Efferent Divisions of the Peripheral Nervous System: Autonomic and Somatic Motor Divisions

The Efferent Division of the Peripheral Nervous System

Overview of Efferent Neurons

The efferent division of the peripheral nervous system carries commands from the central nervous system (CNS) to muscles and glands throughout the body. It is divided into two main subdivisions: the somatic motor division and the autonomic division.

• Somatic Motor Neurons: Control skeletal muscles; actions are mostly voluntary.

• Autonomic Neurons: Control smooth muscle, cardiac muscle, many glands, and some adipose tissue; actions are involuntary.

The Autonomic Division

Subdivisions and Functional Roles

The autonomic division is responsible for regulating involuntary physiological functions. It is divided into two anatomically and functionally distinct branches:

• Sympathetic Branch: Known as the "fight-or-flight" system, it prepares the body for stressful or energetic activity.

• Parasympathetic Branch: Known as the "rest-and-digest" system, it promotes maintenance activities and conserves energy.

• These branches are best distinguished by their relative activity under different physiological conditions.

Autonomic Reflexes and Homeostasis

Autonomic reflexes are crucial for maintaining homeostasis and work in concert with endocrine and behavioral state systems.

• Autonomic Reflex: Involves sensory information from somatosensory and visceral receptors.

• Central Control Centers: Located in the hypothalamus, pons, and medulla; these centers integrate sensory input and coordinate autonomic, endocrine, and behavioral responses.

• Some autonomic reflexes are spinal reflexes and do not require integration in the brain.

Anatomical Organization of Autonomic Pathways

Autonomic pathways typically consist of two efferent neurons arranged in series:

• Preganglionic Neuron: Cell body located in the CNS; projects to an autonomic ganglion.

• Postganglionic Neuron: Cell body located in the autonomic ganglion; projects to the target tissue.

• Autonomic Ganglion: A cluster of neuronal cell bodies outside the CNS, acting as a mini-integration center.

Sympathetic vs. Parasympathetic Anatomy

• Sympathetic Branch: Originates in the thoracic and lumbar regions of the spinal cord; ganglia are located in chains along either side of the vertebral column.

• Parasympathetic Branch: Originates in the brainstem and sacral spinal cord; ganglia are located on or near target organs.

• Vagus Nerve: Contains about 75% of all parasympathetic fibers, carrying sensory information from internal organs to the brain and output from the brain to organs.

Dual and Antagonistic Control

Most internal organs receive dual innervation from both sympathetic and parasympathetic branches, which typically exert antagonistic effects:

• One branch is excitatory, the other inhibitory.

• Some exceptions exist, such as sweat glands and most blood vessels, which receive only sympathetic innervation (tonic control).

• Cooperative control occurs when both branches work on different tissues to achieve a common goal.

• The response in target tissue is often determined by the type of neurotransmitter receptor present.

Chemical Communication in the Autonomic Division

Neurotransmitters and Receptors

Autonomic neurons use a variety of chemical signals to communicate with target tissues:

• Both sympathetic and parasympathetic preganglionic neurons release acetylcholine (ACh) onto nicotinic cholinergic receptors (nAChR) on postganglionic cells.

• Most postganglionic sympathetic neurons secrete norepinephrine (NE) onto adrenergic receptors on the target cell.

• Most postganglionic parasympathetic neurons secrete ACh onto muscarinic cholinergic receptors (mAChR) on the target cell.

• Exceptions include sympathetic cholinergic neurons (postganglionic) on sweat glands, which secrete ACh, and nonadrenergic, noncholinergic neurons that use other neurotransmitters.

Types of Autonomic Receptors

• Adrenergic Receptors: Respond to catecholamines (NE and epinephrine); divided into alpha (α) and beta (β) subtypes.

• Alpha Receptors: α1 cause muscle contraction or secretion; α2 decrease cAMP, causing smooth muscle relaxation or decreased secretion.

• Beta Receptors: β1 respond equally to NE and epinephrine; β2 are more sensitive to epinephrine; β3 are more sensitive to NE.

• Muscarinic Receptors: Stimulated by parasympathetic pathways; subtypes M1–M5 exist, with M2 and M3 being primary on parasympathetic targets.

