The Special Senses: Anatomy & Physiology Chapter 14 Study Notes

The Special Senses

Overview of Sensory Systems

The human body detects and interprets environmental changes through specialized sensory receptors. These receptors initiate nerve impulses that are processed by the cerebral cortex, resulting in the experience of sensation.

• Sensory receptors detect and respond to stimuli.

• Activation of receptors initiates a nerve impulse.

• Signals are interpreted by the cerebral cortex.

• Sensation is the conscious experience of these signals.

Sensory Receptors

Distribution and Types

Sensory receptors are distributed throughout the body and are classified as general or special senses.

• General senses: Pressure, temperature, pain, touch, and sense of position (proprioception).

• Special senses: Vision, hearing, equilibrium, taste (gustation), and smell (olfaction).

Classification of Sensory Receptors

• Mechanoreceptors: Respond to movement (touch, stretch, pressure).

• Nociceptors: Respond to tissue damage (painful stimuli).

• Thermoreceptors: Respond to heat and cold.

• Proprioceptors: Detect position of body parts.

• Chemoreceptors: Respond to chemicals (e.g., taste, smell).

General Senses

Sense of Touch

Tactile corpuscles are specialized receptors for touch, found mainly in the dermis and around hair follicles. Sensitivity depends on the number of receptors present.

• Baroreceptors in large arteries monitor blood pressure and trigger regulatory responses.

Sense of Pressure

Deep touch receptors are located in subcutaneous tissues, near joints, muscles, and other deep tissues.

Sense of Pain

• Pain receptors are free nerve endings found in skin, muscles, joints, and most internal organs.

• The brain itself has no pain receptors.

Sense of Temperature

• Thermoreceptors are free nerve endings widely distributed in the skin.

• Separate receptors exist for heat and cold.

• Thermoreceptors also occur in the hypothalamus to help regulate body temperature.

Sense of Position

• Proprioceptors are located in muscles, tendons, and joints.

• They relay impulses about body part positions to the cerebellum for coordination.

Introduction to the Special Senses

Five Special Senses

• Olfaction (smell)

• Gustation (taste)

• Vision

• Equilibrium (balance)

• Hearing

Smell (Olfaction)

Olfactory Organs

Olfactory organs provide the sense of smell and are located in the nasal cavity on either side of the nasal septum. They consist of two layers:

• Olfactory epithelium: Contains olfactory receptors, supporting cells, and basal (stem) cells.

• Lamina propria: Contains areolar tissue, blood vessels, and nerves.

Olfactory Receptors and Glands

• Olfactory glands secrete fluids that coat the olfactory organ surfaces.

• Olfactory receptors are highly modified neurons that detect dissolved chemicals via odorant-binding proteins.

Olfactory Pathway

• Olfactory epithelium → Olfactory nerve fibers → Olfactory bulb → Olfactory tract → Central nervous system

Olfactory Discrimination

• Humans can distinguish thousands of chemical stimuli.

• The CNS interprets smells by the pattern of receptor activity.

• Olfactory receptors undergo considerable turnover, but their number declines with age.

Taste (Gustation)

Taste Receptors and Taste Buds

Taste (gustation) provides information about foods and liquids. Taste receptors are distributed on the tongue, pharynx, and larynx, clustered into taste buds associated with epithelial projections called papillae.

Types of Lingual Papillae

Type	Function	Taste Buds
Filiform papillae	Provide friction	None
Fungiform papillae	General taste	~5 per papilla
Circumvallate papillae	General taste	~100 per papilla

Taste Bud Structure

• Contains basal cells and gustatory (receptor) cells.

• Gustatory cells extend taste hairs through the taste pore.

• Cells survive only about 10 days before replacement.

• Monitored by cranial nerves that synapse in the solitary nucleus of the medulla oblongata, then relay to the thalamus and primary sensory cortex.

Taste Sensations

• Four primary taste sensations: Sweet, Salty, Sour, Bitter

• Additional sensations: Umami (savory, characteristic of broths and cheese; sensitive to amino acids, peptides, nucleotides) and Water (detected by water receptors in the pharynx).

Gustatory Discrimination

• Dissolved chemicals contact taste hairs and bind to gustatory cell receptors.

• Receptor cells release neurotransmitters, generating action potentials in sensory afferent fibers.

Vision: The Eye

Extrinsic Eye Muscles

• Six muscles attach to the exterior surface of the eyeball: superior, inferior, lateral, and medial rectus muscles; superior and inferior oblique muscles.

• These muscles move the eye in various directions and rotate the eye.

Nerve Supply to the Eye

• Optic nerve (II)

• Oculomotor nerve (III)

• Trigeminal nerve (V)

• Trochlear nerve (IV)

• Abducens nerve (VI)

Accessory Structures of the Eye

• Palpebrae (eyelids): Protect and lubricate the eye.

• Conjunctiva: Epithelium covering inner surfaces of eyelids (palpebral) and outer surface of eye (ocular).

Lacrimal Apparatus

• Produces, distributes, and removes tears.

• Lacrimal gland: Secretes tears containing lysozyme (antibacterial enzyme).

• Tears flow through lacrimal puncta, canaliculi, sac, and nasolacrimal duct.

Structure of the Eye

Three Layers (Tunics) of the Eyeball

Layer	Description
Sclera	Outer fibrous layer; tough, white, protective
Choroid	Intermediate vascular layer; delivers oxygen and nutrients
Retina	Deep inner layer; contains photoreceptors

Sectional Anatomy of the Eye

• Fibrous layer: Sclera and cornea

• Vascular layer (uvea): Choroid, ciliary body, iris

• Neural layer (retina): Neural and pigmented parts

Vascular Layer Functions

• Choroid: Delivers oxygen and nutrients to the retina.

• Ciliary body: Contains muscle to adjust lens shape for focusing.

• Iris: Colored part; changes pupil diameter to control light entry.

Pupillary Muscles

• Pupillary constrictor muscles form concentric circles around the pupil.

• Contraction decreases pupil diameter (increased light intensity, parasympathetic stimulation).

*Additional info: Further details on the retina, photoreceptors, and image formation are covered in subsequent slides not included here. The notes above provide a comprehensive overview of the special senses as outlined in Anatomy & Physiology Chapter 14.*