automatic nervous system

Automatic Nervous System Overview

The autonomic nervous system (ANS) controls involuntary bodily functions, including the regulation of smooth muscle, cardiac muscle, glands, and fat tissues without conscious thought. It is primarily below the level of consciousness and essential for maintaining homeostasis within the internal environment of the body.

Importance of Homeostasis

The autonomic nervous system helps to manage homeostasis of internal environments, responding to both internal and external threats that could disrupt this stability. The focus in this discussion will primarily be on the efferent (outgoing) pathways.

Structure and Pathways of the ANS

The difficulty in understanding the ANS lies in its complex arrangement, which involves afferent (sensory) and efferent (motor) pathways.

  • Central Nervous System (CNS): Comprises the brain and spinal cord.
  • Peripheral Nervous System (PNS): Includes visceral receptors sending information to the CNS, which relays information out through two primary divisions: sympathetic and parasympathetic pathways.
Efferent Pathways

The ANS acts through two main pathways:

  1. Sympathetic Pathway: Often associated with the body's fight or flight response, active in stressful situations.
  2. Parasympathetic Pathway: Associated with rest and digest functions, promoting recovery and conservation of energy.
The Enteric Nervous System

This subsystem governs gastrointestinal function and can independently regulate digestive processes. It has its own integration center and communicates with both the central nervous system and autonomic pathways, allowing for autonomous control yet still receiving higher-level input.

ANS Divisions

Sympathetic Division
  • Consists of neural pathways that originate from thoracic and lumbar spinal cord (hence often referred to as the thoracolumbar division).
  • Structure: Sympathetic ganglia lie alongside the vertebral column, forming a chain that communicates with various effector organs.
  • Functional Characteristics:
    • Activates under stress, increasing heart rate, blood flow to muscles, and pupil dilation.
    • Energy expenditure is prioritized, diminishing digestion and conservation processes.
Parasympathetic Division
  • Originates from the brainstem and sacral spinal cord, referred to as the craniosacral division.
  • Structure: Ganglia are located close to or within the target tissue, resulting in shorter postganglionic fibers.
  • Functional Characteristics:
    • Dominates during restful periods. Promotes digestion, secretion of saliva and enzymes, and urinary function while decreasing heart rate.

Neurotransmitters and Receptors

Sympathetic Division
  • Preganglionic Neurons: Release acetylcholine (ACh) as their neurotransmitter and have unmyelinated postganglionic fibers that may release norepinephrine (NE) or acetylcholine.
  • Postganglionic Neurons: Generally adrenergic (releasing norepinephrine) stimulating receptors that cause physiological responses in the target organ, with effects influenced by receptor subtype (alpha vs. beta).
Parasympathetic Division
  • Preganglionic Neurons: Release acetylcholine, synapsing with postganglionic neurons which typically also release acetylcholine (cholinergic fibers).
  • Effects of Cholinergic Activation: Generally promote stimulation of salivation, lacrimation, urination, digestion, and defecation (the SLUDD system).

Functional Overview

Both divisions maintain a baseline level of activity, termed tonic activity, to finely tune bodily functions:

  • Autonomic Antagonism: This term describes the opposing actions of the sympathetic and parasympathetic systems. For example:
    • Sympathetic: Increases heart rate
    • Parasympathetic: Decreases heart rate.

Summary of Autonomic Functions

  • Sympathetic Functions: Fight or flight, increasing heart rate, inhibiting digestion, dilating pupils, and redirecting blood to skeletal muscles.
  • Parasympathetic Functions: Rest and digest, regulating normal bodily functions, promoting digestion and excretion.

Lab Activity

Students will take part in practical exercises to demonstrate the functioning of the autonomic nervous system, focusing specifically on blood pressure regulation and reflex tests.

Important Definitions and Concepts

  • Homeostasis: The state of steady internal conditions maintained by living things.
  • Afferent Pathway: Sensory nerves that send signals to the central nervous system for processing.
  • Efferent Pathway: Motor nerves that send signals from the central nervous system to the effectors.
  • Preganglionic Fiber: A neuron that transmits impulses from the CNS to a ganglion.
  • Postganglionic Fiber: A neuron that transmits impulses from a ganglion to an effector.
  • Adrenergic and Cholinergic Fibers: Terms that describe the type of neurotransmitter released by postganglionic neurons in the sympathetic and parasympathetic divisions, respectively.