Neurophysiology: the study of how neurons receive and transmit information
The human brain contains between 10 billion and 100 billion neurons, and twice as many neuroglia ("glia" means "glue" in Greek). The neurons electrochemically transmit nerve impulses.
Neurons have a thick central area with a nucleus. A membrane that is specialized in carrying nerve impulses surrounds each cell. Each neuron also consists of a cell body (soma) and numerous tubelike fibers. The longest fiber, the axon, transports impulses from the cell body to other neurons. It can be up to one meter long. Small, branching fibers (dendrites) pick up the impulses from the axon of other neurons and transmit them to the cell body of its own neuron. The synapse is the point at which the branch of a neuron transmits an impulse to the branch of another neuron. Sensory dendrites, however, do not receive impulses. Instead, they transmit information dealing with temperature, touch, etc. These impulses are formed by special receptors in the skin. Neurons may form synapses with thousands of other cells.
Some axons are coated with a white fatty substance called myelin. This insulates the fiber and speeds up impulse transmissions along its surface by 10 to 100 times the previous speed. Tightly-packed axons coated with myelin form white matter, and those without myelin sheaths form gray matter.
The neuroglia act as supports for the neurons. Some neuroglia protect the brain from injured or diseased neurons by digesting them. They also form myelin, guide developing neurons, take up chemicals involved in communication, and help maintain the environment around the neurons.
Two types of signals are used to process nerve impulses: electrical and chemical. Electrical impulses generate a signal within the cell while chemical processes transmit the signal to another neuron or muscle cell.
As in every other cell, neurons contain potassium (positive), sodium (positive), and chloride (negative) ions. However, neurons have a higher concentration of chloride ions than potassium or sodium, and are therefore negatively charged (polarized). This high concentration of chloride ions is due to the fact that the cell membrane is more permeable to potassium than sodium, and sodium pumps within the cells pump sodium out of the cell. Outside of the neuron, however, the concentrations of the ions are exactly opposite. This charge differential represents stored electrical energy.
When depolarization occurs, the charge differential reverses, and an impulse is created. Depolarization is a quick change in membrane permeability. When a stimulant is received by the neuron, the membrane allows a sudden inflow of sodium ions. This changes the charge of the cell from negative to positive. The concentration change triggers more reactions along the membrane, moving the impulse through the cell. After the refractory period (ionic concentration returns to normal), the neuron is again ready to send impulses.
When an electrical impulse reaches the tip of the axon (right before the synapse), it stimulates small vesicles containing chemical neurotransmitters. These chemicals are relased into the synaptic cleft (the space between the neurons), and then attach to receptors on the other neuron. This cell then depolarizes and starts the electrical impulse cycle.