Which Sympathetic Neurotransmitter Raises Heart Rate and Constricts Vessels During the Fight‑Or‑Flight Response?
The sympathetic nervous system (SNS) uses two primary neurotransmitters to orchestrate the fight-or-flight response: norepinephrine (noradrenaline) and epinephrine (adrenaline). Their roles are complementary but distinct.
Norepinephrine (Primary Neurotransmitter)
- Source: Postganglionic sympathetic nerve endings (except sweat glands).
- Receptor Action:
- α₁-Adrenergic: Vascular smooth muscle contraction → vasoconstriction → ↑ systemic vascular resistance (SVR) & blood pressure.
- β₁-Adrenergic: Cardiac stimulation → ↑ heart rate (chronotropy), ↑ contractility (inotropy) → ↑ cardiac output.
- β₂-Adrenergic (minor): Bronchodilation, hepatic glycogenolysis.
- Role: Direct, rapid, localized neural control of vasculature and heart.
Epinephrine (Primary Hormone)
- Source: Adrenal medulla (modified postganglionic neurons) → bloodstream.
- Receptor Action:
- β₁ > β₂ > α₁ affinity at physiological concentrations.
- β₁: Potent ↑ heart rate & contractility.
- β₂: Vasodilation in skeletal muscle/liver (↑ perfusion), bronchodilation.
- α₁ (high dose): Vasoconstriction in skin, splanchnic, renal beds.
- Role: Systemic, sustained amplification of SNS response; metabolic mobilization (glycogenolysis, lipolysis).
Integrated Fight-or-Flight Hemodynamics
| Parameter | Primary Mediator | Effect |
|---|---|---|
| Heart Rate ↑ | NE (neural) + Epi (hormonal) | β₁ stimulation |
| Contractility ↑ | NE + Epi | β₁ |
| SVR ↑ (Vasoconstriction) | NE (α₁) | Skin, splanchnic, renal beds |
| Muscle Blood Flow ↑ | Epi (β₂) | Vasodilation in active muscle |
| Bronchodilation | Epi (β₂) | Airway dilation |