Sympathetic Example

Autonomic Nervous System and Cardiac Innervation

Overview of Autonomic Nervous System (ANS) Innervation

  • The autonomic nervous system innervates organs such as the heart, providing motor functions to various muscle types and glands.
  • It consists of sympathetic and parasympathetic branches, which regulate bodily functions unconsciously.

Heart as a Focus Example

  • The heart is innervated by autonomic pathways that influence cardiac muscle function.
  • Key components include:
    • SA Node: Sinoatrial Node, primary pacemaker of the heart.
    • AV Node: Atrioventricular Node, secondary pacemaker.

Central Regulation of Heart Function

  • Brain and Spinal Cord Involvement:
    • Motor functions initiated from the brain stem and spinal cord.
    • Cardioacceleratory Center: Located in the brain stem, regulates heart rate and myocardial contractility.

Sympathetic Nervous System Activation

  • Sympathetic activation is referred to as the 'fight or flight' response.
  • It leads to:
    • Increased heart rate to deliver more oxygen to tissues.
    • Increased myocardial contractility to pump more blood effectively.
  • To activate the sympathetic pathway:
    • Information is sent from the brain stem down to the spinal segments T1 to T2.

Pathway from CNS to Heart

  • At spinal levels T1 to T3:
    • Gray Matter: Contains autonomic cell bodies in the ventrolateral horn.
    • The autonomic signal leaves via the ventral root and links with the dorsal root to form a typical spinal nerve.
  • Descending fibers can target ganglia:
    • Sympathetic Chain Ganglia: Includes inferior, middle, and superior cervical ganglia.
    • Preganglionic neurons synapse at varying levels within the sympathetic chain.

Characteristics of Neurons in Sympathetic Pathway

  • Preganglionic Neurons:
    • Myelinated and synapse at sympathetic chain.
    • Release Acetylcholine (ACh) onto postsynaptic cells at nicotinic receptors.
  • Postganglionic Neurons:
    • Longer neurons that exit the spinal nerve and travel via the vagus nerve and other pathways to the heart.
    • Release Norepinephrine (NE) onto target receptors in the heart.

Receptor Binding and Physiological Effects

  • Postganglionic Action: Norepinephrine acts on adrenergic receptors:
    • Alpha-1 Receptors: Primarily found in vascular smooth muscle.
    • Beta-1 Receptors: Specifically located in the heart.
  • Upon binding to Beta-1 receptors:
    • Norepinephrine activates a second messenger cascade, increasing intracellular calcium levels.
    • Result: Increased contractile force in cardiac tissue, enhancing the heart's pumping effectiveness.
  • Chronotropic and Ionotropic Effects:
    • Heart rate is increased (chronotropic effect).
    • Contractility is also enhanced (ionotropic effect).

Summary of Fight or Flight Response in Heart Function

  • The entire process facilitates an increased cardiac output, primarily during stress and physical exertion to support the body's immediate energy needs.