Efferent Fibers and the Autonomic Nervous System

Nervous System Overview

General Structure of the Nervous System

  • Main Divisions: Afferent and Efferent fibers
  • Efferent Divisions:
    • Somatic Nervous System (SNS): Controls voluntary movements of skeletal muscles.
    • Autonomic Nervous System (ANS): Controls involuntary bodily functions, divided into two major subdivisions:
    • Sympathetic Nervous System (SNS): Mediates ‘fight or flight’ responses.
    • Parasympathetic Nervous System (PNS): Mediates ‘rest and digest’ responses.

Bell-Magendie Law

  • Afferent fibers enter the CNS through the dorsal root.
  • Efferent fibers leave the CNS through the ventral root.
  • Gray Matter: Unmyelinated fibers including cell bodies and dendrites.
  • White Matter: Myelinated axons.

Efferent Components of the Peripheral Nervous System

  • Somatic (Motor): Controls skeletal muscle movements.
    • Voluntary Control: Body movements such as walking.
    • Involuntary Control: Reflex actions.
  • Autonomic: Controls smooth and cardiac muscles of internal organs and glands.
    • Controlled Involuntarily (Visceral).
    • Key Branches:
    • Sympathetic (SNS): Prepares the body for stressful situations.
    • Parasympathetic (PSNS): Calms the body back to rest.
    • Enteric: Governs digestive organs (not covered extensively).

Autonomic Nervous System (ANS)

  • Tonic Control: Both branches are active to varying degrees, often having antagonistic effects.
  • Dual Innervation: Both SNS and PSNS innervate the same organs but typically oppose each other.
  • Monoinnervation: Certain organs receive input from only one branch:
    • SNS: Sweat glands, adrenal glands, kidneys, blood vessels.
    • PSNS: Lacrimal glands.

Characteristics of Efferent Neural Circuits

  1. Preganglionic Cell Body Location:
    • SNS: Thoracolumbar region (lateral gray horn).
    • PSNS: Craniosacral region (brain, sacrum).
  2. Preganglionic Axon Length:
    • SNS: Short.
    • PSNS: Long.
  3. Location of Ganglion:
    • SNS: Pre- and para-vertebral ganglia.
    • PSNS: Terminal ganglia close to target organs.
  4. Preganglionic Neurotransmitter/Receptor:
    • Both utilize Acetylcholine (ACh) at ganglion with Nicotinic receptor.
  5. Postganglionic Axon Length:
    • SNS: Long.
    • PSNS: Short.
  6. Postganglionic Neurotransmitter/Receptor:
    • SNS: Norepinephrine (NE) with Adrenergic receptors.
    • PSNS: Acetylcholine (ACh) with Muscarinic receptors.
  7. Targets and Effects:
    • Impacts cardiac muscle, smooth muscle, and glands with various effects based on neurotransmitter-receptor interactions.

Neurotransmitter Types in the PNS

  • Cholinergic (ACh):
    • Subtypes: Nicotinic and Muscarinic.
  • Adrenergic (NE):
    • Subtypes: Alpha (α-1, α-2) and beta (β-1, β-2).

Efferent Synapse Patterns

  • Autonomic Pathways: Follow a two-neuron pattern in series:
    1. Preganglionic Neuron (CNS ➞ Ganglion).
    2. Postganglionic Neuron (Ganglion ➞ Target).

Overview of ANS Functions

  • Sympathetic: Adrenal glands stimulate release of norepinephrine and epinephrine affecting blood flow, heart rate, and energy mobilization.
  • Parasympathetic: Reduce heart rate, increase digestion, and enhance glandular secretion.

Summary of Differences

  • Somatic Pathways:
    • Nonganglionic directly innervates skeletal muscles via ACh (Nicotinic).
  • Sympathetic Pathways:
    • Short pregnglionic with ganglia in the CNS; long postganglionic fibers release NE (Adrenergic).
  • Parasympathetic Pathways:
    • Long pregnglionic fibers which release ACh at target (Muscarinic receptors).

Important Effects on Organs

  • Heart: SNS increases heart rate; PSNS decreases heart rate.
  • Pupils: SNS dilates; PSNS constricts.
  • Digestive tract: SNS inhibits activities; PSNS stimulates activities.

Practical Application

  • Review the functions, pathways, and neurotransmitters for both divisions of the ANS to prepare for the exam.