Elaborate neuromuscular junctions are not present in smooth muscle. Often neurotransmitter is released only in close proximity to the muscle such that the neurotransmitter, which may be acetylcholine or norepinephrine, must diffuse to the muscle cells to interact with receptors on the cell membrane. Either of these neurotransmitters may be excitatory or inhibitory depending on the receptors present on that particular smooth muscle cell. Because smooth muscle has spontaneous activity, neuronal input only serves to modify that activity rather than initiating it as in skeletal muscle. Local tissue factors, hormones and mechanical stretch can cause action potentials and thus contraction in smooth muscle.
Smooth muscle is capable of active regeneration after injury.
The heart is made of specialized muscle tissue with some similarities to both smooth and skeletal muscle. It is involuntary and mononucleate as is smooth muscle. Cardiac muscle is striated like skeletal muscle which means that it has microscopically visible myofilaments arranged in parallel with the sarcomere structure described above. These filaments slide along each other during the process of contraction in the same manner as occurs in skeletal muscle. Cardiac muscle fibers branch and have a single nucleus per cell.