Autonomic Nervous System

Autonomic Nervous System (ANS)

Overview of ANS

• Composed of motor neurons that:

  • Innervate smooth muscle, cardiac muscle, and glands

  • Primarily affect visceras (internal organs)

  • Operate under subconscious control

Divisions of ANS

• Sympathetic Division

  • Activates the body for 'fight or flight' responses

  • Increases heart rate and redirecting blood flow to essential areas during stress or danger

• Parasympathetic Division

  • Responsible for 'rest and digest' activities

  • Conserves energy and slows heart rate after meals

Anatomical Structure of ANS

• Two Major Divisions:

  • Sympathetic:

  • Origin: Thoracolumbar (T1-L2)

  • Neurons: Short preganglionic fibers, long postganglionic fibers

  • Ganglia: Close to the vertebral column (Sympathetic chain ganglia and Collateral ganglia)

  • Parasympathetic:

  • Origin: Craniosacral (brainstem and sacral regions - S2 to S4)

  • Neurons: Long preganglionic fibers, short postganglionic fibers

  • Ganglia: Close to or within target organs

Comparison of Somatic and Autonomic Systems

• Somatic Nervous System:

  • Controls skeletal muscles; voluntary movement

  • Directly innervates effectors (muscles) with a single motor neuron releasing ACh

• ANS (Autonomic Nervous System):

  • Involuntary control of effectors; utilizes a two-neuron chain (preganglionic and postganglionic neurons):

  • Preganglionic: Releases ACh

  • Postganglionic (Sympathetic): Releases norepinephrine (NE)

  • Postganglionic (Parasympathetic): Releases ACh

Physiological Responses

• Parasympathetic Responses:

  • Maintains homeostasis; lowers blood pressure, heart rate, and breathing while increasing gastrointestinal activity

  • Example: After eating, relax and digest, blood pressure and heart rate drop

• Sympathetic Responses:

  • Mobilizes body during activity; increases heart rate, blood flow, and breathing rates

  • Example: During stress, increased heart rate and mental alertness

Ganglia Location in the ANS

• Sympathetic Ganglia:

  • Sympathetic chain ganglia (parallel to vertebral column)

  • Collateral ganglia located in front of the aorta (Celiac, Superior mesenteric, Inferior mesenteric)

• Parasympathetic Ganglia:

  • Near organs of innervation (Ciliary ganglion, Pterygopalatine ganglion, Submandibular ganglion, etc.)

Cranial Outflows of the Parasympathetic Division

• Cranial Nerves Functions:

  • CN III (Oculomotor): Constriction of pupils, focusing

  • CN VII (Facial nerve): Secretion from nasal, lacrimal, and salivary glands

  • CN IX (Glossopharyngeal): Innervates the parotid gland

  • CN X (Vagus): Decreases heart rate, constricts bronchioles, and increases digestive mobility and secretions

Key Differences in Effects

• Parasympathetic Activation Characteristics:

  • Localized, short-lived effects since ACh is quickly degraded

  • Less divergence, affecting fewer organs at a time (one or a few postganglionic neurons)

• Sympathetic Activation Characteristics:

  • Longer-lasting and widespread effects due to slower inactivation of norepinephrine

  • Greater divergence; a single preganglionic neuron can activate multiple postganglionic neurons at different levels

  • Involvement of the adrenal medulla releasing epinephrine and norepinephrine into circulation for systemic effects

This detailed understanding of the Autonomic Nervous System highlights the distinct roles and functioning of the sympathetic and parasympathetic divisions within human physiology, emphasizing the importance of both in maintaining homeostasis and responding to environmental needs.