Peripheral Nervous System Efferent Division Study Notes

Peripheral Nervous System Efferent Division

Learning Objectives

  • Understand the direction of information transmission by the efferent nervous system.
  • Identify the two divisions of the efferent nervous system and the functions they control.
  • Distinguish key differences between autonomic and somatic nervous systems, including:
    • Voluntary vs. involuntary control.
    • Origin sites of nerves in each system.

Efferent Division Nervous System

  • Efferent Nervous System Direction:
    • Transmits information from the Central Nervous System (CNS) to effector organs (muscles and glands).

Divisions of Efferent Nervous System

  • Autonomic Nervous System (ANS)
    • Controls involuntary actions: heart rate, digestion, respiratory rate, etc.
  • Somatic Nervous System (SNS)
    • Controls voluntary actions: movement of skeletal muscles.

Key Differences Between Autonomic and Somatic Systems

  • Control:
    • Autonomic: Involuntary control.
    • Somatic: Voluntary control.
  • Sites of Origin of Nerves:
    • Autonomic: Nerves originate from the brainstem and spinal cord (thoracic and lumbar regions).
    • Somatic: Nerves originate in the spinal cord (brainstem for cranial nerves).

Autonomic Nervous System

Autonomic Nerve Pathway

  • Structure of a typical autonomic pathway:
    • Two-neuron chain:
    • Preganglionic Neuron: Originates in the CNS (brain/spinal cord), its axon extends to a ganglion.
    • Postganglionic Neuron: Located in the ganglion, its axon extends to the effector organ.

Neurotransmitters in Autonomic Nervous System

  • Preganglionic Neurons: Release acetylcholine (ACh).
  • Postganglionic Neurons:
    • In the sympathetic division, these release norepinephrine (NE).
    • In the parasympathetic division, these also release acetylcholine (ACh).

Divisions of the Autonomic Nervous System

  • Sympathetic Division:
    • Responsible for the "fight or flight" response.
    • Increases heart rate, expands airways, inhibits digestion, dilates pupils.
  • Parasympathetic Division:
    • Responsible for the "rest and digest" response.
    • Decreases heart rate, constricts airways, stimulates digestion, constricts pupils.

Balance of Sympathetic and Parasympathetic Responses

  • Dual Innervation of Organs: Most organs receive both sympathetic and parasympathetic nerve fibers, allowing for a balance between excitation and inhibition.

Examples of Organ Responses

  • Sympathetic Activation:
    • Heart: Increases heart rate and contractility.
    • Lungs: Bronchodilation (widening of the air passages).
    • Digestive Tract: Inhibition of digestive processes.
    • Eye: Dilation of pupils (mydriasis).
  • Parasympathetic Activation:
    • Heart: Decreases heart rate.
    • Lungs: Bronchoconstriction (narrowing of the air passages).
    • Digestive Tract: Stimulation of digestive processes.
    • Eye: Constriction of pupils (miosis).

Somatic Nervous System

Somatic Nerve Pathway

  • Structure of the somatic pathway:
    • Consists of a single neuron that originates in the spinal cord and terminates directly at the target skeletal muscle.
    • The somatic motor neuron releases acetylcholine (ACh) directly onto skeletal muscle fibers to trigger contraction.
  • Origin: Neurons originate in the ventral horn of the spinal cord.
  • Termination: Neurons terminate directly at the neuromuscular junction (NMJ) of the skeletal muscles.