Special senses

VISION:

The eyes are responsible for vision, analyzing the form, intensity, and color of light reflected from objects.

Organization of the eye:

The eye is a fluid-filled sphere that houses the lens and is surrounded by three layers:

• The fibrous tunic (outermost layer) consists of the sclera (the white of the eye), which extends to the transparent cornea at the front.
• The vascular tunic (middle layer), which provides the eye with a blood supply and nutrients, includes the choroid, ciliary body, and iris.
• The retina (innermost layer) consists of two parts,  the outer pigmented layer and the innermost neural layer containing three kinds of neurons.

Alternatively, the eye can be subdivided into the anterior and posterior segments:

• The posterior segment contains the vitreous chamber/body.
• The anterior segment is further divided (by the iris) into the anterior and posterior chambers.

Knowledge check: Label the parts of the eye

Optional Activity:  Click for an overview of MAJOR structures of the eye

Aqueous humor:

Produced in the posterior chamber, aqueous humor flows through the pupil into the anterior chamber.

Knowledge check: Trace the pathway of aqueous humor in the eye

Knowledge check: Recall the function of aqueous humor

Optics of the eye:

The cornea and lens focus the visual field image by refracting light to form an inverted and reversed miniature image on the retina.

Knowledge check: Trace the pathway of the visual signals from the retina to the brain

Vision Disorders and Diseases:

Understanding the functions of the cornea, lens, and retina is essential for recognizing how different ocular diseases impact vision.

Knowledge check: Assess your knowledge of diseases affecting vision

Eye-Opening Questions to Check Your Understanding:

1. What are the eye’s three tunics and their major subparts?
2. What are the eye’s segments and its anterior segment’s two chambers?
3. How does the eye develop in the embryo?
4. How does aqueous humor form, circulate through the anterior eye, and drain away?
5. What are the layers of the neural retina as defined by photoreceptor cells, bipolar cells, and ganglion cells?
6. What are the causes of these disease states: glaucoma, cataracts, macular degeneration, and detached retina?

HEARING and EQUILIBRIUM:

The ear consists of three main parts: the outer ear, the middle ear (which is filled with air and contains the smallest bones in the body), and the inner ear (which is responsible for hearing and balance).

Organization of the inner ear:

The inner ear, located within the temporal bone, consists of the bony labyrinth that contains the fluid-filled membranous labyrinth, a series of interconnected, epithelium-lined tubes and chambers. The labryinth is divided into the cochlea, which enables hearing, and the vestibular organs (the saccule, utricle, and semicircular ducts), which aid in balance and spatial orientation. The functions of these components are mediated by specialized hair cells that act as mechano-electrical transducers, converting the mechanical energy of detected movement and sound vibrations into electrical signals that are interpreted by the brain.

Knowledge check: Demonstrate your understanding of the inner ear’s anatomical structure

Optional Activity:  Click for an overview of MAJOR structures of the ear

Acoustics of the ear:

The ear converts sound waves into electrical signals for the brain to interpret.

Knowledge check: Test your knowledge on how the ear processes sound

Disorders and Diseases of Hearing and Equilibrium:

Understanding the ear’s anatomy and physiology is essential for diagnosing conditions that lead to hearing loss and balance disorders.

Knowledge check: Assess your knowledge of diseases affecting hearing (and balance!)

Sound Check: Additional Questions to Consider

1. Can you define the external, middle, and internal ear?
2. Can you define and distinguish the bony and membranous labyrinths of the internal ear? Perilymph versus endolymph?
3. How do the hair-cell epithelia in the internal ear monitor fluid vibrations to sense balance and hearing? That is, work out the location, structure, and functions of the cristae and maculae.
4. Can you describe the spiral organ of Corti in the cochlea and how it signals the vestibulocochlear nerve of the auditory pathway?
5. What are the causes of these disease states: congenital deafness and conductive versus sensory hearing loss?

OLFACTION (SMELL) and GUSTATION (TASTE):

Smell relies on olfactory chemoreceptors in the upper nasal cavity’s olfactory epithelium, while taste is managed by taste buds on the tongue that detect and transmit flavor information to the brain. A detailed examination of the structure and organization of these sensory systems is essential for understanding their functions.

Olfactory organization:

Smell receptors are located in the olfactory epithelium at the top of the nasal cavity, where specialized olfactory neurons act as sensory receptors. Neurons extend their axons through the cribriform plate to the olfactory bulb; they are supported by sustentacular and basal cells, which can differentiate into new neurons and support cells. Underneath the olfactory epithelium are Bowman’s glands, which produce secretions that dissolve odorants, allowing them to interact with approximately 350 types of G-protein-coupled receptors. Notably, each neuron expresses only one type of olfactory receptor, and all neurons with the same receptor type synapse at the same glomerulus in the olfactory bulb.

Knowledge check: Describe where odorant receptors are located within the olfactory system.

Gustatory organization:

The tongue is covered in papillae, with taste buds primarily located on the larger vallate papillae near the terminal sulcus, as well as on fungiform and foliate papillae. Grooves around these papillae are filled with fluid from serous salivary glands, maintaining the taste buds’ function. Each vallate papilla has about 250 taste buds made up of gustatory, supportive, and basal stem cells.

Knowledge check: Identify the five basic tastes detected by the human gustatory system.