Final answer:
A friend scaring you speeds up your heart rate due to activation of the sympathetic nervous system, which increases cardiac muscle contraction to provide more oxygen to muscles. The sympathetic and parasympathetic systems modulate heart rate through the release of neurotransmitters, and hormones can also influence heart rate.
Step-by-step explanation:
When a friend sneaks up behind you and scares you, leading to a sped-up heart rate, cardiac muscle contraction is influenced by the sympathetic nervous system. The reason for this increase is to provide more oxygen to your muscles during what your body perceives as a potential emergency situation.
\ The increased heart rate is a result of norepinephrine being released onto the cardiac muscle cells, binding to adrenergic receptors that cause the cells to depolarize more rapidly. Conversely, the parasympathetic nervous system can release acetylcholine (ACh), which binds to muscarinic receptors and slows the heart rate by causing cells to hyperpolarize, making it harder for them to reach the threshold for depolarization.
Besides the autonomic nervous system, hormones and other factors like blood pressure changes and temperature can affect the rate of cardiac muscle contractions. For example, the hormone adrenaline, released during fight-or-flight responses, increases the contraction rate of cardiac muscle, thereby speeding up the heartbeat.