Final answer:
The heart is innervated by sympathetic and parasympathetic fibers, with receptors such as β1-adrenergic affecting depolarization and heart rate. The sympathetic nervous system's cardiac nerves from T1-T4 segments innervate various parts of the heart, leading to increased heart activity. Overall heart rate and contraction are regulated through the interplay of neurotransmitters with specific receptors and autonomic control by the medulla oblongata.
Step-by-step explanation:
The human heart is innervated by both sympathetic and parasympathetic fibers, which modulate the cardiac activity. The innervation includes various receptors that respond to neurotransmitters. The β1-adrenergic receptors (A1 effect) on the heart are activated by norepinephrine, causing the heart cells to depolarize faster, which leads to an increase in heart rate. β2-adrenergic receptors (B2 effect) are linked to Gs proteins, leading to increased intracellular cAMP and result in heart muscle contraction and smooth muscle relaxation. β1 receptors also increase intracellular Ca²+, affecting muscle contraction including in the heart. The β1-adrenergic receptors (B1 effect) particularly influence the heart by speeding the rate of depolarization, thus increasing heart rate and force of contraction.
The cardiac plexus and nerves stemming from the sympathetic ganglia (T1-T4) innervate the sinoatrial (SA) and atrioventricular (AV) nodes, as well as the atria and ventricles. The ventricles receive greater sympathetic than parasympathetic innervation. Moreover, autonomic innervation of the heart is controlled by the medulla oblongata, with the cardioaccelerator increasing activity and the vagus nerve (parasympathetic) slowing it down. Overall, the autonomic nervous system is vital for regulating heart rate and force of contraction, influenced both by the neurotransmitters released as well as the specific receptors they bind to.