Understanding the Autonomic Nervous System and Its Divisions

AUTONOMIC NERVOUS SYSTEM (ANS) ✓ Autonomic nervous system (ANS) largely responsible for homeostasis ✓ It is based largely on reflexes. Sensory afferent inputs and motor efferent outputs  Innervates smooth and cardiac muscles and glands ✓ It operates automatically but components can be influenced by higher-order processes AUTONOMIC VISCERAL REFLEX ARC The ANS is divided into 3 divisions ✓ Sympathetic nervous system  Parasympathetic nervous system  SYMPATHETIC & PARASYMPATHETIC Enteric nervous system ✓ Sympathetic – accelerator / Parasympathetic – brake  Opposing / antagonistic effects ✓ Organs or tissues that receive impulses from both systems have dual innervation – not all are coinnervated by both systems ✓ Complex reflex pathways are integrated by these 2 systems: ✓ Sensory receptors ✓ Afferent pathways (convey sensory impulses to brain) Regulatory networks ✓ Efferent pathways (convey messages to organs and tissues) Target organs SYMPATHETIC & PARASYMPATHETIC ✓ Both systems have sets of 2 connecting motor (efferent) neurones for conduction of impulses between the CNS and the effector organ ✓ Preganglionic neurone – cell body located in the CNS with its axon extending to an autonomic ganglion and synapsing onto the postganglionic neurone ✓ Postganglionic neurone – located outside the CNS within an autonomic ganglion (either the paravertebral sympathetic chain or other plexuses) and innervates the effector organ Sympathetic Nervous System Responsible for the classic “fight or flight” response – body’s response to perceived threat or danger: ✓  rate & force of cardiac contraction ✓ Constricts blood vessels (vasoconstriction) /  BP – blood flow shunted to skeletal muscles and heart ✓ Releases sweat Dilates pupils Relaxes airways (bronchodilation)  glucose (fuel for the body) Liberates adrenaline (epinephrine) ✓ Also known as the thoracolumbar division ✓ Preganglionic neurones are located in the lateral horn of the gray matter – 1st thoracic (T1) to 3rd lumbar (L3) segment of spinal cord ✓ Preganglionic neurones arranged in a rostral-to-caudal viscerotopic organisation ✓ Acetylcholine (ACh) is excitatory neurotransmitter of preganglionic neurones ✓ Preganglionic axons exit spinal cord via ventral root of a spinal nerve ✓ Separates from ventral root and forms a white ramus ✓ Enters paravertebral ganglia/ ✓ sympathetic chain or trunk ✓ Will synapse onto ✓ postganglionic neurones at the ganglion on same level; at different level; or pass through without synapsing to form a splanchnic nerve which synapses with postganglionic neurones within the prevertebra In addition to the paravertebral sympathetic chain there are prevertebral/collateral ganglia ✓ Situated between sympathetic trunk and target organ ✓ Not paired or segmentally arranged ✓ Occur only in abdomen and pelvis ✓ All lie anterior to vertebral column and mostly on the abdominal aorta AUTONOMIC PLEXUSES ✓ Autonomic plexuses are formed from sympathetic and parasympathetic fibers that innervate and regulate the overall ✓ activity of visceral organs ✓ Complex, interconnected neural networks formed by ganglia and axons found close to major ✓ arteries ✓ Are named according to organs they supply or to blood vessels along which they are found PREVERTEBRAL GANGLIA & AUTONOMIC PLEXUSES ADRENAL MEDULLA ✓ Unique variant of SNS ✓ Modified sympathetic ganglion whose cells are similar to sympathetic postganglionic neurones ✓ Preganglionic autonomic fibres synapse directly in gland ✓ Secretes adrenaline and noradrenaline directly the blood stream nerve the into SYMPATHETIC NERVOUS SYSTEM ✓Axons of postganglionic neurones are unmyelinated; they exit ganglion via the grey ramus ✓They are long and travel to various tissues via the peripheral nerves or major blood vessels Noradrenaline is the excitatory neurotransmitter of postganglionic sympathetic neurones that supply all target tissues