Efferent Neurons: Motor Output Pathways of the Nervous System

Definition

Efferent neurons (motor neurons) carry signals from the CNS (brain/spinal cord) to effectors (muscles, glands). They are the final common path for neural control of the body.

Classification

1. Somatic Efferent (Voluntary)

• Alpha motor neurons: Innervate extrafusal skeletal muscle fibers → force generation.
• Gamma motor neurons: Innervate intrafusal fibers (muscle spindles) → sensitivity tuning.
• Cell bodies: ventral horn (spinal cord), cranial nerve nuclei (brainstem).
• Neurotransmitter: Acetylcholine (ACh) at neuromuscular junction (nicotinic receptors).

2. Autonomic Efferent (Involuntary) – Two-Neuron Chain

• Preganglionic: CNS → autonomic ganglion (ACh, nicotinic).
• Postganglionic: Ganglion → effector.
  • Sympathetic: Thoracolumbar (T1–L2); NE (adrenergic) mostly; ACh for sweat glands.
  • Parasympathetic: Craniosacral (CN III, VII, IX, X; S2–S4); ACh (muscarinic).

Key Functions

• Somatic: Voluntary movement, posture, reflexes (stretch, withdrawal).
• Autonomic: HR, BP, digestion, pupil size, sweating, sexual function, metabolic homeostasis.

Reflex Arc

Sensory neuron → interneuron → efferent neuron → rapid, unconscious motor response (e.g., knee-jerk, withdrawal).

Clinical Correlates

• Upper motor neuron lesion (UMN): Spasticity, hyperreflexia, Babinski.
• Lower motor neuron lesion (LMN): Flaccidity, atrophy, fasciculations, areflexia.
• Autonomic dysreflexia, Horner's, Adie's pupil, neurogenic bladder.