Chapter 17 – Special Senses

Overview of Special Senses

The special senses include olfaction (smell), gustation (taste), vision, hearing, and equilibrium. Each sense has specialized structures and pathways for detecting and processing stimuli.

• General Purpose: To detect and interpret environmental stimuli for perception and homeostasis.

Olfaction (Smell)

Olfaction is the sense of smell, mediated by specialized receptors in the nasal cavity.

• General Function: Detection of airborne chemicals (odorants) and transmission of signals to the brain.

• Layers of Tissue in Nasal Cavity:

  • Mucous layer: Traps odorants and protects epithelial cells.

  • Basal stem cells: Regenerate olfactory receptor cells.

  • Epithelial cells: Provide structural support.

  • Bipolar cells: Olfactory receptor neurons.

  • Bowman's glands: Secrete mucus.

  • Connective tissue: Provides support and nourishment.

  • Cribriform plate: Part of the ethmoid bone; olfactory axons pass through.

  • Olfactory bulb: Receives input from olfactory receptor cells.

• Neurons in Nasal Cavity: Bipolar olfactory receptor neurons.

• Innervation: Cranial nerve I (Olfactory nerve).

• Steps of Smelling an Odorant:

  1. Odorant binds to receptor on olfactory cilia.

  2. Activation of G-protein coupled receptor initiates action potential.

  3. Signal transmitted to olfactory bulb, then to olfactory cortex.

• Receptor Location: On the surface of olfactory cilia.

• Pathway: Olfactory receptor cells → Olfactory bulb → Olfactory cortex.

• Integration: Cerebrum processes odorant information.

Gustation (Taste)

Gustation is the sense of taste, involving taste buds located on the tongue and oral cavity.

• General Function: Detection of dissolved chemicals (tastants).

• Types of Tongue Papillae:

  • Circumvallate: Large, dome-shaped; contain many taste buds.

  • Fungiform: Mushroom-shaped; scattered across tongue.

  • Filiform: Thread-like; do not contain taste buds.

  • Foliate: Leaf-like; found on lateral tongue.

• Taste Buds: Contain gustatory cells and supporting cells.

• Structure of Taste Buds:

  • Composed of gustatory (taste) cells and basal cells.

• Taste Pathways:

  • Sweet, Bitter, Umami: G-protein coupled receptors; neurotransmitter release upon tastant binding.

  • Salty and Sour: Ion channels; sodium or hydrogen ions enter cells, causing depolarization.

• Neurotransmitters: ATP, serotonin, or other neurotransmitters released depending on taste type.

Visual Sensation

Vision is the process of detecting and interpreting light through the eyes.

• General Function: Conversion of light into neural signals for image formation.

• Layers of the Eye:

  • Sclera: Outer protective layer.

  • Choroid: Vascular layer; supplies nutrients.

  • Retina: Contains photoreceptors (rods and cones).

• Cornea and Sclera Conditions:

  • Astigmatism: Irregular curvature causing blurred vision.

  • Myopia: Nearsightedness.

  • Hyperopia: Farsightedness.

• Iris Structure:

  • Contains melanocytes affecting eye color.

  • Controls pupil size via smooth muscle.

• Pupillary Muscles:

  • Dilation: Pupillary dilator muscle (sympathetic activation).

  • Constriction: Pupillary sphincter muscle (parasympathetic activation).

• Photoreceptors:

  • Rods: Sensitive to low light; black and white vision.

  • Cones: Detect color; require more light.

• Phototransduction: Conversion of light to electrical signals.

• Wavelengths: Cones detect specific wavelengths (blue, green, red).

Hearing and Equilibrium

Hearing and equilibrium are mediated by structures in the ear that detect sound waves and head position.

• Difference: Hearing detects sound; equilibrium detects balance and spatial orientation.

• Receptors: Hair cells in cochlea (hearing) and vestibular apparatus (equilibrium).

• Regions of Ear:

  • Outer ear: Collects sound waves.

  • Middle ear: Transmits vibrations via ossicles.

  • Inner ear: Contains cochlea and vestibular system.

• Bony and Membranous Labyrinth:

  • Bony labyrinth: Surrounds membranous labyrinth; contains perilymph.

  • Membranous labyrinth: Contains endolymph; houses sensory receptors.

• Cochlea:

  • Organ of Corti: Contains hair cells for sound detection.

  • Stereocilia vs. Kinocilium: Stereocilia are shorter; kinocilium is a single, longer cilium.

  • Action Potential: Bending of stereocilia opens ion channels, generating action potentials.

• Sound Wave Characteristics:

  • High/low pitch, loud/soft volume.

• Vestibule:

  • Contains utricle and saccule; detects linear acceleration.

• Semicircular Canals:

  • Detect rotational movement; hair cells clustered in ampullae.

Autonomic Nervous System (ANS)

Overview of ANS

The autonomic nervous system regulates involuntary functions, including heart rate, digestion, and respiratory rate. It is divided into sympathetic and parasympathetic divisions.

• Somatic vs. Autonomic Motor Neurons:

  • Somatic: Directly innervate skeletal muscle.

  • Autonomic: Involve a two-neuron chain (pre- and postganglionic).

• Autonomic Ganglia: Clusters of cell bodies in the PNS; preganglionic and postganglionic neurons synapse here.

Sympathetic Division

The sympathetic division prepares the body for 'fight or flight' responses.

• General Layout: Short preganglionic, long postganglionic fibers.

• Ganglia Types:

  • Sympathetic Chain Ganglia

  • Collateral Ganglia

  • Adrenal Medulla

• Neurotransmitters: Preganglionic release acetylcholine; postganglionic release norepinephrine (except sweat glands).

• Receptors: Adrenergic (norepinephrine) and cholinergic (acetylcholine).

• Effects: Increased heart rate, pupil dilation, decreased digestive activity.

Parasympathetic Division

The parasympathetic division supports 'rest and digest' functions.

• General Layout: Long preganglionic, short postganglionic fibers.

• Ganglia Types:

  • Terminal ganglia: Embedded in target organ tissues.

  • Intramural ganglia: Located within the walls of target organs.

• Neurotransmitters: Both pre- and postganglionic neurons release acetylcholine.

• Receptors: Muscarinic and nicotinic cholinergic receptors.

• Effects: Decreased heart rate, increased digestive activity, pupil constriction.

• Cranial Nerve X (Vagus): Provides ~75% of parasympathetic outflow.

Dual Innervation

Some organs receive input from both sympathetic and parasympathetic divisions, allowing fine control of physiological functions.

• Examples: Heart, lungs, digestive tract.

Key Tables

Types of Tongue Papillae and Taste Buds

Comparison of Sympathetic and Parasympathetic Divisions

Key Equations and Concepts

• Action Potential Generation: Where is voltage, is current, and is resistance. Additional info: Used to describe electrical activity in neurons.

• Phototransduction: Additional info: Sequence of events in rod/cone cells during light detection.