Special Senses

Receptors are structures that detect stimuli. They range in complexity from the relatively simple dendritic ending of a sensory neuron to complex structures called sense organs whose nerve endings are associated with epithelium, connective tissue, or muscular tissue. 

Receptors in the body monitor both external and internal environmental conditions and conduct information about those stimuli to the central nervous system.

The receptive field of a receptor is the entire area through which the sensitive ends of the receptor cell are distributed. 

There is an inverse relationship between the size of the receptive field and our ability to identify the exact location of a stimulus.

If the receptive field is small, precise localization and sensitivity are easily determined. In contrast, a broad receptive field only detects the general region of the stimulus.

All receptors act as transducers, which are structures that transform the energy of one system into a different form of energy. 

Receptors may be either tonic or phasic. 

• Tonic receptors respond continuously to stimuli at a constant rate.

• Phasic receptors detect a new stimulus or a change in a stimulus that has already been applied, but over time their sensitivity decreases.

• Phasic receptors can undergo a change called adaptation, which is a reduction in sensitivity to a continually applied stimulus.

Classification of Receptors

• Receptor Distribution

  • The receptors for general senses are subdivided into two categories: 

    • Somatic receptors are housed within the body wall; they include receptors for external stimuli, including chemicals, temperature, pain, touch, proprioception, and pressure. 

    • Visceral receptors are located in the walls of the viscera; they respond to chemicals, temperature, and pressure and are sometimes also called interoceptors or visceroceptors.

  • The receptors for the special senses are located within sense organs and housed only in the head. The five special senses are gustation (taste), olfaction (smell), vision, equilibrium, and hearing (audition).

• Stimulus Origin

  • Exteroceptors detect stimuli from the external environment.

  • Interoceptors, also called visceroceptors, detect stimuli in internal organs (viscera).

  • Proprioceptors are located in muscles, tendons, and joints. They detect body and limb movements, skeletal muscle contraction and stretch, and changes in joint capsule structure.

• Modality of Stimulus

  • Chemoreceptors detect chemicals such as specific molecules dissolved in fluid in our external and internal environments,

  • Thermoreceptors respond to changes in temperature.

  • Photoreceptors are located in the eye, where they detect changes in light intensity, color, and movement.

  • Mechanoreceptors respond to touch, pressure, vibration, and stretch.

  • Baroreceptors detect changes in pressure within body structures.

  • Nociceptors respond to pain caused by either external or internal stimuli. 

    • Somatic nociceptors detect chemical, heat, or mechanical damage to the body surface or skeletal muscles.

    • Visceral nociceptors detect internal body damage within the viscera due to excessive stretching of smooth muscle, oxygen deprivation of the tissue, or chemicals released from damaged tissue.

Tactile receptors are the most numerous type of receptor. They are mechanoreceptors that react to touch, pressure, and vibration stimuli.

 They are located in the dermis and the subcutaneous layer. 

Tactile receptors range from simple, dendritic ends that have no connective tissue wrapping (called unencapsulated) to complex structures that are wrapped with connective tissue or glial cells.

Unencapsulated Tactile Receptors

• Unencapsulated receptors have no connective tissue wrapping around them and are relatively simple in structure. 

• The three types of unencapsulated receptors are:

  • Free nerve endings are terminal branches of dendrites. They are the least complex of the tactile receptors and reside closest to the surface of the skin, usually in the papillary layer of the dermis.These tactile receptors primarily detect pain and temperature stimuli, but some also detect light touch and pressure.

  • Root hair plexuses are specialized free nerve endings that form a weblike sheath around hair follicles in the reticular layer of the dermis. Any movement or displacement of the hair changes the arrangement of these branching dendrites, initiating a nerve impulse.

  • Tactile discs, previously called Merkel discs, are flattened nerve endings that function as tonic receptors for fine touch. These receptors are important in distinguishing the texture and shape of a stimulating agent. Tactile discs are associated with special tactile cells (Merkel cells), which are located in the stratum basale of the epidermis. Tactile cells exhibit a small receptive field and communicate directly with the dendrites of a sensory neuron.

Encapsulated Tactile Receptors

• Encapsulated receptors are covered either by connective tissue or glial cells. Encapsulated tactile receptors include:

  • Krause bulbs are located near the border of the stratified squamous epithelium in the mucous membranes of the oral cavity, nasal cavity, vagina, and anal canal, where they detect light pressure stimuli and low-frequency vibration.

  • Lamellated corpuscles, previously called Pacinian corpuscles, are large receptors that detect deep pressure and high-frequency vibration. The center of the receptor houses several dendritic endings of sensory neurons wrapped within numerous concentric layers of flat, fibroblast-like cells. This structure ensures that only deep-pressure stimuli will activate the receptor.

  • Ruffini corpuscles detect both continuous deep pressure and distortion in the skin. These are tonic receptors that do not exhibit adaptation; they are housed within the dermis and subcutaneous layer.

