Cardiac Function+ Autonomic Nervous System

Chapter 17: The Heart and Autonomic Nervous System

Overview of Cardiac Function

  • The heart is a muscular organ that acts as a pump, circulating blood throughout your body. This vital process ensures that oxygen, nutrients, hormones, and waste products are transported to and from tissues and organs.

  • Heart Muscle Cells (Cardiomyocytes):

    • These specialized cells possess an inherent ability to generate their own electrical impulses, meaning they can initiate their own beats without external nervous system input. This property is known as automaticity or intrinsic rhythmicity.

    • Mnemonic for Intrinsic: "INdependent TRICk: Starts its own beat."

  • Sinoatrial (SA) Node:

    • Often called the heart's main natural pacemaker, this small cluster of specialized cells located in the right atrium generates electrical impulses at the fastest rate, setting the rhythm for the entire heart.

    • These impulses spread across the atria, causing them to contract, and then pass to the ventricles, leading to their contraction.

    • Mnemonic for SA Node/Pacemaker: "Starts All: The heart's alarm clock."

Control of Heart Rate

  • While heart muscle cells can beat on their own, the brain, specifically centers in the medulla oblongata, finely tunes heart rate to meet the body's changing demands. This regulation is primarily carried out by the autonomic nervous system (ANS).

Autonomic Nervous System (ANS)

  • The ANS is a division of the peripheral nervous system that regulates involuntary bodily functions, such as heart rate, digestion, respiratory rate, pupillary response, and urination. It operates without conscious thought.

    • Mnemonic for Involuntary: "IN-the-background: Happens without your will."

  • It has two main, often opposing, parts:

    1. Sympathetic Nervous System

    • This is often referred to as the "fight-or-flight" system. It prepares the body for stressful situations or physical activity by mobilizing energy resources and increasing physiological arousal.

    • Mnemonic for Fight-or-Flight: "Speedy: Sympathetic Survives Stress."

    • Functions (think "action-ready"):

      • Increases heart rate and myocardial contractility to pump blood more forcefully.

      • Increases breathing speed and depth by dilating bronchioles to allow more oxygen intake.

      • Sends more blood to skeletal muscles while diverting blood away from non-essential organs.

      • Slows down digestion by inhibiting gastrointestinal motility and secretions.

      • Also causes pupil dilation, sweating, and glucose release from the liver.

    • Mnemonic for Sympathetic Functions: "Hurry, Run, Move, Don't Eat!" (Heart Rate up, Respiratory Rate up, Muscle blood flow up, Digestion down)

    1. Parasympathetic Nervous System

    • This is the "rest-and-digest" system. It conserves energy as it slows the heart rate, increases intestinal and gland activity, and relaxes sphincter muscles in the gastrointestinal tract.

    • Mnemonic for Rest-and-Digest: "Peaceful: Parasympathetic Promotes Peace."

    • Functions (think "calm-down"):

      • Decreases heart rate and reduces myocardial contractility to conserve energy.

      • Decreases breathing speed and depth by constricting bronchioles.

      • Sends less blood to muscles and redirects it to digestive organs.

      • Speeds up digestion by stimulating gastrointestinal motility and secretions.

      • Also causes pupil constriction and stimulates saliva and tear production.

    • Mnemonic for Parasympathetic Functions: "Halt, Relax, Mellow, Digest!" (Heart Rate down, Respiratory Rate down, Muscle blood flow down, Digestion up)

Balance of Nervous Systems

  • These two systems typically work in opposition, providing antagonistic control over many organs to maintain a state of homeostasis (a stable internal environment). The body dynamically shifts which system predominates based on the current physiological needs and environmental demands.

  • But one takes charge depending on the situation:

    • During Stress or Exertion: The sympathetic nervous system becomes dominant, preparing the body for action.

    • Mnemonic: "Stress = Sympathetic (win)."

    • During Relaxation or Recovery: The parasympathetic nervous system becomes dominant, promoting body maintenance and energy conservation.

    • Mnemonic: "Relax = Parasympathetic (win)."