with cerebrospinal fluid, aqueous humor is produced from capillaries, circulates, and then enters the venous circulation. Normally, the rate of formation by the ciliary processes is equal to the drainage into the scleral venous sinus; thus, a normal intraocular pressure is maintained. Glaucoma results from the blockage of aqueous humor drainage
Lacrimal Apparatus
is associated with each eye; it produces, collects, and drains lacrimal fluid (figure 16.9). Lacrimal fluid contains water, sodium ions, antibodies (see section 22.8), and an antibacterial enzyme called lysozyme. This Page 624fluid lubricates the anterior surface of the eye to reduce friction from eyelid movement; continuously cleanses and moistens the eye surface; helps prevent bacterial infection; and provides oxygen and nutrients to the corneal epithelium The production and movement of lacrimal fluid occurs as follows (figure 16.9): A lacrimal gland, which is about the size and shape of an almond and located within the superolateral depression of each orbit, continuously produces lacrimal fluid that drains through short ducts to the eye surface. Blinking, which occurs about 15 to 20 times per minute, but less frequently when we are focusing on something such as reading a book, "washes" the lacrimal fluid over the eyes. Gradually, the lacrimal fluid is transferred to the lacrimal caruncle at the medial surface of the eye. The lacrimal fluid drains into the lacrimal puncta (pungk′tă; sing., punctum; to prick), which are two small openings on the superior and inferior side of the lacrimal caruncle. (If you examine the lacrimal caruncle within your own eye, each punctum appears as a "hole.") Each lacrimal punctum has a lacrimal canaliculus (kan-ă-lik′yū-lūs; small canal) that drains lacrimal fluid into a rounded lacrimal sac. The fluid drains from the lacrimal sac into a nasolacrimal duct. This duct drains lacrimal fluid into the nasal cavity, where it mixes with mucus and then moves into the pharynx (throat) and is swallowed. Excess production of lacrimal fluid produces tears.
Phototransduction
is the converting (or transducing) of light energy into an electrical signal. Photoreceptor cells (rods and cones) are the specific cells within the neural layer of the retina that engage in phototransduction. We first describe the anatomic details of photoreceptor cells (and other cells of the neural layer of the retina) and then discuss the process of phototransduction and the initiating of nerve signals that are sent to the brain.
Olfaction
is the sense of smell, whereby volatile molecules (called odorants) must be dissolved in the mucus in our nasal cavity to be detected by chemoreceptors. We use this sense to sample our environment for information about the food we will eat, the presence of other individuals in the room, or potential danger (e.g., spoiled food, smoke from a fire) Compared to many other animals, our olfactory ability is much less sensitive and not as highly developed. Consequently, we do not rely as greatly on olfactory information to find food or communicate with others. Yet we do have the ability to distinguish one odor among thousands of different ones, a capability that we may appreciate as we walk through a garden of flowers.
Referred pain
occurs when sensory nerve signals from certain viscera are perceived as originating not from the organ, but from somatic sensory receptors within the skin and skeletal muscle (see section 14.5a). Numerous somatic sensory neurons and visceral sensory neurons conduct nerve signals on the same ascending tracts within the spinal cord (figure 16.3). As a result, the somatosensory cortex in the brain (see section 13.3c) is unable to accurately determine the actual source of the stimulus, and thus the stimulus may be localized incorrectly.
Convergence
of the eyes is the voluntary contraction of the extrinsic eye muscles to move the eyes medially. (The extreme case of this is when your eyes are cross-eyed such as occurs when you try to focus on your finger a few inches from your eyes.) This positions the eyes so that the image of the object being viewed is directed onto the fovea centralis. Individuals with extrinsic eye muscles that are weaker in one eye than in the other may be unable to converge the eyes and as a result will have diplopia (di-plō′pē-ă) or double vision.
Bulbous corpuscles
or Ruffini corpuscles, are dendritic endings of sensory neurons ensheathed within connective tissue that are housed within the dermis and subcutaneous layer. They are tonic receptors that detect both continuous deep pressure and distortion in the skin.
modality
or form of a stimulus is provided by a given type of sensory receptor relaying sensory input along designated sensory neurons to specific regions of the CNS. For example, the sensory receptors of the eye (the retina) initiate nerve signals along the optic nerve to the occipital lobe (visual cortex), and sensory receptors of the ear (spiral organ) initiate nerve signals along the cochlear branch of the vestibulocochlear nerve to the temporal lobe (auditory cortex). In comparison, baroreceptors within the aorta send nerve signals along the vagus nerve to the cardiovascular center within the brainstem as part of blood pressure regulation. The brain is like a switchboard, and it interprets the source based upon which "line" the signal arrived.
Vitreous humor
or vitreous body, is the transparent, gelatinous fluid that completely fills the posterior cavity. This permanent fluid is produced during embryonic development and helps to both maintain eye shape and support the retina to keep it flush against the back of the eye
Tactile corpuscles
previously called Meissner corpuscles, are receptors formed from highly intertwined dendritic endings of sensory neurons enclosed by modified neurolemmocytes, which are then covered with dense irregular connective tissue. They are housed within the dermal papillae (which are projections of the dermis; see section 6.1b), especially in the lips, palms, eyelids, nipples, and genitals. Tactile corpuscles are phasic receptors for discriminative touch to distinguish texture and shape of an object and for detecting light touch.
Tactile discs
previously called Merkel discs, are flattened dendritic endings of sensory neurons that extend to tactile cells (Merkel cells), which are specialized epithelial cells located in the stratum basale (deepest layer) of the epidermis. These discs function as tonic receptors for light touch. (Note that tactile cells are the only specialized tactile receptor cells; the remaining tactile receptors are simply the dendritic endings of sensory neurons
lamellated corpuscles
previously called Pacinian corpuscles, are large, leaf-shaped tactile receptors composed of several dendritic endings ensheathed with an inner core of neurolemmocytes and outer concentric layers of connective tissue. They are phasic receptors found deep within the reticular layer of the dermis of the skin; in the hypodermis of the palms of the hands, soles of the feet, breasts, and external genitalia; and in the walls of some organs. The structure and location of lamellated corpuscles allow them to function in coarse touch, and sensing continuous deep-pressure and high-frequency vibration stimuli.
General sense
receptors are distributed throughout the body and are simple in structure. The receptors for general senses are subdivided into two categories based upon their location in the body and include somatic sensory receptors and visceral sensory receptors. Somatic sensory (or somatosensory) receptors are tactile receptors housed within both the skin and mucous membranes, which line the nasal cavity, oral cavity, vagina, and anal canal. These sensory receptors monitor Page 613several types of stimuli, including texture of an object, pressure, stretch, vibration, temperature, and pain. Somatic sensory receptors are also within joints, muscles, and tendons and include joint receptors, muscle spindles, and Golgi tendon organs, respectively. These detect stretch and pressure relative to position and movement of the skeleton and skeletal muscles. A Golgi tendon organ (see figure 14.22), for example, monitors stretch of a tendon when a muscle contracts. Visceral sensory receptors are located in the walls of the viscera (internal organs) and blood vessels. These sensory receptors detect stretch in the smooth muscle within the walls of internal organs (e.g., stretch of the stomach wall), chemical changes in the contents within their lumen (e.g., a change in carbon dioxide levels in the blood), temperature, and pain. In comparison, receptors of the special senses are located only within the head and are specialized, complex sense organs. The five special senses are olfaction (smell), gustation (taste), vision (sight), hearing (audition), and equilibrium (head position and acceleration).
Because the tympanic membrane is 20 times greater in diameter than the oval window,
sounds transmitted across the middle ear are amplified 20-fold