Question

Describe the following for skeletal, smooth, and cardiac myocytes.

1. How each cell receives a signal
2. Excitation-contraction coupling
3. Pacemaker cells
4. Repolarization (cardiac only)

Answer 1

1.

Cells receive signals in chemical form such as various signaling molecules which binds to an appropriate receptor on a cell surface, resulting in chain of events that carries the signal to the cell interior. Cells signaling molecules includes chemical signals such as neurotransmitters,hormones, peptides, nucleotides which binds to its appropriate receptors and shows their action on that cell.

Cardiac myocytes receive signals from pacemaker cells

Smooth muscle and Skeletal muscle cells receive their signals from motor neurons in the form of neurotransmitters and other signaling molecules

2.

Excitation contraction coupling is the process by which the chemical signal converted into mechanical activity. When action potentials reaches cells it triggers release of calcium from sarcoplasmic reticulum and causes sarcomere shortening and muscle contraction.

In cardiac muscle, ECC is dependent on a phenomenon called calcium-induced calcium release, which involves the influx of calcium ions into the cell, triggering further release of ions into the cytoplasm.

In skeletal muscle, the influx of sodium ions causes an initial depolarization; however, in cardiac muscle, the influx of calcium ions sustains the depolarization so that it lasts longer.

smooth muscle is excited by external stimuli, which causes contraction by releasing calcium into cytoplasm.

3.Pacemaker cells

These are the cells which generates action potential and maintains a rhythmic activity

Cardiac pacemaker cells are present in SA node, AV node which provides impulses and maintains heart rate by generating continuous action potentials.

Skeletal pacemaker cells are stimulated by motor neurons

Smooth muscle pacemaker cells are responsible for contraction which is triggered by Ca2+ transients that activate inward currents that spread through the gap junctions to provide the depolarization signal

Eg__interstitial cells of Cajal (ICCs) (gastrointestinal and urethral SMCs) or atypical SMCs (ureter)

4. Cardiac repolarisation

action potential generated is then  terminated by repolarisation which involves opening of potassium channels and thus allowing K+ to leave the cell and causing the membrane potential to return to negative