A large segment of the nervous system is called the autonomic system. It operates at a subconscious level and controls many functions of the internal organs, including level of pumping activity by the heart, movements of the gastrointestinal tract, and glandular secretion. Neurons are cells that act like tiny batteries which power electrical circuits within the body. They can transmit impulses from cell to cell for communication, control, and interpretation of sensory input.
In a typical neuron dendrites respond to stimuli through input to the neuron cell body. When the stimulus is large enough the cell body sends a one way electrochemical impulse through the axon to the next neuron or to some effector such as a muscle. The most important part of a neuron is its cell membrane. Using active transport and special Na+/K+ ion pumps in the membrane of the axon a neuron creates a resting potential of -70 millivolts. The cell essentially becomes a tiny battery ready for action. The action is the action potential -- a wave of electro-chemical activity which sweeps down the axon in one direction, changing its polarity. Depolarization leads to further depolarization as special electrically sensitive protein gates swing open allowing Na+ ions to enter the cell and K+ ions to escape.
Once depolarized the Na+/K+ pumps quickly re-polarize the axon by using ATP to transfer 3 Na+ ions back out of the cell while simultaneously ushering 2 K+ ions into the cell. The three to two ion imbalance helps create the -70 millivolt resting potential. Unlike a typical electrical connection, where components of a circuit must meet and physically touch, there is a special gap between neurons which stops the action potential from continuing. This gap acts like a switch and is essential for homeostasis and regulation of the nervous system.
The synapse involves chemical diffusion of a special messenger molecule called a neurotransmitter from one neuron to the next.
The axon terminals release neurotransmitters from special vacuoles which flood across the synapse which are then are taken up by receptors in the dendrites of the next neuron. Membranes are polarized or, in other words, exhibit a resting membrane potential. This means that there is an unequal distribution of ions (atoms with a positive or negative charge) on the two sides of the nerve cell membrane. This potential generally measures about 70 millivolts (with the INSIDE of the membrane negative with respect to the outside). So, the resting membrane potential is expressed as -70 mV, and the minus means that the inside is negative relative to (or compared to) the outside. It is called a resting potential because it occurs when a membrane is not being stimulated or conducting impulses (in other words, it's resting). An action potential is a very rapid change in membrane potential that occurs when a nerve cell membrane is stimulated. Specifically, the membrane potential goes from the resting potential (typically -70 mV) to some positive value (typically about +30 mV) in a very short period of time (just a few milliseconds).
There are two types of neurotransmitters: Excitatory - neurotransmitters that make membrane potential less negative (via increased permeability of the membrane to sodium) therefore, tend to 'excite' or stimulate the postsynaptic membrane and Inhibitory - neurotransmitters that make membrane potential more negative (via increased permeability of the membrane to potassium), therefore, tend to 'inhibit' (or make less likely) the transmission of an impulse.
Located in the body there are eight major endocrine glands that secrete chemical substances called hormones. Hormones are transported in the extra cellular fluid to all parts of the body to help regulate cellular function. For instance thyroid hormone increases the rates of most chemical reactions in all cells. In this way, thyroid hormone helps to set the tempo of body activity. Insulin controls glucose metabolism; hormonal system mainly controls metabolic functions.
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