Chapter 15

FIGHT or FLIGHT Response

Divisions of the ANS

Two Divisions of the Autonomic Nervous System

1. Sympathetic Division:

   • Prepares the body for physical activity.

   • Increases heart rate, blood pressure, airflow, and blood glucose.

   • Reduces blood flow to skin and digestive tract.

2. Parasympathetic Division:

   • Calms functions; reduces energy expenditure.

   • Facilitates digestion and waste elimination.

   • Known as “resting and digesting.”

Autonomic Tone

Balance of the ANS

• Autonomic Tone: Represents the background activity balance.

• Parasympathetic Tone: Maintains smooth muscle tone, resting heart rate about 70-80 bpm.

• Sympathetic Tone: Partial constriction of blood vessels to maintain blood pressure.

Visceral Reflexes

Homeostatic Negative Feedback Loop

• High Blood Pressure Regulation:

  1. Detected by baroreceptors.

  2. Signaled to CNS by afferent neurons.

  3. Efferent signals to heart, resulting in decreased heart rate to lower blood pressure.

  • Vagus Nerve: Transmits inhibitory signals to the heart.

Autonomic Output Pathways

Components of the ANS

• Composes parts of both central and peripheral systems:

  • Control nuclei in the hypothalamus and brainstem.

  • Motor neurons in the spinal cord and peripheral ganglia.

  • Fiber systems travel through cranial and spinal nerves.

Transmission Across Neurons

Neuronal Pathways

• Autonomic Pathway: Signal reaches target organ through two neurons, passing through an autonomic ganglion.

  • Presynaptic Neuron: Has a cell body in the CNS, synapses with a ganglionic neuron.

Effectors in the ANS

Distinction Between Somatic and Autonomic

• Somatic Effectors: Control skeletal muscle with only one motor neuron.

• Visceral Effectors: Control smooth muscle, cardiac muscle, and glands, utilizing two neurons.

Sympathetic Division Overview

Characteristics of the Sympathetic Division

• Known as the thoracolumbar division, arises from T1-L2 spinal segments.

• Short preganglionic, long postganglionic fibers.

• Leads to sympathetic chain ganglia.

• Distributed to all body levels.

Sympathetic Nerve Pathways

Transmission from Sympathetic Ganglia

• Pathways: Axons exit via gray and white rami.

  • Preganglionic fibers (myelinated).

  • Postganglionic fibers (unmyelinated).

Pathways of Sympathetic Missions

Courses of Postganglionic Fibers

• Routes:

  • May enter and synapse immediately or travel along the chain to different ganglia.

  • Some bypass ganglia, continuing as splanchnic nerves.

Synaptic Pathways of Sympathetic Division

Postganglionic Fibers

• Potential to follow three pathways:

  • Synapse immediately in ganglion.

  • Ascend/descend, synapsing at other levels.

  • Bypass the chain to continue to collateral ganglia.

Target Organs of the Sympathetic Division

Organs Affected

• Includes organs from the heart to the intestines, glands, and blood vessels; extensive innervation ensuring optimal function during stress.

Pathways for Sympathetic Functions

Four Main Pathways of Sympathetic Fibers

• Includes spinal nerve, postganglionic sympathetic, splanchnic nerve, and adrenal medulla pathways.

Spinal Nerve Route in Sympathetic Division

Spinal Nerve Pathway

• Postganglionic fibers exit ganglion, entering spinal nerve, then targeting sweat glands and blood vessels.

Direct Postganglionic Sympathetic Route

Targeting Specific Organs

• Direct paths to heart, salivary glands, and thoracic blood vessels without returning via grey rami.

Splanchnic Nerve Pathway

Splanchnic Nerves

• Fibers passing through ganglia without synapsing, leading to collateral ganglia instead.

Adrenal Medulla Pathway

Influence of the Adrenal Medulla

• Stimulates release of epinephrine and norepinephrine, aiding in the fight-or-flight response.

