The Function of the Junction
NERVOUS SYSTEM
The nervous system instructs muscles to contract through electrical signals that flow from neuron to neuron. However, no physical contact exists between neurons and target tissues such as muscle. Therefore, the electrical signal of neurons must be turned into a chemical signal that can diffuse across the space (called the synaptic cleft) between the neuron membrane and that of the muscle. Chemical receptors on the muscle cell detect the chemical and activate a signaling cascade that will lead to the regeneration of the electrical signal within the muscle cell. This regenerated electrical signal causes the contraction. In the following section, each component of this junction, called a synapse, and signal transduction mechanisms are discussed.
Anatomy of a Word
signal transduction
Signal transduction is a biological process in which a cell converts one type of impulse or signal into another. For example, a neuron creates an electrical impulse that is converted to a chemical by neurotransmitters, then transferred to a muscle cell that converts the chemical back into an electrical impulse.
Motor Nerve
Signals from the nerves result in the contraction of skeletal muscles. In neurons, differences in ions between the inside and the outside of the cells result in a membrane voltage, with one side of the plasma membrane being more or less positive than the other. Think of this as being similar to the way a battery in your remote control has a positive and negative end (pole).
As the neuron receives a stimulus, membrane channels allow ions to move across the membrane and cause the voltage of the membrane to change. This localized change of the membrane influences neighboring membrane channels, and also opens channels in a wavelike progression that flows along the axon of the neuron toward its target. When this moving change of voltage (action potential) reaches the end of the nerve, neurotransmitters stored in vesicles fuse with the terminal (presynaptic) membrane and are released into the space between the neuron membrane and the target cell.
Think of this process as similar to what happens when speech is converted to sign language. The medium is different but the message is the same. A thought or sentence can be spoken aloud, converted to sign language and signed (communicating the same information), then converted back to spoken language and spoken aloud.
For motor neurons that stimulate skeletal muscle contraction, the neurotransmitter secreted is acetylcholine. Thus, the electrical signal of the nerve has been transduced into a chemical signal that can diffuse across space.
Skeletal Muscle Membrane
To regenerate the electrical signal in the muscle cells, neurotransmitter receptors are localized in close proximity to the neuron terminal on the muscle membrane. In skeletal muscle, these acetylcholine receptors are named nicotinic receptors. When 2 molecules of acetylcholine bind to the receptor, sodium and potassium ions are allowed to flow into and out of the cell through the nicotinic receptor. This localized change in voltage also leads to changes in the membrane farther along the muscle surface. These regions have membrane channels like those along neurons, which facilitate a wavelike action. Once activated by the nicotinic receptors, the action potential is regenerated and flows over all of the muscle membrane. Because the plasma membrane of skeletal muscles invaginates—folds into a sac or cavity—into the muscle cell, forming tunnels called T-tubules, the action potential spreads rapidly and completely throughout the muscle cell and results in a muscle contraction.
Motor Units
For muscles to contract, energy (in the form of ATP) must be expended. To conserve energy and use only as many muscle cells as are required to accomplish a task, muscles are divided up into functional units called motor units, which consist of a single motor neuron and all of the muscle cells that the neuron is connected to and controls. To initiate a contraction, the central nervous system activates only a few motor units and then progressively activates more and more until the work is accomplished.
What is an example of a motor unit?
For a muscle whose primary function is strength (quadriceps group), there may be several hundred muscle cells connected to a single motor neuron. However, for muscles that require less strength and more control, such as the ciliary muscle that controls the shape of the lens in the eye, only a few muscle cells are attached per neuron.