Chemical Senses, Hearing, Equilibrium, and Peripheral Nervous System: Study Notes for Anatomy & Physiology

Chemical Senses: Smell & Taste

Introduction to Chemical Senses

The chemical senses include olfaction (smell) and gustation (taste), which detect chemical stimuli in the environment. These senses are essential for detecting food, hazards, and communicating with the environment.

Olfaction (Smell)

• Olfactory receptors are excited by airborne chemicals dissolved in the nasal mucus.

• The olfactory epithelium is located in the roof of the nasal cavity and contains the sensory cells for smell.

• Olfactory epithelium covers the superior nasal concha.

• Receptor cells are surrounded by columnar supporting cells and replaced by stem cells.

• Olfactory receptor cells are bipolar neurons.

Structure of Olfactory Epithelium:

• Apical dendrite ends in a knob with long cilia (olfactory cilia) covered by mucus.

• Axons are unmyelinated and grouped as fascicles to form filaments of the olfactory nerve (CN I), which pass through the cribriform plate to the olfactory bulb.

Physiology:

• Olfactory signals are variable and processed in the olfactory bulb, hypothalamus, amygdala, and other limbic system areas.

• Sensory afferent fibers are found in the dorsal root ganglion.

Gustation (Taste)

• Taste receptors are excited by food chemicals dissolved in saliva.

• Taste buds are the sensory organs of taste, found in papillae on the tongue.

• Types of papillae: fungiform (looks like mushroom), foliate, and circumvallate (has the most taste buds).

Structure of Taste Buds:

• Each taste bud has two types of cells: gustatory (taste) cells with microvilli (hairs) projecting through a taste pore, and basal cells (stem cells replacing taste cells).

• Microvilli are the receptor membranes.

Taste Sensations:

• Five basic taste sensations: sweet, salty, sour, bitter, and umami.

Physiology of Taste

• Chemicals dissolved in saliva enter the taste pore, activating gustatory hairs and causing graded depolarization, which leads to neurotransmitter release and action potential generation.

• Afferent fibers from the anterior 2/3 of the tongue travel via the chorda tympani (branch of facial nerve), while fibers from the posterior 1/3 travel via the glossopharyngeal nerve.

• All afferent fibers synapse in the medulla, then project to the thalamus and gustatory cortex in the insula.

Clinical Note: Losing sense of taste is less common than losing sense of smell, as taste has three nerves and smell has one.

Hearing and Equilibrium

Structure of the Ear

The ear is divided into three major areas: external, middle, and inner ear. The external and middle ear are involved in hearing only, while the inner ear is involved in both hearing and equilibrium.

External Ear

• Consists of the auricle (pinna) and external acoustic meatus.

• Auricle is the visible part of the ear; lobule is the lower part (where earrings go).

• External acoustic meatus leads to the auditory canal.

• Canal lined with skin, hairs, sebaceous glands, and ceruminous glands (producing cerumen/earwax).

Middle Ear (Tympanic Cavity)

• Contains the tympanic membrane (eardrum) and three auditory ossicles: malleus, incus, stapes.

• Middle ear wall has two openings: oval window and round (cochlear) window.

• Pharyngotympanic (Eustachian) tube connects the middle ear to the nasopharynx, equalizing pressure.

• Otitis media: inflammation/infection of the middle ear, common in children.

Inner Ear (Labyrinth)

• Consists of bony labyrinth (vestibule, cochlea, semicircular canals) and membranous labyrinth (filled with endolymph).

• Vestibule contains equilibrium receptors called maculae.

• Semicircular canals contain equilibrium receptors called cristae ampullaris.

• Cochlea contains the spiral organ (organ of Corti), the receptor for hearing.

Physiology of Hearing

• Sound is a pressure disturbance; wavelength is the distance between two crests and is constant for a particular tone.

• Frequency: number of waves passing a fixed point in a given time.

• Amplitude: subjective interpretation of sound intensity (loudness).

• Sound transmission: external acoustic meatus → tympanic membrane → ossicles → oval window → perilymph in vestibular duct → cochlear duct → tympanic duct → basilar membrane vibrates.

Organ of Corti

• Composed of supporting cells and hearing receptor cells called cochlear hair cells.

• Hair cells have stereocilia; movement leads to depolarization and neurotransmitter release.

