Autonomic Nervous System and Special Senses: Study Guide

Autonomic Nervous System (ANS)

Overview of the ANS

The Autonomic Nervous System (ANS) is a division of the peripheral nervous system that regulates involuntary physiological processes, including heart rate, blood pressure, respiration, digestion, and sexual arousal. It operates largely below the level of consciousness and is essential for maintaining homeostasis.

• Somatic Nervous System vs. ANS: The somatic nervous system controls voluntary movements via skeletal muscles, while the ANS controls involuntary functions of smooth muscle, cardiac muscle, and glands.

• Dual Innervation: Most organs receive input from both sympathetic and parasympathetic divisions, allowing precise regulation.

Preganglionic and Postganglionic Neurons

• Preganglionic Neuron: The first neuron in the autonomic pathway; its cell body is in the CNS and its axon synapses with the postganglionic neuron in an autonomic ganglion.

• Postganglionic Neuron: The second neuron; its cell body is in the ganglion and its axon extends to the effector organ.

Sympathetic and Parasympathetic Divisions

• Sympathetic Division (Thoracolumbar): Originates from thoracic and lumbar spinal cord segments (T1-L2). Prepares the body for "fight or flight" responses.

• Parasympathetic Division (Craniosacral): Originates from brainstem (cranial nerves III, VII, IX, X) and sacral spinal cord (S2-S4). Promotes "rest and digest" activities.

Feature	Sympathetic	Parasympathetic
Origin	Thoracolumbar (T1-L2)	Craniosacral (Brainstem, S2-S4)
Ganglia Location	Near spinal cord (paravertebral)	Near or within target organs
Neurotransmitters	Acetylcholine (preganglionic), Norepinephrine (postganglionic)	Acetylcholine (both)
General Function	Fight or flight	Rest and digest

Role of the Adrenal Medulla

• The adrenal medulla acts as a modified sympathetic ganglion. Preganglionic sympathetic fibers stimulate it to release epinephrine and norepinephrine into the bloodstream, amplifying sympathetic effects.

Autonomic Ganglia

• Sympathetic Ganglia: Paravertebral (sympathetic chain) and prevertebral (collateral) ganglia.

• Parasympathetic Ganglia: Terminal ganglia located near or within target organs.

Cholinergic and Adrenergic Receptors

• Cholinergic Receptors: Bind acetylcholine. Two types:

  • Nicotinic: Found on all postganglionic neurons and adrenal medulla cells; always excitatory.

  • Muscarinic: Found on all parasympathetic target organs; can be excitatory or inhibitory.

• Adrenergic Receptors: Bind norepinephrine and epinephrine. Two main types:

  • Alpha (α) receptors

  • Beta (β) receptors

Autonomic Plexuses and Reflexes

• Autonomic Plexuses: Networks of sympathetic and parasympathetic fibers that innervate organs (e.g., cardiac, pulmonary, celiac plexuses).

• Autonomic Reflexes: Involuntary responses mediated by the ANS (e.g., baroreceptor reflex for blood pressure regulation).

Neurotransmitters: Acetylcholine and Norepinephrine

• Acetylcholine (ACh): Released by all preganglionic neurons and parasympathetic postganglionic neurons.

• Norepinephrine (NE): Released by most sympathetic postganglionic neurons.

Autonomic Tone

• Refers to the baseline level of activity in the sympathetic and parasympathetic divisions, allowing for rapid adjustments in organ function.

Sympathetic and Parasympathetic Responses

• Sympathetic: Increases heart rate, dilates pupils, inhibits digestion, stimulates glucose release.

• Parasympathetic: Decreases heart rate, constricts pupils, stimulates digestion, promotes energy storage.

Special Senses

Olfaction (Sense of Smell)

• Anatomy of Receptors: Olfactory receptors are located in the olfactory epithelium of the nasal cavity.

• Odor Threshold: The minimum concentration of a substance required to be detected as an odor.

• Adaptation: Decreased sensitivity to odors with prolonged exposure.

• Olfactory Pathway: Olfactory nerves → olfactory bulb → olfactory tract → olfactory cortex (temporal lobe).

Gustation (Sense of Taste)

• Anatomy of Taste Buds: Taste buds are located on papillae of the tongue, soft palate, and pharynx.

• Basic Taste Sensations: Sweet, sour, salty, bitter, umami.

• Gustatory Pathway: Taste receptor cells → cranial nerves VII, IX, X → medulla oblongata → thalamus → gustatory cortex.

Vision

• Accessory Structures: Eyelids, eyelashes, lacrimal apparatus, extrinsic eye muscles.

• Three Layers of the Eyeball:

  • Fibrous Tunic: Sclera and cornea.

  • Vascular Tunic: Choroid, ciliary body, iris.

  • Nervous Tunic: Retina.

• Convex and Concave Lenses: Convex lenses converge light (used to correct hyperopia), concave lenses diverge light (used to correct myopia).

• Myopia (Nearsightedness): Image focuses in front of retina; corrected with concave lenses.

• Hyperopia (Farsightedness): Image focuses behind retina; corrected with convex lenses.

• Astigmatism: Irregular curvature of cornea or lens causing blurred vision.

• Structure of Retina: Contains photoreceptors (rods and cones), bipolar cells, ganglion cells.

• Rods and Cones: Rods detect dim light; cones detect color and detail.

• Dark and Light Adaptation: Adjustment of eyes to changes in light intensity.

• Visual Pathway: Retina → optic nerve → optic chiasm → optic tract → thalamus → visual cortex.

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

• Glaucoma: Increased intraocular pressure damaging the optic nerve.

Hearing and Equilibrium

• Anatomy of the Ear:

  • External Ear: Auricle, external auditory canal, tympanic membrane.

  • Middle Ear: Auditory ossicles (malleus, incus, stapes), auditory tube.

  • Inner Ear: Cochlea (hearing), vestibule and semicircular canals (equilibrium).

• Auditory Pathway: Cochlear hair cells → cochlear nerve → brainstem → thalamus → auditory cortex.

• Physiology of Equilibrium:

  • Static Equilibrium: Sensed by the vestibule; detects head position relative to gravity.

  • Dynamic Equilibrium: Sensed by semicircular canals; detects rotational movements.

• Meniere’s Disease: Disorder of the inner ear causing vertigo, tinnitus, and hearing loss.

Example: When you stand up quickly, your autonomic reflexes adjust blood pressure to prevent dizziness (baroreceptor reflex). When you enter a dark room, your eyes undergo dark adaptation, increasing sensitivity to light.