Peripheral Nervous System - Efferent Division

Peripheral Nervous System - Efferent Division

• The efferent division carries signals from the central nervous system (CNS) to target tissues or organs.
• The pathway is made of efferent neurons.
• The peripheral nervous system (PNS) efferent division has two major parts:
  • Autonomic neurons
  • Somatic motor neurons
• Autonomic nervous system (ANS) has two branches:
  • Sympathetic
  • Parasympathetic
• Autonomic neurons control internal organs, glands, some adipose tissue, cardiac, and smooth muscles.
• Somatic motor neurons control skeletal muscles.

Objectives

• Describe the physiological function of the autonomic division and its branches.
• Compare and contrast the structure and chemical communication of the sympathetic (SNS) and parasympathetic (PSNS) branches.
• Explain the synthesis and breakdown of autonomic neurotransmitters.
• Detail the structure and secretions of the adrenal medulla.

SNS vs PSNS

• The sympathetic nervous system (SNS) dominates during stressful situations (fight-or-flight response).
• The parasympathetic nervous system (PSNS) dominates during resting activities (rest-and-digest).
• Homeostasis is maintained when both systems work together (dynamic balance).

Autonomic Reflexes

• The hypothalamus, pons, and medulla initiate autonomic, endocrine, and behavioral responses.
• They maintain homeostasis through communication with the endocrine and behavioral systems.
• Information from sensory receptors is relayed to the CNS control centers (hypothalamus, pons, and medulla) to stimulate responses.
• Responses may be autonomic, endocrine, or behavioral.

Autonomic Control Centers

• These centers monitor and regulate involuntary body functions, including blood pressure, body temperature, respiration, and water balance.
• The hypothalamus, pons, and medulla are involved.
• The centers receive sensory inputs, process them, and trigger responses through the ANS.

Autonomic Pathways

• Autonomic pathways involve a series of two neurons (preganglionic and postganglionic)
• They synapse in an autonomic ganglion before reaching the target tissues.
• Preganglionic and postganglionic neurons and their synapses are involved in each path.

SNS and PSNS Differences

• SNS and PSNS differ in their CNS origin, ganglion locations, and the length of their ganglionic axons.
• SNS: thoracic and lumbar spinal cord segments; ganglia near spinal cord; short preganglionic, long postganglionic neurons
• PSNS: brainstem and sacral spinal cord segments; ganglia near or on target organs; long preganglionic, short postganglionic neurons

SNS and PSNS Responses

• SNS and PSNS often innervate the same organs.
• Frequently antagonistic to each other.
• The table displays the effector organs, sympathetic responses, and parasympathetic responses. The table also indicates the receptor types involved.

SNS and PSNS Neurotransmitters and Receptors

• SNS preganglionic neurons release acetylcholine (ACh).
• Most SNS postganglionic neurons release norepinephrine (NE) which binds to adrenergic receptors,
• PSNS postganglionic neurons release ACh, which binds to muscarinic receptors.

Neuroeffector Junctions

• Autonomic postganglionic axons are varicosities which release neurotransmitters across the surface of a target.
• Varicosities are the string-like structure on the target tissue surface.

Neurotransmitter Release and Removal

• Neurotransmitters like NE are synthesized, released, and removed at a neuroeffector junction
• NE is synthesized by neurons, released into the synapse, and removed by diffusion, reuptake, or enzyme degradation.

Autonomic Neurotransmitters

• ANS neurotransmitters and their properties are listed including receptor types. Receptor sensitivity to neurotransmitters is also indicated.

Adrenal Medulla

• The adrenal medulla is a modified sympathetic ganglion.
• Chromaffin cells in the medulla secrete epinephrine (adrenaline) directly into the bloodstream.
• Epinephrine is a neurohormone with widespread effects.

Summary

• The peripheral nervous system is divided into somatic motor neurons and autonomic neurons.
• Autonomic neurons are further divided into sympathetic and parasympathetic nerves.
• Neurotransmitters travel from one neuron to the next, ending at target cells and organs.

Test your knowledge of the efferent division of the peripheral nervous system. This quiz covers the physiological functions of the autonomic nervous system, including the sympathetic and parasympathetic branches, as well as the roles of autonomic and somatic motor neurons. Assess your understanding of neurotransmitter synthesis and the adrenal medulla's function.