Nerve Signals, Muscle Control, and Synaptic Function
Nerve Membrane Potential and Nerve Transmission
General
All nerve signals are transmitted by nerve fibers, either in the brain, spinal cord, or peripheral nerves. These long nerve fibers are called axons. The axon is a tubular structure bounded by a cell membrane and filled with a liquid called axoplasm. Within the membrane of all nerve fibers, an electrical potential of about -90 millivolts exists. This is known as the membrane potential, generated by ionic concentration differences across the cell membrane.
Neuromuscular Unit
All skeletal muscles are controlled by nerve fibers that originate in the spinal cord. Sensory nerve fibers are found in the posterior horns of gray matter, while motor fibers leave the anterior horns. The junction between the nerve fiber and the muscle is called the neuromuscular junction. The neuromuscular junction is the connection between the end of a nerve fiber and the striated muscle fiber. The nerve fiber branches at its end to form a complex called the branched terminal, also known as the endplate. This endplate invaginates into the muscle fiber but remains entirely outside the muscle membrane. The invagination of the muscle membrane is called the synaptic trough, and the space between the end of the nerve fiber and the muscle is called the synaptic cleft or space. This space is occupied by a gelatinous substance called neurotransmitters, which are responsible for transmitting the impulse. In the case of striated muscle, the neurotransmitter is called acetylcholine.
The Neuron
The neuron is the fundamental unit of the nervous system. It is a cell whose function is the conduction and transmission of nerve impulses. Neurons have a characteristic that is, that of not reproducing, even if enlarged. Neurons are equipped with extensions. Most are short and multiple, called dendrites. Besides these, it has a longer one called an axon, which carries the impulse to another neuron or effector organ that will undertake the order. The axon of a neuron is covered by different sheaths. In this state, they are called nerve fibers, which conclude in the telodendron or branching in the nerve terminal. The cell body consists of a spherical nucleus with a nucleolus surrounded by cytoplasm (perikaryon) and wrapped, in turn, by the cell membrane. In the cytoplasm are mitochondria, the Golgi apparatus, and two distinct features: Nissl granules and neurofibrils. The former play a metabolic role in neurons and disappear when they are tired or upset. The neurofibrils traverse the cytoplasm and extend to the extensions; their functions are poorly known. The nerve fiber or axon is an extension of the body of the neuron. The axon sometimes presents a fatty sheath of myelin and sometimes also has a more delicate membrane, the neurilemma. Both serve as insulating sheaths to prevent irradiation of the nerve impulse. Besides, the neurilemma acts in nerve fiber regeneration.
Synapses
The synapse is the junction between two neurons. Through this binding, signals are transmitted from one neuron to the next neuron or a muscle. When a nerve impulse reaches the neuromuscular junction, it stimulates the nerve terminal membrane, causing many small vesicles of acetylcholine, which are stored in the nerve terminal or telodendron, to break and release acetylcholine into the synaptic cleft. Acetylcholine acts on the muscle membrane, increasing the permeability to sodium ions, which enter the muscle fiber, changing the action potential. As the inside was negative, and the entry of sodium (which is positive) results in the cell’s electrochemical potential changing, which is called depolarization. This local depolarization (at the neuromuscular junction) sends an action potential that moves in both directions along the fiber. In turn, the action potential traveling along the fiber produces contraction. If the acetylcholine secreted by the nerve endings remained indefinitely in contact with the muscle fiber membrane, it would transmit a continuous series of pulses. However, cholinesterase (an enzyme) located on the membrane surface in the synaptic trough unfolds the enzyme acetylcholine into acetic acid and choline. Therefore, almost immediately after acetylcholine has stimulated the muscle fiber, it is destroyed. This allows the membrane to be repolarized and ready again for stimulation as soon as a new nerve impulse arrives.
Membrane Potential or Resting Potential
Every excitable cell in the body has an electrochemical equilibrium on either side of its plasma membrane, given by various ions such as sodium, potassium, chloride, and magnesium, found in various concentrations inside (intracellular) and outside (extracellular). This concentration difference is given by different circumstances: 1. the total amount they are in, and 2. the electrical difference that exists. Of all the ions we saw, we are only interested in two: sodium and potassium. Sodium is found in greater numbers outside of the cell; that is to say, sodium (Na) is extracellular, and the only way it can enter the cell is through a mechanism called the sodium pump. Potassium (K) is found in greater numbers within the cell and may come and go freely. This results in the outside of the cell being positive with respect to the inside. If we placed two electrodes at the extracellular or intracellular level, we would obtain a value of zero. But if we put one inside and one outside, the inside will give us a value of about minus 90 (-90 millivolts).