The Autonomic Nervous System

Chapter 11d: The Autonomic Nervous System

Overview of the Autonomic Nervous System

  • The autonomic nervous system (ANS) consists of:

    • Afferent (sensory) components: Carries sensory signals to the central nervous system (CNS).

    • Efferent (motor) components: Carries motor fibers to autonomic effectors. The focus here is on efferent components.

  • Major functions include regulating:

    • Heartbeat

    • Smooth muscle contraction

    • Glandular secretions

    • These functions are essential for maintaining homeostasis.

  • Two primary divisions of the ANS:

    • Sympathetic Division: Noted for the “fight-or-flight” response.

    • Parasympathetic Division: Associated with the “rest and digest” response.

  • ANS pathways comprise separate neural pathways for sympathetic and parasympathetic actions.

  • Many autonomic effectors are dually innervated, which allows precise control of functions.

Structure and Function of the ANS

Sympathetic Division

  • Anatomy:

    • Sympathetic chain ganglia typically located along the anterior surface of the vertebral column.

    • Chain runs from the second cervical vertebra to the coccyx, with 22 sympathetic chains per side:

    • 3 cervical ganglia

    • 11 thoracic ganglia

    • 4 lumbar ganglia and 4 sacral ganglia

  • Functions:

    • Inhibit salivation,

    • Constrict blood vessels,

    • Stimulate secretion of norepinephrine and epinephrine,

    • Dilate pupils,

    • Relax airways,

    • Stimulate glucose production and release.

    • Facilitates physiological responses to stress.

Parasympathetic Division

  • Anatomy:

    • Preganglionic neurons originate in the brain stem and spinal cord.

    • Postganglionic neurons are located in ganglia near or within target organs.

  • Functions:

    • Promote salivation, digestion, urination, and sexual arousal,

    • Constrict airways,

    • Slow the heart rate,

    • Stimulate bile release.

Effectors of the ANS

  • Autonomic Effectors:

    • Cardiac muscle

    • Smooth muscle

    • Glands

  • Somatic Nervous System Effectors:

    • Skeletal muscle

Efferent Pathways and Ganglia

Somatic Nervous System (SNS)

  • Characteristics:

    • Contains a single motor neuron pathway.

    • Cell body located in the CNS.

    • Thick, myelinated group A fibers extend from the spinal/cranial nerves to skeletal muscles.

Autonomic Nervous System (ANS)

  • Characteristics:

    • Uses a two-neuron chain:

    1. Preganglionic Neuron: Located in the CNS and has a thin, lightly myelinated preganglionic axon.

    2. Postganglionic Neuron: Located in autonomic ganglia outside of the CNS, has a nonmyelinated postganglionic axon extending to the effector organ.

Organs Innervation

  • Somatic Nervous System Components:

    • Upper motor neurons in the primary motor cortex.

    • Somatic motor nuclei present in the brain stem.

  • Visceral Motor Neurons:

    • Located in the hypothalamus.

  • Visceral Effectors Include:

    • Smooth muscle

    • Cardiac muscle

    • Glands

Neurotransmitter Effects

Somatic Nervous System

  • All somatic motor neurons release acetylcholine (ACh).

  • Effects are always stimulatory.

Autonomic Nervous System

  • Preganglionic fibers release acetylcholine (ACh).

  • Postganglionic fibers can release:

    • Norepinephrine (NE)

    • Acetylcholine (ACh)

  • The effect can be either stimulatory or inhibitory, depending on the type of receptors present on the target organ.

Comparative Structure of Motor Neurons

  • Somatic Nervous System:

    • Single neuron to effector organs.

    • Heavily myelinated axon (ACh to skeletal muscle).

  • Autonomic Nervous System:

    • Two-neuron chain to effector organs.

    • Lightly myelinated preganglionic axons leading to ganglia, then nonmyelinated postganglionic axon to effectors (ACh, Epinephrine, Norepinephrine).

Functional Overview of the ANS

  • Sympathetic vs. Parasympathetic Functions:

    • The sympathetic division acts during stressful situations whereas the parasympathetic division dominates during restful conditions.

    • Both divisions continually exert an influence on visceral effectors, maintaining a state of balance conducive to homeostasis.

  • Sympathetic Functions:

    • Maintains normal physiological functions under resting conditions,

    • Its major role is in “emergency” reactions, supporting fight-or-flight responses.

  • Parasympathetic Functions:

    • Acts as a dominant control mechanism of autonomic effectors most of the time,

    • Slows heartbeat and promotes digestion and elimination via your “rest and repair.”

Homeostatic Imbalances of the ANS

  • Hypertension:

    • Caused by overactivity of sympathetic vasoconstrictor response to stress.

    • Treatment involves adrenergic receptor-blocking drugs.

  • Raynaud's Disease:

    • Characterized by exaggerated vasoconstriction in fingers and toes resulting in an initially pale, followed by cyanotic (blue) appearance, which is painful.

    • Managed with vasodilators.

  • Autonomic Dysreflexia:

    • Uncontrolled activation of autonomic neurons, often present by individuals with quadriplegia or spinal cord injuries above T6,

    • Leads to life-threatening spikes in blood pressure requiring immediate attention.