Autonomic Nervous System and Special Senses: Study Notes

Chapter 14 – The Autonomic Nervous System and Homeostasis

Autonomic Reflexes and Visceral Reflex Arcs

The autonomic nervous system (ANS) regulates involuntary physiological functions through reflexes called autonomic or visceral reflexes. These reflexes are essential for maintaining homeostasis by controlling smooth muscle, cardiac muscle, and glandular activity.

• Autonomic Reflex: An involuntary response mediated by the ANS, typically involving internal organs.

• Visceral Reflex Arc Components:

  • Receptor

  • Sensory neuron

  • Integration center (often in CNS)

  • Motor neuron (preganglionic and postganglionic)

  • Effector (smooth muscle, cardiac muscle, or gland)

Somatic vs. Autonomic Nervous Systems

The somatic nervous system (SNS) and autonomic nervous system (ANS) differ in their structure, function, and control mechanisms.

• Overall Functions: SNS controls voluntary movements; ANS regulates involuntary functions.

• Types of Sensory Information: SNS receives input from skin, muscles, and joints; ANS receives input from internal organs.

• Control Center: SNS is controlled by the cerebral cortex; ANS is regulated by the hypothalamus, brainstem, and spinal cord.

• Motor Pathway: SNS uses a single neuron from CNS to effector; ANS uses two neurons (preganglionic and postganglionic).

• Neurotransmitters: SNS uses acetylcholine; ANS uses acetylcholine and norepinephrine.

• Effectors: SNS targets skeletal muscle; ANS targets smooth muscle, cardiac muscle, and glands.

Divisions of the Autonomic Nervous System

The ANS is divided into sympathetic and parasympathetic divisions, each with distinct anatomical and functional characteristics.

• Sympathetic Division (Thoracolumbar): Prepares the body for 'fight or flight' responses.

• Parasympathetic Division (Craniosacral): Promotes 'rest and digest' activities.

Sympathetic Division

• Preganglionic Neurons: Located in the lateral horns of the thoracic and upper lumbar spinal cord (T1–L2).

• Sympathetic Chain Ganglia: Lie alongside the vertebral column; relay signals to organs above the diaphragm.

• Collateral (Prevertebral) Ganglia: Located anterior to the vertebral column; innervate organs below the diaphragm.

• Adrenal Medulla Pathway: Preganglionic fibers stimulate the adrenal medulla to release epinephrine and norepinephrine into the bloodstream, prolonging sympathetic effects.

• Degree of Response: Sympathetic responses are widespread and long-lasting due to hormone release; parasympathetic responses are more localized and short-lived.

Parasympathetic Division

• Preganglionic Neurons: Originate in the brainstem (cranial nerves III, VII, IX, X) and sacral spinal cord (S2–S4).

• Terminal Ganglia: Located near target organs; intramural ganglia are within the walls of target organs.

• Organs Affected:

  • CN III (Oculomotor): Eye muscles

  • CN VII (Facial): Salivary and lacrimal glands

  • CN IX (Glossopharyngeal): Parotid gland

  • CN X (Vagus): Heart, lungs, digestive tract

  • S2–S4: Lower digestive tract, urinary bladder, reproductive organs

Neurotransmitters and Receptors

• Cholinergic Receptors: Bind acetylcholine; include nicotinic and muscarinic receptors.

• Adrenergic Receptors: Bind norepinephrine and epinephrine; include alpha and beta receptors.

• Neurotransmitter Release:

  • Preganglionic neurons (both divisions): Acetylcholine

  • Postganglionic neurons: Sympathetic—norepinephrine (except sweat glands: acetylcholine); Parasympathetic—acetylcholine

• Receptor Locations:

  • Postganglionic neurons: Nicotinic receptors

  • Parasympathetic target tissues: Muscarinic receptors

  • Sympathetic target tissues: Alpha and beta adrenergic receptors

  • Sweat glands: Muscarinic receptors (sympathetic)

Interaction Between Parasympathetic and Sympathetic Divisions

• Dual Innervation: Most organs receive input from both divisions, allowing fine control of organ function.

• Autonomic Tone: The baseline level of activity in autonomic neurons, allowing for quick adjustments.

• Structures Without Dual Innervation: Adrenal medulla, arrector pili muscles, sweat glands, most blood vessels.

Main Effects of ANS Divisions on Organ Systems

System	Sympathetic Effect	Parasympathetic Effect
Cardiovascular	Increases heart rate, constricts blood vessels	Decreases heart rate
Respiratory	Bronchodilation	Bronchoconstriction
Digestive	Decreases secretion/motility	Increases secretion/motility
Urinary	Relaxes bladder	Contracts bladder
Integumentary	Stimulates sweat glands	No effect
Reproductive (penis)	Ejaculation	Erection
Eye (pupil)	Dilates pupil	Constricts pupil

Clinical Application: Beta Receptor Inhibitors

• Beta Blockers: Inhibit beta-adrenergic receptors, reducing heart rate and blood pressure; used to treat hypertension.

Chapter 15 – The Special Senses

Sensory Receptors and Senses

Sensory receptors detect environmental changes and convert them into electrical signals, a process called transduction. Senses are classified as general or special based on their distribution and the type of stimuli detected.

• General Senses: Widely distributed; detect touch, pain, temperature, proprioception.

• Special Senses: Localized in the head; include olfaction, gustation, vision, hearing, and equilibrium.

Olfaction (Smell)

• Olfactory Receptors: Bipolar neurons in the olfactory epithelium of the nasal cavity; detect odorants.

• Brain Areas: Olfactory bulb, olfactory cortex, limbic system.

