9B Homeostasis and the Autonomic Nervous System: Anatomy & Physiology I Study Notes

Homeostasis: Introduction to the Autonomic Nervous System

Overview

The autonomic nervous system (ANS) is a critical component of the human body's ability to maintain homeostasis. It regulates involuntary physiological processes, including heart rate, blood pressure, digestion, and respiratory rate, by controlling smooth muscle, cardiac muscle, and glands.

• Motor neurons of the ANS innervate smooth muscles, cardiac muscle, and glands.

• Adjustments made by the ANS ensure optimal support for normal activities.

• Operates via subconscious control; also called the involuntary nervous system or general visceral motor system.

Organization of the Nervous System

Central and Peripheral Divisions

The nervous system is divided into the central nervous system (CNS) and peripheral nervous system (PNS). The PNS includes sensory (afferent) and motor (efferent) divisions. The motor division is further subdivided into the somatic nervous system (SNS) and autonomic nervous system (ANS).

• Efferent pathway: Carries signals from CNS to effectors.

• Both SNS and ANS contain motor fibers but differ in:

  • Effectors

  • Efferent pathways and ganglia

  • Target organ responses to neurotransmitters (NTs)

Somatic Nervous System

Characteristics

The somatic nervous system controls voluntary movements by innervating skeletal muscles.

• Effectors: Skeletal muscles

• Cell body in CNS; single, thick myelinated axon travels in spinal or cranial nerves directly to skeletal muscle

• All somatic motor neurons release acetylcholine (ACh); always stimulatory

Autonomic Nervous System

Characteristics

The ANS controls involuntary functions by innervating cardiac muscle, smooth muscle, and glands.

• Effectors: Cardiac muscle, smooth muscle, glands

• Two-neuron chain:

  • Preganglionic neuron: Cell body in CNS; thin, lightly myelinated axon to ganglion

  • Postganglionic neuron: Cell body in ganglion (PNS); nonmyelinated axon extends to effector

• Preganglionic fibers release ACh; postganglionic fibers release norepinephrine (NE) or ACh; effect can be stimulatory or inhibitory depending on receptor

Divisions of the Autonomic Nervous System

Parasympathetic vs. Sympathetic

The ANS is divided into two main branches: the parasympathetic and sympathetic divisions. Most visceral organs receive dual innervation from both divisions, which usually produce opposite effects.

• Parasympathetic: Maintenance functions, conserves energy

• Sympathetic: Mobilizes body during activity

• Dual innervation: All visceral organs served by both divisions, but have opposite effects

Role of the Parasympathetic Division

Rest-and-Digest

The parasympathetic division is responsible for conserving energy and carrying out maintenance activities.

• Directs digestion, diuresis, defecation

• Promotes relaxation (e.g., reading after a meal)

• Decreases blood pressure, heart rate, and respiratory rates

• Increases gastrointestinal tract activity

• Constricts pupils; accommodates lenses for close vision

Role of the Sympathetic Division

Fight-or-Flight

The sympathetic division prepares the body for stressful or emergency situations.

• Mobilizes body during activity (exercise, excitement, emergency, embarrassment)

• Increases heart rate; causes dry mouth, cold and sweaty skin, dilated pupils

• Enhances mental clarity

• During vigorous activity:

  • Shunts blood to skeletal muscles and heart

  • Dilates bronchioles

  • Causes liver to release glucose

Key Anatomical Differences

Parasympathetic Division

• Craniosacral origin: Brain stem, sacral spinal cord

• Long preganglionic, short postganglionic fibers

• Ganglia located in or near visceral effector organ

Sympathetic Division

• Thoracolumbar origin: Thoracic, lumbar spinal cord

• Short preganglionic, long postganglionic fibers

• Ganglia lie close to spinal cord

Cooperative Effects

Some organs receive coordinated input from both divisions for specific functions.

Division	Effect on Genitalia
Parasympathetic	Vasodilation; responsible for erection of penis or clitoris
Sympathetic	Ejaculation of semen in males; reflex contraction of vagina in females

Sympathetic (Vasomotor) Tone

Blood vessels are only innervated by sympathetic fibers, which control blood pressure even at rest.

• Maintains continual state of partial constriction

• Directs blood flow to areas where needed (e.g., heart, skeletal muscles during emergency)

• Vasoconstriction of skin blood vessels minimizes bleeding during stress or exercise

Parasympathetic Tone

The parasympathetic division dominates the heart and smooth muscle of digestive and urinary tract organs.

• Slows heart rate; dictates normal activity of digestive and urinary tracts

• Activates most glands except adrenal and sweat glands

• Sympathetic division can override during stress

• Drugs that block parasympathetic responses increase heart rate and cause fecal and urinary retention

Unique Roles of the Sympathetic Division

• Adrenal medulla, sweat glands, arrector pili muscles, kidneys, and most blood vessels receive only sympathetic fibers

• Thermoregulatory responses to heat:

  • Body temperature rises: sympathetic nerves dilate skin vessels and activate sweat glands

  • Body temperature drops: blood vessels constrict

• Release of renin from kidneys: Increases blood pressure

• Metabolic effects: Increases metabolic rate, blood glucose, mobilizes fats for fuel, increases mental alertness and muscle contraction strength

Localized Versus Diffuse Effects

Division	Effect Duration	Mechanism
Parasympathetic	Short-lived, highly localized	ACh quickly destroyed by acetylcholinesterase (AChE)
Sympathetic	Longer-lasting, bodywide	NE inactivated more slowly; NE and epinephrine from adrenal medulla have prolonged effects

Control of ANS Function

Brain Stem and Spinal Cord

• Reticular formation: Direct influence over ANS

• Medullary centers: Control heart rate, blood vessel diameter, GI activity

• Midbrain: Controls pupil and lens muscles

• Spinal cord: Reflexes for defecation and urination

Hypothalamus

• Main integrative center for ANS activity

• Anterior region: parasympathetic; posterior region: sympathetic

• Controls heart activity, blood pressure, body temperature, water balance, endocrine activity

• Emotional responses (rage, fear, pleasure) via limbic system activate hypothalamus for fight-or-flight

Cerebral Cortex

• Cortical input may modify ANS subconsciously via limbic system structures on hypothalamic centers

• Voluntary cortical control of some visceral activities is possible

Disorders of the Autonomic Nervous System

Common Disorders

• Hypertension (high blood pressure):

  • Overactive sympathetic vasoconstrictor response to stress

  • Heart works harder; artery walls subject to increased wear and tear

  • Treated with adrenergic receptor-blocking drugs

• Ulcers:

  • Increased sympathetic vasoconstrictor response to stress

  • Decreases blood flow to stomach wall