Notes on Efferent Division: Autonomic and Somatic Motor Control

Chapter 11: Efferent Division: Autonomic and Somatic Motor Control

Overview of the Efferent Division of the Peripheral Nervous System

• Definition: Efferent neurons transmit commands from the Central Nervous System (CNS) to muscles and glands of the body.
• Two Subdivisions:
  • Somatic Motor Neurons:
  • Function: Control skeletal muscles.
  • Characteristic: Mostly voluntary activity.
  • Autonomic Neurons:
  • Function: Control smooth muscle, cardiac muscle, glands, and some adipose tissues.
  • Characteristic: Mostly involuntary activity.

The Autonomic Division

• Key Subdivisions:
  • Sympathetic Branch: Often referred to as the “fight-or-flight” system.
  • Parasympathetic Branch: Known as the “rest-and-digest” system.
• Anatomical Distinction:
  • The sympathetic and parasympathetic branches can be distinguished anatomically and by their activity during certain physiological situations.

Homeostasis and Autonomic Reflexes

• Integration with Other Systems:

  • Autonomic reflexes operate alongside endocrine and behavioral systems to maintain homeostasis.

• Autonomic Reflex Definition: A functional pathway involving:

  • Sensory Input: From somatosensory and visceral receptors.
  • Integration Centers: Located in the hypothalamus, pons, and medulla, which integrate sensory information.
  • Motor Output: Results in autonomic, endocrine, and behavioral responses.

• Neural Mechanisms:

  • Hypothalamus also contains neurons that act as sensors for homeostatic control centers.
  • Some reflexes do not require brain integration (e.g., spinal reflexes).

Antagonistic Control within the Autonomic System

• Mechanism of Action: Most internal organs are controlled by both the sympathetic and parasympathetic divisions, where:
  • One division excites an organ while the other inhibits it.
• Exceptions:
  • Sweat glands and most blood vessel smooth muscles receive only sympathetic innervation (tonic control).
  • Cooperative control where both divisions can act on different tissues to achieve a common physiological goal.
• Receptor Types: The response in target tissues is often determined by the neurotransmitter receptor involved.

Autonomic Pathways Structure

• Composition of Pathways:
  1. Preganglionic Neuron:
  • Location: CNS with a cell body in the spinal cord.
  • Projection: Extends from CNS to an autonomic ganglion.
  • Synapses: Connects with postganglionic neuron in ganglion.
  1. Postganglionic Neuron:
  • Location: Autonomic ganglion with a cell body in the ganglion.
  • Projection: Extends from ganglion to target tissue, synapsing with target cells.
    • Ganglion Definition: A cluster of neuronal cell bodies located outside the CNS, serving as mini-integration centers.

Origin of Sympathetic and Parasympathetic Branches

• Sympathetic Origin:
  • Arises from thoracic and lumbar regions of the spinal cord.
  • Sympathetic ganglia form two chains adjacent to the vertebral column.
• Parasympathetic Origin:
  • Originates in the brain stem.
  • Leaves the brain via cranial nerves, especially the vagus nerve, which constitutes about 75% of parasympathetic fibers.
  • Ganglia are located on or near target organs.

Chemical Signal Usage in the Autonomic Nervous System

• Preganglionic Transmission:
  • Preganglionic neurons (both divisions) release acetylcholine (ACh) onto nicotinic cholinergic receptors.
• Postganglionic Transmission:
  • Most postganglionic sympathetic neurons release norepinephrine (NE) onto adrenergic receptors.
  • Most postganglionic parasympathetic neurons secrete ACh onto muscarinic cholinergic receptors.
  • Exceptions:
  • Sympathetic cholinergic neurons (sweat glands) secrete ACh.
  • Nonadrenergic, noncholinergic neurons release other neurotransmitters.

Neuroeffector Junction

• Definition: The synapse between postganglionic autonomic neurons and target cells, which features a varicosity (swelling) at the neuron’s end.
  • Modulation: Release of neurotransmitters in this junction can be modulated based on physiological needs.

Autonomic Receptors and Their Subtypes

• G Protein-Coupled Receptors: Include multiple adrenergic receptor subtypes:
  • Alpha (α) Receptors:
  • α1: Increases intracellular IP3, leading to muscle contraction/secretion.
  • α2: Decreases cAMP, causing smooth muscle relaxation.
  • Beta (β) Receptors:
  • β1: Responds to norepinephrine equally as epinephrine, increasing cAMP.
  • β2: More responsive to epinephrine than norepinephrine, increasing cAMP.
  • β3: More responsive to norepinephrine than epinephrine.
• Muscarinic Receptors: Located on target cells, stimulated by parasympathetic pathways.

Comparison of Neurotransmitter Properties in Autonomic Divisions

Comparative Overview of Sympathetic and Parasympathetic

The Role of the Adrenal Medulla

• Definition: Acts as a neuroendocrine tissue and modified sympathetic ganglion innervated by sympathetic preganglionic fibers.
• Function:
  • Secretes epinephrine (a neurohormone) directly into the bloodstream.

The Somatic Motor Division

• Structure of Somatic Motor Pathway: Composed of a single neuron from the CNS (brain or spinal cord).
  • Characteristics:
  • Myelinated and very long fiber.
  • Always excitatory, forming connections to single skeletal muscle fibers.
• Neuromuscular Junction (NMJ):
  • Synapse between the somatic motor neuron and skeletal muscle fiber characterized by the motor end plate on the muscle cell membrane.
  • Contains acetylcholinesterase (AChE) in the synaptic cleft.
• Nicotinic Receptors:
  • Present in the skeletal muscle; chemically-gated channels with two binding sites for ACh to induce muscle contraction.