Physiology of the Autonomic Nervous System

What is the autonomic nervous system (ANS)?

The autonomic nervous system is the part of the nervous system that operates largely without conscious control and innervates smooth muscle, cardiac muscle, glands, and viscera to maintain homeostasis.

What are the primary functions of the ANS?

The ANS regulates heart rate, blood pressure, intestinal motility, bladder emptying, sweating, and pupil diameter to maintain internal balance.

What organs are innervated by the ANS versus the somatic motor system?

The ANS innervates smooth muscle, cardiac muscle, and glands, whereas the somatic motor system innervates skeletal muscle.

How does the neural pathway of the ANS differ from the somatic motor system?

The somatic motor system uses a single neuron from the CNS to muscle, while the ANS uses a two-neuron chain: a preganglionic neuron and a postganglionic neuron.

What is a preganglionic neuron?

A preganglionic neuron is the first neuron in the ANS pathway; its cell body is in the CNS and its axon synapses in a peripheral ganglion.

What is a postganglionic neuron?

A postganglionic neuron is the second neuron in the ANS pathway; its cell body is in a ganglion and its axon innervates the target organ.

What is a ganglion?

A ganglion is a collection of neuronal cell bodies located outside the CNS and is the synapse site between preganglionic and postganglionic neurons.

How do ANS synapses at target organs differ from somatic neuromuscular junctions?

ANS synapses are diffuse chemical synapses with varicosities releasing neurotransmitter over a wide area, while somatic synapses are highly specialized and localized.

What are the characteristics of postganglionic axons?

Postganglionic axons are usually unmyelinated, conduct impulses slowly, and can have excitatory or inhibitory effects on target tissues.

What are the major divisions of the peripheral ANS?

The ANS is divided into the sympathetic nervous system, parasympathetic nervous system, and enteric nervous system.

Where does the sympathetic nervous system originate?

The sympathetic nervous system originates from the thoracolumbar region of the spinal cord (T1–L3/L4).

Where does the parasympathetic nervous system originate?

The parasympathetic nervous system originates from the brainstem and the sacral spinal cord (S2–S4), known as the craniosacral system.

What is the enteric nervous system?

The enteric nervous system is an extensive network of neurons within the gastrointestinal tract wall that can control gut function independently of the CNS.

What neurotransmitter is released at all autonomic ganglia?

Acetylcholine (ACh) is released by all preganglionic neurons at autonomic ganglia.

What neurotransmitter is released by parasympathetic postganglionic neurons?

Parasympathetic postganglionic neurons release acetylcholine (ACh) at target organs.

What neurotransmitter is usually released by sympathetic postganglionic neurons?

Sympathetic postganglionic neurons typically release norepinephrine (NE) at target organs.

What is the sympathetic exception to norepinephrine release?

Sympathetic neurons innervating sweat glands and some skeletal muscle blood vessels release acetylcholine instead of norepinephrine.

What enzymes degrade acetylcholine and norepinephrine?

Acetylcholine is degraded by acetylcholinesterase, while norepinephrine is degraded by monoamine oxidase (MAO) and catechol-O-methyltransferase (COMT).

What is a co-transmitter in the ANS?

A co-transmitter is a secondary signaling molecule released alongside the primary neurotransmitter that modifies the target tissue response.

What types of co-transmitters are used in the ANS?

Co-transmitters include peptides (VIP, NPY), purines (ATP), and gases such as nitric oxide.

What is the primary function of the sympathetic nervous system?

The sympathetic nervous system prepares the body for stress through the “fight or flight” response.

What are common effects of sympathetic activation?

Increased heart rate and blood pressure, pupil dilation, elevated blood glucose, increased free fatty acids, and heightened arousal.

Why do sympathetic effects last longer than parasympathetic effects?

Sympathetic responses last longer because epinephrine and norepinephrine are released into the bloodstream by the adrenal glands.

How does the sympathetic system contribute to homeostasis during non-stress conditions?

It regulates routine functions such as thermoregulation via skin blood flow and pupil dilation in low light.

What is the general function of the parasympathetic nervous system?

The parasympathetic system promotes “rest and digest” functions by conserving energy, slowing heart rate, and stimulating digestion.

How do the sympathetic and parasympathetic systems interact?

They often act antagonistically to balance organ function and maintain physiological homeostasis.