AP II-UNIT 1 WEEK 1

Chapter 16 - Senses

Overview of Sensory Modalities

The senses play a crucial role in our interaction with the environment, allowing us to perceive and respond to various stimuli. This chapter will provide a detailed overview of key sensory modalities including hearing, equilibrium, and vision.

Hearing

Hearing is one of the most vital senses, allowing us to communicate, enjoy music, and be alerted to danger.

Anatomy of the Ear

Outer Ear:

• Auricle (Pinna): The visible part of the ear, shaped to collect sound waves effectively.

• Lobule: The soft lower part of the ear, often adorned with piercings.

• Auditory canal: A tube that channels sound waves to the eardrum.

• Tympanic membrane (eardrum): A thin membrane that vibrates in response to sound waves, initiating the hearing process.

Middle Ear:

• Ossicles: Three small bones - Malleus (hammer), Incus (anvil), and Stapes (stirrup) - that transmit sound vibrations from the tympanic membrane to the inner ear.

• Auditory (Eustachian) tube: A canal that connects the middle ear to the nasopharynx, helping to equalize pressure on both sides of the tympanic membrane.

• Tympanic cavity: The air-filled space behind the eardrum that houses the ossicles.

Inner Ear:

• Cochlea: A spiral-shaped organ responsible for converting sound vibrations into neural signals.

• Semicircular ducts: Structures that detect head rotation and assist with balance.

• Vestibule: The central part of the inner ear involved in balance.

• Cochlear nerve: Transmits auditory information from the cochlea to the brain.

• Vestibular nerve: Carries balance-related information to the brain.

Pathway of Sound

Sound waves travel through the ear as follows:

1. Funneled by the auricle into the auditory canal, causing the tympanic membrane to vibrate.

2. Vibrations are transmitted to the ossicles (Malleus, Incus, Stapes) which amplify the sound.

3. The Stapes pushes on the oval window, creating waves in the perilymph fluid of the cochlea.

4. The Basilar membrane undulates, opening potassium (K+) channels on the stereocilia of hair cells.

5. Depolarization of hair cells leads to neurotransmitter release, stimulating cochlear neurons, which send auditory signals to the brain.

Recognition of Sound

• Pitch: Determined by the frequency of sound waves, measured in Hertz (Hz). Higher frequency corresponds to higher pitch.

• Loudness: Determined by the amplitude of sound waves, measured in decibels (dB). Greater vibrations lead to higher stimulation of hair cells and increased perceptions.

Tympanic Reflex

A protective response involving the tensor tympani muscle and stapedius muscle, which contract in response to loud sounds to prevent damage to the inner ear.

Auditory Projection Pathway

Auditory signals pass through the following structures:

1. Cochlear nuclei - the first relay center in the brainstem.

2. Superior olivary nucleus - integrates signals from both ears, aiding in sound localization.

3. Inferior colliculus - processes auditory information further.

4. Thalamus - acts as a relay station before the signals reach their destination.

5. Primary auditory cortex - located in the temporal lobe, where auditory perception occurs.

Disorders Related to Hearing

Middle-Ear Infection

• Otitis media: Inflammation or infection of the middle ear, often due to bacteria or viruses. Symptoms include ear pain and fever.

• Tympanostomy: A surgical procedure wherein small tubes are inserted into the eardrum to relieve pressure and drain fluid, often used for chronic infections.

Deafness

• Conductive deafness: Caused by blockage, damage, or dysfunction in the outer or middle ear, affecting sound conduction.

• Sensorineural deafness: Results from damage to the hair cells in the cochlea or the auditory pathways, commonly caused by noise exposure, aging, or infection.

Equilibrium

Equilibrium is critical for maintaining balance and stability during movement.

Inner Ear Structures

• Vestibule: Houses saccule and utricle, which are involved in static equilibrium.

• Semicircular Ducts: Three ducts (Anterior, Lateral, Posterior) that assess angular acceleration and help with dynamic equilibrium.

Types of Equilibrium

• Static Equilibrium: Detection of head position relative to gravity, controlled by the utricle and saccule.

• Dynamic Equilibrium: Detection of motion and acceleration, both linear (utricle and saccule) and angular (semicircular ducts).

Sensory Organs

Utricle and Saccule:

• Maculae: Specialized regions within these organs containing hair cells embedded in the otolithic membrane, which detect linear acceleration and head tilt.

Detecting Angular Acceleration

• Semicircular Ducts: Located in three planes, they have hair cells in the ampullae that detect rotational movement of the head, aiding in balance.

Vestibular Pathways

Information from the vestibular system is projected to:

• Cerebellum: Coordinates balance and proprioception.

• Sensory areas: Helps integrate sensory input.

• Other brain centers: Involved in motor coordination and spatial awareness.

Motion and Equilibrium Disorders

• Vertigo: An inappropriate sense of motion often caused by dysfunction in the vestibular system, leading to dizziness and disorientation.

• Motion Sickness: A conflict between visual and vestibular signals, leading to symptoms such as nausea and dizziness. Treatment options include medications like Dramamine and Meclizine.

Vision

Vision will be discussed in subsequent sections, highlighting its complex processes and importance in daily life.