  • Tactile corpuscles, previously called Meissner corpuscles, are physically different from the unencapsulated tactile discs. Tactile corpuscles are large, encapsulatedoval receptors. They are formed from highly intertwined dendrites enclosed by modified neurolemmocytes, which are then covered with dense irregular connective tissue. Tactile corpuscles are phasic receptors for light touch, shapes, and texture. 

Our sense of taste, called gustation, permits us to perceive the characteristics of what we eat and drink.

Gustatory (taste) cells are taste receptors housed in specialized sensory organs termed taste buds on the tongue surface.

On the dorsal surface of the tongue are epithelial and connective tissue elevations called papillae, which are of four types:

• Filiform papillae are short and spiked; they are distributed on the anterior two-thirds of the dorsal tongue surface. These papillae do not house taste buds and, thus, have no role in gustation.

• Fungiform papillae are blocklike projections primarily located on the tip and sides of the tongue. They contain only a few taste buds each.

• Vallate papillae are the least numerous yet 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.

• Foliate papillae are not well developed on the human tongue. They extend as ridges on the posterior lateral sides and house only a few taste buds during infancy and early childhood.

Gustatory Discrimination

• The tongue detects five basic taste sensations: salty, sweet, sour, bitter, and umami.

Gustatory Pathway

• Primary neuron axons from gustatory cells extend to the CNS from the tongue through paired cranial nerves VII and IX.

• Primary neurons synapse in the nucleus solitarius of the brainstem.

• Secondary neurons travel from the nucleus solitarius and synapse in the thalamus.

• Tertiary neurons travel from the thalamus and terminate in the primary gustatory cortex in the insula of the cerebrum.

Olfaction is the sense of smell.

Within the nasal cavity, paired olfactory organs are the organs of smell. They are composed of several components. 

An olfactory epithelium lines the superior part of the nasal septum (an aggregate area of about 5 square centimeters). This specialized epithelium is composed of three distinct cell types:

• Olfactory receptor cells (also called olfactory neurons), which detect odors.

• Supporting cells, which sandwich the olfactory neurons and sustain and maintain the receptors. 

• Basal cells, which function as stem cells to replace olfactory epithelium components.

Internal to the olfactory epithelium is an areolar connective tissue layer called the lamina propria. 

Included with the collagen fibers and ground substance of this layer are mucin-secreting structures called olfactory glands (or Bowman glands) and many blood vessels and nerves.

Olfactory Receptor Cells

• Olfactory receptor cells are bipolar neurons that have undergone extensive differentiation and modification. 

• At the apical surface of each neuron, the neck and apical head together form a thin, knobby projection that extends into the mucus covering the olfactory epithelium. 

• Projecting from each knob into the overlying mucus are numerous thin, unmyelinated, cilia-like extensions called olfactory hairs, which house receptors for airborne molecules.

• These olfactory hairs are immobile and usually appear as a tangled mass within the mucous layer. 

• Deep breathing causes the inhaled air to mix and swirl, so both fat- and water-soluble odor molecules diffuse into the mucous layer covering the olfactory receptor cells.

• Receptor proteins on the olfactory hairs detect specific molecules. Airborne molecules dissolved in the mucous lining bind to those receptors. Depending on which receptors are stimulated, different smells will be detected.

Olfactory Discrimination

• The olfactory system can recognize as many as eight different primary odors as well as many thousands of other chemical stimuli.

•  Primary odors are those that are detectable by a large number of people, such as camphorous, fishy, malty, minty, musky, and sweaty. 

• Secondary odors are those produced by a combination of chemicals and not detected or recognized by everyone.

Olfactory Pathways

• Olfactory nerve (CN I) axons are discrete bundles of olfactory neuron axons that project through foramina in the cribriform plate and enter a pair of olfactory bulbs inferior to the frontal lobes of the brain. 

• Neurons within the olfactory bulbs project axon bundles, called olfactory tracts, to the primary olfactory cortex in the temporal lobe of the cerebrum.

Visual stimuli help us form specific detailed visual images of objects in our environment. 

The sense of vision uses visual receptors (photoreceptors) in the eyes to detect light, color, and movement.

Accessory Structures of the Eye

• The accessory structures of the eye provide a superficial covering over its anterior exposed surface (conjunctiva), prevent foreign objects from coming in contact with the eye (eyebrows, eyelashes, and eyelids), and keep the exposed surface moist, clean, and lubricated (lacrimal glands).

• Conjunctiva

  • A specialized stratified squamous epithelium termed the conjunctiva forms a continuous lining of the external, anterior surface of the eye (the ocular conjunctiva) and the internal surface of the eyelid.

  • The space formed by the junction of the ocular conjunctiva and the palpebral conjunctiva is called the conjunctival fornix.

  • The conjunctiva contains numerous goblet cells, which lubricate and moisten the eye. 

  • In addition, the conjunctiva houses numerous blood vessels that supply the avascular sclera (“white”) of the eye, as well as abundant free nerve endings that detect foreign objects as they contact the eye.

