Final answer:
Smooth muscle cells in blood vessels respond to autonomic nervous system inputs, controlling vessel constriction and dilation. The sympathetic division primarily drives vasoconstriction, while cholinergic and NO-releasing neurons can induce vasodilation. Hormonal signals and local factors also modulate smooth muscle cell activity, and these cells can proliferate through hyperplasia.
Step-by-step explanation:
Smooth muscle cells in the walls of blood vessels act as integrating centers, managing various inputs that affect their activity. These muscle cells respond to involuntary control by the autonomic nervous system, including both the sympathetic and parasympathetic divisions. The sympathetic tone of blood vessels arises primarily due to the sympathetic division since the systemic circulatory system largely lacks parasympathetic input, except in specific areas that can induce muscle relaxation.
The vasomotor centers, which are not anatomically distinct, orchestrate the tone or contraction of these smooth muscle cells within the tunica media of vessels. This process affects peripheral resistance, blood pressure, and flow, ultimately influencing cardiac output. Neurotransmitters like norepinephrine, released by sympathetic neurons, primarily mediate the vasoconstriction. Some neurons, however, induce vasodilation in brain and skeletal muscle vessels by releasing acetylcholine, which triggers endothelial cells to release nitric oxide (NO), leading to vasodilation. Some neurons may also directly release NO to cause vasodilation.
Inputs from hormones and locally secreted substances also influence smooth muscle cell activity. Multiunit smooth muscle cells, found in structures like large blood vessels and respiratory airways, exhibit individual contraction responses to autonomic nerve or hormonal stimuli, without the spread of contraction due to a lack of gap junctions. In contrast to cardiac and skeletal muscle, smooth muscle can undergo both hypertrophy and hyperplasia, proliferating to increase the number of cells.