Lec 10: Autonomic Nervous System

Autonomic Nervous System (ANS)

Overview

The Autonomic Nervous System (ANS) is a critical component of the peripheral nervous system responsible for regulating involuntary physiological processes, including heart rate, blood pressure, respiration, digestion, and glandular activity. It operates largely below the level of consciousness to maintain homeostasis.

Somatic vs. Autonomic Nervous Systems

Comparison and Key Differences

• Somatic Nervous System (SNS): Controls voluntary movements by innervating skeletal muscles. It uses a single motor neuron to transmit signals from the CNS to the muscle, with acetylcholine (ACh) as the neurotransmitter.

• Autonomic Nervous System (ANS): Regulates involuntary functions by innervating smooth muscle, cardiac muscle, and glands. It uses a two-neuron chain (preganglionic and postganglionic neurons) and can use ACh or norepinephrine (NE) as neurotransmitters.

Both systems are part of the motor (efferent) division of the peripheral nervous system.

Functional Organization

• CNS (Central Nervous System): Brain and spinal cord integrate information.

• PNS (Peripheral Nervous System): Cranial and spinal nerves connect the CNS to the rest of the body.

• Motor (Efferent) Division: Includes the somatic motor division (skeletal muscle) and the autonomic nervous system (smooth muscle, cardiac muscle, glands).

Functions of the ANS and Visceral Reflex Arcs

Vital Body Functions and Reflexes

• The ANS controls essential involuntary functions such as heart rate, digestion, respiratory rate, pupillary response, urination, and sexual arousal.

• Visceral reflex arcs involve sensory input from internal organs (viscera) and skin, processed by the CNS, and motor output via autonomic pathways to target organs.

• These reflexes help maintain homeostasis by adjusting organ function in response to internal and external changes.

Divisions of the Autonomic Nervous System

Main Divisions

• Sympathetic Division: Prepares the body for 'fight or flight' responses. Increases heart rate, dilates airways, and inhibits digestion.

• Parasympathetic Division: Promotes 'rest and digest' activities. Slows heart rate, stimulates digestion, and conserves energy.

• Enteric Nervous System: A network of nerves within the digestive tract that controls gastrointestinal motility and reflexes independently, but is modulated by the sympathetic and parasympathetic divisions.

Structural and Functional Features of the ANS

Neuronal Pathways

• Preganglionic Neuron: Cell body in the CNS; axon extends to an autonomic ganglion.

• Postganglionic Neuron: Cell body in the ganglion; axon extends to the target organ.

• Neurotransmitters released at synapses include acetylcholine (ACh) and norepinephrine (NE).

Dual Innervation

• Most organs receive input from both sympathetic and parasympathetic fibers, allowing precise regulation of organ function.

• For example, the heart receives sympathetic input to increase rate and parasympathetic input to decrease rate.

Sympathetic vs. Parasympathetic Divisions

Structural Differences

• Sympathetic: Preganglionic neurons originate in the thoracic and lumbar spinal cord; short preganglionic and long postganglionic fibers; ganglia located near the spinal cord (sympathetic chain ganglia).

• Parasympathetic: Preganglionic neurons originate in the brainstem (cranial nerves III, VII, IX, X) and sacral spinal cord; long preganglionic and short postganglionic fibers; ganglia located near or within target organs (terminal ganglia).

Functional Differences

• Sympathetic: Mobilizes body systems during activity (fight or flight).

• Parasympathetic: Conserves energy and promotes housekeeping functions during rest (rest and digest).

Neurotransmitters and Receptors in the ANS

Preganglionic and Postganglionic Neurotransmitters

• Preganglionic neurons (both sympathetic and parasympathetic) release acetylcholine (ACh).

• Postganglionic sympathetic neurons typically release norepinephrine (NE) (adrenergic fibers), but some release ACh (e.g., sweat glands).

• Postganglionic parasympathetic neurons release acetylcholine (ACh) (cholinergic fibers).

Receptor Types

• Cholinergic Receptors: Bind ACh. Two main types:

  • Nicotinic receptors: Found on all postganglionic neurons and adrenal medulla cells; always excitatory.

  • Muscarinic receptors: Found on target organs of parasympathetic fibers and some sympathetic targets (e.g., sweat glands); can be excitatory or inhibitory.

• Adrenergic Receptors: Bind NE and epinephrine. Two main classes:

  • Alpha (α) receptors: Subtypes α1 (smooth muscle contraction, vasoconstriction) and α2 (inhibition of NE release, modulation of sympathetic outflow).

  • Beta (β) receptors: Subtypes β1 (increased heart rate and contractility), β2 (bronchodilation, vasodilation in skeletal muscle), β3 (lipolysis in adipose tissue).

Summary Table: Somatic vs. Autonomic Nervous Systems

Feature	Somatic Nervous System	Autonomic Nervous System
Control	Voluntary	Involuntary
Effector Organs	Skeletal muscle	Smooth muscle, cardiac muscle, glands
Number of Neurons	One (from CNS to effector)	Two (preganglionic and postganglionic)
Neurotransmitter	Acetylcholine (ACh)	ACh, Norepinephrine (NE)
Effect of Nerve Damage	Paralysis and loss of muscle function	Loss of homeostatic control, but not paralysis

Key Terms and Definitions

• Autonomic Ganglion: A cluster of nerve cell bodies in the ANS where preganglionic and postganglionic neurons synapse.

• Dual Innervation: The phenomenon where most target organs receive input from both sympathetic and parasympathetic divisions.

• Cholinergic Fiber: A nerve fiber that releases acetylcholine.

• Adrenergic Fiber: A nerve fiber that releases norepinephrine or epinephrine.

Example: Heart Rate Regulation

• Sympathetic stimulation: Increases heart rate and force of contraction via β1-adrenergic receptors.

• Parasympathetic stimulation: Decreases heart rate via muscarinic cholinergic receptors.

Additional info: The enteric nervous system is sometimes called the "second brain" due to its autonomy and complexity in regulating digestive functions.