Final answer:
Smooth muscle contractility is regulated by neural and hormonal inputs and local factors, with responses being either stimulatory or inhibitory based on specific neurotransmitter receptors and signaling pathways involved.
Step-by-step explanation:
The control of smooth muscle contraction can be influenced by the autonomic nervous system (ANS) and various hormones. The neurotransmitter receptors associated with smooth muscles and the intracellular signaling pathways they activate determine whether the response will be stimulatory or inhibitory.
Neural stimulation by the ANS can either contract or relax smooth muscle. Sympathetic neurons release neurotransmitters like norepinephrine, which typically induce a contraction response in smooth muscle by interacting with the smooth muscle receptors. However, other neurons may release substances like acetylcholine or nitric oxide (NO), leading to the relaxation or dilation of smooth muscle as seen in blood vessels. This process is essential in regulating blood flow and pressure within the cardiovascular system by controlling the diameter of blood vessels.
Hormonal stimulation such as adrenaline can also affect smooth muscle activity. Adrenaline, when released during stress or exercise, can increase the contraction rate of cardiac muscle, reflecting how hormonal signals can alter muscle activity.
In some cases, smooth muscle activity can be triggered by local factors such as the stretching of the muscle in the walls of visceral organs. This is known as the stress-relaxation response.