Smooth Muscle

Question 1

What are the two main types of smooth muscle and describe their structure and function.

Answer 1

1. Single unit - muscles are interconnected by gap junctions, allowing the muscle fibres to contract as a single unit.
2. Multi-unit - muscles consist of individual muscle fibres that are not linked by gap junctions and contract independently of one another.

Question 2

Where is single unit smooth muscle found?

Answer 2

Gastrointestinal (GI) tract, urinary bladder, uterus, and small blood vessels.

Question 3

Where is multi-unit smooth muscle found?

Answer 3

Large arteries, large airways (trachea and bronchi), the iris and ciliary body of the eye (controlling pupil size and lens shape), and arrector pili muscles in the skin (responsible for “goosebumps”).

Question 4

State two ways a rise in cytosolic calcium can occur.

Answer 4

1. Electrical stimulation
2. Chemical (hormonal/neurotransmitter) stimulation

Question 5

Describe how electrical stimulation leads to a rise of cytosolic smooth muscle.

Answer 5

1. Depolarization of the smooth muscle membrane occurs when there’s an influx of positive ions, often sodium (Na+) or calcium (Ca2+), through voltage-gated channels.
2. This depolarization opens voltage-gated calcium channels (VGCCs) on the sarcolemma (plasma membrane of the muscle cell).
3. Calcium flows into the cytosol from the extracellular space, raising cytosolic calcium concentration.

Question 6

Describe the key events responsible for excitation-contraction coupling in smooth muscle.

Answer 6

1. Increase in cytosolic calcium.
2. Binding of calcium yo calmodium.
3. Activation of myosin light chain kinase.
4. Phosphorylation of myosin light chain - enables myosin to bind to actin filaments.
5. Once phosphorylated, myosin interacts with actin to generate the sliding filament mechanism, producing contraction.
6. Relaxation - When calcium levels decrease, myosin light chain phosphatase dephosphorylates myosin.

Question 7

Draw a table to compare smooth muscle and skeletal muscle.
Cell shape, nuclei, striations, control, t-tubules and SR, actin to myosin ratio, calcium binding, functional differences.

Answer 7

• in notes *

Question 8

Give an example of clinical manipulations of smooth muscle.

Answer 8

Bronchodilators: Medications like beta-2 agonists (e.g., albuterol) relax the smooth muscle in the airways, commonly used to treat asthma or COPD.