Anatomy and Physiology I Unit 15 ReviewThe Autonomic Nervous System

Overview and Structure

• The autonomic nervous system (ANS) regulates involuntary functions of the body maintains homeostasis
• Consists of two main divisions: sympathetic and parasympathetic
  • Sympathetic division activates "fight or flight" response prepares body for stressful situations
  • Parasympathetic division promotes "rest and digest" functions helps body conserve energy
• ANS innervates smooth muscle, cardiac muscle, and glands throughout the body
• Autonomic neurons are efferent (motor) neurons that carry signals away from the central nervous system (CNS) to target organs
• ANS pathways involve two neurons: preganglionic and postganglionic
  • Preganglionic neurons originate in the CNS (brain or spinal cord) synapse with postganglionic neurons in autonomic ganglia
  • Postganglionic neurons extend from the ganglia to the target organs
• Autonomic ganglia are clusters of neuronal cell bodies located outside the CNS (paravertebral or prevertebral ganglia)

Sympathetic Division

• Sympathetic division mobilizes body's resources during stress or emergency situations (exercise, emotional stress, or danger)
• Originates from thoracolumbar region of spinal cord (T1-L2)
• Preganglionic neurons are shorter than postganglionic neurons in sympathetic pathways
• Sympathetic ganglia are located close to the spinal cord (paravertebral ganglia form sympathetic chain on either side of vertebral column)
• Activates "fight or flight" response increases heart rate, dilates bronchioles, inhibits digestion, and stimulates glucose release
• Causes pupillary dilation, increased sweating, and piloerection (goosebumps)
• Releases norepinephrine (primary neurotransmitter) from postganglionic nerve endings binds to adrenergic receptors on target organs

Parasympathetic Division

• Parasympathetic division promotes "rest and digest" functions helps body conserve energy during relaxed states
• Originates from cranial nerves (III, VII, IX, X) and sacral region of spinal cord (S2-S4) termed "craniosacral outflow"
• Preganglionic neurons are longer than postganglionic neurons in parasympathetic pathways
• Parasympathetic ganglia are located near or within the target organs
• Slows heart rate, constricts bronchioles, stimulates digestion, and promotes urination and defecation
• Causes pupillary constriction, increased salivation, and erectile function
• Releases acetylcholine (primary neurotransmitter) from postganglionic nerve endings binds to muscarinic receptors on target organs
  • Exceptions: Sweat glands and adrenal medulla receive sympathetic cholinergic innervation

Neurotransmitters and Receptors

• Acetylcholine (ACh) is the primary neurotransmitter released by preganglionic neurons in both sympathetic and parasympathetic divisions
  • ACh binds to nicotinic receptors on postganglionic neurons stimulates action potentials
• Norepinephrine (NE) is the primary neurotransmitter released by postganglionic neurons in the sympathetic division
  • NE binds to adrenergic receptors (α1, α2, β1, β2) on target organs
  • Adrenal medulla releases epinephrine and norepinephrine directly into the bloodstream as hormones
• Acetylcholine (ACh) is also the primary neurotransmitter released by postganglionic neurons in the parasympathetic division
  • ACh binds to muscarinic receptors (M1-M5) on target organs
• Neuropeptides (e.g., neuropeptide Y, vasoactive intestinal peptide) co-released with primary neurotransmitters modulate synaptic transmission
• Neurotransmitters are degraded or recycled after release to terminate signaling
  • Acetylcholinesterase breaks down ACh into acetate and choline
  • Norepinephrine is recycled by reuptake into the presynaptic terminal

Autonomic Reflexes

• Autonomic reflexes are involuntary responses to stimuli that help maintain homeostasis
• Involve sensory receptors, afferent neurons, integration centers (e.g., hypothalamus, brainstem), efferent neurons, and effector organs
• Examples of autonomic reflexes:
  • Baroreceptor reflex: Maintains blood pressure by altering heart rate and vascular tone in response to changes in arterial pressure
  • Pupillary light reflex: Constricts pupils in response to bright light protects retina
  • Accommodation reflex: Adjusts lens curvature for near vision in response to visual input
  • Cough reflex: Protects airways by expelling irritants or obstructions from the respiratory tract
• Autonomic reflexes can be modulated by higher brain centers (e.g., limbic system, cerebral cortex) in response to emotional or cognitive factors

Disorders and Dysfunctions

• Autonomic dysfunction can result from damage to autonomic nerves, ganglia, or central control centers
• Examples of autonomic disorders:
  • Orthostatic hypotension: Excessive drop in blood pressure upon standing due to impaired sympathetic vasoconstriction
  • Horner's syndrome: Ptosis (drooping eyelid), miosis (constricted pupil), and anhidrosis (lack of sweating) on one side of the face due to sympathetic nerve damage
  • Diabetic autonomic neuropathy: Damage to autonomic nerves due to chronic high blood sugar affects multiple organ systems (cardiovascular, gastrointestinal, genitourinary)
  • Multiple system atrophy: Neurodegenerative disorder affecting autonomic functions, movement, and balance due to loss of neurons in the brain and spinal cord
• Autonomic disorders can be diagnosed through various tests (e.g., tilt table test, sweat test, heart rate variability analysis)
• Treatment options depend on the specific disorder may include medications, lifestyle changes, or supportive care

Clinical Applications

• Knowledge of the autonomic nervous system is essential for understanding and treating various medical conditions
• Autonomic pharmacology involves the use of drugs that target autonomic neurotransmitters and receptors
  • Sympathomimetic drugs (e.g., epinephrine, norepinephrine) mimic or enhance sympathetic activity used to treat anaphylaxis, cardiac arrest, or bronchospasm
  • Parasympathomimetic drugs (e.g., pilocarpine, bethanechol) mimic or enhance parasympathetic activity used to treat glaucoma, urinary retention, or Alzheimer's disease
  • Sympatholytic drugs (e.g., propranolol, clonidine) block or reduce sympathetic activity used to treat hypertension, anxiety, or migraine
  • Parasympatholytic drugs (e.g., atropine, scopolamine) block or reduce parasympathetic activity used to treat bradycardia, motion sickness, or overactive bladder
• Autonomic testing can help diagnose and monitor various conditions (e.g., diabetic neuropathy, Parkinson's disease, postural orthostatic tachycardia syndrome)
• Autonomic neuromodulation techniques (e.g., vagus nerve stimulation, spinal cord stimulation) are being explored as potential therapies for conditions such as epilepsy, depression, and chronic pain

Key Takeaways

• The autonomic nervous system (ANS) regulates involuntary functions maintains homeostasis through sympathetic and parasympathetic divisions
• Sympathetic division activates "fight or flight" response parasympathetic division promotes "rest and digest" functions
• ANS pathways involve preganglionic and postganglionic neurons that release neurotransmitters (acetylcholine, norepinephrine) to target organs
• Autonomic reflexes are involuntary responses to stimuli that help maintain homeostasis (e.g., baroreceptor reflex, pupillary light reflex)
• Autonomic dysfunction can result from damage to nerves, ganglia, or control centers causes disorders such as orthostatic hypotension or diabetic neuropathy
• Knowledge of the ANS is essential for understanding and treating various medical conditions through autonomic pharmacology, testing, and neuromodulation techniques
• The ANS plays a crucial role in maintaining the body's internal balance and responding to external challenges