ANS

Autonomic Nervous System Overview

  • The autonomic nervous system (ANS) is responsible for involuntary control of the body.

  • It regulates smooth muscle, cardiac muscle, and glands, influencing visceral functions such as heart rate, blood pressure, digestion, and urination.

Division of the Autonomic Nervous System

Sympathetic Division

  • Known for the 'fight or flight' response during extreme situations such as fear or exercise.

  • Increases heart rate, blood pressure, and breathing rate.

  • Causes dilation of pupils and bronchioles while reducing GI tract motility.

  • Preganglionic neurons are found in the thoracolumbar region (T1 to L2) of the spinal cord.

  • Neurotransmitter at preganglionic to postganglionic synapse is acetylcholine; postganglionic neurons primarily release norepinephrine.

Parasympathetic Division

  • Controls routine maintenance functions associated with 'rest and digest' activities.

  • Active when the body is at rest, focusing on energy conservation and housekeeping tasks like digestion.

  • Preganglionic neurons originate from the brainstem or sacral spinal cord (S2 to S4).

  • Synapses with postganglionic neurons occur near or on target organs, releasing acetylcholine both at preganglionic and postganglionic levels.

Comparison of Sympathetic and Parasympathetic Systems

Origins

  • Sympathetic: Thoracolumbar (T1-L2)

  • Parasympathetic: Craniosacral (brainstem and sacral region)

Neuron Chain

  • Sympathetic: Two-neuron chain (preganglionic and postganglionic) with ganglia near the spinal cord; preganglionic fibers are short, postganglionic long.

  • Parasympathetic: Two-neuron chain with ganglia located near/within target organs; preganglionic fibers are long, postganglionic short.

Neurotransmitters

  • Sympathetic: Acetylcholine at preganglionic to postganglionic synapse; norepinephrine at postganglionic to target organ.

  • Parasympathetic: Both preganglionic and postganglionic neurotransmitter is acetylcholine.

Functional Overview

Sympathetic Responses

  • Mobilizes body for activity – increased heart and breathing rates, blood flow to muscles, and inhibition of non-essential functions.

Parasympathetic Responses

  • Conserves energy; promotes digestion and waste elimination.

Structure of Autonomic Pathways

  • Chain of neurons in sympathetic system contains more ganglia than parasympathetic. Sympathetic has trunk ganglia located alongside the vertebral column.

  • Preganglionic neurons exit the spinal cord via the ventral roots, traveling through the rami communicantes.

Special Pathways and Organs

  • Preganglionic fibers from T1-T4 ascend to synapse in the superior cervical ganglion for head innervation.

  • Preganglionic fibers from T5-L2 travel to prevertebral ganglia (e.g., celiac, superior mesenteric) for abdominal organs.

  • The adrenal medulla, a major sympathetic organ, secretes epinephrine (adrenaline) and norepinephrine when stimulated by sympathetic fibers.

Reflex Arcs and Sensory Input

  • General visceral sensory neurons monitor organ sensations (stretch, temperature, chemical changes).

  • Cell bodies for these neurons located in dorsal root ganglia.

  • Participate in visceral reflex arcs such as defecation and micturition reflexes.

Higher Integration Centers

  • The hypothalamus serves as the main integration center for the ANS, directing sympathetic (lateral/posterior) and parasympathetic (medial/anterior) functions.

  • The reticular formation influences autonomic responses, with nuclei in the medulla and periaqueductal gray matter.

Voluntary Influence

  • Some control over autonomic functions can be exerted by the cerebral cortex during states such as meditation.