Final answer:
A stretch-activated contraction is a response in smooth muscle cells due to membrane distortion, often observed in organs needing to stretch like the bladder. This process differs from contractions regulated by the autonomic nervous system or hormones. Single-unit smooth muscle cells have gap junctions enabling synchronized contractions, unlike multiunit smooth muscle cells.
Step-by-step explanation:
A stretch-activated contraction describes the opening of stretch-activated channels in smooth muscle cells in response to a distortion of the cell membrane that is commonly observed in organs that require the capacity to stretch and maintain tension despite changes in volume, such as the urinary bladder or blood vessels. This mechanism differs from contractions initiated by the autonomic nervous system, which may be influenced by hormones, or can occur spontaneously. In certain smooth muscle tissues, like single-unit smooth muscle found in the viscera, stretching of the muscle cell can lead to a contraction through the opening of these channels—an example of a stress-relaxation response that accommodates expansion of the organ.
Furthermore, the presence of gap junctions in single-unit smooth muscle allows for the synchronization of contractions, making these cells function as a cohesive unit. Contrastingly, multiunit smooth muscle cells lack these gap junctions, allowing for more fine-tuned control as each muscle cell can contract independently, typically in response to stimulation from autonomic nerves or hormones but not from stretching.