Excitation-Contraction Coupling
Let's think about how a signal at an open space like this cleft would have only one-way signaling. On which membrane, presynaptic or post-synaptic, would the receptors be for the ACh neurotransmitter? Now you can explain why the signal only goes one way and not back the other way from where the ACh was released.
Video 1. Excitation-Contraction Coupling video (opens in new window).
Synaptic transmission is referred to as chemical transmission because it involves the use of chemicals called neurotransmitters to relay the message from the neuron to its effector cell. These neurotransmitters are synthesized in the soma and stored in synaptic vesicles in the presynaptic neuron's synaptic end-bulb. When the presynaptic neuron fires an action potential, this potential spreads across the neurolemma. When the neurolemma of the synaptic end-bulb depolarizes, special voltage-gated calcium gates are triggered to open. When the calcium gates open, calcium ions diffuse into the synaptic end-bulb. These calcium ions trigger the exocytosis of the neurotransmitter substance from the presynaptic neuron's synaptic vesicles. The neurotransmitter substance diffuses across the synaptic cleft and binds with specific receptors in the cell membrane of the effector cell forming a neurotransmitter-receptor complex. The formation of the neurotransmitter-receptor complex causes a change in ion gates in the plasma membrane of the effector cell. The neurotransmitter is quickly taken up by the presynaptic neuron or broken down by specific enzymes in the cleft, so the effector cell is only stimulated once per presynaptic action potential. Another way that the neurotransmitter becomes less able to stimulate receptors is that it simply diffuses out from the cleft.
The effect neurotransmission has on the effector cell depends on the type of neurotransmitter substance used and the specific type of effector cell. For example, the neurotransmitter acetylcholine (ACh) has an excitatory effect on skeletal muscle cells. This means the formation of the ACh-receptor complex in the sarcolemma causes sodium gates and sodium ions diffuse into the sarcoplasm, depolarizing the muscle cell. However, ACh has an opposite effect on cardiac muscle tissue. In cardiac muscle tissue, the formation of the ACh-receptor complex causes potassium gates to open and the cardiac muscle cell hyperpolarizes. Another neurotransmitter substance, norepinephrine, and causes the opposite effect on the heart.
