Efferent Division: Autonomic and Somatic Motor Control – Study Notes

Efferent Division of the Peripheral Nervous System

Overview of Efferent Pathways

The efferent division of the peripheral nervous system transmits commands from the central nervous system (CNS) to muscles and glands. It is divided into two main subdivisions: the somatic motor neurons, which control skeletal muscles and are mostly voluntary, and the autonomic neurons, which control smooth muscle, cardiac muscle, many glands, and some adipose tissue, and are mostly involuntary.

The Autonomic Division

Subdivisions of the Autonomic Division

The autonomic division is further divided into two branches:

• Sympathetic branch – Responsible for the "fight-or-flight" response.

• Parasympathetic branch – Responsible for the "rest-and-digest" response.

These branches are anatomically distinguishable and are best differentiated by their activity in various physiological situations.

Autonomic Reflexes and Homeostasis

Autonomic reflexes are crucial for maintaining homeostasis. They work in conjunction with the endocrine and behavioral state systems. Sensory information from somatosensory and visceral receptors, as well as hypothalamic sensors, is integrated in the hypothalamus, pons, and medulla, which serve as homeostatic control centers. Motor output from these centers leads to autonomic, endocrine, and behavioral responses. Some autonomic reflexes are spinal reflexes and do not require brain integration.

Autonomic Control Centers

The hypothalamus, pons, and medulla are key autonomic control centers. They regulate functions such as eating behavior, water balance, temperature control, blood pressure, and urinary bladder control.

Antagonistic and Cooperative Control

Most internal organs are under antagonistic control, meaning one autonomic branch is excitatory while the other is inhibitory. Some organs, like sweat glands and most blood vessels, 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 tissues is often determined by the type of neurotransmitter receptor present.

Autonomic Pathways Structure

Autonomic pathways consist of two efferent neurons in series:

• Preganglionic neuron: Cell body in the CNS, projects to an autonomic ganglion outside the CNS.

• Postganglionic neuron: Cell body in the autonomic ganglion, projects to the target tissue.

Ganglia also contain interneurons and act as mini-integration centers.

Sympathetic vs. Parasympathetic Origins

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

• Parasympathetic: Originates in the brainstem and sacral spinal cord; ganglia are located on or near target organs. The vagus nerve contains about 75% of all parasympathetic fibers.

Chemical Signaling in the Autonomic Nervous System

Both sympathetic and parasympathetic preganglionic neurons release acetylcholine (ACh) onto nicotinic cholinergic receptors. Most postganglionic sympathetic neurons secrete norepinephrine (NE) onto adrenergic receptors, while most postganglionic parasympathetic neurons secrete ACh onto muscarinic cholinergic receptors. There are exceptions, such as sympathetic cholinergic neurons innervating sweat glands.

Neuroeffector Junctions and Varicosities

The neuroeffector junction is the synapse between a postganglionic autonomic neuron and its target cell. Neurotransmitters are released from varicosities, which are swellings at the end of the neuron. The release and removal of neurotransmitters, such as norepinephrine, involve exocytosis, receptor binding, reuptake, and enzymatic degradation.

Autonomic Receptor Subtypes

Autonomic receptors are G protein-coupled and have multiple subtypes:

• Adrenergic receptors (sympathetic):

  • Alpha (α) receptors: α1 (muscle contraction/secretion), α2 (decrease cAMP, smooth muscle relaxation)

  • Beta (β) receptors: β1 (responds equally to NE and E), β2 (more sensitive to E), β3 (more sensitive to NE)

• Cholinergic receptors (parasympathetic):

  • Muscarinic receptors (M1, M2, M3, M4, M5)

  • Nicotinic receptors (NN, NM)

Receptor	Found in	Sensitivity	Effect on Second Messenger
α1	Most sympathetic target tissues	NE > E	Increases IP3 and Ca2+; increases PKC
α2	GI tract, pancreas	NE > E	Decreases cAMP
β1	Heart muscle, kidney	NE = E	Increases cAMP
β2	Certain blood vessels, smooth muscle	E > NE	Increases cAMP
β3	Adipose tissue	NE > E	Increases cAMP
NN	Postganglionic autonomic neurons	ACh	Opens nonspecific cation channels
NM	Skeletal muscle	ACh	Opens nonspecific cation channels
M1, M3, M5	Nervous system, parasympathetic targets	ACh	Increases IP3 and Ca2+; increases PKG
M2, M4	Nervous system, parasympathetic targets	ACh	Decreases cAMP; opens K+ channels

Agonists and Antagonists of Neurotransmitter Receptors

Receptor Type	Neurotransmitter	Agonist	Antagonists	Indirect Agonists/Antagonists
Muscarinic	Acetylcholine	Muscarine	Atropine, scopolamine	AChE inhibitors: neostigmine
Nicotinic	Acetylcholine	Nicotine	α-bungarotoxin (muscle), TEA (ganglia), curare
Alpha (α)	NE, E	Phenylephrine	Alpha-blockers	Stimulate NE release: ephedrine, amphetamines; Prevent NE uptake: cocaine
Beta (β)	NE, E	Isoproterenol, albuterol	Beta-blockers: propranolol, metoprolol

Comparison of Sympathetic and Parasympathetic Branches

Feature	Sympathetic	Parasympathetic
Point of CNS Origin	First thoracic to second lumbar segments	Midbrain, medulla, second to fourth sacral segments
Location of Peripheral Ganglia	Paravertebral chain, outlying ganglia	On or near target organs
Neurotransmitter at Target Synapse	Norepinephrine	Acetylcholine
Receptors on Target Cells	Adrenergic	Muscarinic

The Adrenal Medulla

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

The Somatic Motor Division

Somatic Motor Pathways

Somatic motor pathways consist of a single neuron originating in the CNS (brain or ventral horn of spinal cord). These neurons are myelinated, very long, and always excitatory. Each terminal branch innervates a single skeletal muscle fiber. The synapse between a somatic motor neuron and a skeletal muscle fiber is called the neuromuscular junction (NMJ).

Neuromuscular Junction Structure and Function

The NMJ consists of axon terminals, motor end plates on the muscle membrane, and Schwann cell sheaths. The motor end plate contains high concentrations of nicotinic acetylcholine receptors (nAChR). Acetylcholinesterase (AChE) in the synaptic cleft rapidly degrades ACh to terminate the signal.

Mechanism of Neuromuscular Transmission

When an action potential arrives at the axon terminal, voltage-gated Ca2+ channels open, causing synaptic vesicles to release ACh into the synaptic cleft. ACh binds to nAChR on the motor end plate, opening nonspecific monovalent cation channels. The net influx of Na+ depolarizes the muscle fiber, leading to muscle contraction. ACh is then degraded by AChE.

Key Equations

• Neurotransmitter breakdown (acetylcholine):

$\mathrm{Acetylcholine} + \mathrm{H_2O} \xrightarrow{AChE} \mathrm{Acetate} + \mathrm{Choline}$

• Second messenger (cAMP) pathway:

$\mathrm{ATP} \xrightarrow{Adenylyl\ cyclase} \mathrm{cAMP} + \mathrm{PP_i}$

Summary Table: Postganglionic Autonomic Neurotransmitters

Division	Neurotransmitter	Receptor Types	Synthesized from	Inactivation Enzyme	Varicosity Membrane Transporters for
Sympathetic	Norepinephrine (NE)	α- and β-adrenergic	Tyrosine	Monoamine oxidase (MAO)	Norepinephrine
Parasympathetic	Acetylcholine (ACh)	Nicotinic and muscarinic cholinergic	Acetyl CoA + choline	Acetylcholinesterase (AChE)	Choline