Peripheral Nervous System (PNS) Divisions

Overview of the Nervous System

The nervous system is divided into several functional and anatomical divisions, each responsible for specific roles in the body. Understanding these divisions is essential for grasping how the body controls movement, sensation, and involuntary functions.

• Central Nervous System (CNS): Consists of the brain and spinal cord; processes and integrates information.

• Peripheral Nervous System (PNS): All neural structures outside the CNS; connects the CNS to limbs and organs.

• Somatic Nervous System: Controls voluntary movements via skeletal muscles.

• Autonomic Nervous System (ANS): Regulates involuntary functions such as heart rate, digestion, and respiratory rate.

Examples of actions: Reflex withdrawal from pain (somatic), heart rate increase during stress (autonomic).

Somatic vs. Visceral (Autonomic) Motor Neurons

Differences in Structure and Function

Motor neurons transmit signals from the CNS to effectors. Their structure and function differ between the somatic and autonomic systems.

• Somatic Motor Neurons: Single neuron pathway from CNS to skeletal muscle; always excitatory.

• Visceral (Autonomic) Motor Neurons: Two-neuron chain (preganglionic and postganglionic) from CNS to smooth muscle, cardiac muscle, or glands; can be excitatory or inhibitory.

Example: Somatic motor neuron causes muscle contraction; autonomic motor neuron can increase or decrease heart rate.

Speed of Signal Transmission

Somatic vs. Autonomic Motor Neuron Speed

Signal transmission speed varies between the somatic and autonomic systems due to differences in neuron structure.

• Somatic Motor Neurons: Myelinated axons allow rapid signal transmission.

• Autonomic Motor Neurons: Preganglionic fibers are myelinated, but postganglionic fibers are unmyelinated, resulting in slower transmission.

Additional info: Myelination increases conduction velocity by allowing saltatory conduction.

Functions of the Sympathetic and Parasympathetic Nervous Systems

Sexual Function and Other Roles

The sympathetic and parasympathetic divisions of the ANS have distinct and sometimes opposing effects on organ systems, including sexual function.

• Sympathetic Division: Prepares the body for 'fight or flight' responses; increases heart rate, dilates pupils, inhibits digestion.

• Parasympathetic Division: Promotes 'rest and digest' activities; slows heart rate, stimulates digestion, supports sexual arousal.

• Sexual Function: Parasympathetic division mediates erection; sympathetic division mediates ejaculation.

Autonomic Tone

Definition and Importance

Autonomic tone refers to the baseline level of activity in autonomic motor neurons, which allows for rapid adjustments in organ function.

• Maintains homeostasis by balancing sympathetic and parasympathetic activity.

• Example: Blood vessel diameter is regulated by autonomic tone.

Nervous System Divisions: Direction and Neuron Count

Signal Transmission Pathways

Each nervous system division sends information in specific directions and utilizes a characteristic number of neurons.

• Somatic Pathway: One neuron from CNS to effector.

• Autonomic Pathway: Two neurons (preganglionic and postganglionic) from CNS to effector.

Effectors of Somatic and Autonomic Nervous Systems

Target Tissues

• Somatic Nervous System: Skeletal muscles.

• Autonomic Nervous System: Smooth muscle, cardiac muscle, glands.

Visceral Motor Division: Functions and Characteristics

Overview

The visceral motor division (ANS) controls involuntary functions and is subdivided into sympathetic and parasympathetic divisions.

• Sympathetic Division: Thoracolumbar origin; short preganglionic, long postganglionic fibers.

• Parasympathetic Division: Craniosacral origin; long preganglionic, short postganglionic fibers.

PNS Divisions: Preganglionic Origins, Neuron Lengths, Ganglia Locations, and Responses

Structural Features

• Sympathetic: Preganglionic neurons originate in thoracic and lumbar spinal cord; ganglia near spinal cord.

• Parasympathetic: Preganglionic neurons originate in brainstem and sacral spinal cord; ganglia near or within target organs.

Parasympathetic Nervous System: Craniosacral Division

Pathways and Functions

• Craniosacral Division: Preganglionic neurons arise from cranial nerves III, VII, IX, X and sacral spinal cord (S2-S4).

• Functions: Stimulates digestion, slows heart rate, promotes glandular secretion.

Sympathetic Nervous System: Thoracolumbar Division

Pathways and Functions

• Thoracolumbar Division: Preganglionic neurons arise from T1-L2 spinal segments.

• Functions: Increases heart rate, dilates bronchi, inhibits digestion.

Receptors in the Autonomic Nervous System

Nicotinic and Muscarinic Receptors

• Nicotinic Receptors: Located on all postganglionic neurons; always excitatory.

• Muscarinic Receptors: Located on target organs of parasympathetic division; can be excitatory or inhibitory.

Adrenergic Receptors

• Alpha (α) Receptors: Generally excitatory; found in blood vessels, causing vasoconstriction.

• Beta (β) Receptors: Can be excitatory or inhibitory; β1 increases heart rate, β2 causes bronchodilation.

Significance of β Receptors: Targeted by drugs to treat hypertension, asthma, etc.

Main Regions Controlling the ANS

Central Control Centers

• Hypothalamus: Major control center for autonomic functions.

• Brainstem: Regulates heart rate, respiration, and digestive activities.

• Spinal Cord: Integrates some autonomic reflexes.

ANS Reflexes

Examples and Mechanisms

• Baroreceptor Reflex: Maintains blood pressure by adjusting heart rate and vessel diameter.

• Pupillary Light Reflex: Controls pupil diameter in response to light intensity.

• Gastrointestinal Reflexes: Regulate digestive processes.

Mechanism: Sensory input is processed by CNS, which sends motor output via autonomic pathways to effectors.

Summary Table: Comparison of Somatic and Autonomic Nervous Systems