Adrenal Gland Functionality

Divisions of the Adrenal Glands

• Adrenal Cortex: Secretes steroid hormones.

• Adrenal Medulla: Functions as sympathetic ganglion, secreting hormones critical for stress response.

Sympathetic System Summary

Characteristics

• Generally shorter preganglionic and longer postganglionic axons from T1-L2.

• Preganglionic axons branch extensively, allowing greater effects.

Parasympathetic Division Overview

Characteristics of the Parasympathetic Division

• Arises from craniosacral regions: brain and sacral areas of spinal cord.

• Long preganglionic and short postganglionic fibers.

Pathways of the Parasympathetic Division

Overview of Fiber Pathways

• Long preganglionic fibers in specific cranial nerves and sacral spinal cord; generates vessels that target organs.

Cranial Nerves in the Parasympathetic Division

Functions of Cranial Nerves

• Each cranial nerve (III, VII, IX, X) influences specific organs, enhancing functions like salivation and digestion.

Summary of the Parasympathetic System

Key Features

• Long preganglionic and short postganglionic fibers; ganglia located near or within target organs.

Neurotransmitters Overview

Autonomic Functioning Mechanism

• Variable effects due to different neurotransmitters and receptor types.

Cholinergic Neurons

Role of Acetylcholine (ACh)

• Secreted by all preganglionic and some postganglionic neurons, particularly in parasympathetic divisions.

Cholinergic Receptor Types

Receptor Varieties

• Muscarinic Receptors: Generally excitatory/inhibitory, found in most target organs.

• Nicotinic Receptors: Excitatory effects at autonomic ganglions and neuromuscular junctions.

Adrenergic Receptors Overview

Norepinephrine Function

• Adrenergic receptors classified as alpha (α) and beta (β), each stimulating or inhibiting responses.

Alpha Receptor Functions

Types of Alpha Receptors

• Alpha-1: Excitatory, interacts with blood vessels.

• Alpha-2: Inhibitory effects, modulation of sympathetic vs parasympathetic activities.

Beta Receptor Responses

Types of Beta Receptors

• Beta-1: Increases heart activity.

• Beta-2: Induces relaxation in smooth muscles, especially in lungs.

• Beta-3: Related to lipolysis reactions.

Autonomic Effects on Glandular Secretion

Impact of Autonomic Activity

• Secretion rates modulated by blood vessel dynamics—vasodilation enhances secretion, while vasoconstriction inhibits.

Modulatory Substances

Additional Neurotransmitters

• Other substances like enkephalins and nitric oxide modulate the actions of ACh and NE, influencing muscle tone and blood flow.

Neurotransmitter Actions

Reception Mechanism

• Dual action depending on receptor presence—ACh binds to either type of receptor influencing responses.

Dual Innervation Explanation

Interaction of ANS Divisions

• Antagonistic Effect: One division opposes the other (e.g., heart rate).

• Cooperative Effect: Both divisions contribute separately to an overall effect.

Iris Control without Dual Innervation

Unique Control Example

• The iris is influenced differently by parasympathetic and sympathetic inputs for pupil size regulation.

Unique Sympathetic Control

Single Innervation Example

• Some targets receive only sympathetic input, crucial for functions such as blood flow regulation.

Sympathetic Vasomotor Tone Regulation Mechanism

Baseline Activity

• Baseline sympathetic activity determining blood vessel constriction or dilation based on firing frequency, allowing rapid response adjustments.

Overview of Autonomic Reflexes

Components of Reflex Arcs

• Pre-programmed responses manage visceral functions, enabling ANS control.

Example of Autonomic Reflex

Specific Reflex Functions

• Bladder reflex: Stretch activates sensory pathways triggering motor responses for contraction and relaxation of sphincters.

Central Control of Autonomic Function

CNS Influence on ANS

• Higher brain regions influence ANS through emotional responses and core primitive functions.

Neuroendocrine Regulation of Stress

HPA Axis Dynamics

• Interaction of hormones and nervous functions impacting response to stressors, influencing physiological reactions and functions.