Autonomic Nervous System (ANS)

General Terminology

The Autonomic Nervous System (ANS) is a division of the peripheral nervous system responsible for regulating involuntary physiological functions, including heart rate, digestion, respiratory rate, and glandular activity. It operates largely below the level of conscious control and is divided into two main branches: the sympathetic and parasympathetic nervous systems.

• Autonomic Nervous System (ANS): Controls involuntary functions by regulating smooth muscle, cardiac muscle, and glands.

• Visceral Motor Division: Another term for the ANS, emphasizing its control over visceral (internal organ) effectors.

• Sympathetic Nervous System: Prepares the body for 'fight or flight' responses; increases heart rate, dilates pupils, inhibits digestion.

• Parasympathetic Nervous System: Promotes 'rest and digest' activities; decreases heart rate, stimulates digestion, conserves energy.

Visceral Reflex Arcs

Components and Function

A visceral reflex arc is a neural pathway that controls involuntary responses of internal organs. It consists of sensory input, integration in the CNS, and motor output via the ANS.

• Visceral Reflex Arc: Pathway for automatic responses to internal stimuli, such as blood pressure regulation.

• Autonomic Ganglion: Cluster of neuron cell bodies in the peripheral nervous system where preganglionic and postganglionic neurons synapse.

• Preganglionic Neuron: The first neuron in the autonomic pathway; its cell body is in the CNS and its axon projects to an autonomic ganglion.

• Postganglionic Neuron: The second neuron; its cell body is in the ganglion and its axon projects to the target cell (effector).

• Target Cell: The effector cell (e.g., smooth muscle, cardiac muscle, gland) that responds to autonomic stimulation.

Sympathetic Nervous System

Anatomy and Pathways

The sympathetic nervous system is responsible for mobilizing the body during stress or activity. Its neurons originate from the thoracolumbar region of the spinal cord.

• Thoracolumbar Division: Sympathetic neurons arise from the thoracic and lumbar spinal cord segments (T1-L2).

• Sympathetic Ganglion: Site of synapse between preganglionic and postganglionic neurons; includes sympathetic chain and collateral ganglia.

• Sympathetic Chain: A series of interconnected ganglia running alongside the vertebral column.

• Sympathetic Chain Ganglion: Individual ganglia within the sympathetic chain.

• Collateral Ganglion: Ganglia located outside the sympathetic chain; innervate abdominal and pelvic organs.

• White Ramus Communicans: Myelinated nerve fibers carrying preganglionic axons from the spinal nerve to the sympathetic ganglion.

Parasympathetic Nervous System

Anatomy and Pathways

The parasympathetic nervous system conserves energy and promotes maintenance activities. Its neurons originate from the craniosacral regions.

• Craniosacral Division: Parasympathetic neurons arise from the brainstem (cranial nerves) and sacral spinal cord segments (S2-S4).

• Parasympathetic Ganglion: Ganglia where preganglionic and postganglionic neurons synapse, often located near or within the target organ.

• Vagus Nerve: Cranial nerve X; provides extensive parasympathetic innervation to thoracic and abdominal organs.

• Terminal Ganglion: Parasympathetic ganglia located within or very close to the target organ.

Autonomic Neurotransmitters and Receptors

Types and Functions

Neurotransmitters and their receptors mediate communication between neurons and target cells in the ANS. The main neurotransmitters are acetylcholine and norepinephrine.

• Acetylcholine (ACh): Released by all preganglionic neurons and most parasympathetic postganglionic neurons; binds to cholinergic receptors.

• Epinephrine: Hormone and neurotransmitter released by the adrenal medulla; acts on adrenergic receptors.

• Norepinephrine: Main neurotransmitter released by sympathetic postganglionic neurons; acts on adrenergic receptors.

• Adrenergic Receptor: Receptors that bind norepinephrine and epinephrine; divided into alpha and beta types.

• Alpha Receptor: Subtype of adrenergic receptor; generally causes constriction of blood vessels and increased blood pressure.

• Beta Receptor: Subtype of adrenergic receptor; generally causes relaxation of smooth muscle, increased heart rate, and dilation of airways.

• Cholinergic Receptor: Receptors that bind acetylcholine; divided into muscarinic and nicotinic types.

• Muscarinic Receptor: Cholinergic receptor found on target cells of parasympathetic postganglionic neurons; mediates slower, prolonged responses.

• Nicotinic Receptor: Cholinergic receptor found on all postganglionic neurons and skeletal muscle; mediates fast, excitatory responses.

Example: ANS Neurotransmitter Pathways

• Sympathetic pathway: Preganglionic neuron releases ACh (acts on nicotinic receptors), postganglionic neuron releases norepinephrine (acts on adrenergic receptors).

• Parasympathetic pathway: Preganglionic neuron releases ACh (acts on nicotinic receptors), postganglionic neuron releases ACh (acts on muscarinic receptors).

Additional info:

• The ANS maintains homeostasis by balancing sympathetic and parasympathetic activity.

• Disorders of the ANS can affect blood pressure, heart rate, digestion, and other vital functions.