Autonomic nervous system

The Autonomic (Visceral) Motor System

  • Maintaining Homeostasis

  • Chapter 20 of Neuroscience 3rd ed.

  • Neuroscience Online, Section 4, Chapters 1-4

The Autonomic Nervous System (ANS)

  • Main Components

    • Sympathetic System

      • Prepares the body for 'Fight or Flight'.

      • Increases heart rate, blood pressure, and respiratory rate.

    • Parasympathetic System

      • Promotes 'Rest and Digest' functions.

      • Decreases heart rate, blood pressure, and respiratory rate.

    • Functionality

      • Complimentary systems that regulate bodily functions autonomously.

  • Hypothalamus

    • Integrates sensory feedback to coordinate mechanisms for maintaining homeostasis (e.g., metabolism, temperature).

    • A key player in energy balance and drinking behavior.

  • Enteric Nervous System

    • Regulates the gastrointestinal tract.

Neurotransmitters of the Autonomic Nervous System

  • Sympathetic Nervous System: Norepinephrine and Epinephrine.

  • Parasympathetic Nervous System: Acetylcholine (ACh).

  • Effects on organs:

    • Sympathetic: Increases heart rate, respiratory rate, and blood pressure; pupil dilation.

    • Parasympathetic: Decreases heart rate, respiratory rate, and blood pressure; promotes digestion.

Structure of Sympathetic and Parasympathetic Divisions

Parasympathetic Division

  • Exits from Cranial and Sacral regions.

  • Long myelinated preganglionic nerves end near targets.

  • Short, unmyelinated postganglionic nerves.

  • Acetylcholine is the main neurotransmitter.

  • More focused activation.

Sympathetic Division

  • Exits from Thoracic and Lumbar regions.

  • Short myelinated preganglionic nerves to sympathetic ganglia.

  • Long, unmyelinated postganglionic nerves.

  • Norepinephrine is the main neurotransmitter.

  • 'All or None' activation model.

Organization of the Sympathetic Nervous System

  • Control Areas

    • Preganglionic spinal outflow organized to sympathetic ganglia.

    • Sympathetic trunk and prevertebral ganglion illustrated.

Autonomic Control of Cardiovascular Function

  • Blood pressure sensed by mechanosensors in the aorta.

  • Chemosensors monitor blood chemistry.

  • Modulation based on sensory feedback:

    • Sympathetic activation speeds heart rate and strengthens contractions (NE).

    • Parasympathetic activation slows heart rate and relaxes contractions (ACh).

Enteric Nervous System

  • Comprises neural networks regulating smooth muscle contractions in the gut.

  • Uses diverse peptide neurotransmitters.

  • Modulated by sympathetic and parasympathetic inputs.

Hypothalamus Functions

  • Main regulator for homeostasis.

  • Integrates a variety of sensory and contextual information.

  • Projections to different brain areas coordinate behaviors.

Appetite Regulation by the Hypothalamus

  • Hormonal Cues:

    • Leptin (from fat cells) signals satiety.

    • Ghrelin (from stomach) signals hunger.

    • Insulin (from pancreas) also promotes satiety.

  • Hypothalamic circuits determine energy balance, with specific antagonistic actions.

Thermoregulation

  • The hypothalamus regulates body temperature through feedback loops.

  • Functions to conserve heat or promote heat loss based on environmental factors.

  • Activation of specific hypothalamic nuclei coordinates thermogenic responses such as shivering.

Circadian Rhythms

  • Controlled by the Suprachiasmatic Nucleus (SCN), the 'master clock'.

  • SCN entrains local clocks in other tissues according to light cycles.

  • Melatonin secretion by the pineal gland is regulated by light conditions.