Gustation (Taste)

• Taste Receptors: Located in taste buds on the tongue, soft palate, and pharynx.

• Five Basic Tastes: Sweet, sour, salty, bitter, umami.

• Brain Areas: Gustatory cortex in the insula, thalamus.

The Eye and Vision

Accessory Structures

• Eyebrows/Eyelids: Protect the eye

• Conjunctiva: Mucous membrane lining eyelids and covering sclera

• Lacrimal Apparatus: Produces and drains tears

Major Eye Structures

• Sclera, cornea, iris, ciliary body, choroid, suspensory ligaments, lens, retina, macula lutea, fovea centralis, optic disc

Layers of the Eyeball

• Fibrous tunic: Sclera (protection), cornea (light refraction)

• Vascular tunic: Choroid (nourishment), ciliary body (lens shape), iris (pupil size)

• Neural tunic: Retina (photoreception)

Fibrous Tunic: Sclera vs. Cornea

• Sclera: Provides structural shape and rigidity, serves as an attachment for extrinsic eye muscles, and blocks stray light.

• Cornea: Transparent, allows light entry, bends and focuses light, highly sensitive to protect the eye.

Vascular Tunic Structures

• Choroid: Delivers oxygen and nutrients, absorbs excess light.

• Ciliary Body: Alters lens shape for accommodation, secretes aqueous humor.

• Iris: Regulates pupil size and light entry.

Humors of the Eye

• Vitreous Humor: Gel-like, fills posterior segment, maintains eye shape.

• Aqueous Humor: Watery, fills anterior segment, maintains intraocular pressure.

• Aqueous Humor Pathway: Produced by ciliary body → posterior chamber → anterior chamber → drains via canal of Schlemm.

• Glaucoma: Increased intraocular pressure due to impaired drainage; can damage optic nerve.

• Corneal Transplant: Not rejected due to avascularity and immune privilege of the cornea.

Retina

• Pigmented Layer: Absorbs stray light, supports photoreceptors.

• Neural Layer Cells: Photoreceptors (rods and cones), bipolar cells, ganglion cells.

• Optic Nerve: Formed by axons of ganglion cells.

• Photoreceptors:

  • Rods: Sensitive to dim light, peripheral vision

  • Cones: Color vision, high acuity

• Retina Regions:

  • Macula Lutea/Fovea Centralis: High concentration of cones; sharpest vision.

  • Optic Disc: No photoreceptors; 'blind spot.'

• Location of Photoreceptors: Cones – macula lutea and fovea centralis; Rods – peripheral retina.

• Optic Disc as Blind Spot: Lacks photoreceptors, so no image detection occurs here.

• Macular Degeneration: Loss of central vision due to degeneration of the macula lutea.

Lens

• Structure: Biconvex, transparent, flexible.

• Function: Focuses light on retina; changes shape for accommodation.

• Cataracts: Clouding of the lens, leading to vision impairment.

Physiology of Vision

• Refraction: Bending of light by cornea, lens, and humors to focus on retina.

• Focal Points:

  • Normal (emmetropia): on retina

  • Myopia: in front of retina

  • Hyperopia: behind retina

• Accommodation: Lens changes shape to focus on near or distant objects.

• Near Object: Lens thickens, ciliary muscle contracts, suspensory ligaments relax.

• Distant Object: Lens flattens, ciliary muscle relaxes, suspensory ligaments tighten.

• Dark/Light Adaptation: Adjustment of photoreceptors to changes in light intensity.

• Color Blindness: Inherited deficiency in one or more types of cones.

Visual Pathway

• Light → photoreceptors → bipolar cells → ganglion cells → optic nerve → optic chiasm → optic tract → thalamus → primary visual cortex (occipital lobe)

Hearing and Equilibrium

Ear Structures

• External Ear: Auricle, external acoustic meatus, tympanic membrane

• Middle Ear: Malleus, incus, stapes, auditory (Eustachian) tube

• Inner Ear: Cochlea, vestibule, semicircular canals

Functions of Ear Structures

• Auricle: Collects sound waves

• External Acoustic Meatus: Channels sound to tympanic membrane

• Tympanic Membrane: Vibrates in response to sound

• Malleus, Incus, Stapes: Transmit and amplify vibrations to inner ear

• Auditory Tube: Equalizes pressure between middle ear and atmosphere

• Cochlea: Contains receptors for hearing

• Vestibule: Detects static equilibrium and linear acceleration

• Semicircular Canals: Detect rotational (angular) movements

Hearing

• Hearing Receptors: Hair cells in the organ of Corti (spiral organ) within the cochlea

• Basilar Membrane: Base for hair cells

• Tectorial Membrane: Hairs of hair cells are embedded here

• Cranial Nerve: Vestibulocochlear nerve (CN VIII)

• Properties of Sound:

  • Pitch: Determined by frequency (Hz)

  • Loudness: Determined by amplitude (dB)

• Auditory Pathway: Cochlea → cochlear nerve → brainstem → thalamus → primary auditory cortex (temporal lobe)

Hearing Disorders and Equilibrium

• Conductive Deafness: Impaired transmission of sound to inner ear (e.g., earwax, otosclerosis)

• Sensorineural Deafness: Damage to hair cells or auditory nerve (e.g., loud noise exposure)

• Equilibrium Structures:

  • Static Equilibrium: Vestibule (utricle and saccule)

  • Linear Movements: Vestibule

  • Angular Acceleration: Semicircular canals

• Motion Sickness: Mismatch between visual and vestibular input, causing nausea and dizziness.