  • The eyebrows are slightly curved rows of thick, short hairs at the superior edge of the orbit along the superior orbital ridge. They function primarily to prevent sweat from dripping into the open eyes. 

  • Eyelashes extend from the margins of the eyelids and prevent large foreign objects from contacting the anterior surface of the eye.

  • The eyelids, also called the palpebrae, form the movable anterior protective covering over the surface of the eye.

  • Closing the eyelids covers the delicate anterior surface of the eyes and also distributes lacrimal fluid (tears) to cleanse and lubricate this surface. 

  • Each eyelid is formed by a fibrous core (the tarsal plate), tarsal muscles, tarsal glands, the palpebral part of the orbicularis oculi muscle, the palpebral conjunctiva, and a thin covering of skin. 

  • Tarsal glands, previously called Meibomian glands, are sebaceous glands that produce a secretion to prevent tear overflow from the open eye and keep the eyelids from adhering together. 

  • The eyelids’ free margins are separated by a central palpebral fissure.

  • The eyelids are united at medial and lateral palpebral commissures.

  • At the medial commissure is a small, reddish body called the lacrimal caruncle that houses ciliary glands. 

  • Ciliary glands are modified sweat glands that form the thick secretory products that contribute to the gritty, particulate material often noticed around the eyelids after awakening.

• Eye Structure

  • The eye is an almost spherical organ that measures about 2.5 centimeters in diameter.

  • Orbital fat cushions the posterior and lateral sides of the eye, providing support, protection, and vasculature. 

  • The hollow interior of the eye is called the vitreous chamber. 

  • It is posterior to the lens and is filled with a gelatinous, viscous fluid called the vitreous body, or vitreous humor.

  • Three principal layers form the wall of the eye: 

    • The fibrous tunic (external layer), which has an anterior cornea and a posterior sclera, helps focus light on the retina; protects and maintains the shape of the eye; and provides the attachment site for the extrinsic eye muscles (sclera).

    • The vascular tunic (middle layer) has three regions: the choroid, which distributes vascular and lymphatic vessels inside the eye; the ciliary body, which assists lens shape changes with the suspensory ligaments and secretes aqueous humor; and the iris, which controls pupil diameter.

    • The retina is composed of an outer pigmented layer and an inner neural layer that houses all of the photoreceptors and their associated neurons.

    • The three cell layers in the neural layer are (1) photoreceptors (rods and cones) that are stimulated by photons of light; (2) supporting cells and neurons that process and begin to integrate incoming visual stimuli; and (3) an inner layer of ganglion cells that conduct impulses to the brain.

    • The retina has a posterior, yellowish region called the macula lutea, which houses a concentration of cones. Vision is most acute at a depression within the center of the macula lutea called the fovea centralis.

    • The lens is a hard, deformable, transparent structure bounded by a dense fibrous, elastic capsule.

Visual Pathways

• The optic nerves formed by ganglionic axons that project from each eye through the paired optic foramina converge at the optic chiasm.

• The optic tract is the continuation of nerve fibers from the optic chiasm. Each optic tract is a composite of ganglionic axons originating from the retinas of each eye.

Receptors in the ear provide for the senses of equilibrium and hearing.

External Ear 

• The external ear is a skin-covered, cartilage-supported structure called the auricle.

• The external acoustic meatus directs sound waves to the tympanic membrane, a delicate epithelial sheet that partitions the spaces of the outer and middle ear. Its vibrations provide the means for transmission of sound wave energy to the inner ear.

Middle Ear 

• The tympanic cavity (middle ear) is an air-filled space occupied by three small auditory ossicles: the malleus, the incus, and the stapes, which transmit sound wave energy from the outer to the inner ear.

Inner Ear 

• The inner ear houses the structures for equilibrium and hearing. Specialized receptors called hair cells are housed in the membranous labyrinth, which lies within a cavernous space in dense bone called the bony labyrinth.

• Hair cells are the sensory receptors for equilibrium and hearing. When their apical surface stereocilia, and sometimes kinocilia, are displaced, a change occurs in neurotransmitter release and the firing rate of the monitoring sensory neuron.

• The saccule and utricle are membranous sacs of the vestibular complex that contain areas of hair cells called maculae, which detect acceleration or deceleration of the head in one direction.

• The semicircular canals of the vestibular complex house the membranous semicircular ducts, each with an ampulla that houses the hair cells for detecting rotational movements of the head.

• Equilibrium stimuli from the vestibule and semicircular canals are conducted through the vestibular branch of the vestibulocochlear nerve (CN VIII).

• Hearing organs are housed within the cochlea.

• Sound is perceived when impinging sound waves cause vibrations of the tympanic membrane, resulting in auditory ossicle vibrations that lead to vibrations of the basilar membrane. Movement of the basilar membrane causes stereocilia distortion, which is perceived as sound.

• Sound waves transmitted in the fluid of the inner ear are detected by hair cells of the spiral organ. Impulses are conducted through the cochlear branch of the vestibulocochlear nerve (CN VIII) to the cochlear nuclei in the brainstem.