Neurons: Communicating Cells
Much of the brain is white matter, with myelinated fibers—totaling more than a hundred kilometers in length—connecting neural structures. Gray matter consists of the cell bodies of neurons and glia. Neurons receive electrochemical signals through dendritic fibers, process them in the cell body, and forward them through axonal fibers. Our brain uses hundreds of kinds of neurotransmitters (molecules), crossing over thousands of synapses (gap junctions) per neuron as part of the brain’s 100 billion neurochemical symphony.
When the cell body receives sufficient stimulation, the neuron fires an electrochemical impulse through its axon to dendrites on other neurons. The process begins with (1) presynaptic stimulation (voltage potentials on a neuron’s dendrites). If the stimulation reaches a threshold in the axon hillock, then (2) an action potential travels from the cell body down the axon.
When the action potential reaches the axon terminal, (3) neurotransmitters are released into the synaptic cleft (a space of about 0.02 micrometers) between the axon and the postsynaptic terminal. These neurotransmitters then (4) lock onto receptor sites on the postsynaptic terminal and stimulate the receiving dendritic spine to (5) propagate a postsynaptic potential. Neurotransmitters are typically of three kinds: amino acids, peptides, and monoamines.
Many axons are coated with myelin—lipid-based electrical insulation—allowing signals to travel at 100 meters per second instead of at five meters per second. Our brains are not fully myelinated until we are in our late twenties, and myelina- tion continues at least until we are in our late thirties and likely into our sixties. What we do influences how our brains are myelinated. So, for example, a world- class pianist has myelinated structures to support the rapid and fine motor movements involved in playing the piano. In patients with multiple sclerosis, the immune system attacks myelin, causing devastating motor impairments.
We have examined typical neural communication. There are, however, hundreds of types of neurons. Some communicate using electricity only, and many have receptor sites directly on the cell body. There are also inhibitory neurons that inhibit target neurons instead of stimulating them. Brain cells are also remarkably plastic and dynamic—dendritic spines and axon terminals can move around within seconds. Moreover, not all neurotransmitters are packaged into synaptic vesicles; some are gases, and it is often unclear if a substance is a neurotransmitter or not. The chemical universe of the brain is still poorly understood, not only with respect to neural signaling but also with regard to hormonal and glial signaling.