• Impulses from cochlea travel via spiral ganglion to the cochlear branch of the vestibulocochlear nerve (CN VIII) and then to the auditory cortex in the temporal lobe.

Deafness

• Conductive deafness: damage to the outer or middle ear where sound is conducted.

• Sensorineural deafness: damage to the sensory neurons.

Equilibrium

• Vestibular receptors (maculae) respond to static equilibrium (position of head).

• Receptors in semicircular canals (cristae ampullaris) monitor dynamic equilibrium (head movements).

• Maculae respond to changes in velocity or acceleration, not constant motion.

• Vestibular fibers project to the vestibular nuclear complex in the brainstem or cerebellum.

Peripheral Nervous System: Nerves and Reflexes

Structure of Peripheral Nerves

• Nerves are parallel bundles of peripheral axons surrounded by connective tissue layers:

  • Endoneurium: around one axon

  • Perineurium: around one fascicle

  • Epineurium: around the whole nerve

• If the cell body is intact, axons of peripheral nerves can regenerate.

• Wallerian degeneration: axon and myelin sheath disintegrate after damage due to lack of nutrients; Schwann cells proliferate and form a cord for axon regrowth.

• Oligodendrocytes do not regenerate.

Cranial Nerves

• 12 pairs of cranial nerves; first two attach to forebrain, others to brainstem.

• Serve head and neck, except the vagus nerve (serves thorax and abdomen).

• CN I (Olfactory): sensory, originates from olfactory receptor cells in nasal cavity.

• CN II (Optic): sensory, fibers arise from retina to form optic nerve.

• CN III (Oculomotor): motor, controls eye movement.

• CN IV (Trochlear): motor, controls eye movement.

• CN V (Trigeminal): both, largest cranial nerve, facial sensation and chewing.

• CN VI (Abducens): motor, controls eye movement.

• CN VII (Facial): both, facial expression and taste.

• CN VIII (Vestibulocochlear): sensory, hearing and balance.

• CN IX (Glossopharyngeal): both, swallowing and taste.

• CN X (Vagus): both, serves neck, thorax, and abdomen.

• CN XI (Accessory): motor, serves neck muscles.

• CN XII (Hypoglossal): motor, serves tongue muscles.

Spinal Nerves

• 31 pairs of spinal nerves, named according to their origin:

  • 8 cervical

  • 12 thoracic

  • 5 lumbar

  • 5 sacral

  • 1 coccygeal

• Each spinal nerve connects to the spinal cord by a dorsal root (sensory) and a ventral root (motor).

• Spinal nerves pass laterally to cord and unite to form spinal nerves.

Nerve Plexuses

• Ventral rami (except T2-T12) branch and join to form nerve plexuses: cervical, brachial, lumbar, and sacral plexuses.

• Plexuses serve the limbs; most branches are cutaneous or muscular.

• Cervical plexus: serves neck and diaphragm (phrenic nerve).

• Brachial plexus: serves upper limb (axillary, musculocutaneous, radial nerves).

• Lumbar plexus: serves lower limb (femoral, obturator nerves).

• Sacral plexus: serves buttocks, lower limb, pelvic structures (sciatic nerve).

Plexus	Main Nerves	Regions Served
Cervical	Phrenic	Neck, diaphragm
Brachial	Axillary, musculocutaneous, radial	Upper limb
Lumbar	Femoral, obturator	Anterior thigh, medial thigh
Sacral	Sciatic, tibial, common fibular	Posterior thigh, leg, foot

Dermatomes

• Dermatome: area of skin innervated by the cutaneous branches of a single spinal nerve.

Reflexes

• Spinal reflexes are somatic reflexes mediated by the spinal cord.

• Muscle spindles provide information about muscle length and are involved in stretch reflexes.

• All stretch reflexes are monosynaptic and ipsilateral.

• Flexor or withdrawal reflexes are polysynaptic.

• Crossed extensor reflexes are contralateral and polysynaptic.

Reflex Type	Synapses	Example
Monosynaptic	One	Stretch reflex (e.g., patellar reflex)
Polysynaptic	More than one	Flexor withdrawal, crossed extensor reflex

Additional info: Academic context and terminology have been expanded for clarity and completeness. Tables have been inferred and constructed to summarize plexus and reflex types.