Functional Divisions and Organization of the Peripheral Nervous System (PNS): Somatic and Autonomic Nervous Systems

Functional Divisions of the Peripheral Nervous System

Overview of the PNS

The Peripheral Nervous System (PNS) consists of all nervous tissue outside the central nervous system (CNS). It is functionally divided into the Somatic Nervous System (SNS) and the Autonomic Nervous System (ANS), each with distinct roles and effectors.

• Somatic Nervous System (SNS): Controls voluntary movements of skeletal muscles.

• Autonomic Nervous System (ANS): Regulates involuntary functions, including smooth muscle, cardiac muscle, and glandular activity.

Somatic Nervous System (SNS)

Effectors and Control

The SNS is responsible for voluntary control of skeletal muscles. It involves a single motor neuron pathway from the CNS to the effector muscle.

• Effectors: Skeletal muscles

• Control: Voluntary (conscious) control

• Motor Neurons: Large, fast, thickly myelinated Type A nerve fibers

Organization of the Somatic Nervous System

The main control center for the SNS is the primary motor cortex of the cerebrum. The lower motor neuron extends from the CNS directly to the skeletal muscle effector.

• Upper motor neurons: Located in the primary motor cortex or brainstem

• Lower motor neurons: Extend from the CNS to the skeletal muscle

Autonomic Nervous System (ANS)

Effectors and Control

The ANS regulates involuntary activities of smooth muscle, cardiac muscle, and glands. It is divided into the sympathetic and parasympathetic divisions, which often have opposing effects on target organs.

• Effectors: Smooth muscle, cardiac muscle, glands

• Control: Involuntary (subconscious) control

• Motor Neurons: Two-neuron chain (preganglionic and postganglionic neurons)

Organization of the Autonomic Nervous System

The ANS uses a two-neuron pathway: a preganglionic neuron (originating in the CNS) synapses with a postganglionic neuron in an autonomic ganglion outside the CNS. The postganglionic neuron then innervates the effector.

• Control center: Hypothalamus

• Preganglionic neuron: Originates in CNS, synapses in autonomic ganglia

• Postganglionic neuron: Extends to effector

Somatic vs. Autonomic Nervous System: Key Differences

• Effectors: SNS targets skeletal muscle; ANS targets smooth muscle, cardiac muscle, and glands.

• Control: SNS is voluntary; ANS is involuntary.

• Neural Pathway: SNS uses a single neuron; ANS uses a two-neuron chain.

• Nerve Fiber Types: SNS uses Type A fibers; ANS uses Type B (preganglionic) and Type C (postganglionic) fibers.

Divisions of the Autonomic Nervous System

Sympathetic Division (Thoracolumbar Division)

The sympathetic division prepares the body for heightened activity ("fight or flight"). Preganglionic neurons originate in the thoracic and lumbar regions (T1–L2) of the spinal cord. Sympathetic ganglia are located near the spinal cord.

• Sympathetic chain (paravertebral) ganglia: Innervate head, limbs, thoracic cavity

• Prevertebral (collateral) ganglia: Innervate abdominopelvic organs

• Preganglionic fibers: Short

• Postganglionic fibers: Long

Parasympathetic Division (Craniosacral Division)

The parasympathetic division conserves energy and promotes maintenance functions ("rest and digest"). Preganglionic neurons originate in brain nuclei of cranial nerves III, VII, IX, X and sacral spinal cord segments. Parasympathetic ganglia are located near or within target organs.

• Terminal (intramural) ganglia: Located near or inside effectors

• Preganglionic fibers: Long

• Postganglionic fibers: Short

Somatic & Autonomic Nervous System Organization: Comparative Overview

This table summarizes the organization, neurotransmitters, and effectors of the somatic and autonomic nervous systems.

System	Neural Pathway	Neurotransmitter(s)	Effector(s)
Somatic	Single neuron (CNS to muscle)	Acetylcholine (ACh)	Skeletal muscle
Autonomic - Sympathetic	Two neurons (preganglionic & postganglionic)	ACh (preganglionic), Norepinephrine (NE) or Epinephrine (E) (postganglionic)	Smooth muscle, cardiac muscle, glands
Autonomic - Parasympathetic	Two neurons (preganglionic & postganglionic)	ACh (both neurons)	Smooth muscle, cardiac muscle, glands

Neurotransmitters and Receptors in the PNS

Somatic Motor Neurons

• Release acetylcholine (ACh) at neuromuscular junctions

• Effect is always excitatory, causing muscle contraction

Autonomic Motor Neurons

• All preganglionic neurons (both divisions) release ACh

• Postganglionic parasympathetic neurons release ACh

• Most postganglionic sympathetic neurons release norepinephrine (NE)

• Response (excitation or inhibition) depends on receptor type

Cholinergic Neurons and Receptors

• Cholinergic neurons: Release ACh

• Nicotinic receptors: Found on skeletal muscle, all postganglionic neurons, adrenal medulla; always excitatory

• Muscarinic receptors: Found on all parasympathetic targets and some sympathetic targets; effect can be excitatory or inhibitory

Adrenergic Neurons and Receptors

• Adrenergic neurons: Release norepinephrine (NE)

• Adrenergic receptors: Alpha (α) and beta (β) types

• α1 and β1 receptors: Generally excitatory

• α2 and β2 receptors: Generally inhibitory

• Heart: β1 receptors increase heart rate and contraction

• Respiratory airways: β2 receptors relax smooth muscle and dilate airways

Sympathetic vs. Parasympathetic Effects on Target Organs

The following table summarizes the effects of sympathetic and parasympathetic stimulation on various organs:

Target Organ	Parasympathetic Effects	Sympathetic Effects
Blood vessels	No effect	Dilation to skeletal muscles & heart; constriction to viscera
Salivary glands	Stimulates watery salivation	Inhibits watery salivation
Sweat glands	No effect	Stimulates sweating
Genitals	Stimulates erection	Causes glandular secretion, contraction, & ejaculation
GI Tract	Increases motility & gland secretion; dilates sphincters	Decreases motility & gland secretion; constricts sphincters
Heart	Decreases heart rate, force of contraction, & BP	Increases heart rate, force of contraction, & BP
Respiratory Airways	Constricts bronchioles	Dilates bronchioles
Eye (pupil)	Constricts pupil	Dilates pupil

Dual Innervation and Autonomic Tone

Most visceral organs receive dual innervation from both sympathetic and parasympathetic divisions, allowing for precise regulation. Effects are typically antagonistic, but some structures (e.g., sweat glands, adrenal medulla, arrector pili muscles, most blood vessels) receive only sympathetic input.

Extent and Duration of Autonomic Activity

Sympathetic Activity

• Extensive divergence: Each preganglionic neuron may synapse with 30+ postganglionic neurons, causing widespread effects.

• Effects of NE last several seconds; hormonal release from adrenal medulla prolongs effects.

Parasympathetic Activity

• Limited divergence: Each preganglionic neuron synapses with few postganglionic neurons, leading to localized effects.

• Effects at muscarinic receptors are brief due to rapid inactivation of ACh by acetylcholinesterase (AChE).