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.