Table: Properties of Autonomic Neurotransmitter Receptors

Receptor Type	Location	Sensitivity	Effect	Second Messenger
α1 Adrenergic	Blood vessels, smooth muscle	NE > E	Contraction, secretion	Increases IP3, Ca2+
α2 Adrenergic	Presynaptic terminals	NE > E	Decreased secretion, relaxation	Decreases cAMP
β1 Adrenergic	Heart, kidney	NE = E	Increased heart rate, renin release	Increases cAMP
β2 Adrenergic	Bronchioles, blood vessels	E > NE	Relaxation of smooth muscle	Increases cAMP
β3 Adrenergic	Adipose tissue	NE > E	Lipolysis	Increases cAMP
Muscarinic (M2, M3)	Parasympathetic targets	ACh	Varied (decreased heart rate, increased secretion)	Increases IP3, PKC

Additional info: Table entries inferred from standard physiology sources.

The Adrenal Medulla

Role and Function

The adrenal medulla is a neuroendocrine tissue that acts as a modified sympathetic ganglion. It is innervated by sympathetic preganglionic fibers, and its postganglionic neurons (chromaffin cells) lack axons and secrete epinephrine (a neurohormone) directly into the blood.

The Somatic Motor Division

Structure and Function

The somatic motor pathway consists of a single neuron originating in the CNS (brain or ventral horn of spinal cord). This neuron is myelinated, very long, and always excitatory. Its terminal branches are close to the target, and each terminal innervates a single skeletal muscle fiber.

• Neuromuscular Junction (NMJ): The synapse between a somatic motor neuron and a skeletal muscle fiber.

• Motor End Plate: Specialized region of the muscle cell membrane at the NMJ.

• Acetylcholinesterase (AChE): Enzyme in the synaptic cleft that breaks down ACh.

Neuromuscular Junction and Receptors

• Nicotinic Receptors (NM): Chemically-gated channels with two binding sites for ACh; always excitatory, leading to muscle contraction.

• Similar to NN receptors found on neurons.

Comparison of Somatic Motor, Sympathetic, and Parasympathetic Divisions

Anatomy, Neurotransmitters, and Receptors

Division	Neurons	Neurotransmitter	Receptor Type	Target Tissue
Somatic Motor	One neuron (CNS to muscle)	ACh	Nicotinic (NM)	Skeletal muscle
Sympathetic	Two neurons (preganglionic and postganglionic)	Preganglionic: ACh Postganglionic: NE (mostly)	Preganglionic: Nicotinic (NN) Postganglionic: Adrenergic (α, β)	Cardiac muscle, smooth muscle, glands
Parasympathetic	Two neurons (preganglionic and postganglionic)	Preganglionic: ACh Postganglionic: ACh	Preganglionic: Nicotinic (NN) Postganglionic: Muscarinic (M2, M3)	Cardiac muscle, smooth muscle, glands

Additional info: Table entries inferred from standard physiology sources.

Key Terms and Definitions

• Autonomic Neurons: Neurons of the autonomic nervous system (visceral motor system).

• Sympathetic Branch: Division of the autonomic nervous system responsible for "fight-or-flight" responses.

• Parasympathetic Branch: Division responsible for "rest-and-digest" activities.

• Antagonistic Control: Opposing effects of sympathetic and parasympathetic branches on target organs.

• Preganglionic Neuron: First neuron in autonomic pathway, cell body in CNS.

• Autonomic Ganglion: Cluster of neuronal cell bodies outside the CNS.

• Postganglionic Neuron: Second neuron in autonomic pathway, cell body in ganglion.

• Acetylcholine (ACh): Neurotransmitter released by somatic motor and autonomic preganglionic neurons.

• Nicotinic Cholinergic Receptors (nAChR): Receptors for ACh on postganglionic neurons and skeletal muscle.

• Adrenergic Receptors: Receptors for NE and epinephrine on target tissues.

• Muscarinic Cholinergic Receptors (mAChR): Receptors for ACh on parasympathetic target tissues.

• Neuromuscular Junction (NMJ): Synapse between somatic motor neuron and skeletal muscle fiber.

• Motor End Plate: Specialized region of muscle membrane at NMJ.

• Acetylcholinesterase (AChE): Enzyme that degrades ACh in the synaptic cleft.

Summary of Learning Outcomes

• Describe the physiological role of the autonomic division and its branches.

• Compare and contrast the anatomy and chemical communication of the sympathetic and parasympathetic branches.

• Describe the structure of the neuromuscular junction.

• Compare the anatomy, neurotransmitters, and receptors of somatic motor, sympathetic, and parasympathetic divisions.