Smooth Muscle
- Smooth muscle contains thick and thin filaments that are not organized into sarcomeres.
- Smooth muscle appears homogeneous rather than striated because it lacks sarcomere banding.
- Multiunit smooth muscle consists of cells that behave as separate motor units.
- A motor unit is a functional group controlled by one motor neuron, so separate motor units can be activated more independently than strongly coupled muscle cells.
- Multiunit smooth muscle is found in the iris, ciliary muscle, and vas deferens.
- Multiunit smooth muscle has little or no electrical coupling between cells.
- Multiunit smooth muscle is densely innervated and is controlled mainly by neural input, including autonomic input.
- Unitary smooth muscle is also called single-unit smooth muscle.
- Unitary smooth muscle is the most common type of smooth muscle.
- Unitary smooth muscle is found in the uterus, gastrointestinal tract, ureter, and bladder.
- Unitary smooth muscle is spontaneously active and can show slow waves and pacemaker activity.
- Slow waves are rhythmic changes in membrane potential that can influence smooth-muscle excitability.
- Pacemaker activity is spontaneous rhythmic depolarization that helps generate repeated activity.
- Hormones and neurotransmitters modulate unitary smooth muscle rather than acting as the only control mechanism.
- Strong electrical coupling allows unitary smooth muscle to contract in a coordinated organ-level pattern.
- Vascular smooth muscle has properties of both multiunit and single-unit smooth muscle.
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